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The Chemistry of the Carbon-In--Pulp Process

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The Chemistry of the Carbon-In--Pulp Process THE CHEMISTRY OF THE CARBON-IN--PULP PROCESS Michael David Adams A Thesis submitted to the Faculty of Science University of the Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Doctor of Philosophy Johannesburg 1989 ABSTRACT Several conflicting theories of the adsorption of aurocyanide onto activated carbon presently exist. To resolve the mechanism, adsorption and elution of aurocyanide are examined by several techniques, including Mossoauer spectroscopy, X-ray photoelectron spectro­ scopy, X-ray diffractometry, Fourier Transform Infrared spectrophotometry, ultraviolet-visible spectrophotometry and scanning electron microscopy. The evidence gathered indicates that, under normal plant conditions, aurocyanide is extracted onto activated carbon in the form of an ion pair M n* [Au(CN) 2 3 n , and eluted by hydroxide or cyanide. The hydroxide or cyanide ions react with the carbon surface, rendering it relatively hydrophilic with a decreased affinity for neutral species. Additional adsorption mechanisms are shown to operate under other conditions of ionic strength, pH, and temperature. The poor agreement in the literature regarding the mechanism of adsorption of aurocyanide onto activated carbon is shown to be due to the fact that different mechanisms operate under different experimental conditions. The AuCN produced on the carbon surface by acid treatment is shown to react with hydroxide ion via the reduction of AuCN to metallic gold with formation of Au (CN) 2 , and the oxidation of cyanide to cyanate. Other species, such as An(CN)5 and Ag(CN)g adsorb onto activated carbon by a similar mechanism to that postulated for Au( C N ) 2 . Ion association of MAu(CN ) 2 salts in aqueous solution is demonstrated by * aans of potentiometric titration and conductivity measurements, and various associated species of KAu(CN), salts are shown to occur in organic solvents by means of infrared spectrophoteaietric and distribution measurements. A kinetic model was developed for elution of aurocyanide from activated carbon and was found to predict gold elution performance successfully using the Zadra procedure. The influence of the surface chemistry and structure of activated carbon on adsorption of aurocyanide was investigated by characterization of activated carbons that were synthesized or oxidized under various conditions. Synthetic polymeric adsorbents with characteristics similar to activated carbons were also studied. The evidence suggests that a large nicropore volume is important in providing suitable active sites for adsorption. Another important factor is the presence of basic functional groups within the micropore, which act as solvating agents for the ion pair. The aim is to provide a self-consistent adsorption mechanism that accounts for all observations presented in the literature. Interprrtation of results in terras of preconceived ide..s, and neglect of observations of other authors has greatly contributed to current disagreement in the literature. Declaration I declare that this Thesis is ray own, unaided work. It is being submitted for the degree of Doctor of Philosophy. It has not been submitted before for any degree or examination in any other University. 2- 3 i day of , 1989 Acknowledgements I wish to express my sincere gratitude and appreciation to my supervisors. Prof R.D. Hancock and Prof R.G. Copperthvaite for their enthusiasm and help; my wife, Jenni, for her support; my colleagues, Dr C.A. Fleming, Dr R.L. Paul, Dr M.J. Niccl, Dr B.P. Green, Dr P.J. Harris amongst others, for the many hours of useful discussions; Prof Dr F.E. Wagner of the Physik-Depart-ment, Technische Universitat Hunchen, for running the Mossbauer spectra and for engaging in a useful collaboration regarding this t ec h n i q u e ; Mr P. Ellis and Mrs I. Klingbiel, for their patience with running the Scanning Electron Microscope; Mr D.E. Imes, for running the X-ray Photoelectron s p e c t r a ; Mr P.W. Wade, for his time and help with the Molecular Mechanics calculations; Mrs M. Arinto, Mrs M.R. Hazell and Ms T. McArthur, for their assistance in the typing of this manuscript; the Council for Mineral Technology (Mintek), for allowing me to undertake this research during ray period of employment with them; my parents, for their encouragement through the years. i Contents page CHAPTER 1 INTRODUCTION 1 1.1 The Adsorption of Aurocyanide onto Activated Carbon - An Historical Perspective 1 1.2 The Carbon-in-pulp Process - Process Chemistry and Unknown Areas 3 a; Leaching of Gold from the Ore 3 b) Adsorption of Aurocyanide onto Activated Carbon l c) Carbon Acid Washing and Elution 7 d) Gold Electrot'inning or Precipitation 8 e) Carbon Reactivation 8 f) Other