The Surface Chemistry Spodumene, Lepidolite And

THE SURFACE CHEMISTRY AND FLOTATION OF

SPODUMENE, LEPIDOLITE AND ASSOCIATED SILICATES

IN THE PRESENCE OF DODECYLAMINE

A thesis submitted in fulfillment of the requirements of the degree of Doctor of Philosophy of the University of London

Wi Ifred Ch i sha Lombe

Department of Mineral Resources Engi neering Royal School of Mines Imperi a I Col lege University of London February 1983 2.

Abstract

A study has been made of the surface chemistry and flotation characteristics of spodumene, lepidolite, beryl, microcline and muscovite in the presence of aqueous solutions of dodecyI amine. The investigation included measurements of the solubility, cation exchange and eIectrokinet?c properties of the silicates. Several methods of modifying the surface chemistry of the minerals were studied and these included washing the silicates in HCI, NaOH and NaF, the addition of polyvalent metal species and the addition of starch polymers.

The mechanisms of action of these additives have been elucidated and the results critically compared with those reported in the literature.

In the absence of modifying agents, the electrophoretic mobilities of the silicates were very similar. Hallimond tube tests indicated that selective separation of the minerals, under these conditions and in the presence of amine, was not possible. Washing the silicates in HCI, NaOH and NaF produced electrophoretic mobilities that depended on the structure and composition of the minerals. Hallimond tube tests in the presence of potato and maize starches indicated that the amine flotation of spodumene could be selectively depressed at high pH.

Dodecylamine adsorbed on the silicates by coulombic attraction and the formation of hydrophobic associations, but at low concentrations, the cation exchange properties of the silicates determined the adsorption behaviour. Starch adsorbed 3.

on the minerals by the formation of hydrogen bonds between its alcoholic groups and silanol-type sites at the silicate/ water interface.

Starch and dodecylamine did not enhance the adsorption of each other at the si Iicate/water interface. They interacted in solution, by coulombic attraction and hydrophobic association, to form a complex which had a hydrophilic surface. CompIexation, rather than adsorption at the silicate/water interface, is believed to be the main mechanism of starch and amine removal from solution. 4.

AcknowIedgements

I would like to express my gratitude to Dr H L Shergold for giving me invaluable guidance and encouragement during the last three years.

My thanks are also due to my colleagues in the Mineral

Resources Engineering Department for numerous discussions, to the members of staff of the Department for assisting me at various stages of the work, and to Heather for making the scribbles readable.

I am indebted to the University of Zambia for financial support during the course of the project.

Finally my thanks are especially due to my wife, Christine, for her patience and understanding, and to our children,

Chisha and Muchindu who are too young to understand the long hours spent away from them. 5

CONTENTS

Page

Abstract 2

Acknowledgements 4

List of Contents 5

Li st of Fi gures 9

Li st of Tab Ies I 4

1 INTRODUCTION 15

1.1 Sources and uses of lithium 16 1.2 Structure of silicate minerals 17 1.3 Flotation of lithium silicates 20 1.4 Selectivity in silicate flotation 22

1.5 Surface modification in silicate flotation 24 1.5.1 Modification by polyvalent metal cations 24 1.5.2 Modification by fluoride, HCI and NaOH 25 1.5.3 Modification by starch and dextrins 26 1.6 Aims of the project 28

2 SOLUTION EQUILIBRIA AND SURFACE CHEMISTRY 30

2.1 Stability of minerals in aqueous solutions 31

2.1.1 Stability of a Iuminosi Iicates 32 2.1.2 The solubility of silica 34 2.1.3 The solubility of alumina 38 2.2 Electrical double layer effects 44 2.3 Adsorption of surfactants and polymers at the mineral/ water interface 53 2.3.1 Adsorption of surfactants 53 2.3.2 Polymer adsorption 55 2.3.2.1 General considerations 55 2.3.2.2 Polymer adsorption isotherms 57 6

Page

2.3.2.3 Polymer adsorption mechanisms 58 2.4 Aqueous chemistry of n-dodecyI amine 62 2.5 The structure and chemistry of starch 66 2.5.1 The structure of starch 66 2.5.2 Dextrins 67

3 MATERIALS AND EXPERIMENTAL METHODS 71

3.1 Materi a Is 72 3.1.1 Mi neraIs 72 3.1.2 Reagents 74 3.2 Experimental methods and techniques 75 3.2.1 Elemental analysis 75 3.2.2 Dissolution studies 75 3.2.3 Cation exchange measurements 76 3.2.4 Electrokinetic measurements 77 3.2.5 Methods of preparing dodecylamine and starch solutions 79 3.2.6 Hallimond tube flotation tests 82 3.2.7 Adsorption measurements 82 3.2.8 Viscosity measurements 84 3.2.9 Surface tension measurements 86 3.3 Analytical methods 87 3.3.1 Infrared spectroscopy 87 3.3.2 Determination of ammonia 88 3.3.3 Determination of dodecylamine 89 3.3.4 Determination of starch, dextrins and British gum 90 3.3.5 Analysis by atomic absorptiometry 91

4 DISSOLUTION AND CATION EXCHANGE STUDIES 94

4.1 Dissolution studies 95 4.2 Cation exchange studies 103

5 ELECTROKINETIC STUDIES 108

5.1 General considerations 109 Page

5.2 Effect of pH on the electrophoretic mobi lity of the silicate minerals ||0 5.3 Effect of acid and alkaline washing on the electrophoretic mobility of the silicate m i ne ra I s I 16 5.4 Effect of aging on the electrophoretic mobility of the si Iicates 125 5.5 Effect of fluoride on the electrophoretic mobility of the silicates |31 5.6 Effect of aluminium chloride on the electrophoretic mobility of the silicates 139

6 THE ADSORPTION OF AMINE AT THE SI LICATE/WATER INTERFACE AND ITS EFFECT ON THE HALLIMOND TUBE FLOTATION OF THE SILICATES 150

6.1 Adsorption studies 151 6.1.1 Effect of amine concentration 151 6.1.2 Effect of pH on the adsorption of amine by the si Iicates 162 6.2 Hallimond tube studies in the presence of amine |64

7 ADSORPTION OF STARCH AT THE SILICATE MINERAL/ WATER INTERFACE AND ITS EFFECT ON THE HALLIMOND TUBE FLOTATION OF THE SILICATES WITH DODECYLAMINE 167

7.1 Adsorption of starch polymers 168 7.1.1 Effect of equilibrium starch concentration on the starch adsorption density 168 7.1.2 Adsorption mechanism of starch 171 7.1.2.1 Electrokinetic properties of starch granules 171 7.1.2.2 Effect of pH on the adsorption of starch by spodumene 174 7.1.2.3 Effect of methylation on the adsorption density of starch 177 7.1.2.4 Adsorption of H+ and OH ions by the si Ii cate mi neraIs 180 7.2 Viscosity studies 188 7.3 Hallimond tube flotation tests in the presence of starches 188 Page

7.3.1 Hallimond tube flotation tests in the presence of potato starch 188 7.3.2 Hallimond tube flotation studies in the presence of maize starch, dextrins and British gum 194 7.3.3 Effect of starch preparation method on flotation recovery 197

8 INTERACTION OF STARCH AND DODECYLAMINE WITH EACH OTHER AND THE MINERAL SURFACE 209

8.1 Coadsorption of starch and dodecylamine on spodumene 201 8.2 Complex formation between starch and dodecylamine 208 8.2.1 Effect of concentration of amine and starch on complex formation 209 8.2.2 Binding isotherms of dodecylamine to starch 215 8.2.3 Influence of hydrophobic interactions on complex formation 219 8.2.4 Study of the starch-dodecyI amine complex by infrared spectroscopy 224 8.3 A possible mechanism of depression of the minerals by starch 232

Conclusions 239

References 246 FIGURES

Page

I.I Structure of the silicate minerals

2.1 Aqueous silicate species in equiIibriurn with a solution saturated with respect to amorphous si Iica at 25°C 37 2.2 Aqueous a Iumi ni urn species in equi Iibri urn wi th a solution saturated with respect to gibbsite

(a-AI(0H)3(s)) at 25°C 41 2.3 The Stern-Graham model of the electrical double

layer in the presence of non-specific and Rn specific adsorption --,u 2.4 Solubi lity of n-dodecyI amine as a function of pH 64 -4 2.5 Logarithmic-concentration diagram for I x 10 M total dodecylamine 64 2.6 Simplified structure of starch polymers 68 2.7 Possible dextrinisation reactions 70

3.1 Effect of pH on the effective cell'length in I0"3M NaCl 79 3.2 Calibration curve for the determination of ammon i a 89 3.3 Calibration curve for the determination of dodecylamine 91 3.4 Calibration curves for the determination of potato starch, dextrins, British gum and maize starch

4.1 Spodumene: Effect of dissolution time and pH on the concentration of Li in the leach solution 96 4.2 Spodumene: Effect of dissolution time and pH on the concentration of Al in the leach solution 97 4.3 Spodumene: Effect of dissolution time and pH on the Si concentration in the leach solution 98 4.4 Effect of pH on the aluminium concentration in the leach solution 99 4.5 Effect of pH on the Si concentration in the leach solution 101 4.6 Effect of pH on the Al/Si mole ratio in the leach solution 101 5.1 Effect of pH on the electrophoretic mobility of muscovite and lepidolite in IO~3M NaCI 5.2 Effect of pH on the electrophoretic mobility of spodumene, beryl and microcline in IO~3M NaCI 5.3 Effect of NaCI concentration on the electrophoretic mobility of spodumene at pH 9 5.4 Effect of Li CI on the eIectrophoretic mobility of lepi do Iite 5.5 Effect of LiCI on the eIectrophoretic mobility of spodumene 5.6 The effect of acid and alkaline washing on the electrophoretic mobility of muscovite in IO~3M NaCI 5.7 The effect of acid and alkaline washing on the eIectrophoretic mobility of lepidolite in IO~3M NaCI 5.8 The effect of acid and alkaline washing on the electrophoretic mobility of spodumene in I0~3M NaCI 5.9 The effect of acid and alkaline washing on the electrophoretic mobility of beryl in I0~3M NaCI 5.10 The effect of acid and alkaline washing on the e I ectrophoreti c mobility of microcline in I0~3M NaCI 5.11 Grouping of elements according to their ionic potenti a Is 5.12 Effect of aging on the electrophoretic mobility of lepi do Iite in I0"3M NaCI 5.13 Effect of aging on the electrophoretic mobility of spodumene in I0~^M NaCI 5.14 Effect of aging on the electrophoretic mobility of beryl in 10"3m NaCI 5.15 Hypothetical solubility of kaolinite at congruent dissolution as a function of pH 5.16 Effect of pH on the electrophoretic mobility of muscovite in the presence of 5 x I0~4M NaF 5.17 Effect of pH on the electrophoretic mobility of lepidolite in the presence of 5 x I0~4M NaF 5.18 Effect of pH on the electrophoretic mobility of spodumene in the presence of 5 x I0~^M NaF 5.19 Effect of pH on the electrophoretic mobility of beryl in the presence of 5 x I0~4M NaF 5.20 Effect of pH on the electrophoretic mobility of microcline in the presence of 5 x I0~4M NaF Page

5.21 Logarithmic-concentration diagram for fluoride species at 5 x IO~4M fluoride concentration 135 5.22 Effect of AICI^ on the eIectrophoretic mobility of muscovite as a function of pH 140 5.23 Effect of AICI^ on the electrophoretic mobi Iity of lepidolite as a function of pH 140 5.24 Effect of A1C13 on the eIectrophoretic mobility of spodumene as a function of pH 141

5.25 Effect of AICI 3 on the electrophoretic mobility of beryl as a function of pH 141 5.26 Effect of AICI 3 on the eIectrophoretic mobility of mi croc line as a function of pH 142 -4 5.27 Logarithmic-concentration diagram for I x 10 M AICI -z 144

6.1 Adsorption isotherms of dodecylamine on the silicates at pH 9 152 6.2 Comparison of the adsorption density and electrophoretic mobility data at pH 9 153 6.3 The surface tension of dodecylamine solutions at pH 9 161 6.4 Effect of pH on the adsorption density of dodecylamine at several surfactant concentrations 163 6.5 Effect of pH on the flotation recovery of the silicates in 3 x dodecylamine solutions 165

7.1 Adsorption density of potato starch on spodumene and beryl as a function of equilibrium concentration at pH 10 169 7.2 Adsorption density of potato starch on microcline, lepidolite and muscovite as a function of equilibrium concentration at pH 10 169 7.3 Adsorption density of maize starch and British gum on spodumene as a function of equilibrium concentration at pH 10 170 7.4 Adsorption densities of the dextrins on spodumene as a function of equilibrium concentration at pH 10 172 7.5 Electrokinetic properties of starch granules in the absence and presence of salt as a function of pH 172 7.6 Effect of pH on the adsorption density of potato starch on spodumene 175 12

Page

7.7 Adsorption density of potato starch on methylated spodumene at pH 9 178 7.8 'Adsorption' of H+ and OH on spodumene, beryl and microcline as a function of pH 182 7.9 Effect of pH on the concentration of Li+ in solution for spodumene 182 7.10 Intrinsic viscosities of starch polymers 186 7.11 Effect of 40 ppm potato starch on the flotation recovery of spodumene, beryl, microcline and lepidolite in 3 x I0~^M amine 190 7.12 Effect of HCI washing on the flotation recovery of microcline in the presence of 40 ppm potato starch and 3 x I0"5M amine 192 7.13 Effect of HCI washing on the flotation recovery of lepidolite in the presence of 40 ppm potato starch and 3 x I0~5M amine 192 7.14 Effect of HCI washing on the recovery of beryl in the presence of 40 ppm potato starch and 3 x I0"5M amine 193 7.15 Effect of 40 ppm maize starch on the flotation recovery of spodumene and lepidolite in 3 x I0"5M amine 195 7.16 Effect of 40 ppm maize starch on the recovery of microcline and beryl in 3 x I0~^M amine 195 7.17 Effect of British gum, white and yellow dextrins on the flotation recovery in 3 x I0~^M amine 196 7.18 Effect of starch concentration on the flotation recovery of spodumene at pH 11.3 and 3 x I0~^M amine 198 7.19 Effect of different methods of starch preparation on the flotation recovery of spodumene in 40 ppm solutions and 3 x I0~^M amine 198

8.1 Effect of equilibrium amine concentration on the adsorption density of amine on spodumene in the presence of potato starch at pH 9 202 8.2 Effect of potato starch concentration on the adsorption density of amine from fixed surfactant concentrations at pH 9 204 8.3 The surface tension of dodecylamine in the presence of 40 ppm potato starch at pH 9 207

8.4 Effect of amine concentration on the abstraction of amine from 40 ppm potato starch solutions at several pH values 210 8.5 Effect of amine concentration on the abstraction of potato starch from 40 ppm solutions at several pH values 8.6 Effect of amine concentration on the abstraction of starch from different starch concentrations at pH 9 8.7 Effect of the amount of amine abstracted on the amount of starch abstracted at several pH values from 40 ppm potato starch solutions 8.8 Effect of the amount of amine abstracted on the amount of potato starch abstracted from different starch concentrations at pH 9 8.9 Scatchard plot for the binding of amine to potato starch at several starch concentrations and di fferent pH vaIues 8.10 Scatchard plot for the binding of amine to potato starch at pH 10 8.11 Surface tension of dodecylamine at pH 10 8.12 Turbidity of dodecylamine solutions containing 40 ppm potato starch at pH 7 8.13 Effect of n-dodecanol concentration on potato starch uptake from 40 ppm solutions at pH 9 8.14 Comparison between the amount of amine abstracted by complex formation with the^ distribution of amine species at I x 10 M amine 8.15 Infrared spectrum of Nujol oil 8.16 Infrared spectrum of dodecylamine 8.17 Infrared spectrum of starch 8.18 Infrared spectra of potato starch-DA precipitate and potato starch-DA mixture 8.19 Wavenumber regions in which the indicated groups gave rise to absorption bands in the infrared spectra 8.20 Effect of equilibrium potato starch concentration the abstraction of starch by complex formation at pH 9 8.21 Effect of equilibrium potato starch concentration the abstraction of starch at pH 9 and in the presence of spodumene 14

TABLES

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3.1 Sources of minerals used 72 3.2 XRF analysis of the minerals 73 3.3 Analysis of the cations in the minerals studied 73 3.4 Surface areas of the fractions used in the cation exchange tests 76 3.5 Surface areas of the mineral fractions used in adsorption experiments 83

4.1 Mole ratio of soluble aluminium and silicon in equiIibri urn with the si Ii cate mi neraIs I 02 4.2 Cation exchange capacities of muscovite and lepidolite 103 4.3 Cation exchange capacities of spodumene and mi croc line 104

5.1 Effect of washing the silicate minerals in IM HCI and IM NaOH on the molar ratio, Al/Si, in solution I 16 5.2 Effect of acid and alkaline washing on the fraction (of total sample content) of Fe and Mn dissolved 124 5.3 Effect of acid and alkaline washing on the molar ratio, Al/Si, in the solid residue 124

6.I Effect of cation exchange properties on the adsorption density of DA from I x I0~4M DA solutions 156

7.1 Relative viscosities of dextrin solutions as a function of concentration at 25°C 187 15.

CHAPTER ONE

INTRODUCTION 16.

I.I Sources and uses of lithium

Lithium is found mostly in igneous rocks such as the granitic pegmatites of N.Carolina and S.Dakota. The main lithium mineral in these deposits is the silicate spodumene, although the other lithium silicates IepI do lite and petalite, and the phosphate amblygonite, are sometimes present. The most common 'gangue' minerals are beryl, quartz, microcline and muscovite (I), which are sometimes also recovered as commercial products. Commercial production of Iithiurn mineraIs took place as early as 1898 from spodumene deposits in S.Dakota but it was not until 1953, with the application of the froth flotation process at Kings mountain, N.Carolina, that spodumene became a major source of lithium (2). Some lithium is, however, obtained from brines like those of Searles lake at Trona,

California, as crude di lithium - sodium phosphate (2) but flotation concentrates of the lithium minerals remain the main commercial source of the metal.

Lithium has a high specific heat, electrochemical potential and affinity for oxygen. These properties make It suitable for use in heat transfer applications, as anode material In the manufacture of batteries and as a deoxldiser In the manufacture of copper alloys and stainless steel (2, 3, 4).

Other Important uses are in the chemical engineering industry where it is employed as a catalyst in the production of synthetic rubbers and as a base for the manufacture of a series of metallic greases of exceptional viscosity stability over a wide range of temperatures (2, 4). Lithium is also finding increasing application in the aerospace industry as an alloying agent with Mg because of its light weight, its density being only half that of water, and as nuclear fuel (2, 3).

I.2 Structure of silicate minerals

Si licates are the most abundant minerals in the earth's crust. Although they are sometimes important as commercial minerals, they also constitute the waste minerals in many ores. All the silicate minerals possess one general similarity; -4 they consist of (SiO^) tetrahedra in which a silicon atom is shared by four oxygen atoms. For each Si-0 bond in the tetra hedron the total bonding potential of the oxygen atom may not -4 be utilised. The (SiO^) tetrahedra are therefore able to polymerise, by oxygen sharing, to give a diversity of structural modifications. The different structures generally determine the chemical and physical properties of this class of minerals.

The structures have been summarised in Fig I.I.

Both spodumene (LiAISi^) and beryl (Be^A 12 (S i 60f Q)) belon to the metasilicate group in which the Si-O^ tetrahedra share two oxygen atoms. In spodumene the tetrahedra polymerise into two-dimensional chains that run parallel to the c axis. The octahedral chains, which are also parallel to the c axis, contain two cation sites Ml and M2 (5). Ml is a regular octahedron accommodating the cations in 6-fold coordination while the M2 site is a relatively distorted octahedron and is occupied by Li+ cations also in 6-fold coordination. Number of Structural unit Structural Silicate class oxygen atoms formula shared

0 (SiO, Orthosilicate

6 (Si207) " Pyrosilicates

Metasilicate chains pyroxene

Metasilicate 2 n (Sio3) n " rings

Metasilicate

2h Y (Si^O^) chains ^— (amphibole)

(Si^)2 Layer silicates

4 Three dimensional (SiC^)0 Framework network silicates

Fig I.I Structure of the silicate minerals 19.

The main feature of the beryl structure is that the silica tetrahedra close to form metasilicate rings. These 3+ 2 + rings are stacked above one another with the AI and Be cations linking successive rings. The aluminium is octahedrally coordinated while each beryllium cation occupies a distorted tetrahedron. In both spodumene and beryl there is apparently no substitution of Al3+ in the tetrahedral positions (I, 5, 6). Lepidolite (KLi AI(OH,F)_AI SiO.J and muscovite r 2 3 '0'

(KAfa S'^0|o(oh^) be long to the mica group of silicates. In this group the silica tetrahedra share three oxygen atoms resulting in two-dimensional sheet structures. The sheets formed by hexagonal rings of tetrahedra are stacked along the c axis with all the tetrahedra in one sheet pointing in the same direction.

However, the tetrahedra in successive sheets all point inward 2+ 3+ and are linked by either Mg or Al hydroxides to form a composite tetrahedraI-octahedraI-tetrahedraI (t-o-t) sheet. If 3+ 4+ there is no isomorphic substitution of Al for Si in the tetrahedral positions the t-o-t sheets are uncharged. In both

lepidolite and muscovite, however, isomorphic substitution in the tetrahedral position takes place so that the composite t-o-t sheets have a net negative charge which is balanced by + +

Li and K interlayer cations in 12-fold coordination.

The sharing of all four oxygen atoms gives rise to a

3-dimensionaI network of tetrahedra such as in quartz (SiC^) or microcline (KAISfaOg). The structure of quartz is electrically 4+ 3+ neutral but in microcline substitution of Si by Al in the te+rahedral position takes place so that K atoms are required to balance the resultant negative charge. Distinctions between several types of potassium-feldspars are usually made (7) corresponding to different structural modifications. The mi croc line used in this study was identified by XRD as a mixture of maximum and intermediate microclines, the maximum form being the major component.

I.3 Flotation of lithium silicates

Spodumene: In the flotation of spodumene from pegmatite ores both cationic and anionic col lectors are often used at various stages of the process following a wide variety of conditioning procedures. Baarson et al (8) summarised the main process details as

(a) flotation of the other silicates from spodumene;

(b) flotation of spodumene from the other silicates;

(a) Flotation of the other silicates from spodumene

Quartz, feldspar and mica minerals have been bulk-floated from spodumene with an amine collector and spodumene depressants of starch and dextrin (9, 10). The iron minerals, which are also depressed by starch and dextrin if present, were next removed (10) by flotation at low pH with sodium resinate and hydrofluoric acid as a spodumene depressant.

(b) Spodumene flotation from the other silicates

Spodumene has been floated away from the other pegmatite silicates with oleic acid in the pH range 7 to 9. Concentrations 21.

2+2 + of inorganic salts particularly those of Ca and Mg must, however, be low (II, 12, 13, 14). As In (a) prior conditioning in HF and other acids, and NaOH, is reported to aid selectivity,

The initial stage is usually the flotation of micaceous minerals with an amine collector at acid pH values. This is then followed by the flotation of spodumene from quartz and feldspar at low pH values with a petroleum sulphonate collector

(15) or in neutral pulps with oleic acid (16). The above methods, however, do not recover lepidolite which if present reports with the micas.

Lepidolite: Lepidolite is not as widely distributed in pegmatites as spodumene and its composition apparently varies considerably (17). In laboratory flotation tests (18) the mineral has been separated from quartz and feldspar with amine at a pH of 3.4. The South African Institute of Metals (19) also reported successful small scale flotation tests in which lepidolite was floated from muscovite, quartz and feldspar with a mixed amine/petroleum sulphonate collector at pH 9. Other tests (17) indicate that the mineral may be separated from muscovite by floating the muscovite with oleic acid at pH 8 following conditioning with HF.

Beryl and microcline: It is of interest to compare the above flotation methods with those reported for the other pegmatite silicates. Beryl, for example, has been (20) selectively floated from the other silicates with oleic acid at neutral pH following conditioning in fluoride solutions. Feldspars, on the other hand, have largely been recovered (21, 22) with amine collectors at low pH also following HF treatment to depress quartz.

I.4 Selectivity in silicate flotation

The main observation that can be made about the reported conditions for selective silicate flotation is that the same collectors have been widely used to float different minerals within similar pH ranges. For example oleic acid has been used to float spodumene, beryl and muscovite under slightly alkaline conditions while amines have been used to float microcline, quartz and the micas at low pH values. It is therefore apparent that each of the constituent minerals will float to some degree with both anionic and cationic collectors. This is perhaps not surprising for two reasons:

(a) Similarity in surface chemistry: On the basis of the structural properties outlined in section 1.2 it is clear that during breakage of spodumene, beryl and microcline an increasing number of SI-0 bonds wi I I be ruptured with the result that the points of zero charge, pzc, of these minerals will be low and close to each other (23, 24). It is therefore reasonable to expect a close similarity in the surface chemistry of spodumene, beryl and microcline, and perhaps between lepidolite and muscovite.

(b) Non-selectivity of collectors: The col lectors that are used to float these silicate minerals (alkyl sulphates and sulphonates, amines and oleic acid) are not particularly selective. The adsorption mechanism of such collectors at the si Iicate/water 23.

interface is mainly by coulombic attraction between the collector ion and the mineral surface bearing an oppositely charged double layer, followed by the formation of hydrophobic associations at sufficiently high adsorption densities. Several authors

(23, 24, 25, 26) have summarised the results of a series of investigations in which it has been established that for a large number of silicates (adsorption) flotation behaviour, in the presence of a Iky I sulphates, sulphonates and amines is controlled by electrostatic considerations and that (adsorption) flotation ceases if the mineral particles and the collector ion carry the same charge. Knowledge of the pzc of the silicates is therefore critical and similar pzc values are not likely to lead to selectivity of flotation.

Although oleic acid is capable of chemisorption, its mechanism of adsorption on silicate minerals has never been unambiguously defined. It is believed (24, 27, 28, 29) that fatty acids in general interact with polyvalent metal sites in the mineral surface resulting in the formation of an insoluble phase of metal soap at the mineraI/water interface. This mechanism is, however, complicated by the fact that fatty acids are weak acids, the undissociated form having a low solubility.

Under some conditions of pH, therefore, it is not possible to determine which form of the collector is the adsorbing species.

Further the carboxylate ion can also function as a counter ion and the formation of hydrophobic associates at the mineraI/water interface is possible. Even when chemisorption takes place the amount of fatty acid abstracted is often equivalent to several layers so that it is difficult to ascertain whether chemisorption is followed by a physical adsorption process or only metal soap is present (24), except in a few cases (29) where quantitative information on the metal soap in equilibrium with fatty acid has been determined. Fuerstenau and Raghavan (23) suggested that at pH 8, chemisorption of oleate was related to oleate- aluminium interactions on spodumene. Since all a Iuminosi Iicates contain aluminium such a mechanism is not likely to lead to selective flotation unless the surfaces of the silicates are selectively changed prior to the addition of the oleate.

I.5 Surface modification in silicate flotation

1.5.1 Modification by polyvalent metal cations

The pzc of most silicate minerals is at acid pH values.

Anionic collectors are therefore only used in the flotation of silicates in the presence of multivalent metal cations or other activating species. The multivalent cations specifically adsorb in the pH range where hydrolysis to low charged hydroxy complexes is possible, resulting in a surplus of positive charge at the mineraI/water interface. The main suggested mechanisms of cation adsorption are (27, 30, 31):

(a) water formation between the hydroxy I of the hydrolysed

complexes and H+ ions from the interface;

(b) H-bonding of the complex to the mineral surface; and

mineraI/water interface. 25.

Examples of these mechanisms are given in section 5.6. The mechanisms help explain how superequivalent adsorption of metal cations occurs at the mineraI/water interface although it is not known which mechanism is operative. Unfortunately most of the work on activation with polyvalent metal cations has been conducted on quartz. It is reasonable to expect, however, that similar considerations can be applied to a Iuminosi Iicate minerals, and that polyvalent metal ions will adsorb on silicate minerals under similar conditions to those observed with quartz.

Use of polyvalent metal ions is therefore unlikely to lead to improved selectivity between different silicates.

1.5.2 Modification by fluoride, HCI and NaOH

Fluoride in its various forms (NaF, HF, and NagSiF^) is one of the most widely used modifying agents in silicate flotation.

Perhaps its best known use is in the activation of the cationic flotation of feldspar from quartz at pH 2 to 3 (cf section 1.3)

(17, 23, 32, 33). In the absence of fluoride, quartz and feldspar show practically identical flotation responses in the presence of amine. Several mechanisms of action of fluoride have been suggested. These mechanisms include the formation and adsorption of aluminium fluoride (32, 33) and fIuorosi Iicate

(34) complexes at the a Iuminosi Iicate/water interface. It is also generally believed (33) that the main effect of fluoride on the silicates is to produce an increase in the negative zeta potentials of the silicates. The formation and adsorption of negatively charged aluminium fluoride and fluosilicate complexes

(cf section 5.5) help to explain how an increase in the negative zeta potential of silicates is possible in the presence of fluoride. At low fluoride concentrations, however, the formation of such complexes is not possible. Under these conditions the mechanism of action of fluoride is not known.

