SELECTIVE DISSOLVED AIR FLOTATION OF FINE MINERAL PARTICLES A thesis submitted for the degree of Doctor of Philosophy of the University of London and the Diploma of Imperial College by JAIME ANTONIO SOLARI SAAVEDRA Department of Mineral Resources Engineering Royal School of Mines Imperial College London SW7 May 1980 To Chela and Enrique, my parents. 1 ABSTRACT The limitations of the froth flotation process in the separation of fine particles have been discussed with particular reference to the influence of particle and bubble sizes on the bubble-particle attachment mechanism. The fundamentals of the dissolved air flotation (DAF) process have been reviewed and its application to the selective recovery of fine mineral particles has been investigated using cassiterite/quartz suspen- sions. The dissolved air flotation of fine quartz and cassiterite parti- cles has been studied in the presence of cationic (dodecylamine) and anionic (sodium dodecyl sulphate, sodium sulphosuccinamate) surfactants in a batch DAF system that produced bubbles of 50 um average diameter. Evidence is presented that show that the fundamental mechanism of bubble-particle attachment in dissolved air flotation is the adhesion of bubbles to hydrophobic particles upon collisions. Hydrophilic particles were not floated by the DAF process irrespective of their degree of aggregation. Flotation through bubbles forming from supersaturated solution on the solid phase was studied and estimated to be of little significance as a bubble-particle attachment mechanism in DAF. The dissolved air flotation of both oxide minerals correlated well with the adsorption behaviour of surfactants at the oxide mineral/solution interface as determined by adsorption, electrokinetic and suspension stability measurements. Mineral recovery by DAF was maximized when the particles were hydrophobic and the suspension exhibited a degree of destabilization. Long-chain surfactants were found to accomplish this twofold objective. Results are presented that show that the adsorption of dodecylamine on quartz and of the anionic surfactants on cassiterite and stannic oxide is primarily the result of electrostatic attraction for the surface followed by hydrophobic chain-chain associations at high adsorption densities and surfactant concentrations. Separation tests showed that fine cassiterite particles could be selectively recovered from quartz suspensions by the dissolved air flotation process. The factors influencing cassiterite recovery and the selectivity of the separations were the surface chemical characteris- tics of both minerals in the presence of the collector and parameters associated with the operation of the dissolved air flotation system. 3 ACKNOWLEDGEMENTS I wish to express my sincere thanks to my supervisor, Dr. R.J. Gochin, for his guidance, encouragement and unfailing support over the past three years. I should also like to extend my gratitude to Dr. H.L. Shergold and Dr. J.A. Kitchener for many useful suggestions and discussions throughout the course of this project. I am also grateful to my colleagues and members of the Department of Mineral Resources Engineering for their invaluable comments, assistance and friendship. The support of the World University Service (U.K.) in the form of a grant that enabled me to carry out this work is gratefully acknowledged. I am also indebted to Professor M.G. Fleming and Dr. H.L. Shergold for the provision of financial assistance during part of this project. Finally, I should like to thank all who helped me in the preparation of this work, Particularly, Janis, who managed to decipher my handwriting and carefully typed this thesis, and Enrique, Claudio and Barry for their invaluable help with photography. Special thanks to Maite for her inexhaustible support and patience. 4 CONTENTS Page ABSTRACT 1 ACKNOWLEDGEMENTS 3 LIST OF CONTENTS 4 LIST OF FIGURES 8 LIST OF TABLES 12 1. INTRODUCTION 13 1.1 The recovery of fine mineral particles by flotation 14 1.2 Problems in froth flotation 18 1.3 The floatability of fine particles 20 1.4 Bubble generation in flotation systems 25 1.5 Dissolved air flotation 27 1.6 Aim of the project 30 2. THE DISSOLVED AIR FLOTATION PROCESS 32 2.1 Introduction 33 2.2 Air dissolution in DAF 33 2.3 Bubble formation in DAF 35 2.3.1 General considerations 35 2.3.2 Mechanism of bubble formation in DAF 37 2.3.3 Bubble size in DAF 41 2.3.4 Influence of DAF operation parameters on microbubble production 44 2.3.4.1 Mean diameter of microbubbles 45 2.3.4.2 Bubble-number density 46 2.3.4.3 Rise-time of microbubbles 48 2.3.4.4 Fraction of air released 48 2.4 Particle-bubble attachment in DAF 51 2.5 Industrial practice 54 2.5.1 Applications 54 2.5.2 Equipment 55 2.5.3 Design 56 2.5.4 Performance 58 2.6 Flotation of minerals by