The Effect of Organic Impurities on the Precipitation of Alumina Trihydrate in the Bayer Process

THE EFFECT OF ORGANIC IMPURITIES ON THE PRECIPITATION OF ALUMINA TRIHYDRATE IN THE BAYER PROCESS

Lakshman Dissanayake Jayaweera

A thesis submitted for the degree of Doctor of Philosophy in Chemical Engineering

The University of New South Wales Australia

November 1981 Form 1 WAIVER

THE UNIVERSITY OF NEW SOUTH WALES

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New South Wales, and has not been submitted previously to any other university or technical institution for a degree or award.

~~~~ (Lakshman Dlssanayake Jayaweera) ACKNOWLEDGEMENTS

I gratefully acknowledge the award of a research scholarship by the Aluminium Development Council, which supported me during this work.

I express my gratitude to Emeritus Professor J.S.

Ratcliffe, formerly of the School of Chemical Engineering,

The University of New South Wales, for glvlng me the opportunity to undertake this work.

I am indebted very much to my supervisor, Associate

Professor R.G. Robins, for his guidance, encouragement and constructive criticism which improved this work far beyond that which would have been possible otherwise.

I also wish to thank Mr. A.G. Willard of the School of Mining Engineering, The University of New South Wales, for his help in size distribution analysis.

My sincere thanks are also due to Mrs. K. Nasev of the School of Chemical Engineering & Industrial Chem istry, Miss Viera Piegerova, School of Metallurgy, Mrs. P.

Sirimanna, School of Chemistry, the Laboratory staff of the

Sydney Water Board, the Technical staff of the Queensland

Alumina Ltd., Gladstone, and all others associated with this work, for their assistance.

Finally I would like to thank my wife for her unfailing patience.

Dedicated to my parents. ABSTRACT

The Bayer process has been the most successful industrial process used in the extraction of alumina tri hydrate from bauxite. During the process, organic matter accumulates in the recycled caustic liquor as impurities.

These impurities have a wide range of molecular weight and are made up of degraded and oxidised vegetable matter.

The impurities are believed to have a deleterious effect on the precipitation of alumina trihydrate, either by adsorbing on to the surface of the seed particles of alumina trihydrate or coprecipitating other compounds, thereby preventing further crystal growth and leading to finer particles. The rate of decomposition of the alumina trihydrate in the subsequent calcination process is also affected.

In this work studies were conducted in which an attempt was made to separate and identify some of the organlc impurities present in a particular solution of re cycle Bayer liquor from Queensland Alumina Limited. The total organic content of this liquor was found to be ln the range 25 - 30 g/1 expressed as carbon. Eighty percent of the total carbon was below 1000 molecular weight; five to seven percent was above 10,000 molecular weight. Of the low molecular weight compounds (< 500) sodium formate,

'sodium acetate and sodium oxalate, with small quantities of

sodium propionate and sodium valerate were identified.

The adsorption properties of various organic salts

present in the Bayer liquor on alumina trihydrate seed was

investigated to explore whether such adsorption might be linked with the crystallisation process. It was found, however, that low molecular weight compounds such as sodium oxalate, sodium formate, sodium acetate, sodium carbonate, sodium succinate and sodium benzoate as well as the fractions of molecular weight over 1000, were not significantly adsorbed. There was evidence to suggest some of the inter mediate molecular weight compounds did adsorb onto the seed but experimental difficulties hampered this examination.

The solubility of sodium oxalate in sodium hydroxide, sodium aluminate, and Bayer Plant liquor was deter mined over a range of compositions and temperatures. Sodium oxalate was found to be present in the Bayer Plant liquor at a supersaturated level.

A series of batch crystalliser experiments to determine the effects on the formation of alumina trihydrate of the various organic impurities as additives was investi gated. Although no significant effects on the kinetics of precipitation of alumina trihydrate were found, the particle size of the crystals were finer for sodium aluminate solutions supersaturated with sodium oxalate. The other low molecular weight impurities and fraction with molecular weight greater than 1000 had no deleterious effect on the crystallisation process, but caused some variations ln the rate of reaction. TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS

ABSTRACT LIST OF TABLES

LIST OF FIGURES

INTRODUCTION 1

1. Aluminium production in Australia 1

2. Future process development in the 10 aluminium industry

PART I CHAPTER 1

BAYER PROCESS PLANT OPERATION 13

1.1 The Basic Process 13

1.2 Plant Operation at Queensland Alumina Ltd., 16 Glad stone

1.3 Accumulation of Organic Impurities ln Bayer 24 Process Liquor

1.4 Removal of Organic Impurities from Bayer 28 Process Liquor in Plant Practice

CHAPTER 2

CHEMISTRY OF THE BAYER PROCESS 37

2.1 Occurrence of Bauxite 38

2.2 Structure of Alum9na Trihydrate 46

2.21 Nomenclature 47

2.22 Structure of Crystalline Alumina Trihydrate 47 2.221 Hydragillite 4-7

2.222 Bayerite 51

2.223 Boehmite 52

2.224- Diaspore 54-

2.3 Bayer Alumina Trihydrate and its Phase Trans 55

formations

2.4- Extraction Stage in the Bayer Process 60

2.4-1 Factors Affecting the Extraction 64-

2.4-2 Effect of Organics on the Dissolution of 67

Bauxite

2. 5 Separation of Red Mud 68

2.6 Desilication 69

CHAPTER 3

ORIGIN, IDENTIFICATION, EFFECT AND REMOVAL 72

OF ORGANIC IMPURITIES IN THE BAYER PROCESS

3.1 Origin of Organic Impurities 1n Bayer Process 72

Liquor

3.2 Humic Substances 1n the Environment 74-

3.21 Theories on the Formation of Humic Substances 75

3.211 Felbeck's Hypothesis 76 a) Plant Alteration 77

b) Chemical Polymerization 77

c) Cell Autolysis 77

d) Microbial Synthesis 77

3.22 Occurrence of Humic Substances in Minerals 78

and its Relationship to Bauxite

3.23 Characterization of Humic Substances 79 3.231 Functional Groups in Humic Substances 80

a) Oxygen Containing Functional Groups 81

b) Nitrogen Containing Functional Groups 86

3.232 Use of Physical Methods for Characterization 86

of Humic Substances

a) Spectroscopic characterisation: 87

I Visible, spectroscopic method 88

II Ultra violet spectroscopic 89

method

III Infra-red (IR) spectrophoto 90

metric method

IV Nuclear Magnetic Resonance 93

(N.M.R.) Spectrometry

V X-ray analysis 95

b) Molecular Method 96

I Vapour pressure osmometry 96

II Ultra centrifugation 97

III Gel filtration 97

IV Other methods 102

3.24 Chemical Structure and the Degradation of 102

Humic Substances

3.241 Various Models suggested for Chemical 102 Structure of Humic Substances

a) Flaig's Model 102

b) Felbeck's Model 102

c) Finkle's Model 105

d) Haworth's Model 105

e) Fulvic Acid Structure 105 3.242 Degradation of Humic Substances 107

3. 3 Identification and Characterization of 111

Organic Impurities ln Bayer Process Liquor

3.4 Effect of the Organic Impurities on the Rate 117

of Precipitation and the Particle Size

Distribution of Alumina Trihydrate

3. 5 Behaviour and Mechanism of Inhibition of 125

Organic Impurities in the Bayer Precipitation

3.6 Removal of Organic Impurities from Bayer Liquor 132

3.61 Activated Carbon Method 133

3.62 Ion Exchange Method 134

3.63 Oxidation Method 134

3.64 Heat Treatment 136

3.65 Seeding Method 136

CHAPTER 4

CRYSTALLISATION PROCESS OF ALUMINA TRIHYDRATE

4.1 Al - Na H 0 System 138 2o3 2o - 2

4.11 Solubility of Alumina Trihydrate ln Pure Caustic 140

and Bayer Plant Liquor

4.12 Stability of Sodium Aluminate Solution 142

4.13 Super Solubility Concept 144

4.2 Spontaneous Precipitation of Alumina Trihydrate 145

from Sodium Aluminate Solution

4.3 Precipitation from Aluminate Solution Seeded 146

with Alumina Trihydrate (Bayer Process

Crystallisation)

4.31 Kinetics Relation for Seeded Precipitation 146

4.32 Mechanism of Bayer Process Crystallisation 149 4.321 Induction Period 1n Seeded Crystallisation 150

4.322 Nucleation 151

a) Kinetics Relation for Nucleation

4.323 Crystal Growth 155

4.324 Agglomeration 158

4.325 Crystal Breakage and Attrition 161

4.33 Major Factors which Affect the Over-all 162 Process of Crystallisation in the Bayer

Process

4.4 Summary of Crystallisation 162

PART II

CHAPTER 5

EXPERIMENTS RELATED TO THE SEPARATION, IDEN- 164

TIFICATION, ADSORPTION AND SOLUBILITY OF SOME

OF THE ORGANIC CONSTITUENTS OF RECYCLE BAYER

PROCESS CAUSTIC LIQUOR

5.1 Separation and Identification 164

5.11 Separation and Analysis of Organic Constitu 165 ents Present in the Recycle Bayer Process

Liquor

5.112 Determination of Total Organic Carbon 1n the 168

Bayer Process Liquor

a) Using Beckman Carbon Analyser 168

b) Using Coleman Carbon Analyser and 171 Potassium Permanganate Titration Method

5.113 Use of Distillation Method 173 5.114 Use of Gas Chromatography 175

a) Direct Injection of Ether Extract

b) Injection of Esterified Ether Extract

5.115 Infra-Red Analysis 186

5.116 Use of Membrane Process for Characterization 191

5.12 Separation and Analysis of Organics which are 200

associated with the product Alumina Trihydrate

5.121 Methanol Treatment followed by Hot Water 201

De sorption

5.122 Cold Water Wash followed by Hot Water Desorption 203

5.2 Adsorptive Properties of Various Organic Corn- 209

pounds in the Bayer Process Liquor on the Alumina Trihydrate Seeds

5.21 Preparation of Columns and Solutions 211

5.22 Equipment, Procedure and Results 212

5.3 Solubility of Sodium Oxalate in the Pure 226

Caustic and Aluminate Solutions at Varied

Temperature

5.4 Summary of Experiments discussed in 233

Chapter 5

CHAPTER 6

EXPERIMENTAL STUDIES ON THE EFFECT OF 237

ORGANIC IMPURITIES DURING THE CRYSTALL

ISATION OF ALUMINA TRIHYDRATE

6.1 Batch Crystallisation Unit 237

6.2 Preparation of Caustic Aluminate Solution 240

6.3 Seed Preparation 241 6.4 Addition of Impurities 241

6.5 Procedure 242

6.6 Solution Analysis 243

6.7 Particle Size Analysis by Zeiss Type Micro 244 Videomat-2

6.8 Optical and Electronmicrography 244

6.9 Range of Conditions Studied 248

6.10 Results and Discussion 248

6.1~1 Effect of Sodium Oxalate 269

a) Induction period 272

b) Rate of decomposition 272

c) Particle size distribution 274

d) Optical and electron micrograph 275

6 .10.2 Effect of Organic Impurities Extracted from 276 the Bayer Process Liquor

a) Effect on the decomposition rate 277

b) Effect on the particle size distribution 278

6 .10.3 Effect of Sodium Acetate, Formate and 278

Succinate

CHAPTER 7

CONCLUSIONS 294

SUGGESTION FOR FURTHER WORK 295

REFERENCES 299

Appendix I 310 for the Al-Fe-Si-H o Thermodynamic Stability Diagrams 2 System in Relation to the Bayer Process Appendix II

Use of KMn0 Titration Method for Determination 321 a) 4 of Total Organic Carbon Present in the Bayer Process

Liquor b) Use of Coleman Carbon Analyser for Determination 327 of Total Organic Carbon Present in the Bayer Process

Liquor

Appendix III

Use of Zeiss Micro Videomat for Particle Size 330

Analysis of Product Alumina Trihydrate List of Tables

1(a) Economic grade bauxite ln Australia 1(b) Known bauxite reserves in the world

2. Production details for bauxite, alumina and aluminium metal in Australia 3. Alumina refineries in Australia and their designed capacities 4. Aluminium consumption in Australia in comparison with selected other countries 1.1 Composition of bauxite used at Q.A.L., Gladstone 1.2 Chemical composition of recycled caustic soda obtained from the Queensland Alumina Ltd., plant 1.3 Chemical composition of product alumina 2.1 Mineralogical properties of bauxites 2.2 Aluminium bearing minerals present in the bauxite and its Al o content 2 3 2.3 Composition of typical bauxites 2.4 Nomenclature of aluminium hydroxides 2.5 Crystalline form of product trihydrate obtained from Sakamoto's decomposition experiments 3.1 Distribution of oxygen in humic substances 3.2 Main infra-red absorption bands of humic substances 3.3 Summary of molecular weights of humic substances 3.4 Percentage of total humic matter retained by each ultra filtration membrane 3.5 "Building block" compounds (mg/1) identified ln Bayer process liquor 3.6 Low molecular weight degradation products present in low temperature digestion Bayer liquor

3.7 Sodium oxalate concentration after alumina trihydrate precipitation

3.8 Crystalline sodium oxalate ln product alumina trihydrate

5.1 Results of the Beckman Carbon Analyser

5.2 Results of the Coleman Carbon Analyser 5.3 Results of the permanganate titration method

5.4 Titration results and the formic acid content of the distillate collected from the acidified Bayer process liquor

5.5 Various conditions applied in the G.C. analysis of methyl esters of Bayer process liquor

5.6 Results of G.C. analysis 5.7 Main I.R. absorption bands of the organic extract from the Bayer process liquor

5.8 Relative intensities of selected absorption bands for all systems

5.9 Total organic carbon composition of the Bayer process liquor in relation to the various

molecular fraction

5.10 Total organic content 1n the extracts desorbed from the product alumina trihydrate

5.11 Comparison studies of the U.V. results of different Bayer process liquor fractions after passing through the alumina trihydrate

seed column 6.1 Plant and laboratory crystallisation conditions 6.2 Details of experiments on crystallisation

6.3 Crystallisation yield

6.4 Summary of the results of crystallisation

experiments with oxalate impurity charge

Appendix I.1 Free energy data at 25°C for the various

species involved in the Al-Fe-Si-H 0 system 2 Appendix II.1 Results of the total organic carbon deter

mination using permanganate titration method

Appendix II.2 do Appendix III.1 Measuring facilities of Zeiss Micro videomat-2

Appendix III.2 Typical example of the results of the

particle size analysis of alumina trihydrate

using Zeiss microvideomat-2 List of Figures

Fig. 1 World and Australian alumina production

2 Application of aluminium

3 The reduced energy requirements 1n the smelting

of aluminium over 80 years

1.1 The Bayer process basic circuit

1.2 Process flow chart of Queensland Alumina Ltd.,

Glad stone

1.3 Sodium oxalate solubility in Bayer process liquor

1.4 Kaiser patent on control of sodium oxalate in the

Bayer process liquor

1.5 - do -

1.6 - do -

2.1 X-Ray diffraction patterns of bauxite

2.2 Structure of hydragillite

2.3 Model of hydragillite

2.4 Electron-micrograph of hydragillite crystal

2.5 Structure of bayerite

2.6 Structure of boehmite

2.7 Structure of diaspore

2.8 Diagram of equilibrium system of Al o -H at 2 3 2 o higher pressure

2.9 Equilibrium ratios for trihydrated alumina over

a temperature range of 100 to 140°C

2.10 Equilibrium ratios for monohydrated alumina over

a temperature range of 100 to 260 0 C

2.11 Rate of dissolution of a) hydragillite b) boehmite

and c) diaspore 2.12 Silica cycle 1n the Bayer process 3.1 Formation of humic acids

3.2 Visible spectra of a) humic acid b) fulvic acid

3.3 Ultraviolet spectra of a) humic acid and

b) fulvic acid

3.4 Infrared spectra of a) humic acid and b) fulvic

acid 3.5 Infrared spectra of original humic acid obtained

from various sources

3.6 Infrared spectra of original fulvic acid

obtained from various sources

3.7 Flaig's model of structure of humic substances

3.8 Felbeck's model of the structure of humic

substances

3.9 Finkle's model of the structure of humic

substances

3.10 Structure of fulvic acid

3.11 Degraded carboxylic acids obtained under

alkaline permanganate oxidation

3.12 Process of transformation of organic substances

in soil

3.13 Influence of Bayer process liquor impurities

on rate of precipitation

3.14 Influence of organic matter on precipitate quality

3.15a Effect of glucoisosaccharinate on alumina tri

hydrate yield (synthetic digest)

3.15b Effect of glucoisosaccharinate on alumina tri

hydrate precipitation (synthetic digest) 3.15c Particle size distribution analysis. Effect of

glucoisosaccharinate on precipitation of alumina trihydrate (synthetic digest).

3.15d Effect of glucoisosaccharinate on particle size (synthetic digest)

3.16 The behaviour of sodium oxalate in a Bayer plant over a period of one year

4.1 Phase diagrams in the Na 0-Al 0 -H 2 2 3 20 system 4.2 Mier's super solubility concept

4.3 Kossel model for crystal growth 4.4 Screw dislocation mechanism

5.1 Separations and analysis for the organic com pounds present in the Bayer process liquor

5.2 Separations and analysis for the organic com pounds present in the Bayer process liquor

5.3 Distillation of volatile organic acids from acidified Bayer process liquor

5.4 Schematic diagram of the typical gas chroma tograph

5.5 Gas chromatogram of the Bayer process liquor -

direct injection of ether extract, Condition I

5.6 Gas chromatogram of the Bayer process liquor -

direct injection of ether extract, Condition II

5.7 Gas chromatogram of the Bayer process liquor

methyl esters of impurities, Condition I

5.8 Gas chromatogram of the Bayer process liquor -

methyl esters of Bayer impurities, Condition II 5.9 Results of infrared analysis of the Bayer

process impurities in comparison with natural

humic and fulvic acids

5.10 Stirred cell used for the separation of organ1c

impurities over 1000 molecular weight

5.11 Immersible type of membrane used for the separ

ation of organic impurities over 10,000 molecular

weight

5.12 Results of the pH - titration of the higher mole

cular compounds greater than M.wt. 10,000 extracted

from the Bayer process liquor

5.13 Desorption and analytical methods used to deter

mine the organic impurities contaminated with

the plant product alumina trihydrate

5.14 X-ray diffraction analysis of the alumina tri

hydrate after a methanol wash

5.15 Infra-red analysis of the hot water desorbed

extract of the product alumina trihydrate

5.16a Sketch of the experimental unit used for the

adsorption studies

5.17a Laboratory equipment used for the adsorption

studies

5.17b Adsorption studies of sodium oxalate, succinate,

acetate and benzoate on alumina trihydrate seed

5.18 Adsorption studies of organic impurities

extracted from the Bayer process liquor on

alumina trihydrate seed

5.19 - do - 5.20 Adsorption studies of Bayer process liquor on alumina trihydrate seed. Elementary sketch

of the experiment.

5.21 Adsorption of Bayer process liquor on alumina trihydrate seed - comparison studies of the

collected fractions using the U.V. spectrometer

5.22 Solubility of sodium oxalate in the pure caustic solution at different concentrations and

temperature

5.23a Solubility of sodium oxalate in caustic solution

and in the sodium aluminate solution

5.23b Solubility of sodium oxalate in distilled water

at elevated temperatures

6.1a Photograph of the laboratory batch crystalliser

6.1b Schematic diagram of laboratory batch crystalliser

unit

6.2 Effect of organic impurities on crystallisation of alumina trihydrate. Experiment No. 1.

6.3a Effect of organic impurities on crystallisation of alumina trihydrate. Experiment No. 2.

6.3b - do -

6.4a Effect of organic impurities on crystallisation of alumina trihydrate. Experiment No. 3.

6.4b Effect of organic impurities on crystallisation of alumina trihydrate. Experiment No. 4.

6.5a Effect of organic impurities on crystallisation of alumina trihydrate. Experiment No. 5.

6.5b - do - 6.6a Effect of organic impurities on crystallisation

of alumina trihydrate - Experiment No. 6

6.6b - do -

6.7a Effect of organic impurities on crystallisation

of alumina trihydrate - Experiment No. 7

6.7b - do -

6.8a Effect of organic impurities on crystallisation

of alumina trihydrate - Experiment No. 8

6.8b - do -

6. 9a Effect of organic impurities on crystallisation

of alumina trihydrate - Experiment No. 9

6.9b - do -

6.10 Particle s1ze distribution of product alumina

trihydrate, from Experiment No. 1

6.11 - do - from Experiment No. 2 6.12 - do - from Experiment No. 3 6.13 - do - from Experiment No. 5 6.14- - do - from Experiment No. 6 6.15 - do - from Experiment No. 9 6.16 Microphotograph of product alumina trihydrate

without impurity charge (magnification 50X)

6.17a - do - (magnification 200X)

6.18 Microphotograph of product alumina trihydrate

with sodium oxalate impurity charge - from

Experiment No. 6 (magnification 50X)

6.19 - do - (magnification 200X)

6.20 Microphotograph of seed alumina trihydrate

(magnification 50X) 6.21 Microphotograph of product alumina trihydrate

with sodium acetate impurity charge from

Experiment No. 5 (magnification 50X)

6.22 - do - (magnification 200X)

6.23a Electron microscopic photograph of product

alumina trihydrate with sodium oxalate impurity

charge (magnification 150X)

6.23b - do - (magnification 300X)

6.24a Electron microscopic photograph of product

alumina trihydrate without any impurity charge

(magnification 150X)

6.24b - do - (magnification 300X)

6.25 Electron microscopic photograph of seed

alumina trihydrate (magnification 150X)

6.26 Electron microscopic photograph of product

alumina trihydrate with organic impurity charge

(M.wt.> 1000) (magnification 150X)

6.27 Electron microscopic photograph of product

alumina trihydrate with mixture of organic

impurities extracted from the Bayer process

liquor (magnification 150X)

Appendix I.1 Potential -pH diagrams for Al-Fe-H 2o system at 200°C

I.2 Potential -pH diagrams for Al-Fe-H D 2 at 150°C

I.3 Potential -pH diagrams for Al-Fe-H 2o at 100°C I.4 Potential -pH diagrams for Al-Fe-Si-H 0 2 sys t em a t 25oc

I.5 Temperature -pH diagram for Al-H 0 system 2 II.1 Stability field diagram for manganese species

at 25°C INTRODUCTION

1. ALUMINIUM PRODUCTION IN AUSTRALIA

2. FUTURE PROCESS DEVELOPMENTS IN THE ALUMINIUM INDUSTRY 1

INTRODUCTION

1. ALUMINIUM PRODUCTION IN AUSTRALIA

Although aluminium products have been manufac tured within Australia since 1914 in the form of die-cast, rolled and extruded products, the source material was

either imported primary metal or secondary metal from local

scrap, and since domestic per capita consumption was low there was no inducement to establish a primary metal

industry. The second world war became the main cause of

a great expansion in the use of aluminium, especially for

aircraft, and world supplies were inadequate and Australia

could not obtain more than a fraction of its requirements

of the metal from overseas.

In 1940 the Commonwealth Government proposed

the establishment of a combined aluminsrefinery and

aluminium smelter within Australia and selected Bell Bay in

Tasmania as a site which met the basic requirement of

adequate and reliable power, fresh water and an adjacent

deep water port. The plant came into production in 1955

with a capacity of 25,000 tonnes of aluminium per annum.

Bauxite as a plant raw material was totally

imported from Malaya, Indonesia and Indian sources but the

discovery of huge reserves of bauxite in Australia from

1955 onwards brought a dramatic change to the industry.

The opportunity to develop a full integration of the

industry from the mining of the basic resource to the pro

duction of primary metal and the establishment of

secondary industries without reliance on overseas sources 2

saw the beginning of a mass1ve expansion which has attracted a large investment, both financial and technological, and established self-reliance for Australia, domestically, and recognition as a major supplier of resources and end products in export markets.

The various groups which had been responsible for these bauxite discoveries had to look to the experienced world producers to assist in development for two ma1n reasons. Firstly, the technical complexity of production which needed the know-how of experts and, secondly, because of the huge scale of production from Australian deposits which would have to be absorbed into world markets.

The world industry has had relatively few

large-scale integrated producers, with North American

companies producing about half of the world's total

aluminium. The four world giants in aluminium are Alcan

Aluminium Limited (Canada), Alcoa Aluminium Company of

America, Kaiser Aluminium and Chemical Corporation and

Reynolds Metals Company. Next in size come Swiss

Aluminium Limited and Pechiney (France). All of these world leaders except Reynolds are now associated with the

industry in Australia.

Although the beginning of the Australian

aluminium industry was very small and difficult, today

Australia has built up a large and valuable industry

which is still expanding. Today, the contribution to the

Australian economy by the aluminium industry is consider

able. In 1960 - 61 the total imports of bauxite,

unwrought aluminium waste and scrap, and of worked shapes

were valued at $21.8 million. By comparison, only 3

$1 million value of these commodities were exported in

1960 - 61. By 1965 - 66, this imbalance between imports and exports had been reversed and in 1973, the aluminium industry made a contribution exceeding $300 million to

Australis's overseas earnings. In the past years, the growth in export earnings has been equally as dramatic and the value of exports in 1979 exceeded $900 million.

The aluminium industry is also contributing to the domestic economy of Australia. Australia stands in first place in both the mlnlng of bauxite and the production of alumina. The industry has a total value of production of some $1,300 million which is based on an investment of

$2,200 million and provides direct employment to over

16,000 Australians.

As already mentioned, before 1950 there were no known economic deposits of bauxite in Australia, then in quick succession three major deposits were recognised; one at Weipa in Queensland, another at Gove in the Northern

Territory and the third in the Darling Range in Western

Australia. The total of economic grade deposits in

Australia are estimated to be more than 4000 million

(see Table 1a).

The international importance of the discovery of these reserves can be shown by taking into consideration the world total known bauxite reserve (see Table 1b).

These figures are fairly speculative as all of the known fields have not been thoroughly prospected or proven. 4

million tonnes Weipa 2700

Darling Ranges 800

Gove 300

Kimberley District 200

TABLE 1a: Economic grade bauxite in Australia (Ref.1)

Country Size of reserve (million tonnes)

Australia 4000

Brazil 3000

Guinea 2400

China 1000

Jamaica 400

Switzerland 350

U.S.S.R. 300

Hungary 300

U.S.A. 300

Ghana 290

Guyana 290

India 190

Yugoslavia 188

Greece 100

Dominican Republic 40

TABLE 1b: Known bauxite reserves 1n the world (Ref. 1) 5

Production (OOO's tonnes)

Year Bauxite Alumina Aluminium

1966 1827.1 307.0 92.0 1967 4243.6 854.4 92.3 1868 4955.1 1309.5 97.3 1969 7921.1 1931.0 132.1 1970 9256.3 2152.2 205.6 1971 12732.7 2712.6 223.6 1972 14437.0 3068.1 205.7 1973 17595.9 4088.9 207.3 1974 19994.3 4899.2 219.3 1975 21003.5 5128.9 213.9 1976 24083.5 6205.8 232.3 1977 26070.0 6659.0 247.6 1978 24299.2 6775.7 263.4 1979 27584.0 7416.0 269.6

TABLE 2: Production details for bauxite, alumina and aluminium metal in Australia (Ref. 2)

Million tonnes

1960 62 64 66 68 70 72 74 76 78 80 82

Fig. 1 World and Australian alumina production 6

Following the discovery of bauxite in Australia the Australian aluminium industry has recorded a spectacular

growth in the last decade. Table 2 published by the

Aluminium Development Council illustrates the pattern taken

in the use of bauxite ln the aluminium industry in Australia.

Presently the Australian aluminium industry also accounts for a sizeable fraction of the world's product as

shown in Fig. 1.

There are five major companies involved in the

aluminium industry in Australia.

1) Alcan Australia Ltd. (owned 70% by Alcan Aluminium Ltd.,

of Canada and 30% by Australian institutional investors)

produces aluminium and semi-fabricated and fabricated products.

2) Alcoa of Australia Ltd. (owned 51% by Aluminium

Company of America, 20% by Westmines Investments

Pty.Ltd., 16.6% by B.H. South Ltd., 12% by North

Broken Hill Holdings Ltd., and 0.4% by AUC Nominees

Pty.Ltd., and Cushion Trust Ltd., Mines bauxite,

produces alumina, aluminium and semi-fabricated

products.

3) Comalco Ltd. (owned 45% by Kaiser Aluminium and

Chemical Corporation of U.S.A., 45% by Conzinc Rio

tinto of Australia Ltd., and 10% by Australian and

New Zealand public shareholders), mines bauxite,

produces aluminium, semi-fabricated and fabricated

products.

4) Nabalco Pty.Ltd., (owned 50% by Swiss Aluminium

Australia Ltd., and 50% by Gove Alumina Ltd., a 7

consortium of Australian Companies headed by

CSR Ltd.), mines bauxite and refines alumina at

Gove, Northern Territory.

5) Queensland Alumina Ltd. (owned 21.4% by Alcan

Queensland Pty.Ltd., 20% by Aluminium Pechiney

Australia Pty.Ltd., 30.3% by Comalco Ltd., and

28.3% by Kaiser Alumina Australia Corporation,

refines bauxite into alumina.

Commercial mining has been ln operation at both

Weipa and ln the Darling Range since 1963 and in Gove since 1971. The total bauxite mined in all of these areas in 1977 was over 26 million tonnes.

There are four alumina refineries operating in

Australia at present. The first commenced production in

1963 at Kwinana in Western Australia. A second refinery, now the world's largest, came into operation in March 1967 at Gladstone, Queensland. In 1972, Australia's third alumina refinery came into operation at Pinjarra in Western

Australia, and the fourth of the refineries is located at

Gove in the Northern Territory and also began production in

1972. Present Australian capacity for alumina production is more than 6.5 million tonnes and Australia is the world's largest exporter of alumina. The design capacities of the four major refineries are shown in Table 3.

Alumina production in Australia is forecast to reach 10 million tonnes/year by 1980/83 with an aluminium content of 5 million tonnes. The home market is unlikely to reach more than 500,000 tonnes per year of aluminium by

1982/83. This surplus production would indicate that either 8

Company Capacity Outlets (million tonnes)

Alcoa 1.4 Exported-and Alcoa (Kwinana) Smelter at Point Henry, Victoria

Alcoa 2. 2 Exported-and Alcoa (Pinjarra) Smelter at Point Henry, Victoria

Queensland 2. 0 225,000 tonnes to Alumina Comalco, Bell Bay, (Gladstone) 100,000 tonnes to Alcan, Kurri Kurri, balance exported

Nabalco 1.0 Exported (Gove)

Total 6.6

TABLE 3: Alumina refineries in Australia and their designed capacities.

Australia will have to depend on an export market or a further development of integrated aluminium industry within the country. However, home consumption of aluminium has increased significantly as shown in Table 4.

The potential demand for aluminium ln the near future shows good prospects of a significant increase.

The applications of aluminium are expanding immensely in the various fields shown in Fig. 2. The 1976 model U.S. passenger car had an average aluminium content of about

40 kg. By 1983, it is expected to be between 80 and

100 kg. This increase will represent an additional market of 500,000 tonnes by 1983 in the U.S.A. 9

Building and Construction 22.0%

Fig. 2 Applications of aluminium

The other traditional markets for aluminium, such as containers and packaging, building and construction and electrical applications are expected to grow together with the general worldwide increase of per capita consumption. With their market potential, the world aluminium industry is facing an excellent opportunity to bring the supply versus demand balance in order.

Over the last 20 years, world aluminium consumption has had an average growth rate of 9% per annum which is approximately double the rate of growth of Gross

National Product in the main industrialised countries.

This has been the hard won result of the scale and quality of the development and the effort applied, together with a clear recognition of the marketing approach required, which is an approach that is quite different from that 10

adopted in the marketing of most other metals. Aluminium

is essentially a new material, forcing its way into the market by demonstrating its low cost and technical advan tages over existing materials and, in some cases, making

totally new applications possible. Proper price recovery

and the ability to maintain a profitable operation is the

key to the industry's ability to finance its future growth.

2. FUTURE PROCESS DEVELOPMENTS IN THE ALUMINIUM INDUSTRY

Although the aluminium industry has passed through

a very short history, the technical advancement made within

that short period is remarkable. The related research that

has been carried out throughout the world to bring this

industry up to the successful state of today is enormous,

but it cannot be said that the aluminium industry today has

completely achieved its technical capabilities. There are

many problems to be solved in the area of conservation of

energy in both alumina production and in aluminium smelting.

At the present time there are two major energy

sources available for use in the aluminium industry. The

first is electrical energy which is obtained either through

generation from hydro power or by burning coal. The

second is energy derived from oil in the form of fuel oil,

diesel oil and petrol, while in the medium to longer term,

other energy sources such as nuclear energy, may be devel

oped on an economic basis. The aluminium industry claims

to have been continuously working to reduce the amount of

energy which is consumed in a production of aluminium.

Figure 3 shows the energy requirement in the

smelting stage of producing aluminium has been reduced 11

Kwh/Kg. of Al

50~------401------301\~~------~ 20~~~~~~~====:- 101------

oL-----~----~----~--~~--~ 1890 1910 1930 1950 1970 1980 years

Fig. 3 The reduced energy requirements in the smelting of aluminium over 80 years (2). since aluminium was first smelted on a commercial scale.

Further reduction of the consumption of energy is one of the major tasks today that the aluminium producers are facing. It is hoped that the figure can be reduced to below eight kilowatt hours per kilogram of metal.

In order to reduce the energy requirement ln the production of alumina, some technical ideas are now being developed. The Germans have developed a tubular reactor for the extraction process in which higher extraction temperatures are used and very low reaction times can be achieved and reduced capital and fuel costs are claimed (4).

Another area of further development is in minimising the heavy losses of caustic incurred during the separation of red-mud from the aluminate liquour, and consumed by silica in the bauxite. According to the information obtained from Queensland Alumina Ltd., consumption (or loss) of caustic soda per tonne alumina produced is about 0.15 tonne due to formation of various forms of sodium alumina silicate. A challenging task is also ahead 12

for today's alumina producers to increase the output of alumina trihydrate in the precipitation stage and also to increase dissolution of bauxite in the extraction stage.

The availability of high grade (low silica content) bauxite resources is limited. These resources will probably last only for another 30 - 40 years depending on the demand for aluminium, and then the alternatives will be the relatively high silica content clay minerals. The process that has been adopted for extraction of high grade bauxite (the Bayer process) obviously cannot be used economically for the low grade minerals. Although there are several alternatives processes developed, these pro cesses are still economically not viable, but particularly in the United States there is much research effort on these alternative processes. CHAPTER 1

BAYER PROCESS PLANT OPERATION 13

1.1 THE BASIC PROCESS

While many processes have been devised for the

production of pure alumina from bauxite, clay and other

minerals the one which is used predominantly and almost to

the exclusion of the others is the Bayer process invented

by Carl Joseph Bayer and patented by him in 1888.

In this process the bauxite is first digested

with caustic soda solution under pressure and at elevated

temperatures. The alumina is extracted in the form of

soluble sodium aluminate which leaves behind most of the

impurities such as iron oxide, titanium dioxide and silica

as an insoluble residue. The clear, filtered sodium

aluminate solution is diluted and cooled, and ''seed"

crystals of alumina trihydrate are added. The sodium

aluminate solution hydrolyses on the surface of the seed

to form crystalline alumina trihydrate. The alumina trihydrate ~ finally filtered off and calcined to anhydrous

alumina.

The dissolution process for the var1ous con-

stituents of bauxite can be represented by the following

equation:

Monohydrate (boehmite type)

2) Al 0 .3H 0 + 2Na0H + xNaOH Extraction> 2NaA1o + 2 3 2

Trihydrate (gibbsite type) 14

3) xS10 + YA1 0 + 2Na o + aH 0~xS10 yAl o .2Na o. 2 2 3 2 2 2 2 3 2 aH 2o

Irrespective of the type of bauxite (whether predominantly boehmite or gibbsite) processed, it is the trihydrate which is precipitated during decomposition of the sodium aluminate solution and not the monohydrate.

Figure 1.1 shows a simplified flow-sheet of the

Bayer process which has not changed appreciably from the time it was first developed in the 1890's.

The first plant came into operation 1n France in 1895 and it was followed by one in the United Kingdom

1n 1896. A more detailed account of the original process is that the ore is crushed, ground and mixed with recycled sodium aluminate solution and additional caustic soda and the mixture then charged into autoclaves and heated to dissolve the bauxite under pressure. The output slurry from the autoclave system consists of a rich sodium aluminate liqueur and an insoluble residue which is diluted and cooled. The residue (known as red mud) is separated by settling and filtration, washed free from caustic and discarded. The clear filtered liqueur is further cooled and passed to the precipitators for decomposition to precipitate alumina trihydrate. This was the basic method covered by the original Bayer patent. Bayer also investigated the effect of seeding a super saturated liqueur of sodium aluminate, stirred with alumina tri hydrate as seed and as a result further deposition of 15

r- Soda make-up Alumina I

-'-- Evaporation

' Calcination Old I Ore Ore liquor crushing crushinq

Hydrate filtration

I Seed hydrate

Extraction Decomposition ~~

V r------~ New Mud 1 iquQJ: Dilution ~ .r Cooling separation washing~ '-....-~.....::!'---''IL-----,,-, Water ... Mud washings

' 11 Mud to waste

Fig. 1.1 The Bayer process basic circuit 16

alumina trihydrate on the seed took place.

The alumina trihydrate produced in this operation can therefore be regarded as being in two parts. A portion of the slurry of trihydrate and "old" liquour is filtered to give the product hydrate for conversion to alumina, the remainder is separated from liquour either by settling or filtration and returned to the system as seed hydrate.

The product hydrate is washed virtually free from aluminate liquour and calcined in furnaces at 1200°C -

1300°C to give the final alumina. The remaining liquour after the precipitation of trihydrate is recycled v1a an evaporation plant to the bauxite extraction circuit.

All Bayer plants are based on this basic pattern although there may be some differences in the technical and engineering methods which vary according to the type of raw material being used and individual factory preferences.

There are a number of separate cycles within this broad process and these are considered later individually to show how they lead to particular features of process and plant

design.

1.2 PLANT OPERATION AT QUEENSLAND ALUMINA LTD.,GLADSTONE

This plant also follows the same general pattern

of the basic Bayer cycle, but there are many technical and

engineering methods that have been adopted which differ to

some extent from those adopted elsewhere.

The raw material used in the plant is bauxite

from Weipa which typically consists of the two forms; monohydrate boehmite (Al .3H 0) and the trihydrate 2o3 2 gibbsite (Al .3H 0). Alumina in the trihydrate form 2o3 2 17

1s much more readily soluble in sodium aluminate solution of given soda concentration and temperature than the monohydrate. Generally in the Bayer process, A10 .3H 3 2 o is attacked by concentrated caustic soda solution at 125°C, where as Al .H 0 requires a temperature of 2o3 2 over 200°C but the solubility equilibrium for both types depends mainly on the caustic soda concentration, temperature and caustic ratio.

The bauxite used at the Queensland Alumina Ltd., plant has the composition shown in Table 1.1.

TCA- 55.21% (Total chemical alumina)

TAA- 49.21% (Total available alumina extractable)

Total SiO - 5.3% 2

Quartz 1.0%

Reactable Si0 - 4.3% 2

TOOC 0.25% (Total oxidisable organic carbon)

P205 0.08%

Moisture 13 - 15% Ti0 2.6% 2

Table 1.1 Composition of bauxite used at QAL Gladstone

The flow sheet shown in Figure 1.2 indicates some of the more important conditions used in the process as adopted at Gladstone. The whole process can be divided hj Water Water 1-'· \.Q . Coal Steam ------7oo#1tis- - I-' . Steam generation soda N Caustic

0 ttJ 1-oi Evaporation 11 0 0 13,000 T.P.D. l.l 0 (tl Bauxites P! 0 (/l f-':;:l (/l 0 0 Digestion ~ (tl hj f-':;:l I-' Grinding P205 P! rt 0 rt 1-'l ~ f-'· P> 0 rt () :;:l 1-'· ::J' (/l 0 P> :;:l 1-oi 350 T.P.D. t1 rt c~lBO g/ P> 0 (/l Ho 0 (tl 1-h p, 0 P> lO 101°C Fresh Lime 0 ~ ~ Pregnant caustic liquor ;;at er :::s (/l (]) Filtration rt (]) z 1-'· :::s Lime ~8,000g.p.m "PJ 0 (/l - = 187 ) I-' slaking 9;r 0 (/l P> OA~ l 0 0 :::s ) A. p, 65°C P> ~------:;,. Red mud I-' C=l87 g/1 A/C=0.315 to waste ~ Precipitation :;,. 1-'· 180 g. solids/l...... _ :::s · 28,000 g.p.m . 0 P> 0 11 t-1 rt :;,. p, Hot water 1-' Primary N 0 wa~w~:~i~~~e G) w ;;....._ I-' "'-...:-F_.....,.. t1 Pl '< p, Filtrate (/l Oxalate rich ~ rt Lime (]) 0..., 1-'· ,_, \.Q (l) ::J' rt Cal::-iuxr: oxalate LLJto wa.st"'· I-' CXl 19

into the following major operations:

1) Grinding of bauxite and removal P 2o5

2) Digestion (extraction)

3) Flashing stage to recover heat

4) Desilication and separation of red mud (clarification)

5) Precipitation of alumina trihydrate

6) Classification and thickening

7) Filtration

8) Calcination of trihydrate

9) Concentration of recycled caustic liquor.

Bauxite is firstly ground to a particle s1ze of

-10 mesh and then mixed with lime slurry to remove P2o5 so as to prevent phosphate being formed in the product alumina. In the presence of lime, calcium phosphate is formed which is settled with the red mud at a later stage.

Addition of lime at a later stage will also increase the

"strength" of caustic liquor by reacting with Na 2co 3 present in the caustic liquor to form caustic soda. The total input of bauxite 1s about 550 tons per hour.

After treating the bauxite with lime slurry it

1s passed into the digestor where it is leached with concentrated caustic soda solution at a pressure of 500 psl and a temperature of 245 0 C. This caustic solution is a mixture of recycled liquor recovered from the final stages of the process and "make-up" caustic which is added to increase the concentration to 230 g/litre soda (expressed as Na ). It also contains dissolved alumina up to a 2co 3 ratio of Al 0 /Na (A/C 0.360 by weight). Steam at a 2 3 2co 3 = pressure of 700 psi is directly injected into the digestor to 20

attain the required temperature for extraction which occurs during a retention time of about 15 minutes. These conditions are controlled to suit the particular mixture of both gibbsite and boehmite in the bauxite from Weipa.