Relatively less controversy is associated with the effects other acids like HCI on the surface chemistry of a Iuminosi Iicates. It has generally been found that the main effect of acid washing, as in the case of fluoride, is to increase the negative zeta potential of the silicates. This has been attributed (17, 35, 36) to the selective dissolution of aluminium to leave a silica-rich surface. NaOH washing, on the other hand, does not appear to significantly affect the surface properties of silicates because of near-stoichiometric dissolution of the component oxides (36). It is not known, however, whether these observations are generally applicable to the majority of silicate minerals.

1.5.3 Modification by starch and dextrins

Organic modifying agents such as starch and dextrin have been widely used to improve selectivity in the flotation of mineral particles. Indeed there are few flotation methods in which starch, in one form or another, has not been tried (37).

Other than the above application in the depression of spodumene, starch has also been used in the amine flotation of siliceous gangue from iron ores in which the macromolecule is added to depress the iron oxides (37, 38, 39). Why selective depression is obtained in this system is not clearly understood. A number of workers (37, 40) have shown that depression is associated with the type of starch, the way it is solubilised and solution conditions such as pH and the presence of inorganic cations. It has also been demonstrated (37, 40) that hematite adsorbs more starch than quartz at pH 7. Balajee and Iwasaki (40) suggested that depression was related to the formation and adsorption of a binary starch-amine complex at the mineraI/water interface. They did not, however, present any meaningful experimental evidence to support the hypothesis. Further, they did not consider why the binary complex was hydrophilic on one mineral but hydrophobic on another (24).

Starch has also been shown (41) to depress the fatty acid flotation of calcite and barite in preference to fluorite although the greatest starch adsorption in this system occurred on fluorite. The reason for this is not known though it appears that the high adsorption of starch on fluorite could be related to enhanced formation of hydrogen bonds with this mineral compared to calcite and barite. The formation of hydrogen bonds and coulombic attraction have also been postulated as adsorption mechanisms of starch on quartz and hematite (42). In contrast

Somasundaran (43) concluded that hydrogen bond formation did not significantly affect the adsorption of starch on calcite but that complex formation between starch and surface calcium species took place. If starch has an affinity for Ca2+ species it would be of interest to know why the adsorption and flotation behaviour of calcite and fluorite are widely different in the presence of the macromolecule. The presence of starch has been found (43) to increase the adsorption of oleic acid on calcite particles and vice versa. Similarly, dodecyIammoniurn chloride has been observed

(40) to enhance the adsorpti on of British gum on quartz. It would appear that although the mineral particles adsorb more surfactant, they still remain hydrophilic. Somasundaran (43) has attributed this behaviour to the formation of a helical starch/oleate clathrate on the mineral surface. Unfortunately he did not give any experimental evidence for the existence of such a complex in his system. It is not possible, therefore, from current knowledge, to state whether starch simply inhibits the adsorption of collector at the mineraI/water interface or that there is complex formation between starch and the collector, and that this produces depression. Recently, Kitchener (44) appropriately described the present state of knowledge about the role of starch in the depression of minerals as 'inadequate'.

Clearly more work has to be done before it is possible to predict whether starch can be used to separate any particular group of mi nera I s.

I.6 Aims of the project

The main aim of the project was to study the aqueous surface chemistry of spodumene, lepidolite, beryl, microcline and muscovite with a view to predicting the conditions under which the selective amine flotation of the lithium minerals might be obtained. In particular the main aspects of the work were: 29.

(a) Determination of whether or not the minerals possessed

sufficient differences in their surface properties to act

as a basis for the selective flotation of the lithium

minerals with dodecylamine as the collector.

(b) Determination of whether or not the surface properties of

the lithium minerals can be modified so that selective

flotation can be obtained. The effects of the following

surface modification procedures were studied:

(i) acid/alkaline washing;

(ii) fIuoride/hydrofIuoric acid treatment;

(iii) the presence of multivalent cations; and

(iv) the presence of organic modifiers such as starch

and dextrin.

used in (b).

The studies involved in the project included the determination of the dissolution, cation exchange and electrokinetic

properties of the minerals. Measurements were made of the

adsorption of starch and amine at the mineraI/water interface

and the effect that such adsorption had on the flotation of

the minerals. The interaction between amine and starch in

aqueous solution was also studied. CHAPTER TWO

SOLUTION EQUILIBRIA AND SURFACE CHEMISTRY 31.

2. I ' Stab? I ? ty of mi riera IS i ri aqueous so I utions

Generally, minerals have polar surfaces because they contain atoms which are not fully coordinated. In aqueous solutions, the unsaturated atoms become hydrated.as a result of interaction with water dipoles. The extent of hydration will be determined by the difference between the energy of hydration of the ion and the specific absorption energy of the ion for the mineral substrate (45). If the ion - water bond is stronger than the ion - mineral bond the ion will pass into solution. This dissolution process eventually leads to an equilibrium of the form:

A B , . — mAl?+ . + nB ~m, (2.1) m n (s) — (aq) (aq)

Assuming a chemically pure and compositionaI Iy invariant solid phase the equilibrium can be defined by the solubility product

k m so • K;q)) K;q>r

where the brackets are assumed to denote activities of the enclosed species, which can be converted to concentrations by use of activity coefficients. If the ions do not undergo further reactions the solubility of the mineral may be calculated from the solubility product. In many systems, however, the ions undergo hydrolysis and complex formation so that knowledge of the solubility product alone is not adequate for solubility computati ons. 32.

2.1.1 Stab TIity of a f umiiibsif\cafes

There is little information in the literature on the solubility of a Iuminosi Iicates in water. This is due to the slow reaction kinetics of the dissolution reactions (46) the experimental difficulties in making calorimetric measurements

(47)and the fact that in many cases the exact nature of the complex reactions is not known. In contrast, therefore, to the dissolution of for example salt-type minerals, thermodynamic data on the free energy of formation of a Iuminosi Iicates is insufficient. Several approaches (47,48,49)have been made to circumvent the problem. V

In one such approach (47) the standard Gibbs free energy of formation, AG°^, of the silicates of interest were estimated from the standard Gibbs free energies of formation of the elements, AG° ., with the aid of an equation of the form: f,'

(2.3)

where

a, b = constants

c = the extrapolated value for the AG°^. of the mineral

of interest

x = the independent variable describing the rank of a

particular £AG° 'n w^nio corioc o-f yap.°

values arranged in descending order of magnitude.

The basis of the equation is that although a set of oxides o and/or silicates can be used to calculate AG for the formation reaction of the si licate of interest, the set is not unique. A series of EAG° . values can thus be obtained for different combinations of oxides and/or silicates. These

EAG°, . values will exhibit an exponential trend consistent with • T' equation (2.3).

A further approach also utilises the principle of con- servation of free energy to compute EAG°^. . for the formation reaction. Thus, in those cases where the standard enthalpy of formation, H° ., and the standard entropy, S?, of the constituents T , I I are known, use has been made (48,49)of the relation

SAG0, . = EAH°, . - TES? (2.4) f,i f,i i

Where data is only available at temperatures higher than

298.I5°K, the van't Hoff equation

dink = AH°

has been used (48,49) for extrapolation to room temperature.

Although in principle, it is therefore possible to estimate the solubility products of a!uminosi Iicates, rarely are the exact compositions of their solid phases known. In many minerals cations may replace each other to varying degrees with the effect that the solid phase is not independent of the solution composition (50). The free energy and solubility of a mineral has also been found to vary with its crystaI Iinity

(50) with the different structural modifications expected to give different solubi Iities (53). More important has been the 34.

observation that a Iuminosi Iicates show incongruent dissolution*

(48,50,51) because solid phases like gibbsite and kaolinite

have relatively lower stability constants. For this reason

the parent a Iuminosi Iicate is unlikely to be the solid phase

governing equilibrium solution composition. Also at low pH

the formation of a si lica solid phase may have to be

considered (48). These considerations introduce the possibility that many a Iuminosi Iicates are metastable in aqueous solutions

(50). Since the kinetics of dissolution are usually not known

it is not easy to predict which solid phases govern solution

composition. These difficulties make the detailed calculation

of thermodynamic solubility for a Iuminosi Iicates less certain.

For qualitative purposes it is often useful to consider that

the solubility characteristics of a Iuminosi Iicates are

dependent on the solubility of its two component oxides, namely

Si02 and A^Og.

2.1.2 The so Iub i Ii ty of s i Ii ca

Although a considerable amount of work has been done on

the solubility of silica, there is lack of general unanimity

in the values that have been reported. Indeed, prior to 1960

there was speculation that quartz might exhibit no true

equilibrium solubility in water at ordinary temperatures (52).

The reasons for the discrepancies are partly due to the

followi ng:

(i) Equilibrium is established only very slowly (50);

* The term incongruent is generally used if a mineral during dissolution reacts to form a new solid. (ii) The ground surface of silica possesses a highly disordered

structure of several hundred A° thick, which can lead to

abnormally high solubilities (53, 54), and

(iii) Different silica modifications have widely different

solubi Iities (53).

It appears (52),however, that certain forms of silica like crystalline quartz and amorphous silica, in which the surface layer has been removed by etching in NaOH, exhibit a solubility definable in the sense of a dynamic equilibrium between silica in solution and in the solid phase. The equilibrium reaction has been reported as (29, 52, 54)

Si02x(s) * 2H2° = Si(0H,4(aq> + Si02x-I(s) (2"6)

The reaction has been reported (53)to be retarded by the adsorption of silicic acid on the solid phase, thus obscuring true equilibrium. Further, the solubility has been found to be catalysed by the presence of (OH ) ions (52,55). The catalytic effect has generally been assumed to derive from the weakening of the oxygen bonds, due to a resultant increase in the coordination number of the silicon atom to more than four. The increase in apparent solubility at pH 9 has thus been explained on the basis of the equi librium

Si(OH)^ + OH ^ SiO(OH)3 + H20 (2.7)

between monosilicic acid and the monosilicate ion. Although it is known that further increase in the pH results in the formation of polymeric silicate species, the amount of silica which can exist in solution under these conditions has not yet

been definitely established (29).

The generally accepted view is that the dissolution of

silica in water is, in effect, a depolymerisation reaction

through hydrolysis. The 'solubility1 is then the concentration of monosilicic acid reached as a steady state in the polymerisation-

depolymerisation equilibrium.

The term polymerisation is generally used to describe the

reactions of silicic acid that result in an increase in the molecular weight of the silica, due to the condensation of

si Ianol groups (52):

- Si OH + HOSi — -SiOSi - +HgO (2.8)

This happens when the monomer, Si(OH)^, is present at

concentrations greater than the solubility of amorphous silica, and in the absence of a solid phase on which the silica might

deposit (52). In acid media or in the presence of salts, the

colloidal particles aggregate to form 3-dimensionaI gels whereas in basic solutions they remain as sols due to mutual electrical repulsion.

If it is assumed (50,54) that the concentration of mono-

si I icic acid is constant in a saturated solution of silica,

it is possible to construct the solubility diagram shown in

Fig 2.1 using the following equilibria:

Si0o, , . + 2Ho0 — Si(0H)4 log k = -2.7 (2.9) 2(s, amorphous) 2 Aqueous silicate species in equilibrium with a solution saturated with respect to amorphous si I ica at 25°C (Data from ref 50) 38.

+ Si(0H)4— Si0(0H)~ + H log k = -9.46 (2.10)

2 + + Si0(0H)~ Si02(0H) " H log k = -12.56 (2.11)

2 + 4Si (OH) .^zf: Si/10.(0H) " + 2H + 4Ho0 log k = -12.57 (2.12) 4 4 6 6 2 a

The solubility of monosilicic acid/amorphous silica is used to represent the upper limit of dissolved silica because the transformation of amorphous silica to quartz only occurs in geological time. Although further multinuclear species other

2- than Si^OgtOH)^ are possible, there is some uncertainty concerning their exact nature. 2.1.3 The solubility of alumina

The Al20^ - H20 system is usually considered with gibbsite

(a - AI(OH)-., ) as the solid phase because this is the stable 3(s) r form of aluminium hydroxide at ordinary temperatures. The dissolution of gibbsite is complicated by the extensive

3+ hydrolysis of the Al cation, particularly in acidic media.

Although this hydrolysis is perhaps one of the most studied, there has been marked disagreement on the formulae of the species that are formed. Controversy has arisen because of the severe demands placed on the accuracy of the data so that distinctions can be made between the various possible species present (56, 58).

In earlier experiments, like those of Schofield and Taylor

(57), the kinetics of dissolution were not followed. It is now recognised that although the formation of the initial hydrolysis products at low ligand number*, n, is rapid (58), the formation of large polymers at higher n values, and the precipitation of the hydroxide are kinetically slow processes

(56, 57). It is likely, therefore, that in those experiments in which a low aluminium concentration was obtained by dilution from more concentrated solutions, a metastable state with regard to the precipitation of gibbsite probably existed. Frink and Peech (59) have published data on the first hydrolysis constant which decreased on dilution, due to this effect.

Further, the extent of hydrolysis will vary with the concentration of aluminium, so that at these dilutions, the hydrolysis product may not be uniquely defined (58).

In view of the numerous, and sometimes disconcordant data that have been reported, the following general basis was used to establish the solubility diagram presented in Fig 2.2.

2+ + (i) The mononuclear species, AI(OH) and AI(0H)2 are significant at relatively low metal concentrations where they are soon followed by precipitation of the hydroxide (58). In

* For acidic and basic aqueous solutions, the ligand number, n, is defined as (58)

+ n = [h ] - rri|_| and n =. .MQH - [oH^j respectively,

where m^, itIQ^ = analytical concentrations of acid and base [H+] , = measured (equilibrium) acidity or basicity

mm = concentration of the polyvalent metal species, m

At neutra H, the complete expression for n is

- n = - [OH ] - MH a significant contribution, Nazarenko and Nevskaya (60) proved -5 that at 5 x 10 M total aluminium concentration only the

2+ AI(0H) , AI(OH)* and AI(0H)3 species were present. Of these

AI(OH) has been reported by a number of authors with the data being reasonably consistent. On the assumption that for all n values, at low metal concentrations only monomeric species are present, Stol et al (61) have synthesised a titration curve that agrees reasonably well with experimental data.

(ii) With increasing aluminium concentrations, the bulk of the evidence indicates the predominance of the dimeric species 4+ A^tOH^ over monomers (56, 58, 6i, 62). There is relatively 5 + less agreement for the species AI^(0H)^ although statistical factors (58) have shown that it probably exists. There is, also, general unanimity (56, 58, 61) that at high aluminium concentrations, n approaches values of about 2.5. This has resulted in limiting the number of possible species with this ratio to one containing 14 + I AI atoms (58). The species

AI|^(OH)^ been considered more likely on the evidence of its existence in basic aluminium salts (58) and X-ray data

(56). The intermediate polymeric species are not widely supported, partly because of their absence in similar hydrolys schemes (56, 58).

(iii) The presence of AI(OH)^ in basic media has not been di sputed.

The data (50)used to construct the solubility diagram in

Fig 2.2 are presented below: >I » TOX

cn o

Fig 2.2 Aqueous aluminium species in equilibrium with a solution saturated with respect to

gibbsite (a - AI(0H)3(s)) (Constants from ref 50) log k at 25°C

Al3+ + HgO ^ AI(OH)2+ + H+ k, = -4.97 (2.13)

3+ + + A| + 2HgO Al (0H>2 2H $2 = -9.30 (2.14)

3+ + A| + 3Ho0 ^r AI(OH),, , + 3H 6, = -15.00 (2.15) 2 3(aq) 5

3+ + A| + 4HgO — AI(0H)4 + 4H 64 = -23.00 (2.16)

2A13+ + 2HgO AI^(0H)g+ + 2H+ B2 2 = "7,7° (2.17)

3+ + + B = 13 94 3AI + 4H20 ^ AI3(0H)4 + 4H 4 3 ~ - (2.18)

3+ 7 + I3AI + 32H20 Al|3(0H) 2 + 32H B32 |3 = -98.73 (2.19)

+ 3+ AI (OH).., , + 3H ^T A| + 3Ho0 k = 8.50 (2.20) 3(s) 2 so

The figure shows that the concentrations of the large polymeric species are significant only at low pH values. In

general, the concentration of a I I the species, except AI(0H)4, decreases with an increase in pH. The region where precipitation -4

of gibbsite is thermodynamicaIly possible in a I x 10 M AICI3

(used in the experiments) is bounded by the vertical dashed lines on the diagram. At high pH values the aluminate ion,

AI(0H)4, predominates and accounts for the increase in solubility with pH at pH values above about 8.

The stability diagrams shown in Figs 2.1 and 2.2 can be used to make qualitative predictions of the behaviour of a Iuminosi Iicates in aqueous solution. For example, at high pH it is reasonable

to expect the solubility of a Iuminosi Iicates to increase as

both oxide components are relatively unstable under these

conditions. A further use is that they indicate the limiting

solid phases which might be present in a saturated solution.

Thus the concentrations of aluminium or silica in solution

would not be expected to exceed the solubility limits of gibbsite

or amorphous silica, respectively, without the precipitation of

these soli d phases.

It is known, however, that the presence of aluminium ions

in solution lowers the solubility of silica (63). It has been

postulated that this is due to the formation of aiuminosi Iicate

sites on the surface of si lica which inhibit further dissolution.

The presence of soluble silica has also been reported (63) to

lower the solubility of alumina, due to a simi Iar effect. At

low silica concentrations, however, this does not appear to be

the case.

Brace and Matjevic (64) studied the coprecipitation of

silica with aluminium hydroxide. Their results showed that

the precipitation boundaries of aluminium solutions in the -4 -3

presence of low silica concentrations (<5x10 -2x10 M)

were typical of those established for gibbsite.

At higher silica concentrations (> 2 x I0~3M), however, the

precipitation pH range was wider due to the coprecipitation of

silica and alumina in the pH range 2-10. They related this

behaviour to the ease of polymerisation of silicic acid in this

pH range. Hingston and Raupach (65) also concluded, from a

study of the kinetics of adsorption of silicic acid on aluminium hydroxide, that the adsorbed layers could form by polymerisation of si I icic acid on the hydroxide surface.

These and other studies leave little doubt that dissolved si Ii ca is adsorbed on a Iumi na (and a Iumi ni urn on si Ii ca) particles, particularly in weakly alkaline solutions, forming an a Iuminosi Iicate surface. The interaction has been postulated to be of the form (24, 64, 65

Al(OH), + HOSi(OH) Al(0H)20Si(0H)3 + H20 (2.21) surf surf

If gibbsite, therefore, precipitates from a saturated a Iuminosi Iicate solution, the surface of the colloidal particles is likely to be covered with an a Iuminosi Iicate layer, whereas if the silica concentration is high coprecipitated a Iuminosi Iicates are feasible.

2.2 Electrical double layer effects

When a mineral is added to water its surface acquires an electrical charge due to the ionisation of surface sites or the preferential adsorption of ions. Since the system as a whole must remain electrically neutral a layer of oppositely charged ions (counter ions) forms in the solution adjacent to the mineral surface. The combined effect of thermal agitation and electrical forces prevents the counter ions forming a compact layer at the solid surface and produces a diffuse layer in which the concentration of excess counter ions decreases in an exponential manner with distance from the solid surface. For each class of minerals the ions which give rise to the surface potential referred to as potential determining ions (PD1) are unique. For salt-type minerals 2+ they are the ions which make up the ionic lattice, e.g. Ba

2- and SO^ are potential determining for BaSO^. For oxides they are considered to be H+ and OH . Addition of a freshly crushed oxide surface to water can be considered to result in the formation of a surface acid whose dissociation characteristics wi I I be dependent on the pH, thus,

MOH , — MO (2.22) surf surf

MOH MOH (2.23) surf 2 surf

where M denotes a surface metal site. At a particular pH the net charge on the surface will be zero. This pH value is known as the point of zero charge (pzc). A distinction is often made between the pzc and the pH at which the zeta potential is zero. The latter condition is known as the iso-electric point (IEP) and it equals the pzc in the absence of specific adsorption.

Parks (66) has postulated a different mechanism for the charging of oxide surfaces which involves partial dissolution of the oxide and the formation of hydroxylated metal species in solution, followed by adsorption of these complexes. For example, for the metal oxide MO: 2M(0H) (2.24) 3(aq)

M(OH) (2.25) (aq)

3-m M(OH) (2.26) m(aq)

Distinguishing between these two mechanisms is not possible because both will involve the same change in pH, and in the latter case, the soluble metal species will be below the limits of detection in the pH range of interest.

The dependence of on the concentration of the potential determining ions is given by the Nernstian expression:

a/ t = kT In a,o (2.27) o ze— where

k = Boltzmann's constant

T = absolute temperature

z = valence of potential determining ion

e = electronic charge

a = activity of PDI

aQ = activity of PDI at the pzc.

The classical statisticaI-mechanicaI theory of the electrical double layer was independently proposed by Gouy and Chapman. The basic assumptions made were that: 1 the solid surface is flat

2 the counter ions are point charges

3 the solvent dielectric constant applies up to the particle

surface, and

4 in moving a charge from solution to the surface only

coulombic work is involved.

The Boltzmann distribution law was assumed valid:

n.(x) = nQ exp(-z.et(x)/kT) (2.28)

where n. = concentration of species at a point (x) in

solution where t = ^(x)

n = concentration of ions in the bulk solution, o

From (2.28) and the Poisson equation, the charge density at the interface is given by the Gouy-Chapman equation as:

a 2n ekT s f o *) sinh( f^o) (2.29) ^ 7T ' 2kT where a= surface charge

e = dielectric constant of the solvent.

At low potentials, i.e. zetQ << 2kT or tQ << 25mV the potential at a point x from the surface is given by:

ip = ijj exp(-icx) (2.30) Equation (2.30) shows that the potential decreases exponentially with distance from the surface, '/K is referred to as the

'thickness' of the double layer and is given by the equation

2 / (2.31)

The total ionic concentration, I, is given by

2 (2.32)

2 Setting n z = 21 in equation (2.31) gives

2 /k (2.33)

Hence the thickness of the double layer is inversely related to the ionic strength. As the ionic strength is increased,

drops more rapidly.

The Gouy-Chapman theory of the diffuse double layer suffered from a number of defects, the most notable being its inability to recognise effects derived from the

'discreteness of charge' and specific adsorption. The quantity z.eifKx) in the Boltzmann equation (2.28) interpreted as the work done in bringing the ion from infinity to a point x in the diffuse layer is not unambiguous. When the ion is inserted at point x, the surrounding charges will rearrange so as to maintain a 'self-atmosphere' (67, 68). The contribution due to this discreteness-of-charge effect to the mean potential as defined in equation (2.28) is not often i ncluded.

Secondly, an ion in aqueous media is solvated, carrying a strongly bound inner hydration shell and an outer sphere in which the water dipoles will be oriented. Further, the solid surface itself will carry at least one layer of oriented water molecules. Clearly such an ion can only approach the surface to within a finite distance.

To overcome these defects, Stern (69) proposed a model for the inner part of the double layer in which some ions might be assumed to be anchored to the surface through close-range forces. The model was later refined by Graham

(70) who distinguished between the locus of closest approach of hydrated ions (the Stern plane) and that of specifically adsorbed ions, which are expected to be partially dehydrated at least in the direction of the surface. Graham called the two planes the outer and inner Helmholtz planes, OHP and IHP, respectively. Specificity in this sense means adsorption which depends on the nature rather than merely the charge on the surface or ion (67). Further, it can represent adsorption either in deficit or excess of the amount which would be expected at the surface from simple coulombic considerations.

Thus for finite adsorption, a|Hp < aQ, and for superequivalent adsorption a^p > cM^I) (see Fig 2.3 for notation). Specific adsorption depends on several parameters; the most important is generally considered to arise from solvation effects with those ions that are bereft of primary hydration shells showing greater specificity. 50

Stern Gouy layer layer b I HP OHP I © A© % o

9n© ©co-ion s 9©

i Cl) i© © ucounte r ions i© ©i specifically adsorbed ions

(a) (b)

Fig 2.3 The Stern-Graham model of the electrical double layer in the presence of (a) non-specific and (b) specific adsorption 51

To account for the above considerations the Boltzmann distribution law may be modified, so that:

n. (x) = noexp(-(z.eiMx) - <$>)/kT) (2.34)

where (J) = term allowing for specific 'chemical1 forces.

Although the successful application of the Stern-Graham electrical double layer of Fig 2.3 to quantitative interpretation of experimental data is widely reported (see also section 2.3.1), it must be noted that there probably exists no unique description of chemical and electrostatic energies of adsorption at the oxide surface, with different models being capable of fitting the same experimental data (72).

In the case of silicate minerals, the formation of an electrical double layer at the solid/liquid interface is considered to be controlled by the broken bonds of both

Si-0 and M-0 oxides, where M is Al for a Iuminosi Iicates such as those used in this study. Thus when immersed in water the surface of these minerals may tend to behave as that of a composite oxide with H+ and OH being potential determining

(23,24):

S i 0H | - — -Si-0 + H+ (2-35)

surf surf

+ H+ , -H+

M —OH ^ —M OH ^ —M 0 (2.36)

surf surf surf Although equations (2.35) and (2.36) are analogous to the charging mechanism for simple oxides given before, it is doubtful that the Nernst equation is valid for these complex oxides which may show progressive degradation in solution, rather than a reversible-electrode type of equilibrium.

Because the surface properties of silicates are a function of the number and type of bonds broken at the surface, a number of studies have attempted to rationalise these properties on the basis of crystal-chemicaI considerations. Lai (73) and

Deju and Bhappu (74) postulated that the IEP of silicates is a function of their O/Si ratio and that as the ratio increased more oxygen was available for bonding to cations other than

Si, therefore the IEP increased. On the other hand, Smolik,

Harman and Fuerstenau (36) considered the IEP to be related to the Al/Si ratio in the cleavage plane, and used this relationship to explain their streaming potential results on sillimanite, kyanite and andalusite. At the finer particle sizes used in electrophoresis experiments, they argued that the fractional

Al/Si bond density in the bulk influenced the IEP more than at coarse sizes because the particles had a more random surface arrangement of atoms due to extended grinding.

The results of these and many other studies (66) suggest that the approximate IEP of a limited number of a Iuminosi Iicates can be empirically predicted, as long as extensive isomorphous replacement has not occurred. Thus in the case of the non- sheet silicates used in the present study, microcline may be expected to have a low IEP (about pH 2), reflecting an increased tendency to rupture the Si-0 bonds during grinding (23). For 53

spodumene and beryl the predicted IEP are also low but somewhat higher than those of the framework silicates (23,24). For the sheet silicates, however, isomorphic replacement is likely to be extensive so that similar predictions cannot be made (66).

2.3 Adsorption of •' surfactants and po I ymers at the minera I / water i riterface

In the flotation of the lithium silicate minerals amines are often used as col lectors and starch or dextrin is added to improve the selectivity. It is therefore useful to briefly consider how surfactants and macromolecules are adsorbed at the mineraI/water interface.

2.3.1 Adsorption of Surfactants

The adsorption of long chain ionic surfactants in the

Stern layer can be represented by a Langmurian type of equation of the form:

0 + | = C exp ((zet0 4>)/kT) (2.37)

where 0 = the fractional surface coverage,

c = surfactant concentration in bulk solution,

(f> = the specific adsorption potential, if specific chemical interactions are not dominant then adsorption will be determined largely by the magnitude of the surface charge. Adsorption at the pzc where i|> zero generally indicates that the adsorption energy is dependent on some specific interaction term, . Another way of representing equation (2.37) is in the form:

r = 2rC exp (-AG° , /RT) (2.38) r ads

where T = the adsorption density,

r = the radius of the adsorbed counter ion,

Aâds = sân<ârc' free energy of adsorption.

Fuerstenau and his associates (75-77) have used this Stern-Graham type equation to explain the adsorption of long chain amines, a Iky I sulphates and a Iky I sulphonates at the oxide/water interface. In these cases adsorption occurs predominantly under the pH conditions where the surfactant ions can function as counter ions. At low surfactant concentrations the ions adsorb by ion exchange with other counter ions in the diffuse layer. Under these conditions the Stern potential (zeta potential) is assumed to remain constant. As the bulk concentration is increased, the adsorbed ions begin to form hydrophobic associations which then provide an additional adsorption mechanism. The onset of hydrophobic interactions produces a marked increase in the adsorption and results in a reduction in the magnitude of the Stern potential and charge reversal. Patches of associated surfactant ions have been termed hemimice lies by

Gaudin and Fuerstenau (78) and the concentration at which they form the critical hemimice lie concentration, CHMC, by analogy with the CMC in bulk solution.

In this type of adsorption process, AG^^ can be subdivided

(75,79) as follows: AG° = AG°. , + * (2.39) ads elect

where = the electrostatic contribution to adsorption,

± zeif/ .

Although 4 's made up of a number of contributory terms (see for example ref. 75), it appears that the main contribution arises from van der Waai's-type interactions between the hydrocarbon chains, AG° . Values of approximately -kT have 'HM' been obtained (76,77) for AGj^. This is of the same order of magnitude as that obtained from miceI Iization studies in bulk solution.