Features of the CIP Process 9 1.3 The Structure and Chemistry of Activated Carbon 10 a) Physical Structure of Activated Carbon 10 b) Chemical Stricture of Activated Carbon li 1.4 The Chemistry of Gold - An Introduction 13 1.5 Current Processes for the Recovery of Gold 17 CHAPTER 2 EXPERIMENTAL PROCEDURE 19 2.1 Reagents and Chemicals 19 2.2 Activated Carbon Adsorption studies 20 a) Equilibrium Adsorption 20 b) Elution of the Carbon 21 c) Precipitation of AuCN 21 d) Decomposition of AuCN 22 e) Oxygen Effect Experiments 22 f) Gold(XII) Cyanide Experiments 23 g) Silver and Mercury Experiments 23 h) Miscellaneous Techniques 24 (i) X-ray diffractometry 24 (ii) Scanning electron microscopy 24 (iii) Fourier Transform infrared Spectrophotometry 24 (iv) 197Au Mossbauer spectroscopy 25 (v) x y photoelectron spectroscopy (XPS) 33 2.3 Solution Studies 35 a) Pctantiometric Titration 35 by Conductivity Measurements 35 c) Infrared Spectrophotometry 36 d) Distribution Experiments 36 e; Determination of Chloride Concentration in Organic Phases 37 Determination of Dielectric Constant 37 g) Determination of Water Content by Automatic Karl-Fischer Titration 38 2.4 Molecular Mechanics Calculations 38 a) The Force Field 38 by Conformations of Polyether in Organic Solvents 39 2.5 Activated Carbon Elution Studies 40 a) Elution Mechanism Experiments 40 by Kinetic Experiments 41 2.6 Activated Carbon Surface Chemistry and Structure Studies 4 5 ay Carbons and Adsorbents Used 45 by Synthesis of Activated Carbons 45 cy Oxidation of Activated Carbons 47 dy Synthesis of Polyxanthene and Polyquinone 47 ey Adsorption of Aurocyanide 48 fy Techniques for the Characterization of Physical Properties 49 gy Techniques for the Characterization of Chemical Properties 51 CHAPTER 3 THE MECHANISM OF ADSORPTION OF AUROCYANIDE ONTO ACTIVATED CARBON 53 3.1 The Mechanism of Adsorption of Aurocyanide onto Activated Carbon - A Literature Review 53 ay Summary of Factors Influencing Adsorp- vii tioji of Aurocyanide onto Activated Carbon 53 b! Summery of Mechanisms Postulated for Adsorption of Aurocyanide onto Activated Carbon 54 c, Summary of Major Inconsistencies Present in Modern Theories ..or Adsorption of Aurocyanide onto Activated Carbon 55 3.2 Novel Studies on the Nature of the Adsorbed Species 56 a) Effect of Gold Concentration on the Carbon on the Nature of the Adsorbed Species 57 b; Elution of Aurocyanide from Activated Carbon with Sodium hydroxide 59 c) Effect of the Treatment of Loaded Carbon with Hot Acid on the Nature of the Adsorbed Gold Species 62 (i) SEM and XRD studies of gold species on carbons subjected to acid and base treatments 62 (ii) Mossbauer spectroscopic study of gold species on carbons subjected to acid and base treatments 70 (iii) Kinetics of AuCN precipitation from aqueous solution 77 (iv) Decomposition of AuCN in aqueous solution 78 <*} Thermal Stability of Adsorbed Gold Species on Activated Carbon 84 B) Relation Between the Effects of Oxygen and Ionic Strength on the Adsorftion of Aurocyanide onto Activated Carbon 98 f) X-ray Photoelectron Spectroscopic Study of Adsorbed Gold Species on Activated Carbon 115 9) Fourier Transform Infrared Spectroscopic Study of Adsorbed Aurocyanide Species on Activated Carbon 128 3.3 Mechanisms of Adsorption of Other Complex Metal-ion Species onto Activated Carbon 145 a) The Mechanism of Adsorption of Au(CN) ^ onto Activated Carbon 145 b) The Mechanicms of Adsorption of Ag(CN) 2 and Ag* onto Activated Carbon 154 c) The Mechanisms of Adsorption of Hg(CN) 2 and HgCl2 onto Activated Carbon 172 d) The Mechanism of Adsorption of AuCl ~4 onto Activated Carbon 182 3.4 Summary and Conclusions 182 C H APTER 4 ION ASSOCIATION O F MAti(CN) , SALTS IN VARIOUS SOLVENTS 190 4.1 Ion Association of HAu(CN) 7 Salts in Various Solvents - A Literature Survey 4.2 The Study of Ion Association - A Literature Survey 4.3 Ion Association of MAu(CN ) 2 Salts in Aqueous Solution a) Potsntiometric i'itration of Aqueous Solution of HAufCN;2 195 h) Conductimetric Study of Aqueous Solutions of KAu(CN)? and N a A u (CN) 2 Ion Association of MAu(CN ) 2 Salts in Organic Solvents aj Infrared Spectra cf MAu(CN) 2 Salts in Organic Solvents Distribution of MAu(CNj 2 Salts between Aqueous Solutions and Organic Solvents 212 a; Effect of Solvent 224 b) Effect of Diluent 234 c) Effect of Cation 238 4.6 Extraction of Aurocyanide Ion Pairs by Poly(oxyethylene) Extractants 251 Solvent Extraction of M #A u (CN) 2 Ion Pairs by Triton X-10C h) Molecular Mechanics Calculations of the Complexation of Alkali Metal Cations by Polyethers 258 1% 4.7 Summary and Conclusions 266 CHAPTER 5 THE MECHANISM OF ELUTION OF AUROCYANIDE FROM ACTIVATED CARBON 269 5.1 Introduction 269 a) Processes for Elution of Aurocyanide frcm Activated Carbon 269 b) Chemistry and Mechanism of Elution of Aurocyanide from Activated Carbon
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