Usually a weighed quantity of about 9 tons of bauxite per minute is continuously mixed with about 20,000 US gallons of caustic liquor. The reacted slurry which has reached a caustic ratio, A/C (Al 0 /Na ) of 0.65 after 2 3 2co 3 digestion is now passed through a series of flash evapor ators, where steam and heat are recovered from the system.

The temperature is reduced down to the boiling point at atmospheric pressure of about 105°C after passing through the flash evaporators.

The liquor stream then passes through hydro cyclones where a sandy residue is separated and pumped out and the undissolved fine mud is separated out of the liquor by settling in single deck thickeners each 100 ft. in diameter. The settled mud is re-washed in several stages as shown in the flow sheet to recover part of the caustic liquor in the mud. The concentration of caustic in the feed to the thickener is reduced to 180 g/litre as it will be mixed with low concentration caustic solution which is recovered from the mud washing as shown in the flow sheet. The dilution of the caustic concentration lS optimised to separate soluble silica along with the red mud.

Usually the red mud is flocculated with a sorghum starch solution added at the rate of 1 ton of starch for each

500 tons of Al produced. 2o3 The overflow from the mud settlers passes through

Kelly filters to remove the fine material carried over with 21

the liquor and the clear solution ("pregnant mother liquor") is further cooled to about 82°C.

The next stage of the process involves the precipitation of alumina trihydrate from the mother liquor in tall open topped tanks which stand 100 ft. high and 25 ft.

in diameter. The precipitation is aided by seeding with

crystals of previously precipitated hydrate which are added to the tank and agitated for about 36 hours. The seed

crystals are added in a slurry of 130g of solid hydrate

per litre of pregnant liquor. It is essential at this point

to maintain the desired particle size of the alumina tri-

hydrate as it increases during the decomposition process.

Precipitation occurs mainly by growth on existing particles

to yield agglomerated polycrystals. The growth of the

crystals depends mainly on the temperature, the rate of

decomposition and the supersaturation of the solution, the

particle size and the proportion of seed hydrate, the

impurities present in the liquor and the rate of agitation.

The crystals formed are kept ln suspension by agitation.

The precipitators consist of 11 tanks arranged in series,

each with a capacity to hold about 60,000 U.S. gallons.

The overflow passes through by gravity and the liquor passes

out from the last tank at 65°C with a caustic concentration

of 187 g/litre and A/C = 0.375. Between tanks 4 and 5

there is a heat exchanger which brings about a temperature

0 drop of about 14 C.

The liquor from the precipitators which contain

the final required particle size range of crystals is

settled and the trihydrate crystals separated into three

size ranges in a series of three classification tanks. 22

The primary classifier collects the coarse fraction which becomes the product hydrate; the crystals from the secondary and the tertiary classifier are returned to the first precipitation tank as seed crystal after passing through an oxalate removal stage.

The crystals which are settled in the tertiary and secondary thickeners are contaminated with sodium oxalate which retards the growth of alumina trihydrate when these crystals are used for seeding, and as a result an undesirably fine and slowly settling alumina trihydrate precipitates. This situation is avoided by washing the

sodium oxalate from the hydrate seed crystals before they are used in the first precipitation tank. The fine cry-

stalline seed containing sodium oxalate is filtered and washed with hot water to dissolve the oxalate. The tri hydrate crystals from the final filter are then used for

seeding.

The overflow from the tertiary thickener which

has a concentration of 187g of caustic soda per litre and A/C = 0.375 is mixed with fresh caustic soda and passed through evaporators to build up the required concentration

of 230 g/1 and A/C is about 0.360. The chemical composition

of the spent liquor is as shown in Table 1.2.

The alumina trihydrate crystals which are

settled in the primary thickener contain mainly the coarse

particles. These are fed in to the hydrate washing system

which comprises a plurality of hydrate wash tanks and a

filter. After hydrate washing the alumina hydrate is

subjected to calcination. 23

g/litre 1. Alumina Al 0 2 3 84.3

2. Caustic soda (as Na ) 234.5 2co 3

3' Carbonate soda ( -do- ) 24.1 4. NaCl 16.19

5. Si0 2 0.53

6. P205 0.088

7 . V205 0.74

8 . Na 0.31 2so 4

9 . Sodium oxalate 5.51

10. Total organic soda (as carbon) 23.85

Fe .005 11. 2o3

Table 1.2: Chemical composition of recycled caustic soda obtained from the Queensland Alumina Ltd. plant (5)

When alumina trihydrate lS calcined it first

loses its water of crystallisation, forming the active

y - type alumina at 400 - 600°C which will adsorb water.

Since moisture is undesirable in the alumina used in the reduction furnace, calcination is carried to 1200 - 1350°C

in order to form the inert a - alumina. This operation

is carried out in oil-fired rotary kilns of about 200 ft.

long. Dust leaving the kiln is collected by an electro- static precipitator and fed back to the kiln. The alumina is finally cooled and conveyed to storage bunkers

and then despatched to the reduction factories.

The chemical composition of alumina produced is 24

shown 1n Table 1.3.

Maximum Typical allowable

S10 0.030% 0.02% 2

Fe 0 . 0 3 0% 0.016% 2o3

Na 0 . 70% 0. 35% 2o

Loss on ignition 0 . 8% 0.7-0.8% (300-1000 oc)

Screen

+ 100 mesh 7.0% 2-3%

+ 325 mesh 90% 92%

Table 1.3 Chemical composition of product alumina

1.3 THE ACCUMULATION OF ORGANIC IMPURITIES IN THE BAYER LIQUOR

in all Bayer process plants one of the major problems in the precipitation stage is the accumulation of organic impurities. Most of the other impurities originally present in the bauxite such as silica, iron

and titanium are easily separated from the liquor before the precipitation of the alumina trihydrate. An explanation of the separation of iron and aluminium is presented

in the form of thermodynamic stabilities in Appendix I.

As shown in Table 1.1, bauxite contains about

0.25% of organic carbon which is in the form of various humic, fulvic and lignin materials mainly due to decayed

vegetation. These organics are extracted into the Bayer

liquor in the autoclave extraction. Some o£ these organics 25

altered by chemical reactions and many converted into the

sodium salts of various carboxylic acids. Furthermore a certain percentage of starch added for the red mud floccu

lation is also added to the Bayer liquor as a semi-soluble organic compound. Because of the need to re-cycle caustic

liquor these organic compounds accumulate giving it a typical brown coloration. In the Queensland Alumina Plant analysis has shown that the total organic can reach about 160 -

, whereas the sodium oxalate would 190 g/litre as Na2co 3 per litre as Na . Although it reach about 3.0 to 5.00g 2co 3 is generally known that there are a great number of differ

ent organic compounds present in the liquor apart from

sodium oxalate, no detailed quantitative or qualitative

study has been made to identify these compounds. Two

Bayer plants processing the same Jamaican bauxite, with

different temperature conditions of 135°C and 240°C have

shown an organic carbon content of 8.5 g/litre and

15 g/litre (as carbon) respectively in the recycled liquor.

Among the organic salts formed during the process . .some of

them are likely to reach saturated concentrations in the

pregnant liquor. For example, the solubility of sodium

oxalate decreases with an increase of pH. Yamada et.al.(6)

in their work, reported the solubility of sodium oxalate 1n

various concentrations of plant liquor as shown in

Figure 1.3. It was also shown from his work that sodium

oxalate is supersaturated in the solution under the Bayer

process condition thus causing a tendency to crystallise

with the aluminium trihydrate as the concentration of sodium

oxalate in the Bayer liquor reaches about 3 to 4 g/litre.

The equilibrium solubility of sodium oxalate in the Bayer 26

liquor at 60°C is about 2 g/litre .

.-I ...... 4.0 M.R . 3.0 0'>

() 3.0 ~ 0 () 2JJ 0 (\) 70C n:l so·c :z; 1.0 9-o·c 0 0 u 1 0 120 140 160

Na o concentration (g/1) 2 Fig. 1.3 Sodium oxalate solubility in Bayer process liquor (6).

The concentration of the total organic sub-

stances ln the Bayer process liquor reaches a certain level

after a period of time and such level is called the

"saturated concentration". Usually the saturated concen-

tration varies from plant to plant according to the

conditions applied and the type of bauxite treated. Yamada

et.al. (6) reported the following values of saturated con-

centration for the bauxite used in various plants in the

world.

Bauxite Digestion Saturated temp. cone. oc g/l South East Asia 150 8.1

Australia - I 150 17.2

Australia - II 230 11.3

South Pacific Island 150 24.6

Africa 150 5. 5 27

Although there are no routine analyses to deter mine the various organic impurities in Bayer process liquor,

the total organic carbon analysis indicates the rate at

which the organics accumulate. These accumulated organic

impurities are known to cause several process problems. The

main problem is the inhibiting effect of the organics on hydrate precipitation.

The organic impurities are reported to affect

trihydrate precipitation in the following ways:

a) they change the equilibrium solubility

values thus affecting both the rate of precipitation as well

as the precipitation yield.

b) Depending on the nature of the impurity

and the conditions of precipitation, they may increase or

decrease the nucleation rate. For example, it is widely

believed that sodium oxalate builds up to a critical super

saturation and then gives rise to a finer precipitate of

trihydrate suggesting increased nucleation. The mechanism

of such fine formation is believed to be in the following

manner; after reaching the critical supersaturated level of

sodium oxalate it begins to crystallise as very fine

precipitate of sodium oxalate and thus interfere in the

agglomeration of alumina trihydrate in the precipitation

stage by acting as nuclei for trihydrate crystallisation.

c) Impurities are also believed to be inter

fering with the induction period and the crystal growth,

for example, by getting adsorbed on the seed surface as

has been observed in other crystallisation systems.

A product of suitable size distribution lS

desirable for the subsequent reduction step. The slze 28

distribution is also important in the performance of the settling, filtration and calcination operations of the process where an unreasonable rate of fine recirculation can cause considerable operational difficulties and loss of product by leading to the return of significant quantities of hydrate to the digestion circuit.

There may also be some advantage to having certain accumulated organic impurities in the Bayer process liquor depending on the nature of the impurity. For example, higher molecular compounds are believed to aid red mud separation to sediment. Furthermore, as a result of the rise of the solubility of sodium aluminate, premature decomposition will be stopped by the presence of some organlcs.

1.4 THE REMOVAL OF THE ORGANIC IMPURITIES FROM BAYER LIQUOR IN PLANT PRACTICE

Although there are several works reported ln regard to the removal of organics from Bayer process liquor which will be discussed in a later chapter, only one method has been adopted with some success in plant operation.

The Kaiser Aluminium Company of America (7) has patented a method to remove some of the organlcs present in Bayer process liquor (mainly the sodium oxalate).

This method has been adopted by several plants in the world including Queensland Alumina Ltd., at Gladstone. The aim of this method of control of organics lS to obtain the desired particle SlZe of the alumina trihydrate as well as to obtain a better recovery. Normally, during the crystallisation of alumina trihydrate the sodium oxalate 29

and other organic salts having similar properties to sodium oxalate is precipitated from the liquor as very fine crystalline particles to provide active surface sites for the nucleation of alumina trihydrate particles and/or

retard the agglomeration andgrowth of alumina trihydrate

particles thereby resulting in an undesirably fine and

slowly settling alumina trihydrate precipitate.

The Kaiser patent covers the procedure for wash

ing seed crystals from the final sedimentation stages with

hot water prior to recycling to the crystallisers.

It is claimed that the removal of the sodium

oxalate contaminant from the seed fraction of the alumina

trihydrate promotes better particle size control during

precipitation, increased capacity of the precipitation system,

more efficient operation of the classification system, a

purer seed trihydrate and a purer product alumina. This

application also provides for treating the oxalate-rich

wash~ngs to precipitate all the oxalate as calcium oxalate

which can be further processed and also for recovery of

caustic soda which may be recycled in the process.

In the crystallisation stage the liquor is likely

to be saturated with respect to both alumina trihydrate and

sodium oxalate. The oxalate which crystallises tends to

nucleate a finely divided alumina trihydrate precipitate.

The Kaiser patent reports that the finer fractions of the

trihydrate particles from the classification stage are high

in sodium oxalate whereas the coarser fraction is substan-

tially lower in sodium oxalate. Therefore in plant practice

the sodium oxalate is removed only from a portion of the

finer fraction of alumina trihydrate particles which is 30

then used for seeding. The flow diagram for this oxalate removal step which is basically the same in most of the plants in the world, can be traced in Figs. 1.4 - 1.6 which

show a relationship of the washing techniques and the

oxalate removal system of the Kaiser patent with respect

to the precipitation and classification systems of the

Bayer process.

Figure 1.4 is a schematic drawing in which it

can be seen that the pregnant Bayer process liquor and alumina

trihydrate seed particles are added through lines 2 and 3

respectively to a precipitator tank No. 1. After the

precipitation cycle, the slurry containing alumina tri

hydrate and solid sodium oxalate is fed to a coarse particle

thickening system No. 10 through lines 6 and 7. In the

No. 10 tank the fine particles are separated from the

coarse hydrate particles by elutriation and then fed to the

fine particle thickening system No. 30 with the major

portion of the spent liquor. Whereas the relatively

coarse hydrate slurry is fed to the hydrate washing system

No. 24 which is comprised of hydrate wash tanks and a

filter (not shown). After hydrate washing this product

alumina hydrate is subjected to calcination.

The fine particle thickening system No. 30 is

comprised of a plurality of thickeners or classifiers, from

which the fine alumina trihydrate particles are separated

from the major portion of the spent liquor. The overflow

of spent liquor, including some suspended solids, is recycled

to the digestion step of the process. The fine trihydrate

particles and the solid oxalate from the underflow are fed

to the seed washing system. The fine alumina trihydrate

Bayer Bayer

......

recycled recycled

of of

P.t P.t

ctl ctl

)..! )..! 0 0

P.t P.t

)..! )..!

(J) (J)

.J-1 .J-1

·.-! ·.-!

(J) (J)

u u

(J) (J)

0 0

u u

:>;U :>;U

p:; p:;

.jJ .jJ

rcl-1-l rcl-1-l

.-I .-I

1 1

liquor liquor

Ul Ul

0 0 u u

0 0

Ul Ul

(J) (J)

l-1 l-1

-I-) -I-)

(J) (J)

u u

(J) (J)

u u

digestion digestion

:>;P.t :>;P.t

I):; I):;

.-I .-I ro ro

liquor liquor

to to

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Primary Primary

from from

thickener thickener

(tray) (tray) Slurry Slurry 34-

and solid oxalate are subjected to a series of washings in hot water to dissolve sodium oxalate. Thus the alumina trihydrate is separated from the liquor containing dissolved sodium oxalate and the washed seed is recycled to the precipitation step.

The liquor containing dissolved oxalate is either discarded or is sent to an oxalate - precipitation system where calcium oxalate is precipitated. This is normally done by addition of lime thereby giving a caustic solution which can be recycled to the process. The precipitate of calcium oxalate lS converted by calcination to regenerate the lime.

The flowsheet ln Figure 1.5 shows more detail of the particle thickening system. Here it can be seen that the product from thickeners 11,32 and 33 is progressively smaller in particle size, and are referred to as primary, secondary and tertiary thickener trihydrate. The primary thickener trihydrate is of largest particle size and is not normally used as seed material.

In Figure 1.6 there is shown diagrammatically a seed washing system in more detail than in Figures 1.4 and

1.5 for washing the alumina trihydrate particles in the tertiary or tray thickener underflow. The underflow is fed to a rotary belt filter No. 70 wherein the spent liquor

is removed from the seed until the cake contains approx-

imately 80% solids by weight. Several water sprays wash the filter cake with water (cold water, less than 32 0 C).

The amount of water used lS preferably just sufficient to substantially remove all of the spent caustic liquor (the primary filtrate) which can then be recycled in the process. 35

The filter cake from filter No. 70 is agitated with hot water in tank No. 71 at a temperature of 50°C to 90°C, to dissolve the oxalate. The resulting slurry is then filtered on a second rotary belt type filter No. 72 after which the filter cake is again washed with water to remove

substantially all of the dissolved oxalate. The washed

seed is fed to a repulper wherein the seed is reslurried with liquor and returned to the precipitation tanks. The

filtrate from filter No. 72 is oxalate-rich and can be dis

carded or suitably treated (for example, by adding lime to

precipitate calcium oxalate with the recovery of caustic

soda, or by evaporation to precipitate sodium oxalate). The

filtrate from filter No. 72 may be recycled to the dissolver

No. 71 to maintain the trihydrate fluid.

In extensive testing of washing, tertiary (tray)

thickener trihydrate in a system such as that depicted ln

Figure 1.6, it has been found that alumina trihydrate

particles containing 0.4 to 6.5% by weight of sodium

oxalate can be washed to yield a washed alumina seed having

only up to 0.25% by weight of sodium oxalate. The second

ary filtrate from filter No. 72 in the flow sheet (Figure

1.6) contains in the range of 6-32 g/1 sodium oxalate.

The process described above is a typical

application in the Bayer process to reduce the concentra

tion of sodium oxalate and some of the other organic salts

which reach the level of supersaturation after number

of process cycle. But this method does not remove all the

organic matter which can be poisonous to the crystallisation.

On the other hand, the effect of sodium oxalate on

trihydrate crystallisation is itself highly speculative. 36

It is essential to justify its poisonous effect, before any development work is done to remove oxalate from the

Bayer process liquor on a commercial basis. CHAPTER 2

CHEMISTRY OF THE BAYER PROCESS 37

The ma1n ore used for the production of aluminium

1s bauxite. The main compounds present in the bauxite are aluminium oxide hydrates together with oxides and hydrates of iron, silica, titanium and other elements.

Physically the bauxites could be represented either as soft loose aggregates or hard massive rock deposits with its colour changing with iron content from almost white right through to a dark brown. Each bauxite deposit lS distinctive depending on the way in which the var1ous elements are combined and also the physical properties of the ore.

Therefore careful studies have to be conducted for each type of deposit before the optimum conditions are chosen for the

stage of extraction and subsequent precipitation.

There are three main mineralogical forms of alumina present in the world bauxite deposits. They are as follows:

a) Gibbsite: (Al .3H 0) which is easily 2o3 2 extracted in the Bayer process

b) Boehmite: (yA1 o .H 0) which can be extracted 2 3 2 using higher temperatures and caustic concentrations in the

Bayer process

c) Diaspore: (aA1 .H 0) this phase is rare in 2o3 2 Australian bauxite and is only extracted under relatively extreme extraction conditions.

In most of the bauxites of Europe and Northern # Asia the alumina monohydrate minerals predominate. Through- out most of the rest of the world, the bauxites contain alumina largely as the trihydrate gibbsite with varying but relatively small proportions of the monohydrate boehmite. 38

Bauxite from Jamaica is principally gibbsitic but some of it contains as much as 20% boehmite. Major impurities in bauxite ores include iron oxides, aluminium silicates, quartz and titanium dioxides.

Australian bauxites, depending on the deposits, represent both gibbsitic and boehmitic forms. For example, the bauxite mined from Weipa contain about 20% of boehmite, whereas the bauxite from the Darling Ranges is mainly gibbsitic, but low in the total available content of alumina which is about 30 - 35%.

2.1 OCCURRENCE OF BAUXITE

It is estimated that about eight percent of the

earth's crust is aluminium, occurring mainly in combination with silica in the form of various silicate minerals. Under

specific conditions these minerals are weathered to the

hydrous oxides from which alumina can be extracted.

The definition of bauxites from the refiner's

point of view is a mineral deposit of aluminium 1n which

aluminium is present predominantly as a hydrate or a mlx

ture of hydrates and hydrous oxides. It also contains the

other oxides of iron, silicon and titanium and traces of

compounds of some other elements.

The silica is mainly present 1n the form of

hydrous aluminium silicate and a portion of it is found

as quartz.

Specific weathering conditions such as high

temperatures and seasonal variation in the weather is

believed to be the main requirement for the formation of

bauxite. Gently sloping land forms, minimal denudation, 39

tropical or sub-tropical vegetation, and a rock texture which lS susceptible to leaching are considered as the other requirements.

Under these conditions and with the increase 1n acidity due to the presence of carbonic acid solutions derived from rain, and organic acids supplied by plants, the action of bacteria and other organisms leach out the more soluble potassium, sodium, calcium and aluminium silicates, thereby leaving behind a residue richer in the more insoluble hydrous oxides of aluminium and iron.

Depending on the predominance of iron or aluminium oxides the laterites are classified as ferruginous or aluminous.

If the laterite is rich in aluminium it is called bauxite.

From the mineralogical point of view, the residual rocks, in which the alumina trihydrate and monohydrate minerals gibbsite, boehmite and diaspore predominate are classified as bauxite. Other residual minerals are developed in a unique type of rock weathering which produces bauxite, and these minerals, kaolinite, hallosite, goethite, hematite, magnetite, anatase, quartz and some phosphatic and manganiferrous minerals may form the lesser constituents of bauxites.

As mentioned earlier. the bauxites form only under special climatic, hydrological and biological con ditions. Bauxitization occurs only above the water level, where there is alternating downward movement of acid solutions and upward movement during drier periods of alkaline solutions in rocks which contain sufficient sodium and calcium aluminium silicates or calcaseous rocks contain ing small amounts only of soda, ferric oxide, silica and 40

alumina but with the latter oxides in the ratio 1 : 1. In bauxites which are still being formed the predominant alumina hydrate mineral is gibbsite. The tropics and sub-tropics appear to provide the proper climatic conditions, since it lS only in these areas that gibbsitic bauxites are known to be forming today. In Malaya, where gibbsitic bauxites are forming from metamorphosed acid volcanics and sediments on a peneplained area, the annual range of temperature is from

68 to 94°F, the rainfall from 85 to 110 ln. and the moisture content of the bauxites from 5 to 14%. In the Hawaiian

Islands, where gibbsitic bauxites are forming from alkaline basalts and andesites on the wet side only of volcanic 0 domes, the annual range of temperature is from 62 to 77 F, the rainfall from 60 to 150 in., and the moisture content of the bauxites from 26 to 41%. While in Jamaica, where

gibbsitic bauxites are forming on karsted tertiary limestone

plateaux, the annual range of temperature is from 60 to 80°F,

the rainfall from 50 to 95 in., and the moisture content of

the bauxite from 20 to 23%.

Climatic variation and diastrophe, during or

after bauxitization probably account for subsequent mineral-

ogical changes of gibbsitic bauxites. Under a drier envir-

onment boehmite may be produced from gibbsite and hematite

from goethite, while under reducing conditions siderite,

marcasite, pyrite and chamoisite may develop from goethite,

hematite and magnetite and kaolinite from hallosite.

Summaries of the physical, crystallographic and

optical properties of the three alumina hydrate minerals,

gibbsite, boehmite and diaspore, have been published by 41

Hose (8) and are shown in Table 2.1. X-ray diffraction patterns of typical bauxite are shown ln Figure 2.1.

X-ray diffraction patterns (a) and (b) Fig. 2.1 are of diasporic bauxites derived from limestones from

Greece. Pattern (b) shows the lines of hematite and anatase, which are the normal lesser constituents in these bauxites and (a) is of thermally metamorphosed low-grade diasporic bauxite in which the kaolinite has been converted to chlorite which was identified optically as daphnite.

X-ray diffraction pattern (c) is of boehmitic bauxite derived from limestone from France, whereas pattern (d) is of fine-grained red bauxite derived from limestone in Jamaica. Pattern (e) of coarsely crystalline gibbsite occurs as pods up to 4 ft. ln length in a low-grade supergene manganese deposit in Minas Gerais.

These gibbsite crystals fluoresce yellow under short wave ultraviolet light. These coarse gibbsite crystals give a very strong d = 4.82 line and a weaker d = 4.34 than usual for gibbsite in gibbsitic bauxites.

Usual alumina bearing minerals present ln the bauxites have been published by Kuznetsov and Derevyarn kin (9) with its formulas as shown in Table 2.2.

As the aluminium industry developed, bauxite exploration was stimulated and resulted in the discovery of immense deposits in tropical countries. Known bauxites reserves throughout the world, including Australia, are shown earlier in Table 1a and 1b.

Although the main components of all the ores are identical, there are many variations in the percentage of each compound according to the location. Table 2.3 4-2

{a)

(b)

B B (c)

(d)

G A-anatase H-hallosyte B-boehmite He-hematite C-calcite K-kaolinite Ch-chlorite M-magnetite G-gibbsite P-phosphate (e) Go-goethite Q-quartz Diaspore G GG G G ""G9G.Ar-. GTG,., ~VWV~\fk.-.1~

Fig. 2.1 X-Ray defraction patterns of bauxites 43

Mineral Gibbsite Boehmite Diaspore

Chemical formula

65.4 85 85

34.6 15 15

Crystal system Monoclinic Orthorhombic Orthorhombic

Space group

Unit axis a8.62 2.85 4.40 lengths,A b5.06 12.2 9.39 c9.70 3.69 2.84 angle 85° 26 I

X-ray diffraction 4.82 6.11 3. 9 9 d values in A 4.34 3.16 2. 56 and intensities 4.30 2. 3 5 2. 3 2 of strongest 2.44 1. 86 2.13 lines 2.37 1.85 2.08 2.03 1. 45 1. 63 1.98 1.31 1. 48 1. 79

Index of refrac 1.568 1. 649 1. 702 tion nD 1.568 1.659 1.722 1.587 1.665 1.750

Cleavage (001)perfect (010) (010)perfect

Hardness (Moh) 2.5-3.5 3.5-4 6.5-7

Density 2.42 3.01 3.44

Temp. of decomposition

TABLE 2.1 Minerological properties of bauxites (8) Mineral Formula Content of Al o wt.% 2 3

1) Corundum Al o 100 2 3 2) Diaspore, boehmite AlOOH 85 3) Al(OH) 65.1+ Hydragillate, baerite 3

I+) Al o .2H o 39.5 Kaolinite 2 3 .2Si0 2 2

5) Al o .1+H 0 31+.7 Hallosyte 2 3 .2Si0 2 2 6) Al o .1+Si0 .H 0 28.3 Pyrophyllite 2 3 2 2

7) Allophane mA1 o .pH o 2 3. 5 - 1+1.6 2 3 .nsio 2 2 8) K o .2H 0 38.5 Muscorite 20.3Al 2 3 .6Si0 2 2

9) Hydrous K< o J[OHJ .nH 0 25 33 muscorite 1Al 2[CSi,Al) 4 10 2 2 -

10) Chamosite I+FeO.Al o .1+H 0 13 20.1 2 3 .3Si0 2 2 -

TABLE 2.2 Aluminium bearing minerals present in the bauxite and its Al o content (9). 2 3 Composition % Grade of Ore Proportion of Loss on Country % Trihydrate or Monohydrate Silica Available (Gibbsite) (Boehmite Reactive Free Iron Ignition Alumina Diaspore)

Australia

Darling Range 30-35 Major Nil 1-2 18-22 15-20 20

Mitchell Plateau 40-45 Major Small 2-5 20 20

Gove 50 Major Small 2-6 0.4 13-18 26

Weipa 52 40% 10% 3 2 . 5 7 26

Guinea 50 Major Small 1.2 0.8 24 25 Jamaica 46-50 Major Small 2. 5 17-22 26-27 Suriname 46-50 Major 4-5 1-2 8-11 26-28 Yugoslavia 58-64 Major 2-3 16-22 Hungary 50-60 Major 1-8 15-20 13-20 Greece 35-60 Major 0.5-5 15-30

Dominican 46-48 Major &mall 1-5 20 23-26 Republic United States 50 Major 11-13 4-7 27 India 50-55 Major 1-3 7-10 28-29

-1=" CJ1 TABLE 2. 3 Composition of typical bauxites 46

published by Kirke (10) shows the composition of bauxite taken from throughout the world.

The relative degree of hydration is an important economic feature of bauxite. The extraction of alumina from bauxite containing boehmite and diaspore (alumina monohydrate) is more difficult and costly than from those consisting largely of gibbsite (alumina trihydrate). As

shown in Table 2.3, most of the deposits from the tropical reg1ons, including Australian deposits, can be considered as the most economical deposits due to the higher percent age of gibbsite.

The higher content of silica will also decide the economy of the bauxite deposits. For example,the bauxite from the Darling Range has 1 - 2% reactive silica and 18 - 22% free silica. This would also increase the cost of operation in the process.

Other factors such as the location of the deposits, for example, whether it is a surface deposit or an interior deposit, will affect the quality of bauxite.

Normally surface deposits contain relatively high levels of organic materials and their decomposition products during the refining process tend to be compounded by the low grade.

2.2 STRUCTURE OF ALUMINA TRIHYDRATE

There are three forms of aluminium hydroxide which are the amorphous, gel and crystalline forms. There are two groups of the crystalline aluminium hydroxide that have been identified as follows: 47

Group 1 - alumina trihydrate, which exists in three

different crystalline modifications

Group 2 - alumina monohydrate (oxide-hydroxide -

AlO(OH)) which exists in two crystalline

modifications.

2.21 NOMENCLATURE

There is some confusion as to the nomenclature relating to aluminium hydroxide. Table 2.4 summarises various names which have been ascribed to the hydroxides.

This table is a result which evolved by some agreement at a symposium in West Germany in 1957 (11).

2.22 STRUCTURE OF CRYSTALLINE ALUMINA TRIHYDRATE

2.221 Gibbsite (hydragillite)

Most of the bauxite deposits found in the tropical regions of the world contain gibbsite as the major component. This is in the form of trihydrate which is crystallised as product during precipitation in tbe Bayer process. Megaw et.al.(12), Saalfeld et.al.(13) and

Wells (14) have independently investigated the structure of gibbsite.

In Wells' work, he has described the alumina trihydrate as a tri-clinic structure, whereas in Megaw's work it has been described as a monoclinic (pseudo-hexagonal) structure. The products usually obtained from the Bayer process precipitation support Megaw's conclusion.

Kuznetsov (9) has reported that the structure of gibbsite of brucite Mg(OH) , is very similar to the structure 2 CD

acid

hydroxide

names

trihydrate

dihydrate

monohydrate

alumina

Other

Aluminium Orthoaluminic Bauxite Alumina

Aluminium Randomite Alumina

Bauxite

Baverite

Alurnina

Meta

(])

ro (J)

UJ ro

(!) UJ

(!) ~ !-l

(.9

·.-!

·.-I

Pl!-l

.-I

.£:

Haber

(18)1925

and

1930

(11)

(17)

Frary

alumina

alurnina

trihydrate

monohydrate

Edwardsi Jeffries

Alpha Beta

hydroxides

Alpha Beta

aluminium

(16)

alumina 1934

alumina

Milligan

alumina

alurnina

trihydrate

Nomenclature

Gamma

Alpha

Gamma

monohydrate

Alpha

Weiser

2.4

TABLE

(15)

Boehmite)

Gibbsite

Bayerite

Bohmite

Hydrargillite

Nordstrandite

(or

1957

cEJl.aspore

Symposium

(!)

;:;..,

X X (J)

l-4

::r:

'Cl

'Cl ·.-I

·.-I

(]) ~

!-l >::

;:;.., !-l ::s

.j.l

'Cl

.£:

'Cl

·.-I

·.-I .-I

·.-I ·.-I

::s

E ~

;:1

.-I

·.-I ·.-I 49

except for minor differences.

The basic structural unit of gibbsite is a layer consisting of two sheets of closely packed, negatively charged, hydroxyl ions bound together by positively charged aluminium ions which occupy two-thirds of the interstices between the hydroxyl sheets as shown in Fig. 2.2. Growth occurs by superposition of other layers and lateral extension of the layers. Figure 2.3 shows a model of the gibbsite

structure in which it can be seen that there are no alumin-

ium ions between the second and third layers of the hydroxyl

ions and the hydroxyl ions of the third layer are directly over rather than lying in the depression of the second layer.

The stacking arrangement of the double sheet layers can be

1 I I described by the symbol: AB : BA : AB : BA and so on, I where A and B denote the hydroxyl sheets in different positions as shown in Figure 2.2, and the vertical dashes represent the boundaries between the layers. In the

"c" -plane the large hydroxyl ions are approximately hexa-

gonal close packed. The small hexagonally distributed aluminium ions are surrounded by six hydroxyl ions, three above and three below. A characteristic of the structure is the hole in the centre of each hexagon of the aluminium

ions.

According to Van Nordstrand et.al.(19) gibbsite grows as hexagonal platelets aligned parallel to the layer

in the lattice. According to Oomes et.al.(20) gibbsite crystallises as hexagonal plates below 40 0 C, and above 60 0 C as hexagonal bars. Hence the rate of growth on the direction of the C - axis apparently depends more on temperature so

B .B

-----A

Al at 1/3 2/3

Al at 1/6 5/6

Fig. 2.2 Structure of hydragillite

c- plane

Fig. 2. 3 Models of hydrogillite

Fig. 2.4 Electron-micrograph of hydragillite crystal 51

than the rate of growth in the direction of the other axis.

The hexagonal plate structure is easily recognised in

Figure 2.4 which is an electron micrograph of a small gibbsite crystal.

According to Megaw et.al.(12), Betexin (21) and

Ormont (22) (who reported separately) the unit cell of gibbsite consists of eight ions of aluminium and twenty- four of hydroxyl which corresponds to eight units of

The dimensions of the unit cell lattice have been 0 reported by Betexin et.al. as a= 8.624°A, b = 5.060A ,

0 0 ~ c = 9.7A , S = 94 34 (21) whereas according to Ormont s = 85° 26~.

2.222 Bayerite The hydroxide bayerite was named after the founder of the Bayer process, K.J. Bayer. Unlike gibbsite, bayerite lS not found ln nature (except for a very few, rare occurrences). The existence was first reported by

Fricke (23). Aranderenko et.al. (24) reported the possible formation of bayerite by passing carbondioxide through a solution of sodium aluminate containing 200 g/litre of

A1 0 at room temperature without any seeding. Bayerite 2 3 also can be produced by heating gibbsite to 200°C under 2 pressures in the vicinity of 30,000 kg/cm (25). According to Fricke (23), bayerite can also be found ln an intermediate form between bayerite and gibbsite.

According to Montoro (26), bayerite is also crystallised in a hexagonal form in which the unit cell consists of two ions of aluminium and six hydroxyl lons,

cell dimensions are a = 5.01°A and c = 4.76°A. and the 0 0 52

Funaki et.al. (27) has reported similar results with a difference of 0.01°A in all dimensions. Sasvari (28) has suggested that the structure of bayerite is monoclinic, but that the unit cell consists of four molecules of

Al(OH) instead of two. Belov (29) has suggested the 3 structure of the crystal of bayerite as shown in Figure 2.5.

Fig. 2.5 Structure o£ bayerite (29)

2.223 Boehmite

This mineral was named after Byoma who was the first person to discover it. Boehmite is usually found in nature as diaspore boehmite or as the boehmite hydragillite type bauxite. Crystals of boehmite can be obtained by heating either hydragillite, bayerite or 53

- alumina, with water in an autoclave or in a hydro- thermal bomb (30,31,32). Formation of boehmite has also been reported by Diana et.al. (33) as a result of heating a mixture of aluminium nitrate and nitric acid at 320 -

360°C under a pressure of 200 - 300 atm.

Boehmite crystallises in the rhombic system.

The structure of the crystal of boehmite is believed to be

as shown in Figure 2.6 (9).

Fig. 2. 6 Structure of boehmite

In boehmite the refractive index was found to be in the range of 1.654 - 1.661 (34) and the density . -3 0 of pure boehm1te 3.014 gm cm at 20 C (35).

Cryst~l structure was reported by Betexin (21) and Diana et.al. (33) as having a unit cell with dimensions: 54

0 a = 3.78 A c = 2.85A0 0 0

The crystalline structure of boehmite is very similar to

the structure of lenidocktrite (goethite) FeOOH as shown in

Figure 2.6. E very F e +3 ('ln b oe h . mlte Al +3 lon. ) lS

surrounded octahedrally by six ions of oxygen (21).

2.224 Diaspore

Diaspore has a similar composition to boehmite

and it is normally found in diaspore or in the mixture of

diaspore-boehmitic type bauxites. It is also found as the mineral in a pure form in the Naksos region of Greece and

Koksoi Brod (Sverdlovski region) and Aktas (Republic of

Usbekistan) of U.S.S.R., where it is combined with corundum

and aluminium silicate.

The artificial preparation of diaspore was first reported by Laubengayer and Weisz (31) and it can be formed from boehmite or y-kaolinite by heating at temperature of 280°C - 450°C, for a time interval of 50 - 200 hours

ln the presence of natural or artificial diaspore (2%) as seed material.

Diaspore as boehmite crystallises in the rhombic system in a rhombo-dipyramidal habit. The structure

(which corresponds to the formula H Al ) is reported to 4 4o8 have the following unit cell dimensions:

a = 4.40 0 A b = 9.39A0 c = 2.84A0 (21,33). 0 0 0

The typical structure lS represented diagrammatically ln

Figure 2.7. 55

Fig. 2.7 Structure of diaspore

2.3 BAYER ALUMINIUM TRIHYDROXIDE AND ITS PHASE TRANSFORMATIONS

The crystalline aluminium hydroxide obtained under Bayer process conditions is essentially the trihydroxide gibbsite (mono-clinic form),but in some work it has been reported that there are other compounds (boehmite and diaspore) precipitated in the process.

A portion of the phase diagram of the Al - 2o3 H 0 system taken from Kennedy (36) is shown in Figure 2.8. 2 Kennedy reported that at one atmospheric pressure hydrag illite slowly breaks down to boehmite at about 90°C.

However, he states that the transition probably takes place at much lower temperatures but at rates which are much slower. According to Laubengayer and Weisz (31) this transition occurs at 155°C whereas Ginsberg et.al. (37) have reported temperatures in the range of 150 - 200°C for the transition. 56

3 As shown in Figure 2.8, at pressures of 26 x 10 2 to 40 x 10 3 kg/cm , and with heating up to 260°C, hydragillite can be transformed to diaspore, but at lower pressures hydragillite is first converted into boehmite and then to diaspore.

2 kg/cm 40,788 Hydragillite (Gibbsite) Diaspore + Water Al 0 .3H 0 2 3 2 30,591

20,394

Metastable Boehmite

0~----~----~~--~~----_. ______100 200 300 400 500 600°C

Fig. 2.8 Diagram of equilibrium system of Al o - H 0 at higher pressures (36) 2 3 2

In the Bayer process it is usually assumed that the primary product obtained in the precipitation is gibbsite, but there are also some conflicting reports regarding the possibility of forming boehmite during precipitation at the higher temperatures in the range of temperature where precipitation occurs. Thermodynamic stability calculations also indicate that both gibbsite and boehmite are stable under Bayer process conditions. 57

The use of thermodynamic stability diagrams to explain

stabilities under Bayer process conditions is reported in

Appendix I.

Sakamotos (38) has reported in his work that

crystallisation studies carried out under conditions close

to those employed in the Bayer process yielded products

containing both gibbsite and bayerite (at both 30°C and

50°C), but bayerite was not formed at 80°C. Bayerite was

formed at the initial stage of the experiments where the

temperature was kept constant at 30 0 C or 50 0 C, but later

the proportion of bayerite to gibbsite decreased with time.

More bayerite formed at 30°C than at 50°C.

Furthermore, almost an equal amount of gibbsite

and bayerite was obtained in this work when bayerite was

used as the seed at 30°C up to a time limit of 24 hours.

Bayerite seeds for Sakamoto's experiments were prepared by

bubbling carbon dioxide into sodium aluminate solution at

Boehmite seeds were produced by treating Bayer

process gibbsite hydrothermally for 2 hours at 200 0 C. The

results obtained in his experiments are presented in

Table 2.5.

According to Sakamoto's results, the ratio of

gibbsite : bayerite changes slightly with the type of seed

added. For example, when bayerite seed is added, the quantity of bayerite formed during the early stages

increases and when gibbsite is added, the quantity of bayerite formed decreases. However, the precipitated hydrate is not all bayerite when bayerite seed is added.

When alumina monohydrate (boehmite) is added, there is no great change in the ratio of the two hydrates, but the rate 58

Temp. Time Decompn., Amount precipitated oc hr. %

Gibbsite Seed 30 24 16 48 40 G > B 72 55 G > B 96 60 G>> B

50 24 36 G>> B 48 50 G>> B 72 55 G>> B

80 24 16 G 48 22 G 96 27 G Bayerite Seed 30 24 22 G B 48 50 G > B 72 62 G > B

50 24 30 G > B 48 51 G>> B 72 57 G>> B Boehmite Seed 30 24 3 48 19 G = B 72 32 G > B 96 55 G>> B

60 24 6 G>> B 48 29 G>> B 72 53 G>> B 96 56 G>> B No Seed 30 72 0 96 11 120 33 G > B 144 48 G >>B 168 58 G >>B

80 72 0 96 12 G 120 18 G 144 21 G

TABLE 2.5 Crystalline forms of product trihydrate obtained from Sakamoto's decomposition experiments. G gibbsite B bayerite 59

of precipitation decreases. It can therefore be concluded that the kind of seed has a definite effect on the rate of precipitation but does not have a great effect on the crystalline form of the precipitated trihydrate.

Chistyakova (39) has shown that bayerite is the

stable solid phase in equilibrium with a solution correspond-

ing to Bayer process concentration at 20°C. With increas-

lng temperature there was an increase in the proportion of

gibbsite.

According to Arakelyan et.al. (40), only gibbsite

is precipitated from sodium aluminate solutions at 65°C,

regardless of the nature of seed (boehmite or gibbsite ).

However, a t t tempera ures Of 95°c , b oe h ml"t e lS a 1 so preclp-·

itated when boehmite seed is used and gibbsite when gibbsite

seed is used.

Kavina et.al. (41) have detected a small amount

of bayerite in an industrial Bayer process precipitation.

According to them, the presence of bayerite in Bayer process

precipitate plays a significant role in the increase in the

relative amount of fines in the product. They consider the

action of atmospheric carbon dioxide on the cold solution

to be a major factor in the formation bayerite under indus-

trial conditions.

In summary it can be concluded that there have

been many conflicting reports regarding the conditions of

formation and the stability of the three different forms of

crystalline aluminium hydroxide. However, the results of

most of the work suggest that gibbsite is the stable

crystalline form under. Bayer process precipitation conditions. 60

Although the formation of bayerite takes place at the

initial stage of precipitation, most of the workers cited

believe that there is a rapid transformation of this bayer

ite into gibbsite under the usual Bayer process precipitation conditions, thus explaining the absence of bayerite in the

Bayer process precipitation product.

2.4 THE EXTRACTION STAGE IN THE BAYER PROCESS

Generally, alumina occurring in bauxite ln the

trihydrate form is readily attacked at low temperatures by

liquors of relatively low soda concentration, while mono hydrate alumina is economically extracted only at higher temperatures using liquors of higher soda concentration.