An alternative theory of ionic surfactant adsorption is that proposed by Cases et a I (25,80). Adsorption of the surfactant is assumed to occur by 2-dimensionaI phase changes, initially on high energy lattice sites, and later on the deposited first layer. The adsorption isotherm is based on the Fowler adsorption model, and predicts that at the onset of condensation, an infinite slope is observed but as the second layer condenses onto the first, the slope will tend to zero.

An interesting model which incorporates both the Fuerstenau and Cases approach has recently been proposed by Scamehorn et al (81).

2.3.2 PoIymer adsorption

2.3.2.1 Generai considerations

The factors generally considered to be important in polymer adsorption are summarised below (82,83): 56

(i) Equilibrium is established slowly. This is because a large number of configurations at the solid solution interface are possible, each polymer molecule possessing large numbers of potentially adsorbing groups with a high degree of mutual independence. The probability of simultaneous adsorption or desorption of the groups is therefore low.

(ii) Polymers are adsorbed more strongly from poor solvents than from good solvents. Further, polymers with polar groups are readily adsorbed by polar adsorbents. Examples include the adsorption of polythene oxide on silica (84), starch on some salt-type minerals (41) and hydrolysed polyacryI amides (85) on quartz, alumina and other minerals.

(iii) The extent of adsorption of polymers is generally a function of molecular weight, if adsorption occurs on a non-porous surface, the maximum amount of adsorbed polymer, A , is ' max' related to the molecular weight, M, by

A = kMa (2.40) max

where k and a are constants. Apart from the nature of the adsorbent, the molecular weight effect depends to a large degree on the critical miscibility temperature (82,86). In the region of the theta temperature (or in theta solvents) higher molecular weight fractions are only sparingly soluble whereas above the theta temperature (or in good solvents) the effect is diminished.

(iv) The adsorbed phase is regarded as a fully extended linear chain lying on the particle surface. Although several 'monolayers' are common, the concept of monolayer cannot be strictly applied to the adsorption of polymers. For example, in good solvents many polymers wi II exist as expanded random coils with a much larger radius of gyration than that of smaller coils in poor solvents. If adsorption occurs from good solvents, relatively fewer anchor segments are utilised; the adsorbed layer is also of considerable thickness, with many loops and chains fanning out into the solution phase. For this reason some authors consider zeta potential measurements in the presence of polymer solutions to be of limited value, due to the uncertain effect of the loops on the position of the slipping plane (87).

2.3.2.2 Polymer adsorption'isotherms

The most commonly used isothern in polymer adsorption is the Langmuir equation or modifications of it. For example

Fuerstenau and Wie (88) assumed the Langmuir isotherm in order to estimate the standard free energy of interaction between molybdenite and dextrin. Frequently, however, modifications of the form

= KC (2.41) v(l- )V

have been introduced to account for the fact that the polymer is attached by v segments to the surface. C is the bulk solution polymer concentration and K is a constant.

Next various statistical assumptions may be introduced into equation (2.41). One such resultant model is that due to

Simha, Frisch and Eirich (89): 2k, 0 0 e /v I^CC) (I - 0) (2.42)

where kj, k^ = constants.

The main assumptions are that

Ci) the polymer forms localised monolayers on the surface

with each active centre binding only one segment,

(ii) the polymer solution is infinitely dilute, and

Ciii) there is a Gaussian distribution of end to end distances;

the polymer molecule being considered to be a flexible

random coiI.

The values predicted by equation (2.42) are generally lower

than those derived from the Langmuir equation.

Further theoretical treatments have been proposed, notably that due to Silberberg, in which it has been recognised that the assumption that v is independent of 0 is unlikely to be valid. Lipatov et al (82) have summarised some of the approaches.

The complexity of the equations makes their utilisation difficult and their inclusion in this work has therefore been considered unnecessary. In view of the complicated theoretical treatments it is somewhat embarrassing that polymer adsorption data appear to fit the simple Langmuir equation as well as any other.

2.3.2.3 Polymer adsorption mechanisms

It is well established in flocculation studies that aggregation of finely divided suspensions occurs predominantly through an interparticle bridging effect by the adsorbed polymer chains. In contrast, the polymer adsorption mechanisms that 59

make possible the bridging effect are diverse in character. This is probably due to the fact that adsorption in polymer systems is influenced by many factors which include, for example, the presence and type of functional groups in the structure, and the molecular configuration which the polymer molecules adopt in a particular solution environment. For example, it is known that the adsorption characteristics of a non-ionic polymer can be significantly improved by incorporating a small proportion of ionic groups in the polymer chain, which serve to increase the mean extension of the chain, whereas the ionic state of a polyelectrolyte can inhibit its adsorption at the solid/liquid interface (90,91).

The term polyelectrolyte is commonly used for polymeric flocculants to distinguish them from inorganic coagulants like alum. Although many polymers, both synthetic and natural products, are polyelectrolytes, there are many examples of others which have no electrolyte character like polyethylene oxide

(PEO) and unmodified starch (87). In the case of starch, it appears that under some conditions of pH, ionisation of the hydroxy Is present in the structure can lead to substantial electrolyte character.

Although numerous polymer adsorption mechanisms have been proposed in the last few decades, the important considerations relevant to the mechanisms are relatively few and are summarised below (85,87,91,92).

(a) General electrostatic forces

As a general rule, an ionised polymer is always strongly adsorbed on a surface of opposite charge and also tends to be 60

repelled from one of the same charge. Schulz and Cooke (37), for example, observed that substitution of aminoethyl groups in starch markedly increased its adsorption on quartz. A similar effect was reported by Balajee and iwasaki (42) although they did not specify the functional group(s) involved. Numerous other examples of this mechanism have been reported particularly with hydrolysed polyacrylamides (PAM).

Electrostatic attraction can also occur on surfaces bearing a charge of the same sign as the polymer provided that charge

reversal is first obtained at the mineraI/water interface by the specific adsorption of an oppositely charged species. Since heavy metal cations can produce charge reversal on quartz and silicates, they facilitate the adsorption of anionic polymers on these minerals. Unfortunately it is often not possible, in such cases, to determine whether the cationic species forms a chemical bond with the polymer or merely acts by reversing the

Stern potential,

(b) ' Cherhica I bond formation

In principle it is feasible for polymers to adsorb by a chelating or coordination reaction depending on the type of functional groups they carry, e.g. poIyethylenimine on copper carbonate minerals (87). In practice, however, there are few high polymers known to adsorb by this mechanism (91).

Somasundaran (43) suggested that starch adsorbs on calcite by complex formation between the polymer and surface calcium species. it is reported (93) that hydroxides of multivalent cations can interact with carbohydrates through hydroxylic proton abstraction, to form cation-containing chelates or 61

alcohola+es. For example Fe^+ has been reported to form stable chelates with sugars In mildly alkaline solutions (93). This might explain why starch Is abstracted in greater quantity on hematite than on quartz at alkaline pH values (42), although it

Is also possible that this Is due to differences In surface potentials of the minerals, and hence different degrees of mutual repulsion.

Cc) Hydrogen bonding

This mechanism is common with polymers in which non-ionised

functional groups such as -OH and -C0NH2 form H-bonds with surface si lanols or other oxide/hydroxide sites on the mineral surfaces. It has been suggested, for example, that PEO (84),

PAM (94) and polyvinylalcohol (PVA) (95) adsorb on isolated silanol sites on silica by H-bonding, and that adsorption is inhibited by extensive hydroxy I ation such as that present on aged silica. It has also been found (84) that adsorption Is strongly dependent on the extent of the Ionisatlon of the silanols, decreasing with increase of pH. This suggests that the bridging proton Is donated by the si lanol sites.

In the case of starch polymers, a hydrogen bonding mechanism has been postulated for the adsorption of starch on salt-type minerals (41) and quartz (42). Somasundaran (43), however, suggested that the hydrogen bonding of starch on calcite is probably negligible,

(d) Hydrophobic bonding

This occurs in those cases where some or all of the functional groups have sufficient amphipathic character to lead to Interaction with a hydrophobic particle surface. Rubio and Kitchener (84) and Tadros (95) postulated that the adsorption of PEO and PVA, respectively, on silica was aided by hydrophobic associations between ethylene groups and si loxane bonds on the surface. It has also been suggested that dextrins adsorb by hydrophobic interactions on molybdenite (88) and coal (96). In these systems adsorption capacity appears to be little influenced by pH.

(e) Dipolar'interactions

In their studies on the adsorption of PAM on fluorite,

Slater and Kitchener (92) postulated that adsorption may occur by the interaction of dipoles of the amide group with the electrostatic field on the mineral surface. In a later paper, however, these authors suggested that further experimental verification of the mechanism might be required.

Although the above adsorption mechanisms may individually apply in some systems, in many others they compliment one or the other. For example, Tadros, and Rubio and Kitchener provided evidence that PVA and PEO adsorbed on si Iica by both H-bonding and hydrophobic interactions. Balajee and Iwasaki also suggested that starch adsorbed on negatively charged quartz by a combination of hydrogen bonding and electrostatic forces, if cationic functional groups were present in the starch. Finally,

Somasundaran proposed that the adsorption of starch on calcite

involved both electrostatic attraction and a complexation reaction.

2.4 Aqueous chemistry Of n-dodecyI am? ne

The effectiveness of alkylamines as collectors in flotation systems is dependent on the solution chemistry of the amine group as well as the size of the hydrocarbon group. With the exception of quarternary compounds, amines are weak bases so that depending on the pH both cationic and neutral species will be present in solution. Smith (97) has shown that the coadsorption of both amine species produces the most hydrophobic surfaces.

Dodecylamine (DA) is a 12-carbon primary amine whose

formuI a may be written as CHg.CH2(CH2)Q.CH2.CH2NH2. The diameter of the charged head is reported to be about 3.7°A (26). The dissolution and hydrolysis of a primary amine can be represented as (26):

RNH,+ RN1-L, . + H+ K = 2.5 x I0"H (2.43) 5 2(aq) a

4 RNHor . + hLO — RNH* + Off K. = 4.3 x I0~ (2.44) 2Caq) 2 3 b

+ 9 RNhL, . + Ho0 ^RNH, + 0H~ K = 8.6 x I0" (2.45) 2(s) 2 3 so

5 RNH2CS) - RNH2(aq) Ks|=2xl0" (2.46)

A mass balance on amine in a saturated solution gives

S"[RNH2(aq)] + bV] (2"47)

where S is the solubility. Combining equations (2.43), (2.46) and (2.47) gives

S = K (I + [H+] ) (2.48) S| K a

This equation is expressed graphically in Fig 2.4. With the combined use of the above equations, the logarithmic/concentration diagram of Fig 2.5 can also be constructed. At pH values below 64

1 1 1 r

Solubi Iity Ii ne

c O

(D +- C 0o o£Z o

CD O

Fig 2.4 Solubility of dodecylamine as a function of pH

CD o

Fig 2.5 Logarithmic concentration diagram for I x IO~4M total dodecylamine -4 + 10.6, in a I x 10 M solution, the cationic amine ion (DA ) predominates and at pH values below 10 precipitated amine will be absent. The concentration of soluble unionised amine decreases logarithmically with a decrease in pH from pH 10.

Recently, it has been argued (98,99) that pre-micellor associations and ion-molecular complexes enhance surface activity and hence significantly contribute to the flotability of minerals. A number of properties of surfactant solutions, such as electrical conductivity, have been reported (98,100) to deviate from their expected behaviour in sub-mi cellar concentrations, particularly for higher homologues. These deviations have been reported (100) to be accounted for by dimerisation. However, stability data for dimers is generally lacking and not self-consistent. This is probably due to the general problem of quantitatively studying the self-association of long chain molecules at the low concentrations characterised by the CMC. Additional complications arising from inter-ionic interactions have never been properly defined. Further, it has not been fully explained why the formation of multimers appears to be relatively unimportant, in view of the well known stability of much larger aggregates, namely micelles.

With regard to ionmolecular complexes, Somasundaran and

Ananthapadmanabhan (98) claim that formation of the DA complex

(RNH^RNH^)* in solution coincides with maximum surface activity as observed by surface tension and flotation response. From their results, however, it is not possible to distinguish

+ between the more established theory of coadsorption of DA ion and molecule, and the adsorption of the proposed complex at the air/water or solid/liquid interface. Further, in their derivation of the stabi lity constant of the complex, these authors used the dielectric constant for water, i.e. 78.

It is well known that in the immediate vicinity of an ion, water dipoles are strongly oriented and that complete dielectric saturation may be observed. Hence for the primary hydration shell, the dielectric constant is reduced from its bulk to a value which may be as low as 6 (31,101). This consideration makes the value of their derived constant to be in considerable doubt.

In summary, although the existence of dimers and ionmolecular complexes is recognised, their inclusion in solution equilibria is not possible without more reliable stability data.

2.5 The Structure and chemistry of starch

2.5.I The structure of starch

Starch is a glucose polymer in which the glucose units are bonded mainly through a-D-(l—>4) linkages with a-D-(l-*-6) branch points. Carbon, hydrogen and molecular weight determinations have confirmed that it may be represented by the formula

(C-H.-CL) (102). Starches usually contain small amounts of 6 10 5 n 1 impurities such as fatty acids (up to 0.6$) and phosphorus

(up to 0.2$), the latter being more common in tuber starches than in cereal starches (103). Two principal types of carbohydrate polymers are present in starches: amylose and amylopectin.

Amy lose is essentially a linear-chain polymer containing mostly the a-D-(l—>4) glucosidic bonds. Its average molecular 4 5 weight lies in the range 4 x 10 - 3 x 10 , depending on the type of the parent starch. The content of amylose also varies with the botanical source; most starches contain 14-30$ of the linear polymer (104). It Is character!sed chiefly by its high iodine binding capacity (105) and Its relative tendency to retrograde from aqueous solutions compared to amylopectin (|06).

Amylopectin is considerably branched and contains both the a-D-CI—*4) and a-D-(l—»-6) bonds (104). It has a much higher 6 6 molecular weight (I x 10 - 5 x 10 ) than amylose and retrogrades only slowly from aqueous solutions, probably due to steric hinderanee (104). Simplified structures of the starch components are presented In Fig 2.6.

The term retrogradation Is used to describe the ability of starch to associate, particularly In poor solvents, probably by IntermolecuIar hydrogen bonds (104). The aggregates that are formed lead to a partial crystallisation of the starch molecules out of solution. Among the many factors that

Influence retrogradation are the molecular weight of the polymer and its concentration, type of solvent, temperature and pH of the solutlon.

2.5.2 Dextrins

The term dextrins has been loosely applied to extensively degraded starch products In which cleavage of the molecular chains results in products that are intermediate between the parent starch and oIigasaccharides. Several types of dextrins are obtained depending on the type of treatment and the amount of rearrangement and repolymerisation allowed.

White dextrins are produced when starch is heated at low temperatures (79-l20°C), low acidities and for short times. The (a) Segment of amy lose

(b) Segment of amylopectin

Fig 2.6 Simplified structure of starch polymers polymer is a result of mainly hydrolytic action (107). Yellow dextrins are produced at higher temperatures (I50-220°C) with higher amounts of acid catalyst. The depolymerisation reaction is followed by transglycosidation and repolymerisation as shown in Fig 2.7. This results in extensive branching of the molecules and a reduced tendency to retrograde (106, 107). British gums are produced by heating starch with little or no acid at high temperatures (I30-220°C) for relatively long periods. The products of dextrinisation are shown in Fig 2.7. Although several

options at C2, C^ or fa are available for the recombination reaction, it has been reported (108) that the a-D-(I—>6) linkage is expected to form more easily due to stereochemical CH OH CH OH

Fig 2.7 Possible dextrinisation reactions: (a) hydrolytic scission; (b) transglycosidation; (c) recombination (107) 71

CHAPTER THREE

MATERIALS AND EXPERIMENTAL METHODS 3.1' Materials •

3.1.1' Mi rierals' •

High purity samples of the silicate minerals were obtained from Ward's Natural Science Establishment, Rochester, New York.

Their sources were as follows:

Table 3.1 Sources of minerals used.

Mineral Source

Lepidolite Keyston, South Dakota Spodumene Keyston, South Dakota Beryl New England Microcline Keyson, South Dakota Muscovite Stoneham, Maine

Several hand-picked pieces of each mineral, measuring about 5x3 cm, were washed free of surface impurities by

leaching for several hours in 50$ HCI after which they were repeatedly rinsed with distilled water and allowed to air dry.

The samples were then wrapped in paper to minimise contamination during hand crushing with a hammer, followed by stage grinding to different sizes in a laboratory 'Tema' agate mill. The ground fractions were stored in tightly covered glass containers and kept under reduced pressure until required.

The purities of the minerals were checked by XRF and elemental analysis. Results of the XRF scans are given in

Table 3.2. 73

Table 3.2 XRF analysis of the minerals

Mineral Major Intermediate Minor Trace > 5% 0.5-5% 0.05-0.5% < 0.05%

Lepidolite SI K, Rb, AI Cs, Mn Fe, Pb, Sr, S, Sn, Ca

Spodumene SI, AI Fe, Pb, Sn, Rb, Ca

Beryl SI, Al Fe Pb, Zn, Cs, Ca, S, Br

Ml croc IIne SI, Al K, Rb Fe, Ca, Cs, Gr

Muscovlte SI, Al K, Rb, Fe Zn, Pb, Ni

Elemental analysis results are given In Table 3.3.

Table 3.3 Analysis of the cations in the minerals studied

Element • <% jfjement present in jf e fn'inera I

Muscovite Lep? do Iite Spodumene BeryI Mi croc 11ne

SI 21.00 29.00 29.67 30.33 28.33 Al 17.67 9.43 14. 17 9.27 9.5

Li 0.093 1.20 3.47 < 167 ppm n.d. Fe 3.07 0.07 0.07 0.58 0.05 Na 0.30 0.06 0.26 290 ppm 440 ppm

Ni < 333 ppm < 333 ppm < 333 ppm < 333 ppm < 333 ppm

Mg 400 ppm < 27 ppm 43 ppm 43 ppm < 27 ppm Pb < 0. 10 < 0. 10 < 0. 10 < 0. 10 < 0. 10 Zn 0.07 277 ppm 267 ppm 0.06 133 ppm

Mn 0.07 0. 1 1 333 ppm 133 ppm n.d.

Sr < 500 ppm < 500 ppm < 500 ppm < 500 ppm < 500 ppm Be n.d. n.d. n.d. 2.51 n.d. K 9.77 9.87 n.d. n.d. 14

n.d. = not detected Reasonable agreement was obtained between the values in Table 3.3 and theoretically expected values assuming the minerals were

'ideal'. Further indication that the samples were reasonably pure was obtained by XRD analysis. Several weak peaks were identified as quartz,aIbite and topaz, in association with the major minerals. These associations suggest that the minerals were from typical pegmatite deposits (I).

3.1.2 Reagents

Pure n-dodecyI amine was obtained from Koch-light

Laboratories Ltd, and was used without further purification.

Potato and maize starches, together with yellow dextrin, were of technical grade. No information concerning their purity was supplied by the manufacturers (BDH). The white dextrin, also supplied by BDH, was alcohol precipitated and contained less than 4$ reducing sugars. British gum was supplied by Tunnel

Avebe Starches Ltd, Gillingham, Kent. All the other reagents used were of 'Analar' grade.

High purity conductivity water was prepared by passing distilled water through a mixture of anion and cation exchangers

(Amberlite, BDH), and finally through a column of activated charcoal before redistillation. The water thus prepared had a conductivity of I x 10 6 mho and contained no surfactant material as indicated by bubble persistence tests.

The pH was measured with a PYE Unicorn pH meter, model 292.

Buffer solutions were prepared as solutions of potassium hydrogen phthalate (0.05M), equal amounts of potassium and sodium dihydrogen phosphates (0.025M each) and borax (0.0IM). The respective pH values were 4.01, 6.86 and 9.18. Solutions of NaOH or HCI of appropriate strength were used for pH adjustment.

3.2' Experimental'methods and techniques

The experiments were either carried out at a room temperature of 20°C ± 2°C or at the temperature specified In the relevant experimental procedure. Glassware was cleaned by chromic-sulphuric acid followed by repeated washing with distilled water. In some cases where the glassware had been in contact with surfactant or organic solutions, it was soaked in a solution of concentrated NaOH and alcohol prior to washing with water and chromic-suIphurlc acid treatment. This procedure produced completely hydrophilic glass surfaces.

3.2.1 Elemental analysis

0.300 g of the -45ym fraction of each mineral powder was fused with an excess of sodium peroxide (2.5g). The fusion mixture was dissolved in acidified distilled water and made up to I litre, such that the solution contained 5$ V/v HCI.

A further 0.300 g was digested in HCI overnight, filtered and made up to I litre. The fusion so Iutions were used for

Identification of a I I the elements shown in Table 3.3 except for sodium, for which the acid-digested sample solutions were used.

3.2.2 PISsoluti on studies

I g samples of the -45ym fractions of the minerals were weighed into 100 ml conical flasks. Distilled water (50 ml) with the required pH value was added to each of the flasks and the suspensions equilibrated on a mechanical shaker for the required time. Both the initial and final pH values were recorded. The suspension was then centrifuged prior to determination of the lattice cations in the supernatant.

3.2.3''Cat? on'exchange'measurements

The -53, + 45ym fractions were used in the determination.

The surface areas of the respective mineral fractions, as determined by the single point method using a 'Monosorb' apparatus (Quantachrom Inc.) are given in Table 3.4. Three independent measurements were made for each determination.

Table 3.4 Surface areas of the fractions used in the cation exchange tests

2 -I Mi nera I S, m g

Muscovite 14.4 Lepi do Ii te 1.9 Spodumene 0.5 Microcline 0.4 Beryl 0.2

A preliminary investigation revealed that the particles -4 could not be suspended directly in I x 10 M salt solutions because of the release of the cation of interest by the mineral samples, in some cases. Subsequently, accurately weighed I g samples of each mineral were suspended in IM NlfaCI for I h (109) in stoppered centrifuge tubes and rotated on an end-over-end agitator. The suspension was then centrifuged and decanted. This procedure was repeated four times after which the mineral powder was washed several times with methanol to remove occluded

NH^CI, dried and reweighed. The exchange capacities were then -4 determined as the ability of I x 10 M chloride salts to desorb + NH^ ions from the saturated mineral powders. Parallel experiments, in which no cations were present, were also

+ conducted. In both cases the NH^ -saturated powders were repeatedly resuspended in salt solutions (distilled water in

+ the latter case) until little or no NH^ ions passed into

+ solution. The amount of NH^ in each washing was determined and the exchange capacity deduced as the total NH^* exchanged

in salt solution less that exchanged in distilled water. The exchange experiments were conducted at pH 7, using hydroxide solutions of the particular cation to adjust the pH. Earlier dissolution studies, and the stability data presented in Chapter

2, indicated that there was minimum dissolution of the silicates at this pH value.

3.2.4 Electrokinetic measurements

The electrophoretic mobility measurements were made with a Rank Brothers' MK II microelectrophoresis apparatus using a

flat quartz cell. A suspension of 0.005 g ml ' was made up with distilled water and adjusted to the required pH prior to equilibration for three days on a mechanical shaker. The final

pH was noted immediately before each measurement. At least

20 particles were timed at both stationary levels and the mobi lity value computed from a mean of the readings. In most of the experiments an ionic atmosphere of 10 5M NaCl was maintained.

Preliminary tests showed that neither Na+ nor CI had significant specific adsorption effects on any of the minerals studied.

The eIectrophoretic mobility was calculated from the equation

U = vj_ (3. I) V where v = particle velocity

V = applied potential

L = effective interelectrode distance

The effective interelectrode distance was obtained from the formuI a

L = RKA (3.2)

where R = resistance across the cell

K = specific conductivity of the suspension

A = cross-sectional area of the cell.

R was measured with a Marconi Instruments Universal A.C. bridge model T.F. 2700. A Portland Electronics conductivity meter

(model P 310), with a cell constant of 0.53 was used to measure

K. Values of L thus obtained were plotted against pH, for the particular ionic strength used, as shown in Fig 3.1. Below a pH of 4 and above a pH of 10, L was not found to be constant.

This contradicts recent reports (29, I 10) in which L has been assumed to be constant below and above these pH values. Fig 3. was used to obtain values of L within the pH range studied. PH

Fig 3.1 Effect of pH on the effective cell length in 10 8 M NaCI

3.2.5 Methods of preparing dodecy1 amine arid starch solutions

DodeCyI ami ne Solutions. DodecyI amine solutions were prepared by dissolving a weighed quantity in a slight stoichiometric excess of

HCI (10 8 M HCI). Gentle warming was used to increase the rate of dissolution.

Starch so Iutions. The effectiveness of starch and related polymers as froth flotation depressants is strongly dependent on the solubi Iisation procedure used. Several methods of preparation have been reported in the literature. Chang et al (39) prepared starch solutions by digesting the starch in an autoclave at I20°C for I h.

More recently, Afenya (III) prepared starch solutions by boiling the polymer at I00°C for 20 mins. Balajee and Iwasaki (42 ) preferred causticising their starches in 0.5N NaOH. It is likely that all of these techniques would produce solutions of different properties from the same raw materials.

For solubi IisatI on of a polymer, the solvent envlronment must be favourable in thermodynamic terms; the polymer must prefer polymer-solvent interactions compared to polymer-polymer associations.

For most starches, water Is a poor solvent. It appears that this is associated with the uniformity of the linkages and the linearity of the molecules. As a general rule polysaccharides in which the sugar units are wholly or primari ly linked by the a-D-(l—>4) glycosidic bonds are Insoluble in water due to their preference for partial crystallisation (retrogradation). Contrari ly, polysaccharides in which a-D-(l—bonds are present are soluble to varying extents.

It has been suggested ( I 12) that the greater solubility results from the extra degrees of freedom provided by the equatorial bond between the C^ and C^ carbon atoms. This results in higher solution entropies for such polymers and hence lower free energies of solution.

The main consideration, therefore, Is that for most starches cold-water solubi Iisation Is determined by the amylose: amypectin ratio and not merely the solvent characteristics. The use of hot water only produces swelling of otherwise intact starch granules which at sufficiently high temperatures will burst to form gels

(103). Upon cooling the gels are expected to remain In the associated state (104). The depressive properties of such starch solutions must therefore be considered doubtful. Indeed, this is supported by the results presented in later sections. 81

Perhaps the best method for solubilising starches is by

dispersing them in dimethyl sulphoxide (103,113). However, the

use of such a reagent is inhibitive for industrial applications.

It is also probable that the reagent Is not sufficientIy poor to allow for reasonable amounts of adsorption of the polymer at the

sol id/liquid interface (||4).

Good solubi Iisation characteristics of starches can be obtained by dispersing the starch granules in urea or sodium

hydroxide, even at room temperature (42, 103). It appears that the main reason for this is the unlikely existence of the intermolecu Iar

hydrogen bonds, that are responsible for associative effects, in these solvents (86). Further, subsequent ionisation of the hydroxyls

contribute to the stability of the solutions (112). However, it appears that the use of high concentrations of sodium hydroxide can

lead to degradative effects and the formation of carboxylic groups

C103), apart from the disadvantage of working with a high salt concentration upon neutralisation to the normal working pH values.

For the above reasons, starch solutions were prepared, in the

present work, by causticising at low NaOH concentrations (0.025M),

using low stirrer speeds (300 rpm for 30 min). As the effects

discussed above are concentration-dependent (e.g. retrogradation)

relatively dilute solutions at 0.01$ w/v (dry basis) were prepared.

Such starch solutions were observed to be optically clear and

remained indefinitely stable. Fresh starch solutions were, however,

prepared daily to minimise biological degradation effects. To avoid retrogradation, the solutions were not neutralised until

Immediately before use. 82

3.2.6 Ha II imorid tube ' f Iotation ' tests

Microflotatlon tests were carried out in a modified Hallimond tube equipped with an extended stem to minimise mechanical carryover.

A Ig sample of -212 + 106 ym mineral was conditioned with 200 ml of amine solution at the required pH in a 250 ml beaker for 20 mins. For tests in the presence of starch, the starch was added and conditioned with the mineral before the addition of amine.

After conditioning the suspension was transferred to the magnetically stirred Hallimond tube by use of an extended stem

funnel. Flotation was carried out for I min using CO^-free nitrogen at a flow rate of 25-30 ml min Both the sinks and floats were

collected, dried and weighed. This procedure enabled an overall

particle recovery to be calculated; this was always greater than

96$. The pH of the suspension was measured both before and after

flotation. Generally there was little or no difference between

these two values.

3.2.7 Adsorption measurements

Adsorption densities were determined from the difference in

concentration before and after contacting the mineral particles with

the reagents. The -45ym fraction of each mineral was used in these

tests. The corresponding surface areas, measured as a mean of

three independent determinations, are given in Table 3.5. Table 3.5 Surface areas of the mineral fractions used in adsorption experi ments

c 2 -I Mi nera I S, m g

Muscovi te 14.5 Le p i do Ii te 3.4 Spodumene I .3 Mi croc Ii ne I .2 Bery I 0.8

The adsorption tests were carried out in 100 ml Erlenmeyer flasks. Minimum rates of agitation were used so that the mineral particles were kept in suspension, without undue mechanical breakdown and a subsequent increase of surface area of the powder.

tn the amine adsorption tests 0.5g of mineral was conditioned with 50 ml of amine solution for Ih. A similar procedure was used to determine the adsorption of starch and dextrins except that Ig samples were used. Preliminary tests established that equilibrium was obtained with the amine and starch/dextrin within

10 min of conditioning. Reversibility of the adsorption was investigated by using different sample weights.

