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Recovery of Particulate Gold by Coal-Oil Agglomerates

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Recovery of Particulate Gold by Coal-Oil Agglomerates RECOVERY OF PARTICULATE GOLD BY COAL-OIL AGGLOMERATES A Thesis by JUANCHO PABLO S. CALVEZ B. Sc. Metallurgical Engineering Submitted for the degree of Master of Science School of Chemical Engineering & Industrial Chemistry Faculty of Engineering UNIVERSITY OF NEW SOUTH WALES March 1998 11 CERTIFICATE OF ORIGINALITY I hereby declare that this submission is my own work and to the best of my knowledge, it contains no materials previously published or written by another person, nor material which to a substantial extent has been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgment is made in the thesis. Any contribution made to the researchby others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledgedin thethesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in theproject's design and conception or in style, presentation andlinguistic expression is acknowledged. lll ABSTRACT Fundamental and applied studies were undertaken to evaluate the underlying ' principles by which gold is recovered by coal-oil agglomerates. The utilization of coal-oil agglomerates as a potential environmentally safe alternative to conventional methods of recovering gold has been patented over a decade ago but has not gained its ground commercially in the mining industry. This study then was conducted to further improve the understanding of the process. Experiments were conducted in a 1.5 liter baffled glass vessel agitated by four­ axial blade impellers. Each experiment was carried out in two phases: first, the formation of agglomerates and second, the agitated mixing of agglomerates and ground gold bearing samples. Carbonaceous materials used for agglomeration were bituminous coal, lignitic coal and graphite while the agglomerating liquids were diesel oil and kerosene. Gold bearing samples employed for the fundamentals studies were synthetic mixtures of cyanide leaching residues (or pure silica) and pure gold powders while the materials for the applicability tests were real gold bearing samples - copper concentrates and amalgamation tails - from the Philippines. Tests to determine the effects of different variables - oil:coal ratios, agglomerate:ore ratios, pH, impeller speed, particle suspension, coal particle size, ore particle size - on gold recovery were conducted. The effects of sulfides were also evaluated. The mechanism by which gold are collected by the agglomerates was investigated by the use of scanning electron microscope. IV Results of the fundamental studies revealed that rate of gold recovery was increased by increasing agglomerate:ore ratio (AOR) or decreasing oil:coal ratio (OCR). At AORs of 1 and 0.13, 99.8% and 99.3% gold recoveries were attained after 15 and 120 minutes, respectively. Increasing agglomerate:ore ratio correspondingly increases the number and surface area of agglomerates available for gold capture. At 30 minutes mixing time, gold recoveries at OCRs of 0.269 and 0.403 were 99% and 91 %, respectively. Decreasing the amount of oil relative to constant weight of coal (decreasing OCR) produced smaller agglomerates thereby increasing their number and available surface area. At pH range of 4 - 12, there was no significant difference in gold recoveries while at pH of 2, recovery was adversely affected specifically at limited mixing time. Whereas superficial organic contamination or oxidation influences gold hydrophobicity, there may have been an initial cleaning effect of sulfuric acid (pH regulator), that lessens the hydrophobicity of the gold surfaces. By extending mixing time from 30 to 60 minutes, however, gold recovery was increased considerably due to the ease by which gold could be re-contaminated. Increasing impeller speed increased gold recovery due to enhancement of inter­ particle collisions and gold particle suspension. The final recovery at 300 rpm was notably low at 64% as compared to over 90% attained at 500 rpm and above. Agglomerate strength and stability as dictated by the size of coal particles, the type and amount of oil, and the type of carbonaceous material also influenced gold recovery. With weaker agglomerates, formed by using coarser coal particles, lighter oils, over-critical amount of oil or less hydrophobic carbonaceous agglomerating material, gold recovery was adversely affected when V fine agglomerate particles (possibly loaded with gold) were abraded from the main agglomerates and were lost with the tailings. At up to 5% sulfides in the feed, gold recovery was not significantly altered. While the agglomerates mostly recovered chalcopyrite, pyrite and galena, sphalerite, due to the oxidized nature of the sample, gave poor recovery. Increasing pH lowered pyrite recovery that at pH of 12 virtually no pyrite was recovered. Experiments with real gold bearing samples revealed that increased liberation size and impeller speed raised gold recovery. Too much sulfides had an adverse