Notable in modern practice lS the increasing importance of mixed ores with alumina present in both trihydrate and monohydrate form.

The extraction conditions are normally chosen to

be suitable for the economics of the plant as a whole and

not as dictated solely by the extraction of alumina.

For mixed monohydrate and trihydrate ores as

found in Jamaica, Queensland or Ghana, economic factors

such as plant site, determine whether they should be treated

for recovery of the gibbsite only or whether the generally more expensive recovery of the boehrnite is justified. If the latter, then two processes are available. In the

first, the ore is considered as being boehrnite alone and the extraction conditions arranged accordingly. In the

second, gibbsite is extracted ln a first extraction step

and the residue re~worked for extraction of the boehrnite.

This is particularly applicable where the proportion of 61

monohydrate is small but the ore cost is high due to the

long distance of transport. This latter procedure is the

normal practice in the United Kingdom when using Ghana ore

although a plant in Ghana would probably use a single

gibbsite extraction process alone. The Queensland and

Jamaican ores contain much higher proportions of monohydrate

and the single high-temperature process is used in their pro-

cessing.

Extraction conditions are usually chosen to

facilitate the process of removal of silica. Silica corn-

bined as clay and other silicates dissolves early in the

extraction process. Silica present in the bauxite as quartz

is generally not attacked during extraction at lower tern-

peratures but is attacked to an increasing extent at higher

temperatures. Silica dissolved during the extraction

process must be eliminated from the solution as sodium

alumino silicate, and is therefore responsible for loss of

soda from the circulating liquor and for reduced recovery

of alumina from the ore. The choice of extraction condi-

tions is usually such that quartz attack is at an economic minimum, and that optimum desilication of the product liquor

lS affected.

Extraction conditions must also be selected so that the product liquor can readily be recycled into the

feed liquor. Furthermore, the extraction conditions must

be chosen, in such a way so that premature decomposition will not take place.

Generally, for economic operation, extraction of monohydrate is carried out at temperatures above 180 0 C,

and with relatively concentrated solutions containing up to 62

caustic Na The solubility of 320 g/litre free of 2o. trihydrate is higher and extraction can be carried out at lower temperatures up to 150°C using weaker solutions. At temperatures above about 140 0 C, and at a rate increasing with temperature and soda concentration, trihydrate is converted to monohydrate, so that a solution prepared under trihydrate conditions, if containing alumina in excess of that soluble under monohydrate conditions, if heated above

140°C, will deposit monohydrate.

The solubilities of alumina under extraction condition for both trihydrate and monohydrate in the form of equilibrium ratio (weight Al /weight Na 0) curves are shown 2o3 2 in Figures 2.9 and 2.10. Pearson (43) has reported that extraction obeys a kinetic law similar to that governing decomposition. In practice extraction is based on the attainment of the required extraction efficiency under production conditions for the particular ore, and also desilication of the solution after alumina extraction. Pearson's equation for extraction rate is as follows:

2 dx (xt - xoo) -dt = kAt------(aoo + xoo)2 where K is a parameter dependent on temperature only; At is 2 the area of solid bauxite surface, m /litre at time t, hours : xt and X are the molar concentrations of NaAl0 00 2

and a is the at time t and at equilibrium, respectively; 00 molar concentration of NaOH at equilibrium.

Kuznetsov and Derevyarnkin (9) in this work have recommended a very similar rate formula for extraction as 63

0 N

...... _~ 1.55 ("') 0 N r-1 .:1! --:- 1. 35 ! 0 :j 1.15 ('(j l-1 ~ ·~ 0.95 ,.Q ·.-! r-1 ·.-! &o. 75 fil 0 40 80 120 160 200 240 Free soda concentration g/1 Na o 2

Fig. 2.9 Equilibrium rates for trihydrated alumina 0 over a temperature range of 100 to 140 C (42)

1.45 0 N ('(j ~ 1.25 ("') 0 N r-1 .:1! --:- 1. 05 - ! 0 ·.-! ~ 0.85 l-1 ~ •.-! ~ 0.65 ·.-! r-1 ·.-! i 0.45 0 50 100 150 200 250 Free soda concentration g/1 Na o 2

Fig. 2.10 Equilibrium ratios for monohydrated alumina over a temperature range of 100 to 250°c (42) 64

follows:

dx KA ( y - x) dt =

where A = area of the solid phase

y concentration of Al in the inter- = 2o3 mediate phase

X concentration of Al in the whole mass = 2o3 of the solution

K = constant.

By considering A and to be constant for a short interval of time, the equation could be formulated as follows:

dx ( y X) dt = K1 - and by integrating the above formula:

X = y 1 K has the units of t- . This formula lS well in agree- 1 ment with the results of the experiments conducted by

Herrman ( 44).

2.41 FACTORS AFFECTING THE EXTRACTION

According to Kuznetsov and Derevyarnkin (9) the following are the main factors affecting the rate of extraction of aluminium from bauxite.

1) The physical structure of the bauxite

2) The mineralogical composition of bauxite

3) The surface properties of the bauxite,

including surface area

4) Temperature

5) Stirring rate

6) Activating substances

Kuznetsov and Derevyarnkin have reported that (as shown 1n 65

Figures 2.11 (a,b,c) the rate of extraction is not greatly dependent on temperature, but basically more dependent on diffusion, and he gives the following equation: dx DA (x - x) (x - x) dt = KA s = --0 s

dx where rate of dissolution of bauxite dt = K = coefficient of the rate of dissolution which is equal to the ratio of diffusion

coefficient (D) and the thickness of the

diffusion layer (o)

A = surface area of the crystals of gibbsite in bauxite which are in contact with the

solution of Al ln = equilibrium concentration 2o3 the caustic solution

of Al in the solution at X = concentration 2o3 the given time.

Kuznetsov and Derevyarnkin have also shown diffusion to be related to temperature by the equation

RT 1 D = N . 311dll R = gas constant N = avagadro number 11 = viscosity of solution d = diameter of particles. With an increase ln temperature, viscosity decreases and therefore the diffusion increases. Furthermore, when the temperature is increased, solubility of aluminium hydroxide is also increased, thereby the difference of

But it should also be considered that with the increase of concentration of the aluminate solution (at

m m

2 2

1 1

c c

210°C 210°C

230°C 230°C

175°C 175°C

150°C 150°C

X-

diaspore. diaspore.

0 0

c) c)

2 2

boehmite boehmite

b) b)

c c

0 0

230°C 230°C

210°c 210°c

150 150

11s

gibbsite gibbsite

b b

1 1

I I

(Hours) (Hours)

D-

a) a)

TIME TIME of of

I I

0 0

I I

dissolution dissolution

of of

2 2

J J

Rate Rate

211 211

Fig. Fig.

210°C 210°C

1 1

120°C 120°C

150°C 150°C

a a

% %

0- ·-

. .

11 11

0 0

~/ ~/

Dissolution Dissolution

80 80

60 60

40 40

20 20 100 100 67

constant caustic ratio), viscosity is increased, and so the rate of diffusion is decreased. However, in this process as the Xs - X is increased, the isotherm will be increased and in this manner XS - X increases faster than the change of viscosity, and so will keep the rate of dissolution

increasing.

Kuznetsov and Derevyarnkin have also reported

from other literature the effect of some of the mineral

impurities on the rate of dissolution. The effect of

silicates, iron, sulphide, titanium and carbonate bearing minerals and lime on the dissolution of bauxite are reported.

Impurities such as silicagel and kvartz have a negative

effect on the rate of dissolution but the presence of iron

containing minerals favour the dissolution.

2.42 EFFECT OF ORGANICS ON THE DISSOLUTION OF BAUXITE

Accumulated organic material in the recycle

caustic liquor is also believed to affect the extraction

rate. Kuznetsov and Derevyarnkin (9) have reported in

their publication some of the work done in relation to this

proposition. According to them some of the organic corn-

pounds present in the recycled caustic liquor promote the

extraction. It was found that aluminate solution taken

from the plant and used to leach bauxite is more effective

than the synthetic solution. Organic impurities with

hydroxyl radicals appear to activate the dissolution,

whereas the rest of the organic is believed to be increas

ing the solubility of bauxite in the aluminate solution.

Sodium salts of oxalic and acetic acids have no effect on

the dissolution. However, organic impurities which are 68

soluble 1n benzene reduce the rate of dissolution.

It has also been found that different fractions of similar organic compounds have different effects on the dissolution of bauxite in caustic soda. For example, higher molecular weight compounds reduce the rate at 210°C whereas they accelerate the rate over 230°C. Sodium oxalate reduces the rate of dissolution considerably between

210 and 230 0 C.

According to Kuznetsov and Derevinkin (45) the electronmicroscopic observation of crystals of aluminium hydroxide indicates a uniform dissolution of the particles in the solution during the extraction in the presence of organic impurities. They believe that this may be due to adsorption of some of the organic on to the surface of the hydroxide crystals, thereby preventing further contact between the caustic and the hydroxides crystals.

2.5 SEPARATION OF RED MUD

Separation of the red mud from the alumina rich liquor from the Bayer process is carried out by sedimenta tion followed by classification and filtration. This part of the process has an enormous influence on the plant cost and product quality. Firstly to produce a rich aluminate liquor sufficiently free from suspended solids to prevent contamination of the hydrate deposited later. The second objective is to remove as much as practicable the adhering caustic liquor from the mud before disposal, using a minimum amount of wash water.

The efficiency or the rate of sedimentation depends on many factors such as the mineralogical composi- 69

tion of the solids, the conditions that were applied in the extraction stage) the dispersion and other conditions in the sedimentation stage itself.

The mud always possesses some degree of flocculation depending on the type of ore and the extraction conditions but it is universal practice to increase this by the addition of starch ln some form (for example, bran, potato starch or maize starch) pre-treated with caustic solution.

2.6 DESILICATION

The control of silica in the Bayer process circuit is most important to ensure that the required purity of the alumina is obtained. Adamson (4) in his report, has illustrated the role of silica in the form of a silica cycle in the Bayer process as is shown here in Fig. 2.12.

Silica normally exists ln bauxite in two main forms: firstly as reactive silica and as minerals of the kaolinite type which are rapidly attacked by caustic liquor under the extraction conditions; secondly, silica exists as quartz which is not readily attacked by caustic liquors at low temperatures but increasingly attacked at higher temperatures causing corresponding loss of alumina and caustic soda.

Simultaneously with the solution of the silica, desilication of the solution occurs by deposition of rela tively insoluble triple compound (xA1 .y Na 0.z Si0 ). 2o3 2 2 The rate of deposition is accelerated by the presence of the triple compound in the solid phase acting as a seed and by high temperature. 70

Si0 in ore 2 l Extraction & I I Evaporation Dilution I I

solid 3Na o 2 3Al 0 2 3 Extract I 5Si0 15-30g Si0 Decomposition 2 2 I I Hydrat e 2H 0 0.012% Sio 2 2 in Al o 2 4

Conditioning Retention at 105°C Hot liquor Solid I J Cooler I 0 . 55g.Sio 2 l I Triple compound per 100g NA20 discarded with red mud

Fig. 2.12 Silica cycle in the Bayer process

It is essential in the process control of a

Bayer plant to reduce the silica in the liquor passing to

the crystalliser to such a value that contamination of

the hydrate produced during crystallisation is minimised.

This desilication process is only partially completed in

the extraction plant and hence it is necessary to have a

conditioning step in the process. According to Adamson (4),

the desilication of sodium aluminate solutions appears to

follow a monomolecular rate law of the form

where S is the concentration of silica in the solution.

S is independent of temperature, The equilibrium silica 00 but is proportional to the alumina and free soda concen-

trations. 71

The primary desilication product is believed to be the triple compound itself. Hence the best desili-

cation conditions are to dilute the extraction slurry to the new liquor strength and ratio thereby keeping the silica

deposition potential at a maxlmum and maintaining a high temperature as practicable until the silica content is reduced to the desired level. In practice the alm lS a

figure of not greater than 0.55 - 0.60 Si0 per 100 g of 2 free soda in the solution.

The role of silica in the Bayer process can be

explained by the use of the stability diagrams calculated

by the author and presented in Appendix I. CHAPTER 3

ORIGIN, IDENTIFICATION, EFFECT AND REMOVAL OF THE ORGANIC IMPURITIES IN THE BAYER PROCESS 72

It is commonly accepted in the alumina industry that organic impurities adversely affect the precipitation of alumina trihydrate. These observations are, however, mostly based indirectly on plant operating experience and very little direct information is available on actual identification and

effects of these impurities. It was firstly Pearson (43) who discussed the

poisoning effect of impurities on the seed and the consequent

slowing down and even bringing to a complete stoppage the

precipitation rate of alumina trihydrate. He also stated

that effective poisons are high molecular weight organic

substances containing hydroxyl groups like saponin, green

arabic, cane sugar~ etc., but that sodium oxalate appeared

to have little effect. Dissolved calcium and iron salts

are the most common inorganic impurities which cause poison

ing according to Pearson who also stated that the effective

ness of poisons decrease with increasing temperatures.

However, his statements in regard to this matter were with

out any experimental evidence and were merely based on

conjecture relating to plant experience.

3.1 ORIGIN OF ORGANIC IMPURITIES IN BAYER PROCESS LIQUOR

Organic matter enters the recycled caustic liquor

from two main sources. 1) Organic matter present in the bauxite

2) Starch added as a flocculating agent in

the red mud separation stage.

Bauxite usually contains from 0.1 to 0.3% by

weight of organic carbon, but occasionally up to 0.6% is 73

found when surface bauxites are mined. It is generally believed that the organic carbon is present in the form of humic substances.

Uteley (46) has reported on the or1g1n and nature of organic matter found in Arkansas bauxite. He

believes that the bulk of the organic matter comes from the

overburden, and especially from the lignitic clays. It is

believed to be composed mostly of humic acids, humates and

their oxidation products. The oxidation products of these

humic acids, after extraction with sodium hydroxide, may be

degraded into many different compounds. However, these con

clusions were not based on any direct experimental results.

Other possible sources of organic carbon in

Bayer process liquor are the flocculants and antifoams used

in the process. The contribution of these organics to the

total organics present in the liquor is believed to be small

and this presentation will therefore concentrate only on the

organic matter originating from the bauxite.

Kirke (10) has reported on the likely organic

impurities in the Darling Range bauxite and their behaviour

in the Bayer process. He believes that their organic material is derived largely from decomposed vegetation and roots and consists of many substances, such as humus, lignin,

cellulose and protein. Under the alkaline oxidative condi

tions existing in the Bayer process these complex organic

compounds break down through a series of stages as outlined

below to simple compounds such as the sodium salts of

succinic, acetic and oxalic acids and carbon dioxide. Pre-

dominant among these salts is sodium oxalate . 74

Humus cellulose etc. ! Humates + co 2

l Acids + Coloured Humic co 2 l + Polyhydroxy Compounds co 2 ~ + C0 Oxalic, Acetic Acids 2 l + H 0 C0 2 2

These suggestions by Kirke are based only on the behaviour of humic substances in the environment as described by previous workers.

3.2 HUMIC SUBSTANCES IN THE ENVIRONMENT

Humic substances are probably the most widely distributed natural products on the earth's surface, occurring in soils, lakes, rivers and 1n the sea. In spite of their extensive distribution, much remains to be learned about their origins) synthesis, chemical structure and reactions.

The organ1c matter of soils and waters consists of a mixture of plant and animal products in various stages of decomposition, and of substances synthesized biologically and/or chemically from their breakdown products and of microorganisms and small animals and their decomposing remains.

Schnitzer and Khan (47) have classified the 75

organic compounds ln soil into two groups:

(a) non-humic substances, and

(b) humic substances.

Non-humic substances include compounds that

exhibit still recognizable chemical characteristics. To this

class of compounds belong carbohydrates, proteins, peptides,

amino acids, fats, waxes, resins, pigments and other com

paratively low-molecular-weight organic substances.

The bulk of the organic matter in most soils and

waters consists of humic substances. These are amorphous,

brown or black, hydrophilic, acidic, polydisperse substances

with molecular weights ranging from several hundreds to tens

of thousands. Based on their solubility in alkali and acid,

humic substances are usually divided into three main fractions:

(a) humic acid, which is soluble in dilute alkaline solution

but is precipitated by acidification of the alkaline extract,

(b) fulvic acid, which is that humic fraction which remains

in the aqueous acidified solution, i.e. it is soluble in both

acid and base, (c) the humic fractions that cannot be extract

ed by dilute base or acid, which are referred to as humins.

3.21 THEORIES ON THE FORMATION OF HUMIC SUBSTANCES

The mechanism of formation of humic substances is

still a highly speculative matter. It has been suggested

that they result from bacterial and chemical degradation of

plant tissue followed by secondary processes in the soil.

These latter processes include polymerization of the plant polyphenols, condensation of these with amino acids and meta

bolites from soil micro-organisms and finally complexing 76

with sesqui-oxides and silica. A diagrammatic repres- entation of these processes is shown in Figure 3.1.

Lignin and lignin:--like Plant polyphenols materials acting as reacting with amino polyphenols to complex acids in the plants with amino acids and soils

Humic Acids

Lignin oxidation products Cellulose polysacharides being de-polymerized and ~sugar utilizable by eo-polymerized with other micro-organisms---- plant phenols and amino shikimic acids--~~ acids polyphenols which react with amino acids

Fig. 3.1 Formation of Humic Acids

3.211 FELBECK'S HYPOTHESIS

Felbeck (48) in 1971 suggested four possible

mechanisms for the synthesis of humic substances. 77

a) Plant Alteration

According to this hypothesis, fractions of plant tissue (especially lignified tissue) are changed to form humic substances at or near the soil surface. The higher molecular weight fractions, humins, are formed first, then bacterial degradation takes place to form humic acids, fulvic acids and finally carbon dioxide and water. b) Chemical Polymerization

This hypothesis assumed that plant materials are

attacked by bacteria to form small molecules. These mole-

cules are used by microbes to synthesise products such as

phenols and am1no acids which are then excreted. Humic sub-

stances are formed as products of chemical oxidation and

polymerization of these excretions.

c) Cell Autolysis

Humic substances are formed from plant and

bacterial cell autolysis as the products of random conden

sation and free radical polymerization of cellular debris.

These reactions are assisted by autolytic enzymes.

d) Microbial Synthesis

This hypothesis assumes that plant tissue is

used as a source of carbon and energy by microbes to produce

high molecular weight humic-like substances. These sub

stances are syntherized by microbes intercellularly and

only released into the soil when microbes die and their cells

are lysed. This stage is followed by extracellular

degradation to form humic acids, fulvic acids and finally

carbon dioxide and water. 78

3.22 OCCURRENCE OF HUMIC SUBSTANCES IN MINERALS AND ITS

RELATIONSHIP TO BAUXITE

Humic substances are accumulated in surface and

sub-surface soil layers, in and beneath marsh deposits, 1n-

shore and in beach sands. It is believed that the humic materials are leached from decaying plant materials or humus

on the land surface and transported by surface and sub-surface

waters 1n a soluble or colloidally dispersed form to sub

surface sand environments or to brackish or saline waters

where flocculation or precipitation of the humic substances

is triggered by various physical-chemical mechanisms. One

or probably a combination of the following mechanisms may

be responsible for precipitation:

a) the adsorption or complexing of dissolved cations such as Al+ 3 , Fe+3 and Mg 2+

b) complexing with clay minerals and

c) a lowering of pH.

Dissolved aluminium is involved in the important

processes of rock weathering and soil development, but there

has not been much literature available to explain the

occurrence of these humic substances in the clay minerals

such as bauxite. However, supporting evidence to this

question can be gained from the work done on the association

of humic substances with aluminium in soil and natural water.

Pauli (49) described strong complexes formed by humic acid

and aluminium. Kavrichev et.al. (50) demonstrated the

possible binding of a measurable amount of dissolved alumin

ium with dissolved organic matter, in soil water in the

normal pH range. 79

Lind and Hem (51) reported the possibility of forming aluminium complexes with naturally occurring organic compounds, such as humic substances, in natural water. The functional groups in these humic substances are most likely to be involved in metal complexation reactions, since protons are released during such reaction (47). The extent of complexation of a metal thus depends not only on the stability constant of the metal-ligand complex but also on the acid dissociation constant of the acidic functional group which serves as the complexing ligand.

Hoefs (52) in his book went so far as to say that

"many geological model cases which have been treated as pure inorganic systems are not realistic at all, because, especially in sedimentary geology, there are no pure systems without organic matter or micro-organisms". Also Schnitzer (53) stated that there is increasing evidence that the most impor tant component of soil solution is fulvic acid, as it is likely it affects practically all reactions occurring 1n soils.

There is strong evidence to suggest that the humic substances present in bauxite are 1n a very complex form a part of which is associated with both aluminium and iron present in the bauxite. During the Bayer process extraction, these humic substances are leached into the caustic liquor producing a brown colouration, probably in a very different structure to the occurrence in the mineral before processing.

3. 2 3 CHARACTERIZATION OF HUMIC SUBSTANCES

Schnitzer and Khan (47) have reported on the 80

distribution of the major constituent elements in humic compounds. Functional groups analyses shed light on the

functional groups ( H, -OH, and occurrence of major -co 2 = C = 0 ). The predominant elements are carbon and oxygen with the carbon content of humic substances ranging from

50 - 60%, oxygen content from 30 - 35% and percentages of hydrogen and nitrogen ranging from approximately 4 - 6% and

2 - 4% respectively. The sulphur content may vary from close to zero to between 1 - 2%.

3.231 FUNCTIONAL GROUPS IN HUMIC SUBSTANCES

The literature available on humic substances in the environment and in bauxite in particular indicate that these substances are very complex in structure and properties.

The structure and properties of these humic substances depend on the location, geographical and geological condi tions such as climate, weathering, and soil conditions and also on the vegetation of the area. The analytical results obtained on humic substances from the same location differ in structure and properties due to the wide range of complexity. Therefore most of the reports made ln the last decade on this subject have been speculative. However, it is generally accepted that humic substances are complex polymers which possess both carboxyl, phenolic, hydroxy and acidic functional groups.

In normal systematic organlc analysis and in instrumental analysis, very often the sample has to be chem ically treated, so that it suits the particular method of analysis, but in doing so, the structure of these humic 81

organics are likely to be changed. Due to such complexity of humic substances, many workers have given priority to determining the functional groups quantitatively, as such determination is a useful guide for further investigation of the structure, but most of this work has been limited to humic substances in natural waters and soils.

(a) Oxygen containing functional g~oups

The most possible oxygen containing functional groups believed to be present in humic substances are the carboxylic and phenolic groups. There have been varlous methods used in determining the functional groups in humic substances. Many workers in the early stage used poten tiometric titration methods to characterize acidic functional groups to determine the concentration of carboxyl and phen olic hydroxyl groups, but this method was not successful due to the complexity of the humic substances, and particularly due to 1the presence of components in the humic substances which may take a longer period to complete full oxidation.

Therefore the end point of potentiometric titration is always inaccurate.

The concentration of carboxyl and phenolic hydroxyl groups in humic substances is more often determined using a calcium acetate exchange reaction and the barium hydroxide total acidity reaction. The specificity of these reactions however has been questioned. The author believes that these methods can be successfully applied only

after the humic substance is extracted as a separate fraction,

as otherwise the other low boiling carboxylic acid groups and

the inorganic acid groups such as sulphate and chloride

could interfere. 82

The calcium acetate method is based on the reaction between calcium acetate and humic acids as follows:

Before the reaction a total acidity of the humic acids is detected by the barium hydroxide method and then after the addition of calcium acetate, the acetic acid is titrated with a standard solution of sodium hydroxide. In this method there is an error claimed to be due to the reaction

of calcium acetate with some of the more weakly acidic

phenolic groups. Perdue (54) claimed from his work that calori-

metric titration is the most accurate method of determining

the carboxylic and phenolic content in humic substances.

The method of titration calorimetry was recently used by

Perdue to characterize the acidic functional group of humic

acids. This work was done as part of an overall evaluation

of techniques for· characterization of acid-base and metal-

ligand reactions of humic substances in river water. Perdue

has determined by this method the concentration of carboxyl

and titrable phenolic hydroxyl groups, their respective and the average average enthalpies of ionization (~H),a pK of the titrable phenolic hydroxyl groups. His studies a have been limited to river water humic substances.

According to Perdue, the thermometric titration

curves, in which the amount of heat evolved is determined

for a series of titrant additions (sodium hydroxide

solutions), were established. In these curves the initial

slope of approximately -56 kJ/mol has clearly indicated

that carboxyl groups are being titrated, while the non 83

linear portion of the titration has indicated weaker acidic functional groups (presumably phenolic hydroxyl groups) as being titrated in the latter part of the titration.

Although these methods have been used satisfactor ily to characterise the humic substances in natural water, they must be further extended for examining the organic materials present in such as clay minerals.

For determining the total hydroxyl in humic substances, acetylation with acetic anhydride in pyridine has been recommended by Brooks et.al.(55). This method which is being widely used can be represented by the equations: + (CH C0) 0 CH COOR + CH COOH ROH 3 2 3 3

H 0 + (CH C0) 0· 2CH COOH 2 3 2 3

The excess anhydride is hydrolyzed to acetic acid, which is

titrated with standard base. The phenolic hydroxyls are

calculated as follows: Total acidity - carboxyl group

acidity = phenolic hydroxyl group acidity. There is no

direct method to determine alcohol hydroxyls considered

that alcohol hydroxyl groups are less reactive than

phenolic hydroxyl groups. Estimates of alcoholic hydroxyl

groups can be obtained indirectly by taking (total hydroxyl -

phenolic hydroxyl= alcohol hydroxyl).

The determination of carbonyl CC = 0) groups

present in humic substances is reported by Fritz et.al.(56).

This procedure was based on the reaction of humic substances

in methanol - 2 - propanol with an excess of hydroxylamine. 84

R1 R1 ~ ~ C 0 + NH 0H.HC1 --~- C - NOH + HCl + H 0 = 2 2 / R/ - R2 2

The unreacted hydroxylamine is titrated with standard

perchloric acid solution. Brown (57) has developed a method

for estimating the carbonyl group which is based on their with sodium boro hydride (NaBH ). reduction to - CH 20H 4 The reduction is carried out in alkaline solution and the the unused NaBH is estimated hydrogen liberated from 4 manometrically.

However, the most widely used method for carbonyl

group determination is based on the formation of derivatives,

especially oximes and phenyl-hydrazones (58).

From various functional group analyses, the value

of the proportion of oxygen in the functional groups of humic

substances was collected and published by Schnitzer and

Khan ( 4 7). Table 3.1 indicates that between 68% and 91%

of the oxygen in humic acids and humins can be accounted for

1n the functional groups, whereas more than 90% of oxygen

1n fulvic acids is similarly distributed. In general, most

of the oxygen in humic substances, especially 1n fulvic acid, groups, phenolic -OH and C = 0 groups is present 1n -co 2H accounting for most of the remaining oxygen. Most of the Oxygen Oxygen Carboxyl Phenolic Alcoholic Carbonyl Methoxyl accounted OH OH for

% of oxygen

Soil HA1 s 32.9 43.8 10.2 13.6 21.4 1.5 90.5 36.8 26.1 24.9 15.2 7.8 -a 74.0 35.4 13.6 38.0 12.7 4.1 -a 68.4 35.6 42.2 24.7 0.9 23.4 -a 91.2 33.6 44.8 17.1 -a 14.8 1.4 78.1 Coal HA 28.7 49.1 16.2 -a -a 9.4 74.4 Soil FA 1 S 47.3 57.5 19.3 11.5 5.8 -a 94.1 44.8 65.0 11.8 12.9 11.1 1.7 102.5 47.7 61.0 9.1 16.4 3.7 1.0 91.2 Soil humins 33.8 36.0 9.9 -a 22.7 1.9 70.5 a 31.8 26.1 12.1 - 28.7 1.5 68.4

~otdetermined TABLE 3.1 Distribution of oxygen in humic substances {47)

CO (]1 86

systematic errors that occurred in the determinations of the individual oxygen-containing functional groups are accumulated in these values. However, the available data show that most of the oxygen in humic substances occurs in H, -OH and C 0. functional groups such as -co 2 = (b) Nitrogen Containing Functional Groups

There has been a complete review on the distribu tion and nature of nitrogen in humic substances by Bremner

(59,60).

The nitrogen content of humic substances ranges from about 1 - 6%. The presence of amino acids, amino

sugars and ammonium compounds was observed by Sowden (61) using a "Technicon" amino acid auto analyzer. There is also

evidence to show that after acid hydrolysis of humic

substances about 20 - 55% of the nitrogen consists of amino

acid nitrogen, and that 1 - 10% is amino sugar nitrogen

(61- 65). However, the nature of the solutions used for

the extraction of humic material may have significant effects

on the total nitrogen content and on the nitrogen distribu-

tion in acid hydrolyzates. For example, Bremner (64 - 65)

found that humic acids extracted by 0.5 N sodium hydroxide

and by 0.1 M sodium pyrophosphate (pH 7) differed markedly

from each other in total nitrogen content and in nitrogen

distribution after acid hydrolysis, the former having the

higher nitrogen content and higher proportion of acid soluble

and amino acid nitrogen.

3.232 THE USE OF PHYSICAL METHODS FOR CHARACTERIZATION OF

HUMIC SUBSTANCES

The characterization of humic substances using 87

physical methods such as spectroscopy, electrometric titra tion and molecular weight measurements, has become more attractive due to the complexity of other chemical procedures.

The use of the following main methods of character ising humic substances have been discussed by Schnitzer and

Khan (47). a) Spectroscopic methods:

i) Visible spectroscopic method

ii) Ultra violet spectroscopic method

iii) Infra red method

iv) Nuclear magnetic resonance (NMR) spectrometric method

v) Electron spin resonance (ESR) spectrometric method

vi) X-ray analysis b) Molecular methods:

i) Vapor pressure osmometry

ii) Ultracentrifugation

iii) Gel filtration a) Spectroscopic characterisation

The different regions of electromagnetic spectrum were used by soil scientists for the characterisation of humic substances. These methods have a number of attractive features: (a) they are non-destructive (b) only small sample weights are needed (c) they are experimentally simple and do not require special manipulative skills.

Some of the disadvantages in these methods were the overlapping of the results due to the similar characteristics of some of the functional groups. Secondly, these methods are very much limited to qualitative determination 88

for the same reason.

i) Visible Spectroscopic Method

Humic substances, like many relatively high mole cular weight materials, yield generally uncharacter,isTic

spectra in the visible region (4u0 - 800~m ) as shown ln

Figure 3.2. Absorption spectra or neutral, alkaline and

acidic aqueous solution of humic acids and fulvic acids are

featureless showing no maxlma or mlnlma.

Light absorptlon of humic substances appears to

increase wiTh increases in (a) the degree of condensation of

the aromatic rings that they contain (b) the ratio of carbon

in aromatic "nuclei 11 to carbon in alipnatic or alicyclic

side clJ.ains (c) total car·bon content and (d) molecular

weight. Usually~ adsorption spectra of humlc substances

in the visible region of tne spectrum do not provide

detailed lnformation on their chemical structure. However,

Konova (66) has approached this problem by using a ratio of

optical densities at 465 and 665 m~ wavelength. According

referred to as E /E is independent of the to him this ratio, 4 6 concentration of the humic compound, but varies for humic

substan8es from different soil type. Brown humic acids

have an E /E ratio of about 5, whereas the ratio for grey 4 6 humic acids ranges from 2.2 to 2.8. He claims that the

progressive humification and increased condensation are in the E /E ratio so that the indicated by a decrease 4 6 ratio could serve as an lndex of humification. This method

of characterisation received a great deal of attention as

a successful ntethod of separation as some differences of

various humic substances could be determined even though the 89

two separate groups could not be identified 1n molecular terms.

1.6

1.4

:>,1.2 -!J ·.-I ~ 1.0 Q) 'D r-1 r1) 0 ·.-I -!J 0.6 0.. 0 0.4

0.2

400 500 600 700 Wave length mlJ Fig. 3.2 Visible spectra of (A) Humic Acid and (B) Fulvic Acid

ii) Ultraviolet Spectroscopic Method (200 to 400 IDlJ)

Absorption in the UV region normally occurs due

to the presence of multiple bonds and of unshared electron

pairs in the humic substances. Among those linkages which

confer the colour to the organic substance are those called

chromophores and the linkages which by themselves do not

confer colour but which increase the colour of chromophores

are referred to as auxochromes. The typical chromophores

known to occur in humic substances are C = C and C = 0,

auxochromes that are likely "LO be present are C - OH,

C - NH and others. The use of UV absorbtion spectra has 2 not been wide because the interpre~ation of the spectra is 90

difficult due to the presence of more than one chromophore.

Ultra-violet spectra of most humic substances are featureless, because there is no sharp maximum in the absorption spectra. In most cases, a broad peak lS obtained probably due to the whole complex mixture. It is also reported that the optical density decreases as the wavelength increases as in Figure 3.3 (47). Occasionally an indication of a maximum lS observed in the U.V. region but this cannot be used as an accurate method for characterisation without

separating the mixture of humic substances to some extent.

:>, 0.5 +-' ·rl (}) !=: 0.4 (]) '\j rl 0.3 rU 0 ·rl +-' 0. 2 p_, 0 0.1

200 250 300 350 400 Wave length, fill

Fig. 3.3 Ultraviolet spectra of (A) HA and (B) FA

iii) Infra-red (IR) Spectrophotometric Method: (4000 to 1 650 cm- or 2.5 to 15.4 fill)

Infra-red methods have been very attractive ln

yielding useful information on the structure of humic

substances. These methods are based on the following

principle. The masses of atoms, and the forces holding

them together, are of such magnitude that the usual vibra- 91

tions of organic molecules interact with electromagnetic energy so as to absorb and radiate in the I.R. region of the electromagnetic spectrum. Some of the vibrational bonds are associated with specific groupings in the molecule such as the carbonyl group carboxylate anions, hydrogen bonded - OH groups, aromatic C = C groups, etc., thus producing a peak at a characteristic wavelength (67). The position and intensity of an absorption bond can be used to confirm the presence of a particular group and to obtain information on its molecular structure. Conversely, the absence of a strong absorption group is often indicative of the absence of that group in the molecule, provided that no other effects operate which could shift the absorbtion bond to other regions of the spectrum.

Schnitzer (53) has reported on the infra-red spectra obtained from humic acids and fulvic acids as shown in Figure 3.4. The absorption bonds are listed ln

Table 3.2. These bonds appear to be broad and Schnitzer claims that this is because of extensive overlapping of individual absorption peaks.

~ 0 ·rl00 00 ·rl~------,s 00 ~ ~ ~ ~

~ ~ ~ 0 ~ ~ ~ 4000 3200 2400 1800 1400 1000 700 Wave Number cm-1

Fig. 3.4 Infra red spectra of (A) HA and (B) FA (53) 92

-1 Frequency cm Assignment

3400 Hydrogen-bonded OH

2900 Aliphatic C-H stretch

1725 C 0 of H, C 0 stretch of = co 2 = ketonic carbonyl

1630 Aromatic C = C Hydrogen - bonded C = 0 of carbonyl, conjugated with carbonyl, COO

1450 Aliphatic C - H

1400 COO , aliphatic C - H

1200 C - 0 stretch or OH deformation

of H co 2 1050 Si - 0 of silicate impurity

TABLE 3.2 Main infra-red absorption bands of humic substances 93

Other workers have also reported similar results

(68) for soil humic compounds as are shown in Figures 3.5 and 3.6. In most of those reported works sharp peaks were not obtained due to the overlapping as mentioned earlier.

This has been a prime drawback to using I.R. analysis for structural determination. I.R. analysis, however, has given a lead to the determination of functional groups present in soil humic substances.

As shown in Figures 3.4, 3.5 and 3.6, the most striking difference between the two spectra of humic acids and fulvic acids lies in the intensities of the bonds in 1 1 the 2900 - 2800 cm- region and in the 1725 cm- bond.

The humic acids appear to contain more aliphatic C - H groups -1 than do fulvic acids. The 1725 cm bond is very strong in the case of fulvic acids, but only a shoulder for humic acids, and so substantiates the chemical data which shows that fulvic acids contain considerably more H groups co 2 than do humic acids.

The I.R. spectra reflect the preponderance of

groups, that is, H, OH, oxygen - containing functional co 2 and C = 0 in humic substances which may, at least 1n part, be responsible for the relatively poor definition of the

I.R. spectra of these compounds. However, it is evident that, in general, spectra of humic substances of diverse origin are very similar, which may indicate the presence of essentially similar chemical structures, differing mainly in the contents of functional groups. iv) Nuclear Magnetic Resonance (NMR) Spectrometry

There has been work reported by Barton and

Schnitzer (69) on the use of N.M.R. spectrometric methods ln 94

4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600 -1 Frequency (ern ) Fig. 3.5 Infrared spectra of original humic acids (A) humic acid-I (B) humic acid-II (C) sodium salt of "A" (68)

4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 600

Fig. 3.6 Infrared spectra of original fu1vic acid (A) fulvic acid type I (B) fulvic acid type II (C). sodium salt of "A" (68). 95

studying humic substances. The application of N.M.R. to organic molecules is concerned largely with proton resonance.

The resonance frequency varies slightly for hydrogen in

different molecules, and for hydrogens in the different

environments in the same molecule, so that different types of

hydrogen in an unknown structure can be distinguished in a

N.M.R. spectrum. It is only in the liquid or dissolved

state that fine structure, due to the chemical shift, can be

observed. The most widely used solvents for this purpose

are CC1 and CDC1 . However, pure humic substances are not 4 3 soluble in these solvents, unless they are chemically

treated. Therefore the use of N.M.R. spectroscopy has been

limited to methylated fractions or to the degradation

products of humic substances soluble in organic solvents.

However, there has been useful information reported by Barton

and Schnitzer by producing the N.M.R. spectra of methylated CC1 and CDC1 . The most fulvic acids fraction dissolved in 4 3 remarkable fact about these spectra was the absence of

aromatic and olefinic protons.

Schnitzer and Skinner (70) recorded N.M.R. spectra

of methylated fulvic acid fractions separated by gel filtra-

tion. These spectra do not exhibit aromatic protons,

indicating that either the aromatic "nuclei" or cores of the

fulvic acid fraction are fully substituted by atoms other

than hydrogen or that the relaxing effects of the spins of

impaired electrons (free radicals) interfere with the N.M.R.

measurements.

v) X-Ray Analysis

X-ray defraction analysis has been used to charac

terise carbon arrangement in humic substances as to whether 96

it occurs predominantly in condensed "nuclei" or in loose open structural arrangements. Kasatochkin et.al. (71) reported from X-ray studies that humic substances contain flat condensed aromatic networks to which side chains and functional groups are attached. There has been more work reported on this aspect by Kononova in 1956 (72), Tokudome and Kanno (73), Kodama and Schnitzer (74) in their independ ent work.

It was claimed in the above work that the diffrac tion patterns suggest structural similarities between soil humic materials and carbon black and coal of various ranks.

A structure of condensed aromatic networks of humic material was suggested after comparing some of the lines of the

X-ray diffraction pattern with high ranks of coal in which some bonds occur due to ordering of condensed aromatic layers normal to their carbon planes. Since naturally occurring humic substances are non-crystalline, the use of this X-ray diffraction method was found to have only limited application in structure determination. b) Molecular Weight Method

Molecular weights ranging from a few hundred to several million have been reported for humic substances.

Among the methods used in determining molecular weight are vapour pressure osmometry, ultra centrifuge and gel filtration. Membrane separation has also been applied very recently ( 7 5) . i) Vapour Pressure Osmometry

Vapour pressure osmometry lS being used increas ingly for measuring number - average molecular weight (Mn) 97

of water-soluble humic substances, especially fulvic acids, but when measured in water, number-average molecular weight values for polyfunctional substances like fulvic acid may be erroneous because of the dissociation of any acidic functional groups. Although this dissociation can be mini- mized by measurement in organic solvents, humic substances are usually insoluble or only slightly soluble in these media.

These difficulties have been overcome by Hansen and

Schnitzer (76) by a correction system for experimentally

determined values of Mn and pH, which makes possible calcu-

lation of accurate Mn values in aqueous solution.

ii) Ultracentrifuge

The ultracentrifuge method is used to measure

molecular weight, size and shape of high-molecular weight

organic substances such as proteins. Attempts to use this

technique on humic materials have, so far, met with only

limited success. Many of the techniques that have been

developed for proteins did not work well with humic

materials ( 4 7) . The cause is mainly due to the complexity

of humic substances.

iii) Gel Filtration (Gel Permeation Chromatography) Gel filtration has been very successful in measur- (M ) of humic sub ing weight - average molecular weights w stances. This method is being widely used due to the inexpensiveness of the apparatus and the use of small

sample size. Gel permeation chromatography (GPC) is a type of

liquid partition chromatography in which the chromatographic

medium is made up of particles of a gel, the pores of which

are filled with a liquid (the stationary phase). The mobile 98

phase is also liquid, and when a mixture of solutes is passed through the column it is found that the sequence of emergence of the components of the mixture from the column is in order of their decreasing molecular size. For practical purposes, this method is used for determining the molecular weight of similar components in similar environ ments. Three mechanisms have been suggested for the separation processes involved in G.P.C. (77) based on: a) the steric exclusion theory

b) the restricted diffusion theory

c) thermodynamic theories.

G.P.C. has been used both for fractionation

and measurement of molec1.1lar weights of humic substances. In early work carried out before 1967, it was

found that charged and mainly aromatic humic acid molecules

are adsorbed by the gels used, making it impossible to

deduce reliable molecular weights from elution curves.

Several eluants were suggested by Swift and Posner (78)

to overcome this difficulty. They showed that fractiona-

tion of humic substances solely on a molecular weight basis M Na B o can be obtained on sephadex gels using 0.025 2 4 7 solution or an alkaline buffer solution containing a large

amino cation to depress gel solute inter-action. Khan and

Friesen (79) also obtained molecular weight fractionation

of humic acids on sephadex gels using a glycine - NaOH

buffer at pH 10 as eluant. The application of G.P.C. to the measurement

of molecular weights of humic substances in sediments, soils

and natural waters has been reported. The data show that 99

the molecular weights of humic substances in sediments range from 700 to over 200,000 in soils from 300 to over

200,000 and in natural waters from 700 to over 50,000.

Schnitzer and Skinner (70) point out that molecular weights of humic materials measured by gel filtra tion may be between 2 and 10 times higher than those measured by other methods such as vapour pressure osmometry and freezing point depression. This disagreement was found to be very high in the case of the upper range of molecular weight fractions. Gels are usually calibrated with carbohydrates or proteins of known molecular weight and shape. Many workers have assumed that these calibra tions were also valid for humic substances of which the molecular dimensions were not known. The basic error could originate from the above assumption, as the proteins and carbohydrates differ very much from humic substances.

There have been suggestions to overcome this difficulty by calibrating gels with humic fractions of accurately known molecular weight, size and shape.

