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 by 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 electro- phoretic 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 electro- phoretic 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