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

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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 by 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 DECLARATION RELATING TO DISPOSITION OF THESIS H'-- 1 :T-:cr">~l-'A..t .~,. ~-p;·/' be· Thl.s I·s to cert1"£y that I ____ ,_,n.,_.,;,_f~." '' ur .. :t.~: •..·' ~............... :u.. <.~;J.:.c: ..... fi,J,.~ .....t<......... 1ng a .•., ; f) candidate for the degree of... ........ l::~A.:.tr-.:: .. :.................................................... am fully aware of the policy of the University relating to the retention and use of higher degree theses, namely that the University retains the copies of any thesis submitted for examination, "and is free to allow the thesis to be consulted or borrowed. Subject to the provisions of the Copyright Act (1968) the University may issue the thesis in whole or in part, in photostat or microfilm or other copying medium." In the light of these provisions I grant the University Librarian permission to publish, or to authorise the publication of my thesis, in whole or in part, as he deems fit. I also authorize the publication by University Microfilms of a 600 word abstract in Dissertation Abstracts International (D.A.I.). \ \ ' ' )\ . \ ·,, \ Szgnature ........ >v7\:·\:::.,:.c.·..... ~c •. ·'"::: ..• :: . .. :c·. ··-··;· ....... '<\ ~\ '•<,'\.\ Witness ..... .. ::.Q... :..... :... ::;:;~l>:~:.'<::~: ... ~ ......................... .. '). ' J Date ....... lJ.j/J:jJ:,/................................................... Candidate's Certificate This is to certify the work presented in this thesis was carried out in the School of Chemical Eng- ineering and Industrial Chemistry at The University of 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 tri- hydrate 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
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