Cameron et.al. in 1972 (80) calibrated sephadex

G-100 for humic acids. They used humic acid fractions of very low polydispersity obtained by preparative gel permea tion chromatography and measured the molecular weight of each fraction by ultracentrifugation. The published calibration curves were regarded as being approximately equivalent to curves obtained with a series of truly mono disperse macromolecular species. However, the application of such a calibration curve to broadly polydisperse unfractionated humic materials requires the further assump tion that such a system is made up of a series of monodis- 100 perse species. Alderdice et.al. (81) ln 1978 calibrated sephadex G.200 for humic acids by a cross calibration procedure. Darmutji (77) in 1978 in her work fractionated humic acid samples by gel permeation chromatography, and the relationship between average molecular weight and optical absorptivity in the visible - ultraviolet region was determined. Humic acid samples were satisfactorily frac tionated by the gel permeation chromatographic method in this work. Absorption spectra were measured for each fraction and it was found that absorptivities increase with decreasing molecular weight. The molecular weight results were established using a similar calibration curve to that published by Cameron et.al. (80).

Lever (75) used gel permeation methods to characterise the organic matter present in Bayer process liquor. His work also was limited to the gel filtration curves due to the non availability of suitable known compounds for calibration.

Gel filtration appears to have good application prospects to the problem of identifying the organics in

Bayer process liquor but it needs additional development before it can be used confidently for measuring the molecular weight of these substances. The method is based on the assumption that molecular size is proportional to molecular weight for similar compounds in similar environ ments. It would be useful to investigate this proportion in regard to the humic substances, as they are so complex ln structure. However, until that stage is reached, it is advisable to determine molecular weight of humic materials by at least two different but independent methods. 101

Value Method Reference

-1000 Isothermic (82) distillation

984-1294 End group analysis (83) 1336 Equivalent weight (84)

1350 Equivalent weight (85)

1235-1445 Equivalent weight (86) 1624 Freezing point (87)

1392 X-ray (88)

5000-7000 Dialysis (89)

4500-26,000 Diffusion (90)

14,000-20,000 GPC (91) 5000-100,000 GPC (92)

10,000-200,000 GPC (93)

25,000 Sedimentation, (94) viscosity

36,000 Viscosity (95)

47,000-53,000 Osmometry (96) 53,000 Sedimentation (97)

6600-100,000 GPC (98) & (81) 500 to over 10,000 Ultra filtration (75)

membrane

TABLE 3.3 Summary of Molecular Weights of Humic Substances 102

iv) Other methods

There have been other methods such as end-group analysis, viscometry) isothermic distillation, equivalent weight, dialysis and diffusion, used for measuring molecular weights of humic substances. Table 3.3 summarises the available information on the various molecular weight deter mination of humic substances.

3.24 CHEMICAL STRUCTURE AND THE DEGRADATION OF HUMIC

SUBSTANCES

3.241 Various Models suggested for Chemical Structure of

Humic Substances

In regard to the structure of humic substances various models have been suggested 1n the past decade which are discussed below.

Flaig's Model

Flaig 1n 1960 (89) suggested that this source of humic substances in soil is plant liquor which is depoly merized, demethylated, oxidized and eo-polymerized with amino acids to form a heteropolymer as given in Fig. 3.7.

Felbeck's Model

Felbeck (48) concluded from his experiments, which were based on hydrogenolysis of humic acid from muck soil, that a significant part of the humic macromolecule has a heterocyclic structure containing both oxygen and nitrogen as the principal hetero-atoms. A partial structure is given in Figure 3.8. 103

! 0~ ----AocH3 -y~OH y ~~V ~H~ ~H~ HC~ HC~ I I CH CH 3 3

0 -oOH11 0

H C ---·---C-CH3 4 H .

R represents an amino acid residue

Fig. 3. 7 Flaig's model of the structure of humic substances. 104

l 0 CH30o~"'OCH3 _,., o H---0 H·--- 0 r./ 11 , 6-?it o~c(') o-('~ / AN)l c H~Y,,0)l.c H 4Jl~1)l.c H{ C-R l C=O I N-H I I R-C-H c=oI l

R and R1 represent alkyl groups

Fig. 3.8 Felbeck's model of the structure of humic substances.

CH==CH-COOl-1

OH OH \

H ...... --+---C C A 6H R~OH OH n

R represents H, OH or COOH

Fig. 3.9 Finkle's model of the structure of humic substances. 105

Tinkle's Model

Finkle in 1965 (99) suggested that humic acids are formed in soils by a sequence of reactions which begin with the liberation and/or synthesis of hydroxycinnamic acid from the decay of plant litter. Micro-organisms de- carboxylate these substances, producing hydroxystyrenes which are converted to humic acids by oxidation, polymerization and eo-polymerization reaction as shown in Figure 3.9.

Haworth's Model

Recently, Haworth (100) in 1971 suggested that the typical humic acid molecule has a complex aromatic core with chemically or physically attached polysaccharides, proteins, metals and phenols, as shown below:

Peptides)------?'Carbohydrates ,' ... , " , I : .... ',1 core( I , , I , ', I / ', I ," 'I . l' Metals .,_------~Pheno lC acids

The mode of attachment is uncertain. The protein core attachment appears to be stable towards chemical and biological attack. Haworth suggested that humic substances are linked to polypeptides by hydrogen bonds, which can be broken by hot water.

Fulvic Acid Structure

Ogner and Schnitzer in 1971 (101) and Khan and

Schnitzer in 1971 (102) suggested that fulvic acid is made up of phenolic and benzene carboxyl acids linked by hydrogen bonds to form a polymeric structure of considerable stability.

The proposed structure is given in Figure 3.10 (47 and 103). 106

Th~ structure is characterized by voids or holes of different dimensions where organic molecules,

such as alkanes, fatty acids, inor'ganic compounds, metal

lons and oxides can be accommodated. Any weakening of

hydrogen bonds breaks up the structure and liberates both

the building units and any included compounds (47).

0 0 OH ~ 0 o~-1 OH 11 I C-oH 0 HO-~ oH----o=c*c-OH·-··O=C*CtDH 0 0 / /OH c--Y o==c . OH·---o=c c:::::::- 0 HO 'ow· 1 I OH I C-C>H OH OH I I OH 11 I I I I I 0 I I I I I I I 0 --70 I 11 c -oH H HO-C*OH 0 0 ... OH HO-C C=O····HO-~ 8 C=-0 I OH

Fig. 3.10 Structure of fulvic acids 107

3.242 The Degradation of Humic Substances

The chemical structure of humic substances has been the subject of numerous investigations over a long period of time. Because of the chemical complexity of humic materials) many workers have used degradative methods, hoping to produce compounds that could be identified and whose structures could be related to those of the starting materials. However, very few approaches have been successful.

The major methods that have been and are being used

1n structural investigations on humic substances were hydrolysis with H 0, acid hydrolysis, alkaline hydrolysis, oxida 2 tive degradation and reductive degradation (47).

Hydrolysis of humic substances with water means boiling with water and analysing the extract for organics, similar to the method in which polysaccarides yield glucuronic acid (47) as acid hydrolysis.

The boiling water also extracts small amounts of ether soluble materials which contain (a) p-hydroxy benzoic,

HC=O COOH

OCH Q ~OH ~ 3 OH OH ~OCHOH 3 OH (a) (b) (c) (d)

Under acid hydrolysis, it was claimed that in addition to the p-hydroxy benzoic, vanillic acids and vanillin, there are also small amounts of syringic acid: 108

C0 H 2 OCH 3

HC0 3 OCH3 .. OH

Jakab et.al. (104) has reported the degraded products ob tained when a humic substance is leached with 5NaOH at 170°C.

They have detected over thirty phenolic compounds by chromatographic methods and suggested that these compounds obtained in the alkaline hydrolysis is shown as follows:

~OH ~OH HA OH ~OH ~H OH~H OH

The conditions applied in the Bayer process are,

1n fact, alkaline hydrolysis and alkaline oxidation.

Schnitzer and Khan (47) have reported on the oxidation of humic substances by potassium permanganate under alkaline conditions. They have identified the presence of aliphatic mono- and dicarboxylic and benzoic carboxylic acids. Aliphatic monocarboxylic acids that have been isolated and identified include acetic, propionic, isobu- tyric, isovaleric, n-valeric, isocaproic, n-caproic and n-heptanoic acids. Aliphatic dicarboxylic acids resulting from the alkaline permanganate oxidation of humic substances include oxalic, malonic, succinic, glutaric, adipic, pimelic and suberic acids. The following benzene 109

T02H T02H TH3 TH3 Y02H yH3 C0 H yo H I 2 2 CH TH2 ?HC0 H yH.CH co2H TH2 TH(CH2)2C02H I(CH2) 4 (CH2) 5 3 2 2 I CH CH CH CH CH CH 3 3 3 TH2 3 3 3 CH 3

(1) (2) (3) (4) (5) (6) (7) (8)

CO H ?0 H T02H I 2 T02H T02H 2 T02H T02H C0 H I(CH2) 2 (CH2) 3 fCH) 4 I(CH2) 5 (CH2) 6 2 TH2 I co H co H bo H C0 H C0 H C0 H 2 2 2 2 2 2

(9) (10) (11) (12) (13) (14) (15)

6 co2H6 Q (Xco2H 2lco2:6 QtC02H C0 H C0 H H0 C C0 H C0 H 2 2 2 2 2 C0 H C0 H C0 H 2 2 2 (16) (17) (18) (19) (20) (21) (22) Ho2yYcoCO H 2H HOM02H C0 H 2 (23) (24) (25) (26)

(1) Acetic (13) Adipic (25) Benzene penta carboxylic (2) Propionic (14) Pimelic (26) Benzene hexa carboxylic (3) Iso-butyric (15) Suberic (4) Iso-valeric (16) 0-phthalic (5) N-va1eric (17) M-phthalic (6) I so-caproic (18) P-phthalic (7) N-caproic (19) 1,2,3 -benzene tri carboxylic (8) N-heptanoic (20) 1,2,4 -benzene tri carboxylic (9) oxalic (21) 1,3,5 -benzene tri carboxylic~ (10) Malonic (22) 1,2,3,4 - benzene tetra carboxylic (ll) Succinic (23) 1,2,3,5 - benzene tetra carboxylic (12) Glutaric (24) 1,2,3,5 - benzene tetra carboxylic

Fig. 3.11 Degraded carboxylic acids obtained under alkaline permanganate oxidation.

) )

3 3

-OCH -OCH

, ,

I I

-H -H

, ,

Polymers Polymers

Oxidation Oxidation

rings rings

-C -C

(

the the

o o

& &

between between

• •

+ +

chain chain

mostly mostly

(105) (105)

side side

I I

the the

soil soil

Oxidation Oxidation

Polymers Polymers

C-linkages C-linkages

in in

demethylation demethylation

Dehydrogenation Dehydrogenation

Polymers Polymers C-linkages C-linkages

in in

products products

parts parts

------~~co

substances substances

dehydrogenation dehydrogenation

degradation degradation

I I

& &

10-30% 10-30%

molecular molecular

Lignin Lignin

degradation degradation

organic organic

a=r=o=m~at=~=·c~~~~~=-

high high

of of

3 3

of of

Oxidation Oxidation

of of

Lignin Lignin

Oxidation Oxidation

dehydrogenation dehydrogenation

chain chain

demethylation demethylation

o o

and and

side side

-C,-H,-OCH

+ +

~ ~

the the

Oxidation, Oxidation,

Decomposition Decomposition

transformation transformation

of of

compounds compounds

microorganisms microorganisms

I I

for for

I I

Polymers Polymers

1-3% 1-3%

Protein Protein

Process Process

Aliphatic Aliphatic

supply supply

3.12 3.12

Energy Energy

Fig. Fig.

~------~ ~------~

(Cellulose) (Cellulose)

and and

acids acids

microorganisms microorganisms

of of

autolysis autolysis

supply supply

Amino Amino

50-60% 50-60%

3 3

growing growing

Energy Energy Carbohydrates Carbohydrates 111

carboxylic acids have been identified among the oxidation products~ o-phthalic acid, m-phthalic acid, p-phthalic acid, 1,2,3 -benzene-tricarboxylic acid, 1.2.4 -benzene tricarboxylic acid, 1,3,5 -benzene-tricarboxylic acid,

1,2,3,4 -benzene-tetra carboxylic acids, 1,2,3,5 -

benzene-tetra carboxylic acid, benzene-penta carboxylic

acid and benzene hexacarboxylic acid. Excluding oxalic

acid, the aliphatic and aromatic compounds identified

account for up to 4.3% of the original weight of humic

substances. The structure of the degraded compounds

obtained under alkaline permanganate oxidation are

illustrated ln Figure 3.11.

The degradation of organic matter in the soil has

been reported by many workers as a very complex process.

It could vary greatly according to the conditions applied.

The complexity of the processes that may be involved in

transformation of organic materials in soil is illustrated

ln Figure 3.12 originating in the work of Flaig (105).

It is evident from the above collected work that

the organic matter present in bauxite shows a similar type

of complex system during its degradation when attacked by

the caustic liquor in the Bayer process. The degraded

products obtained in the Bayer process are also believed

to be similar to the products illustrated in Figure 3.11.

3.3 IDENTIFICATION AND CHARACTERISATION OF ORGANIC

IMPURITIES IN BAYER PROCESS LIQUOR

There has been very little work reported ln rela

tion to the identification of organic materials present in

Bayer process liquor. 112

One of the earliest studies of the organics present

ln Bayer process liquor was carried out in 1954 by Breuer

(106). He identified the presence of eight organic com

pounds, but the technique he employed was not sufficiently

modern to give accurate results, although his findings were important.

In most alumina plants, any organic materials which reach supersaturation and can be oxidised by potassium permanganate are termed oxalate. Therefore, most methods of control of organic matter in the process assume that it is sodium oxalate.

Sata and Kazama (107) have identified formic,

acetic and oxalic acids in Bayer process liquor. They have used steam distillation to separate these organic acids with low boiling point and column chromatographic methods for oxalic acid. The details were not mentioned in their work, therefore it was difficult to assess the reliability of their results. They have also reported the presence of oxalic acid in a sample of scale taken from a precipita tor in an alumina plant by x-ray diffraction methods.

The most detailed and up-to-date work is reported by Lever (75) on the subject of identification of organics ln Bayer process liquor. In Lever's work the major organic constituents in Bayer process liquor have been classified in the following three groups: (a) the humic matter, consisting of the coloured high molecular weight organics extracted from the bauxite and their initial degradation products (b) the humic "building block" organic compounds which are mainly the benzene carboxylic acids and phenolic acids and (c) the low molecular weight degradation products 113

such as formic and oxalic acids.

The organics in group (a) (humic matter) were

characterized by measuring their molecular weight distri-

butions by ultrafiltration and gel filtration chromato-

graphy. Firstly, the Bayer process liquor was acidified to pH= 2, and the organic matter was extracted into n-butanol, after which the butanol extract was subjected to a number of washings in order to make the extract almost

salt free. It was then fractionated by ultrafiltration,

successively, through the PMIO, DM5, UM2 and UM05 membranes

(Amicon Ultrafiltration membranes). Lever used two types of Bayer process liquor for his investigations. These were from the Bayer process plants processing Jamaican bauxite; one from a low temperature (135°C) digestion plant and the other from a high temperature (240°C) digestion plant.

The results obtained are reported in Table 3.4 below.

Membrane M.W. cut off Liquor Digestion Temp. 135°C 240°C

PM10 10,000 2 0

DM5 5,000 14 12

UM2 1,000 75 76

UM05 500 9 12

TABLE 3.4 Percentage of Total Humic Matter Retained by each Ultrafiltration Membrane 114

In his gel filtration chromatographic method, Lever has only been able to use the shape of the gel filtration curve as a fingerprint to characterise the humic material.

Because of the polydispersity of the fractions, calibration by this method could not be used for any accurate molecular weight distribution measurements.

The organics in group (b), or the suspected organ1c compounds which are mainly the benzene carboxylic acids and phenolic acids, were separated by using gas chromatography and identified by their retention times and comparison of their infra-red spectra to those of known compounds. Firstly, the Bayer process liquor was acidified to pH = 2 and then extracted with diethyl ether. The extract was then methylated with diazomethane. The methylated sample was subjected to gas chromatography using a Perkins

Model 3920 flame ionisation detector, 1.82m x 3.18mm O.D. stainless steel column containing 3% OV-17, 80 - 100 mesh

Gaschrom Q, programmed from 100° to 320°C at 8°C/min and nitrogen flow of 30 ml/min. The results obtained are illustrated in Table 3.5.

The organics 1n group (c) 1.e. the Low Molecular

Weight Degradation Products, were extracted into butanol and then butylated. The butyl esters were subjected to gas chromatographic analysis. Butyl esters were used since the methyl esters of the low molecular weight degradation products were found to be highly volatile. These results are given in Table 3.5.

The work done by Lever is considered to be the most up-to-date and successful in identification of organic 115

Liquor Digestion Temperature No. Compound 135°C 240°C

1 Glutaric acid 10 20 2 Pentanedicarboxylic acid 540 100 3 2-Hydroxy benzoic 10 70 4 Hexanedicarboxylic acid 150 250 5 Dihydroxybenzoic acid 10 10 6 Pentanetricarboxylic acid* 340 10 7 Hydroxybenzenedicarboxylic acid 70 470 8 1,2,4-benzenetricarboxylic acid 70 260 9 1,3,5-benzenetricarboxylic acid 50 410 10 Methyl-benzenetricarboxylic acid 25 250 11 Ethyl-benzenetricarboxylic acid 150 360 12 Dihydroxybenzenedicarboxylic acid 13 Hydroxybenzenetricarboxylic acid lOO 10 14 1,2,4,5-benzenetetracarboxylic acid 140 490 15 1,2,3,5-benzenetetracarboxylic acid 120 990 16 Methyl-benzenetetracarboxylic acid 40 200 17 Benzenepentacarboxylic acid 460 1580 18 Benzenehexacarboxylic acid 640 570

* Tentatively identified Identified 2925 6050 Total from integrator count 3600 6500 % of compound identified 81 93

TABLE 3.5 "Building Block" compound (mg/1) identified in Bayer process liquor (Ref. 75). 116

impurities in Bayer process liquor, but there are limitations to Lever's methods which are described below.

Compound Concentration (mg/1)

Formic acid 2290 Acetic acid 4440 Lactic acid 180 Oxalic acid 2530 Succinic acid 1420

TABLE 3.5 Low Molecular Weight Degradation Products Present in Low Temperature Digestion Bayer Liquor

In characterisation of the organ1cs 1n group (a), the Bayer process liquor was first acidified to pH = 2 and later extracted into butanol. By doing this, there could be changes in the molecular structure of the humic matter present in the Bayer process liquor caused by change of pH. These could have been polymerization of the organic matter resulting in different quantitative results for the higher molecular compounds. Secondly, there is the question as to whether all of the particular organic matter enters the ether extract. A survey of the solubil-

ity of benzene carboxylic acids indicates that they are less

soluble in ether than in aqueous solution. Therefore quantitative results obtained by Lever in identifying the organics of group (b) could be in error.

The most recent work on identifying organics in

Bayer process liquor has been by Paul (108) who claims the

presence of sodium oxalate and an organic compound similar to glucoissaccharinate in the Bayer process liquor. He has

not given details of the analytical methods used in identify-

ing this compound, but he considers that the starch added 117

to the Bayer process liquor is the main source of organic impurity. Therefore a close relationship between the organic compounds present in the Bayer process liquor and the degraded products of starch under alkaline attack has been brought to attention in his work. According to

Paul, starch consists of amylase and amylopectin. Amylase, the labile fraction of starch and cellulose compounds, undergoes degradation in hot sodium hydroxide solution, generating saccharinates as the main degradation products

(109).

3.4 EFFECT OF THE ORGANIC IMPURITIES ON THE RATE OF

PRECIPITATION AND THE PARTICLE SIZE DISTRIBUTION OF

ALUMINA TRIHYDRATE

There have been several works reported on the investigation of the behaviour of organic impurities of

Bayer process liquor and its effect on the crystallisation of alumina trihydrate. However, the literature has revealed that there has not been a complete work to deter mine the actual mechanism of those organics in the precipi tation system. Furthermore, some of the workers have looked into the kinetics of precipitation, by using some organics which were not even present in the Bayer process liquor. For example, Ivekovic and the others(110) have examined the rate of precipitation of alumina trihydrate ln the presence of isopropyl alcohol, n-butyl alcohol, isovaleryl alcohol, glycol and glycerol. The same workers have also investiated the effect of the presence of starch.

According to them only glycol accelerated the precipitation of alumina trihydrate at concentrations covered by the 118

~experiments, the other substances retarded precipitation.

Glycerol in a concentration of about 0.2 M/1, glucose in a

concentration of 6.7 g/1 and starch in a concentration of

6.7 g/1 prevented the precipitation.

Sato (111) has examined the decomposition rate of

sodium aluminate solution with the addition of glucose,

sugar and starch. It was revealed from his experiments

that the addition of glucose or sugar retards the decompo

sition, and that an increase ln the amount of starch added

promotes decomposition. Further, it was found that the

retarding action of glucose is stronger than that of sugar.

In the Bayer operation starch is added to the

process as a flocculant in the separation of red mud.

Therefore the above workers have conducted their experi

ments on the basis that the starch is present ln the

pregnant Bayer process liquor as an impurity. However, it

is important to note that the amount of starch added is

negligible. Usually it is only about one ton of starch

added to every 500 tons of Al produced. That that is 2o3 means it is only about 0.1 gm of starch that is present in

1 litre of Bayer process liquor. Furthermore, 80% of the

starch added is carried away by the red mud during the

separation of red mud. Consequently it is very unlikely

that .02 gm of starch per litre could affect the rate of

precipitation, but there could also be argument for the

accumulation of starch in the Bayer process liquor after a

series of recycles. The starch would then not be retained

as starch because it has to go through an extraction con

dition at high temperatures and pressure in an alkaline

solution and would be converted into a degraded product. 119

Kuznetsov et.al. (112) has investigated the decom position of aluminate solutions in the presence of aluminium salts, iron salts, iron hydroxide and oxalic acids. He found that oxalic acid, iron salts, Feso , Fe Cso ) and 4 2 4 3 Fe o accelerate decomposition of aluminate solutions. He 2 3 explained the above mechanism in the following manner.

Crystals of aluminium salts and oxalic acid react with the aluminate solution in their vicinity and render it neutral.

During this process, colloidal aluminium hydroxide is formed, its particles becoming crystallisation centres. Therefore the aluminium hydroxide gel formed in the solution areas adjoining the crystals of aluminium salts and oxalic acid

(and evidently on their surfaces) serves as the actual seed.

According to Kuznetsov et.al., the above mechanism is only applied to aluminium salts and oxalic acid and the mechanism for iron impurities has not been investigated. The amount of salts of aluminium and iron present in the Bayer process liquor was negligible when compared to the amount of organics present in the liquor as impurities.

There has been detailed work on the effect on the crystallisation of alumina trihydrate by Kelly (113). His studies covered the influence of sodium oxalate on the induction periods, initial decomposition rate, equilibrium concentration and crystal shape. Micro-photographs of the products were presented. His major conclusion was that the sodium oxalate does not affect the kinetics of crystallisa tion. Another interesting work was carried out by Sato and Kazama (107) who investigated the influence of organic matter, sodium carbonate and sodium chloride on precipitation 120

of alumina trihydrate. His experiments were carried out with the addition of different charges of organic impurities from 0 - 8 gm per litre, Na from 0 - 90 gm per litre 2co 3 and NaCl from 8 - 30 g/litre. He has obtained the organic impurities by extraction using n - butanol under the H so 2 4 acid phase thus neutralised by means of NaOH. Results from his experiments prove that the precipitation yield de clines with the lncrease of addition of impurities.

However, it was not clear how the rate of crystallisation was affected in the presence of impurities. It was not possible to obtain a clear idea as to the actual effect of the impurities on the rate of crystallisation because details of figures and experimental and analytical methods applied were not available. Some of his results are illustrated in Figures 3.13 and 3.14.

In his investigation he has also analysed the particle size of the products obtained in crystallisation.

Figure 3.14, which was taken from his work, indicates the organic matter causes the particle size of the precipitate to become fine and increase the fixed Na o content in the 2 crystals. He also believes that those organic matters were probably included within the crystals.

The most recent work on the effect of impurities on the Bayer precipitation was carried out by Paul (108) who studied the effect of glucoisosaccharinate on Bayer precipitation. Glucoisosaccharinate has been synthesized in the laboratory. He claims that the addition of this compound to a concentration of 0.8 g/litre as organlc carbon has lowered the precipitation yield by 17% and increased the quantity of particles finer than 20 um by 23%. Organic Matter

a,o Carbon ro equivalent .-I (!) 45 ·r-l :>.. s:: 0 ·r-l .j.l rtl .j.l ·r-l 40 Cl ·r-l u (!) 1>-1 p.,

0 20 40 60 80 100 0 2 4 6 8 Concentration of impurities (g.p.l.)

NaCl Minor Inorganic Impurities 50

o'P ro .-I (!) 45 ·r-l :>.. s:: 46.5 hrs. 0 ·r-l .j.l rtl .j.l ·r-l 40 0! ·r-l u (!) hrs. 1>-1 p.,

10 20 30 40 1st 2nd 3rd (level) Concentration of impurities (g.p.l.)

Fig. 3.13 Influence of Bayer process liquor impurities on rate of precipitation. 122

(!) Particle size Fixed Na o impurity N 2 ·r-1 Ul 100 l--1 (!) 0.31 5 4-1 80 0 0.30 o\O

.jJ ..c: 60 tJl 0.29 ·r-1 olo (!) .jJ :s: ..c: (!) :> -~0.28 ·r-1 40 (!) .jJ :3: ro 200 mesh ,...; 0 0.27 § N ::s ro u 20 z 0.26 mesh

0 0.25 0 2 4 6 8 0 2 4 6 8

Concentration of Organic Matter (carbon equivalent: g.p.l.)

Fig. 3.14 Influence of organic matter on precipitate quality.

His results suggest that, unlike pure sodium oxalate,

certain types of organic compounds similar to glucoiso-

saccharinate exist in Bayer process liquor, which have an

adverse effect on the precipitation of alumina trihydrate.

Paul also claims that the glucoisosaccharinate,

which has a molecular structure similar to glucose (or

sugar) interacts with aluminate molecules on the surface of

the trihydrate to form soluble compounds thereby stabilizing

the aluminate liquor around the alumina trihydrate seed

crystal surface. It is known that when an induction period 123

Al o 2 3 yield g/1 60

0 0.2 0.4 0.6 0.8 1.0 Glucoisosaccharinate cone. as organic carbon/L

Fig. 3.15(a) Effect of glucoisosaccharinate on A1 o yield (synthetic digest) • 2 3

0.36

0.34

Finish ratio 0.32 Al 0/TC 2 0.30 0 0.2 0.4 0.6 0.8 1.0

Glucoisosaccharinate concentration as organic carbon/L

Fig. 3.15(B) Effect of glucoisosaccharinate on alumina trihydrate precipitation (synthetic digest) 124

100

90 0 ::: control 80 H QJ rJ ::: 0. 4 g/1 Org .Carbon ~ 70 ~ A -.::: 0.8 g/1 Org .Carbon ·ri 60 <0 @50 ..c: .j.l H 40 QJ r-1 r-1 ~ 30 UJ

.j.l 20 ..c: tJ'l ·ri ~ 10

dP 0 1 10 100 Diameter in microns

Fig. 3.15(c) Particle size distribution analysis Effect of glucoisosaccharinate on precipitation of Al(OH) (synthetic digest) 3

= 30..}Jm H • QJ 0 20 ,, .j.l = QJ 80 s jj, =10 ,, ctJ ·ri <0

~ ctJ :5 60 H QJ r-1 r-1 ctJs UJ

.j.l ..c: tJ'l ·ri 20 QJ ;;: cr.o

0 0.2 0.4 0.6 0.8 1.0 Glucoisosaccharinate concentration as organic carboh/L Fig. 3.15(d) Effect of glucososaccharinate on particle size (synthetic digest) 125

precedes precipitation, outgrowths appear on the seed crystal surfaces. Inter-particle collisions, caused by mixing, will dislodge and break them to form secondary nuclei, resulting in the generation of excessive fines and lower precipitation yield. His work has been largely concentrated on the effect of precipitation yield and the particle size and some of the results are illustrated 1n

Figures 3~15a,b,c and d. However, no mention is made of the effect of this compound on the rate of precipitation.

3.5 BEHAVIOUR AND MECHANISM OF INHIBITION OF ORGANIC

IMPURITIES IN THE BAYER PRECIPITATION

Most of the work carried out has been related to the effect of organics on the rate, yield and particle size in the Bayer precipitation. Results were interpreted either as precipitation curves, yield or particle size distribution.

There has been little attempt to explain the behaviour and the mechanism that takes place within the precipitation system in the presence of organic impurities. Kelly (113) in his work made an attempt to explain the role of impurities in the precipitation systems. He looked into the effect, and the behaviour, of the following three impurities; sodium oxalate, soluble starch and magnes1um ions. The rate of precipitation was studied by using the electrical resistance vs time data. He reported from his work the impurities influence on the induction period, initial decomposition rates, equilibrium concentra tion and crystal sizes.

Kelly suggests that the role of sodium oxalate on the crystallisation is unimportant. Although the size 126

distribution is not affected by sodium oxalate, it is suggested that the increased induction period results from the dissolution of sodium oxalate which tends to inhibit and retard the decomposition of sodium aluminate molecules.

Consequently, the increase in stability of the molecules acts on the increased induction period.

Additional starch would appear to affect crystallisation resulting in a prolonged induction period, decreased decomposition rates, higher equilibrium concentration and coarser products. Kelly believes these results are attributable to the shielding or enveloping of potential growth surfaces by the starch.

It was claimed in his work that the presence of magnesium added as 4MgC0 .Mg(OH) .5H 0 tends to retard the 3 2 2 decomposition rates and cause a marked 1ncrease in alurnina equilibrium concentrations. No effects on induction periods and product crystal size were noted. These results are interpreted to arise from the adsorption of magnesium ions by colloidal Al(OH) particles with consequent interfering 3 effects on the conversion of this colloidal material to the crystalline product.

Yamada and others (6) have looked into the behaviour of sodium oxalate and measured the solubility of sodium oxalate in a Bayer plant liquor at different concentrations and temperatures. Their results were previously illustrated in Figure 1.3. It is clear that the sodium oxalate represents a very low solubility value under the Bayer precipitation conditions, therefore there is always a tendency for supersaturation of Bayer process liquor with sodium oxalates. 127

In their work, eo-precipitation of sodium oxalate with alumina trihydrate was reported. Their observations were mainly based on the variation of sodium oxalate content

in the plant liquor after the addition of seeds and also the sodium oxalate analysis done on product alumina trihydrate itself. Their test results are listed in

Table 3.7.

Na c o initial ln Al(OH) (%) 2 2 4 3 cone. ( g/1) 0.05 0.3 0.6 0. 9

3 . 0 2. 8 2. 4 2 . 3 2 . 3 2. 5 2. 5 2. 5 2. 2 2.15 2. 3 2. 3 2 • 3 2. 0 2 • 0

Precipitation time 48 hr.

TABLE 3.7 Na cone. after alumina trihydrate 3c2o4 precipitation

According to Yamada and others (Fig. 1.3) the solubility of sodium oxalate in the Bayer plant liquor under the Bayer precipitation conditions is analysed to be

1. 8 g/li tre. However, as shown in Table 3.7, from his experimental results the sodium oxalate concentration in the Bayer liquor lS higher than the solubility value

(1. 8 g/litre). He claims that the codeposition rate of sodium oxalate with the alumina trihydrate, increases with the increase of the oxalate content in the liquor and also in the seed. He also observed that when the sodium oxalate content in the alumina trihydrate is lower than 0.3%, and the oxalate concentration in the liquid is lower than

2.5 g/1, the sodium oxalate does not precipitate with the alumina hydrate for 48 hours. 128

Brown and Cole (114) reported the behaviour of sodium oxalate in the Burntisland Bayer plant in Scotland with special emphasis on (a) the cyclic nature of the sodium oxalate behaviour (b) the distribution of crystalline sodium oxalate in the alumina trihydrate produced, and the effect of crystalline sodium oxalate on the calcination of the trihydrate to alumina.

According to them, the driving force for crystallisation of sodium oxalate from Bayer process liquor is the oxalate supersaturation which is defined as follows:

c c c oxalate oxalate equilibrium supersaturation concentration solubility in liquor

They arrived at an equation which satisfactorily describes the equilibrium solubility of sodium oxalate in

Burntisland spent liquor following the experimental work carried out to determine the equilibrium solubility of sodium oxalate.

2 C . .b . 7.62 exp.(.012T- 0.016FS- 0.011 - ) equl1 l rlum = co 3 solubility

where T is liquor temperature in °C

FS is liquor free soda concentration expressed

as g/1 Na 2o 2 - lS liquor carbonate concentration expressed co 3 as g/1 Na 2o

They have established the equilibrium solubility of sodium oxalate at 50°C as 1.2 g/litre for a typical plant spent liquor of the following composition: 129

Alumina 75 g/litre

Free Soda 145 g/litre

Carbonate 15 g/litre

The behaviour of sodium oxalate ln the alumina plant was monitored over a period of one year by analysing weekly composite samples of

a) spent liquor for dissolved sodium oxalate

concentration

b) hydrate seed for crystalline sodium oxalate,

and

c) kiln feed (product hydrate) for crystalline

sodium oxalate.

The results of their monitoring experiments are illustrated in Figure 3.16. As the dissolved sodium oxalate in spent liquor rises, there is a corresponding decrease in the solid sodium oxalate present in the hydrate and in the kiln feed. These trends are reversed when the

dissolved sodium oxalate reaches a critical concentration.

Over the monitoring period of one year this behaviour pattern

was repeated three times.

They examined the crystalline sodium oxalate within

the hydrate particles and it was claimed that the finer end

of the hydrate seed distribution would be most contaminated

with crystalline sodium oxalate. They observed the presence

of sodium oxalate needles amongst the final hydrate, using

the scanning electron microscope. According to them the

product trihydrate, when fractionated, has shown the follow

ing distribution of crystalline sodium oxalate within the

hydrate particle. 130

Spent liquor Dissolved 4 sodium oxalate -----~ (g/1) 1

H drate seed

Solid 0.3 sodium 0.2 oxalate -----L 0.1 (wt.%) I

Kiln feed (Product Hydrate) .04 Solid .03 -----~ sodium .02 oxalate .01 (wt.%)

Time (one year period)

Fig. 3.16 The behaviour of sodium oxalate in a Bayer plant over a period of one year 131

Size Weight Percent Fraction Sodium Oxalate (~m) %

0-32 .046

32-63 .033

63-90 .032

90 .027

TABLE 3.8 Crystalline Sodium Oxalate in product alumina trihydrate

They suggest that the increasing concentration of sodium oxalate towards the fine end of the distribution is consistent with the greater trapping of the oxalate crystals by the finer hydrate particles during precipitation, but the evidence seems insufficient to conclude that the oxalate promotes fine formation. There is no clear indication given from this result as to the effect on the particle s1ze distribution of the product trihydrate in the presence of sodium oxalate.

Solymar and Zsindely (115) also investigated the behaviour of organic impurities in the Bayer liquor. Their work was done by extracting the organic impurities 1n the following manner. The Bayer liquor was acidified by adding

HCl and some of the organic matter was precipitated during the acidification which was graded as the humic fraction.

Model tests were carried out by adding the precipitated humic matter and they claimed decreased precipitation rates with increased organic impurities. They also reported the diminution of the digestion effect due to a carbon content from 2 to 3 g/1. During a further increase 1n the organic substance concentration, however, the degree of digestion 132

was not diminished further. However, the above report

on digestion effect was not based on any det;ailed experimental

evidence. They also reported on the influence of the humic

acid fraction on the sedimentation of red mud and on the

precipitation. According to them the unfavourable effects

are only noticeable after attaining the critical carbon

~------content of 2 to 3 g/1. It is also claimed that the increase

of viscosity of aluminate liquor, containing sodium carbonate,

is noticeable with small amounts of organic substances, but

when the amount of organic is increased, the viscosity

decreases again. They syggest that this appearance depends

on the influence of the humic acids increasing the soda

solubility. They also suggest organic substances increase

not only the solubility of soda, but also other salts,

thereby increasing the stability of aluminate liquor, thus

reducing precipitation yield. However, it was not quite

clear from their experimental results as to the increase

of the solubility of the other salts.

Tikhonov et.al. (116) reported that high content of

organic impurities in the Bayer process liquor increase the

induction period to about 4 - 6 hours. They explained that

the action of organic substances is due to adsorption on the

surface of the trihydrate seeds, leading to partial break

down of contact between the hydroxide crystals and the liquid

phase, so that crystal growth is hindered.

3.6 REMOVAL OF ORGANIC IMPURITIES FROM BAYER LIQUOR

There have been few workers who report on the

problem of removing organic impurities from Bayer process

liquor. Most of the studies have been limited to laboratory 133

scale investigations and no commercial application has taken place. The only method used in industry was that based on the Kaiser patent described in Chapter I. This may have been due to the fact that firstly there has been no complete investigation to justify the poisoning effect of the organic impurities, and, secondly, most of the remain ing methods are costly and the economics of such an operation would not be sound. The present method of removal (i.e.

Kaiser patent (7)) has been quite successful but it removes only the organic which reaches supersaturation and adheres onto the fine trihydrate particles.

The following methods were suggested by various workers to remove the organic impurities in the Bayer process liquor: 1) use of activated carbon and pulverised coal as adsorp

tion material

2) ion exchange method

3) oxidation method, anodic oxidation, oxidation under

pressure

4) heat treatment

5) seeding method.

3.6.1 ACTIVATED CARBON METHOD

Early in 1936, Iskolsky et.al. (117) reported the removal of organic impurities from Bayer liquor using activated carbon and they claimed that the activated carbon adsorbed the organic present in the liquor. Their findings were not used commercially as the economics of such operation were doubtful. Further, it was necessary to investigate the after-effects of the activated carbon itself (by contamina- 134 tion) on the precipitation of alumina trihydrate. They also claimed that the stability of aluminate solution was affected by the addition of activated carbon resulting 1n premature precipitation. The premature precipitation is also believed to be due to the nucleation effect of carbon particles.

Recently (1971) Loeffler (118) examined various adsorption methods for the removal of organic impurities.

Dolomite, active bauxite, active alumina, carbon and active carbon, were subjected to the examination as adsor- bants. Active carbon proved to be very effective. He had also looked into the use of pulverised and granulated coal for their activity. The economics of using coal as an adsorbant to remove organic impurities were studied.

3. 6. 2 ION EXCHANGE METHOD

Romanov et.al. (119) reported from his findings that an1on type ion exchange resins could successfully remove organic impurities from Bayer process liquor. He stated

(Cl-) type ion exchange resins are capable of remov1ng 30% of the organic impurities present in the Bayer liquor, whereas the strong (OH-) base ion exchange resins could remove almost 70% of the organics. Both types of resins used were stable in caustic medium and the complete decol- ouration of the Bayer process liquor was observed with the

(OH-) base ion-exchange. However, it was not clear whether the sorption could be carried out successfully above 60°C, under Bayer conditions, as his experiments were limited to only 40 0 C.

3. 6. 3 OXIDATION METHOD

This method is based on oxidation of the organic 135

matter to C0 under various conditions. Lapp and Kuznet- 2 sov (120) investigated electrolytic oxidation of humic

substances in alkali and soda solution which was carried out in glass electrolyzers equipped with nickel and nickel plated anodes, and, in some cases, with stainless steel anodes. They determined the optimum conditions for, and the composition of final products of the electrolytic oxidation of humic substances in alkali and soda solutions.

It was concluded that the oxidation of humic substances takes place much more effectively in soda solutions than ln

alkaline solutions.

Loeffler (121) reported on his tests on the anodic

oxidation of organic impurities using a cell which was

previously designed for electrolytic purification of waste

water. Pb0 - stratified anodes were used. The tests 2 2 were carried out at current densities of 10 - 100 amp/m

and resulted in a consumption of 10 kwh/t Al o , for a 2 3 brightening of 15% during a liquor temperature of 83 0 C.

At current densities of about 1000 Amp/m 2 , an energy con-

sumption of 140 Kwh/t Al resulted with a new anode. 2o3 However, Loeffler pointed out that in order to use the cell

to decolour Bayer process liquor, the cell would have to be

reconstructed.

According to a Japanese patent (122) the oxidation

of humic substances could be carried out with oxygen under

high pressure. A similar type of work was repeated by

Loeffler (123) and the oxidation of the organic substance

in the Bayer liquor was suggested to be carried out under

digestion conditions of bauxite under pressure and tempera-

ture with oxygen. He claims that it is possible to achieve 136

a satisfactory discolouration of the Bayer liquor uslng this method. He pointed out, however, that at the reaction a corrosion may occur on the autoclaves so that it will be necessary to execute corrosion investigations prior to the technical application of the process.

3.6.4 HEAT TREATMENT There have been suggestions by various workers (9)

that if the recycled caustic liquor is heated to 400°C, . most of the organic impurities will decompose into co 2 Although this method is simple, the economics of the process

would requlre further investigation. Jung abd Loeffler (124) suggested from their find

ing that the organic impurities could successfully be removed

directly within the bauxite itself by roasting before the

digestion stage within the temperature range of 400 - 500°C.

However, any structural changes of the bauxite which could

affect the extraction conditions have to be studied further.

For example, during such operation, hydragillite, which

could have been extracted at low temperature, may be con

verted to boehmite which can be extracted only at high

temperatures.

3.6.5 SEEDING METHOD The methods mentioned earlier do not find commer

cial application due to the heavy cost involved. However,

the method based on the Kaiser patent (7) has been

successfully applied in the industry to remove the organics

which reach supersaturation and tend to coprecipitate with

alumina trihydrate. This method has been described in

detail in Chapter 2. 137

Yamada et.al. (6) reported from their findings that removal of sodium oxalate could be performed by adding sodium oxalate as seeds to precipitate sodium oxalate and some of the humic acids which have low solubility values in Bayer liquor. They have not investigated the possible coprecipitation of alumina trihydrate along with the sodium oxalate crystals as seeds, thereby again the problem of possible fine formation arises. CHAPTER 4

CRYSTALLISATION PROCESS OF ALUMINA TRIHYDRATE 138

The crystallisation (precipitation) of alumina tri hydrate is one of the most important steps in the production of alumina from bauxite. The process is also known as the decomposition of sodium aluminate liquor. The process is controlled under the two prime criteria of a high recovery of product and a desirable product size distribution. In practice, it is always found that those criteria are contrad ictory to each other since when the conditions are selected to obtain a high recovery of product then the product contains a large percentage of fines. On the other hand, if the conditions are chosen to obtain the best particle size distribution, then the product yield is very poor. Therefore in the industry the process lS optimised on the basis of past experience.