effect on gold recovery. Microscopic examination of loaded agglomerate showed that gold particles penetrated within the sub-surface up to 60 µm depth. Vl ACKNOWLEDGMENT I would like to convey my heartfelt appreciation and gratitude to the following persons and entities, without them, this study would not have been a reality: To Associate Professor Tam Tran, my supervisor, for his dedicated guidance and support during the whole course of my research. To the Australian Agency for International Development (AusAID), for granting me a scholarship to pursue this MSc degree. To Dr. Patrick Wong, my former co-supervisor, for his technical support during the initial phase of this study. To the staff of Centre for Minerals Engineering, Jin Song and Ling Lau, for their invaluable assistance and to my fellow postgraduate students, for helping me cope up with the rigors of laboratory works. To the personnel and staff of the Metallurgical Technology Division of the Philippine Mines and Geosciences Bureau, for the unselfish help they accorded me during the conduct of my experiments in the Philippines. To the Filipino Students Society of UNSW, for sharing with me the ups and downs of being away from home. vii To the Couples for Christ - Singles for Christ Ministry here in Australia, for supporting me spiritually and sharing with me the joys of being together in fellowships and service. To my family and friends, for their prayers and words of encouragement that inspired me to do my best in hurdling this another milestone of my career. And most especially to our Good Lord, the Almighty King, for always being there - watching and guiding me in all my endeavors. Vlll TABLE OF CONTENTS i, Page ABSTRACT 111 ACKNOWLEDGMENT VI LIST OF FIGURES Xlll LIST OF TABLES XVI LIST OF APPENDICES XVIll LIST OF PUBLICATIONS lX CHAPTERl INTRODUCTION 1 CHAPTER2 LITERATURE REVIEW 5 2.1 INTRODUCTION 5 2.2 GOLD MINERALOGY 5 2.3 GOLD RECOVERY PROCESSES 7 2.3.1 Cyanidation Process of Gold Extraction 9 2.3.1.1 The Chemistry of Cyanidation 9 2.3.1.2 Treatment of Gold Ores by Cyanidation 10 2.3.1.3 Methods of Recovering Gold from Cyanide Leach Liquor 11 2.3.1.3.1 Zinc Cementation 11 2.3 .1.3 .2 Carbon Adsorption Process 12 2.3.1.3.3 Resin Technology 16 2.3.1.3.4 Ion Flotation 18 IX 2.3.2 Non-Cyanide Lixiviants for Gold Dissolution 20 2.3.2.1 Thiourea 21 2.3.2.2 Thiosulfate 22 2.3.2.3 Thiocyanate 23 2.3.2.4 Halides 24 2.3.3 Gravity Concentration 25 2.3.4 Froth Flotation 27 2.4 AGGLOMERATION OF FINE COAL WITH OIL 32 2.5 COAL-OIL AGGLOMERATION FOR GOLD RECOVERY 36 2.5.1 Patents for Coal-Oil Agglomeration Process 37 2.5.1.1 The CGA Process 38 2.5.1.2 The CARBAD Process 39 2.5.1.3 The Bateman Process 40 2.5.2 Application of Coal-Oil Agglomeration on Gold Recovery 41 2.6 THEWETTABILITYOFGOLDBYWATER 45 CHAPTER 3 SUMMARY OF LITERATURE REVIEW AND RESTATEMENT OF OBJECTIVES 48 CHAPTER 4 RESEARCH METHODOLOGY 52 4.1 MATERIALS 52 4.1.1 Gold-Bearing samples 52 4.1.1.1 Synthetic Mixtures 52 4.1.1.1.1 Gold Powder - Cyanidation Residue Synthetic Mixture (Blend A) 52 4.1.1.1.2 Gold Powder-High Purity Silica Mixture (Blend BJ 53 4.1.1.2 Real Gold-Bearing Samples 54 4.1.2 Carbonaceous Materials 54 4.1.2.1 Bituminous Coals 54 X 4.1.2.2 Graphite 55 4.1.2.3 Lignitic Coal 56 4.1.3 Agglomerating Liquids 56 4.1. 4 Chemical Reagents 56 4.2 EQUIPMENT AND INSTRUMENTATION 57 4.2.1 Experimental Apparatus 57 4.2.2 Sample Preparation Equipment 58 4.2.3 Analytical Equipment 58 4.2.4 Scanning Electron Microscope 59 4.3 EXPERIMENT AL DESIGN 59 4.3.1 General Procedures 59 4.3.1.1 Agglomeration 60 4.3.1.2 Gold Recovery Stage 60 4.3.2 Preliminary Experiments 61 4.3 .3 Experiments to Determine the Effects of Various Parameters in Gold Recovery 62 4.3.3.1 Agglomerate : Ore Ratios 62 4.3.3.2 Oil: Coal/Graphite Ratios 63 4.3.3.3 pH 63 4.3.3.4 Particle Suspension 64 4.3.3.5 Coal Particle Size 65 4.3.4 Experiments to Determine the Effects of Sulfides 66 4.3.5 Laboratory Experiments Conducted in the Philippines 68 4.3.5.1 Tests on Real Gold Bearing Samples 68 4.3.5.2 Tests on Philippine Lignitic Coal 70 4.3.6 Determination of the Extent of Gold Attachment/ Penetration in the Agglomerate 71 4.4 ANALYTICAL PROCEDURES 72 X1 4.4.1 Analysis of Heads and Metallurgical Products 72 4.4.2 Ash Determination 74 4.5 SIMULTANEOUS PULP AND AGGLOMERATE SAMPLING PROCEDURE 74 4.6 METAL RECOVERY CALCULATIONS 75 CHAPTERS RESULTS AND DISCUSSION 77 5.1 RESULTS OF PRELIMINARY EXPERIMENTS 77 5.2 EFFECTS OF AGGLOMERATE:ORE RATIOS 80 5.3 EFFECTS OF OIL:COAL RATIOS 83 5.3.1 Effects of Oil:Coal Ratios on Agglomerate Size, Surface Area and Number 84 5.3.2 Effects of Agglomerate Surface Area and Size on Gold Recovery 86 5.3.3 Summary of Effects of Different Oil:Coal Ratios on Gold Recovery 88 5.3.4 Effects of Oil:Graphite Ratios on Gold Recovery 89 5.4 EFFECTS OF pH ON GOLD RECOVERY 90 5.5 EFFECTS OF PARTICLE SUSPENSION 92 5.6 EFFECTS OF COAL PARTICLE SIZE ON AGGLOMERATION AND GOLD RECOVERY 95 5.7 EFFECTS OF SULFIDES ON GOLD RECOVERY AND RECOVERY OF SULFIDES BY COAL-OIL AGGLOMERATES 96 5.
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