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Myths and Realities in Artisanal Gold Mining Mercury Contamination

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Myths and Realities in Artisanal Gold Mining Mercury Contamination MYTHS AND REALITIES IN ARTISANAL GOLD MINING MERCURY CONTAMINATION by GUSTAVO ANGELOCI SANTOS A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Mining Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) April 2013 © Gustavo Angeloci Santos 2013 ABSTRACT The world faces a major gold rush currently. Worldwide 15 million people work directly in artisanal gold mining in more than 55 countries. The usual method of extraction is amalgamation and cyanidation, or very commonly a combination of both. Significant amounts of contaminants are released to rivers and soil, generating environmental and health concerns. The majority of artisanal miners are involved in micro-mining, but it is believed that the majority of contamination comes from small to large artisanal operations. Larger operations involve less people, but process much more material, re- leasing much more contaminants as well. The present work explains how contamination is generated, through a com- prehensive analysis of the labor division in small-scale mining sites around the world, and also analyses aspects of different intervention approaches. Different approaches are taken by different groups working on the problem around the world. This work evaluates the most common kinds of interven- tion, with special attention to technical and legal aspects in trying to elimi- nate mercury use in gold extraction. The study is conducted by comparing different interventions strategies to evaluate the myths and realities from an engineering perspective. Special attention is paid to the evaluation of alterna- tives to amalgamation process for gold such as intensive cyanidation, borax direct smelting and chlorination. Accordingly to the location in which the technology is introduced, different methods can be applied, but only cyanide remains a proven method to deal with complex ores. Important aspects such as education, training, financial aid and understand- ing of a community before intervention are also shown to be essential for success. The association of Small-scale Gold Mining and potential gold concentrate consumers is pointed to as a possible solution, applicable where concentrates can be shipped to a smelter. Association with large companies is cited as an alternative means to reduce contaminants generated by gold extraction as well. ii Table of Contents TABLE OF CONTENTS Abstract ........................................................................................................................ ii Table of Contents ..................................................................................................... iii List of Tables .............................................................................................................. v List of Figures ............................................................................................................ vi List of Acronyms .................................................................................................... viii Acknowledgements ................................................................................................... ix 1. Introduction ......................................................................................................... 1 2. Mercury in Artisanal Mining.............................................................................. 3 2.2. How Mercury is Used in ASGM ............................................................... 7 2.2.1. Gold Ore Processing Centers and Mercury ................................ 10 2.3. Mercury Releases and Emissions ............................................................ 19 2.3.1. Metallic Mercury .............................................................................. 19 2.3.2. Methylmercury ................................................................................. 25 1.2.1. Mercury Cyanide .............................................................................. 29 3. ........ Definitions and Differences Between Artisanal Mining and Small-scale Mining ........................................................................................................................ 32 3.2. Micro Gold Mining ................................................................................... 38 3.3. Small-scale Gold Mining .......................................................................... 40 3.3.1. SGM Contamination ....................................................................... 47 4. The Legal Approach ......................................................................................... 51 iii Table of Contents 5. The Technological Approach.......................................................................... 60 6. Myths and Realities in Gold Extraction ........................................................ 64 6.1. Concentration is the Solution .................................................................. 66 6.1.1. Gravity Concentration .................................................................... 70 6.1.2. Flotation ............................................................................................ 76 6.2. Amalgamation of Concentrates ............................................................... 77 6.3. Replacing Mercury ..................................................................................... 89 6.3.1. Conventional Cyanidation .............................................................. 89 6.3.2. Intensive Cyanidation ..................................................................... 93 6.3.3. Borax Extraction ............................................................................. 99 6.3.4. Chlorine Leaching ......................................................................... 106 6.3.5. Electro-Leaching............................................................................ 110 6.3.6. Ore Characterization in AGM .................................................... 117 7. Conclusion ....................................................................................................... 123 References .............................................................................................................. 128 Appendix I – Cazaderos Plant Pictures ............................................................ 154 Appendix II – Cazaderos Ore XRD Result ..................................................... 157 Appendix III – Cazaderos Centrifuge Sample Assay ..................................... 158 Appendix IV – Fire Assay Fluxes ...................................................................... 159 Appendix V – Characterization of an AGM Ore ............................................ 160 iv List of Tables LIST OF TABLES Table 3-1. Processing Capacity Definition ......................................................... 35 Table 3-2. Countries With Documented ASGM Activities (Adapted from: Veiga et al., 2004) .................................................................................................... 37 Table 5-1. Suggested Steps for Intervention ...................................................... 63 Table 6-1. Arguments for not Using a Retort at GMP Sites (Adapted from Veiga et al, 2006a) ................................................................................................... 83 Table 6-2. Recoveries for Direct Smelting Tests ............................................. 104 Table 6-3. Tests of Electro-leaching of Pure Gold ......................................... 113 Table 6-4. Electro-leaching Test Conditions .................................................... 115 Table 6-5. Electro-leaching Tests Results ......................................................... 115 Table 6-6. Feed and Recovery for GRG Test .................................................. 119 Table 6-7. Conditions of Flotation Tests .......................................................... 120 Table 6-8. Results of Flotations Tests ............................................................... 120 Table 7-1. Myths and Realities Compilation ..................................................... 125 v List of Figures LIST OF FIGURES Figure 2-1. Mercury Released by ASGM (2010 - Mercurywatch.com) ............. 6 Figure 2-2. Processing Center in Zimbabwe With Stamp Mills and A) Knudsen-Type Centrifuge B) Copper-Amalgamating Plate (Photo: M. Veiga) .................................................................................................................................... 13 Figure 2-3. Percolation Tanks in Kadoma, Zimbabwe, Leaching Mercury- Contaminated Tailings With Cyanide .................................................................. 13 Figure 2-4. Flow Sheet of a Typical Whole Ore Amalgamation in Antioquia Cordy et al., 2011) ................................................................................................... 16 Figure 2-5. Chilean Mill With Sluice Boxes Covered With Wool Carpets in Ecuador ..................................................................................................................... 18 Figure 2-6. Mercury Deposition by Year in Ice Cores (Krabbenhoft and Schuster, 2002) ......................................................................................................... 21 Figure 2-7. Mercury Cycle (Morel et al., 1998) ................................................... 27 Figure 3-1. Schematic Definition of Different Types of Mining Operations34 Figure 3-2. Countries With Documented
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