The mechanism of alumina trihydrate precipitation has been extensively studied by a great number of authors who have tried to evaluate the importance of the measurable

factors such as temperature, caustic concentration, degree

of supersaturation, nature and quantity of seed, impurities

in the liquor and agitation speed as well as to establish

mathematical expressions to describe the kinetics of the

trihydrate precipitation.

In the Bayer process the conditions for both the

extraction process and the precipitation stage can be clearly Na delineated from phase diagrams relating to the 2o - Al - H 0 system. One of the most applicable phase 2o3 2 diagrams was published by Wefers (125), illustrated in

Figure 4.1, in which is plotted equilibrium curves using the 40 Bayer Extraction ~ Bayer Precipitation 35

Temperature 0 C 30

(Y) 0 1:\1 rl 2 5 -< .J-l ~ ([) C) H 20 ([) p_,

.J-l ,.>::; bO ·rl 15 :s([)

rv.(J/ / / / /

0 5 10 15 20 25 40 Weight percent NA2 0 I-' Figure 4.1 Phase Diagrams NA Al 0 - H 0 system w in the 2 0- 2 3 2 tD 140

data obtained from the past workers. The range of conditions usually applied in the extraction and precipitation step of the Bayer process are shown as shaded areas in

Figure 4. L

4.11 SOLUBILITY OF ALUMINA TRIHYDRATE IN PURE CAUSTIC AND BAYER PLANT LIQUOR

Process optimisation of the extraction or the crystallisation steps of the Bayer process requires accurate information on the solubility of alumina trihydrate 1n sodium hydroxide solution. Although many studies of the

Al o - Na o - H 0 system have been reported there are 2 3 2 2 discrepancies in the solubility values obtained.

Misra (11) reviewed the solubility determination published by previous workers and also some additional solubility data which was obtained from several alumina producers.

Various equilibrium equations have been proposed in the past to interpret the solubility data. Pearson (43) suggested the following empirical equilibrium constant to represent solubility data.

c + c [ NaA1o 1 NaOH 2 j K = [cNaA1o 2][ a H2o] n where C represents the molar concentration of the subscripted

and a H is the activity of water. The exponent species 0 2 of the activity term was g1ven by n = 2.67 - 0.0133t, where t = temperature 0 C. The above equation does not clearly explain the 14-1

fundamental solubility relationship and is only a useful form for data correlation. Pearson did not give a value for the equilibrium constant K or clearly explain how successfully this correlation fitted experimental data.

Russell et.al. (126) used the following equation to represent the equilibrium constant:

Employing the equation he defined the equilibrium constant as follows:

K =

The symbols have the same meaning as previously and the equilibrium constant is related to temperature by the

Vant Hoff equation:

- 734-0 + 4-.11 log K = 4-.574-T where T is the temperature in °K.

The most recent empirical relation to correlate

solubility data was proposed by Misra (11) in 1970 for the temperature range of 25 - 100°C and for the caustic

of 30- 320 g/litre (Na 0): concentration of range 2

ln Re = 6.2106- 24-86.7/T + 1.0875 CNa /T 20 where =

NaOH concentration g/1 Na = 2o

T = temperature °K

He claimed that the solubility of hydragillite in caustic

solution is dependent on the caustic concentration and the 142

Furthermore~ impurities such as Na , NaCl temperature. 2co 3 and organic impurities tend to increase the solubility value.

For example, in Bayer plant liquor with the same caustic concentration the solubility values are typically 15 percent higher than that of the pure solutions.

Adamson et.al. (127) suggested a relationship to interpret the solubility data 1n plant liquor but in the temperature range of 40 - 60°C:

"i'~ log C 2.4186 + 0.02113t + (1.788- 0.004t). log Al 0 =- 2 3

0 where t = temperature C = initial concentration g/1 = equilibrium concentration g/1.

Juhasz (128) proposed a relation for plant liquor which is applicable to the temperature range of 40 - 68°C of 80 - 196 g/1 Na as and for caustic concentrations 2o follows:

i' log CAl = 2o3

where concentration is in g/1 and temperature is in °C, but which is applicable only to particular plant liquors.

4.12 STABILITY OF SODIUM ALUMINATE SOLUTION

diagrams for the Al - Na The various phase 2o3 2o - H (Fig. 4.1) system gives information about the stability 2o of the sodium aluminate solution under given conditions. 143

Using these it is possible to predict whether the solution is supersaturated with Al o and also the amount of Al o 2 3 2 3 which will be precipitated at a given temperature.

Kuznetsov (9) suggested the following formula 1n relation to the degree of supersaturation

Caustic ratiot s = = Caustic rat1o eq where S = degree of supersaturation concentration of Al o in super- = 2 3

saturated solution at the given time

Al eq concentration of Al at equilibrium 2o3 = 2o3 Caustic ratio =

He also suggested that the stability of the sodium aluminate solution depends very much on the degree of supersaturation.

Stability decreases with the increase of S. The other factors on which the stability will depend are the caustic ratio (Al 0 /Na 0), temperature, seed, impurities and 2 3 2 agitating speed. However, the apparent stability of highly supersaturated caustic aluminate solution is shown by the prolonged induction periods (up to several days) for precipitation from unseeded liquors of Bayer process composition

(in the absence of alumina trihydrate seed). Furthermore, the high solid to solution interfacial tension of -2 1,250 ergs. cm (129) suggests that the possibility of spontaneous homogenous nucleation occurring in the industrial process is remote. 144

4.13 SUPER SOLUBILITY CONCEPT

In order for crystallisation to take place, there should be a state of imbalance with a decrease in chemical potential between the bulk of the solution and the crystalline surface. In other words, the solution should be supersaturated. Miers (130) postulated that the region of supersaturation can be divided into two parts as shown in

Figure 4.2. According to the theory, crystal growth occurs when the concentration lies above line A and nucleation takes place only in the labile region (above line B). Referring to Figure 4.2, any point below the saturation curve A (for example, point E) is unsaturated and will not promote or yield crystal material. Point D in the metastable region will promote crystal growth and will drop to concentration E if seed is added, but will remain unchanged at point D if no such seed crystals are present. Concentrations above the supersaturation curve such as point C will spontaneously crystallise to ultimately yield concentration E by both nucleation and growth mechanisms.

/B Labile c / region 0 / A

>::: I 0 ,/ ·o-1 +l ...... m / f..l .,....D +l ..... >::: , 9 Stable Q) 0 - region >::: - - 0 u I I I F 0

Temperature

Fig. 4.2 Miers supersolubility concept 145

4.2 SPONTANEOUS PRECIPITATION OF ALUMINA TRIHYDRATE FROM SODIUM ALUMINATE SOLUTION

Generally sodium aluminate solution will remain stable even at the higher levels of supersaturation for long periods so that crystallisation of alumina trihydrate has a very long induction period.

Vrbasky et.al. (131) studied the kinetics of the spontaneous precipitation of alumina trihydrate from unseeded aluminate solution. He suggested that the spontaneous precipitation system shows an auto-catalytic characteristic nature with a pronounced induction period, during which no precipitation occurs then a steady state and a period of delayed precipitation. These authors also developed an empirical equation for calculating the maximum precipitation rate for the time when this precipitation takes place and for the amount of the precipitate at equilibrium for glven aluminate solutions. They also reported that during the spontaneous precipitation system there can exist a very long induction period depending on the caustic ratio

The induction period becomes longer with the decrease of the caustic ratio Al /Na 2o3 2o.

According to Vrbasky and others the spontaneous precipitation behaves more like an autocatalytic reaction at lower caustic ratio and less so at higher caustic ratio, thus the precipitation rate seems to follow a simple logarithmic law which is of first order relationship. This is to be expected since in a higher caustic ratio solution

(high supersaturation), there is a very rapid nucleation at the beginning of precipitation and then the rate. is con- 146

trolled by growth and therefore appears to follow a first order relation.

4.3 PRECIPITATION FROM SODIUM ALUMINATE SOLUTIONS SEEDED

WITH ALUMINA TRIHYDRATE (BAYER PROCESS CRYSTALLISATION

During the production of alumina trihydrate us1ng the Bayer process, it is the usual practice that the previously pre·pared alumina trihydrate is added to the supersaturated solution of sodium aluminate to act as a seed crystal. In fact, Bayer, in his original work, showed that the rate of decomposition could be accelerated by adding some seed material to act as nuclei as well as by providing new surface for crystal growth. In his patent, he recommended only alumina trihydrate and possibly isomorphic chemical compounds to be used as the seed material for the crystallisation of alumina trihydrate. Although it is now known that many other compounds such as other aluminium salts, iron hydroxide, titanium oxides and cryolite can be used as seed. However, in plant practice, for the sake of purity and economy, alumina trihydrate is used as the seed material.

4. 3 1 KINETICS RELATION FOR SEEDED PRECIPITATION

There have been several relationships proposed by various workers 1n the past to interpret the kinetics of crystallisation of alumina trihydrate with seeding, but most of these relationships have been unable to explain all the stages in the crystallisation systems including the induction period. The rate equations suggested by past workers were mainly based on the change in weight of trihydrate precipi- 147

tated with time as follows:

dW dt = K f (S) where

S lS the supersaturation of the aluminate solution given by R - R or CAl e 0 - c* and K is the rate constant 2 3 dependent on the temperature, surface area of the seed and caustic concentration of the solution.

A simple first order reaction equation was proposed by Hermann and Stipetic (129) for the kinetics of precipitation of alumina trihydrate 1n the Bayer process:

dW ~': K (C - C ) dt =

An identical type of equation was suggested by

Kuznetsov (9) in his work. Shimosato (132) suggested the effect of seed as one of the factors in his equation to explain the kinetics of seeded precipitation as follows:

dC - dt = K/Cs + r

Where the seed concentration c s r the activity ratio of alumina trihydrate

precipitated to seed trihydrate

the initial alumina concentration

I K & K reaction rate constants dependent on

temperature and the caustic concentration.

Pearson (43) suggested the following equation for the kinetics of precipitation:

dX dt = 148

(The same equation was suggested for the extraction stage as mentioned in Chapter 2 of this thesis). Where K

1s a parameter dependent upon temperature, At is the area 2 of seed surface (m /litre) at timet (hours), xt and x* are the mole concentration of NaA10 at time t and 2 at equilibrium respectively and a * is the molar concentration of NaOH at equilibrium. It has been shown that the rate of reaction is proportionate to the total surface area of the trihydrate particles present at any given time.

Solymar and Zambo (133) presented some express1ons for the kinetics of precipitation, using experimental results from different sources. They showed that in the tempera-

0 ture range of 30 - 80 C the rate of precipitation in the initial part of the process is proportional to the square and later to the first power of supersaturation.

Jamey (134) from his isothermal precipitation experiments suggested two distinct stages for the crystallisation. In the first stage, at the beginning of the precipitation where the ratio of Al /Na of the solution 2o3 2o is higher than 0.92 or (Al /Na >0.54), the rate of 2o3 2co 3 precipitation was constant and, in the second stage, the rate was decreasing according to the law,

dC dt = where C is the alumina concentration in which n 1s rather high, comprised between 8 and 10. He suggested that 1n the first stage in the hydrolysis of the sodium aluminate plays an essential part, while the second stage represents probable polymerisation of the aluminate ions followed by crystal growth. 149

In most of the previous works the induction period is neglected from the crystallisation system in the equations that have been developed. For example, Jamey described two stages of the precipitation system without mentioning the induction period in the system.

Misra (11) constructed a mathematical model for the precipitation process by combining appropriate population, mass and energy balance relations with kinetic models for the various crystallisation mechanisms. His model was solved for a batch unit and the solution has been shown to agree with the results obtained from a laboratory precipita tor which was used to study the effect of different variables on the product rate and size distribution.

4. 3 2 MECHANISM OF THE BAYER PROCESS CRYSTALLISATION

The mechanism of the crystallisation stage in the

Bayer process is highly speculative. It is not known whether it is a physical or chemically controlled reaction, but, at present, there is more evidence to conclude that it is a chemical reaction.

The crystallisation of alumina trihydrate with seeding is usually followed by an induction period, nuclea tion, crystal growth, agglomeration and crystal breakages, and therefore it is regarded as a complex system.

Misra (11) explained the precipitation of alumina trihydrate as a process involving new crystal formation by nucleation, and crystal breakage and growth of the crystals by agglomeration and deposition from the super-saturated caustic aluminate solution.

Maricic and Markovcic (135) explained from their 150

work that the decomposition of sodium aluminate solution by seeding with hydragillite crystals occurs simultaneously in the following ways:

a) crystal growth

b) formation of firm binded agglomerates, and

c) creation of new crystallites.

They claimed that the formation of new crystallites 1s characteristic for the beginning of the decomposition while the crystal growth is predominant at the latter stage.

An investigation of the decomposition of sodium aluminate solutions or the crystallisation process, must be separated into the above-mentioned partial processes, the kinetics of which in relation to various physico-chemical factors must be first studied separately. As a result it should be possible to select favourable conditions for acceleration of the overall process with greater precision and better utilization of the accelerating factors.

Lj.. 3 21. Induction Period in Seeded Precipitation

A very long induction period is usually noted in the case of spontaneous precipitation, but a considerably decreased induction period is found with the use of seeding.

Tikhonov et.al. (116) carried out some systematic work of the induction periods in seeded crystallisation of alumina trihydrate. They claimed that the following major factors will affect the duration of the induction period.

a) Temperature: induction period decreases appreciably with increasing temperatures

b) Initial ratio (Al 0 /Na 0) of the solution: 2 3 2 induction periods decrease with increased initial ratio 151

c) Seed surface area: induction period decreases with increased seed surface area

d) The purity of the solution: impurities such as organic substances accumulated in the Bayer process liquor increase the induction period.

They also suggested that 1n plant practice where a high caustic ratio and high seed surface are applied, the induction period can be regarded as being negligible.

4. 3 22 Nucleation

Several workers have studied nucleation in caustic aluminate solutions in the presence of seed. Formation of nucleus in the seeded crystallisation process is known as secondary nucleation, and secondary nucleation of alumina trihydrate can occur during the crystallisation step of the

Bayer process. Therefore for effective control of product size distribution, it is necessary to be able to control the rate of secondary nucleation or the formation of new crystals.

Maricic and Markovcic (135) reported that the crystallisation of alumina trihydrate in the presence of seed occurs simultaneously in the following three different ways:- a) creation of new crystallites (secondary nuclea

tion)

b) crystal growth

c) formation of firm-binded agglomerates (inter

crystallisation).

They also claimed from their experiments that the secondary nucleation is characteristic for the beginning of the decomposition while the crystal growth is predominant at 152

the later stage.

They also investigated the nucleation 1n the presence of different size ranges of seeds and reported that the creation of new crystallites is caused only by fine seed particles and independently from the coarser one which plays quite a passive role.

Hlobik et.al. (136) later revised their prev1ous work by adding that the nucleation can be induced even with coarse grained alumina trihydrate. They also reported that the induced nucleation is greater the more supersatur ated the decomposition solution. Some approach to the actual mechanism of induced nucleation was made in their work.

Scott (137) concluded from the results of previous workers that the crystallisation process has the following stages: a) growth on the surface of the parent crystals b) formation of dendrites at the surface of the crystals c) breaking away of the dendrites to give new particles.

The new nuclei so formed tend to form large loosely bound flocculates which can be broken under very slight mechanical pressure.

Misra and White (138) suggested that broken frag ments formed by attrition of the seed crystals could be one of the sources for new crystals formation, when there is a high agitation rate in the crystalliser, but that it was not the case at lower agitation rates. They also reported that the number of fine particles generated decreased progressively with increasing temperatures from 40°C to 70°C. At 75°C and above there was negligible nucleation. Furthermore, 153

they noted that nuclei will not form readily when seed particles are about, but will form copiously when they are present. This would suggest a secondary nucleation mechanism in the Bayer process.

Brown (139) reported that the secondary nucleation of alumina trihydrate in seeded caustic aluminate solutions of industrial composition is closely associated with the crystal growth process via a surface nucleation mechanism.

When crystallisation is preceded by an induction period, the seed crystals develop and give birth to large numbers of secondary nuclei. However, under the condition of high supersaturation and absence of impurities, the initial crop of secondary nuclei grow dendritically. These dendrites then break away from crystal surfaces to form an additional crop of secondary nuclei. Brown also reported that in the caustic aluminate solution of industrial composition, the crystal counts obtained under more vigorous conditions of agitation indicated that the number of secondary nuclei formed

is directly proportional to the length of the induction period. According to his results, at sufficiently large

seed charge (300 g.p.l. at 60 0 C), where there was no measur-

able induction period, the secondary nucleation was not noted.

Scanning electron microscopy was used in Brown's experimental studies to identify the secondary nucleation. He

also suggested that in the industrial Bayer precipitation

step, the impurities can adsorb on the faces of the seed

crystal and suppress crystal growth by a surface nucleation

mechanism and divert the nucleation process to slower

growing, but presumably less contaminated crystal surfaces.

Exactly how the impurities act was not clearly explained 154

ln his studies, but he suggests that the impurities may alter the characteristics of the adsorbed layer of the solute molecules and interface with the reaction which must take place at crystal surfaces to convert the aluminate ions prior to incorporation in into the neutral Al(OH) 3 molecules the crystal lattice. a) Kinetic relationships for nucleation

A generalised nucleation rate equation for the industrial Bayer crystallisation system was suggested by

Branson et.al. (142) as follows:

dNo Rate B = f (S,T,X,Y,Z) = 0 = dt where = total number of particles

t = time T = temperature °C

~~ s = supersaturation cc - c ) X = some property representing the presence of solid crystalline material Y = some properties connected with the history of the solution Z = hydro-dynamic conditions (agitation, geometry, etc.)

A quantitative consideration of factors such as Y and Z are usually omitted and the suggested relationship for nucleation rate is as follows: dN 0 B = f (S,T,X) 0 = dt

A form of nucleation rate was suggested (11) after representing property X as the total surface area as follows. dN Rate B 0 = 0 = dt = 155

when s = supersaturation (C - c*) KN = rate constant dependent on the temperature

A = total crystal surface area.

4. 32 3 Crystal Growth

Growth of alumina trihydrate particles in super saturated caustic aluminate solutions is considered to be entirely surface reaction controlled. Qualitative evidence for this is provided by the very low growth rate and the negligible effect of agitation on growth.

Usually growth occurs by deposition of the solute from the supersaturated solution on to the surface of the crystal seed including both the newly formed crystallites and the added seed crystals.

The overall process of crystal growth on a seed crystal or nucleus surrounded by the supersaturated solution can be considered basically as a mass transfer with surface reaction operation and treated as a diffusional step in series with a surface reaction step. In the first stage the solute molecules diffuse through the solution to the surface of the growing crystal. In the second stage, after reaching the surface they must be incorporated into the crystal lattice.

It is believed that the supersaturation is the driving force for both the steps.

Lyapunov and Kholmogortseva (14) determined the growth of alumina trihydrate particles in an aluminate solution by the linear growth of the crystal faces. Some quantitative growth rate measurements were reported, but the data is very limited. This method eliminated the error caused by increase of the particle size owing to aggregation 156

of the particles. They also reported that the growth rate of alumina trihydrate particles in aluminate solution of equal degrees of supersaturation greatly increases with an increase of temperature from 50 to 65°C. The rate of growth of the alumina trihydrate particles increases with increase of the degree of supersaturation of the aluminate solution to a greater extent at 65°C than at 50°C.

Misra and White (138) reported that the growth is found to depend on the square of the supersaturation and it also increased with increasing temperature, but was independent of particle size. They developed the following kinetic relationship for growth rate.

dL G = dt =

The effect of temperature on the rate constant KG was correlated by an Arrhenius type relation. 6 3 = 1.96 x 10 Exp.(- 14.3 x 10 /RT)

Then the growth rate can be re-written as follows: 6 3 G = 1.96 x 10 Exp.(-14.3 x 10 /RT)(c-c*)

The activation energy was found to be 14.3 K.cal/g-mole and this value suggests that a surface reaction mechanism

is rate controlling step for crystal growth in this system.

In an attempt to relate the variation of nucleation with growth conditions, Misra and White suggested the following possibility in the crystallisation system. If nucleation occurs by the growth of small crystallites on the crystal, which then break off) the nucleation rate would be proportional to growth rate. Thus a dependence of nucleation on

the second power of supersaturation (as with growth) is a 157

Figure 4.3 Kossel Model for Crystal Growth

Figure 4.4 Screw Dislocation Mechanism 158

possibility. However, their experimental data did not fit this proposition closely.

Two types of growth mechanism models are suggested in the literature (11). First is the Kosel model of crystal growth and second the screw dislocation mechanism. Accord ing to the Kosel model, crystal growth occurs by the deposition unit by unit of successive strips (Figure 4.3). This con- tinues until the particular layer is completed. In order for growth to continue a new layer must then be initiated. This is accomplished by a process of two dimensional surface nucleation which provides the starting point for continued growth on a new plane.

The screw dislocation mechanism takes place when the supersaturation level is in the region of one per cent higher than saturation~ the growth pattern may be initiated by a screw dislocation method (141) as shown in Figure 4.4.

Therefore, in the Bayer precipitation system, the possible mechanism is that crystal growth first occurs by filling the

Kosel sites. As the step reaches the low supersaturation level, a spiral growth pattern results and the dislocation is continually regenerated as each surface layer is corn- pleted. This obviates the need for surface nucleation to initiate a new layer.

4. 324 Agglomeration In the precipitation of alumina trihydrate it is generally recognised that under certain favourable conditions

(e.g. low agitation rate) the finer particles (< 20~m size) agglomerate into larger particles in addition to the ordinary crystal growth mechanism. Pearson (43) suggested that 159

agglomeration occurs through flocculates followed by cement ation, and the occurrence of some such phenomenon is con firmed by the fact that the agglomerate can be split, at least partly into their component crystals by treatment with sodium hydroxide solution. He also claimed that the agglomeration is favoured by (i) fine seed (ii) high seed concentration (iii) high temperature and (iv) slow stirring rate, but no quantitative evidence was given to confirm this.

Agglomeration is usually desirable since it results

1n the coarsening of the product at the expense of the unwanted fines, although there are some trial and error techniques being applied in the industry to promote the agglomeration, there are no basic studies related to this mechanism. Maricic and Markovcic (135) observed the formation

of firmly bound agglomerates in the crystallisation system which they named "intercrystallisationtr. They also claimed

that this mechanism is exclusively characteristic for the

particle size-range <20 microns (especially for the particle

s1ze 10 microns). Scott (137) reported considerable agglomeration of

particles less than 20 ]Jm in the size under plant supersat

uration conditions and found that the extent of agglomeration

is inversely proportional to the number of particles

originally present. The process of agglomeration is generally accepted

to be composed of the following three steps.

1) Firstly, relative motion between the particles allows

them to collide, but this need not occur provided the

particles are within a range of influence. 160

2) Secondly, attractive forces between the particles allow the particles to remain attached (but again, it is not necessary to be in direct contact). During this process the particles may also separate again due to agitation of the precipitate.

3) Finally, ln the suspension ln which the particles are simultaneously growing by deposition (e.g. crystallisation) deposition can cement the particles together. Such deposition could occur by growth in the space between the two particles and also possibly by the bridging action of the growth layer extending from the surface of one particle to the other.

The process of agglomeration is believed to take the following form in its actual set-up.

Reversible original individual particles of ..._ Aggregate suspension - (surface forces)

Irreversible Agglomerate (Cementation by crystallisation)

As shown ln the process, aggregation or the initial cluster-

ing step is reversible. In an aggregate the force which holds the particles together is weak so that the breakdown of the aggregate to the initial constituent particles is 161

easily achieved with little energy requirement. Furthermore the aggregate retains the identity of the individual con stituent particles, but an agglomeration is formed by the cementing action of additional deposition by crystallisation and therefore the agglomeration is not reversible.

4.325 Crystal Breakage and Attrition

It is generally believed in the Bayer crystallisation system that crystal breakage can be significant enough to affect the crystal size distribution during the precipitation, which is usually undesirable. A high degree of agitation promotes the crystal breakage and the breakage is closely related to the properties of crystals mainly hardness, crystal habit and crystal growth pattern (for example, den drite growth is more easily broken).

The laws governing the crystal breakage can be compared to processes such as grinding, but the limiting case of such comparison with the grinding mechanism is the attrition which takes place in the crystallisation system.

Attrition means the chipping off of a small fragment from the parent crystal, but this leaves the parent crystal prac tically unaltered in s1ze. For example, 200 small particles of 2 ~m size or 1000 particles of 1 Vm size can be generated from a 50 ~m crystal without significantly changing the size of the parent crystal (i.e. 50 ~m particle). This effect will generate a number of fine particles without changing the larger particles to any significant extent, but gross break age takes place with the complete change in the present crystal size by breaking into a number of small ones.

Although the operating conditions can be chosen to 162

avoid particle breakages, attrition is a difficult one to avoid. Furthermore, attrition cannot be differentiated from secondary nucleation, which is a function of super saturation. However, the mechanism of attrition has not been investigated to any extent.

4. 3 3 MAJOR FACTORS WHICH AFFECT THE OVERALL KINETICS OF DECOMPOSITION

The major factors that affect the overall kinetics of crystallisation are as follows: 1) Temperature

2) Caustic concentration

3) Initial and final caustic ratio

4) Quality and quantity of seed

5) Method and rate of agitation

6) Main impurities present in the Bayer

process liquor.

The selection of optimum conditions in the crystall isation process in order to obtain the desired particle size distribution and the maximum product output should be con trolled by studying the effect of the above major factors, on the induction period individually: nucleation crystal growth, agglomeration, crystal breakage and attrition. The effect of organic impurities on the crystallisation process has been widely discussed in Chapter 3.

4.4 SUMMARY OF CRYSTALLISATION

The main objective in the crystallisation stage is to select optimum conditions in order to obtain desired particles and the maximum output of product. The major driving force in the crystallisation of alumina trihydrate 163

is the degree of supersaturation. The industrial Bayer crystallisation process is followed by secondary nucleation, crystal growth, agglomeration, breakage and attrition which takes place simultaneously. An induction period has also been noted at the beginning of the process.

Secondary nucleation is accepted as the principal mechanism by which new crystals are generated in an agitated suspension. The crystal growth is a surface controlled mechanism, the main driving force for which is the degree of supersaturation, and also the increase of temperature.

Agglomeration is a process which depends very much as a) particle size of the seed b) seed concentration c) temperature and d) agitation. The process of agglomeration takes place through a stage of aggregation and then to the final stage of agglomeration. The process of aggregation is reversible whereas the agglomeration is irreversible.

The major variables for the overall crystallisation process are: temperature, caustic concentration, caustic ratio, quantity and quality of seed, the method and rate of agitation and finally the major impurities present in the

Bayer process liquor. CHAPTER 5

EXPERIMENTS RELATED TO THE SEPARATION, IDENTIFICATION~

ADSORPTION AND SOLUBILITY OF SOME OF THE ORGANIC CON-.

STITUENTS OF RECYCLE BAYER PROCESS LIQUOR 164

In this project the experimental work was con ducted in the following four areas:

1) Separation and identification of the organics present ln both recycle caustic liquor and product alumina trihydrate crystals.

2) Adsorption on alumina trihydrate seed crystals of various pure organic compounds as well as different molecular weight fractions separated from Bayer liquor.

(Since it is most likely that molecules adsorbed to the seed crystal surface will effect crystal growth on that surface).

3) Solubility of sodium oxalate ln sodium alum inate liquor. (It has been shown that sodium oxalate at concentrations near saturation play a significant role as an impurity in the crystallisation process and possibly inhibit crystallisation of alumina trihydrate due to coprecipitation of sodium oxalate on the seed crystals).

4) Crystallisation studies were performed in a batch crystallisator using pure sodium aluminate solutions seeded with clean seed crystals under conditions where the suspect organics were both present and absent.

This chapter describes the experiments relating to 1,2 and 3 above, and Chapter 6 covers the crystallisation experiments.

5.1 SEPARATION AND IDENTIFICATION OF THE ORGANICS PRESENT

IN BOTH RECYCLE CAUSTIC LIQUOR AND PRODUCT ALUMINA

TRI-HYDRATE CRYSTALS

Samples for the investigation work were obtained from the Bayer process operation at Queensland Alumina Ltd., in Gladstone. The recycled Bayer liquor which was taken 165

after the precipitation stage was used as sample. It was dark brown in colour and the composition is given 1n

Table 1.2. The pH of this liquor was greater than 14.

Samples were obtained of alumina trihydrate at the thickening stage and before washing and were collected from both the secondary and tertiary thickeners. These were the samples most likely to be contaminated with organics 1n the Bayer liquor either by coprecipitation or by adsorption.

5.11 SEPARATION AND ANALYSIS OF ORGANICS IN THE RECYCLE

BAYER PROCESS LIQUOR

Recycled Bayer liquor was subjected to various methods of analysis and the main methods are given in

Figures 5.1 and 5.2.

There are many difficulties to overcome in attempt ing to separate the organics from the Bayer process liquor.

The primary problem was that the liquor was highly caustic and most of the organic impurities had much greater solu bility in the caustic solution than in any organic solvent chosen for extraction. Secondly, much of the organic present in the liquor was in the form of the sodium salt of the organic acid and in such form a limited range of extraction and separation procedures is available. It is difficult to extract the organic sodium salts into an organic solvent, so in order to overcome this it was necess ary to convert the caustic solution to acidic conditions.

However, in doing this, some of the complex type organic

impurities may undergo various changes in structure as dis

cussed in Chapter 3 1 but this method was adopted in some cases in this work when the analysis was only limited to the 166

Total organic r-::=-~d caustic C analysis 28.15 gms/litre as ~~~quor ... using Beckman carbon analyser

2.2 g/litre Forw~c Acid Concentrated Distillation Presence of acetic acid acidified liquor identified

Direct G.C. Presence of: Ether extract analysis ~------~------~·1) Acetic Acid Sg/litre 1---....------.~-----' 2) Propionic acid .06 g/litre l 3) Valeric acid .02 g/litre

Diazomethane esterification G.C.Analysis 3 Peaks obtained not identified

Infra red analysis Presence of carboxylic Evaporated to of solids h acid groups and aromatic dryness rings

Figure 5.1 Separations and Analysis for the Organic Compounds present in Bayer liquor. 167

Total organic carbon Recycled Caustic ~------~~~ 28.15 g/litre using Liquor Beckman Carbon Analyser

with C0 Carbonation 2 to neutralise the solution and separate Aluminium Hydroxide

I T.O.C.5% by weight Precipitated of solid trihydrate I Aluminium Hydroxide

Filtered Liquor T.O.C.30.6 g/litre pH 9 of the original liquor

Membrane separat~io::1 with Millipore type molecular cut off.lOOO 1 Concentrate of the higher}I molecular compounds over 1000

Membrane separation with Millipore type .Total Organic Analysis molecular cut off with Beckman Carbon 10,000 A.."lalyser

Results.T.O.C. 6.20 g/litre of original liquor Concentrate M.W. over 10,000

T.O.C. 1. 23 g/litre of original liquor

Figure 5.2 Separation and analysis of organic compounds present in the Bayer process liquor. 168

stable lower molecular organic impurities.

5.112 Determination of Total Organic Carbon Present in

the Bayer Liquor

The organic impurities present in the liquor were very complex in nature and it was decided that some prelim- inary approach had to be made by determining the total organic content present in the liquor. Three methods were used for this purpose:

a) Using Beckman carbon analyser

b) Using Coleman carbon analyser

c) Titration with potassium permanganate and

oxalate to determine the total oxidisable

amount in the liquor. a) Using Beckman carbon analyser

The Beckman carbon analyser is very much based on the Van Hall (143) principle of catalytic conversion of the

(usually C0 ) which is the measurement carbon to a substance 2 by a sensitive detector. Usually this method was very accurate for organics in water and dilute aqueous solution.

The Beckman carbon analyser is used as a standard equipment for the determination of the organic level in water. In this work the Bayer liquor was diluted 10 times in order to get the conditions suitable for the equipment.

This system essentially involves vapourizing the

sample and oxidizing it in a stream of oxygen on a contact swept by oxygen (via a catalyst bed. The resulting co 2 is condenser and water trap) through a non-dispersive infrared

to C0 ) which, in turn, relays a signal analyser (sensitised 2 to a recorder. 169

Two combustion furnaces are employed, one that oxidizes only organlc. carbon at 950 0 C and the other that oxidizes only inorganic carbon at 150°C. Two micro-volume injections of sample (between 20 and 200 microlitres, depending on the range of analysis) are required, and the total organic carbon (TOC) is determined by subtracting the inorganic carbon (TIC) from the total carbon value (TCV). The high-temperature combustion tube contains cobalt-oxide impregnated asbestos packing, while the low temperature combustion tube holds quartz chips wetted with 85 percent phosphoric acid. In the latter tube the acid-treated packing causes the release of from inorganic carbonates and co 2 the water is vapourized, but the temperature at 150°C is too low for the oxidising of organic matter.

Usually determination of soluble organic and inorganic carbon in water can be made by analysis of a sample portion that has been filtered through a glass fibre filter of approximately 0.3 ~m porosity. Particulate carbon levels ln the sample can then be obtained by subtraction of the soluble carbon values from the total values. Where water samples contain particulate material of sufficient size to prohibit injection of the sample via a microlitre syringe, particles with a dimension greater than 100 ~m are unlikely to pass through an ordinary microlitre syringe needle. The total carbon levels are determined after reduction of particle size via treatment with a high-speed blender or in a sonic bath. Sample dilution before analysis is required only when concentrations greater than 4000 mg/1 are present or when samples with high salt, acid or base content are to be analysed. 170

Bayer liquor was diluted 10 times and acidified

precipitated Al(OH) is to pH2 by adding HCl, so that the 3 also Na present in the solution would re-dissolved and 2co 3 C0 . The acidified sample was injected into the evolve 2 equipment after filtering through the glass filter. The samples were injected in 100 microlitres. Firstly, it was only heated to 150°C to determine any inorganic carbon content, then a fresh sample was put in the 950°C chamber to determine the total organic carbon content. After a few runs it was noticed the deposition of inorganic salts was interfering.

The total organlc carbon analyses for the recycle

Bayer process liquor using the Beckman Analyser are shown in

Table 5 .1.

Test Sample Reading at 150°C Reading at 950°C Total dilution chamber mg/litre chamber mg/litre organic carbon @ g/1

1 10 X 120 2870 28.7

2 10 X 100 2890 28.9

3 10 X 150 2830 28.3

TABLE 5.1 Results of Beckman Carbon Analyser

During the calculation of total organic carbon the author has not subtracted the value obtained at 150°C from the total value at 950°C(Table 5.1). Usually the figure obtained at 150°C should be accounted as inorganic carbon or

evolved due to the presence of carbonate. However,in the co 2 this method the samples were acidified to pH2. Therefore any from the inorganic carbonates had already been co 2 evolved, before the test. 171

However, the end results showed that there was about 1000 - 1500 PPM of inorganic carbon present in the f liquor. Theoretically, such an amount of inorganic carbonate cannot be present at pH2. The author suggests the following explanation for the inaccuracy of the results obtained. i) Some of the organic compounds present in the Bayer liquor after acidification are decomposed before 150°C and have been interpreted as inorganic carbonate figures. ii) Although the carbonate in the liquor has reacted with

evolved , there has been a supersaturation of acid and co 2 C0 at this stage. 2 One of the major drawbacks of the Beckman

Carbon Analyser is that it has not been improved and cannot differentiate and compute separately the inorganic carbon from the unstable organics which could be decomposed before

150°C. b) Using Coleman Carbon Analyser and Potassium Permanganate

titration method

Details of the above two methods are provided ln Appendix II. Both methods were unsatisfactory for Bayer process liquor. In using the Coleman Carbon Analyser, sodium carbonate in the liquor overestimates the results, but in order to eliminate carbonate the sample has to be acidified and later it should be in dried form to be used in the equipment. By so doing it is always possible that some of the volatile acids present in the liquor are lost during evaporation. 172

A major drawback in the potassium permanganate titration method is the difficulty in obtaining an accurate end-point due to complexity in nature of the humic substances and also the formation of a number of compounds of manganese. This is extensively described 1n Appendix II.

The results obtained from both methods are

summarised in Tables 5.2 and 5.3.

Sample Total organ1c (as carbon)

I Bay er process liquor 27.6 g/litre

II -do- 27.3 11

III -do- 26.5 11

Table 5.2 Results of the Coleman Carbon Analyser

Sample Total organic oxidised by permanganate (as carbon)

I Bay er process liquor 9.08 g/litre

II -do- 10.59 11

III -do- 11.51 11

IV -do- 9. 5 11

V -do- 6.85 11

VII -do- 6.37 11

Table 5.3 Results of the potassium permanganate titration method 173

5.113 Use of Distillation Method to Identify s6me of the

Volatile Organic Compounds in the Bayer liquor

The distillation method has assisted in obtaining some useful information about volatile organic compounds in the form of acids by acidifying.

250 ml of Bayer liquor was concentrated down to

150 ml and it was acidified to almost pH 1, by adding concentrated hydrochloric acid. During the acidification heat is evolved therefore precautions were taken not to heat the sample over 60°C as volatile organic acids present in the sample may be evolved. This was done by adding HCl in a very small quantity in a cold water bath. Aluminate in the solution is precipitated as alumina trihydrate and it was redissolved in the acidic medium. The sample was now ready for distillation.

A 1-litre distillation flask with a distillation column, cooled by cold water, was used, as shown in Fig. 5.3.

The receiver was immersed in an ice bath in order to minimize any loss of acid vapour. The sample in the flask was heated to the boiling point of the solution (approx. 100°C).

However, prior to the water vapour, HCl vapour was evolved.

The liquor was boiled and some of the vapour condensed and collected in the collecting flask. The condensed liquid had a strong smell which could be due to the presence of either organic acids or hydrochloric acids. Heating continued until the solution in the flask was almost thick. The quan tity of condensed liquid was measured and the sample was

checked with the U.V. absorption, showing absorption at

210 )lffi,, The U.V. peak was determined with various dilutions. 174

water

Gentle Acidified heating Bayer liquor

Fig. 5.3 Distillation of volatile acids from acidified Bayer process liquor.

It was noticed that the condensed solution was

with KMno . The above result is an indication discoloured 4 of the presence of organic acids in the sample.

In order to determine the amount of organic present

in the condensate, permanganate value was determined by titrating a known quantity of the sample of condensate with of KMn0 in the acidic medium, as a standard solution 4 described in Appendix II. The results obtained are as

follows:

Total quantity of Bayer liquor treated 250 ml

Volume after concentration 150 ml

Total volume after acidification 250 ml

Volume of condensed solution collected- 50 ml 175

Molarity Volume Volume Molarity of Formic acid (distillate) (KMnO ) distillate content in the of KMn0 4 4 titrated (calculated Bayer process from results) liquor Ml M1 g/1

.031 5 39.5 .2449 2.25

.031 1 8 .248 2.28

.031 1 7.7 .238 2.18

Table 5.4 Titration results and the formic acid content of the distillate collected from the acidified Bayer process liquor

Usually in the Bayer process liquor, salts of

acetic acid and formic acid are suspected to be present as

impurities. These two acids have low boil~ng points and

with steam they could easily vaporise and condense with

water.

Acetic and formic acids have max1mum absorption 1n

the U.V. range at 210 ~m which is similar to the results

obtained from the sample. Furthermore, neutral solution

of the sample gave a deep red colouration with ferric chloride

confirming the presence of either acetate or formate. Only formic

by KMn0 in acidic medium, therefore the acid is oxidised 4 from the KMn0 titration are taken to be figures obtained 4 the amount of formic acid in the Bayer liquor.

5.114 Use of Gas Chromatography to analyse the Organics 1n

the Bayer liquor

Gas chromatography lS a quick and easy method of

determining the organic compounds in a solution which is vola-

tile at high temperatures. The main requirement is that the

organ1c compounds are stable in their gaseous form. 176

The basic principle of gas chromatography involves the preferential absorption and subsequent desorbtion of volatile materials on to an absorbant substance. In the

G.C. Method the vapour of the organic compound to be analysed is mixed with a carrier gas, usually nitrogen or argon, and the gas mixture is known as the moving phase. It is then passed into a column of absorbent material, the stationary phase, which can be either a solid silicate material or a non-volatile liquid absorbed into a porous solid.

A small sample of the gas to be analysed is injected into the carrier gas stream and the gaseous components are selectively absorbed on to a long column (about 2m) of stationary phase material. They are then eluted one at a time. The presence of the components in the eluent gas is then detected and measured by a device which gives an output on a chart recorder. The peaks can then be identi fied by subjecting known standard gases to the same condi tions as those used when analysing the test sample. The main variables in gas chromatography are the carrier gas flow temperature and the sample quantity. Stationary phase columns may also be changed.

A schematic diagram of a typical gas chromatograph

1s given in Figure 5.4.

The organic impurities in the Bayer liquor are usually in the form of organic salts, therefore they cannot be detected as volatile unless these compounds are converted

into the acidic form

A 100 ml of Bayer process liquor was first concen trated to 70 ml by boiling and then acidified to pH 1 by

adding concentrated HCl. It was noticed during the acid- 177

Differential Flow

To waste or fraction collector

SampleI injection

Column

ics Recorder Column

Fig. 5.4 A schematic diagram of the typical gas chromatograph.

ification that some of the organic present in the solution was precipitated as brown material. The solution was filtered, transferred to a quick-fit liquid/liquid extractor and extracted with 400 ml of diethyl ether (analytical grade), added in small quantities over about ten hours. The brownish-yellow colour of the Bayer solution was gradually transferred to the ether extract, which was then concentrated to 100 ml in a water bath.

During this method of preparation it was possible that some of the organics in the Bayer liquor could change 1n

structure during acidification. However, the objective of this method was to track the most volatile organic acids and usually those acids are not expected to be affected by the

acidification.

Sample injection was carried out in two ways. In

the first method the ether sample was injected directly to the gas chromatograph, whereas in the second method the ether 178

extract was evaporated to dryness and the residue dissolved in methanol and methylated using an etheral solution of diazomethane. Diazomethane was ~prepared in the laboratory from diazald. Methyl esters are more volatile than its carboxylic acids, therefore some of the organic acid which cannot be trapped in the G.C. in acid form (due to the high boiling points) is expected to be trapped when it is methy lated to methyl esters.

a) Direct Method of injecting Ether Extract to the Gas

Chromatograph

The equipment used was Hewlett Packard Model 5710A which has facilities to programme the temperature. The out- going gases are detected either by the flame ionising detector or refraction index detector.

A sample of the ether extract in 5 ~1 was injected into the gas chromatograph equipment with the following conditions:

Type of column:

4 ft. x 18" I.D. stainless steel column

packed with Poropak Q. Mesh size 80-100. Temperature programming:

Initially holding at 100°C for 16 minutes

then with the rising rate of 4°C/min to

250°C holding for 10 minutes.

Temperature of the detector - 300 0 C

Temperature of the injecting port - 100°C

Detector - flame ioniser.

Nine peaks were obtained and to identify them retention time was compared by injecting known compounds such as acetic acid, formic acid, lactic acid, propionic acid, butonic 179

acid, valeric acid and pivalic acid, with different concentrations. The same conditions were applied as previously

in the gas chromatograph when the reference samples were

injected.

In order to confirm the identify of the peaks, the

samples were injected with slightly changed conditions so that the retention times of the peaks were also changed slightly, then, under the same conditions, the known corn- pounds were injected. b) Injection of methyl esters of the extract

The sample was prepared as mentioned previously.