In tests where both the adsorption of starch and amine was determined, 0.5g of solid in 50 ml of solution was used. The mineral powder was conditioned initially with 40 ml of starch solution for 20 min and then 10 ml of dodecylamine solution was added and conditioning continued for a further 40 min. Equilibrium concentrations were determined after centrifuging the solids at relatively low speed. Under certain amine and starch concentrations, a reaction occurred between the two reagents and a precipitate was formed.

Tests were conducted to determine the starch and amine concentrations

in equilibrium with this solid phase. To prevent centrifugation of the unreacted starch, a speed of only 1000 - 1200 rpm was used for a time of h. At the end of this period the supernatant solution was clear.

Some adsorption tests were done on methylated silicate powders to determine the influence of hydrophobicity of the mineral surface on the adsorption of starch. The mineral particles were methylated with trimethyIchlorosi lane, using the method of

Laskowski and Kitchener CI 15). A 0.05M solution of the reagent was made up in pure dry benzene and the particles coated by mixing 20 g of the mineral powder with 100 ml of solution for

30 min. The suspension was then filtered, washed with benzene and

dried under reduced pressure in the presence of activated silica gel and phosphorus pentoxide. Powders treated in such a manner were found to be perfectly hydrophobic when suspended in water and showed no tendency to disperse. However, the aggregates

dispersed when treated with an ultrasonic probe.

3.2.8 V?ScoSity measurements

The viscometric quantities of importance for a dilute

polymer solution of concentration, c are (116):

Cal The relative viscosity, T\ " re I

nre I = n/no C3-3) where n, n are the viscosities of the solution and solvent ' o respectively;

(b) the specific viscosity, nSp5

n = (n - n i/n (3/4) sp lo lo

Cc) the viscosity number obtained as nsp/c, and

Cd) the limiting viscosity number, [nj , (the intrinsic

c viscosity) obtained by extrapolation of hSp/ Infinite dilution, i .e.

[n] = lim Cns /c) (3.5)

c-» 0

Although the intrinsic viscosity can be related to the

viscosity average molecular weight*, Mv, by the Mark-Houwink expression as

a [n] = K(Mv) (3.6)

where K and a are empirical constants for a given polymer-

solvent system, absolute values of Mv are rarely obtained due to the tedious nature of determining K and a, particularly for highly polydisperse systems (116). Viscosity measurements by themselves provide the most convenient method of determining relative molecular sizes.

* There are several ways of defining the molecular weight of a polydisperse system. See for example ref. 117. The measurements reported in this work were carried out

in a modified UbbeIhode-type viscometer, with a nominal constant of 0.01 cS s '. The viscometer had a water efflux time of 83 s and was thermostatted at 25 ± 0.5°C. Kinetic energy corrections were calculated to be negligible by comparison of determined to

known viscosities for several analytically pure reagents (116).

For starches, concentrations of up to 2 g litre ' in 0.05 M

NaOH were investigated. This concentration was found to be too

low for the dextrins for which a maximum concentration of 5 g

litre ' was used.

In all, a total of 3 efflux times were measured to ± 0.01 s

and the mean of the measurements taken to be indicative of the

flow time for that solution. The intrinsic viscosity, [ h], was

determined as the intercept from a plot of Clnnre|/c) against the

starch concentration (c) in g ml although similar values of

n] could be obtained by plotting hSp/c against the concentration.

3.2.9 Surface tension measurements

The surface tension of the aqueous amine/starch solutions

were measured by the drop volume method (118), using an 'AglaT

micrometer syringe attached to a glass capillary of outer radius

0.376 ± 0.001 cm, as measured by a travelling microscope. The

capillary was held perpendicularly and the drop formed on the

capillary tip could be held or released as desired by manipulating

the micrometer. Two drops of solution were initially released

and the difference in micrometer readings noted. A third drop

was formed at 90$ of this difference and held on the tip of the

capillary for 2 min for equi Iibriurn to be estabIished. Longer 87

aging times did not produce any significant difference in surface tension. The drop was then slowly discharged allowing it, as

near as possible, to detach itself. The volume of the drop was

then deduced from the difference in the micrometer readings. in

all 5 such measurements were taken and the volume computed as a

mean of the measurements.

The surface tension, y, was calculated from the Harkins-Brown

equation CI 18):

y = Cmg/r)F (3.7)

where

m = weight of the drop (i.e. density x volume)

g = gravitational acceleration

3 F = correction factor, defined by v /p

v = volume of drop

r = radius of capi Ilary tip.

The outer radius of the capillary measured as above agreed with

that determined by measuring the surface tensions of several

analytically pure reagents of known y.

3.3 Analytical methods

3.3.I 'infrared-spectrometry

Infrared spectra of dodecylamine, potato starch, potato starch-

dodecylamine mixtures and potato starch-dodecyI amine precipitates

were obtained with a Perkin Elmer, model 599B, double beam

spectrometer. The spectrum of dodecylamine was obtained by smearing liquid amine between two NaCl discs and using a NaCl disc as a reference. Samples of starch and starch-amine mixtures and precipitates were presented to the spectrometer as Nujol mulls. Before making each mull the solids were dried over phosphorus pentoxlde and then gently ground In an agate pestle and mortar. Nujol oil was then added and the grinding continued until the solids were completely suspended In the oil. The translucent paste was then squeezed between two NaCl windows.

The KBr disc method of preparing samples for infrared analysis was not used because it was found that the amine extruded out of the discs during the pressing stage.

3.3.2 ' Determination of ammoni a

The method outlined by Vogel (119) using Nessler's reagent

Can alkaline solution of potassium tetralodomercurate(I I)) was used.

When the reagent is added to a dilute ammonium salt solution, the liberated ammonia reacts to form an orange-brown product which can be used for colorimetric determinations. A Perkin Elmer

200 double beam spectrophotometer was used.

Reagents: Nessler's reagent, NH^CI stock solution.

A stock ammonium chloride solution was prepared by dissolving

3.141 g of NH^CI, dried at I00°C, In ammonia-free distilled water, and diluting to I litre. Standard solutions were obtained by diluting suitable aliquots of the stock solution.

' Procedure: 5 ml of the sample solution was pipetted Into a 50 ml volumetric flask and diluted to near-capacity with distilled water. I ml of Nessler's reagent was added and the flasks brought to the mark by addition of more distilled water. 4 8 12 16 20 24

5 NH^ concentration, M x 10

Fig 3.2 Calibration curve for the determination of ammonia

The flasks were left to stand for at least 15 min. before determination of the absorbance at a wavelength of 420 nm, using a reagent blank made with distilled water. The calibration curve obtained for the

NH^CI standard solutions is shown in Fig 3.2.

3.3.3 Determ? hat i on of dodecyI ami ne

The concentration of dodecylamine in solution was determined by a colorimetric method involving a suIphonaphthalene indicator and extraction into chloroform (120). Reagents: Chloroform, bromocresol green and conc. h^SO^.

The indicator solution was prepared by dissolving 0.Ig of solid bromocresol green in 60 ml of a Icohol, acidifying with 0.5 ml of concentrated F^SO^ and diluting to 100 ml with distilled water.

' Procedure: I ml of the indicator solution was added to

5 ml of the sample solution and the whole diluted to 10 ml.

5 ml of chloroform was then added and the mixture inverted rapidly about 100 times, care being taken not to form an emulsion.

After separation of the phases, the absorbance of the organic layer was determined at 416 nm. A calibration curve (Fig 3.3) was obtained by determining the absorbance of amine solutions of known concentrations.

3.3.4 Deterriii nation of Starch, ' dextr? ns'arid'Brit? sh'gum

The method outlined by Dubois et al (121) was used for the analysis of starch and related compounds.

Reagents: 5% w/v phenol, conc. h^SO^.

5 g phenol was dissolved in 100 ml distilled water.

Procedure: 2 ml of starch solution was pipetted into 2 identical 20 ml capacity glass vials and I ml of 5% phenol added.

5 ml of concentrated F^SO^ was then added, directing the stream at the centre of the vial for adequate mixing. The vials were allowed to cool in air for 15 min, after which they were shaken and immersed in a water bath at room temperature for a further

20 min. The absorbance of the solutions was determined at

486 nm. Calibration curves for potato and maize starches, British gum and the dextrins are shown in Fig 3.4. Slight differences in 91

0.8

0.6

CD o c 0.4

<- Q 0.2

12 3 4 5 5

Amine concentration, M x 10

Fig 3.3 Calibration curve for the determination of dodecylamine absorbance were observed in the curves. These differences may have been due to small variations In the moisture content of the compounds.

3.3.5 ' Ana I ys i s by atorhi c' absbrpt? orhetry

The concentrations of the cations (cf Table 3.3) in solution were determined by atomic absorptiometry using a Baird Alpha I spectrometer. Standard solutions were obtained from BDH and diluted to the recommended working range set out in the instrument manual. For the majority of the elements, the 0.8

0.7

0.6

0.5

(D _ . u 0.4 c CD J21 L U)O

0.2

0. I

10 20 30 40 50 60

Concentration, mg litre '

Fig 3.4 Calibration curves for the determination of potato starch, dextrins, British gum and maize starch.

instrument was operated in the absorption mode while

concentrations of the alkaline earth cations were determined by flame emission photometry. In the cases where ionisation problems were encountered, an excess of lanthanum (1000 ppm), as the chloride, was added to both standards and the sample solutions. Where high concentrations of Si were to be determined the standards were prepared in dilute NaOH to avoid precipitation of silica. CHAPTER FOUR

DISSOLUTION AND CATION EXCHANGE STUDIES 4.I' Dissoiutiori studies

The objective of the dissolution experiments was twofold; to assess the time required for equilibrium to be established and to determine whether or not preferential dissolution of any of the lattice cations occurred. The latter was achieved by comparing ratios of the elemental concentrations in the leach solutions with those in the bulk (35, 36, 122).

The results presented in Figs 4.1 to 4.3 for spodumene are typical of those obtained. From the figures it appears that an

'equilibrium' is established in the lithium, aluminium and silicon, dissolution after about 50 h of agitation.

The aluminium concentration in equilibrium with the different silicates was at a minimum at a pH of approximately 7, and this is demonstrated for lepidolite, spodumene, beryl and microcline

in Fig 4.4. These results are consistent with the effect of pH on the thermodynamic solubility of gibbsite presented in section

2.1.3. At low pH values, the presence of A|3+, mono and polynuclea hydrolysis products enhances the solubility and at high pH, the solubility increases again due to the formation of aluminate i ons.

It is highly unlikely, however, that true equilibrium was established. Comparison of Fig 2.2 to 4.4 indicates that these solutions contained at least ca 60 times the total soluble aluminium in a saturated solution of gibbsite. Thus if gibbsite was assumed to be the solid phase controlling the solution composition, these solutions would be supersaturated with respect 96

20 40 60 80 100

Dissolution time, h

Fig 4.1 Spodumene: Effect of dissolution time and pH on the concentration of Li+ in the leach solution Dissolution time, h

Fig 4.2 Spodumene: Effect of dissolution time and pH on the concentration of Al in the leach solution Dissolution time, h

Fig 4.3 Spodumene: Effect of dissolution time and pH on the Si concentration in the leach solution Fig 4.4 Effect of pH on the aluminium concentration In the leach solution to gibbsite. However, the validity of such an assumption is doubtful in the light of the considerations outlined In section

2.1.3. Along similar lines, it is later shown (aging experiments) that the approach to true equilibrium in suspensions containing a Iuminosi 11cates necessitates the presence of a silicate solid phase, rather than gibbsite. Hence 11ttIe quantitative significance can be attached to the above comparison.

It might be noted, however, that most a IuminosI 11cates are metastable states (49) in comparison to their weathering residues like gibbsite or kaolinite. In aqueous solutions, therefore, thei r relatively higher solubi lities wi I I probably lead to a saturated state with respect to gibbsite before precipitation of the latter is initiated. It is also possible that the presence of a disturbed layer on the minerals' surface contributed to the high solubility. Such effects have been widely reported

( 53, 54, 66).

An alternative explanation is that the presence of soluble a IuminosiIicate species In solution contributed to the high solubility of aluminium. Although these condensation-type reactions are feasible in precipitates, it is not known whether they occur in solution and If they do what form they take. If soluble a Iuminosi Iicates were responsible for the high solubility of aluminium in solution, it would be reasonable to expect silica to show similar solubilities. This explanation is therefore di ffi cu11 to Justi fy.

With the exception of muscovite, the silicon concentration

In equilibrium with the minerals was found to be independent of pH at pH values below 9 (Fig 4.5) and was lower than the solubility PH

Fig 4.5 Effect of pH on the Si concentration in the leach solution

1 1 1 1 1 i 1 i 1 1 1 i

o Lepi doli te V Spodumene

\ A Ml croc 11ne - \ O Muscovite • Bery 1

I M i i i . • . i

10 12 PH

Fig 4.6 Effect of pH on the Al/Si mole ratio in the leach so Iution 102

of amorphous silica reported in Fig 2.1. At high pH values, the

amount of silica passing into solution increased markedly. This

was attributed to the formation of the various ionic silicate

species outlined in section 2.1.2.

Stober C53 ) argued that even for NaOH-treated silica,

a reversible solubility equilibrium is never attained due to the

slow readsorption of silicic acid on the mineral. It is possible

that a similar effect was operative in the present work. Another

reasonable explanation for the observed undersaturation is that

consistent with the comments in section 2.1.3, that the solubility

limit of silica is lowered in the presence of aluminium ions.

The results shown in Figs 4.4 and 4.5, and summarised in

Table 4.1 and Fig 4.6, show that although aluminium and silicon

pass into solution, they do not give the same molar ratio in

solution as in the mineral.

Tab ie 4.I: Mo Ie rati o of so IubIe a Iumi n i urn and si Iicon i n equi Iibri urn with the si Ii cate minerals

Experimental Al/Si mole ratio in Mineral Approximate chem formula Al/Si mole leach solution (I) ratio (Table pH 2 pH 5 pH 9 pH 12 3^3)

Lepi dolite KLi (Al(0H,F)2 AlCSi03)3) 0. 34 1.29 0.22 0. 23 0. 41

Spodumene Li A1 (Si 03>2 0. 50 1.28 0.31 0. 36 0. 58

Be ry 1 0. 32 1.29 0. 12 0. 13 0. 35 3 2 3 6

Muscovi te K AI3(Si04)3 0. 87 1.33 0.73 0. 80 1. 19

Mi croc 1i ne K AlSfaOg 0. 35 1.81 0.34 0. 33 0. 44

It appears that preferential di ssoluti on of Al occurs at low

pH. At high pH the ratio in solution is cl ose to that i n the

bulk minerals, although marginally higher dissolution of Al still takes place. At intermediate pH values, more Si tends to dissolve although dissolution is again nearly stoichiometric.

Similar observations have been made by other workers (35, 36,

122). These results suggest the possible effects of acid and alkaline scrubbing of these minerals prior to flotation.

Qualitatively, a silica-rich surface or a near-stoichiometric surface will be obtained, depending on the type of scrubbing used.

4.2 Cation exchange Studies

The cation exchange capacity (CEC) of a mineral may be defined as the total amount of cations held exchangeably by a unit weight of the mineral (123). Although the total amount of cations passing into solution was measured, the results obtained did not necessarily represent total cation exchange capacities, owing to difficulties associated with defining the CEC (123).

Further, it has been assumed that the exchange process was equivalent. This may be regarded as a consequence of the condition of electrical neutrality for the double layer. The results that were obtained are presented in Tables 4.2 and 4.3.

Tab Ie '4.2: Cation exchange capacities of muscovite and lepidolite

Muscovite Lep? doli te

2+ 2+ 2+ + 2+ 2+ „ 2 + . + Ba Mg Ca Na Ba Mg Ca Na

+ Total yeq NH4 6J Q7 6Q>6g 56JQ 5-36 |2.50 11.22 11.48 exchanged

+ yeq NH4 /g 55.52 47.24 39.45 32.83 2.62 11.02 8.49 8.37 Table 4.3: Cation exchange capacities of spodumene and microcline

Spodumene Microcline

2+ 2+ 2+ + 2+2+ 2+ + Ba Mg Ca Na Ba Mg Ca Na

Total yeq NH4 2 6Q 3 Q6 3 Q6 4 Q8 2 55 2Q] 255 3 5? exchanged

+ yeq NH4 /g 3.18 3.51 3.18 4.14 3.12 3.19 2.69 3.67

In the tables the results have been reported on the basis of unit mass rather than unit area. The main reasons for this are:

(i) traditionally all exchange properties are quoted on the

basis of unit weight (109, 123, 124^* so that comparisons

of the results to those obtained with similar minerals

can be made;

(ii) the porosity of the minerals used in the study was unknown;

the effect of this on the exchange properties could not

therefore be assessed;

Ciii) in the layer silicates, although a small amount of exchange

can take place on the sheets themselves, it is recognised

(124,125) that the bulk exchange process takes place at

the wedge-shaped cracks that develop on the edges of the

sheets, at interlayer positions. Thus the effective

exchange area is only a small proportion of the total

surface area for these minerals.

The results obtained for muscovite and lepidolite showed strong and moderate cation exchange properties, respectively.

Spodumene and microcline showed only weak exchange behaviour while beryl had no measureable exchange properties.

To explain why the minerals of Table 4.2 showed good cation exchange characteristics, it is necessary to remember that in sheet si Iicates nonstoichiometric isomorphous substitution of Al for Si in the tetrahedraI sites or Mg for Al in octahedral sites produces a non-specific intrinsic space charge on the surfaces of the sheets which is neutralised by interlayer cations

(66, 124 ). The interlayer cations are not themselves part of the structure, and will consequently hydrate and be exchanged for other cations as they are only loosely held. That muscovite showed stronger exchange ability than lepidolite can perhaps be explained from the aluminium content of the two minerals given in

Table 3.3. For each aluminium atom in a tetrahedraI site one negative charge results. As cation exchange behaviour will be partly dependent on interlayer change density (123), it is not, therefore, surprising that cation exchange activity increases with increasing AI concentration (126, 127).

The selectivity of an exchanger is a measure of its relative affinity for ions. For the cations used, muscovite showed the affinity sequence Ba2 + > Mg2+ > Ca2+ > Na+. Barshad (109) reported a similar order for muscovite, although a different order was obtained on vermiculite. In the case of lepidolite the 2+ 2+ + 2+ selectivity sequence was Mg > Ca > Na > Ba The quantities exchanged in the case of microcline and spodumene were considered to be too small to be significant. Examination of the sequences for the sheet minerals shows that divalent cations were preferred to sodium at the same molarity (124). Further, the exchange sequence of lepidolite for the divalent cations was the reverse of 2 + that observed for muscovite, if i+ is assumed that Mg occupied an anomolous position in the muscovite sequence.

The basis for the affinities of various cations for particular exchange substrates has not been unequivocal Iy estabIished. For the simple oxides, for example, there is apparently little

consistency in the selectivity sequence, even for similar materials

C|24)• Originally, selectivity was discussed in terms of

crystaIlographic radii and the ability of particular cations to

fit into holes postulated as existing in the hexagonal oxygen

lattice (109), |n more recent work, however, the significance of cation hydration has been noted, although not unambiguously.

Thus Shainberg and Kemper (101) have shown that among the alkaline metal cations, those that are least hydrated, and with probably

+ +

incomplete primary hydration shells (e.g. Cs and Rb ) will tend

to be more specifically adsorbed. Several other workers ( |24,

127, 128) have considered the assumption that exchange equilibria

are dominated by (a) coulombic interactions between the counter

ions (in various states of hydration) and the substrate exchange

groups, and (b) the ion-dipole and ion induced dipole interactions

between the counter ions and water molecules (ionic hydration).

Thus the 'normal' affinity sequence (Hofmeister series) of Cs + > K + > Na + > Li + or Ba 2+2+2+> Sr > Ca > 2M g + for alkaline earth cations, has been explained by assuming that the cations

adsorb primarily by coulombic interaction, retaining their primary

hydration sheath. The ion with the smallest hydrated radius will

therefore be held most strongly (101, 124, 127). Other hypotheses

have been proposed in order to explain the reverse sequence. One

plausible interpretation has been that the ordered water molecules 107

close to the surface favour adsorption of ions that can maintain

this order CI24). These constitute ions that are relatively

more hydrated, and show greater specific adsorption. The reverse

sequence is thus associated with structure-promoting surfaces,

while the normal sequence is shown by structure-breaking

surfaces C|24). The ability of a surface to order interfacial

water molecules is obviously related to the polarity and field

strength of the solid (Fig 5.11). Since the field strength decreases

with the increase of Al content (127), lepidolite might be expected

to show the reverse sequence to muscovite as was found. The 2 + anomolous position of Mg in the muscovite sequence might be

related to its presence as an inter layer cation in this

mineral (see Table 3.3).

Although these considerations are not unreasonable, they do

not explain why beryl showed no cation exchange activity.

Measurement of the cations passing into solution indicated that

the process occurred predominantly through the release of mono-

valent cations. This might suggest that the poor cation

adsorption response for beryl was"related to the absence of the

generally weakly held monovalent cations. in aqueous solutions these cations have been widely reported to be easily exchangeable

(23 , 66 , 74 ). The beryllium cation, on the other hand, would

be more tightly held because of its high polarising power. This

conclusion is collaborated by the results reported in later sections on the adsorption of amine at low concentrations and

acid/base titrations. CHAPTER FIVE

ELECTROKINETIC STUDIES 5.1 General considerations

When an electric potential is applied to an aqueous mineral suspension, the particles migrate to the oppositely charged electrode, taking with them part of the double layer enclosed between the solid surface and a zone of shear situated just outside the Stern plane. The potential at this shear plane, known as the zeta potential (?), Is the only potential that can be measured by eIectroklnetic methods. The value of the zeta potential can be obtained from eIectrophoretlc mobility (EM) measurements by application of the He Imholtz-Smoluchowski equati on:

v = EM = (5.1) E 4nn where v Is the velocity of the particle under the influence of an electric field E, in a solution of permitivity e and viscosity n. For aqueous solutions at 25°C, the form

C(rnv) = 12.83 x EM (ym/sec/v/cm) (5.2)

Is often used (67, 110 ).

In most cases, equation (5.1) must be modified to allow for retardation, relaxation and viscoelectrlc effects. It has also been shown (67 ) that the He I mho Itz-Smoluchowski equation Is one of two limiting forms of the Henry's equation

v =-Ee£ (I + Xf (i

a = radius of curvature of particle.

For large

ica the Huckel equation may be used (67 )

_v = eg (5.4) E 6 7TT1

Although Henry's equation includes a retardation effect it

does not include a relaxation effect ( 67 , 129 ). A large number

of workers (130, 131) have proposed modifications to equation

(5.3) in which these effects are considered. A further possibility

that e and n, which are assumed constant in most derivations, may

be influenced by locally high electric fields has been assessed

(132, 133). Hunter (133) concluded that under most conditions the combined effects of e and n are small. However, exact

corrections are not possible until the exact manner in which

n and e depend on E is known.

Further difficulties arise from the uncertain nature of

the location of the slipping plane (and how much solvent is entrapped within it) and discreteness of charge effects (132).

In view of these difficulties, the conversion of EM values to

g potentials was not considered necessary in the present work.

5.2 Effect of pH on the electrophoretic mobility of the silicate mi neraIs

Due to the importance of pH in the charge generation

mechanism for silicates, electrokinetic properties were measured

as a function of pH. The effect of pH on the EM of the minerals is summarised in Figs 5.1 and 5.2. The IEP (zero electrophoretic mobility) of muscovite, spodumene and beryl occurred at pH values of 5.7,

2.8 and 3 respectively. Microcline and lepidolite were still negatively charged at a pH of 2.7. The mobiI ity-pH curves for lepidolite, spodumene, beryl and microcline were very similar to one another. This probably helps to explain the problems of selectivity that are encountered in the flotation of these minerals. Further, the mobilities showed a marked dependence on pH, except perhaps at high pH values. These results are si mi lar to those obtained for quartz and other si Iicates (134,

I35) and suggest that H+ and OH ions are potential determining.

Fig 5.3 shows the effect of NaCI concentration on the mobi lity of spodumene at pH 9. Si mi lar curves to this were obtained for the other si licate minerals. It is apparent that NaCI acts as an indifferent electrolyte for spodumene and the other silicates.

Thus the reduction in EM at high ionic strengths is due to compression of the double layer. Li+ , although present as a lattice cation in spodumene and lepidolite, gave similar results

+ to those obtained with Na . This indicated that it did not have a potential determining role on the minerals. This is illustrated in Figs 5.4 and 5.5.

The IEP values of spodumene and beryl compare favourably with those found by other workers ( 73 , 74 ). Reported IEP values for microcline are at very acid pH values (approximately

1.7) (73), and this is consistent with the negative mobilities obtained in the pH range 2.7 to 11.4 in the present work. In the i 1 1 1 1 r

• Muscovite +4 O Lepidolite

1 1 1 r

Fig 5.1 Effect of pH on the electrophoretic mobility of muscovite, lepidolite and amblygonite in 10 M NaCl

+4 h O Spodumene o • Be ry I > [j Mi croc I i ne \ M +2 p- +-

I 0 o pH 0

LU i=fi- -4

j L

Fig 5.2 Effect of pH on the electrophoretic mobility of spodumene, beryl and microcline in lO^-^M NaCl I 13

Fig 5.3 Effect of NaCl concentration on the eIectrophoretic mobility of spodumene at pH 9 case of the sheet silicates, muscovite and lepidolite, such comparisons are often not meaningful because of the pronounced isomorphic substitutions in both the tetrahedral and octahedral positions. It has been shown in Chapter 4 that the main effect of this is to produce a cation exchange affinity. For these minerals the IEP will therefore be dependent on the type of inter layer cations present and the cation exchange capacity

(66 ). These considerations probably also apply to the non-sheet a Iuminosi Iicate structures to a lesser degree.

The interpretation of IEP values for a Iuminosi Iicates is further complicated by problems of non-stoichiometric dissolution and effect of cleavage (66 ). Apart from determining the extent of cation exchange activity, the type of substituting cation also determines the degree of acidity or basicity of the lattice site (68 ) and its subsequent degree of ionisation in a given I 14

1 1 1 1 1 1 1

+4 0 in 10 NaCI e O 3 > • in I0" LiCI

^ +2 e

-h 0 E 1 1 1 |ii- -Q 10 pH o "cfl 4 6 8 e O

- £ -2 O _c oCL

o ® -4 LU

. 1 1 1 1 — •

Fig 5.4 Effect of LiCI on the e lectrophoreti c mobility of lepi do Iite

Fig 5.5 Effect of LiCI on the electrophoretic mobility of spodumene I 13

aqueous environment (cf section 5.3) (124). On the other hand, the location of the cation relates to cleavage and the degree of exposure of the cation to the solution environment. Although the tendency of cleavage to control cation exposure is probably not lost during the grinding of minerals, the surface of the products will unfortunately be unlikely to assume a similar cation distribution because of the disordered nature of such a surface ( 66). The effect of cleavage and the composite nature of the silicate minerals in which both acidic and basic oxides are present, will lead to non-stoichiometric dissolution and the possibility of readsorption of the hydrolysis products on the mineral surfaces.

It is a little surprising that the EM of the sheet minerals was strongly dependent on pH. It is generally assumed ( 23, 66 ) that the cleavage planes in these minerals are of constant charge and these surfaces predominate. In the case of muscovite it might be argued (cf Table 3.3) that some readsorption of iron hydrolysis species took place under slightly acidic conditions.

Such a mechanism cannot be applied to lepidolite because it contained much fewer polyvalent cations. It would therefore appear that either the surface of the lepidolite sheets contained a considerable number of ionisable groups or that the small proportion of edge sites exerted a significantly large influence on the total amount of surface charge. A pH-dependent surface charge has been observed on talc (23) and strongly hydrophobic si Iica (115). I 16

5.3 Effect of aci d arid a I ka I i ne washing on the e 1 ectrophoreti c mobi I it Fes of the si i icate miriera I s

Table 4.1 showed that at pH 2 non-stoichiometric dissolution of lepidolite, spodumene, beryl, muscovite and microcline produced AI/Si ratios in the leach solution that were higher than in the solids. At pH 12, there was a tendency for the soluble Al/Si ratio to approximate to that in the solids except for perhaps muscovite. To determine whether or not the dissolution of the silicate lattice affected the electrical double layer at the si Iicate/water interface, electrophoretic mobility measurements were conducted on samples which had been washed in IM HCI and IM

NaOH for 16 h. Analysis of the wash liquors produced the results summarised in Table 5.1. The mobility curves of the HCI and NaOH washed minerals are shown in Figs 5.6 to 5.10, inclusively.