bubbles formed from super- saturated solutions ' 60 2.6.1 Vacuum flotation 60 2.6.2 Dissolved air flotation 62 2.7 Summary 66 3. THE OXIDE MINERAL/AQUEOUS SOLUTION INTERFACE 68 3.1 Introduction 69 3.2 Electrical phenomena at the oxide mineral/aqueous solution interface 69 3.3 Collector adsorption at the oxide mineral/aqueous solution interface 75 5 Page 3.4 Phenomena at the solid/liquid interface associated with the aggregation of mineral suspensions 79 3.5 The surface properties and the flotation of cassiterite 82 4. EXPERIMENTAL 88 4.1 Materials 89 4.1.1 Minerals 89 4.1.2 Reagents 91 4.2 Electrokinetic studies 93 4.2.1 Electrophoresis 93 4.2.2 Experimental procedure 95 4.3 Adsorption measurements 96 4.3.1 Experimental procedure 96 4.3.2 Dodecylamine analysis 97 4.3.3 Sodium dodecyl sulphate analysis 97 4.4 Tin analysis 99 4.5 Suspension stability studies 100 4.5.1 Determination of suspension stability 100 4.5.2 Experimental procedure 101 4.6 Dissolved air flotation studies 103 4.6.1 General 103 4.6.2 Apparatus 103 4.6.3 Experimental procedure 105 5.- 8. RESULTS 5. STUDIES ON THE MECHANISM OF BUBBLE-PARTICLE ATTACHMENT IN DISSOLVED AIR FLOTATION 108 5.1 Dissolved air flotation of the quartz/dodecylamine system 109 5.1.1 Electrokinetic studies at the quartz/solution interface 109 5.1.2 Stability of quartz suspensions in the presence of dodecylamine 111 5.1.3 Adsorption measurements of dodecylamine on quartz 115 5.1.4 Dissolved air flotation studies 117 5.1.5 Discussion 122 5.2 Dissolved air flotation of various aggregated systems 126 5.2.1 Dissolved air flotation of the quartz/ iron(III) system 126 5.2.2 Dissolved air flotation of the quartz/ polyethylenimine system 131 5.2.3 Dissolved air flotation of the silica/ polymeric flocculant system 133 5.2.4 Dissolved air flotation of the silica gel/cationic surfactant system 135 6 Page 5.3 Discussion 136 6. THE SURFACE CHEMISTRY AND DISSOLVED AIR FLOTATION OF CASSITERITE 141 6.1 Studies on the cassiterite/sodium dodecyl sulphate system 142 6.1.1 Electrokinetic studies 142 6.1.2 Stability measurements 148 6.1.3 Adsorption measurements 151 6.1.4 Dissolved air flotation studies 155 6.2 Studies on the stannic oxide/sodium dodecyl sulphate system 159 6.2.1 Electrokinetic studies 159 6.2.2 Adsorption measurements 161 6.3 Studies on the cassiterite/Aeropromoter 845 system 165 6.3.1 Electrokinetic studies 166 6.3.2 Stability measurements 168 6.3.3 Dissolved air flotation studies 170 6.4 Discussion 173 7. STUDIES ON THE SEPARATION OF CASSITERITE FROM QUARTZ BY DISSOLVED AIR FLOTATION 189 7.1 Separation studies in the presence of anionic collectors 190 7.2 Separation studies in the presence of oil/collector emulsions 197 7.3 Influence of various parameters on the separation studies 203 7.3.1 Effect of the addition of a modifier 203 7.3.2 Effect of rate of stirring during micro- bubble injection 206 7.3.3 Effect of a cleaner DAF stage 208 7.3.4 Effect of higher solids concentration 209 7.4 Discussion 213 8. OTHER ASPECTS OF DISSOLVED AIR FLOTATION OF MINERAL PARTICLES 220 8.1 Introduction 220 8.2 The influence of various parameters on the recovery of mineral particles by dissolved air flotation 220 8.2.1 Stability of the floated product 220 8.2.2 The effect of stirring 221 8.2.3 The effect of particle size 225 8.2.4 The effect of solids concentration . 226 8.2.5 The effect of saturation and injection pressures 227 7 Page 8.3 Flotation of minerals by bubbles forming from supersaturated solution 230 8.3.1 Introduction 230 8.3.2 Experimental 231 8.3.3 Results 232 8.3.3.1 Experiments on the formation of bubbles from supersaturated solution - The effect of hydrophobicity 232 8.3.3.2 Experiments on the formation of bubbles from supersaturated solution - The effect of gas nuclei 235 8.3.3.3 Experiments on the flotation of minerals by bubbles forming from supersaturated solution 239 8.3.4 Discussion 245 9. SUMMARY AND CONCLUSIONS 254 APPENDICES 263 APPENDIX I. Experiments on the formation of microbubbles in dissolved air flotation systems 264 APPENDIX II. Calculation of the surface charge of cassiterite and stannic oxide 270 APPENDIX III. Photographic study of flotation by bubbles forming from supersaturated solution 272 REFERENCES 275 8 LIST OF FIGURES Page 1.1 Classification of dissolved air flotation systems according to type of pressurization 29 3.1 Stern-Grahame model of the electrical double layer at the solid-liquid interface 31 4.1 Calibration curves for the spectrophotometric determination of dodecylamine (DAC) and sodium dodecyl sulphate (SDS) 98 4.2 Calibration curve for the determination of tin by atomic absorption spectroscopy 98 4.3 Schematic diagrams of (a) saturator, (b) pressure reduction nozzle, (c) Urban's cell, and (d) cell designed in this work 104 5.1 Electrophoretic