5 ]Jl of the sample was injected with the conditions as set out in Table 5.5.

The type of chromatograms obtained from both methods of injection are shown in Figures 5.5, 5.6, 5.7 and

5.8. Figures 5.5 and 5.6 illustrate the peaks obtained from the direct analysis of ether extract under different conditions. Retention times obtained were compared with the known carboxylic acids which have a boiling point in the range 100 - 250 0 C. The known carboxylic acids injected were as follows:

1) Formic acid (formic acid cannot be detected by

the flame ioniser)

2) Acetic acid

3) Lactic acid

4) Propionic acid

5) Butonic acid

6) Pivalic acid

7) Valeric acid 180

Conditions I Conditions II

Column 6'x1/8" stainless 10'x1/8" stainless steel column packed steel packed with with liquid phase SE 30-20% O.V-17,-mesh gas chrom Q Temperature programming

Initial temp. 54°C held for 100°C held for 16 mins. 8 mins.

Rate of temp.

Final temp.

Temperature at detector

Temperature at injecting port

Table 5.5 Various conditions applied in the G.C.analysis of methyl esters of Bayer process liquor.

The reference samples were chosen out of the

carboxylic acids which are normally stable in the volatile phase. Identifications were made by comparing the reten-

tion time with the reference samples. The identified sub-

stances were confirmed by changing the temperature programm-

ing and comparing again with the known samples. The peak

heights were compared with the peak heights of different

concentration of the known samples in the ether mixture.

Such calibrations give an approximate value of the concen-

tration of the identified organic impurities.

Table 5.6 summarises the results and the confirm-

ation arrived at from the chromatogram of ether extract

illustrated in Figures 5.5 and 5.6.

From the results only the presence of acetic,

propionic and valeric acids were confirmed. The presence 100°C

250°C

min.

~1.

4°C/min.

ether

- -

- 5

the

temperature column time

vol.

temperature

Sample

Initial

Holding Rate

Poropak

Final

injection

direct

Conditions:-

·ri

liquor

m 0 m 0

·M

·ri

process

Bayer

the

~ ~

0 ~

0 ~ 0

·ri ·ri

·M

(2)

condition

chromatogram

u w ~ u 0 ~

·ri

·M

extract,

Gas

(1)

5.5

Fig.

~ w

~ N CO I-'

100°C

250°C

mins.

~1.

- 5

(J) m () :> () l-l m

time column temperature

r-i

·.-! 'tl ·.-!

4°C/min.

vol.

temperature

(3)

injection

sample Initial Holding Rate Poropak. Final

direct

liquor

Conditions:-

() ~ 0; 0 0 l-l 0; () m

'tl

·.-! ·.-! ·.-!

(2)

process

Bayer

condition-II

the

and

(J) () () m m

()

.j.l

'tl

·.-!

extract

(1)

chromatogram

ether

the Gas

5.6

Fig.

(J) (J)

l-l

:5 Q

chrom.

gas

54°C

290°C

~1. -

min. mesh

4°C/min.

- 5

(7)

OV-17

temperature time

- volume

esters

temperature

Sample Initial Rate Final Holding

Column

methyl

(6)

liquor-

Conditions:-

compounds

process

Bayer

the

unidentified

Condition

(7)

(1)

chromatogram extract

the

Gas

5.7

(1)

Fig. (6)

100°C

290°C

~1.

20%

mins.

- 5

- 8

(5)

Methyl-esters

temperature time

4°C/min.

volume

temperature

liquor

Sample

Initial

Holding Rate Final

Column

process

Bayer

Conditions:-

compounds

the

Condition

unidentified

(2)

extract

(6)

chromatogram

the

Gas

(1)

5.8

Fig.

Bayer Bayer

uor uor

g/litre g/litre

li li

the the

5 5

Calculated Calculated

.02 .02

in in

concentration concentration

0.06 0.06

time time

similar similar

sample sample

acid acid

has has

Valeric Valeric

Propionic Propionic

Acetic Acetic

Lactic Lactic

Valeric Valeric Propionic Propionic

Acetic Acetic retention retention

Reference Reference

which which

cm. cm.

4 4

2.0 2.0 1.5 1.5

2.0 2.0

2.1 2.1

1.0 1.0

Peak Peak

Height Height

2.6&4.5 2.6&4.5

chart chart

Analysis Analysis

cm. cm.

or or

37.1 37.1 20.8 20.8

47.1 47.1

28.4 28.4

26.7 26.7

23.5 23.5

33.6 33.6

22.4 22.4

31.2 31.2

21.7 21.7

17 17

13.4 13.4

G.C. G.C.

Distance Distance

Retention Retention

time time

of of

Results Results

do do

10x128 10x128

100x128 100x128

Sensitivity Sensitivity

5.6 5.6

Extract Extract

No. No.

TABLE TABLE

2 2

3 3

1 1

7 7

9 9

6 6

4 4

5 5

8 8

3 3 2 2

1 1

Ether Ether

Peak Peak

of of

Column Column

Poropak Poropak

Analysis Analysis

5) 5)

c c

• •

Direct Direct

5 5

100 100

Temg. Temg.

100 100

Temg. Temg.

minutes minutes

4°C/min. 4°C/min.

at at

4°C/min. 4°C/min.

at at

Conditions Conditions

32 32

16 16

250°C 250°C

(Fig. (Fig.

{2) {2)

(1) (1)

to to

for for

Initial Initial

Rate Rate

held held

to to for for

Rate Rate

Initial Initial

held held Sample:- 186

of lactic acid was noticed in some of the G.C. analyses when compared to the retention time of the standard sample of lactic acid as shown in Table 5.6. However, it was not reproducible when the temperature programming condition was altered for confirmation.

Peaks obtained from the methyl ester samples were not identified due to the difficulty of methylating each reference sample to be injected for comparison.

However, it was noted that some of the carboxylic acids in the extract which are not volatile and cannot be trapped by direct injection, are trapped in the form of methyl esters.

5.115 Infra Red Analysis

A sample of the Bayer process liquor was first acidified and extracted into the ether as prepared for

G.C. analysis. The ether extract was then evaporated to dryness in the water bath and a brown wavy type solid material was obtained. An infra-red spectra of the material was obtained on KBr plate.

The spectrum obtained is illustrated in

Figure 5.9. Although infra-red analysis of such an organlc mixture would not characterise it, it definitely did suggest at least some of the functional groups in this mixture.

The major absorption bands from Figure 5.9 are

listed in Table 5.7. These bands are broad presumably because of extensive overlapping of individual absorptions.

For comparison studies, the infra-red spectra of humic acids and fulvic acid published by Schnitzer (53)

lS illustrated in Figure 5.9. Fulvic acid (previously published)(53)

Humic acids (previously published) (53)

Sample

4000 3000 2500 2000 1600 1400 1200 1000 -1 Frequency (cm ) Figure 5.9 Results of the Infra-red analysis of desorbed material from the trihydrate seed. 188

Major absorption bands Possible functional groups of the Bayer liquor sample -1 Frequency cm

3400 - 3100 Hydrogen-bonded OH at 1 3400 cm-

3000 - 2900 Aliphatic C-H stretch 1 2900 cm-

1750 - 1700 C=O of co H, C=O stretch of 2 ketonic carbonyl group at 1 1725 cm-

1650 - 1600 Aromatic C=C, hydrogen bonded C=O of carbonyl, conjugated 1 with carbonyl, coo- at 1630 cm-

-1 1450 - 1400 Aliphatic C-H at 1400 cm

1 1300 - 1260 e-o at 1200 cm-

Table 5.7 Main infra-red absorption bands of the Bayer liquor sample. 189

Major absorption bands occur at about 3400 cm -1 1 (OH-stretching), 2900 cm- (aliphatic C-H stretching), 1 -1 1720 cm- CC= 0 stretching of COOH and ketones), 1620 cm

(aromatic C = C and/or H- bonded C = 0). Referring to the spectra of the humic and fulvic acid, spectra of the organic extract used in the experiment is very similar to the spectra of the humic acids. A unique feature of the spectra of the organic extract from the Bayer liquor is the absorption in 1 the 3400 cm- which is weak and centred in the range 3100 - -1 2900 cm . This feature can be explained as a result of the presence of considerably more aliphatic C - H stretching than does humic acid extracted from the soil. Alternatively, it is due to the small amount of OH - stretching in the organic extract. It is also noteworthy that the absorption 1 band at 1720 cm- 1s stronger in the case of organic extract than the humic acids which is mainly due to the presence of 1 more -COOH groups. The absorption band at 1620 cm- which is due to the aromatic C = 0 and/or H - bonded C = 0 is weaker than in the humic acids, possibly indicating the presence of less aromatic. The absorption at 1440 cm -1 which is prominent in both organic extract and humic acid samples is again due to the presence of H-bending of C-H aliphatic groups. The broad absorption band between 1 1260 - 1300 cm- appeared in the spectra of organic sample, whereas in the spectra of both humic and fulvic acid, this band is not shown. This band is usually due to the C-0 -1 stretching vibration at 1200 cm However, the broad absorption in this region is possibly increased due to the

C-0 stretching from diethyl-ether found in the dried sample as impurities. This was initially prepared by extracting

tO tO

0 0

I-' I-'

-1 -1

Very Very

cm cm

1250 1250

strong strong

Absent Absent

-1 -1

Weak Weak

cm cm

1440 1440

for for

Moderate Moderate

bands bands

-1 -1

cm cm

1600 1600

1640 1640

Moderate Moderate

-1 -1

absorption absorption

Very Very

cm cm

1720 1720

strong strong

Strong Strong

selected selected

-1 -1

cm cm

1800 1800

of of

Absent Absent

-1 -1

Very Very

weak weak

2800 2800 Very Very

cm cm

Strong Strong

2940 2940

intensities intensities

to to

cm-1 cm-1

spectra spectra

1 1

3400 3400

cm-

all all

3200 3200

Relative Relative

shifted shifted

Strong Strong

8 8

5. 5.

Table Table

from from

acid acid

liquor liquor

acid acid

Sample Sample

Bayer Bayer

extract extract

Organic Organic

Fulvic Fulvic Humic Humic 191 the organics ln the Bayer liquor into an ethereal solution.

Table 5.8 summarises the relative intensities of humic acids, fulvic acids and the organic extract from the

Bayer liquor.

In general, the results indicate that the spectra of organic extract is very similar to the spectra of humic acids and fulvic acids, suggesting the presence of essentially similar chemical structures, differing mainly in the contents of functional groups. The presence of carboxylate lons

(COO-) indicates that the organic impurities in the Bayer liquor are found as salts of carboxylic acids.

5.116 Use of Membrane Process for Characterisation

The main objective of the work done in this section was to determine the presence of any higher molecular weight fraction. The Bayer liquor was passed through the ultra filtration type membranes with specific molecular cut off. The concentrates so obtained were examined for total organic carbon to get an idea about the higher molecular compound content in the Bayer liquor. The content of this work and the results are summarised briefly in Figure 5.2.

Usually the membranes used for ultrafiltration have the following characteristics: a) the size of solute retained in the concentrate are over

the molecular weights 1000 and 10,000 b) the effect of osmotic pressure of feed solution on the

efficiency of the ultrafiltration membrane is negligible

c) the mechanism of the separation is based on the molecu

lar screening and the proper pore size, and the pore size

distribution in the membrane play an important role. 192

The Bayer liquor had to be treated before passing through any membrane as most of the commercial type ultrafiltration membranes are affected by highly caustic solutions. Therefore the pH of Bayer liquor was gradually brought down to pH 9. This was achieved by bubbling C0 2 through the solution. Sodium hydroxide in the solution is converted into Na during the process. Sodium carbon- 2co 3 ate has the pH range of 9.

A 1-litre sample of Bayer liquor was poured into a beaker and was bubbled through it for about co 2 20 hours. The pH of the liquor which was 14 initially was dropped down to pH 9. During this process aluminate ions in the liquor were precipitated as aluminium hydroxide due to the change of pH.

The precipitate was filtered and washed and the filtrate was measured for the volume change. One litre of the liquor was changed into 750 ml of filtrate after precipitating aluminium hydroxide. 100 gm of aluminium hydroxide was obtained from one litre. The precipitated aluminium hydroxide was dried and tested for total organlc carbon content using a Coleman carbon analyser.

The filtrate was first passed through a membrane which has a nominal molecular cut-off 500,000 fitted in a stirred cell, thereby obtaining a solution free of fine solid and colloidal material. The filtrate is now ready for membrane separation. a) Separation of molecular weight over 1000

The apparatus used was a stirred cell with a capacity of 100 ml fabricated from clear plastic material, as shown in Figure 5.10. The membrane is fitted to the 193

Figure 5.10 Stirred cell type membrane used for the separation of organic impurities, M.wt. > 1000. 194

bottom of the cell and the pressure is applied to the cell from a nitrogen cylinder connected to it with a regulator.

The membrane used was a millipore type ultrafiltration membrane, type PSAC, with the nominal molecular weight cut off 1000. 100 ml of the filtrate was poured into the cell and nitrogen pressure was regulated at 25 lbs/sq.in. The flux was very slow, only about 1 ml per minute. It was noticed that the colour of flux was lighter than the original and also the concentrate in the cell, indicating that most of the brown colour retains as higher molecular compounds in the cell. When the level of liquor in the cell was dropped, it was refilled with distilled water. The addition of distilled water was continued until only the distilled water was passed through the membrane. This was tested by check

ing the U.V. absorption of the flux at different times, using an ultra violet spectrophotometer. When there was no absorption in the U.V. range by the flux sample it was decided that the separation was complete. The concentrate was diluted to 100 ml in order to equalise back to the original volume taken. The concentrate and the original solution were examined for the total organic carbon content using the Beckman carbon analyser. b) Separation of organic impurities over 10,000

The membrane used in this work was millipore type PTGC membrane with the nominal molecular cut off 10,000.

The filter kit included 5 filter units. Each 2 unit consisted of 11 cm of Pellicon type filtration mem brane permanently sealed to a cylindrical plastic core.

The retentive surface of the membrane faces outward. 195

Pigure 5.11 Immersible type membrane used for the separation of organic impurities, M. wt > 10, 0 0 0 • g/1.

0.805

line

10,000

compound

over

0.1

equilibrium

wt.

HCl

molecular ml.

line

100

high

dotted

volume

compound

Concentration

Total

Horizontal

Concentration

HCl

molecular

0.1

high

the

material

Volume

(3)

organic

titration

the

----

pH 6

5.12

Original

Fig.

(1)

-----

6 3 4

pH 197

In use, the device is lowered into the solution to be pro cessed and is then connected to any convenient vacuum source.

Filtrate passes through the membrane into the core, and thus flows via the vacuum line to a suitable trap. Retained molecules (the retontate) remain in the original sample vessel.

100 ml. of the Bayer process liquor was taken into a beaker and the membrane was immersed fully as shown in

Fig. 5.11. When the vacuum was applied the lower molecular solutes and water pass through the membrane into the collect ing flask. Higher molecular solutes over 10,000 were concen trated in the beaker. The concentrate in the beaker was mixed up with distilled water in order to keep the membrane fully immersed in the solution for successful performance.

The addition of distilled water was continued until the absorption of spectra for filtrate in the ultraviolet region was blank. After the separation was completed, the concen trate was made to 100 ml. and it was tested for total organic carbon content. The sample was later evaporated until the dryness in. the oven settled at 100°C. The solid brown coloured waxy compound was obtained and weighed. A known

concentration of this material was titrated against a solution of 0.1 molar hydrochloric acid us1ng a pH meter to obtain a titration curve. The curves obtained are shown in Fig. 5.12.

Although the curve obtained was not clear, the results show the formation of numbers of equilibrium points indicating the presence of different compounds act separately in the mixture.

The suspected equilibrium points are outlined by the dotted

line. Different functional groups in this complex mixture have reached an equilibrium with HCl at different levels of pH. 198

The determination of total organic carbon content after each membrane separation suggests an idea of the quantity of higher molecular compounds present in the Bayer liquor.

The results obtained from both types of separation are summarised in Table 5.9.

as carbon

Re cycled caustic liquor 28.75 g/litre

Re cycled caustic liquor 30.6 g/litre

after carbonisation

Molecular wt. over 1,000 6. 2 g/litre

Molecular wt. over 10,000 1. 735 g/litre

Molecular wt. below 1,000 24.4 g/litre

Table 5.9 Total organic carbon composition of the Bayer process liquor in relation to the various molecular fractions (determined as carbon g/litre)

During the carbonation of the Bayer process liquor, some of the organlc impurities in the Bayer liquor are adsorbed on to the crystals of precipitated aluminium hydroxide. The total organic carbon analysis on the precipitated aluminium hydroxide using a Coleman carbon analyser 199

indicate that it contains about 5% (by weight) of organic impurities contaminated with aluminium hydroxide. Whether these organics are adsorbed or eo-precipitated due to the decrease in pH is a question for further investigation. How ever, the quantitative results obtained for the higher molecular compounds present in the Bayer process liquor cannot be con sidered as the most accurate due to the loss of some of the organic impurities along with the aluminium hydroxide precip itated previously during the carbonation.

The millipore type membrane used for this work was designed and specified by the manufacturers to produce a mole cular cut off at 1000 and another at 10,000 based on the pore size of the membranes. The accuracy of these results is questionable due to the following drawbacks.

1) The pore size of the membrane cannot be considered as the main basis for different molecular cut off because different organic compounds have different shapes. For example, long carbon chain could easily pass through a small pore slze.

2) During the membrane separation of higher molecular compounds, some of the organic compounds also could go through a structural change, such as polymerisation, colloidal forma tion, breakages due to the change in the pH.

3) In the stirred cell as well as in the immersible type membranes, there is a possibility of forming a concentrated layer of the organic impurities on the top of the existing membrane, thus changing the function of the membrane. Such a formation could easily alter the pore size of the membrane, furthermore the organic compounds concentrated near the membrane as a layer could act as a new membrane. However, 200

the above possibilities are minimised by continuous stirr

lng to a certain extent.

4) Usually the flux of both stirred cell and the immer

sible type membrane are very slow. It was almost in the range of 10 ml - 15 ml per hour. A complete separation takes

even longer than 8 hours. Due to the prolonged period of

contact of membranes with the organics, the membranes are

liable to be changed in their properties at a later stage.

5.12 SEPARATION AND IDENTIFICATION OF ORGANICS WHICH ARE ASSOCIATED WITH THE ALUMINA TRIHYDRATE PRODUCT

It is believed that the organic impurities in

recycled Bayer process liquor inhibit the crystallisation

process either by coprecipitating with the alumina trihydrate

or by adsorbing onto the seed crystal surfaces and thus

affecting crystal growth. In order to assess the extent

of association with the growing crystal, samples were

obtained of alumina plant product at the final stages of

crystallisation and before the washing stage. These

samples which were provided by Queensland Alumina Ltd., from

the tertiary thickener of their Gladstone plant, contained

adherent plant liquor which was thoroughly washed from the

crystals prior to each of the tests carried out.

Various methods of desorption were applied to extract

the organics from the product trihydrate. Figure 5.13 out

lines various methods applied and summarises the results

obtained thereafter. 2Q1

There were two major methods applied to desorb and characterise the organics contaminated with the product alumina trihydrate. The two methods are as follows:

1) Methanol treatment followed by hot water desorption

2) Cold water wash followed by hot water desorption.

5.121 Methanol treatment followed by hot water desorption

The alm of this method of desorption was firstly to

dissolve and separate most of the inorganic contaminants

and caustic from the product alumina trihydrate without

hindering the organic impurities contaminated in the crystals

of alumina trihydrate. Later the organics are extracted

from the crystals. Most of the inorganic salts, such as

caustic and sodium carbonate are dissolved in the methanol.

Usually organic salts are less and sparingly soluble in the

methanol. After the methanol wash the product was dried and

the X-ray defraction analysis was performed. The XRD

analysis usually indicates if there are any crystalline

organic salts deposited along with the alumina trihydrate

seed during the precipitation. The results are discussed

later in this section.

A weighed quantity of the dried sample was extracted

into the hot water. The extract was divided into two

portions, A and B, portion A was evaporated until dryness at

100°C. The dried sample from A was weighed to determine

the amount of impurities extracted into the hot water.

An infra-red spectrum of dried sample of A was 202

Wet crystals from tertiary thickener

Cold water wash & filter

Hot water extraction

Aqueous extract

Methanol Evap. to wash and dryness crystals dried ~xy brown 81 g solid organics L__solid per kg of crystals Membrane separation I.R. Presence of aromatic rings 1000 MW of ana.lys1.s phenolic compounds. No solutes carbohydrate. concen Aqueous Evaporate 3.44 g solid organic per MW trate 1000 kg of crystals

Crystals still Presence of sodium oxalate x-ray diffraction containing confirmed. Many unidenti methanol Analysis fied diffraction lines. insolubles

Hot water extraction

Evap. to ~ 61 g per kg crystals Aqueous extract dryness

Membrane concentration Compounds over MW 1000 of 1000 MW solutes 10 g per kg crystals

Fig. 5.13 "Desorption" of organics from Bayer plant crystalliser product. 203

taken to study the functional groups present in the sample.

The results are discussed later in the section.

The portion B of the hot water extract was first filtered through a membrane with the pore size of about 0 140A with the nominal molecular weight cut off about

500,000, to separate any fine solid particles from the

solution such as alumina trihydrate particles. Then the

filtrate was passed through a millipore type membrane with the nominal molecular weight cut off 1000 in a stirred cell

as shown in Figure 5.10. The liquid ln the stirred cell was washed with distilled water until it was concentrated

in the cell with the compounds over 1000 molecular weight

cut off. The concentrate was dried and weighed to measure the quantity.

5.122 Cold water wash followed by the hot water desorption

The method adopted here differs only from the

previous method by the choice of the washing reagent used.

Methanol was used in the previous method.

The aim of the previous method was to wash the

caustic and sodium carbonate from the surface of the alumina

trihydrate leaving behind most of the contaminated salts of

carboxylic acids with the seeds as they are not soluble in

the methanol. Later when the product is desorbed with hot

water carrying most of the undissolved organic salts to the hot water extract. In this present method, the methanol wash is eliminated from a cold water wash. The cold water

wash usually removes even some of the organic salts along

with the caustic and inorganic salts, leaving behind the corn-

pounds which were mostly adsorbed on to the surface of

alumina trihydrate. 204

A weighed quantity of alumina trihydrate was washed several times with cold water at 10°C until no caustic is present in the washed water. The washed liquor was checked by a pH meter until the pH came down to about 8.

However, allowance had to be made for some of the eo- precipitated organic contaminants being washed away with the cold water leaving the organics which were adsorbed onto the surface of trihydrate products.

After the cold water wash the sample was drained and poured into boiling water which was stirred for about

30 mins. whilst boiling. The hot water extract was then filtered and divided into two portions, namely A and B.

Portion A was evaporated until dry 1n an oven where the temperature was fixed at 100 0 C. The dried sample had a waxy brown appearance and infra-red analysis was made on it using paraffin oil as the base.

Portion B of the extract was passed through a membrane with the molecular weight cut off of about 500,000 to separate any fine solid particles. It was then passed through a millipore type membrane with a nominal molecular weight cut off of 1000. The resulting concentrate was then dried and weighed.

The results obtained from both methods of desorption experiments were not very conclusive. The poisoning effect of the organics in the Bayer process liquor are usually divided into two groups on the basis of its behaviour.

1) The organic impurities which reach supersaturation during crystallisation of alumina trihydrate and eo- precipitates with the product. In this case it is believed that the organic precipitates on the surface of alumina A = Gibbsite B = Sodium Oxalate A A A

A A A A A A A

Fig. 5.14 X-ray diffraction analysis of the product alumina trihydrate after the methanol wash.

N 0 (Jl 206

trihydrate. However, methanol may also have dissolved some of this brown coloured humic acid fraction.

X-ray diffraction patterns take on the above dried

sample of alumina trihydrate after the methanol wash has

indicated the presence of sodium oxalate. Sodium oxalate

is crystallised with the trihydrate. The XRD patterns obtained are illustrated in Figure 5.14. In addition to the peaks obtained due to alumina trihydrate and sodium oxalate, there were a number of peaks due to unknown com pounds which cannot be identified from the available charts.

It is suggested that those unknown peaks are due to some organic compounds which were crystallised with alumina trihydrate.

The results of the XRD patterns also reveal that the sodium oxalate, and some unknown organics, crystallise along with the alumina trihydrate after reaching the level of supersaturation. Usually XRD analysis detects only crystalline forms of a compound and not the adsorbed layer on to solid surface.

Analysis of the hot water extract of the alumina trihydrate samples indicate it contains about 1% by weight higher molecular compounds over nominal molecular weight

1000. This was revealed from the membrane filtration.

Results obtained from the second method of treat ment (cold water wash) are compared with the previous method in Table 5.10. However, an error is encountered in both methods by the inorganic contaminants which were computed to the total organic per kg. of product trihydrate, as it was measured by weight. 207

Methanol wash Cold water wash followed by hot followed by hot water desorption water desorption

TOC per kg of seeds of the 81 g 61 g extract

Molecular wt. > 1000 per kg 3.44 g 10 g seeds

Table 5.10 Total organic content in the extract (by weight) desorbed from the product alumina trihydrate.

In order to justify the possible adsorption of organic impurities onto the alumina trihydrate surface, the following simple calculations were made to determine the amount of organic impurities required to form a complete monolayer on the total surface area of solid alumina trihydrate.

The surface area of alumina trihydrate was determined and varied in the range of 0.1 - 0.2 m2 /g. The highest value of the surface area was taken for these calculations.

The following assumptions were made ln the calculations. 2 The density of organic materials were taken as 1.2 g/cm

The thickness of a mono-layer of organlc

material-50°A 2 Surface area of alumina trihydrate .2 m /g

The weight of organic required to form a] 1.2 g per Kg of alumina mono-layer of organic impurities per =( trihydrate Kg of seed

(X) (X)

0 0

N N

800 800

1000 1000

product product

the the

of of

1200 1200

extract extract

Paraffin Paraffin

1400 1400

desorbed desorbed

Paraffin Paraffin

water water

1600 1600

number number

hot hot

Wave Wave

the the

of of

1800 1800

analysis analysis

trihydrate. trihydrate.

2000 2000

Infra-red Infra-red

aluroina aluroina

2500 2500

5.15 5.15

Paraffin Paraffin

3000 3000

Fig. Fig.

(1) (1) 3500 3500 209

When the above results are compared with Table 5.10, it is clear that the formation of more than a monolayer of organlc on the surface is possible.

Infra red spectra taken on a sample of hot water extract after membrane filtration is shown in Figure 5.15.

The IR analysis was performed for the sample after mixing with paraffin oil. The peaks obtained due to the paraffin oil is marked "Paraffin" and the rest of the peaks are due to the sample. These peaks were studied by using published charts for the frequencies of characteristic groups. The peaks obtained were in the following region of frequencies: -1 1) 3700 = 3400 ern -1 2) 1000 - 1050 ern 3) 700 - 800 ern -1 These results indicate that the IR pattern does not show any similarity towards the humic acids (Fig. 5.9).

However, the sample used for the IR tests was the filtrate after the membrane separation. It is possible, therefore, that humic acid fractions are not present in it.

From the IR results it is suggested that the hot water extract contains phenolic and polynuclear aromatics ln the mixture as the frequencies obtained are in the same visuality and vicinity of the frequencies obtained for the said organic compounds. However, the peaks are not very accurate and not sharp enough to confirm this judgment as it has overlapped and formed very broad peaks due to the complex ity of the mixture.

5.2 ADSORPTIVE PROPERTIES OF VARIOUS ORGANIC COMPOUNDS IN

THE BAYER PROCESS LIQUOR ON THE ALUMINA TRIHYDRATE SEEDS

When crystallising alurnina trihydrate from the Bayer 210 process liquor it is always suspected that some of the organic impurities are adsorbed onto the surface of seed alumina trihydrate. As a result it could modify many surface properties.

This behaviour was explained as one of the major reasons for the retardation of crystal growth of alumina trihydrate and its fine formation. Brown (139) has explained that the build-up of a mono or multilayer on the surface of seed alumina trihydrate by some of the surface active organic impurities in the Bayer process liquor will eventually prevent further deposition of alumina trihydrate on the seed surface.

The above predictions were not supported by any experimental evidence.

In this work, the author has made an attempt to justify the above possibilities. An investigation was carried out to characterise the organic impurities in the

Bayer process liquor according to its adsorption properties.

Alumina trihydrate seed crystals (obtained from

Queensland Alumina Ltd.) were packed into adsorption columns and solutions of various pure organics and also specific molecular weight fraction organics separated from the plant liquor was passed through them. As the adsorptive properties depend on the level of pH and temperatures, the conditions were made similar to the Bayer crystallisation conditions in the experiments. The organic compounds to be tested were dissolved in the NaAl0 solutions and the 2 temperature was set up at 60°C which was very close to the process conditions.

The fractions collected were analysed and the concentrations of organics were determined to compare with the original concentration. The plant liquor was also passed directly through the column to investigate the adsorptive materials 211 present in the liquor.

5.21 PREPARATION OF COLUMNS AND SOLUTIONS

The plant seeds were washed thoroughly ln hot caustic liquor and then in hot water to remove any contam-

inants. The seeds were agaln washed in cold water until the washed liquor failed to absorb any wavelengths of the

U.V. spectra when it was checked by the U.V. spectrophotometer. The seeds were dried at 100°C. Care had to be taken that the seed was not dried over 120°C as it could be transferred into monohydrate.

70 gms. of trihydrate seeds were weighed and

l!! densely packed into a glass column of internal diameter 2 •

A solution of caustic liquor was prepared using analar

grade sodium hydroxide and the concentration was made up to

0. 5M. A weighed quantity of aluminium failings with a purity of 99% was dissolved in the solution for a few hours at 70 0 C. The solution was then filtered and cooled. The proper Bayer concentrations could not be used for these experiments as the trihydrate seeds in the columns can be easily dissolved at higher concentrations of caustic. The higher concentrations of aluminium could deposit some of the trihydrate on the seeds.

The organic salts known to be present in the liquor, i.e. sodium oxalate, sodium acetate, sodium formate, sodium succinate, sodium benzoate and sodium carbonate were dissolved separately, in the previously prepared solution of sodium aluminate and it was made up to the concentration of

.145 g/litre.

The organic compounds extracted from the Bayer process liquor were also made up to the similar concentration 212

1n the caustic aluminate solution. The group of organ1c impurities selected for adsorption studies were as follows: a) Mixture of organics extracted from the ether in which

both the higher and lower molecular compounds

are present b) Higher molecular compounds over 10,000 separated us1ng

membrane filtration as described in section 5.115 c) Higher molecular compounds over 1000 separated using

the same method as in (b) d) Organic impurities below the molecular cut off 10,000

(a filtrate after passing through the 10,000

molecular cut off membrane).

5.22 Equipment Procedure and Results

A schematic diagram and photograph of the experl- mental unit are shown in Figures 5.16 a and b. As shown 1n

Figure 5.16, the storage funnel is connected to a glass column packed with alumina trihydrate seed. The funnel and the column are packetted to pass hot water at 60°C in order·to keep the temperature closer to the Bayer prec1p1- tation conditions.

Fractions are collected from the bottom of the column into a fraction collector which is set up to collect one at each 5 minutes.

The hot water system in the jacket was maintained at 60°C by connecting the tube into a hot water tap in the laboratory. From the hot water tap it is connected to a copper coil immersed in a water bath before it reaches the jacket inlet. This method was adopted to boost the temper-

0 ature to 60 C as normal tap hot water is not always able to

0 produce the temperature of 60 C. The temperature in the 213

Hot water --rf- outlet

Hot Caustic solution water of organic salts

- "'\. ,_~ - -.. ·:,...... - - "!:'"- - ;i

~~-+---+---~_- - Inner tube packed - -..:~ - 4=: - with alumina trihydrate '·- f'l-- .... :·:- Jacket - , ..

- •'"1 - ,•. - \.. ~ - - ·..... - -t!=: - - .._ - ...... - ~- - r-......

Hot water at 60°C ~ :r. ·n.-- ---~,...: _..,...... Fraction collector It,. .. u____ ) I

Fig. 16(a) Sketch of the experimental unit used for the adsorption studies. 214

Fig. 5.16(b) Laboratory equipment used for the adsorption studies. 215

jacket was controlled at 60°C by m1x1ng with cold water from the tap where necessary. However, once the temperature had achieved a steady state, it was kept constant for 6 to 7 hours until the experiments were completed.

A solution of organic compounds to be tested on the trihydrate column was prepared as previously described 1n this section. It was then passed through the column. The solution level in the funnel was kept constant by adding solution when necessary in order to keep the flow rate constant. The solution drained from the column was collected as fractions using a fraction collector which was programmed to collect fractions every 5 minutes. The flow rate was determined by measuring the volume collected every five minutes. The samples collected were compared for concentration with the original organic solution.

A calibration chart was established using the original solution prepared at different dilutions. Absorption in the

U.V. spectra was taken as the measurement. There is also the possibility that the C0 which is absorbed into the caustic 2 aluminate solution should also absorb the U.V. region. In order to avoid this effect, a sample of the original pure caustic aluminate solution was used as the blank solution 1n the U.V. spectrophotometer. The wavelength was set at

210 ~m. In some cases where the U.V. determination was difficult, the total organic value was determined for each fraction by titrating with a standard solution of KMn0 . 4 Although the flow rate was kept constant by keeping the solution level constant, there were small variations from one run to another. This behaviour is explained as being due to the difficulty in obtaining equally packed 216

columns for each run. A separate column was used for each run. The following organic compounds were subjected to investigation:

1) Sodium Oxalate 0.146 g/litre

2) Sodium Formate 0.145 11

3) Sodium Acetate 0.145 11

4) Sodium Carbonate 0.146 11

5) Sodium Benzolate 0.144 11

6) Sodium Succinate 0.146 11

7) Mixture of organic impurities extracted

from Bayer process liquor 0.146 g/litre

8) Higher molecular fraction of nominal

molecular weight over 1,000 present

in the Bayer process liquor 0.146 g/litre

9) Higher molecular fraction of nominal

molecular weight over 10,000 present

in the Bayer process liquor 0.145 g/litre

10) Recycle Bayer process liquor diluted x 10

In the case of recycle Bayer process liquor, the objective was to make an approach to the selective adsorptive properties of the impurities present in the liquor by the alumina trihydrate seeds.

The original Bayer process liquor, after diluting ten times, was passed through the column and the fractions collected were tested by U.V. spectrophotometer. A sample of the original liquor (10 x diluted) was kept in the

'sample' side of the U.V. photometer and the fractions collected were kept in the blank side, and was scanned through the whole range of U.V. spectrum to detect whether 217

any wavelengths had been absorbed. It was anticipated that the appearance of such absorption peaks would indicate that some of the organics were adsorbed by the seeds. Later the adsorption column was regenerated and desorbed with 3M solution of sodium hydroxide and the fractions were collected as desorbed materials.

The result of the adsorption as a function of con centration of the solution and the volume was plotted. The curves obtained are illustrated in Figures 5.17 - 5.19. The results indicate that none of the following compounds were adsorbed on to the surface of trihydrate: sodium salts of acetic, formic, benzoic and succinic acids and sodium carbon ate. Sodium oxalate being adsorbed very slightly. However, the calculations indicate that the amount of sodium oxalate adsorbed onto the surface is hardly sufficient to form even amono-layer on the surface of trihydrate seeds. The higher molecular fractions with the N.M.W. over 1000 and

10,000 acted in a similar manner as sodium oxalate.

The above results indicate that usually the higher molecular fraction generally known as humic fractions, do not act like a poisoning agent in the crystallisation stage as a result of adsorbing onto the seed crystals. If it has any poisoning effect, it may be due to other reasons which have to be further investigated, possibly by eo precipitating with alumina trihydrate.

Figure 5.19 illustrates the adsorption results obtained from the mixture of higher and lower molecular compounds before and after passing through the membrane filtration with cut off 10,000 are adsorbed on to the alumina trihydrate. From the results the calculations were 218

made to determine the amount of organics adsorbed by 1 gm of alumina trihydrate, as shown ln the~ection 5.122. An approximate assumption was made as follows:

Surface area of trihydrate ::0.15m/g2 2 Density of organic materials = 1.2 g/cm Thickness of monolayer of

organic material

The amount of organic adsorbed by the seeds was calculated from the adsorption curves by integrating the area in the adsorbed region. The calculation was limited to the curves in Figure 5.17.

The amount of organic adsorbed = 0.114 gm per

by the seed (integrated from 70 gm of seed (or 4 Figure 5.17) 1.6 X 10- gm/gm

of seeds)

4 2 Volume of the adsorbed layer = 1. 33 3 X 10- cm -8 Thickness of the layer = 8.88 X 10 cm 0 = 8. 8 A The results from the above calculation indicate that the adsorbed layer is not sufficient to form even a monolayer if it is assumed that the thickness of a mono o layer is about 50A.

When the results are summarised it was found that the lower molecular compounds such as the salts of aliphatic carboxylic acids, as well as higher molecular compounds, do not adsorb onto the surface of trihydrate seed, but there was an indication that some intermediate compounds present in the g/1 ~------~ cone.

Initial concentration 0.146 g/1 0.15

(1) 0---0 Sodium oxalate 0.1 (2) o---0 Sodium succinate

(3) x---x Sodium acetate (4) V---V Sodium benzoate

0.05

6 12 18 24 30 36 42 48 54 Vol.ml.

Fig. 5.17 Adsorption studies of sodium oxalate, succinate, acetate and benzoate on alumina trihydrate seed column. N

ml.

volume

the

extract

10,000

1000

from

g/1.

m.wt.>

molecular

column.

0.146

extracted

Higher

trihydrate

impurities

o--0

concentration

(1)V---V (2)

alumina

organic

Initial

liquor

studies

process

Bayer

Adsorption

5.18

Fig.

0.0

0.1

0.15

cone.

g/1 cone. g/1 (1) o-o Organic impurity extract before molecular membrane separation (2) v-v Filtrate of the above extract after passing 0.15 through membrane cut of~lO,OOO 0.146 g/1 initial concentration for No. 1

0.1

-0.082 g/linitialconcentration for No. 2

0.05 V

50 100 150 200 250 volume ml.

Fig. 5.19 Adsorption studies of organic impurities extracted from the Bayer process liquor. 222 organlc materials (extracted from the Bayer process liquor) adsorb onto the surface, when passed through the seed column.

The above statement was supported by the adsorption curves appearing in Figure 5.19 i.e. the organic extract after passing through the membrane. However, there is doubt about the curves in Figure 5.19 as chemical changes may have occurred in the method of preparation of these organic extracts. The organic extract for this experiment appearing in Figure 5.19 was prepared as described in section 5.115.

The Bayer process liquor sample was acidified and extracted into the ether solution. This organic mixture was re dissolved in the caustic aluminate solution for this experiment. During this process it is questionable whether there was any change in the properties of these organics, but this error does not arise for the curves obtained in Figure 5.18 as those higher molecular compounds were directly separated from the Bayer process liquor using the membrane filtration, without acidifying the liquor.

It was found that when the plant liquor containing impurities was passed directly through the column, some of the organics were adsorbed onto the trihydrate seeds. This was evident from the different U.V. adsorption peaks obtained from the fractions collected when compared with the original solution in the U.V. spectrophotometer.

Furthermore, some useful information was collected when the column was regenerated with pure caustic liquor

(Figure 5.20). The fractions so collected were compared with the original caustic liquor in the U.V. spectrophotometer indicating some U.V. absorption peaks. The first few fractions so collected were brown in colour and the later 223

Adsorption Collection of fractions (first fraction to be obtained from 1.6 (brownish) - the bottom) 1.5 (brownish)

1.4 (brownish)

---;o.. 1.3 (brownish)

1.2 (slightly brownish)

..... 1.1 (colourless)

Alumina tri-hydrate adsorption column

De sorption J_ Collection of fractions

2.6 (colourless)

2.5 (colourless) alumina tri hydrate column ~------2.4 (slightly brownish) 2.3 (slightly brownish)

2.2 (brownish)

2.1 (brownish)

3m Ng.OH solution

Fig. 5.20 Adsorption studies of recycled Bayer process liquor on seed alumina trihydrate. Elementary sketch of the experiment. 224

Wavelengths of the peaks 11m Fractions

~~!~i:a!f the 1------.... l 265 [_:__ 1_._1 __.....~1 recycle Bayer . i process liquor r------·~ ~------2_6_2------~~-----1-·-2-----~

262 I 1.3 N.B. Fractions were I I placed in the blank side of the double beam 262 U.V.spectrophotometer, 1.4 whereas the original sample of the recycle Bayer process liquor 260 I was placed in the 1.5 sample side. 260 I I 1.6

Desorption with caustic liquor N.B. Regenerated 272 fractions were f 2.1 placed in the 272 sample side of 2.2 the equipment. [ J r- - 3 M 1----- 2.3 NaOH Sol. I I 220 : [:: 2.4 i 220 I l 2.5 l 218 I 1 2.6 j 270 & 220 '---· L 2.7 l 270 & 230 2.8

Fig. 5.21 Comparison studies of the collected fractions using the U.V. spectrophotometer. was

being

fraction

column

compound

fractions

the

solution

the

peak

Remarks

after

fraction fraction

fraction

coloured brown as fraction

- -

colour

brown

colour

do do

above

-do-

indicating

Slightly

adsorbed

Medium Colourless Yellow Slightly Brown

with obtained Coloured Coloured Coloured

Colourless Coloured

fractions

liquor

Peak

process

230 230

220

& &

liquor

1Jm

218

220 220 260 272 260 272 262 262

262 265

Bayer

tested

270 270

270

Absorption

caustic

different

passin9:

the

com

after

column.

results

(colourless)

(placed

the

U.V.

side

for

1.2 1.5 1.6 1.3

1.4

1.1

liquor

Desorbed

the

studies

trihydrate

-do- -do- -do- -do- -do- -do- -do- -do-

used

blank

equipment)

the

Fraction Fraction Caustic

Fraction Fraction Fraction

Fraction

U. Sample

parison

studies

through

passing

Comparison

used

sample

u.v.