Table 5.1 Effect of washing the silicate minerals in IM HCI and IM NaOH on the molar ratio, Al/Si, in solution

Mole ratio, Al/Si, in solution Mole ratio, Al/Si, n Mineral ft . , n . , " mineral Acidic wash Basic wash

Muscovite 4.25 I.01 0.87

Lepidolite 1.51 0.66 0.34

Spodumene 1.97 0.63 0.60

Beryl 1.22 0.36 0.32

Microcline 1.66 0.40 0.35 Fig 5.6 The effect of acid and alkaline washing on the electrophoretic mobility of muscovite In ICF M NaCl. (The dashed lines are redrawn from Fig 5.1 and Fig 5.2)

Fig 5.7 The effect of acid and alkaline washing on the electrophoretic mobility of lepidolite in 10"% NaCl Fig 5.8 Effect of acid and alkaline washing on the electrophoretic mobility of spodumene in 10 M NaCl

Fig 5.9 The effect of acid and alkaline washing on the electrophoreti mobility of beryl in I0"3M NaCl I 16

Fig 5.10 Effect of acid and alkaline washing on the electrophoretic mobility of microcline in 10 M NaCI

Comparison of Tables 4.1 and 5.1 shows that although the same AI/Si ratios were not obtained in the two cases because of the different conditions used, the conclusions are similar.

Washing muscovite, lepidolite and microcline with acid (Figs

5.6, 5.7 and 5.10 respectively) produced a shift in the electrophoretic mobility Vs pH curve to more negative mobility values. The shift was largest for muscovite, which also exhibited the greatest dissolution effect. The EM of spodumene and beryl (Figs 5.8 and 5.9) was unaffected by washing the minerals in acid solutions. This is surprising in view of the surplus of aluminium leached into solution. Work by other authors (17 , 35 , 36, 122 ) has shown that AI is preferentially leached from silicate minerals in acidic solutions. It appears however, that acidic scrubbing does not necessarily produce a silica-rich surface. In view of the high Al concentration in the leach solution, it would seem that the disturbed silicate surface must contain a somewhat higher AI concentration than the buIk mi neraI.

Table 5.1 further shows that washing the silicate minerals in alkali produced nearly stoichiometric soluble Al/Si ratios except for muscovite and lepidolite, where a slight excess of

Al dissolution took place. In line with this, the shift to negative mobilities in the EM-pH curves of NaOH washed muscovite, lepidolite and microcline was much less than that obtained for the acid washed samples of these minerals. In the case of spodumene and beryl, however, relatively more positive mobilities, than those of the unleached samples, were obtained after NaOH scrubbing. It appears therefore that NaOH scrubbing did not necessarily produce a surface with a composition simila to that of the buIk.

It can be concluded from the discussion of the results in this and preceding chapters that a systematic method of interpreting the behaviour of aluminosi Iicates in aqueous solution does not exist due to their complex nature. A helpful guide might be some property, closely related to the electro- negativity of the lattice ions (and therefore lattice site behaviour), such as the ionic potential, IP (136). The IP may be defined numerically as the ratio of the charge (z) to the ionic radius (r). Fig 5.11 shows a grouping of some elements 2.0- (Q) aquo cations "(b) ^ -Cs(66) \ 3 X 'Rb(8M TV K(S4) •Ba / /

< .Sr (79) o pronounced ,0" "-Ca (79) •Na(# v hydro Iysi s in zj .Fe(72)^ "n • Zr( 67) "O (D •Mg(74) V.Fe(51) -Ti •Li(79) X 7lr ^ ° 0.5 -Al( 63) - ""'Si( 40) • B^ (50J — — """ X X 'p - — " ^ / sol ub le anions 'C X j i « i i i i I 2 3 4 5 Ionic charge

Fig 5.I I Grouping of elements according to their ionic potentials. Figures in brackets are % ionic bond, (a) and (b) denote direction of increase of acidity and ionicity, respectively.

according to their ionic potentials. The information contained

in the figure can be used to qualitatively explain some of the effects that have been observed in the present and other authors' work. Such information has, for example, been used to explain the behaviour of silicate minerals in weathering experiments

(137). The ions have been arranged into three groups: (a) those with low IP (Z/r < 3.0), such as Na+, K+, Ca2+ and Mg2+, which are weakly held by predominantly ionic bonding and are usually observed to readily pass into solution (23 , 44 , 66 (b) those with intermediate IP (3.0 < Z/r < 9.5) such as Al3+, Fe3+ and Be + with a high tendency to hydrolyse and precipitate due to their low solubilities and (c) those with high IP (Z/r > 9.5) such 4+ as Si , which form anionic radicals in solution.

On this basis, for example, the poor cation exchange response of beryl is consistent with the absence of the mobile cations of group (a) in the mineral, as earlier explained. As a corollary, those a Iuminsi Iicates in which such cations are present can be expected to show a finite exchange affinity. It can also be inferred from the direction of the decrease of ionicity that the Si-0 bond is the strongest in a Iuminosi Iicates

(73 f 74 , 138). This is reflected in the fact that cleavage planes in silicate minerals run parallel to the Si-0 groupings.

Quartz, with no tetrahedral substitution by Al, has no pronounced cleavage (I ). Microcline, another framework mineral, has good cleavage due to the fact that AI goes only into certain tetrahedral positions to avoid 'overbonding' the oxygens ( 7 ). As the substitution of AI for Si increases the tetrahedral bond distance, T-0, increases (138) and the bond strength decreases.

In the polymerised tetrahedral structures of a Iuminosi Iicates, the Al-O-Si bond therefore forms the weakest linkage. In spodumene and beryl, there is no such substitution so that the polymerised tetrahedra are relatively stable towards degradative acidic leaching, the Si-O-Si bond probably limiting accessibility to weaker ionic bonds, particularly in minerals with poor cleavage like beryl. For the minerals in which Al-O-Si bonds were present

(i.e. muscovite, lepidolite and microcline) degradative leaching probably contributed to the negative shift in the EM-pH curve during acidic washing. Loughnan (137) reported a similar 123

observation in the weathering of feldspar minerals. From the relative strengths of the AI-0 and Si-0 bonds, it might be speculated that It Is possible to create a somewhat higher concentration of AI on the surface of the mineral during grinding.

The direction of increase of the acidity of the cations is also shown in Fig 5.11. The presence of different cations imparts different degrees of surface acidity/basicity to the minerals.

Acidic lattice sites will be unstable in basic media while basic sites will be leached in acidic media, as long as there is no structural hinderance of accessibility to the sites.

Tables 5.2 and 5.3 show a more detailed analysis of the wash solutions and of the leached residue. It is clear.from these results that for the alkali wash, the basic cations, notably Fe, tended to be conserved In the mineral, but were substantially lost during acid washing. For the minerals with a reasonable concentration of these cations, the eIectrophoretic mobilities were relatively positive following scrubbing in NaOH, but not for lepidolite and microcline which both contained significantly less basic cations.

The effect of cleavage Is also apparent from the tables.

Beryl, with imperfect cleavage lost the lowest amount of available

Fe during acidic washing, compared to say muscovite. Under the same conditions, muscovite and lepidolite had greatly similar electrophoretic mobilities (Figs 5.6 and 5.7) confirming the observation (|39) that the two minerals differed mainly in the structure and composition of the octahedral layer. Table 5.2 Effect of acid and alkaline washing on the fraction (of total sample content) of Fe and Mn dissolved

% cation in leach solution on basis of mineral composition (see Table 3.3) Acidic wash Basic wash Mi nera1 Fe Mn Fe Mn

Muscovi te 23. 12 21 .71 0.19 0.46 Lepido 1ite 8. 1 1 10. 18 1 .54 0.29

Spodumene 7.07 - 1 .33 -

Beryl 3.09 - 0.73 -

Mi crocli ne 7.6 - 2.16 -

Table 5.3 Effect of acid and alkaline washing on the molar ratio, AI/Si, in the solid residue

Molar ratio, Al/Si, in solid residue MlneraI Acidic wash Basic wash In mineral

Muscovi te 0.75 0.85 0.87 Lepi do Iite 0.33 0.33 0.34 Spodumene 0.47 0.54 0.50 Beryl 0.34 0.33 0.32 Mi crocli ne 0.37 0.36 0.35 In summary, it may be concluded that for a Iuminosi Iicates,

if the polymerised tetrahedra do not contain AI atoms, the

minerals will be relatively stable to acidic exposure; more so

if the mineral has poor cleavage. If Al substitutes for Si,

the mineral will probably degradativeIy leach, particularly if it

has good cleavage. At high pH, the tetrahedra themselves, and

consequently the mineral structure, will be unstable.

The shortcomings of the above treatment are illustrated by

the limited nature of the information that can be obtained from

the use of solution composition to study the surface atomic

rearrangement which takes place during grinding. Perhaps

increased use of techniques like ESCA (140) will supply more

relevant information. The effect of cleavage on the surface

composition is also inadequately defined. It is not easy to calculate the distribution of atoms on cleavage planes (|4|).

Even if this could be done, the composition of the actual disturbed

layer would still have to be related to that of the undisturbed

cleavage plane. These considerations are beyond the scope of this

thes i s.

5.4 Effect of aging on the electrophoretic mobility of the si Iicates

It is known (134) that the aging of silica suspensions

results in a drift in zeta potentials to less negative values.

Electrophoretic measurements were therefore made on silicate

minerals which had been aged in 10 NaCl for different times.

The results obtained for lepidolite, spodumene and beryl are

summarised in Figs 5.12 to 5.14 inclusive. Fig 5.12 Effect of aging on the eIectrophoretic mobility of lepi do Iite in IO"3M NaCl

nr • Aged for 2 weeks

+ 4| O Aged for I month A Aged for I week on a mechanical shaker

+2P

-2

-4 r

Fig 5.13 Effect of aging on the eIectrophoretic mobility of spodumene in IO~3M NaCl +4 . O Aged for I month O Aged for 2 weeks

+2 -

-2 .

-4i -

Fig 5.14 Effect of aging on the electrophoretic mobility of beryl in I0_3M NaCI

In general, the EM of the minerals became less negative with an increase in aging time. These observations are therefore similar to those reported for silica (134) and silicate minerals

(142). However, after long aging times (I month) the EM-pH curves displayed a 'hump1 in the pH range 4-8. In simpler minerals

like quartz, the decrease of the negative EM may be related to the

increase in crystal Iinity with aging time. In a Iuminosi Iicate systems, however, it is unlikely that the decrease in EM is due to this effect. It was suggested In section 2.1.3 that due to the presence of soluble silica in saturated a Iuminosi Iicate solutions, any gibbsite particles that precipitate will be covered with a surface layer of an a IuminosiIicate phase. It Is difficult to assess the physical significance of this layer in the absence of the relevant stability data for the a Iuminosi 11cate solid. Further, because the kinetics of dissolution are not known, the actual a Iuminosi Iicate phase that may be formed under the experimental conditions is also not known. For Illustrative purposes It was therefore assumed that the possible a Iuminosi11cate might be similar to kaolinite (64 ). This assumption is reasonable because in soil science studies, It is widely reported (137, 143) that this mineral commonly forms the weathering residue for the a Iuminosi Iicates studied In the present work. This is obviously related to the low solubility of kaolinite (49 , 143).

A solubility diagram similar to that calculated for gibbsite

(Fig 2.2) was determined for kaolinite using the assumption that the mineral dissolves congruently according to the reaction 5.5

(143):

+ 3+ AloSio0c-(0H). + 6H ^r 2AI + 2H.SiO. + H„0 log k = 7.63 (5 2 2 5 4 (s/ )\ 4 4 2

Further reactions were derived by combining equation (5.5) with reactions (2.13) to (2.16) to yield the equations: 129

Al Si (OH) + 4H + H90^2AI(0H) + 2H.SiO log k. = -2.31 Z 2 (s) Z 4 4 1 (5.6)

AI2Si2(0H)4 + 2H + 3H20 — 2AI(0H)2 + 2H4Si04 log fa = -10.97

(5.7)

2AI(0H) + 2H S lo k AI2Si2(0H)4 + 5H20= 3(aq) 4 '°4 9 3 = "22.37

(5.8)

AI2Si2(0H)4(s) + 7H20z^r 2AI(0H)~ + 2H4Si04 + 2H log fa = -38.37

(5.9)

The above equilibria were solved for the species XJ using the observation that

K<0<*] " [H4SI04] (5.10)

The resulting solubility diagram is shown in Fig 5.15.

Comparison of the diagram with Fig 2.2 indicates, as expected, that gibbsite is more stable than kaolinite over a wide pH range. From the concentrations of soluble aluminium obtained in the dissolution studies, it would therefore appear that gibbsite probably does form, especially in the neutral pH range where it is only sparingly soluble. AIuminosi Iicates in general cannot, therefore, dissolve congruently without the precipitation of gibbsite.

However, as dissolution proceeds the concentration of silicic acid in solution increases. On the basis of equation (2.21), increased conversion of the gibbsite surface to an a IuminosiIicate phase will therefore take place. In the gibbsite/kaolinite model I X ho

oCO

Fig 5.15 Hypothetical solubility of kaolinite at congruent dissolution as a function of pH

OJ o 131

system, it may be shown (143), using equations (5.5) and (2.20), that the transition between gibbsite and kaolinite is defined by -5 a silicic acid concentration of G-^SiO^) = 2 x 10 M, i.e. above this concentration the presence of kaolinite, over that of gibbsite,

is thermodynamicaI Iy favoured. Comparison of the theoretical solubility of amorphous silica to the above value also shows that a solid a Iuminosi Iicate like kaolinite is more likely to precipitate, from a saturated a Iuminosi Iicate solution, than silica. In the present experiments therefore where the solubility of silica was not ever exceeded silica was not a stable solid phase.

In the aging experiments the drift towards less negative

EM values might therefore be associated with the presence of a composite gibbsite/aIuminosi Iicate surface on the mineral particles. Presumably, the gibbsite fraction will be higher in the neutral pH range than at acidic pH values, where the solubility of gibbsite increases. Thus the drift in the EM-pH curve upon aging is more pronounced in the neutral pH range than at low pH.

These conclusions are consistent with a number of results obtained in weathering studies and the partial equilibria that exists between the solid phases present in a saturated a Iuminosi Iicate suspension (51, 137, 143).

5.5 Effect of ffuoride on the electrophoretic mob!Iity of the ST Iicates

The wide use of fluoride as a modifying agent in silicate flotation was noted in Chapter I. Its effect on certain types of silicates has been further summarised by Read and Manser (122). 132

In view of the widely reported successful application of this modifier in silicate flotation, its effect on the silicates used in this study was determined. The electrophoretic mobilities of the untreated, acid and alkali washed silicate samples were -4 measured in the presence of 5 x 10 M NaF as a function of pH.

The results obtained with muscovite, lepidolite, spodumene, beryl and microcline are summarised In Figs 5.16 to 5.20. The addition of NaF had no significant effect on the mobilities of the silicate minerals at alkaline pH values, irrespective of whether the samples had been acid or alkali washed. At acid pH values, however, the mobilities were more negative than those obtained in the absence of fluoride for all the samples investigated. In the case of lepidolite and spodumene prior acid or alkaline washing did not produce significantly different

EM values compared to the untreated samples (cf Figs 5.17 and

5.18). With muscovite and microcline (Figs 5.16 and 5.20 respectively), however, prior washing of the samples enhanced the effect of fluoride so that more negative potentials were obtained. The results obtained with beryl (Fig 5.19) were somewhat contradictory with fluoride producing a more negative surface on acid washed material and a less negative one on the alkali washed sample. It would appear, however, that in general, the effect of fluoride on the electrophoretic mobility is greater than that of acid or alkali washing.

Fig 5.21 shows a distribution diagram of the different -4 fluoride species for a total fluoride concentration of 5 x 10 .

The stability constants of the species considered are (122): 133

1 1 1 1 1 1 ' 1 +4

o Untreated sample • HCI washed sample

+2 A NaOH washed sample E

XI \ \ i i i O e 8 10 \ pH- \s v \ \ \ 0 L_ o -2 x Qo . +- u 0 L±J -4 1 1 i i . i i l.i

Fig 5.16 Effect of pH on the electrophoretic mobility of muscovite in the presence of 5 x 10"% NaF

+4 t 1 r

O Untreated sample e U • HCI washed sample N> +2

\ A (/) NaOH washed sample e

1 1 1 1 r 4 6 8 10 XI pH- O o -2 0 o X ocl o -4 _0 LU

Fig 5.17 Effect of pH on the eIectrophoretlc mobility of IepI do lite in the presence of 5 x I0~4M NaF n ' 1 i 1 1 r

O Untreated sample • HCI washed sample A NaOH washed sample

1 1 1 1 i 1 r \ 4 6 8 10

j i | i | i i l

5.18 Effect of pH on the electrophoretic mobi Iity of spodumene in the presence of 5 x 10 4M NaF

5.19 Effect of pH on the electrophoretic mobility of beryl in the presence of 5 x I0~4M NaF 135

T 1 1 1 ' 1 1 1 I

O Untreated sample • HCI washed sample A NaOH washed sample

1 1 1 1 1 1 1 1 4 6 8 10 pH -

j i • » »

Fig 5.20 Effect of pH on the electrophoretic mobility of microcline in the presence of 5 x I0~^M NaF

3 4 5 6 7 pH

Fig 5.21 Logarithmic-concentration diagram for fluoride species at 5 x I0~4m fluoride concentration H + F — HF log k = 3.18 (5.11)

H+ + 2F"— HF~ log k = 3.76 (5.12)

It was further assumed that only [F ] and [HF] were significant within the pH range considered (i.e. pH 2 to 7). The maximum error involved in making such an assumption was calculated to be less than \% for the given pH range.

The figure shows that the concentration of molecular HF increases continuously with a decrease of pH. At a pH of 3.1 the solution contains equal amounts of F and HF. At pH 2.2 the concentration of HF is ten times that of F and vice versa at a pH of 4.1. The fact that fluoride conditioning had no effect on the EM of the silicates in the pH range where F is the predominant species and that there was a progressive increase towards more negative zeta potentials below pH 7, where the concentration of

HF increased, suggests that HF, rather than F , is the active species. Similar correlations have been observed for beryl and microcline by other authors (33 , 44 ).

However, although it is generally accepted that HF Is the active species and that the main effect of fluoride is to increase the negative zeta potential of the minerals the reason why such an Increase takes place is not known (24). Several explanations have been proposed.

(I) Buckenham and Rogers (32) considered fIuoride adsorption by a Iuminosi Iicates to proceed by the formation of aluminium fluoride complexes on the mineral surface: AI-OH , + H+ + 2F"— (>I-F: )H+ + OH (5.13) surf / 7. _ , surf

(2) Joy et a 1 (35 ) proposed the exchange of OH for F on surface si lanols:

,Si-OH , + F Yi-F + OH (5.14) surf / surf

(3) Smith and his associates (34, 144, 145 ) considered that the adsorption of fIuorosi Iicate ions, derived from the dissolution of the mineral lattice, on aluminium sites occurred:

AI"0Hsurf + SiF6~ = >-SiF6 + 0H" (5"l5) surf

(4) Warren and Kitchener (33 ) suggested specific adsorption of fluoride to form surface complexes:

^'"^surf + F =>-Fsurf + 0H (5J6)

:AI F + F AI F (5 l7) " surf = / " 2 t " surf

A| H + F A (5 l8) -° surf — "

These models are consistent with the increase of pH that is observed in the presence of fluoride. However, Shergold (24)

+ + has pointed out that exchange of K , Na and other similar cations

+ on the mineral surface, for H in solution, will lead to a similar increase in solution pH. The results shown in section 4.2 demonstrate that such an exchange does in fact occur with the silicates studied in this project. A rise in pH cannot therefore be used as an indication of which mechanism is applicable.

Mechanism 2 is not consistent with the increase of negative EM found in this work and elsewhere ( 33 ). Read and Manser (122) have deduced the probable fluoride species for a wide range of concentrations in silicate systems. The predominant species -3 2 + below a total fluoride concentration of I x 10 M are AlF ,

AIF*, and AIF^. The AIF^ species is present only at a concentration greater than ca 2 x 10 3M. This makes the existence of negatively charged aluminium fluoride complexes on the mineral, as suggested by mechanisms I and 4, doubtful, at the concentration of fluoride used. However, at higher concentrations specific adsorption to form such surface complexes appears feasible. In the I x 10 3 to

-2 I x 10 M concentration range Warren and Kitchener (33 ) reported enhanced adsorption on corundum and microcline but not on quartz, at pH 7 and below. They further found that a fluoride concentration

-2 of 10 M was required to reverse the zeta potential on corundum, at a pH of 5.

The above comments are also relevant to mechanism 3. It has been shown ( 33 , 122 ) that the concentration of f I uorosi I i cate species will be insignificant at the concentration of fluoride used in the present work. Further, aluminium and ferric fluoride complexes are much more stable than the fIuorosiIicate ion so that the latter will not form appreciably unless all the aluminium and iron are complexed first (24, 32 ). These considerations suggest that although the effect of

fluoride has been sometimes ascribed to the formation of surface

fluoride complexes, it might be related to enhanced leaching of

aluminium (44 ), particularly at low fluoride concentrations.

5.6 Effect of a i urn?hi urn chlor?de on the eIectrophoretic mobiIity of the sf f i cate itinera I s

Polyvalent metal ions are known to adsorb at the quartz/ water interface and to produce charge reversal in a pH range which is characteristic of the metal cation ( 30, 146 ). It is

reasonable to assume that a si mi lar effect wi I I occur on the

surface of silicate minerals. To determine whether this is the

case, eIectrophoretic mobility measurements were made in the

presence of aluminium chloride. This metal was chosen because it

is known to form a series of hydrolysis products although the

polymeric species are not well characterised. Furthermore, soluble

aluminium is present in solution because of the dissolution of the

silicate minerals. The results obtained with muscovite, lepidolite, -5 spodumene, beryl and microcline, in the presence of I x 10 M and -4

I x 10 M AICI^ are summarised in Figs 5.22 to 5.26 inclusive.

The general shape of the EM/pH curves for each of the silicate

minerals, in the presence of AICI^, is similar and in agreement

with that reported for quartz ( 30 , 31, 146 ). At pH values

below 3, AICI3 had no effect on the EM of the si licates. As the pH

was increased, however, the EM deviated from that obtained in the

absence of AICI^ and became less negative or positive. Maximum

deviation occurred in the pH range 4-6. With a further increase 140

+4 O I x IO"5M AICI • I x IO"4M AICI

-2

-4

J i L

Fig 5.22 Effect of AICI^ on the electrophoretic mobility of muscovite as a function of pH

Fig 5.23 Effect of AICI3 on the electrophoretic mobility of lepidolite as a function of pH 141

Fig 5.24 Effect of AlCI on the electrophoretic mobility of spodumene as a function of pH

Fig 5.25 Effect of AICI on the electrophoretic mobility of beryl as a function of pH 142

+4 - o > e

>- +2

O e O

0 o -£= CL O -2 - +- o 0 UJ

-4 -

Fig 5.26 Effect of AICI^ on the electrophoretic mobility of microcline as a function of pH

in pH, the mobility returned to that obtained in the absence of

AlCly With an increase in AICI^ concentration, the EM became

more positive and the pH range over which charge reversal was

obtained increased. At high pH, however, the charge reversal was

invariably located at pH 9.1, except for muscovite.

The IEP of muscovite was earlier noted to occur at a pH of

5.7. Fig 5.22 shows that below this pH value, the presence of

AICI^ increased the magnitude of the positive charge on the

mineral. This, together with the above observations, suggested

that AI species were not only adsorbing specifically on all the

silicate minerals, but were potential determining. Although the effects of hydrolysable metal cations on oxides and silicate minerals are reasonably well known, the mechanisms by which these effects occur have not been unambiguously defined. Generally, the pH range in which the reversal of the EM to positive values takes place is coincident with that in which hydrolysed metal species are present (27, 30). Similar observations have been made in a number of related experiments such as coagulation (147, 148) and Hallimond tube flotation studies (149). This has led to the conclusion that the observed effects are produced by hydrolysed metal species of the form

(n l)+ n 2)+ M(0H) " (30, 149) and M(0H)2 " (146) rather than the unhydrolysed metal cation.

-4

Fig 5.27 shows a pH-log concentration diagram for a Ix10 M

AICI3 solution. The presence of polymeric Al complexes has been ignored. It is seen from the figure that the concentrations of

2+ AI(0H>2 and Al(0H) are at a maximum in the pH range 4-6. As this is the pH range in which the maximum positive EM was observed, it is tempting to conclude that the form of the EM curves reflect the concentrations of these species. Indeed, on the basis of similar correlations, Fuerstenau and co-workers

(27, 30, 149), Mackenzie (146) and Healy et a I (150) suggested mechanisms by which the M(0H)^n or MCOH)^1"1 species adsorb on the quartz surface. In these mechanisms the adsorption was assumed to involve a combination of coulombic forces between the positively charged complex and the negative mineral surface, and hydrogen bonds between (OH) groups of the complex ions and those on the mineraI.

The suggested mechanisms can be represented in the folIowing way: 144

x I0"4M AICI Fig 5.27 Logarithmic-concentration diagram for

V /OH OH \ / I . \ M (5.19) M + Al(0H)? Al / ^OH ~7—OH / \0H f surf surf

2+ /OH OH-AI 0H \ / (5.20) M + Al(OH) M

/ \QH / \'O H surf surf

0 Al - OH 0H \ / \ / M + HgO (5.21) M + Al(0H)9 / No" / Vsur f surf u 0 Al - OH \ / \ / M + Al (OH). M OH (5.22)

/ \n" surf / V' surf

0 Al2 - OH 2 + \ / M + Al(OH) M (5.23)

/ \n" / Vsur f surf

M-OH — M-OH I 0 + Al (OH) * — I OH 2 (5.24) I + M-OH 0 Al I , I surf 1 OH — M-OH I surf

where M denotes SI or Al. The hydrogen bonding reactions in

(5.19), (5.20) and (5.24) are analogous to those proposed for the bonding of molecular water on si Iica (151, 152, 153).

Mechanisms (5.19) and (5.24) are also similar except that in

(5.19) the bridging reaction is on two dangling bonds that occur on the same surface site whereas in (5.24) the cation is associated with only one dangling bond. Further reactions

involving cation exchange between hydroxylated Al species and the surface have been suggested ( 148):

v /OH /0 Al (0H)(H20)4

M + AI(0H)(H20)^ ^ M + H30 (5.25) / \0H , ^ surf surf

How easily polymeric hydrolysis products can exchange with protons of the surface hydroxide and whether the exchange is 146

equivalent Is not known.

All the reactions (5.19) to (5.25), except (5.22), will produce superequivalent adsorption within the IHP and subsequent charge reversal as observed In the present results. Reaction

(5.22), however, can not produce charge reversal. This has led some workers (147, 154) +o consider that charge reversal in the pH range 4 to 7 Is due to the adsorption of polymeric species 4+

(possibly Alg(OH)20). However, calculations of the concentrations of such species within this pH range shows that these are extremely small and cannot produce superequivalent adsorption.

From the equilibrium constants reported In Chapter 2, the 5+ 7+ concentrations of the species AI^(OH)^ and AI ^ ^(OH)were -14 -24 calculated to be 3.63 x 10 M and 5.89 x 10 M respectively.

Thus the presence of polymeric species Is unlikely to be responsible for the observed electrokinetic behaviour.

The above mechanisms do not explain why the unhydrolysed

Al3+ cation dees not reverse the surface charge or indeed why

+ 2 +

Al(0H)2 or Al(OH) appear to do so. As outlined In Chapter 4, there Is evidence to suggest that adsorbed polyvalent cations retain at least one layer of their hydration water (101, 124).

Ionic hydration may therefore prevent the close approach of highly charged (highly hydrated) cations to the surface. The lowering of Ionic potentials by hydrolysis decreases Ion-solvent interactions. This results in a smaller hydration sphere for the ion and permits closer approach to the surface so that greater coulombic and short range interactions are possible. On this 3+ basis, the highly hydrated AI cation is expected to reside outside the IHP and therefore show no specific adsorption or charge reversal effect.

Similar considerations have been summarised by James and

Healy ( 31 ) who considered the change in free energy upon adsorption of the species K^G0^., be a combination of the changes in coulombic energy, secondary solvation,

AG • , and chemical affinity, AG , so I v,i 7' chem,i

AG°. . AG° .. + AG° . . + AG°, . (5.26) ads,i coul,i solv.i chem,i

As AG„ , is of opposite siqn to both AG . and AG , , a solv rr a coul chem reduction of charge on the cation by hydroxy Iation favours

specific adsorption by reducing AG°q|v.

The significance of hydrolysis phenomena in explaining the

EM results, therefore, appears to be related to the presence of weakly hydrated Al(lll) species for which there exists a possibility of cooperative hydrogen and coulombic bonding.

Other evidence, however, suggests that the relationship between adsorption of metal cations and hydrolysis is not a simple one. James and Healy ( 31 ) observed that at high pH where the concentration of CoOH+ was extremely low, there was no decrease in the adsorption of Co(III) species on quartz. Figs

4 5.22 to 5.26 show that in the pH range 6 to 9, at I x I0" M AICI3, the si licate particles were positively charged. At a pH of 7, the calculated concentrations of Al(OH)* and Al(0H)2+ are 1.58 x 10 and 3.39 x 10 ''m respectively. These low concentrations are unlikely to produce the high positive mobilities observed at this pH James and Healy ( 31 ) argued that the abruptness of the first charge reversal ( - to + ) in the pH range 3 to 5 is typical of a system in which a critical change in the solution properties is taking place. They therefore related this charge reversal to the presence of a surface-nucleated metal hydroxide precipitate. According to these workers, above the precipitation pH, the quartz particles reflected the properties of the metal hydroxide, dependent on the degree of surface coverage. This theory explains a number of the observations made in the present investigations. For example, a pH of 9 corresponds approximately to the IEP of AI(OH), ( 155). At higher pH (s) therefore both the mineral particles and precipitate have charges of the same sign so that coulombic adsorption of the precipitate is not possible. A reduction in the metal cation concentrations would reduce the amount of metal hydroxide obtained and hence the surface coverage. A shift in the point of zero EM to lower pH values for the composite mineral surface would therefore be expected. For beryl the shift was relatively small (low surface coverage) and for lepidolite (high surface coverage) no positive mobility was observed at low aluminium concentrations.