2.9

2.8 2.4

2.6 2.2 2.7 2.3

2.5 2.1

Bayer

the

5.11

colour

sample

the studies

-do- -do-

-do- -do- -do- -do- -do-

-do-

the

TABLE

solution)

(brown

liquor

Fraction

Original

(placed

for

equipment)

side

Original

5) 7) 3) 8)

2) 1) 4) 6)

14) 15)

10) 11) 12) 13) 226

ones colourless. The results obtained in these experi mental studies are summarized in Table 5.11 and Figure 5.21.

The first part of the results indicated that some of the components in the Bayer process liquor are adsorbed by the seed column. They have the U.V. absorption in the region of 260 to 265 ~m.

The fractions collected from the regeneration stage were regarded as some of the components which were actually adsorbed by the trihydrate column and now being desorbed by the caustic. The major peaks of these fractions were obtained in the region of 270 - 272 vm and 218 to 230 vm.

The first few regenerated fractions were dark brownish ln colour and the later ones gradually became colourless.

5.3 SOLUBILITY OF SODIUM OXALATE IN THE PURE CAUSTIC AND

ALUMINATE LIQUOR AT VARIED TEMPERATURE

Solubility curves for sodium oxalate in aluminate solution at different temperatures and concentrations were established in order to study the saturation level of sodium oxalate at varied Bayer process conditions. These results would also lead to gaining of knowledge as to the behaviour of sodium oxalate in the Bayer process liquor, such as the possible coprecipitation of sodium oxalate with alumina trihydrate. Yamada et.al.(6) had already reported the solubility data for sodium oxalate in the Bayer process liquor at different temperatures.

The experimental work carried out in this thesis has been limited to pure caustic and pure sodium aluminate solutions. Previous work performed by Yamada could be affected by other impurities in the Bayer process liquor which is not the case in these experiments. 227

In the case of solubility in pure caustic solutions a saturated solution of sodium oxalate was prepared

by dissolving an excess of sodium oxalate in the caustic

liquor at different concentrations, initially at room temperature. The concentrations of the solutions were determined by titrating with a standard solution of KMn0 . The 4 same beakers containing the samples were kept in an oil bath

controlled by a thermostat and then the concentration of

0 oxalate was determined at elevated temperatures up to 8 0 C.

In the case of solubility in sodium aluminate

solutions the solutions of sodium aluminate were prepared by dissolving different weights of aluminium in a previously prepared 4m solution of caustic liquor. Sodium oxalate in excess was dissolved ln the aluminate liquor and the concentrations were determined as discussed ln the previous method at room temperature and also at elevated temperatures. These experiments were not conducted in the higher range of caustic ratio, although it was important to do so. Solutions of high caustic ratio were not prepared for the tests as this could lead to the deposition of aluminium hydroxide as a result of supersaturation.

Figure 5.22 illustrates solubility curves plotted for the pure caustic liquor at different concentrations and temperatures.

It was noticed that in the case of pure caustic solution the solubility decreases with the increase of caustic concentration. This behaviour can be explained by looking at the equation for the solubility products of sodium oxalate as follows: 228

Solubility g/1

30 0-0 23°C x-x 40°C Cl-Cl 70°C 28 ~-A 80°C

16 -

10 -

8 -

0 1 2 3 4 molar cone. NaOH Fig. 5.22 Solubility of sodium oxalate in the pure caustic solution at varied concentration and temperature. 229

solubility g/1 5

(1)· 0-0 Pure sol'n of NaOH - 4M 2591 (2) Sodium aluminate sol'n 4M, Al 0 /Na o-0· v--V 2 3 2 (3) X--X Sodium aluminate sol'n 4M, Al 0 /Na 0-0.1159 2 3 2 (4) 0---a Bayer plant liquor of 4M cone. of NaOH (previously published by Yamada et.al.)

4 Oxalate level in the recycle Bayer process liquor

3 -

20 30 40 50 60 70 80 oc temp.

Fig. 5.23(a) Solubility of sodium oxalate in the pure caustic and the caustic aluminate solution. 230

The increase of the sodium 1on concentration in the

solution by increasing the caustic concentration tends to low

down the oxalate ion concentration 1n the solution resulting

1n a low solubility value for oxalate in the solution. The

solubility of sodium oxalate in the caustic aluminate solu-

tion at different temperatures was plotted and illustrated 1n

Figure 5.23(a). The prepared caustic aluminate solutions had the following composition.

1) 4 Molar sodium hydroxide with the caustic ratio

(Al 0 /Na 0) by weight 0.2591 2 3 2 2) 4 Molar sodium hydroxide with the caustic ratio

Solubility g/1

10 20 30 40 50 60 70 80 0 T~.c

Fig. 5.23(b) Solubility of sodium oxalate in distilled water at varied temperature. 231

The solubility values obtained for the pure caustic solution of 4 Molar concentrations at different temperatures

(previously obtained from Figure 5.22) are also plotted here in Figure 5.23 for comparison studies. The results indicate that at the lower temperatures (near room temperature), the solubility values are similar in both cases. At elevated temperatures solubility values are higher in caustic alumin ate solutions than in the caustic solutions for a given temperatu~e value. The above behaviour could be explained by considering this system in the form of a solid-liquid equilibrium. Any decrease in the activity of the solute species in the equilibrium system is accompanied by an increase in solubility. Hence in dilute solutions where the ionic strength (I < 1) is less than the unity, the solubility of a salt is enhanced in the presence of inert electrolyte.

It is suggested here that the aluminate species is most likely to behave as inert electrolyte, resulting in higher solubility values for oxalate in the system.

Solubility curves for sodium oxalate ln the Bayer plant liquor at different caustic concentrations and tempera tures were studied by Yamada et.al. (6). The results have been previously outlined in Figure 1.3. Using these values, the solubility versus temperature was plotted for the Bayer process liquor with a caustic strength of 4 molar sodium hydroxide. The results are shown in the curve 4 of Figure

5.23. Those results are compared with the experimental results obtained by the author using 4 molar concentration of

NaOH with different caustic ratio (Al 0 /Na 0). Curve 1 2 3 2 indicates the solubility of sodium oxalate in 4 molar pure sodium hydroxide solution, Curve 2 indicates the solubility 232

v~lues 4 Molar sodium hydroxide with the caustic ratio

(Al o /Na o wt.) of .2591~ whereas Curve 3 indicates the 2 3 2 caustic ratio of 0.1159. However, it is interesting to

note that Curve 4 is similar to Curve 1 which is for pure

caustic solution. The Bayer plant liquor usually contains

Al0 in the caustic ratio range of (Al 0 /Na 0) 0.2 - 0.3. 2 2 3 2 According to results obtained by Yamada (Figure 1.3) the

aluminate does not have any effect on the solubility of sodium

oxalate in the Bayer process liquor.

Some workers have reported the following figures

being analysed as sodium oxalate in the recycle Bayer process

liquor.

1) 3.7 g/litre Ref. (75)

2) 4 g/litre If If Ref. ( 144)

The analysis was performed at room temperature.

The recycle Bayer process liquor usually contains

about 3.5 molar concentration of caustic liquor and alumin-

ate with the caustic ratio of about 0.25. When these

analytical results are compared with the actual solubility

values of sodium oxalate under those conditions, the values

are much higher than the solubility figures. These results

indicate that the sodium oxalate in the Bayer process liquor

has reached the supersaturation level. The following exper-

iments by the author contribute very much to the above

argument. After determining the solubility values at higher

temperatures the solutions were brought down to room temper

atures and thereaften the concentration of oxalate in the

samples was measured. The final concentration values

obtained were very much closer to the values obtained for 233

elevated temperatures~ thus indicating that no crystallisa tions of sodium oxalate had taken place instead it was in the form of supersaturation. This supersaturation behaviour of sodium oxalate in the Bayer process liquor can lead to various effects on the precipitation of alumina trihydrate, such as eo-precipitation with alumina trihydrate, when trihydrate seed acts like nuclei.

5.4 SUMMARY OF THE EXPERIMENTS DISCUSSED IN CHAPTER 5

Results of the experiments carried out in Chapter 5 indicate that the organics present in the Bayer process liquor are complex in nature. It was found to be a mixture of low molecular compounds and higher molecular compounds.

Total organic carbon content in the Bayer process liquor was analysed and it is usually built up to about 25 - 30 g/litre based on carbon.

Various methods were applied ln an attempt to analyse the organic impurities present in the Bayer process liquor. Distillation method suggested that it contains sodium formate and acetate, whereas its formate content was analysed to be in the range of 2.0 - 2.5 g/litre (as formic acid).

The Bayer process liquor was subjected to various extraction methods. Ether extract of the acidified solution of Bayer process liquor was analysed using gas chromatography. The direct analysis of the ether extract indicated the presence of sodium acetate, propionate and valerate. The ether extract was methy.lated and was injected through a G.C. Although there were some distinct peaks in the chromatogram, it was unable to identify as 234 there were no suitable reference samples available. The infra red analysis performed on the ether extract from the

Bayer process liquor suggested the presence of carboxylic acid radicals and aromatic rings in the organic impurities.

Furthermore, I.R. spectra obtained from the said extract was found to be very similar to the I.R. spectra of the natural humic and fulvic acids reported elsewhere.

Membrane separation was used to characterise the organ1c present in the Bayer process liquor according to the molecular weight range. Two types of membrane were used, one with the molecular weight cut off at 1000, the other with

10,000. The membrane with cut off 1000 was not operable at high pH. Therefore the Bayer process liquor sample was

C0 to bring down the pH to 9. The follow carbonated with 2 lng analyses of the Bayer process liquor were obtained from the membrane separation.

Molecular wt. over 10,000 - 1.8 g/litre (as carbon)

Molecular wt. over 1,000 - 6.2 g/litre (as carbon)

Molecular wt. below 1,000 - 24.4 g/litre (as carbon)

An attempt was made to characterise the organic impurities accumulated with the plant trihydrate product either in the form of adsorbed or coprecipitated material.

Various methods were applied to desorb the contaminated organics selectively from the product trihydrate. The first extract was obtained by washing with methanol and thereafter the dried samples were extracted with hot water. The dried trihydrate before the hot water desorb, was tested with x-ray defraction analysis and it indicated the presence of sodium oxalate and some other unknown compounds in the crystalline form suggesting that they are crystallised with trihydrate 235

product. The hot water desorb of the product trihydrate was fractionated using membrane separation and it was found to contain about 20% of higher molecular organic greater than 1000 M.wt.

The second extract was obtained by washing the alumina trihydrate first with cold water and extracted with hot water. In this extract higher molecular organic greater than 1000 M.wt. was found to be only about 3% out of the total organic content. Furthermore, infra red analysis of the desorb suggest the presence of aromatic rings and the phenolic compounds in the extract.

Experiments were conducted to investigate the adsorptive properties of some of the known organic compounds believed to be present in the Bayer process liquor on alumina trihydrate seed. Among them were sodium oxalate, formate, acetate, carbonate, benzolate, succinate and different molecular weight fractions of the organics extracted from the Bayer process liquor. Adsorption properties of the recycled Bayer process liquor was also investigated by passing through the trihydrate column. The results indica ted that the oxalate, formate, acetate, carbonate, benzolate, succinate and the organic molecular fractions over 1000 were less adsorptive, whereas the intermediate molecular weight fraction below 1000 was considerably adsorptive. When the recycle Bayer process liquor was passed through the trihydrate column, it was found that some of the organics do get adsorbed on to the trihydrate seed.

Solubility of sodium oxalate 1n the pure caustic and aluminate solution was studied at varied caustic concentrations and temperatures. It was noticed that in the case 236

of pure caustic solution the solubility decreased with the increase of caustic concentration. The solubility value

1n the aluminate liquor at ambient temperature was identical to the value obtained in the pure caustic liquor of equal caustic strength. However, at elevated temperatures it was higher in the aluminate liquor than in the pure caustic solution as shown in Fig. 5.23(a).

The plant analysis of sbdium oxalate concentration

1n the Bayer process liquor was compared with the solubility values obtained in these studies and revealed that the sodium oxalate in the Bayer process liquor was present in the super saturated level. CHAPTER 6

EXPERIMENTAL STUDIES ON THE EFFECT OF ORGANIC IMPURITIES DURING THE CRYSTALLISATION OF ALUMINA TRI-HYDRATE 23 7

INTRODUCTION

Studies were carried out to determine the effect of various organic compounds on the crystallisation of alumina trihydrate. The individual effect on the kinetics, crystal habit~ particle size distribution and the crystal mass by major organic impurities found to be present in the Bayer process liquor were investigated. The above kinetic studies mainly involved the effect on the induction period, the crystal growth and the rate of decomposition.

An isothermal batch crystallisation was selected as the most appropriate method for these studies. Although the continuous process was the most suitable method as it reproduced the nearest possible plant condition, it was not possible under these circumstances. The precipitation runs were carried out under isothermal conditions. Most of the other conditions, i.e. temperature, caustic ratio, caustic concentration, seed charge and the impurity charge were very similar to the Bayer plant conditions. The data obtained were interpreted as rate kinetics, particle size distribution, and those results were compared with those of crystallisation runs without any impurities.

6.1 BATCH CRYSTALLISATION UNIT

A photograph of the laboratory batch crystalliser is displayed in Figure 6.1a and a sketch of the unit is in

Figure 6.1b. The crystalliser was a 5 nos. of stainless steel beakers of 750 ml capacity and 500 ml working capacity.

The beakers were closed with PVC type lids which can be lifted easily. The closed operation kept evaporation loss to the minimum and also minimised the contamination of caustic 238

Fig. 6.1a Photograph of the laboratory batch crystalliser. with atmospheric . Propeller type stainless steel co 2 stirrers enter through each beaker, as shown in Figures

6.1 a and b. The stirrers were mechanically rotated by connection into a universal axle coupled to an electric motor through a gear box. All the stirrers were rotating at a speed of 30 r.p.m. which was continually checked by a tachometer.

The crystallisers were kept immersed ln a constant temperature bath (± 0.3 accuracy) and all the runs were carried out isothermally. The temperature in the water bath was controlled by a thermostat and the water bath was insulated from every side, including the top of the tank, during the run. The temperature of the bath and the beakers was checked during each run and the products were kept in

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6.2 PREPARATION OF CAUSTIC ALUMINATE SOLUTION

Caustic aluminate solution for precipitation studies can be prepared 1n two ways. The first is to dissolve pure aluminium metal 1n NaOH solution. The reaction is exothermic with the evolution of hydrogen. The second is by dissolving alumina trihydrate in a caustic solution in a stainless steel autoclave at elevated temperatures in the range 150°C - 175°C.

In this work the first method was used to prepare the caustic aluminate solution. There were some difficulties, such as premature precipitation before the ratio was reached. This was mentioned by Misra (11) when using the

same method. However, the premature precipitation was not

observed in this work, as a result of the exothermic reaction, usually the caustic concentration was increased

due to the evaporation, while keeping the required

Therefore, during the preparation, solubility value was high due to the high concentra-

tion of caustic. When all the aluminium was dissolved the

solution was brought up to the correct concentration of caustic

by adding water. For each run 3 litres of NaOH solution of

appropriate Bayer process concentration (equivalent to

187 g/litre in Na co ) was prepared in a stainless steel 2 3 container. Aluminium foils (equivalent to a caustic ratio Al o /Na co of 0 .. 65) were added to the solution little by 2 3 2 3 little. The aluminium metals used had the purity of 99% which was obtained from Alcan Ltd., in Sydney. During the H was evolved and the boiling was addition of aluminium, 2 noticed in the solution due to the heat. When the addition 241 of aluminium was completed the volume of the solution was almost reduced to about 2300 ml. and water was added to make up a quantity of 3 litres. The caustic aluminate solution was black in colour due to the presence of solid carbon as an impurity. When the solution was passed through a glass fibre filter it was separated and the filtrate became very clear. Special care was taken to avoid even the slightest contamination of solid particle in the container as they could act as nuclei to precipitate alumina trihydrate.

6.3 SEED PREPARATION

The seed was obtained from the Queensland Alumina

Co. Ltd., in Gladstone, and was an industrial type used in their plant for seeding purposes. It was the product alumina trihydrate obtained from the tertiary and secondary thickeners of the Bayer process plant. The average particle size was in the range of 10 - 20 ~m. The seed was washed in cold and also in hot water and then dried for about 6 - 8 hours at 100 0 C. Weighed amounts of seed were usually prewarmed to the test temperature before addition to the crystalliser.

6.4 ADDITION OF IMPURITIES

Impurities used in these studies include most of the organ1c salts commonly present in the Bayer process liquor, i.e. a) sodium oxalate

b) sodium acetate

c) sodium formate

d) sodium succinate

e) higher molecular organic impurities, 242

molecular weight over 1000

f) mixture containing all the organic impurities

present in the Bayer process liquor.

The impurity charge was based on the quantities present in the Bayer process liquor, but, ln some runs, sodium oxalate was added at a higher than normal level of con centration to investigate the effect of supersaturation.

The organic impurities over the molecular weight

1000 used for this experiment were extracted from the Bayer process liquor according to the method described in section 5.116. The organic impurity concentrate so collected after passing through the membranes was oven dried at 100°C before using for this work.

The mixture containing all the organlc impurities present in the Bayer process liquor was extracted using the method adopted in section 5.115.

6.5 PROCEDURE

The crystalliser unit was filled with water and the required temperature was maintained. It was allowed to run for a few hours until it was stabilised at the required temperature. (Most of the runs were at 64°C).

The stainless steel beakers were filled with 500 ml of sodium aluminate liquor and placed in the crystalliser bath until it reached the equilibrium temperature. Weighed quantities of organic compounds were added to the beakers as the impurities and allowed to dissolve. Seed was also prewarmed to the required temperature separately.

Weighed quantities of warmed-up seed were charged into the beakers and stirring began immediately. Every 243 batch of experiments included one or two units (beakers) in the crystalliser without any impurity charge for corn- parison studies.

The precipitation experiments were continued for

24 - 26 hours. When the experiments were terminated, the product trihydrate was separated from the remaining liquor by sedimentation and filtration and the volume of the rema1n- ing liquor was measured. The product was washed and dried at 101°C.

6.6 SOLUTION ANALYSIS

Analysis of Al and Na concentration 1n the 2o3 2o solution was required for obtaining kinetic data. Although the variation in the Na concentration was slight, the 2o change in Al 0 concentration is regarded as the main 2 3 variable in order to plot the precipitation curves.

The atomic absorption method was used for estimating alumina concentration of periodic samples. The samples of aluminate liquor were taken us1ng a syringe at different intervals, usually at 1 or 2 hours. However, in some cases, the intervals were shortened to ~ hr., especially at the initial stag.es of some runs, in order to investigate the induction period. Samples were filtered and diluted 100 times as they were too strong in Al o 2 3 concentration to be tested by atomic absorption method.

Solutions for calibration were prepared using different dilutions of standard solution of aluminium nitrate used for atomic absorption. A fresh batch of calibration was done for every run as the conditions of the atomic absorption unit can be changed easily. 244

6.7 PARTICLE SIZE ANALYSIS BY ZEISS. TYPE MICRO VIDEOMAT 2 The determination of accurate kinetic data from batch crystallisation depends considerably on the accuracy of the particle counting sizing techniques employed. Fairly fast analysis and good resolution were desirable features.

The micro-videomat used for this work has many of these features. Details of the micro-videomat used ln these experiments are described in Appendix III.

Particle sizes in the range of 1 - 120 ~m were measured using the videomat. A major problem in this analy sis was to obtain a sample of uniform particle size distribu tion to be placed on the slide. Larger particles were con centrated on one side of the slide and the smaller particles on another side. This problem was overcome by mixing the product to be analysed in a glycerine - water mixture with a ratio of 1 : 1. In this manner the product was kept ln a uniform suspension due to the high viscosity. A portion of the suspension was transferred on to the slide to be analysed in the videomat.

6.8 OPTICAL AND ELECTRONMICROGRAPHY

The products obtained from the experiments were studied uslng the optical microscope and the electron micro scope. These studies were aimed at looking at the crystal habit, size and the shape of the products trihydrate due to the presence of impurities. Products from several runs were photographed with an optical microscope and using a 35 mm reflex camera.

Micrographs of the samples of various products were taken using the scanning electron microscope. The samples were properly washed and dried, then coated with a gold film. 245

Variables Plant Laboratory

Caustic concentration 185 - 190 185 - 190 (gm/litre Na ) 2co 3

Caustic ratio by weight 0.65 -0.36 0.6 - 0.3 (Al o /Na co ) 2 3 2 3

0 Temperature ( C) 80 - 60 60 - 65 varying at constant temperature

Seed charge 100 - 150 80 - 130 (Al(OH) g/litre) 3

24 negligible

Impurities · Organic Individual impurities organic com (inc.sodium pounds and oxalate), pure runs & NaCl, and also various Na Trace fractions of 2so 4 organics ex tracted from the liquor

Residence time 40 - 60 24 - 26 hrs.or more hours

Type of operation continuous batch type

Type of agitation air mechanical agitation stirring

TABLE 6.1 Plant and laboratory crystallisation conditions. 246

Exp. Cryst- Impurities addition Seed Temp. Dura- No. alliser addi- range tion of No. tion Expt. g/litre g/1 oc hrs.

1 1 Pure solution 130 65-66 24

2 -do- " " " 3 Sodium oxalate 3 " " "

4 -do- 5 " " "

5 Sodium acetate 5 " " "

2 1 Pure solution " 64-65 24

2 -do- 11 11 "

3 Sodium oxalate 5 " 11 11

4 Sodium formate 5 11 11 11

5 Sodium succinate 5 " 11 11

3 1 Pure solution 80 63.5-65 23.5

2 -do- 130 11 11

3 Organic extract from 4 130 11 11 Bayer process liquor M.Wt.> 1000

4 Sodium succinate 5 130 11 11

5 Sodium oxalate 5 80 11 "

4 1 Pure solution 130 26 24

5 1 Pure solution 130 63.5-64 23

2 Sodium oxalate 10 11 11 "

3 Sodium succinate 10 11 11 11

4 Sodium formate 10 11 " 11

5 Sodium acetate 10 11 " "

6 1 Pure solution 130 66-68 24

2 Sodium oxalate 8 11 11 11

3 Organic extract from 10 11 11 " Bayer process liquor M.Wt.> 1000 247

Exp. Cryst- Impurities addition Seed Temp. Dura- No. alliser addi- range tion of No. tion Expt. g/litre g/1 oc hrs. 4 Pure solution 130 66-68 24 (Unwashed seed)

5 Sodium acetate 8 130 " "

7 1 Pure solution 130 64.5-65 29

2 Sodium oxalate 6 " " "

3 -do- 6 " 11 "

4 -do- 6 " 11 "

5 Pure solution 11 " "

8 1 Sodium oxalate 10 130 63.5-64 24

2 Pure solution " 11 "

3 Sodium oxalate 10 11 " 11

4 Pure solution 11 " 11

5 Sodium oxalate 10 11 11 "

9 1 Pure solution 130 63-63.5 24

2 Mixture of organic 5 11 11 11 impurities extracted from Bayer process liquor (without any molecular fractionation)

3 Pure solution " 11 "

4 Mixture of organic 5 " 11 11 impurities extracted from Bayer process liquor (without any molecular fractionation)

5 Pure solution 11 11 11

TABLE 6.2 Details of experiments on crystallisation 248

During coating the spec1mens were rotated and tilted.

The specimens were carefully examined by a scanning microscope with magnifications of 150 and 300 and compared with the seed products obtained from the pure solutions. As many of the samples were similar only representative samples of different results obtained were selected to be micro photographed.

6.9 RANGE OF CONDITIONS STUDIED

The conditions employed 1n the studies were based on the normal operating conditions of Bayer plants. Few changes had to be made to suit laboratory scale experiments.

Table 6.1 compares industrial operations at Queensland

Alumina Plant with the conditions used in this study. Details of the crystallisation runs are summarised in Table 6.2 indi cating the number of runs, type and quantity of impurities added, seed change and the temperature ranges for each run.

6.10 RESULTS AND DISCUSSION OF CRYSTALLISATION STUDIES

The atomic absorption results were used to plot the precipitation curves based on the aluminium content in the solution against time. The results obtained for each run are illustrated in Figures 6.2 to 6.9. Table 6.3 summarises the kinetics of all the crystallisation experiments.

As shown in Figures 6.2 to 6.9, the precipitation curve of each experimental run differs due to the change in the conditions applied i.e. initial caustic concentration) caustic ratio and the temperatures which were not identical throughout the series of experiments. This error is caused because the aluminate liquor was prepared batchwise for 249 each run. The temperatures also varied slightly by at least

1 - 2°C in each run. Therefore the study of impurity can be treated more intelligently if a basis for comparison is avail able. For this reason it was decided to run at least one unit out of the five crystalliser units without any impurity charge.

It was felt that this would prove to be a valuable standard against which the effects of impurities could be better treated.

Particle slze distribution of the products and seed trihydrate were analysed and are shown in Figures 6.10 - 6.15 as cumulative oversize against the particle size using the micro videomat (image analyser). The analyser is able to produce only the particle size count and it was not possible to obtain the weight distribution.

The product obtained from the experimental runs was analysed by the optical microscope and the scanning electron microscope. The microphotographs were taken only from the experiment which indicated some different results and they are shown ln Figures 6.16 - 6.25.

On the basis of the experimental results a number of aspects of the precipitation may be analysed. These include the following factors: induction period, initial and general decomposition rate, equilibrium concentration, crystal growth, crystal habit and particle size distribution. a) Induction period

Induction period lS defined as that interval between the time that seed is added to the supersaturated solution and the time that decomposition is first noted. Many workers in the past have neglected the induction period ln modelling the crystallisation system of the Bayer process. Past 250

workers were not able to obtain an equation for the complete precipitation curve, including the induction period, and it was limited to only the crystal growth.

The induction period is a measure of the stability of the given sodium aluminate solution. This stability is related to the tendency for the aluminate lons to decompose to colloidal aluminium hydroxide, which, in turn, depends on the tendency for the conversion from c6lloidal to crystalline material and deposition of this material on an existing crystal surface. The induction period usually depends on the surface area of the seed, the greater the seed surface the shorter will be the induction period. Wherever possible the induction period was evaluated in these runs.

(b) Initial decomposition rate

This rate is determined at the end of the induction period, i.e. at the beginning of the decomposition period.

The initial decomposition rate is analysed as the grams of dissolved Al o per hour at the initial stage. 2 3 (c) Total decomposition rate

The assumption made for the initial decomposition stage is not valid for the entire curve. The total decomposition rate is based on the overall drop in dissolved alumina. (d) Equilibrium concentration

The alumina concentration at the termination of the runs is designated as "equilibrium concentration".

Theoretical equilibrium concentration can be calculated for a given temperature and the caustic and alumina concentration using the solubility values of Al in the caustic liquor. 2o3 N (Jl

I-'

6.3b

6.4a 6.4b 6.3a

6.2

-do-

-do- -do- -do- -do- -do- -do- -do- -do-

Fig. Fig. Fig.

Fig.

reference

Source

33.5 31

29 32 33 34.7 24

23.2 23 33 34 34.2 32.5 38

g/1

Equilibrium

concentration

57.5

57.5 70 70 57.5 70 70 57.5 70 59 59 59 59 59

g/1

Original

concentration

0.54 0.63

0.57 1.02 0.55 0.97 0.516 0.96 0.55 0.975 0.60 0.43 0.92 0.78 0.78

rate

g/hr

Total

decomposition

rate

3.5

4.0 2.0

6.2 4.25

3.5 5.7 0.9 0.6 0.5 1.0 1.0

g/hr

10.0 10.0

10.0

Initial

decomposition

0 0

0 0 0 0 0 0 0

hr.

1.2 1.0

1.2

1.0 1.2

period

Induction

'4!~ 130 130

130 130

130 130 130 130 130 130

130 130 130

Seed

charge

5 5

5 5 5 - 4 - - 3 5

- - -

g/1

cone.

charge

Impurity

formate succinate succinate

oxalate extract

acetate

oxalate oxalate

Sodium Sodium -do-

Sodium Sodium M.Wt>lOOO

Organic

Sodium Sodium Pure -do- Pure Sodium -do- Pure -do-

Impurity

Type

4 5

4 3 5 3 2 5 1 2 1 2 4 3

No.

alliser

Cryst-

3 2

No.

Expt. Expt. Cryst- Impurity charge Seed Induction Initial Total Original Equilibrium Source of No. alliser charge period decomposi- decomposition concentration concentration reference No. Type of Impurity tion rate rate of Al of Al Impurity cone. g/1 hr.

5 l Pure - 130 1.2 3.0 0.47 57 36 Fig. 6.5a 2 Sodium 10 130 1.2 1.4 0.36 57 oxalate 41 -do-

3 Sodium 10 130 1.2 2.5 0.41 57 39 succinate -do-

4 Sodium 10 130 1.3 1.62 0.41 57 39 6.5b formate Fig.

5 Sodium 10 130 1.3 2.0 0.40 57 acetate 39.5 -do-

6 l Pure - 130 1.0 1.5 0.42 56 36.5 Fig. 6.6a 2 Sodium 8 130 1.0 0.75 0.41 56 oxalate 37 -do-

3 Organic 10 130 2.0 1.15 0.39 56 37 extract -do- M.Wt>lOOO

4 Pure - 130 0.5 0.5 0.36 56 38.5 Fig. 6.6b (unwashed)

5 Sodium 8 130 0.25 0.625 0.40 56 acetate 36.5 -do-

7 1 Pure - 130 0.25 0.75 0.61 54 34 Fig. 6.7a 2 Sodium 6 130 1.0 1.0 0.45 54 32 oxalate -do-

3 -do- 6 130 1.0 0.5 0.47 54 33 Fig. 6.7b 4 -do- 6 130 0.5 1.0 0.47 54 33 -do- 5 Pure - 130 0.4 1.1 • 0.47 54 33 -do-

N c.n N w N (.T1

6.8b

6.9b 6.9a 6.9b 6.9a 6.9b

-do- -do- -do-

Fig.6.Ba Fig. source reference

Fig. Fig.

Fig. Fig. Fig.

experiments

34 34 33 33.5 34 34 30 32 30 34

Equilibrium

concentration

crystallisation

the

57 57 57 57 57 57 of 57

57 57

Original

concentration

Results

6.3

0.48 0.48 0.5

rate 0.49 0.48 0.57 0.56 Total 0.62

0.56 0.57

Table

decomposition

3.5 3.5

2.5 2.5 3.0

1.0 1.1 1.0

1. 1.1

Initial

decomposi-

1.5 0.2 1.5 1.5 0.5 0 1.5 0.5 0 1.5 hr.

period

Induction

130 130 130 130 130 130 130 130 130 130

g/1

Seed

charge

10 10 5 10 10 - g/1 - - -

cone.

charge Impurity

organ- organ-

of Bayer Bayer

extract extract

Impurity

oxalate oxalate oxalate

Impurity from Sodium Sodium ic Sodium Pure Pure from ic Total Type Pure liquor liquor Pure Pure Total

No.

2 3 5 4 2 1 5

1 3 4

alliser

Cryst-

8 9 No.

Expt. (hours)

Time

No.

liquor

experiment

g/1

g/1 g/1

aluminate

acetate oxalate

oxalate

sodium

Crystallisation

Sodium Sodium

Sodium

Pure

6.2

FIGURE

x-x

A-IJ.

0-0

0--I:l

g/1

[Al] 25

(hours)

Time

No.

experiment

solution

g/1

Crystallisation

aluminate

oxalate

6.3a

sodium

Sodium

Pure

FIGURE

0-0

X-X

V-'V

20 10

40 50

70 60

[Al]

gl [Al] g/1 sodium succinate 5 g/1 70 0-0 o- a sodium formate 5 g/1

Time (hours)

FIGURE 6.3b Crystallisation experiment No. 2 N c.n m

N N

-...J -...J

(J1 (J1

25 25

0 0

(hours) (hours)

0 0

Time Time

g/1 g/1

130 130

80 80

3 3

charge charge

No. No.

Seed Seed

experiment experiment

solution solution

do do

aluminate aluminate

Crystallisation Crystallisation

sodium sodium

6.4a 6.4a

Pure Pure

FIGURE FIGURE

o-n o-n

o-o o-o

0 0

40 40

50 50

60 60

g/1 g/1 [Al] [Al] 25

(hours)

Time

No.

g/1.

1000

experiment

wt.>

g/1

- 5

Crystallisation

impurity

succinate

oxalate

6.4b

sodium

Sodium

Organic

FIGURE

X-K

0-.-0 a-o

50 tD (Jl N

(hours)

Time

No.

experiment

solution

g/1

aluminate

Crystallisation

succinate

oxalate

sodium

6.5a

Sodium

Pure

FIGURE

X-

a-D

0-•0

[Al]

g/1 (J)

(hours)

Time

No.

experiment

g/1

Crystallisation

formate

acetate

6.5b

Sodium

FIGURE

0-0

h.-A

[A1]

g/1 (hours)

Time

No.

g/1

experiment

1000;

solution

M.wt.>

g/1

Crystallisation

aluminate

impurity

oxalate

6.6a

sodium

Sodium Organic

Pure

FIGURE

X-X

0-0

IJ-0

60r------~

[Al]

g/1 25

(hours)

Time

------~0

No.

experiment

solution

g/l

Crystallisation

aluminate

6.6b

acetate

sodium

FIGURE

Sodium

Pure

50 60------.

[Al]

g/1 60 [Al] o-o Pure sodium aluminate solution g/1 X-X Sodium oxalate 6 g/1

Time (hours)

FIGURE 6.7a Crystallisation experiment No. 7

N (5) w 60 [Al] 0-0 Sodium oxalate 6 g/1 g/1 x->< - do -

Pure sodium aluminate solution

Time (hours) FIGURE 6. 7b Crystallisation experiment No. 7 m

(J1

(hours)

Time

No.

experiment

solution

g/1

aluminate

Crystallisation

oxalate

sodium

6.8a

Pure

Sodium

FIGURE

0-0

1:.-A

[AI"]

g/1 N

0) 0)

(hours)

Time

No.

solution

experiment

g/1 g/1

10 10

aluminate

oxalate oxalate

sodium

Crystallisation

Sodium Sodium

Pure

6.8b

Fig.

Q-0

O-D x-x

30L--J--~--L--L--~~--~--L--L--J-~~~--L-~--J-~~~--~~--~--~~--~~--~

50 40

60 25

(hours)

Time

Bayer

the

from

No.

extracted

experiment

impurities

g/1

organic

liquor,

Crystallisation

6.9a

Mixture

process

FIGURE

0-0

A-!::.

[Al]

g/1 25

(hours)

Time

No.

experiment

solution

aluminate

Crystallisation

sodium

6.9b

-do-

Pure

FIGURE

0-0 o-P

A-A

[Al]

g/1 269

Usually it was found that the practical values are different from the theoretical values.

(e) Crystal Growth

The theoretical aspect of a crystal growth is widely explained in Chapter 4. The crystal growth in these runs lS started from the initial decomposition stage down to the reach of equilibrium concentration. However, during this process, because the mechanism of the crystal growth is com bined with other factors such as secondary nucleation, crystal breakages and agglomeration, a proper independent crystal growth analysis is not possible. Furthermore, it was found that the effect of impurities on the above men tioned factors is not an easy task in these studies.

(f) Crystal size distribution

In crystallisation systems one of the prime interests is to characterise the population according to Slze. The results obtained from these experimental runs gives a lead to the effect of individual organic impurities on the particle size of the product. In these studies the results with impurity charges are compared with the particle size distri bution of the original seed and also the product obtained without the influence of impurities.

6.10.1 Effect of Sodium Oxalate

Few batches of experiments were performed to invest igate the effect of sodium oxalate in the precipitation system. A summary of the results obtained from the experi ments is given in Table 6.4. In this section the effect of sodium oxalate on the induction period, decomposition rate, equilibrium concentration, crystal growth and habit and the

0 0

--.:1 --.:1

N N

~ ~

~m ~m

was was

~ ~

39 39

~ ~

40 40

15.5 15.5

observed observed

coarse coarse

analyser analyser

45 45

average average

with with

was was

Dstd= Dstd=

microscope microscope

less less

Dstd= Dstd=

the the

product product

image image

Dstd= Dstd=

rate rate

distribution distribution

in in

but but

distribution distribution

the the

of of

the the

whereas whereas

optical optical

formation formation

in in

whereas whereas

formation formation

size size

by by

size size

standard) standard)

size size

~m ~m

whereas whereas

~ ~

~m ~m

the the

~ ~

particles particles

formation formation

16 16

fine fine

formation formation

fine fine

23 23

5 5

5.5 5.5

= =

difference difference

(Comparison (Comparison

detected detected

using using

fine fine

but but

Particle Particle

Dp Dp

Dp= Dp=

Less Less

particles particles

particle particle

Dp= Dp=

No No

Fine Fine

DP= DP=

Finer Finer

Particle Particle

low low

of of

low low

is is

do do

low low

do do

is is

slightly slightly

is is

standard) standard)

kinetics kinetics

eq eq

is is

C C

high high

on on

difference difference

with with

eq eq

I.D.R. I.D.R.

No No

I.D.R. I.D.R. and and

I.D.R. I.D.R.

C C

No No

T.D.R. T.D.R.

(Comparison (Comparison

rate rate

Effect Effect

in in

plus plus

pre

plus plus

* *

excess excess

oxalate oxalate

only only

in in

do do

oxalate oxalate

applied applied

oxalate oxalate

sodium sodium

charging charging

impurity impurity

in in

sodium sodium

Method Method

sodium sodium

Saturated Saturated

excess excess

Supersaturated Supersaturated

sodium sodium

cipitate cipitate

Supersaturated Supersaturated

with with

6 6

8 8

5 5

3 3

10 10

-do-

change change

Oxalate Oxalate

No. No.

4 4

3 3

2 2

5 5

3 3

4 4

3 3

Fig.6.7b Fig.6.7b

Fig.6.7a Fig.6.7a

Fig.6.6a Fig.6.6a

Fig.6.5a Fig.6.5a

Fig.6.4b Fig.6.4b

Fig.6.3a Fig.6.3a

Fig.6.2 Fig.6.2

Unit Unit

Crystalliser Crystalliser

7 7

6 6

5 5

3 3

2 2

1 1

No. No. Expt. Expt. to

with

rate

distribution

oxalate

standard)

charge

size

with

oxalate

formation

the

crystalliser.

(Comparison

with

impurity

Fine

Particle

from

the

solution

the

experiment

with

obtained

range

experiments

the

aluminate

standard)

kinetics

the

charge)

with

trihydrate

-do- difference

-do-

(Comparison

temperature

rate

No heating Effect

crystallisation

impurity

equilibrium

product

the

any

the

operating

the the

rate of

prepared

only

rate

(without

down

was

applied

oxalate

size size

results

-do-

-do- charging

impurity

the

sodium

Saturated

Method

charge.

concentration

oxalate

brought

solution

decomposition

particle

decomposition

was

sodium

impurity

Summary

11 standard

Initial

Aluminium Total Average of Average

= = = =

change

Oxalate

6.4:

temperature

.R.

solution

the

I.D

Dstd T.D.R. c

TABLE

No.

then

Fig.6.8b Unit Fig.6.8a Fig.6.8b

and

Crystalliser

95°C

Supersaturated

Notation:-

No.

Expt. 272 particle size distribution are discussed. a) Induction period

Misra (11) suggested that prolonged induction periods can be due to the presence of sodium oxalate impurity.

Kelly (113) in his work contributed to the above suggestion.

However, the results obtained in this work are contradictory to Kelly's findings as any evidence_ of prolonged induction periods was not noticed at any time due to sodium oxalate impurity. A drawback in Kelly's experiments was found to be that his standard crystallisation runs (experiments with out any impurities) were performed at a different time and were not parallel to the runs with the impurity charge.

In the author's experiments it was noticed that slightly different precipitation curves were obtained for the experi ments under the same conditions, when it was carried out at different times, therefore the results indicate that the slightest change in the conditions could easily change the precipitation results. b) Rate of decomposition

The results show that there is a slight effect, mainly on the initial decomposition rate, by the sodium oxalate impurity. This effect seems to be very much dependent on the degree of supersaturation of sodium oxalate in the aluminate liquor.

It was notice in experiment No. 1 (Fig. 6.2) that the total decomposition rate has been affected compared to the standard. However, due to the lack of sufficient point at the beginning of the experimental curve, it is not possible to show clearly which step of the crystallisation 1s being affected~ whether the induction period, the initial 273

decomposition rate or the crystal growth.

In Experiment No. 2 (Fig. 6.3a), although sufficient points have been taken at the initial stage, due to the high content of aluminium in the solution, there lS a very high initial decomposition rate in all five units of the experiment, therefore it seems to have overruled any effect of impurity on the crystallisation. For this reason the effect of sodium oxalate cannot be properly detected in this batch of experiments.

In Experiment No. 3 (Figs. 6.4 a and b), Experiment

No. 5 (Fig. 6.4a), Experiment No. 6 (Fig. 6.6a) and Experi ment No. 7 (Fig. 6.7a), it was noticed that the initial decomposition rate is slightly affected. All these experiments were conducted in the solution supersaturated with sodium oxalate and many of the experiments showed sodium oxalate was present in excess in insoluble form.

The following experiments were carried out only in the saturated solution of sodium oxalate with excess insoluble.

They are Experiment No. 7 (Fig. 6.7b) and Experiment No. 8

(Figs. 6.8 a and b). It was noticed that in these crystall isation runs, the initial decomposition rate is not affected very much by the sodium oxalate impurity.

The results of the experiments suggest that the initial decomposition rate is affected only when the sodium oxalate in the aluminate liquor reaches the level of super saturation. Furthermore, there is also a possibility for sodium oxalate to deposit on the alumina trihydrate seed particle at the initial stage of precipitation, thus slightly retarding the initial decomposition rate of alumina trihydrate in the 274

system. When the sodium oxalate reaches its equilibrium concentration 1n the aluminate solution, the rate of decomposition of alumina trihydrate takes the normal pattern as in the standard solution. It was also investigated in the previous studies (author's experiment, section

5.122) that the product trihydrate obtained from the industrial precipitation system analysed to be containing deposited sodium oxalate from the X.R.D. analysis.