However, although the results obtained are consistent with the presence of an aluminium hydroxide layer, it is not possible to unambiguously separate the effects of hydrolysed aluminium species and an aluminium hydroxide precipitate, as both the products are present within the pH range in which charge reversal was obtained. This is clearly demonstrated in Fig 5.27. If it is assumed, as has been suggested (27 , 30 ) that charge reversal ( - to + ) in the pH range 3-5 is due to hydrolysed species, some change in the EM curve might be expected as the transition is made towards the presence of metal hydroxide precipitate on the surface of the mineral, at higher pH values. Such a change was not observed, neither has it been noticed in the adsorption studies by other workers (31 ). Further, if the above assumption

is true, coulombic adsorption of AI(OH)^. , which is positive below pH 9, on the positive mineral surface, produced by the

initial adsorption of hydrolysed species, is unlikely. Along similar lines, the presence of AI(OH), in solution does not ^(s)

readily explain how positive metal hydroxide species increased the positive mobility on muscovite (Fig 5.22).

From the results obtained in this work, it is not possible to determine the mechanism of interaction of aluminium at the si Iicate/water interface. The results do, however, suggest that a si milar mechanism is involved with all the si Iicate minerals studied and that after the addition of AICI^ similar eIectrophoreti mobilities are obtained. CHAPTER SIX

THE ADSORPTION OF AMINE AT THE SI LICATE/WATER INTERFACE"

AND ITS EFFECT ON THE HALLIMOND TUBE FLOTATION OF THE SILICATES 151

6.I Adsorption studies

The adsorption of dodecylamine at the silicate mineral/ water interface was measured as a function of amine concentration and pH and the results obtained compared to eIectrokinetic measurements under similar conditions.

6.1.1 Effect of DA cohceritration

The adsorption isotherms of DA on lepidolite, spodumene, microcline and beryl, at pH 9.0, are shown In Fig 6.1. The isotherms are consistent with the generally accepted theory for the adsorption of long chain electrolytes by oxides and silicates outlined in Chapter 2. All the isotherms were sigimoidal and of a similar shape to those designated as S2 in Giles et al classification system (156). This suggests that although the adsorbing species were monofunctionaI, they had moderate van der Waals interactions at the mineraI/water interface (156). The shape of the isotherms also indicates that there was strong competition for the negative substrate sites from inorganic cations present in bulk solution. All the isotherms were independent of the so Iids/1iquid ratio in solution, indicating that adsorption was reversible. This is shown for spodumene in Fig 6.1.

Fig 6.2 compares the EM of spodumene and the amine adsorption density as a function of the equilibrium concentration of dodecylamine at pH 9.0. The similarity in shape of the two curves suggests that both the electrokinetic and adsorption data were concerned with the same surface phenomenon (135). 152

1 1 1 1 1—i—i—i—» 1 t r O Lep i do Ii te a Spodumene 3 Mi croc line D Beryl * Desorption on spodumene Vertically oriented monolayer assuming cross • sectional area of 20°A for hydrocarbon chain ^

-4 IxlO 5x10 1x10 5x10 Equilibrium concentration of amine, M

Fig 6.1 Adsorption isotherms of DA on the silicates at pH 9 1 1 1 r i i i t 1 r 1 12 O Electrophoretic mobi I ity OJ i Adsorpti on

~ 10 - / +4 Region 3 / vo o / / / x / / o +2 4- Region 2 j q^ (/A) 0 -a c 0 O 4- $a_. o 0 XJ to -2 0 c Region 1

-4 • i i * i • i i 1 1 1 2xlO"5 5xlO-5 IxI0"4 5xTO"4 Equilibrium amine concentration, M

Fig 6.2 Comparison of the adsorption density and electrophoretic mobi Ii ty data at pH 9

It is convenient to discuss the isotherms by dividing

them into three different regions as demonstrated in Fig 6.2.

Region I: In this region, there was a slow increase in the

adsorption density with the logarithmic DA concentration. The

slope of the EM curve in the region was, however, not zero as

has been reported in many similar studies ( 77, 110, 135). Furthe

different adsorption levels were observed for the different

silicates; lepidolite apparently adsorbed the most and beryl the

least amount of amine.

Region 2: The region was characterised by a marked increase in

the adsorption density of DA and a corresponding decrease in the EM to less negative values. The mobility subsequently reversed -5 sign at a DA concentration of 8 x 10 M. Although the minerals had different adsorption densities in region I, the equilibrium concentration at which the marked adsorption increase took place

(CHMC) was approximately the same for all the minerals.

Region 3: In this region, the increase in adsorption density with amine concentration became smaller than that in region 2.

At high DA concentration, the isotherms were characterised by a near-zero slope, and convergence to the same adsorption density.

Region 1: The part of the adsorption isotherms in this region revealed several interesting characteristics when compared to those reported in the literature. Firstly, the observed adsorption densities were much higher than those reported for a IkyIammoniurn ions on quartz and alumina (135, 157, 158). At a pH of 9 and a -5 dodecyIammoniurn chloride (DAC) concentration of 5 x 10 M, Li -12 and De Bruyn found an adsorption density of ca 2 x 10 moles

-2 cm on quartz. This is two orders of magnitude less than the adsorption densities shown in Fig 6.1 at similar concentrations.

Secondly, at pH 9, the EM values of lepidolite, spodumene, beryl and microcline were (Figs 5.1 and 5.2) 3, 3.3, 3.5 and 3.5 ym sec '/volt cm ' respectively. The corresponding estimates of the zeta potentials using equation 5.2 are 38.5, 42.3, 44.9 and

44.9 mV, respectively. The similarities in the zeta potential values suggest that the coulombic contribution to the adsorption of amine in the Stern layer should be almost the same for the silicates studied. However, different adsorption densities were obtained. Thus, in the case of beryl and microcline, similar 155

EM values were obtained at pH 9 but the adsorption density of -5

DA on microcline at a concentration of 5 x 10 M, was nearly

twice that on beryl.

The different levels of DA abstraction obtained therefore

suggest that some additional mechanism, other than simple ion exchange in the diffuse layer, was operative. Reference to the

cation exchange results shown in section 4.2 suggests that this additional mechanism is related to the cation exchange properties of the minerals. This is because the order in which the adsorption

densities decrease, i.e. lepidolite > spodumene microcline >

beryl corresponds to the decrease in the cation exchange ability of the si Iicates.

To test the validity of this hypothesis, the concentrations of monovalent lattice cations (Li+ and/or K+) in equilibrium with the minerals, in the presence and absence of DA, were determined.

The results, summarised in Table 6.1, were compared to the

adsorption density of amine at the mineraI/water interface. To

facilitate comparisons the concentrations of the lattice cations

in equilibrium with the minerals were expressed in moles per

unit area of each mineral.

It is clear from these results that the presence of amine in

solution increased the concentration of the lattice cations in equilibrium with lepidolite, spodumene and microcline. Although + + .

beryl contained little or no Li and K (cf Table 3.3), it is

included in Table 6.1 to facilitate comparison of its adsorption

density with the other silicates. If the exchanged cations are

ignored in computing the adsorption densities (i.e. column 2

subtract column 5), the adsorption density (column 6) obtained Table 6.1 Effect of cation exchange properties on the adsorption density of DA from I x 10 4M DA solutions

Mi neraI Total amine 2 Total cations in Cations in so In Total cation Ads density excluding ads (moles/m ) so In (moles/m ) in absence of exchanged cations exchanged DA (moles/m ) (moles/m ) (moles/m2) (Li + K) (Li + K)

-6 -6 -6 -6 -6 Mi croc Ii ne 4.03 x 10 4.78 x 10 3.25 x 10 I.53 x 10 2.5 x 10

-6 -5 -5 -6 -6 Spodumene 4.75 x 10 I.74 x 10 I .53 x 10 2.10 x 10 2.65 x 10

-6 -6 -6 -6 Lep i do Iite 2.68 x 10 8.87 x 10 6.58 x 10 2.29 x 10 3.9 x 10,- 7

-6 Bery I 3.18 x 10 3.18 x 10- 6 for spodumene and microcline (weak exchangers) are similar to

that of beryl (non-exchanger). It is probable therefore that

at low DA concentrations, the predominant mechanism of

adsorption is by cation exchange; this is particularly true for

the minerals with good cation exchange capacities. In the case

of lepidolite, for example, practically all the DA was abstracted -4

through cation exchange (cf columns 5 and 6) at I x 10 M DA.

However, this does not mean that coulombic forces are unimportant; they provide the driving force for the exchange while the presence

and amount of weakly held cations determine the degree of exchange

activity. These results are therefore in agreement with the

observations made in Chapter 4 that monovalent cations, which

are held predominantly by weak ionic forces, are readily exchanged.

The concentrations of AI and Si in solution were not included in

Table 6.1 because they did not change significantly in the

presence of DA.

A further point of interest is whether the DA+ ions should

be considered to be specifically adsorbed or merely concentrated

in the diffuse double layer, as reported by a large number of

authors (26 , 27 , 77 , 135, 157). According to double layer

theory, an ion located in the diffuse layer is not associated

with any discrete exchange site on the surface of the mineral

(101). It is unlikely, therefore, that non-specificaI Iy adsorbed

DA+ ions would lead to an increase of inorganic cations in solution

as demonstrated by the results in Table 6.1.

Shainberg and Kemper (101) have assessed the role of ionic

hydration in electrical double layers. They estimated that the fraction of Li , Na and Ka which have a completely oriented first molecular shell of water Is 0.97, 0.80 and 0.46, respectively. If their model is applied to dodecyIammoniurn and trimethyIammoniurn ions, the fraction of the ions with a complete primary hydration shell is about 0.23 and 0.07, respectively. Leja (159) is of similar opinion, that for

+ monovalent cations larger than K , the effective number of bound water molecules is nearly zero due to their low Ionic potentials.

It appears, therefore, that LI+ and Na+ are completely hydrated In aqueous solutions. This would explain why Li+ did not show any potential determining effect on lepidolite and spodumene,and why Na+ acted as an indifferent electrolyte for the minerals as has also been observed for quartz (78 ). It does not explain, however, why incompletely hydrated alkylamonium ions adsorb non-specificaIly on quartz, at low amine concentrations

( 78 ). It may be that because quartz is a non-exchanger, the low adsorption densities do not lead to significant changes in the

Stern potentials. Thus the slope of the zeta potential - amine concentration curve remains zero at low surfactant concentrations.

In the case of a IuminosiI icates, however, amine adsorption densities, at low concentration, are higher due to finite exchange capacities. The changes in Stern potentials in response to specific adsorption are therefore significant. It Is thus possible that the Increase of slope observed in the EM curve of Fig 6.2 in + region I was related to specifically adsorbed DA ions.

A practical example of the above considerations Is the

+ + + + flotation of salts. For the alkali halides of Li , Na , K , Rb , Cs and NH^ , Fuerstenau and Fuerstenau (160) obtained flotation with dodecyIammonium chloride only where the lattice cation was larger than K + . For the alkaline earth chlorides of Mg 2+, C2a + ,

0 2+ 2+

Sr and Ba , only BaCI2 floated with the same collector. These authors considered that ionic sizes controlled flotation and that DA+ ions could fit into the lattice sites of the floatable saIts.

It has been argued (161), however, that this theory is unlikely because a Iky I sulphates also float KCI but not NaCI, although both lattices contain the chloride ion. A more likely explanation (161) is therefore that salts of NaCI and LiCI are strongly hydrated compared to those of KCI or of larger sized cations (162). DodecyIammonium ions, that are themselves only weakly hydrated, are therefore capable of adsorbing specifically on the floatable salts.

Region 2: According to the generally accepted theory of long chain surfactant adsorption on oxides, the abrupt increase in adsorption density in region 2 is because of the formation of hydrophobic associates (hemi-miceIles) between the surfactant hydrocarbon chains. The formation of hemi-miceI Ies at the mineral surface thus provides an additional adsorption mechanism which is believed (NO, 158, 163) to be driven by entropic changes arising from hydrogen bonds between water molecules.

The hemi-miceIles were conceived, by Gaudin and Fuerstenau

(78 ), as arising from a particular level of surfactant adsorption at the mineraI/water interface. If, therefore, two minerals have different zeta potentials, different CHMC values are expected in region I. The silicates used in this study had different amine adsorption densities at the CHMC, although their zeta potentials were similar at pH 9. If the formation of hydrophobic associations is solely initiated by a particular amine adsorption density, the CHMC values of the silicates should have been different. The similarity of the amine concentration at the CHMC therefore suggests that the adsorption density was not the only critical factor that determined the location of the CHMC.

Smith (26) has suggested that in the dodecyI amine/oxide system, the CHMC depends on the ratio of charged to neutral species both in the double layer and the bulk aqueous phase.

Thus in this system the CHMC would appear to be a bulk solution property, so that van der Waal's associations are not only initiated by a particular DA+ adsorption density at the silicate- solution interface, but by the presence of a particular concentration of neutral molecules in bulk solution. This conclusion agrees with a number of studies involving alky I ammoni urn col lectors ( 32 , 97 ,

135 , 164) in which the greatest surface activity of the surfactant is observed in the presence of unionised amine species.

Region 3: The decrease in slope in the adsorption isotherms at the start of region 3 has been attributed ( 77 ) to the formation of a surface which retards further adsorption of cationic amine.

An alternative viewpoint (25 , 80 , 81 ) is that condensation takes place on increasingly less energetic sites as saturation of the mineral surface is approached.

Fig 6.3 shows the surface tension of DA as a function of concentration at a pH of 9. It is clear that the CMC of DA at -4 this pH occurred at a concentration of 3.2 x 10 M. Comparison —r —I t n

80 1 - 1 x I0"2 NaCI

60 -

40 • H-xO—Q—Ol_o— o

• _J 1 1

5xlO"5 IxlO 4 5x10~4 IxlO"3 5xl0"3

Amine concentration, M

Fig 6.3 The surface tension of dodecylamine solutions at pH 9

with Fig 6.1 shows that saturation adsorption levels occurred at amine concentrations at or slightly above the CMC. Similar observations have been made by a number of other authors ( 81 ,

I 10 , 158).

The adsorption density marked as a monolayer in Fig 6.1 was obtained by assuming that the cross sectional area of the hydrocarbon chain was 20°A2 (134, 161, 165) and that this area was larger than that occupied by the polar head. Using this value a close packed, vertically oriented monolayer would be

—6 —2 formed at an adsorption density of 8.3 x 10 moles m . For the isotherms in Fig 6.1, saturation adsorption therefore occurred at 1.3 monolayers. Similar saturation adsorption densities have been obtained by other workers ( 81 , 110) where they have been explained by the formation of a bi layer surface, in which a second layer of surfactant molecules are oriented with their hydrophilic polar heads towards bulk solution. Such a mechanism would explain the convergence of the adsorption isotherms at high DA concentrations. According to the pseudo-phase model for surfactants (166), beyond the CMC the concentration of micelles increases linearly with increase in surfactant concentration.

The invariant slope of the isotherms therefore shows that micelles do not adsorb to any significant extent on a surface consisting of a bi layer. An alternative explanation is that suggested by

Scamehorn et a I ( 81 ) that only monomer adsorbs. Since the concentration of monomer is constant beyond the CMC saturation adsorption is observed under these conditions.

6.1.2 Effect of pH on the adsorption of amine by the Sit i cates

Fig 6.4 shows the effect of pH on the adsorption density of DA on spodumene at three different concentrations of the surfactant. In general the adsorption density increased with pH and this is to be expected because of the increased coulombic attraction between the mineral and the cationic amine ion. Similar shaped curves have been reported by De Bruyn (157) for the adsorption of amine on quartz. Interestingly, the curves did not display a gradual increase in adsorption density with pH; rather the adsorption density increased more markedly at high pH values. O |.5 x I0"4M A I.0 x I0"4M 0 - • 6 x I0"*5M

Monolayer coverage

6 _

6 7 8 9 10 II 12

Fig 6.4 Effect of pH on the adsorption density of dodecylamine at several surfactant concentrations

Consideration of the solution equilibria of dodecylamine shows that at alkaline pH values the concentration of neutral molecules increases at the expense of a decrease in the concentration of cationic species. The marked increase in adsorption density at alkaline pH values can, therefore, be attributed to the formation of hydrophobic associates in which the neutral amine molecules act as bridges. This theory is consistent with the considerations out Ii ned above and has been we I I expI a i ned by Smi th (26 , 97 , 164 ). The highest concentration of DA investigated was determined -4

by pH-solubi Iity considerations. At 1.5 x 10 M DA there is a

clear tendency towards saturation adsorption in the isotherm at

1.3 monolayers. In this case it is probabIy vaIid to consider

that this corresponds to the formation of a bi layer as explained -4

above. At I x 10 M amine there was a tendency to saturation

adsorption although the packing density was less than a monolayer.

However, the data at high pH is insufficient to unequivocally

confirm this observation. In a study of the effect of pH on the

adsorption density of dodecyItrimethyIammoniurn chloride on

alumina, Doss (158) found that saturation adsorption was dependent

on pH and that plateau adsorption could be obtained at surface

coverages of much less than a monolayer. It appears, therefore, that under some conditions of pH and surfactant concentration

close packed monolayers and bilayers are not formed although

plateau adsorption may be obtained. Under such conditions

saturation adsorption appears to be related to the constant supply

of monomer beyond the CMC ( 81 ) rather than the presence of

bi layers on the mineral surface.

6.2 Ha I Iimond tube studies in the presence of amine

Fig 6.5 shows the results of Hal Iimond tube flotation tests

on spodumene, beryl, microcline, lepidolite and muscovite, in the -5

presence of 3 x 10 M DA. These results are consistent with the

adsorption results and show that good flotation was obtained in the

pH range corresponding to good amine adsorption. As the pH

decreased towards the IEP values of the silicates there was a pH

Fig 6.5 Effect of pH on the flotation recovery of the silicates in 3 x Idodecylamine solutions

decrease in recovery. Muscovite, however, showed an increase in recovery below the IEP. This was probably due to an increase in the cation exchange activity as the pH decreased (cf acid/base titrations, Chapter 7).

The good exchangers, muscovite and lepidolite, showed a more gradual increase in recovery with pH. The poor exchangers, on the other hand, showed a more defined critical pH of flotation at which there was a marked increase in recovery.

At pH values above 12, there was a sharp drop in flotation recovery. This can be attributed to the precipitation of the amine from solution and the low levels of cationic amine present in solution. Unfortunately direct comparison of the Hallimond tube and adsorption data is not possible because of the differences

in the concentration of amine used in the investigations. At the

lowest initial DA concentration used in the adsorption tests -5 (5x10 M) a surface coverage of about 25$ was indicated on -5 spodumene at an equilibrium concentration of 1.4 x 10 M (Fig 6.1). -5

Hence at an initial concentration of 3 x 10 M, which is that used

in the flotation tests, a lower surface coverage is indicated.

This is in line with the observation that only a small surface coverage is required for complete flotation (157, 165) to occur. CHAPTER SEVEN

ADSORPTION OF STARCH AT THE SILICATE MINERAL/WATER

INTERFACE AND ITS EFFECT ON THE HALLIMOND TUBE

FLOTATION OF THE SILICATES WITH DODECYLAMINE 168

7.I Adsorption of starch polymers

The adsorption densities of several different starches on the different silicate minerals were measured as a function of equilibrium concentration and pH. The different polysaccharides used in this study were potato and maize starches, British gum and both white and yellow dextrins.

7.1.1 Effect of equiIibrium Starch'concentration on the starch adsorption density

The adsorption densities of potato starch on spodumene and beryl (Fig 7.1), microcline, lepidolite and muscovite (Fig 7.2) are shown in Figs 7.1 and 7.2 as a function of equilibrium concentration at pH 10. The isotherms are similar to those reported by Shulz and Cooke ( 37) for the adsorption of potato starch on quartz, and are best described as of the Freundlich type. With increase of starch concentration, the adsorption density did not exhibit saturation. Such isotherms have also been reported (82) to apply to the adsorption of polysaccharides on activated coal. All the isotherms were reversible; this was determined by using different solid to liquid ratios in suspension.

This suggested that the adsorbing forces were weak such as those operative in a physical adsorption process. Reversible adsorption isotherms have also been reported for the adsorption of PEO on silica (84) although it is generally agreed that most polymer adsorption isotherms are irreversible because of the difficulty of simultaneous adsorption and desorption.

The adsorption isotherms of maize starch and British gum on spodumene, at pH 10 (Fig 7.3) were similar in shape to those 169

Equilibrium starch concentration, mg I 1

Fig 7.1 Adsorption density of potato starch on spodumene and beryl as a function of equilibrium concentration at pH 10

Equilibrium starch concentration, mg I '

Fig 7.2 Adsorption density of potato starch on mi croc Ii ne, lepidolite and muscovite as a function of equi I i bri urn concentration at pH 10 Equilibrium concentration, mg I '

Fig 7.3 Adsorption density of maize starch and British gum on spodumene as a function of equilibrium concentration at pH 10 171

obtained with potato starch. Those of the dextrins (Fig 7.4),

however, were more Langmuirian and similar in shape to the L2

curves in Giles et al (156) system of classification, with the adsorption densities being an order of magnitude less than those of starch. Published data (82 , 88 , 156) indicate that adsorption

isotherms of low molecular weight polymers are best described by the Langmuir isotherm. The reason for this is, however, not

known although it appears that it may be related to the observation

that low molecular weight polymers appear to lie closer to the surface than those of greater chain length (114). As explained in

Chapter 2, it is possible to fit a variety of theoretical isotherms to the same experimental polymer adsorption data. For this reason

no attempt was made to calculate the parameters of the suggested theoretical isotherms. It was noted, however, that the adsorption

density of the biopolymers on spodumene decreased in the order

potato starch > maize starch ^British gum > dextrins. It is

later shown (viscosity studies) that this was due to differences

in the molecular weights of the polymers. Other studies (37 )

also indicate that dextrins do not adsorb on quartz to the same

extent as whole starches.

7.I.2 Adsorption mechanism of starch

7.I.2.I Eiectrokinetic properties of starch granules

The starch solutions used in the adsorption experiments were

optically clear and could not therefore be used directly in the

determination of the sign of charge of the biopolymers in aqueous

solutions. The reported EM results were obtained for potato 0.05-

0.04

CiM 0.03-

0.02 - X 0 t3 • Yellow dextrin "o 0 O White dextrin x t_ o 0 0.01 ~

20 40 60 80 100

Equilibrium dextrin concentration, mg I '

Fig 7.4 Adsorption densities of the dextrins on spodumene as a function of equilibrium concentration at pH 10

o • In 2.5 x I0"2M NaCl > +2 - \ O In absence of NaCl 0 £

0 10 i i x o PH o -2 - 0 o x oCL -o- -4 .

LU J L _i i i i i i 1 l

Fig 7.5 Electrokinetic properties of starch granules in the absence and presence of NaCl as a function of pH starch granules suspended In water at the desired pH and to which was added, In some cases, NaCI equivalent to that which would be

present as a result of neutralising the causticised starch solutions to similar pH vaIues.

The EM results obtained at different pH values are summarised

in Fig 7.5. It Is evident that starch Is predominantly negatively charged at a I I alkaline pH values, the magnitude of the EM being greater In the absence of added salt. Similar results were

reported by Balajee and Iwasaki (42) and Sterling (167). Apparently the negative charge arises from the dissociation of alcoholic hydroxy I groups present in the starch structure (cf section 2.5).

The magnitude of the negative charge, however, suggests that the alcoholic hydroxy Is in starch are comparatively more acidic than those of monohydric, aliphatic alcohols. For example, the dissociation constant In aqueous solution at 25°C of methanol

i fo and other simple alcohols is ca I x 10 (93). In contrast, the dissociation constants for carbohydrates are reported to lie In -12 -14 the range 10 - 10 (93) and in particular the pk of starch

is approximately 12 (168). Qualitatively, the higher acidity of hydroxy I groups in carbohydrates has been attributed to the greater number of such groups In the polymeric structure (93 ,169 ) and the higher lability of the proton of the hemiacetal hydroxyl

In the anomeric position. The latter condition apparently stems

from the electron-withdrawing effect exerted on the group by the neighbouring ring oxygen (93 ). The monohydric alcohols having no strong electron-withdrawing substituents are relatively weaker aci ds. 174

By analogy with these results, it can be assumed that the causticised starch polymers were negatively charged under the conditions used in the adsorption experiments. It is highly unI ike Iy, therefore, that electrostatic attractions were responsible for adsorption of the polymers on the silicate minerals at the high pH value used, particularly as multivalent cations were absent. These cations can lead to neutralisation and reversal of the negative charge at the mineraI/water interface, thereby aiding the adsorption of starch.

7.I.2.2 Effect of pH on the adsorption of starch by spodumene

Fig 7.6 shows the adsorption density of potato starch on spodumene as a function of pH at three different initial starch concentrations. With a decrease of pH below pH 12, the adsorption density increased linearly, and the same slope was obtained for the different starch concentrations. Similar results have been obtained for the adsorption of starch on quartz (37, 42) and calcite (43). Beyond a pH of 12 the adsorption density decreased markedly.

The decrease of adsorption with increase of pH suggests that adsorption of starch took place via sites whose form strongly depended on pH. The sites on the mineral surface most likely to be involved in the adsorption process are the si lanol sites. The degree of dissociation of these sites would increase with pH. In the case of starch the appropriate sites would be the alcoholic hydroxy I groups which, as discussed above, become more anionic with an increase in pH. It would appear feasible, therefore, that the starch adsorption increased with a decrease of pH because of 75

0.81—

0.6h

0.4 h

0.2 L

Fig 7.6 Effect of pH on the adsorption density of potato starch on spodumene at several starch concentrations 176

the formation of hydrogen bonds between surface silanols and other

M-OH groups, and the starch hydroxy I groups. At high pH values, the progressive dissociation of both sets of hydroxyl sites made hydrogen bond formation unlikely. The increase of mutual repulsion would also be expected to decrease the adsorption density as the pH increased. Another contributory factor could be that with an increase in pH, the solution environment became a 'better' solvent for starch resulting in decreased adsorption of the polymer.

It was mentioned in Chapter 2 that in the adsorption of PEO on silica by hydrogen bonding, the bridging proton appears to be donated by the si lanol site. Bremenko and Sergienko (170) further concluded that adsorption of PEO on silica occurred on strongly acidic silanols which had a maximum capacity for hydrogen bonding due to a high proton lability. In the PEO-silica system the decrease of adsorption density with increase of pH can, therefore, be associated only with a decrease of undissociated si lanol sites with increase of pH.

In the present system, however, both the surface hydroxide and the alcoholic hydroxyls possess both donor and acceptor properties. It is reasonable to expect, therefore, that in weakly alkaline solution multiple hydrogen bonding reactions are possible between, for example, Si-OH and R-OH, Si-0 and R-OH, and Si-OH and R-0 sites, where R-OH is the alcoholic hydroxyl and R-0" its alcohol ate anion. At pH values greater than 9, it is likely that only Si-0 type sites are present on the surface, as found for example on silica (153). In this case adsorption can only take place between Si-0 and R-OH sites. In the neighbourhood of the pk value of starch, adsorption is drastically reduced because all sites are now present only In the dissociated form. It is well established in carbohydrate chemistry that the

acidity of the alcoholic hydroxy Is decrease In the order C2~0H >

C6-OH > C -OH > C4-0H (93). These differences in proton lability would be consistent with the progressive decrease of starch adsorption with increase of pH observed up to pH 12.

7.1.2.3 Effect of methylation on the adsorption density of starch

Adsorption of potato starch on methylated spodumene was carried out to confirm that adsorption occurred by a hydrogen bonding reaction. The reaction between trimethyIchlorosi lane and si lanol groups Is reported as (115, 153):

—;SI -OH , + (CH,),SICI — ^Si -O-Si (CH,)^ + HCI (7.1) / surf 3 3 / 3 3 , surf

It was therefore argued that if adsorption of starch took place on surface silanols, a decrease In adsorption density should result upon replacement of these groups with trimethyI si lyI groups.

Fig 7.7 shows the results obtained. Clearly there was a decrease in the adsorption density of starch upon silylation of the mineral surface although some adsorption was still taking place. This was attributed to the fact that conversion of the surface hydroxy Is was probably not quantitative because the trimethyIsi lyI group Is apparently too large and blocks access to adjacent sites (153). Estimates from similar studies on silica suggest that only 40-50$ of si lanol groups form trimethylsi loxane bonds (115, 153). It is also possible that some rehydration of 178

I .4

.2 _

0.8 -

0.4 -

10 20 30 40 50 Equilibrium starch concentration, mg I

O Untreated spodumene • Methylated spodumene Untreated spodumene at pH 10

Fig 7.7 Adsorption density of potato starch on methylated spodumene as a function of equilibrium concentration at pH 9 the hydrophobic surface took place (||5) at the a Ika)ine pH used,

and that this resulted in the abstraction of a small amount of

hydrogen bonded polymer. The results of Fig 7.7 are, however, in

qualitative agreement with the results of a large number of

studies involving polymers which adsorb by a hydrogen bonding

mechanism and in which a decrease in adsorption was obtained by either a methylation technique or dehydroxi lation by heat

treatment ( 84 , 95 ,114, 170 ).

The formation of alcoholates might be of relevance to the adsorption mechanism, particularly since there was a significant amount of NaCI in solution. A reduction in the negative charge of starch could have, for example, taken place through the formation of cation-containing chelates of the form suggested by Rendleman

(93):

C C

H —0 0

This author reports also that chelates of multivalent cations are more stable than those of monovalent analogues. It might be argued, however, that if formation of such species is significant, beryl with more Fe on the surface than spodumene would have abstracted more starch than the latter. The contrary was, however, observed to be true, suggesting that if formation of these species took place, they were not significant contributors to the adsorption mechanism. 180

7.1.2.4 Adsorption of H+ arid OH ions by the si I icate mirieraI s

Although the preceeding considerations indicate that adsorption of starch occurs by hydrogen bonding, this does not readily explain the observed order of adsorption on the silicates which decreased in the sequence spodumene > microcline > beryl > lepidolite > muscovite. It has been adequately shown ( 84 , 94 ,

95, 170 ) that the adsorption of some synthetic polymers on silica, by hydrogen bonding, is dependent on the surface silanol density. By analogy, it would be expected that the observed order of potato starch adsorption on the silicates would be influenced by similar considerations. Acid/base titrations were therefore conducted to obtain more information on the density of acidic sites on the silicate minerals. Similar studies have been used successfully in the characterisation of silanol groups on different silica modifications (153).

The experimental technique (171) consisted of the potentiometric titration of the mineral suspensions in aqueous electrolyte

(I0~3M NaCl) solutions with H+ or 0H~ ions at 25°C. A blank electrolyte solution of the same volume was then titrated with acid or alkali in the same manner. The difference between the amounts of OH or H+ ions required to produce the same pH in the silicate suspension and the blank sample of electrolyte, gives the amount of OH or H+ ions adsorbed at the silicate/water interface. A gas-tight titration vessel was used and purified nitrogen was passed through to avoid contamination from atmospheric

C02. After each addition of acid or alkali,.the suspension was equilibrated for 30 mins before pH measurement. Preliminary tests 181

showed that a reasonably steady pH value was obtained within

this time limit.- The pH of the suspension was kept lower than

pH 9 to avoid bulk dissolution of the silicate minerals.

The results obtained for spodumene, beryl and microcline are shown in Fig 7.8. Muscovite and lepidolite were not included in these experiments due to their good cation exchange properties.

The adsorption density, r(H-OH), on spodumene was observed to decrease markedly upon methylation of the mineral as would be expected from the above considerations. With a decrease of pH below pH 8, there was a general increase in the r(H-0H ) values.

The order of abstraction on the minerals was spodumene £ microcline

> beryl. As this order is similar to that observed for the adsorption of starch on the minerals, it is tempting to conclude that adsorption of starch is related to particular surface hydroxy I densities on the respective silicates.

However, this order is also similar to that established in the cation exchange studies. Further, comparison of the above data to similar results on quartz (I "71) and alumina (1*72) shows that adsorption densities in the present work are much higher than those obtained in many similar studies, it is therefore likely that there was an exchange of H+ and Na+ ions with monovalent cations present on the mineral surface, as was earlier indicated in the study on the adsorption of DA at low concentrations. If this is the case, which seems likely, the results of Fig 7.8 do not represent true adsorption.

The concentration of monovalent cations in solution, after addition of each titrant 'dose' and equilibration, was determined.

Fig 7.9 shows the results obtained for a suspension of spodumene. 1 —1— 1 1 l i —i ' i > i o Spodumene a Mi crocli ne \ • Beryl

ov \ 3 Methylated spodumene

1 i i . 4 5 6 7 8 9

Fig 7.8 'Adsorption' of H+ and OH on spodumene, beryl and microcline as a function of pH csl i

P 2.5

Fig 7.9 Effect of pH on the concentration of in solution for spodumene 1+ is clear that the concentration of Li +.in solution increased with a decrease of pH below pH 8 due to an increase in cation exchange activity. Furthermore, from the form of the curve and the concentration level of Li+ passing into solution, it is possible that the titration results in Fig 7.8 were significantly influenced by the cation exchange behaviour of the minerals. Thus, although it is possible that the relative (H+-0H ) abstraction observed reflected different degrees of surface acidity or hydroxide concentration, it is not possible to unambiguously separate this from effects derived from the cation exchange properties of the silicates using acid/base titrations. Similar considerations apply to any other method of estimating surface acidity of these minerals in which iongenic species are utilised, irrespective of whether the species are inorganic or organic ions. It is therefore not possible to state with certainty that the different starch adsorptions obtained on the different silicate minerals is related to the density of the silanol sites.

7.2 Viscosity studies

It is known ( 37 , 42 ) that the adsorption characteristics of starch are strongly influenced by the method in which it is prepared. The viscosity studies therefore had two aims: to compare the effects of several methods of solubilising potato starch on the molecular weight and to obtain information on the relative molecular weights of the different starch polymers used in the adsorption and Hallimond tube flotation studies. The utility of viscosity measurements in the determination of relative 184

molecular sizes in polymer solutions has already been noted in

Chapter 3.

The several methods of preparation of starch solutions investigated were:

Method A: This method corresponded to the general solubi Iisation procedure outlined in section 3.2.5, except that larger quantities of reagents as set out in section 3.2.8 were used. The method was also used in the investigation of relative molecular weights as it is later shown that the procedure resulted in the least amount of structural breakdown of the polymer chains.

Method B: About 100 ml of boiling water, containing the requisite amount of NaOH, was added to a beaker containing a weighed quantity of starch dispersed in several cm5 of water. The starch was then swelled gently for several minutes before cooling and volumetric dilution.

Method C: The starch was causticised as in Method A but with simultaneous heating to about 85°C for 30 min.

Method D: The starch was causticised at room temperature as in

A except that the beaker was fitted with stainless steel buffles to permit the formation of high shear rates in the solution. The stirring speed was increased to 1000 rpm and the solubi Iisation time extended to I h.

Method E: Si mi lar to Method C except that no NaOH was added during solubi Iisation. The starch solutions prepared by this method were extremely viscous and it was apparent that extensive gel formation had occurred. The starch solutions prepared by these methods were all

observed to be visibly clear, suggesting that the polymers were

reasonably dispersed.

The results of the viscosity tests are shown in Fig 7.10.

AI I the curves were constructed by the least squares method. The

extrapolated intrinsic viscosities, [n] , for the different

methods of preparation were 295, 286, 278 and 258 litres mg

for methods A, B, C and D respectively. Assuming the applicability

of the Mark-Houwink equation, the differences in the values of

[njindicate that the different agitation and thermal treatments

used did not significantly affect the molecular weights of the

dissolved polymers. The starch solutions prepared by method E

showed non-Newtonian flow properties in the viscometer employed.

This was obviously due to the high degree of association obtained

in this case.

Fig 7.10 also shows viscosity plots for British gum and

maize starch prepared under similar conditions to those used for

the potato starch (method A). The extrapolated intrinsic

viscosities were 25.2 and 162 litres mg ' for the two polymers

respectively. Similar plots could not be constructed for the

dextrins because the differences between the efflux time of the

solvent and solution were small. This is for example demonstrated

in the values of the relative viscosities shown in Table 7.1.

Although in principle higher polymer concentrations than those

in Table 7.1 could have been used, viscosity equations assume

(116, 117) that the density of the solution approximates to that

of the solvent. At high concentration this assumption would no

longer be valid and the viscosity equations become more complex. Concentration of British gum, mg I

Starch concentration, mg I

Fig 7.10 Intrinsic viscosities of starch polymers; the letters refer to the methods of solubiIisation (see text) Table 7.1 Relative viscosities of dextrin solutions as a function of concentration at 25°C

Relati vi scosity Dextrin concen- White dextrin Ye I Iow dextri n tration, mg litre '

1000 1.013 I .009

2000 I .02! 1.019

3000 I .029 1.028

4000 1.041 I .037

5000 I .050 I .045 From the results, it can be concluded that the molecular weights of the polymers used decreased in the order potato starch > maize starch British gum > dextrins. This order is reflected in the adsorption densities of these compounds on spodumene.

In the above considerations it has been assumed that polymer- solvent interactions remained approximately the same for the different starch compounds used. It can be argued that this assumption is not unreasonable because the curves in Fig 7.10 are of a similar form to the Huggins equation (117, 174, 175):

+ s 2c (7 2) vel/c = M H -

Although the Huggins slope constant, 3, may be influenced by a number of factors (e.g. polydispersity, degree of branching, etc) it is thought (117) that for homologous fractions of the same polymer, the value of 3 does not change much as long as the solvent is the same. The similarity of slopes in the results of

Fig 7.10 might therefore provide some justification for the above assumption.

7.3 Ha[IImond tube f1otation tests in the presence of starches

7.3.I Hai f imorid tube flotation tests in the presence of potato starch

The results of Hal Iimond tube flotation tests in the presence -5 of potato starch and 3 x 10 M DA are shown in Fig 7.11. At a starch concentration of 40 ppm, spodumene was more or less completely depressed while lepidolite was about 50$ depressed at 189

^ MicrocIine • Beryl

• Lepi do Iite

O Spodumene

Fig 7.11 Effect of 40 ppm potato starch on the flotation recovery of microclme, beryl, lepidolite and spodumene in 3 x 10 5M DA pH II. In contrast, beryl and microcline showed complete flotation although the pH range for flotation was considerably reduced from that obtained in the absence.of starch (cf Fig 6.5), indicating that some depression did take place even for these minerals. The inability of potato starch to depress microcline to a comparable extent to that obtained for spodumene is surprising since it had been established in the adsorption tests that microcline abstracted a similar amount of potato starch at pH 10. This suggests, therefore, that the ability of starch to act as a depressant is not merely dependent on adsorption density but is influenced by additional factors, such as the configuration of the adsorbed polymer. A great deal of evidence has been accumulated that starch molecules exist in the helical conformation in the presence of organic reagents. Some of this evidence is considered in greater detail later.

Another reasonable explanation for the above observation is that the degree of depression obtained depended partly on the zeta potentials of the silicates. This view is strongly supported by flocculation results obtained in the presence of aery I amide polymers (85 , 176, 177) in which the zeta potentials on the minerals remained negative after adsorption of the polymers. In such cases adsorption of cationic collector ions by a coulombic mechanism is therefore still possible, although it might be influenced by the degree of steric hinderance due to the adsorbed polymer chains. The EM results presented in Fig 5.2 showed that in the pH range II to 12 where complete flotation of microcline was obtained, the negative EM was greatest on this mineral. It is 191

reasonable to conclude, therefore, that although microcline adsorbed relatively large amounts of starch, its lack of depression is related to its relatively larger zeta potential and hence greater amine adsorption ability.

This conclusion is experimentally easy to demonstrate because it was established in section 5.3 that the negative electrophoretic mobilities of microcline and lepidolite were enhanced following acidic scrubbing while that of beryl remained unchanged. Hallimond tube tests were therefore conducted using acid washed specimens of the three minerals. Figs 7.12, 7.13 and 7.14 show the results of these tests. It can be seen that for both microcline and

lepidolite the increase in the negative zeta potential resulted in an extended pH range of flotation and that lepidolite was no longer depressed in 40 ppm potato starch. In the case of beryl, however, there was no change in the flotation recovery of the mineral following acidic washing.

It might, therefore, be concluded from this and the preceeding considerations that the depressant ability of starch probably depends on a number of factors such as the type and number of surface hydroxide sites (i.e. surface acidity/basicity), the conformation of the starch molecules in solution and in the adsorbed state, and the Stern potential on the silicate minerals.

As these factors are all strongly influenced by pH, it is likely that they are highly interrelated. For example, the adsorption density is determined by the number of undissociated hydroxy Is on both the mineral and starch, the polymer configuration in solution and the mutual electrostatic repulsion between polymer and mineral particle. In turn the adsorption density determines 192

100

>q o 0o 40 c o

° 20

j i i i i l 8 m 10 12 14 pH

Fig 7.12 Effect of HCI washing on the flotation recovery of microcline in the presence of 40 ppm potato starch and 3 x 10~5 DA -i 1 1 r

100

>0 o o 0 40

+-0 o Ll_ 20

j i i l

6 8 u 10 12 pH Fig 7.13 Effect of HCI washing on the flotation recovery of lepidolite in the presence of 40 ppm potato starch and 3 x I0"5M DA 193

-i r 100 / I i 9 80

0 i 1 >- i i o 0 > 40 o o 0) c o 20 CD O I 4— / O / ll_

j i i l. 8 u 10 12 ph Untreated mineral in 40 ppm starch

-5, Untreated mineral In 3 x 10 M DA only

HCI washed mineral in 40 ppm starch

Fig 7.14 Effect of HCI washing on the flotation recovery of beryl in the presence of 40 ppm starch and 3 x I0~5M DA the degree of protective colloid action and, therefore, the accessibiIity of collector ions to the ionised sites at the s iIi cate/water i nterface.

It might also be briefly noted that although lepidolite did not show a particularly high starch adsorption density, it was relatively more depressed than both beryl and microcline.

This suggests that the accessible area on this mineral to the starch molecules might have been considerably less than that

available for N2 adsorption.

7.3.2 Hal Iimond tube flotation studies in the presence of maize starch, dextrins and British gum

Figs 7.15 and 7.16 show the flotation recovery of spodumene and lepidolite, microcline and beryl in the presence of maize starch as a function of pH. For comparative purposes the results obtained with potato starch are also shown. It is clear that maize starch is a less effective depressant than potato starch for all the minerals studied. Although complete depression of microcline and beryl had not been obtained with potato starch, the flotation pH range of the two minerals was relatively wider in the presence of maize starch. No change in the flotation recovery of the minerals was obtained in the presence of British gum, white dextrin and yellow dextrin, although relatively larger concentrations

(100 ppm) were used.

This is typified by the results shown in Fig 7.17. This suggests that the ability of the starch polymers to function as depressants is strongly related to their respective molecular weights. Therefore, for the concentration range of British gum i 1 1 « 1 1 r 5235 O Spodumene a Lepi do Iite Spodumene in 40 ppm potato starch 80 Lepidolite in 40 ppm potato starch

10 12 pH Fig 7.15 Effect of 40 ppm maize starch on the flotation recovery of spodumene and Iepido Iite in 3 x 10 M DA

Fig 7.16 Effect of 40 ppm maize starch on the recovery of mi croc line and beryl in 3 x 1DA 196

1 R 1 t 1 » i 100 - 9/a %

80 - 6* 1

D /PA

/ 1 ' 1 - •as. 60 4 •t I \ 1 i >0) / i o o CD 40 - 9 L i c 1 i o A / i '+- 1 (u 1 -oh 20 _ Ll_ / i / i

o / 8i J* i.I.

pH -5 In 3 x 10 M dodecylamine only • In 50 ppm British gum o In 100 ppm British gum In 100 ppm white dextrin O In 100 ppm yellow dextrin A

Fig 7.17 Effect of British gum, white and yellow dextrins on the flotation recovery in 3 x DA and the dextrins used in the tests, no depression was obtained due to the relatively low molecular weights of these compounds

(cf adsorption studies). Further, even for the high molecular weight polymers like potato and maize starches, depression depends on the concentration of polymer in solution. Fig 7.18 shows that as the concentration of polymer in solution increased there was only a small decrease in flotation recovery until a critical concentration of starch was reached, depending on the molecular weight of the starch, at which point a marked decrease in recovery took place.

7.3.3 Effect of starch preparation method of flotation recovery

A limited number of Hallimond tube tests was carried out to demonstrate the effect of the different methods of preparing the starch solutions on the flotation recovery. Fig 7.19 shows the results obtained for potato starch prepared by the several methods described earlier. It is apparent that the depressant ability of starch depended on the manner in which the starch was solubilised and decreased in the order method A > B > C > D. Thus, as the level of shear applied in preparing the starch increased, there was a slight decrease in depressant ability of the starch. The above order is, therefore, consistent with the decrease of molecular weight for the different solubi Iisation procedures, as has already been elucidated. This illustrates the need to maintain low shear forces in solution during the preparation of starch solutions. g 7.18 Effect of starch concentration on the flotation recovery of spodumene at pH II.3 and 3 x IO~5M DA

pH g 7.19 Effect of methods of starch preparation on the flotation recovery of spodumene In 40 ppm starch and 3 x I0~5m DA Potato starch solutions prepared by boiling in water in the absence of NaOH were found to have Iittle and irreguIar depressant ability. This was attributed to a decrease in the number of sites with a latent adsorptive affinity for the mineral particles and the irregular rheological properties of gel systems. These considerations highlight the significance of chain extension that takes place for starch compounds in the presence of NaOH. CHAPTER EIGHT

INTERACTION OF STARCH AND DODECYLAMINE WITH EACH OTHER

AND THE MINERAL SURFACE In the preceeding chapters the adsorption mechanisms of starch and DA have been discussed. The initial aim of the work described in this chapter was to establish the coadsorption behaviour of starch and DA on the silicate minerals. However, the investigations were extended to include a study of the interaction between starch and DA in the absence of mineral particles. This was because the mixing of starch and amine solutions in certain proportions produced a precipitate.

8.I Coadsorption of starch and dodecylamine on spodumene

Fig 8.1 shows the adsorption density of DA on spodumene in the presence of two different starch concentrations at pH 9.

For comparative reasons the adsorption isotherm of DA in the absence of starch and at the same pH value has been included. -4

With an increase in DA concentration beyond about I x 10 M there was a marked increase in the adsorption density of amine in the presence of starch. The increase in the adsorption did not, however, show a tendency towards saturation, as was found for the adsorption of DA on spodumene in the absence of starch.

It would appear that in the presence of 40 ppm starch, a plateau might have been obtained had higher concentrations of amine been used. In the presence of 80 ppm starch, however, there was no indication of a plateau at the highest DA concentration used which was determined by amine so Iubi Iity-pH considerations

(cf Fig 2.4). Furthermore, there was not much difference in the adsorption densities of DA obtained at the two starch Equilibrium concentration of amine, M • In 80 ppm starch O In 40 ppm starch ^ No starch added Monolayer coverage assuming cross sectional area of

20°a2 for hydrocarbon chain

Fig 8.1 Effect of equilibrium amine concentration on the adsorption density of amine on spodumene in the presence of potato starch at pH 9 203

concentrations used.

Further comparison of the isotherms reveal two additional -4 features. At amine concentrations below about 2.4 x 10 M, the adsorption density of DA in the presence of starch was consistently

lower than that obtained in the absence of the macromoI ecule, but at higher concentrations the opposite was true. It would therefore appear that at low amine concentrations, the presence of starch inhibited the adsorption of DA on spodumene while at relatively high concentrations, adsorption of the collector was enhanced. Further experiments were therefore conducted to explain this contrasting behaviour.

The adsorption density of amine was measured as a function of starch concentration at pH 9 and in the presence of two fixed concentrations of amine. The two amine concentrations -4 -4 were I x 10 M and 5x10 M, and these were chosen such that the corresponding equilibrium concentrations would lie in the

'depressed' and 'enhanced' amine adsorption regions (Fig 8.1), -4 respectively. The CMC of DA at pH 9 occurred at 3.2 x 10 M, the lower amine concentration used was therefore below the CMC while the higher concentration was above this value.

The results obtained are summarised in Fig 8.2. It is clear that an increase in the starch concentration resulted in a progressive decrease in the adsorption density of DA on -4 -4 spodumene from I x 10 M DA solutions whereas in 5 x 10 M amine, adsorption of the surfactant increased with an increase in polymer concentration. These results are therefore in agreement with the observations made in Fig 8.1. The decreased -4" o cxi

Initial potato starch concentration, mg I '

Fig 8.2 Effect of potato starch concentration on the adsorption density of amine from fixed surfactant concentrations at pH 9 205

adsorption of amine at low concentrations might be related to the adsorption of starch molecules on adjacent sites, shielding possible DA adsorption (or cation exchange) sites. In contrast, it is not readily apparent why the presence of starch should enhance the adsorption of DA on spodumene at high amine concentrations.

It has been reported (178, 179) that the presence of starch enhances the adsorption of surfactants on mineral particles and that the adsorption of starch itself is enhanced by the presence of surfactant. The reasons for this are not really understood.

Enhancement in the adsorption of British gum 9084 on quartz, in the presence of dodecyIammoniurn chloride (DAC) has been attributed to a reduction in the electrostatic repulsion between the starch and quartz (40). Although a decrease in the electrostatic repulsion between starch and the silicates probably takes place in the presence of amine (cf Fig 6.2), this is unlikely to result in enhanced adsorption of starch. This is because the adsorption of amine at the silicate/water interface usurps the hydrophilic Si-0 sites resulting in a hydrophobic mineral surface. Since starch adsorbs on the silicates by interaction with the hydrophilic Si-0 and Si-OH sites, hydrophobicisation of the mineral surface should result in decreased adsorption of the macromoI ecu Ie at the mineraI/water interface as, for example, observed on methylated quartz samples.

The amine adsorption isotherm in the presence of starch

(Fig 8.1) extends to adsorption densities in excess of a monolayer or a bi layer and continues to increase above the CMC of 206

dodecyI amine. This suggests that the mechanism of amine abstraction differs appreciably from that in the absence of starch. Furthermore, the absence of an infinite slope in the amine adsorption isotherm indicates that the amine is not adsorbing by a condensation-type process at the si Iicate/water interface. It would therefore appear that the amine abstraction mechanism is not one of coadsorption. An interaction between amine and starch in bulk solution to form a soluble or an insoluble complex would be consistent with these results.

To determine whether or not the amine and starch were interacting with each other in bulk solution, the effect of starch on the CMC of DA at pH 9 was studied by surface tension measurements. Fig 8.3 shows the effect of 40 ppm potato starch on the surface tension (y) of DA as a function of the surfactant concentration (C). In comparison to the surface tension results obtained in the absence of starch, two transition points, Tj and

T^, were clearly identified at DA concentrations of about -4 -3

2.4 x 10 and I x 10 M, respectively. Below T| and above T2 the starch had little effect on the surface tension of amine solutions. At Tj, however, the rate of decrease of the surface tension with increase of amine concentration became zero and

remained so until the concentration approached T2, whereupon it increased. Above T^ the rate decreased to zero again. Arai et al (180) obtained similar results in their study of the interaction between polyvinylpyrrolidone and sodium alkyl sulphate by the surface tension method.

From the Gibbs adsorption isotherm the surface excess, T, is proportional to dy/dlnC, I.e. the s]6pe in the y vs log C curve. 207

Fig 8.3 The surface tension of dodecylamine in the presence of 40 ppm potato starch at pH 9

The fact that dy/dlnC (and hence D decreased at Tj due to

the presence of starch, therefore, implies that the added -4 surfactant beyond 2.4 x 10 M DA was consumed either by adsorption onto or by complex formation with starch in bulk solution. Consistent with this is the fact that a precipitate was observed in solutions at amine concentration corresponding to T|. The point Tj may therefore be interpreted as the concentration of DA at which complex formation between starch and

DA is initiated and T2 as that at which it ceases (180, 181).

The latter consideration is supported by the coincidence between the two surface tension curves beyond T^ which implies that dodecyfamine micelles formed under these conditions (180, 181). An interesting deduction may also be made from the similarity of the surface tension values beyond T^ in the absence and presence of starch. Since dy/dlnC provides a quantitative measure of surfactant monomer adsorption at the gas/liquid interface, beyond T^ the number of molecules of amine adsorbed at the interface per unit area was not affected by the presence of starch. Thus there was negligible adsorption of the starch- ami ne complex at the Interface so that the complex had no surfactant properties, I.e. It was predominantly hydrophilic.

Comparison of the adsorption behaviour of DA on spodumene in the presence of starch (Fig 8.1) with the surface tension results shows that the amine concentration beyond which adsorption of amine increased markedly corresponds to that at which the starch-DA precipitate Is formed in solution. It can therefore be concluded that at DA concentrations higher than -4

2.4 x 10 M, not all of the added surfactant adsorbed on spodumene, some of it was consumed by precipitate formation with the starch. These preliminary considerations are reassessed in greater detail in the next two sections.

8.2 Complex formation between starch and dodecylamine

The interaction between starch and DA in the absence of mineral particles was further studied under different conditions of pH and reagent concentrations to elucidate the mechanism of association between the two compounds. Methods used In the study included abstraction measurements (by centrifugation) and infrared spectroscopy. 8.2.1 Effect of concentration of amine and starch on complex formation

Figs 8.4 and 8.5 show the amount of starch and amine removed from solution as a function of surfactant concentration at several pH values. The arrows in the figures indicate the points at which a precipitate was observed in solution. All the data points to the right of the arrows had a precipitate present, but not those to the left. The term precipitate was used to denote a distinct condensed phase which was observed but excluded slight clouding which was noticeable at incipient precipitation. No precipitate formation was observed at pH 4 and 7 at the amine concentration investigated.

The results in Figs 8.4 and 8.5 show that the amount of

DA or starch abstracted increased slowly initially with increase of DA concentration. Beyond a critical amine concentration there was a marked increase in the abstraction of both starch and DA at pH values 9 and 10. Furthermore, with increase of pH from 9 to 10, the concentration of amine required to form the precipitate decreased. At pH 4 and 7, however, there was no sudden increase in the abstraction of both DA and starch.

It is possible that at pH 10 the marked increase in abstraction was caused by both precipitation of the complex and amine. At pH 9, however, the amine concentrations were well below the theoretical solubility of dodecylamine so that the upswing in

DA abstraction can be attributed to precipitation of the complex.

Comparison of Figs 8.4 and 8.1 shows that the amount of DA lo

"o +-0 u 0 +- If) j2l 0 0 tz TD 0

U) j2> 0 0 c 'i 0 0 jz) E zj c

o i-

5x10 1x10 5x10 Equilibrium concentration, M

Fig 8.4 Effect of DA concentration on the abstraction of DA from 40 ppm potato starch solutions at several pH values

isj o CD 40 -o 0 CD E

TD 0 0 30 X -4- 0 u 0 -f- 0 x 0 20

+c- Z3 o E 0

o i—

j i i i i l _l l j l -5 -4 -4 -3 5x10 " Ix10 ' 5x10 I x 10

Equilibrium amine concentration, M

Fig 8.5 Effect of DA concentration on the abstraction of potato starch from 40 ppm solutions at several pH values (legend as in Fig 8.4) 212

abstracted in the absence of spodumene corresponded to that abstracted when spodumene was added. Thus at amine concentrations -4 above 2x10 the main mechanism of amine removal is by complex formation rather than by adsorption.

Figs 8.4 and 8.5 also show that beyond the precipitation point virtually all the starch was removed from solution; nevertheless this did not stop further abstraction of DA at high surfactant concentrations. This is probably explained by the fact that the interaction between starch and DA does not cease until the concentration of amine in solution corresponds to T , as was earlier suggested in the surface tension studies at pH 9.

The effect of different starch concentrations on the abstraction of starch by complex formation was studied. Fig

8.6 shows the amount of starch abstracted by complexation at three different concentrations of the polymer and as a function of equilibrium amine concentration. The results were similar to those of Fig 8.5, with more or less all of the starch In solution being consumed by precipitate formation at all starch concentrations and at high amine concentrations. Notably, however, the amine concentration at which marked abstraction of the polymer took place was approximately constant regardless of the starch concentration. This observation Is similar to reported results of surface tension studies (180, 182) in which the surfactant concentration at which complex formation Is

Initiated is independent of polymer concentration. It would appear that whatever the effect Is responsible for precipitation of the starch-DA complex, It is strongly dependent on a critical O 80 ppm starch A 60 ppm starch • 40 ppm starch

o— -

j i i « • • -i i i l. -5 -4 -4 5x10 ' I x 10 ' 5x10

Equilibrium amine concentration, M

Fig 8.6 Effect of DA concentration on the abstraction of starch from different starch concentrations at pH 9

surfactant concentration rather than that of starch.

Figs 8.7 and 8.8 present the data from Figs 8.5 and 8.6 respectively, as the amount of starch abstracted plotted against the level of DA abstraction. Below the precipitation point, i.e. before all the starch was removed from solution, the relative amounts of starch and DA consumed by complexation were approximately constant irrespective of pH and reagent concentration. Thus below the precipitation point the starch-DA complex was of constant composition. 214

40 •o" o) E

"o 0) +• A pH 10 O (d L. O pH 9 4- l/) xi V pH 7 (d x • pH 4 o

+-itj U)

0 c o3 E <

I 2 3 4 5 Amount of DA abstracted, x 10 M Fig 8.7 Effect of the amount of DA abstracted on the amount of starch abstracted at several pH values from 40 ppm potato starch solutions

Amount of DA abstracted, x I04M

Fig 8.8 Effect of the amount of DA abstracted on the amount of potato starch abstracted from different starch concentrations at pH 9 8.2.2 Binding isotherms of dodecylamine to starch

An alternative way of presenting the data of Figs 8.4-8.6 is by use of quantitative relationships derived for describing the interaction between organic ligands and proteins (163, 183).