In a supersaturated solution of sodium oxalate, the driving force for crystallisation of sodium oxalate has been reported as follows (114):

c = c c oxalate oxalate equilibrium supersaturation concentration solubility in liquor

They have also reported that the following equation satisfactorily described the equilibrium solubility of sodium oxalate in industrial conditions:

-2 0.011C0 ) c = 7.62 exp.(0.012T- 0.016FS- 3 equilibrium solubility

(Further details 1n section 3.5).

c) Particle size distribution

The oxalate appear to have some effect on the final 275

size distribution of the product trihydrate. This con- elusion has been made on the basis of the particle slze analysis found as shown in Figs. 6.10, 6.11, 6.12, 6.13 and 6.14. In most of the runs the product trihydrate was very much finer than the product obtained from the pure aluminate solution.

The above results contradict the results reported by Kelly (113) in his work. He found that oxalate have little effect on the particle size distribution.

It was previously found by the author that sodium oxalate is not adsorbed onto the surface of alumina trihydrate seed (section 5.2). Therefore it is suggested that due to the level of supersaturation of sodium oxalate in the solution, some of the sodium oxalate is likely to be deposited on the surface of the seed, thus retarding the crystal growth. As a result there is a possibility towards the formation of secondary nucleation, followed by the increase of the fine formation. However, fine formation was also noticed in some of the experiments where the solution was only saturated with sodium oxalate. This behaviour can be explained as due to the presence of excess insoluble sodium oxalate, they act as nuclei for the deposition of alumina trihydrate on it. d) Optical and Electron Micrograph

Microphotographs of the product trihydrate with the sodium oxalate impurity charge are shown in Figs. 6.18 and

6.19. The fine formation, due to the sodium oxalate, is 276

also noticed ln these photographs.

The same samples were analysed by the electron scannlng microscope. Some of the photographs are displayed in Fig. 6.23. It can be seen from these photographs that due to the presence of sodium oxalate, some incomplete crystal growth (appearance of small size crystallite) has taken place in these products. However, in the product crystals obtained from the crystallisation experiment without any impurity appeared to be in complete form of growth (crystallites are large). No observation was made of any deposited sodium oxalate crystals along with the product trihydrate. Brown and Cole (114) have identified the presence of needle-shaped sodium oxalate in the product alumina trihydrate obtained in the industrial precipitation system, using the scanning electron microscope. Such eo-precipitation of sodium oxalate was not observed in the author's work.

6.10.2 EFFECT OF ORGANIC IMPURITIES EXTRACTED FROM THE

BAYER PROCESS LIQUOR

The organic impurities tested in the crystallisation experiment are grouped into two categories based on the method of preparation. They are as follows:

1) Organic impurities extracted from the Bayer process liquor

with M.wt. > 1000.

2) Mixture of organic impurities extracted from Bayer process

liquor composed of lower molecular fraction, intermediate

fraction as well as the higher molecular weight fraction.

The organic impurity mentioned in Group 1 was extracted as described in Section 5.116, whereas the organics in Group 2 were extracted as described in Section 5.115.

However, most of the dibasic acids such as oxalic acids, 277

succinic acids~ etc., do not enter the organic extract

Group 2 because they are sparingly soluble in ether. a) Effect on the decomposition.. rate

The results of the precipitation curves obtained in the presence of organic compounds Group 1 (Experiments Nos. 3 and 6) are plotted in Figs. 6.4b and 6.6a.

The results indicate that 4g/litre of organ1c corn- pound of M.Wt.> 1000 (Group 1) have no effect on the rate of ~ crystallisation (Fig. 6.4b) whereas the experiment with

10 g/litre of the same material has slightly prolonged the induction period (Fig. 6. 6a). However, later it seems to follow the same pattern curve as the standard crystallisation curve performed at the same time.

Furthermore, the product yield is also not much different from the standard run (Table 6.3).

The precipitation curves obtained with the addition of organic impurity Group 2 type are illustrated in Fig. 6.9a.

The results are compared with the standard run (Fig. 6.9b) which was performed at the same time. It was noticed that the induction period decreased due to the presence of this group of organic impurities. Furthermore, a sharp increase 1n the initial decomposition rate is noticeable, but from the second hour of the process until the fifth hour, there is a sudden drop in the decomposition rate. After the fifth hour again it takes the normal pattern of the precipitation curve, as in the standard c~rve. This behaviour seems to be somewhat unusual compared to the other crystallisation experiments.

The equilibrium concentration of these curves has reached the same level of the standard at the end of the experiment.

The adsorption experiments conducted by the author, 278

described in Section 5.2 have revealed that the same group of organic extracts (Group 2) of the Bayer process liquor, were moderately adsorptive onto the seed of alumina trihydrate.

Therefore the coincidence of the results of the total experi ments suggests that the Group 2 organic extract has some poisonous effect on the rate of decomposition, although it lS not very severe. Furthermore, the results of both experi ments suggest that it is an intermediate molecular fraction of organic compound M.Wt. <1000 that is most likely to play this role in the precipitation system. It was revealed in these experiments that most of the lower molecular compounds and the higher molecular compounds (M.Wt. >1000) known to be present 1n the Bayer process liquor were already individually tested and they are playing a neutral part in the rate of decomposition, as well as in the adsorption experiments. b) The particle size distribution

The particle size distribution of final product tri hydrate obtained from the crystallisation experiments No. 3 ~

Unit 3, No. 6 - Unit 3 and No. 9- Units 2 and 4, are displayed in Figures 6.12, 6.14 and 6.15 respectively. The analysis reveals that the final products were not being affected by the presence of either type of organic impurity extracted from the

Bayer process liquor.

The optical microscopic and electron microscopic photograph taken of the final products suggests that the experi ments with organic impurity charge shows no difference to the products from the standard experiment. Scanning electron micro scopic photographs are illustrated in Figures 6.26 and 6.27.

6.10.3 EFFECT OF SODIUM ACETATE; FORMATE AND SUCCINATE

The results of the crystallisation experiments in the 279

presence of sodium acetate are displayed in Figures 6.2,

6.5b and 6.6b. The results indicate that there is no evidence as to any poisonous effect caused by sodium acetate.

The particle size analysis of the produce trihydrate obtained from the same experiments, suggest a slight tendency towards the formation of fine particles as shown in

Figures 6.10, 6.13 and 6.14. The fine formation was also observed when the same products were examined through optical microscope (Fig. 6.21).

It was noticed from the crystallisation experiments that the effect of sodium formate and succinate on the crystallisation kinetics is negligible. Figures 6.3b and

6.5b display the shape of the precipitation curves obtained with the presence of sodium formate and Figures 6.3b, 6.4b and 6.Sa show the curve obtained for sodium succinate impurity.

The examination of the results of the particle slze distribu tion of product trihydrate obtained from these runs also reveal nothing particularly striking or unique about these impurities had no effect on the precipitation of alumina trihydrate in the Bayer process. 0

(X)

150

Expt.No.

from

obtained

trihydrate

100

alumina

size)

product

charge charge

Particle

impurity impurity (~)

distribution

acetate

oxalate

size

product

Sodium Sodium Seed

Pure

Particle

6.10

x-x

v-\7

0-0

o-o

FIGURE

100

N ~

::l (]) tll ~ m 5 (]) (]) :>

.jJ u

r-1

·.-l

·.-l 0-0 Sodium succinate impurity charge A-A Sodium formate impurity charge Q-0 Sodium oxalate impurity charge X--X Pure product 0-0 - do -

(~)particle size 150 Fig. 6.11 Particle size distribution of product alumina trihydrate obtained from Experiment No. 2 lOO

0-0 Pure product 0--P Sodium oxalate impurity x--X Organic extract molecular Wt.> 1000

~-ll. Sodium succinate impurity

o\o

(]) N ·r-1 !il

1-l (]) :> 0 50 (]) :> ·r-1 +lm r-l

~ u::I

0 50 lOO 150 Particle size (micron)

FIGURE 6.12 Particle size destribution of product alumina trihydrate obtained from Expt.No. 3 w CO

150

Expt.No.S

from

(micron)

impurity

size

obtained

impurity

impurity impurity

succinate

formate acetate oxalate

Particle

product

trihydrate

Sodium

Sodium Pure

Sodium Sodium

alumina

0-0

1<-X

Cl-0

A-A

0-0

product

distribution

size

Particle

6.13

FIGURE

100

(!) 1:\1

!Jl (!)

(!) > ctl ~ l-1 > 0

()

·r-1

.-l N

150

Expt.No.

from

(micron)

size

obtained

Particle

1000

trihydrate

100

wt.>

alumina

molecular

impurity

product

impurity

acetate

oxalate

product

Sodium

Sodium Pure

Pure Organic

distribution

size

o_o

G-0

V-'if

X-><

Particle

6.14

FIGURE

100

eO ::l i> Q) i> 0 N Q) § Ul f..! Q)

.J-l u o'l'

r-l

·.-l

·.-l (X)

150

No.

extracted

(micron)

Expt.

from

size

previously

liquor

obtained

Particle

mixture

process

100

trihydrate

Bayer

impurity

the

product

-do-

alumins

-do-

from

Pure Organic

product

X-X

0-0

o-o

A-

distribution

size

Particle

6.15

FIGURE

100 286

FIGURE 6.16 Microphotograph of the product alumina trihydrate without impurity charge (magnification SOX)

FIGURE 6.17 Microphotograph of the product alumina trihydrate without impurity charge (magnification lOOX) 287

flif V 4 ~ ..0~6 ,@ _,& .,. Q 4 . 4 ,, 'iP •:'? .,4\, Ill "·. 6 10 lif.i ~ ., " # c; . 'e ~-41 # '.? >\ 0 'I'

lJ •jp

1 :;;» s"' % 4 #· Zl ' * .. "i! * (j ., 4 'i '· 'tit " -:-:. " ~ !i \ $ .At ..fJ. -- FIGURE 6.18 Microphotograph of the product alumina trihydrate with sodium oxalate charge (Expt. 6) (Magnification SOX)

FIGURE 6.19 Microphotograph of the product alumina trihydrate with sodium oxalate charge (Expt. 6) (Magnification 100X) 288

·:>b ,,£(} ~·:~ " . ·~ ·~

;:; ;~ """ Q)

FIGURE 6.20 Microphotograph of the seed alumina trihydrate (Magnification SOX) 289

ff ~ it

* f ' ~. "' "" ""'' '* (;)), 11 %

''* ~ ,. if Q 1f $'

FIGURE 6.21 Microphotograph of the product alumina trihydrate with sodium acetate charge (Expt. 5) (MagnificationSWX)

FIGURE 6. 22 Microphotograph of the product alumina trihydrate with sodium acetate charge (Expt. 5) (Magnification 100X) 290

(a)

(b)

FIGURE 6.23 Electron microscopic photograph of the product alumina trihydrate with sodium oxalate impurity charge (a) (150X) magnification (b) (300X) magnification 291

(a)

(b) FIGURE 6. 24 Electron microscopic photograph of the product alumina trihydrate without any impurity charge. (a) (lSOX) magnification (b) (300X) magnification 292

FIGURE 6.25 Seed alumina trihydrate used in the crystallisation experiments (150X) magnification. 293

FIGURE 6. 26 Electron microscopic photograph of the product alumina i::J;:'ihydrate with the organic impurity charge M.Wt. > extracted from the Bayer process liquor.

FIGURE 6. 27 Electron microscopic photograph of the product alumina trihydrate with organic impurity charge (mixture .extracted from the Bayer process liquor). CHAPTER 7

CONCLUSIONS 294

7.1 CONCLUSIONS

Organic impurities present in the Bayer process

liquor are very complex in nature. These organics were

found to originate from the humic substances present in the

soil. During bauxite leaching the organics are also extracted into the caustic solution. Some of the organics

are degraded into low molecular carboxylic and intermediate

compounds.

Sodium formate, acetate and oxalate were found to be

present as major low molecular compounds. Propionate and

valerate were found in very small quantities. Although there

were many other low molecular compounds, it was not possible

to identify them due to the lack of required techniques.

There was also evidence of the presence of aromatic carboxylic

and phenolic compounds in the Bayer process liquor. Further

more, the infra red spectra of the Bayer liquor extract was

found to be very similar to the spectra of the natural humic

and fulvic acids reported elsewhere. The total organic con

tent was found to be in the range of 25 - 30 g/litre (based

on carbon). 80% of the total organic present was below

1000 molecular weight and about 20% was greater than 1000

whilst 5 - 7% was over 10,000. Any attempt to identify

these organics over 500 molecular weight is not advisable as

it would involve a very complicated procedure. Furthermore,

with a slight change in the medium of the solution rapid

change of the structure of these organfcs can be expected.

It was found that some of the organics were adsorbed

on to the product alumina trihydrate seed. Sodium oxalate

and other groups of unidentified organics were found to be

crystallised with product alumina trihydrate after reaching 295

a supersaturation level.

The experiments on the adsorptive properties of

individual organic compounds present in the Bayer process

liquor on the product alumina trihydrate, reveal that low

molecular organic compounds such as sodium oxalate, formate,

acetate, carbonate, succinate and benzolate were less

adsorptive. Molecular weight fractions over 1000 were also

less adsorptive. Evidence was found to suggest that some

intermediate compounds were comparatively adsorptive on the

alumina trihydrate seed.

Experiments on the solubility of sodium oxalate in

the sodium aluminate liquor at varying temperatures reveal

that the oxalate in the Bayer plant liquor is present in the

supersaturated region.

It was found from the crystallisation experiments

that the presence of sodium oxalate has no adverse effect on

the kinetics of precipitation. However, it was noticed

that the particle size was slightly affected resulting in

fine formation especially when the sodium aluminate solution

was supersaturated with sodium oxalate. Sodium formate,

acetate,succinate, benzolate and the organic fraction

> 1000 M.wt. has no poisonous effect on the crystallisation

process. Some intermediate organic impurities below

1000 M.wt. have a very slight poisonous effect on the rate of decomposition, but the size of the particles was not affected.

7.2 SUGGESTION FOR FURTHER WORK

The studies conducted in this project were aimed at exploring the role of organic impurities and their effect 296

on the precipitation system with a broad v1ew as there has

not been much literature reported elsewhere. Detailed

study of each individual problem was not possible in this

work as it can be seen as a very complex problem. For this

reason the results of these studies can only be used as a

guide for further investigation which should eventually lead

to some solution in the alumina industry. The following

areas of work are therefore recommended for further investi

gation.

1) The characterisation studies of the organics present

1n the Bayer process liquor were not carried out in systematic

order as the author was unable to obtain the ultra-filtration type membranes with varying range of molecular cut-off

which should also be stable in highly caustic medium. It is

suggested, therefore, that future work be carried out us1ng

such membranes. The separation must be performed in the

caustic medium as the slight change of the medium could alter the structure of organics present in the Bayer process liquor.

2) The analysis of the low molecular compounds in this project was not successful. Some of the results (peaks) obtained from the gas chromatographic method could not be identified as there was no full range of reference samples readily available. Further detailed work undertaken in relation to the analysis of the low molecular compounds present in the liquor should perhaps be carried out using a G.C. mass spectrometer.

3) Similar work is also recommended for the organic impurities extracted from the product alumina trihydrate obtained from the plant. 297

4) There are no records whatsoever available on the solu bility of the various fractions of the organics impurities

in the caustic or Bayer process liquor~ Any data on such work would assist in evaluating which fractions of the organics reach supersaturation during the precipitation of alumina trihydrate. Further work on that aspect may be worthwhile for further studies.

5) Adsorptive properties of various organlc impurities on seed alumina trihydrate were studied, however, they were limited to some low molecular compounds and higher molecular compounds M.wt. over 1000. It was impossible to study the adsorptive effect of the intermediate organics between 200 and 1000 M.wt. separately. It is suggested that further work might be carried out if such separation can be achieved using a membrane.

6) In the crystallisation studies undertaken by the author a major drawback was the quality of the seed trihydrate used.

Seed with a very narrow range of particles should have been used. In any future work on crystallisation serious consider ation should be given to this aspect as the slight change of particle size itself can alter the kinetics of the system.

7) The crystallisation studies in this project were batch operation. It could be worthwhile considering the possibil ity of using continuous type crystallisation experiments when the results could be very applicable to industrial practice. 8) Organic impurities used for the crystallisation studies were the material extracted from the Bayer process liquor, however, in future work it is suggested organic impurities 298

desorbed from the product alumina trihydrate obtained from the plant be used. These organics could be very poisonous material.

I 299

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Basic Principles of Bayer Process Design.

Extractive Metallurgy of Aluminium, Vol. 1, 308

New York. Interscience Publishers 1963

(128) Juhasz~ A. Paper presented at the A.I.M.E. Annual

Meeting, Washington, February, 1969

(129) Hermann, E.~ and Stipetic, J. z. Anorg. Alleg.Chem., 262 (1950) 258

(130) Miers, Sir H.A., Journal of Institute of Metals, Vol. 37, (1927) p 331

(131) Vrbasky, T., Ivekovic, H. and Pavlovic D., Can. J. Chem. , 3 6 ( 19 58) p 1410

(132) Shimosato, J., Kogyo Kogaku Zasshi, 66 (1963) 172

(133) Solymar, K., and Zambo, J. Paper presented at the

symposium on Reaction Engineering, Amsterdam, 1964

(134) Jamey, M., Paper presented at the 95th A.I.M.E.

Annual Conference, New York, 1966

(135) Maricic, S. and Markovcic, I. Arhiv, Kern. 27 (1955) 41 (136) Hlobik z., Jungman - Horvart, Maricic, s., Croatica Chimica Acta, (1960) 32

(137) Scott, J., Effect of Seed and Temperature on the

Particle Size of Bayer Hydrate. Extractive Met

allurgy of Aluminium, Vol. 1 Alumina Interscience Publishers, New York 1963

(138) Misra, C. and White, E.T. Journal of Crystal Growth 8 (1971) p 172 - 178

(139) Brown, N., Journal of Crystal Growth, 16 (1972) 163 - 169

(140) Lyapunov, A.N. and Kholmogortseva, E.P. Journal of

Applied Chemistry of the U.S.S.R., v. 30 (1957) p 1306 - 1535 309

(141) Walton, A.G. The formation and properties of

precipitates, Interscience Publishers,

New York 1967

(142) Branson, S.H., Brown, D.E., Healey, G.P. Paper

presented at the symposium on Industrial

Crystallisation, Institution of Chemical

Engineers, London, April 1969

(143) Van Hall) C.E., Safaranko, J. and Stenger, V.A.

Anal. Chem., 35 (1963) p 315

(144) Private correspondence with the Technical Manager of

Queensland Alumina Ltd., Gladstone, 1977,

Analytical report of the recycle of Bayer Process

liquor

(145) Lowson, R.T., Potential -pH Diagrams at temperatures

above 298.16 0 K. Part 3 Australian Atomic Energy

Commission Research Establishment, Lucas Heights,

Oct. 1972

(146) N.B.S. Technical Note 270 - 4

(147) O.J. Kwok, Ph.D. Thesis, University of New South

Wales, Australia (1974)

(148) Pourbaix, M., Atlas of Electrochemical Equilibria

in Solution, Pergamon (1966)

(149) Hardwick, B.A. Ph.D. Thesis, University of New South

Wales, Australia (1972)

(150) Criss, C.M. and Cobble, J.W., J.Am.Chem.Soc., 86,

(1964) 5385 - 5390 and 5390 - 5393

(151) Cobble, J.W. J.Am.Chem.Soc. 86 (1964) 5394 - 5401

(152) Blaedel, W.J., Meloche, V.W. Elementary Quantitative

Analysis, Theory and Practice, 2nd Ed. p.837 - 841

Harper & Row, New York and John Weatherhill, Inc.

Tokyo 310

Appendix I

THERMODYNAMIC STABILITY DIAGRAMS .FOR THE Al~Fe~si-H 2 0 SYSTEM IN RELATION TO THE BAYER PROCESS

The Bayer process lS based on hydrometallurgical re actions wherein the first step bauxite is leached with caustic

liquor at elevated temperatures (between 150 - 225°C). In the second stage the temperature of the leached sodium alumin

ate solution is reduced to 60 - 80°C to precipitate alumina trihydrate, by adding previously prepared alumina trihydrate

seed crystals.

An attempt is made in this section to explain the

course of reactions in the Bayer process using thermodynamic

stability diagrams. Potential -pH, or Pourbaix (148)

diagrams provide a useful means of summarizing a large amount

of thermodynamic data in relation to a system composed of metal

species in an aqueous environment. These diagrams can be

used as a tool to understand the basic principles of various

hydrometallurgical processes. The utility of published

diagrams is unfortunately somewhat limited as they have been

constructed mostly for simple Metal - Water systems and the

single temperature of 25°C. Since many important processes

occur at elevated temperatures, it is desirable to have dia

grams available to depict the different equilibria that exist

at those temperatures. In cases where thermodynamic data

are not available the problem is either a) measuring the

necessary high temperature equilibria, or b) computing those

equilibria by a reliable means.

Criss and Cobble (150, 151) have advanced a 311 technique that allows the second alternative which was the one used in the present study. This technique is based on a correspondence principle relating ionic entropies at room temperature to those at higher temperature.

In the present study, data for the Al-H 0 system at 2 elevated temperatures were available from Lowson's work (145).

The Criss and Cobble (150,151) method was used to plot the diagrams for the Fe-H system at elevated temperatures. The 2o values of free energy data used in these diagrams are listed in Table I .1.

The stability diagrams are illustrated in Figures

I.1, I.2, I.3, I.4 and 1.5 and the equations relevant to the

Bayer process are shown later in this appendix.

The main constituents in the bauxites are the hydroxides and oxides of aluminium, iron, silica and titanium, in which aluminium is predominant. In the Bayer process bauxite is first leached with strong caustic liquor over

It is clearly seen from Fig. I.1 that aluminium present ln the bauxite (in this case alumina monohydrate is considered as the type of bauxite) enters the solution as the equilibrium line between Al .3H and Al0 lies just below 2o3 2o 2 pH 14. As shown from the diagrams iron does not enter the caustic solution as Fe or FeOOH is more stable in that 2o3 reglon under the conditions applied during the leaching.

At elevated temperatures, the equilibrium line between Si0 and Si0 - 2 (Fig. I.4) will be shifted towards 2 3 the left, although it is not shown ln the diagrams plotted for elevated temperatures. Presence of soluble silica which create the scaling problems in the Bayer process can be explained clearly from these diagrams. 312

In the crystallisation stage, the temperature is reduced to about 60°C. Although the stability diagrams for

60°C are not illustrated here, the equilibrium of the species can be predicted from the stability diagrams of 25 0 C and

100°C which are shown in Figures I.3 and I.4 respectively.

It is shown from those diagrams that alumina trihydrate is crystallised at just over pH 14. In order to avoid eo-precipitation of Si0 , the desilication is carried out 2 before the crystallisation stage, as such soluble silica will

be crystallised before alumina trihydrate.

Hydragillite has been considered for low temperature

diagrams, as it is the most stable hydrate below 100°C.

Therefore during the crystallisation stage, irrespective of

the raw material leached, it is the hydragillite which will

be precipitated.

The metallic aluminium region has not been consid-

ered in the diagrams as it was not relevant to the author's

studies.

H 0 System Al - 2 - (1) Al 0 + H 0 Al0 + H+ 2 3 2 = 2

25°C Log [Al0 -J 15.644 + pH (hydragillite) 2 = -

100°C Log [A10 -J 13.964 + pH (hydragillite) 2 = -

Log [Al0 -J - 13.576 + pH (boehmite) 150°C 2 =

[Al0 -J 13.490 + pH (boehmite) 200°C Log 2 = - 313

Calculations for elevated temperatures are not presented in this work due to unavailability of sufficient thermodynamic data for the Si - H 0 system. However, 2 it is understood that in general at elevated temperatures equilibrium lines will be shifted towards the left. (2b)

25°C Si0 + H 0 = Si0 - 2 + 2H+ 2 2 3

2 Log [Si0 - 27.21 + 3 J = - 2pH

Fe - H 0 System 2

( 3) FeOOH + e = HFe0 2

25°C E = - 0.982 - 0.0591 log (HFe0 - ) 2 - 100°C E = - 1. 033 - 0.074 log (HFe0 ) 2

(4) FeOOH + 3H+ + 3e = Fe + 2H 0 2

25°C E = 0.004 - 0.0591 pH

100°C E = 0.0344 - 0.0740 pH

( 5) Fe o + H 0 + 2e = 2HFe0 2 3 2 2

2 25°C E = 1.149 0.0296 (HFe0 -) - - log 2

- 2 150°C E = -1. 2 7 8 - 0.042 log (HFe0 ) 2

- 2 200°C E = 1. 396 0.0469 log (HFe0 ) - - 2 314

( 6) HFe0 - + 3H+ + 2e = Fe + H 0 2 2

25°C E = 0.496 + 0.0296 log (HFe0 - ) 0.0887 pH 2

100°C E = 0.568 + 0.0370 log (HFe0 - ) 0.1110 pH 2

150°C E = 0.618 + 0.0420 log (HFe0 -) 0.1259 2 pH

200°C E = 0.676 + 0.0469 log (HFe0 -) 0.1408 pH 2 -

( 7 ) Fe o + 8H+ + 8e = 3Fe + 4H 0 3 4 2

150°C E = 0.053 0.0839 pH

200°C E = 0.055 0.0939 pH

( 8) Fe o + 2H 0 + 2e = 3HFe0 - + H+ 3 4 2 2

25°C E = 1. 842 0.0296 log (HFe0 - ) 3 + .0296pH 2

150°C E = 2.068 0.0420 log (HFe0 -) 3 + .. 0. 0420pH - 2

200°C E = 2. 2 5 0.0469 log (HFe0 - ) 3 + 0.0469pH - - 2

( 9) =

E = 0.3 - 0.0839 pH

E = 0.31 - 0.0939 pH 315

!'J.Go Temp. 0 C Ref. Formula f K.cal.mol -1

Al o .H 0 -377.5 25 145 2 3 2 (boehmite)

Al .3H 0 -382.4 145 2o3 2 " (hydragillite)

A1o; -196.8 " 145 Si0 (quartz 192.4 " 148 2 silica)

-2 Si0 -212 148 3 "

Hsio; -228.36 " 148

FeOOH -113.166 " 147

90.3 146 HFe0 2 - "

Fe o -117.4 146 2 3 "

Fe o -242.7 146 3 4 "

H 0 (liquid) 56.688 146 2 - "

Fe 0 " 146

TABLE I.1 Free Energy Data at 25°C for the various species involved Eh------i 3

110-2 (1) 2 I I Al203.H20 1 I I I 0 - I ---- I -- (9) -- --- 5 --- HF 0 ------~2... -- -2

8 9 10 11 12 13 14 15 16 17 18 19 20 pH Fig. I.l Potential -pH diagram for Al-Fe-H o system at 200°c 2 2 f- 10- 2 10° 10- 4 I I I 1 - I I (1) I I I Al o .H o Al0 - I 2 3 2 2 I 0 f-. l I I I - Fe o ~ _(7j (9) 2 3 10- 4 - I - _I __ Fe:f -1 - 4 - I - - (5) 10° I - (8) - I I - I -2 1- I I I Fe 1 I I I ~ I I I I I I I l I I I I I 8 9 10 11 12 13 14 15 16 17 18 19 20 pH

Fig. I.2 Potential -pH diagram for Al-Fe-H o system at 150°C 2 Eh 3

2 f-

10- 2 10° 4 10- I 1 - I 1 I I I I Al0 - I I A1203.3H20 2 I 0 f-- I I I (4) I I 4 I I FeOOH 10- -1 - I (1) 10° I I (6) HFe0 - I I 2 I 10° I -2 - I Fe - I I I I I I I, I I I I 8 9 10 11 12 13 14 15 16 17 18 19 20

Fig. I.3 Potential -pH diagram for A1-Fe-H o system at 100°C 2 w ~ CO 20

10°

HFeO...,

(fil

(3)

Al0

10°

(1)

25°c

(2) .1

3H20

system

- 2

2 14

10-

Al203.

I I I I I

I I I

I lsio I I I I I

(2b)

Al-Fe-si-H

for

Sio

10-

I I

I I I I I I I I I I I I I

I I I

diagram

-pH

Potential

(

I.4

Fig.

- -

loo-

r- 1-

3 0 1

-2

-1 320

oC 200 I 4 I -2 I 10- 1 10 10° I I 180 I I I I I \ ' ' 160 I I I I I \ I \ I A10 140 I Al203 .H20 2 \ I \ 120 '\ \ \ \ \ \ '\ lOO \ \ \ \ 80 '\ \Al o • 3H 0 ' 2 3 2 \ \ \ \ \ 60 \ \ \ \ \ \ \ 40 \ \ \ \ \ \ \ \ \ 20 10 11 12 13 14 15 16 17

Fig. I.5 Temperatures -pH diagram for Al-H o system 2 321

APPENDIX II

DETERMINATION OF TOTAL ORGANIC CARBON "PRESENT

IN THE BAYER PROCESS LIQUOR

a) Potassium Permanganate Titration Method

This method was used to determine the total oxidisable organic content in Bayer process liquor, but due to the complex nature of the organic matter, the results obtained were not conclusive. The organic material in the

Bayer process liquor was not completely oxidised during the

normal period of titration. It was noticed that some of the organic present in the liquor was oxidised after the final

end-point reading was noted even with very slow rates of titration.

This method is usually used to determine the

total oxidisable organic in the water (149) and is based on

the oxidation-reduction titration with potassium permanganate

solution. Firstly, the organic substance is oxidised

with excess standard potassium permanganate (KMn0 ) in 1 m 4 sodium hydroxide for about 10 minutes at room temperature

(longer times will cause permanganate to decompose slowly to

When permanganate acts as an oxidant in strongly

basic solution, the product is the manganate ion (Mno -2 ): 4 + e - -2 but if the solution is not strongly basic, Mn0 will 4 disproportionate as follows:

3Mn0 -2 + H 0 ---Mn0 + 2Mn0 + 4PH - 4 2 2 4

Many organic compounds of low molecular weight containing 322

C = C , C- OH, C NH , = 2 C = 0 and -CHO

are oxidised to carbonate by permanganate, but some

organic compounds such as the humic substances are not

oxidised quantitatively to carbonate but yield oxalate and

other intermediates. Therefore in the titration of Bayer

liquor the solution was first acidified and a sufficient

excess of standard sodium oxalate added to reduce all the +2 manganate and permanganate still present to Mn The

excess of oxalate then present in the acid solution was

titrated with a standard solution of potassium permanganate.

In this way all of the original compound was converted to CO an d a ll o e f th +2 2 permanganat e to Mn . Th e ea l lcu a t•lOn of total oxidisable carbon could then be completed as follows:

(A) (B) (C) (D)

(equivalent = Eq.of KMn0 + (Eq.of KMn0 (Eq.of 4 4 oxalate of organic added before used in added before compound) oxidation titration) titration)

From the results obtained it was clear that this method was unsuitable for determining the oxidisable organic carbon content for any accurate purpose. This was mainly due to the complexity of the reaction of potassium permanganate in the basic medium.

The stability fields on Eh-pH diagrams for manganese water were studied to understand the role of potassium permanganate in the reaction with organic matter in Bayer process liquor. The stability field diagram for manganese species at 25°C constructed by Pourbaix (148) is shown in Figure II.1.

The boundaries of the various species are 323

numbered to correspond to the reaction mechanism given below

(only the reactions which are relevant to this work are mentioned here).

1) Mno + H+ + e = MnOOH 2

2) 3MnOOH + H+ + e Mn o + 2H 0 = 3 4 2

3) Mn + 2H+ + 2e 3Mn0 + H 0 3o4 - 2

4) Mn0 + 4H+ + 3e = Mn0 + 2H 0 4 2 2 2 5) Mn0 + 4H+ + 2e = Mn+ + 2H 0 2 2

6) Mn0 + e = Mn0 -2 4 4 2 7) Mn0 + 8H+ + Se = Mn+ + 4H 0 4 2 -2 8) Mn0 -2 + 4H+ + 4e Mn(OH) 4 = 4

9) Mn0 -2 + 5H+ + 4e = Mn(OH) -1 + H 0 4 3 2 -2 0 10) Mn0 + 6H+ + 4e Mn(OH) + 2H 0 4 = 2 2 -2 + 11) Mn0 + 7H+ + 4e = MnOH + 3H 0 4 2 2 12) MnOH+ + H+ Mn+ + H o = 2

13) Mn(OH) 0 + H+ = MnOH+ + H 0 2 2

14) Mn(OH) - + H+ Mn(OH) 0 + H 0 3 = 2 2 2- 15) Mn(OH) + H+ = Mn(OH) - + H 0 4 3 2 2 16) Mn(OH) + 2H+ Mn+ + 2H o 2 = 2 2 17) MnOOH + 3H+ + e = Mn+ + 2H o 2 2 18) Mn o + 8H+ + 2e = 3Mn+ + 4H 0 3 4 2 324

1.6

1.2-

0.8-

0.4 Mn ++ -- -- -

(16)

-1.2·~------~~

-1.6

-2 0 2 4 6 8 10 12 14 16 pH

Fig. II.1 Potential -pH diagram for the system Manganese - water at 2soc. 325

I II III IV V VI

1 .031 .099 1 20 80 8. 3

2 .031 .099 1 21 80 15.5

3 .031 .099 1 21 80 18.0 4 .031 .099 1 20 80 9. 5

5 .031 .099 1 21 81 4.5

6 .031 .099 1 21 80 4.2

TABLE II.1 Titration results

I Molarity of KMn0 4 II Molarity of sodium oxalate

III Volume of Bayer liquor used for titration (ML)

IV Volume of sodium oxalate added before titration (ML)

V Volume of KMn0 added in excess 4 VI Volume of KMn0 used for titration 4 326

Total oxidisable organic in terms A B c D of carbon g/litre

1 0.7513 2.48 .2573 1.98 9.08

2 0.8815 2.48 .4805 2.079 10.578

3 0.959 2.48 .558 2.079 11.508

4 0.7945 2.48 .2945 1. 98 9. 5

5 0.571 2.511 .139 2.079 6.852

6 0.531 2.48 .130 2.079 6.372

TABLE II.2 Calculation of Organic Content

A Equivalent of organic compound

B Equivalent of KMn0 added ln excess 4 Equivalent of KMn0 used for titration c 4 D Equivalent of oxalate added before titration 327

It is evident from the diagram that in the basic medium a number of consecutive reactions are taking place.

The main reaction is the reduction of Mn0 - to Mn0 - 2 (No.6), 4 4 but the diagram shows that reactions Nos. 1,2,3,4,8,9,10 and

11 are also possible. As a result Mno , MnOOH, Mn o and 2 3 4 Mn(OH) can also be formed. Furthermore the following 2 . -2 intermediate ionic specles are also stable, Mn(OH) 4 , Mn(OH) -2 , Mn(OH) -1 and MnOH + . However, formation of such 2 2 intermediate compounds are also dependent on the properties of the organic compounds in the Bayer process liquor.

In the second stage of the titration (after the solution is acidified) the following reactions will take place among the ionic species- Nos. 7,12,13,14 and 15.

Among the solid and ionic species reactions Nos. 5,17,18 and

16 will take place.

At the end of the titration the product is ex +2 pected to be Mn and co , but due to these intermediate 2 reactions, the end-point of the titration cannot be properly identified. Tables II.1 and II.2 indicate some of the results obtained. It is noticeable that the results are not reproducible. Therefore the above method is not useful and cannot be recommended for the determination of total organic content in the Bayer process liquor.

(b) Using Carbon-Hydrogen Analyzer - Coleman Model 33

The Coleman Model 33 Carbon-Hydrogen Analyzer is an automated combustion instrument for the laboratory determination of carbon and hydrogen ln materials dissociable at temperatures below 1100°C. The instrument is comprised of a combustion train 328

and an absorption train. The combustion train consists of a vertically mounted combustion tube and motor-driven

electric furnaces. The furnaces move to specified positions

during each phase of the instrument's operating cycle.

The absorption train consists of four glass absorption tubes containing chemical packings which contain absorption media

for C0 .H 0 and interfering substances. 2 2 One tube pretreats the oxygen sweeping gas to remove possible contamination by or water. co 2

The operating cycle of the instrument is function

ally divided into purge, combustion and absorption. During purge, a stream of oxygen displaces atmospheric gases trapped

in the combustion train. During combustion, the sample is

broken down into its elemental constituents which combine with the oxygen stream. During absorption, the gaseous

combustion products are swept into the absorption train where

carbon (as carbon dioxide) and hydrogen (as water vapour)

are selectively absorbed. Carbon and hydrogen content of

the sample are determined by weighing the respective absorption tubes. In this case only the carbon content was determined.

A measured sample of Bayer liquor was first acid

ified by adding hydrochloric acid and then evaporated until

dryness in the oven, which was set at 11S°C. The amount obtained was then weighed. A portion of the weighed dried

sample was fed into the analyser. The absorption co 2 tube was weighed before and after the experiments and the differ ence in weight indicated the carbon content as carbon dioxide.

The results obtained are as follows: 329

I Bay er liquor sample 27.5 g/litre (as carbon)

II -do- 27.2 g/litre -do-

III -do- 26.5 g/litre -do-

These results indicate that the total organic values obtained by this method are somewhat lower than the results obtained when using the Beckman Carbon Analyser.

The difference can be explained as being due to the loss of

volatile organic acids such as formic and acetic, during the

evaporation in the oven. However, the sample must be

acidified and dried in order to eliminate the carbonate which

is also present in the sample. Therefore this method is

not useful as a satisfactory method for determining total

organic carbon present in Bayer liquor. 330

APPENDIX III

THE USE OF A ZEISS MICROVIDEOMAT-2 FOR THE

ANALYSIS OF PARTICLE SIZE DISTRIBUTION OF

ALUMINA TRIHYDRATE

The Zeiss Microvideomat-2 provides a rapid means of analysing the size of particulate material. This method has been found to be less time consuming and more efficient compared to the other commonly used methods for determining particle size such as the use of sieves, sedimentation equipment, cyclosizer or Coulter counter.

The Zeiss Microvideomat-2 quantitatively analyses the images taken either with the optical microscope or with the electron microscope. The objects which are to be measured are discriminated according to their brightness.

Objects which are not required to be measured can be suppress ed. During the analysis the input is supplied from the microscope. The TV line scans the aerial image from the microscope. The operator manually selects the discriminat-

ing levels. When the brightness deviates from this level, a

square wave pulse is produced with the length of the scan

line within the discriminated image proportional to the time

parameter of the square wave. This discriminated signal pulse selected from the continuous background oscillator pulse

is further processed if required before being fed to the

digital output stage. The combination of vertical oscilla

tor pulses and horizontal TV scan line produces picture

prints, which are scanned 1n 40 milliseconds.

Stereological and photometric parameters can be 331

Parameter Required measuring parameters

Classification with erosion with D 13 inscribed hexagon (with structure analyzer) particle count

Surface area absolute object surfaces classification feature selector

Perimeter classifi perimeter surface cation (with structure analyzer) feature selector

Linear analysis in linear erosion with D 13 X2 direction particle count

Proximate analysis linear dilatation with D 14 in X2 direction particle count

Linear analysis in linear erosion with D 13 in each three directions hexagonal direction (with structure analyzer) particle count

Linear proximate linear dilatation with D 14 in each analysis in three hexagonal direction (with structure directions analyzer) particle count

Hexagonal analysis erosion with D 13 or opening with D 13 and D 14 (with structure analyzer) particle count

Hexagonal proximate dilatation with D 14 or closing analysis in three with D 14 and D 13 (with structure directions analyzer) particle count

Transmittance intensity measurements with D 12

Reflectance intensity measurements with D 12

Total absorbance intensity measurements with D 12 surface area measurement feature selector

Brightness classifi gray threshold given with D 12 cation particle count 332

Parameter Required measuring parameters

Absolute object relative object surface surface reference field size

Volume percentage relative area percentages of the of a phase different object phases

Weight percentages relative area percentage and specific of a phase weights of the required phases

Specific surface intercept count particle count

Projection length particle count reference field height

Chord length in linear erosion with D13 X2 direction

Inscribed hexagon erosion with D 13 (with structure analyzer)

Longest dimension with D 13 (with structure analyzer)

Perimeter perimeter measurement (with structure analyzer)

Average fibre relative object surface thickness (measurement in measuring slit, if necessary) reference field size

Average grain size intercept count reference field height particle count

Dilatation factor intercept count with Shape factor 90° object rotation

inscribed hexagon longest dimension (with structure analyzer)

intercept count linear erosion with D 13

Classification with longest dimension with D 13 longest dimension (with structure analyzer) particle count

TABLE III.1 Measuring facilities of the Microvideomat-2 333

determined with the microvideomat-2 e.g. absolute and relative surfaces of specific areas, lengths and distances of image areas, relative frequency and sizing of particles, intensities and brightness classifications.

Depending on specimen and problem the required measuring parameter can be determined directly by conversion or by a combination of several instrument parameters as follows:

1) The primary measuring parameters are the characteristic measuring results, which are directly supplied by the instru ment and can be interpreted without conversion (e.g. particle counts).

2) By mathematical relationships, a number of other measur ing parameters can be derived and or calculated from the primary measuring parameters, such as average diameter via intercept counts. This requires the calibration of the corresponding instrument parameters.

3) Only the combination of the primary or derived measuring parameters and different instrument parameters offers the great variety of measuring facilities (e.g. sizing).

Table III.1 illustrates most important stereological and photometric measuring facilities available using videomat-2.

The particle size distribution of the product alu mina trihydrate obtained from the crystallisation experiments in this project were analysed using the videomat-2. The maJor problem in these studies was the difficulty in obtain ing a uniform particle distribution on the slide which is to be analysed by the microscope. However, this problem was overcome by dispersing the product to be analysed in a glycerine - water solution. 334

Particle counts

Field Particle oversize 1 2 3 4

j.lm

1 43 53 32 45

5 38 50 30 41

10 36 47 26 38

15 31 45 25 39

20 29 43 20 37

25 28 42 18 34

30 24 38 18 30

35 22 35 17 28

40 16 30 13 24

45 15 28 10 21

50 12 22 6 19

55 10 18 5 13

60 9 15 5 10

65 8 13 4 10

70 6 11 2 8

75 4 9 2 5

80 3 8 3 4

85 2 8 2 3

90 2 7 0 0

95 3 5 0 0

100 0 0 0 0

Total count: 43 53 31 46

TABLE III.2 Particle size count of product alumina trihydrate 335

The particle count of oversize and the average size of the product trihydrate were determined using the microvideomat and an example of the results is shown ln

Table III.2.