In the relationships it is assumed that there are n binding sites per protein molecule and that the sites are identical (i.e. have equal chance of interacting with a ligand molecule). The average number of bound ligand molecules per protein molecule, v, is then related to the equilibrium concentration of free organic ligand by the equation (163)

v/(n - V) = k X f (8.1)

where

k = an equilibrium or intrinsic association constant for

the binding reaction at each site;

= mole fraction of ligand at equilibrium;

f = activity coefficient.

In dilute ligand concentrations X is approximated by C , the e e equilibrium concentration of organic ligand so that

V/(n - v) - kC (8.2) e

Plots of v against fa at constant pH and temperature have been termed binding isotherms (163). Alternatively, equation (8.2) may be put into the form v/c = k(n - v) (8.3)

V and /Ce plotted against v. Such plots have been referred to as the Scatchard plots (183, 184, 185). The intrinsic association constant, k, is then obtained as the slope of the resulting

Ii near curve.

Scatchard plots were obtained for the data of Figs 8.4 to

8.6 and are shown in Figs 8.9 and 8.10. As the molecular weight of potato starch was not known v was defined as the average number of moles of bound DA per g of starch. At pH 9 and 10 the Scatchard plots consisted of two linear segments with a transition at the point where precipitation of the complex was

V observed, as shown by the arrows. Below this point /Ce increased slowly with v, but above the point the increase was marked. At pH 4 and 7, however, the Scatchard plot was associated with only one curve. Furthermore, at low v, the slope of the line was approximately constant (about 500 litre mole ') irrespective of starch concentration and pH. At high v the slopes of the

lines were different; that at pH 10 (6030 litre mole ') being greater than the corresponding value at pH 9 (1530 litre mole ').

It has been suggested (184, 185) that in the case where two

linear segments are present, it is necessary to assume that there are two types of binding reactions at two sets of sites on the polymer, with association constants k| and k^. It would appear, then, that the interaction between potato starch and DA at pH 9 and 10 involved two separate binding mechanisms with a transition between the two mechanisms at the precipitation point. The slope -- i i * 1 At pH 9: 16 O 40 ppm starch • 60 ppm starch - V 80 ppm starch 12 ® 40 ppm starch at pH 4 / A 40 ppm starch at pH 7 ° /a® o 6

8 - v D • /

O 4 - O O• V A 3 A a 3

• • i .i Ixl0 4 5xlO~4 IX10~3 5x1 Cf3 IxI cf2 v Fig 8.9 Sca+chard plot for the binding of DA to potato starch at several potato starch concentrations and different pH values (lines fitted by least squares)

v Fig 8.10 Scatchard plot for the binding of DA to potato starch at pH 10 (lines fitted by least squares) constant, k, is a measure of the affinity of the binding of the organic ligand to the macromoI ecule (163). The large values of k at alkaline pH values and high DA concentrations

(high v) therefore suggests that under these conditions, the binding of DA to potato starch is largely a cooperative process in which a large number of amine molecules are incorporated in the complex within a narrow concentration range of surfactant (163). Further, with increase of pH from 9 to 10, the affinity or cooperative nature of the binding reaction i ncreases.

At pH 7 or low v, the low value of k indicates that the relevant mechanism of interaction is relatively less cooperative and of reduced affinity. Further, the independence of k of pH and starch concentration indicates that under these conditions the affinity of DA molecules for potato starch is approximately the same. This would appear to explain why the composition of the complex below the precipitation point is apparently constant.

A possible low affinity mechanism at low amine concentrations is electrostatic attraction between aminiurn and inorganic cations in the bulk solution and R-0 sites on the starch. At high DA concentrations and pH values the cooperative nature of the interaction and the previously observed effect of pH on the concentration of DA required for precipitating the complex, are analogous to the effect of pH and concentration of amine on the CMC of DA. Under these conditions the driving force for the interaction between starch and DA appears to have been van der Waal's forces of association between the hydrophobic C-H 219

groups on the starch and the amine collector. Alternatively, the amine molecules could coadsorb with the cationic amine ions held by coulombic attraction to the R-0 sites.

8.2.3 Inf I uetice of hydrophobic Interactions on comp lex formation

Further studies of the mechanism of Interaction between starch and DA were made. These involved surface tension and turbidity measurements.

Fig 8.11 shows the surface tension of DA as a function of amine concentration in the absence of starch, at pH 10. In -4 this case, the CMC of DA occurred at about 1.7 x 10 M. Comparison of this value with that required to precipitate the starch-amine complex at pH 10 (Figs 8.4 and 8.5) shows that, like at pH 9, the precipitate forms only at submicellar concentrations, i.e. relative to the CMC of amine in starch-free solutions. This observation Is similar to that reported for complex formation between a number of polymers and surfactants (180, 181, 182).

It has further been shown In these studies that the free energy change for transferring a methylene group from bulk solution to the complex is of the same magnitude (i.e. I.I kT per CH^ group) as that required for the formation of micelles in polymer-free surfactant solutions. The consistent proximity to the CMC, at both pH 9 and 10, of the concentration of DA at which precipitation of the complex took place is therefore strong evidence that precipitation is Induced by van der Waal's forces.

It would appear that at pH 7 precipitation of the complex was not observed because of the relatively low DA concentration range 220

t 1—i—r~r t—i—r

80 "

E O 0 >g* 60 tj D o 0 . - c 40 o

£ 20

' « i i i i—l J I I L -5 -4 -4 5x10 " 1x10 5x10

Amine concentration, M

Fig 8.11 Surface tension of dodecylamine at pH 10

investigated compared to the CMC value which has been reported

(26 ) to be at 1.4 x I0~2M DA at this pH. It can be argued,

therefore, that at concentrations of DA close to the CMC, a

starch-DA precipitate should form.

Fig 8.12 shows the turbidity of DA solutions in the presence

of 40 ppm potato starch, as a function of amine concentration.

The measurements were conducted with an fEELf (Evans

Electroseleniurn Ltd, England) nepheIometer. A 40 ppm potato

starch solution was used as a blank to cancel out errors arising

from light scattered by the bulky potato starch molecules (117).

With increase of DA concentration there was only a small increase

in turbidity but in the region of the CMC the increase in turbidity was marked due to the formation of a precipitate. 221

Fig 8.12 Turbidity of dodecylamine solutions containing 40 ppm potato starch at pH 7

It is therefore clear that at all pH values a starch-DA precipitate is obtained as long as the bulk concentration of amine is close to that required for miceI Iisation in starch-free solutions. It is not completely clear, however, whether at low amine concentrations the small increase in turbidity is solely due to electrostatic interaction between DA+ ions and

R-0 sites, or is influenced by the increasing presence of neutral amine molecules with increase of DA concentration. 222

To further define the respective roles of DA ions and neutral amine molecules, the starch abstraction was measured in the presence of a neutral long chain alcohol, n-dodecyl alcohol, and a strongly ionising cationic surfactant, cetyItrimethylammoniurn bromide (CTAB). The alcohol was prepared in pure benzene and was added to the starch solutions such that the amount of benzene introduced in bulk solution was relatively small and always approximately constant. If the alcohol-starch solution was equilibrated for short periods

(< 16 h) no precipitate was formed and the amount of starch removed from solution was negligible. After prolonged equilibration times (> 48 h), however, a precipitate was observed to form in solution. The results obtained in this -4 latter case are shown in Fig 8.13. In the presence of I x 10 M alcohol or greater, nearly all the starch was precipitated out of solution. In the case of CTAB, however, no precipitate was observed irrespective of the equilibration time. Furthermore, little starch was abstracted from solution in the CTAB concentration range I x I0~5M to I x I0~3M and pH 7 to 10.

These results show two main aspects of the interaction between potato starch and DA. In the starch-DA system precipitation of the complex was observed within about 30 min of mixing the starch and amine solutions. The longer equilibration time required for the alcohol to precipitate the starch therefore shows that in the starch-DA system electrostatic attractions between amine ions and R-0 sites probably aids the kinetics of complex formation. The fact that there is negligible interaction /

i i i i i i i i i i i IxlO-5 5x1 Cf3 IxlO-4

Initial concentration of n-dodecanol, M

Fig 8.13 Effect of n-dodecanol concentration on potato starch uptake from 40 ppm solutions at pH 9

between CTAB and starch suggests that the adsorption of cationic

amine on starch would not result in significant abstraction of

the surfactant from solution. Thus coadsorption of amine ion

and molecule probably takes place even at low amine concentrations.

Furthermore, the fact that a neutral long chain alcohol can

precipitate the starch emphasises the hydrophobic nature of the

precipitat ion mechanism. These considerations agree with a

large amount of evidence which has been accumulated on the

selective separation of starch fractions by precipitation with

hydrophobic organic ligands. Some of this evidence is reviewed

in section 8.3. 224

Fig 8.14 shows a logarithmic concentration diagram for -4

I x 10 M DA In which the distribution of amine species Is compared with the amount of starch abstraction as a function of pH. The data points In the figure were obtained from Fig 8.5 -4 at a constant equilibrium concentration of I x 10 M amine.

Although the data is limited, It is clear that the level of starch abstraction is of a similar form to the variation in

^^2(aq) aS a PR- can Therefore be suggested that the interaction between starch and DA is in many respects similar to the adsorption mechanism of amine on the silicate minerals. At low amine concentrations coadsorption of aminium

ion and amine molecule takes place on the starch molecule but at a particular concentration of amine in solution, van der Waal's associations lead to Increased abstraction of amine and precipitation of the starch-DA complex occurs. The major difference between the two mechanisms might perhaps be the inherent hydrophobic groups on starch, which are capable of engaging in van der Waal's associations with surfactants even in the absence of ionised species. At the mineraI/water Interface, however, the hydrophilic nature of the surface requires that van der Waal's associations + take place on or between adsorbed DA ions.

8.2.4 Study of the starch-dodecylamine complex by infrared spectroscopy

The Infrared spectrum of the precipitated complex was examined mainly to confirm that chemical interactions were absent In the association between starch and DA. This was achieved by comparison of the spectrum of the precipitate with c (0 X

4— —

Fig 8.14 Comparison between the amount of amine abstracted by complex formation_^ith the distribution of amine species at I x 10 M DA

the spectra of DA, potato starch and a potato starch-DA mixture.

The spectra obtained are shown in Figs 8.15 to 8.18. The method used to obtain the spectra is also given in the figures. Fig

8.19 outlines the absorption bands in which vibrations of the indicated chemical groups were observed.

The spectra of Nujol oil and dodecylamine (Figs 8.15 and

8.16) compared well with the reported spectra in the literature

(145, 186, 187, 188). No spectrum of starch could be found in the literature although numerous spectra of sugars have been recorded (188, 189). It is also apparent In the spectrum for starch (Fig 8.17) that there was overlap in the C-H stretching and bending modes with simi lar vibrations from the muI Iing oi I. 100

60 i Fi Im

4000 3500 3000 2500 2000 1800 1600 1400 1200 1000 800 Wavenumber, cm

Fig 8.15 Infrared spectrum of Nujol oil

ho ho CD Wavenumber, cm

Fig 8.16 Infrared spectrum of dodecylamine

k) --n)j Nujol muI I

800 1600 400 200 000 800

Fig 8.17 Infrared spectrum of potato starch

k) n> 00 Wavenumber, cm

Fig 8.18 Infrared spectra of potato starch - DA precipitate and potato starch - DA mixture

m w vo Group

... r. • i

^as £s - CH3

V V 6 as, s s > CH2 1 1 -c-c- V 1 1

JL -NH2

-NH*

V -C-N- 1

V 6 -OH

I i V -c-o-c 1 1

-CH2- OH

• • • 3000 2000 1000

Wavenumber, cm '

Fig 8.19 Wavenumber regions in which the indicated groups gave rise to absorption bands in the infrared spectra

k> LU o 231

It was confirmed, however, by the KBr-disc method, that these

bands were also present in starch. A further feature of the

starch spectrum was its diffuse nature particularly in the

so-called fingerprint region (i.e. 1300 - 900 cm '). It is

reported (190) that band resolution in the fingerprint region

is generally not as good as in the upper regions (functional

region) even for simple sugars, presumably due to interaction between the numerous vibrations that take place within the

region (191). The possible absorption bands in this region

probably include in-plane - OH vibrations, C-0, (^-OH and C-C stretching modes (186, 188, 190).

In the functional region the starch spectrum showed a broad absorption band between 3600 and 3000 cm '. The broadness of the band suggests that there was extensive association of the hydroxyl group with probably both intra and

intermolecular molecular hydrogen bonds being present. However,

it is also apparent that although the starch was dried for long periods under reduced pressure, some hydrate water persisted

in the sample. The presence of hydrate water was strongly supported by the characteristic deformation band of molecular water at 1640 cm ' (192).

The spectra of the starch-DA mixture and the starch-DA precipitate are compared in Fig 8.18. In both spectra the presence of the deformation band of molecular water at 1640 cm ' indicates that some hydrate water was present. The OH/NH^ stretching band at high wavenumbers shows more definition in the case of the precipitate than in the mixture. This may

indicate that the precipitate had relatively less hydrogen 232

bonding associations in this region than the parent starch.

Similarly the fingerprint region showed improved resolution between 1200 and 1000 cm '. Theoretically, improved band resolution arises from a well defined energy of interaction of the infrared radiation with the molecule so that perturbation effects from the field of force of neighbouring molecules are resisted (191). Such a case could have arisen if the precipitated structure was a relatively more refined geometrical structure (cf section 8.3) or extraneous impurities were excluded during precipitation.

The single most important conclusion from the spectra, however, is that the interaction between starch and DA did not involve the formation of any new and chemically different compounds. This conclusion is in agreement with the physical nature of the interaction between starch and DA, as has been out Ii ned above.

8.3 A possible mechanism of depression of the minerals by starch

Fig 8.20 and 8.21 show a comparison between the amount of starch abstracted in the absence and presence of spodumene, respectively, and under identical conditions of pH, starch and amine concentration. In the absence of mineral (Fig 8.20) there is a general increase in the amount of starch abstraction with increase in DA concentration. At high amine concentration -4

(i.e. 5 x 10 M) the starch abstraction curve is nearly vertical, indicating that practically all the starch is removed from solution by precipitation of the complex. In the presence of spodumene -4 A In 5 x 10 M DA -4 I V In 2 x 10 M DA -4 • In 1 x 10 M DA O In 5 IO"5M DA 60 x

10 20 30 40 50

Equilibrium potato starch concentration, mg I

Fig 8.20 Effect of equilibrium potato starch concentration on the abstraction of starch by complex formation at pH 9

Equilibrium potato starch concentration, mg I '

Fig 8.21 Effect of equilibrium potato starch concentration on the abstraction of starch at pH 9 and in the presence on spodumene 234

(Fig 8.21) a similar increase in the starch abstraction with -4 increase in DA concentration takes place. At 5 x 10 M DA, practically all the starch is removed from solution as was observed in Fig 8.20. For comparative reasons the amount of starch adsorbed in the absence of amine is included in Fig 8.21.

At all amine concentrations investigated the amount of starch abstracted was greater than that adsorbed on spodumene in the absence of the surfactant.

Similar results to those of Fig 8.21 and 8.1 have been erroneously interpreted as indicating that mutual enhancement of adsorption of starch and surfactant takes place in the presence of each other ( 40, 43 , 178, 179 ). It is believed that the evidence presented in this thesis is unambiguous; in the presence of each other, starch and DA associate by a physical mechanism resulting in the presence in solution of a complex.

The hypothesis of mutual enhancement of adsorption of surfactant and starch on mineral particles is clearly not valid. On the contrary, the experimental evidence (cf Figs 8.1 and 8.2) suggest that the adsorption of amine on spodumene decreases in the presence of starch and at low surfactant concentrations. In any -4 case, at high amine concentrations (e.g. 5 x 10 M amine at pH

9) there is no free starch, as such, in solution so that

'enhanced' adsorption of starch is impossible.

in attempting to elucidate the mechanism by which starch acts as a selective depressant for certain minerals, it is necessary to account for the adsorption behaviour of the starch- amine complex which on the basis of the present, and similar, evidence (86, 193) appears to be present at a I I starch and DA concentrations.

In the last chapter the main elements of the mechanism of action of starch as a depressant were identified as the

(a) surface coverage of the adsorbed starch;

(b) surface charge on the mineral particles; and

The relevance of (a) and (b) was experimentally demonstrated.

For example, the fact that a critical starch concentration,

irrespective of starch type, was required to depress spodumene

indicates that a particular starch adsorption density is critical for depression. However, although microcline had nearly the same starch adsorption density as spodumene, it was relatively

less depressed by DA because of its higher surface charge.

Condition (c) was not experimentally demonstrated. Consideration of the experimental results, however, suggests that the geometrical structure of the complex is critical. For example, the dependance of the degree of depression on the surface charge of the minerals suggests that the adsorption of the complex at the mineraI/water interface does not completely prevent the adsorption of amine ions on Si-0 type sites. If the complex is adsorbed as a flat coil, it is reasonable to assume that these sites would be obscured from bulk solution. A further difficulty is that of accounting for the position of the hydrophobic regions of the complex following its adsorption. The presence of such regions

in the complex is well supported by the results presented in this thesis. Since complete depression of spodumene is obtained

in the Hal Iimond tube tests, the mineral's surface is presumably hydrophilic. The hydrophobic regions of the complex cannot therefore be oriented towards bulk solution. An alternative assumption is that they are oriented towards the mineral surface. In this case, the complex can only adsorb If the mineral surface itself is hydrophobic. Such adsorption is not, however, expected to be selective. The latter suggestion does not therefore appear likely since selective depression was obtained in the Hal Iimond tube flotation tests of the silicates.

There is a large amount of evidence in the field of starch chemistry that suggests that in the presence of organic complexing agents, starch molecules adopt a helical conformation

Such a geometrical structure in aqueous is possible for starch for which the -OH and C-H functional groups have conflicting solvation characteristics (86). The macromolecu Ie therefore seeks to expose a maximum number of the hydrophi lie -OH groups to the solution environment but with minimum exposure of the hydrophobic C-H groups. Assessment of available hydrodynamic evidence ( 86 , 193) has led to the conclusion that the helical structure of starch molecules in organic aqueous solutions is imposed upon the macromolecule by the hydrophobicity of the organic Iigands.

Along similar lines a number of workers have shown that starch can be precipitated from solution by a wide variety of organic reagents and that the precipitate is a helical clath rate

Several methods have been used to deduce that the organic molecules are coaxial with and enclosed by the starch helix.

For example, optical studies (194, 195, 196) have shown that the precipitate is pleochroic, suggesting that strong alignment of 237

the organic molecules takes place. Other evidence has included

Fourier projections of electron density (196, 197) and crystallographic studies (185, 194, 197,198-201). In the

XRD studies it has been assumed (159, 199, 202) generally that the cylindrical nature of the helix is reflected by a pseudohexagonaI unit cell whose parameters depend on the nature of the enclosed organic molecules (i.e. chain length, degree of branching, etc).

In view of the overwhelming evidence for the helical model, there is little reason to doubt that starch molecules are present in the helical conformation in amine solutions, with the alcoholic hydroxy Is oriented to the bulk solution. Adsorption of the complex on the minerals by hydrogen bond formation between

Si-OH and other surface metal hydroxide sites, and the R-OH group is therefore possible in a similar manner.to the adsorption of starch on the minerals. This theory is reasonable because the alcoholic hydroxy Is are not usurped by the association between starch and amine. The adsorption of the complex on the silicates could not have occurred by electrostatic attraction because EM tests on the precipitate indicated that the complex was essentially uncharged. Little is known about the mechanism of depression of mineral fractions by starch. A comparison of the above theory to other mechanisms is therefore not possible. However, the presence of a helical starch/amine clathrate in solution and its adsorption at the mineraI/water interface by hydrogen bonding would be consistent with a number of observations made in this thesis. In particular the theory would explain why: 238

(a) The surface of the complex remains hydrophilic as was

found in the surface tension studies, although

starch adsorbs a large quantity of amine especially at

high pH and surfactant concentration. Furthermore, because

the complex has a hydrophilic exterior, its adsorption at

the mineraI/water interface is expected to result in a

hydrophilic mineral surface.

(b) Spodumene is relatively more depressed than the other

silicates. By analogy to the adsorption mechanism of

starch elucidated in section 7.1.2 it would appear that

adsorption of the complex is greatest on this mineral

and that this results in a relatively greater degree of

hydrophi Iicity in comparison to the other silicates.

charge of the minerals. Because of its cylindrical

shape the helix adsorbs on the mineral surface presumably

at relatively few anchor points so that some Si-0 sites

still remain exposed. Adsorption of cationic amine on

these sites is therefore possible and increases with the

increase in the negative zeta potential. Thus although

mi croc line adsorbs nearly as much starch as spodumene it

is relatively less depressed than the latter because of

its higher zeta potential. 239

CONCLUSIONS A comparative study has been made of the surface chemistry of spodumene, lepidolite, muscovite, beryl and microcline In aqueous solutions of dodecylamine, with a view to Improving the flotation selectivity between the lithium minerals and the other silicates. The techniques used included measurement of the solubility, cation exchange, electrokinetic, adsorption and flotation properties of the minerals. The effect of surface modification procedures such as acid/alkaline washing, the addition of fluoride and polyvalent metal Ions on the measured properties were determined.

From the results obtained the following conclusions can be made:

DIsSo I ution, efectrokiiietic and cat ion exchange properties

(a) The dissolution behaviour of the a Iuminosi Iicates In aqueous solutions is consistent with the stability of the component oxides. At low pH preferential dissolution of Al occurs. At high pH dissolution is more or less stoichiometric.

(b) The I.EP values of muscovite, spodumene and beryl occur at pH values of 5.7, 2.8 and 3 respectively. Those of lepidolite and microcline are at very acid pH values (less than pH 2.7).

(c) H+ and OH appear to be potential determining for the silicates studied. Al species are specifically adsorbed at the silicate/water Interface. Li+ and Na+ behave as indifferent ions although Li+ Is a lattice cation in spodumene and lepidolite.

This appears to be due to the fact that the two cations are nearly completely hydrated in aqueous solutions. (d) The aging of silicate suspensions results in less negative

EM values. This can be explained by the presence of a composite gibbsite/aIuminosi Iicate surface on the mineral particles. The actual a IuminosiIicate solid that forms is, however, unknown.

(e) The effects of acid and alkaline washing on the EM of the a Iuminosi Iicates depend on the structure and composition of the mi neraIs.

(i) Washing spodumene and beryl in HCI does not significantly affect the EM of the two minerals. This appears to be due to the fact that these minerals do not contain AI in the tetrahedraI positions. The polymerised tetrahedra are therefore relatively stable in acidic media, the siloxane bond probably limiting accessibility to weaker ionic bonds, particularl in minerals with poor cleavage like beryl.

(ii) Washing muscovite, lepidolite and microcline in HCI increases the negative EM values of the minerals. Due to the presence of AI in the tetrahedra, degradative leaching probably contributes to the loss of basic cations resulting in more negative EM values.

(iii) Washing the a Iuminosi Iicates in NaOH does not alter their EM values significantly unless they contain reasonable amounts of basic cations in which case the negative EM decreases.

(f) The presence of NaF has no effect on the EM of the a Iumino- si I icates at alkaline pH values. At acid pH values, however, fluoride increases the negative EM for all the silicates studied. Consideration of the distribution of fluoride species indicates that HF is the active species. 242

(g) Adsorption of Al species results in charge reversal at the a IuminosiIicate/water interface. The pH range of charge reversal is dependant on the concentration of Al in solution. The resulting EM values are, however, similar for all the silicates studied. This indicates that the mechanism of charge reversal is similar for all the minerals. It is not possible, however, to tell from the data whether the charge reversal is due to the adsorption of AI(OH)* AI(OH)2+ or colloidal Al(OH), 2 3(s)

(h) The cation exchange properties of the silicates decrease in the order muscovite > lepidolite > spodumene ~ microcline.

Beryl has no measurable cation exchange affinity. The cation exchange process occurs predominantly through the release, by the a Iuminosi Iicate lattice, of the weakly held (ionically bonded) monovalent cations. The lack of exchange affinity for beryl has been attributed to an absence of such cations in its latti ce.

Adsorption of amine and Starch at the si Ii cate/water interface

(a) The adsorption of DA at the siIicate/water interface is consistent with the theory of coulombic attraction and the formation of hydrophobic associations. However,

(i) at low DA concentration the predominant mechanism of adsorption is by cation exchange particularly if the aluminosi I icate has good cation exchange properties;

(ii) the slope of the EM-DA concentration curve at low amine concentration is not zero for a Iuminosi Iicates. It is considered that this arises from the relatively high DA adsorption densities due to the finite cation exchange capacities of the aluminosi Iicates. (b) Starch adsorbs on the a Iuminosi Iicates by H-bonding between its alcoholic hydroxyls and surface silanols and other

M-OH groups on the mineral. Adsorption decreases with increase in the ionic state of both hydroxyl types. Methylation of the hydroxyl groups on the mineral surface also decreases the starch adsorption density and the interaction between H+ and OH and the mineral surface.

(c) The adsorption density of starch on the minerals decreases in the order spodumene >microcline > beryl > lepidolite > muscovite. It is probable that this order is related to different degrees of surface acidity. Potentiometric acid/base titrations could not, however, confirm this because of the exchange of H+ for other cations on the mineral surfaces.

(d) The adsorption density on spodumene of the polysaccharides used decrease in the order potato starch > maize starch » British gum > dextrin. This order is the same as that of the molecular weights of the compounds.

Hallimond tube flotation of the silicates

(a) The flotation behaviour of the a Iuminosi Iicates in the presence of dodecylamine is consistent with the theory of adsorption of DA at the silicate/water interface. Good flotation is observed above the IEP of the minerals. At pH 12 flotation is drastically reduced due to the precipitation of amine from solution and the low levels of cationic amine present.

(b) Modification by potato and maize starches results in the selective depression of the amine flotation of spodumene and, to a lesser degree, lepidolite at high pH. British gum and the dextrins are ineffective as depressants within the concentration range investigated.

(i) a critical starch concentration, and

(ii) the magnitude of the surface charge on the mineral particles.

(d) Excessive mechanical and/or thermal shear during so Iubi Iisation of the starch results in reduced depressive ability of the polymer. Preparing starch in dilute alkali avoids self-association of the starch chains and improves the effectiveness of starch as a depressant.

Interaction of starch and DA with each other

(a) Starch and DA do not enhance the adsorption of each other at the silicate/water interface. The two compounds interact in solution resulting in the formation of a starch-DA complex which has no surfactant properties. Thus complex formation, rather than adsorption at the si Iicate/water interface, is the main mechanism of starch and DA abstraction from solution particularly -4 at amine concentrations >2x10 M.

(b) The mechanism of interaction between starch and DA is purely physical, involving coulombic attraction and hydrophobic associations. In the CMC region of starch-free amine solutions the starch-DA complex precipitates out of solution. Other hydrophobic Iigands, such as n-dodecanol, can also precipitate the starch. Selectivity considerations

(a) In the absence of modifying agents, the a Iuminosi Iicates have greatly similar eIectrokinetic, amine adsorption and

Hallimond tube flotation characteristics. Selectivity of flotation is not therefore possible without modifying the silicate surfaces.

(b) The similarities in the EM values of the silicates in the presence of fluoride or AICI^ indicates that modification by fluoride or polyvalent metal cations is unlikely to improve the selectivity of flotation of the a Iuminosi licates.

(c) The commercial practice of using starch as a spodumene depressant appears reasonable. Further work with high molecular weight polysaccharides might therefore produce improved flotation results. A further approach to improve selectivity might be to exploit the effect of acid washing on the aluminosi licates prior to the amine flotation. Deer, W A, Howie, R A and Zussman, J, Rock forming minerals, vol 1-5, John Wi ley and Sons, New York, 1962

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Ribbe, P H, The chemistry, structure and nomenclature of feldspars, in Fe1dspar mineralogy, Proceed. Mi nera I Soc. Am, vol 2, 1975

Baarson, R E, Ray, C L and Treweek, H B, Plant practice in non-metallic flotation, in Froth Flotation, 50th Anniversary volume, Ed Fuerstenau, D W, AI ME, 1962, 427-453

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15 Bunge, F H, Flotation of spodumene, US Pat. 2, 748, 938, 1966

16 Falconer, S A and Crawford, B D, Froth flotation of some non-sulphide minerals of strategic importance, AI ME Tech. Pub., No 1754, 1944

17 Manser, R, Handbook of si Iicate flotation, Warren Spring Laboratory, (Crown copyright), 1975

18 Madhaven, T R, Viswanathan, K V and Majumdar, K K, Berieficiation of lepidolite ores, Trans. ' I rid. Inst. Met., _[9, 1966, 224-226

19 Lewis, J M and Levin, J, Concentration tests for the recovery of lepidolite from tailings dumps at Rubcon Mines by electrostatic separation and flotation, Johannesburg: Nat. Inst. Met., Research Rept 226

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