Chemistry and Treatment of Cyanidation Wastes SECOND EDITION

Terry I. Mudder, Ph.D. Michael M. Botz, M.S., P.E. and Adrian Smith, Ph.D.

MJ

MINING JOURNAL BOOKS LTD LONDON

Dedication

Dedicated to the memory of Dr. Adrian Smith and to those who still believe in science and have the courage to apply it.

Acknowledgements There are many people without whose efforts the first and now the second edition of this book would have not been completed. With respect to the first edition, we thank Dr. Ian Hutchison of TRC, Inc., Mr. Jim Whitlock of Whitlock Associates, Homestake Mining Company and Dr. Terry Chatwin from the University of Utah for their substantial contribution of thoughts and ideas to various parts of the book. Furthermore, we thank James Scott formally of Environment Canada and the first recipient of the Adrian Smith International Environmental Mining Award. It is the dedication of Mr. Scott and his co-workers within Environment Canada that has contributed much to our understanding of the environmental implications of the cyanidation process. The cooperation exhibited between Environment Canada and the mining industry worldwide has provided over the years an excellent example worthy of copy. We would like to thank SRK Consulting Inc. for their part in the production of the first edition of the book and Homestake Mining Company for providing important original research on the subject of biological treatment. We would like to also thank Clint Strachan and Rick Frechette for their insights and comments on water balances and management at mining operations. Roger Schulz from the Chemistry Centre in Perth, Western Australia, provided an in-depth review of the analytical chapter with his considerable experience and expertise in cyanide measurement. Dr. Karen Hagelstein provided important knowledge on the ecotoxicological properties of cyanide and its related compounds. We would again like to thank Dr. Adrian Smith, who is no longer physically with us, but whose inspiration and spirit will remain forever real in our hearts. Finally, a special thanks to The Mining Journal and Mining Journal Books for believing this book was worth publishing.

Terry I. Mudder, Ph.D. Sheridan, Wyoming USA

Michael M. Botz, M.S., P.E. Joliet, Montana USA June 2001

Preface The increased awareness of the environmental implications associated with cyanidation has led to a tremendous increase in the knowledge of chemistry, analysis, toxicity and treatment of its process solutions. These four aspects of the cyanidation process are intimately related, with the treatment of cyanidation solutions forming the visible bridge between the mining operation and the receiving system or environments. There is sufficient detail and depth of expertise, experience and information to begin to quantify the impacts of cyanidation and to provide permanent, reliable and environmentally acceptable solutions to long-standing concerns. The chemistry of the cyanidation process is well defined, but its impacts on the characteristics of mine waters and the costs of water treatment have often been underestimated during the initial stages of mine development. Today, however, the mining industry is well aware of the consequences of inadequate environmental management and has invested hundreds of millions of dollars in the development of state-of-the-art tailings disposal and water treatment systems. The analysis of cyanidation process waters has evolved from the free cyanide titration with silver nitrate to the speciation of individual metal cyanide complexes in the part per billion range using specialized ion chromatography methods. Although many analytical methods are available, only a few are capable of producing reliable and accurate data. It is necessary to focus upon and improve existing methodology, while appropriately using analytical methods currently in wide usage. The toxicity of cyanidation solutions is complex, as it involves not only individual compounds, but combinations of compounds which exhibit characteristics much different than any of the individual components. There is now sufficient toxicological information concerning cyanidation process solutions to quantify and calculate acceptable discharge limitations on a siteby-site basis. This approach requires the practical application of both laboratory and field data to provide acceptable levels of environmental protection. The chemistry, analysis and toxicity of cyanidation process solutions are combined for the development and design of suitable and viable water treatment alternatives. A crucial aspect of this is proper evaluation of mine site water balances and implementation of appropriate water management systems. There are many water treatment alternatives that can be used at mining operations, none of which are applicable in all situations. It is hoped that the second edition of this book will aid those involved in the permitting, design and operation facets of the mining industry to ask the appropriate questions and to make justifiable and practical decisions.

Unit Conversions Length 1 inch = 2.540 cm 1 m = 3.2809 feet = 39.37 inches 1 mile = 5,280 feet = 0.6214 km 1 yard = 3 feet = 36 inches Area 1 acre = 43,560 ft2 1 hectare (ha) = 2.471 acres 1 m2 = 10.764 ft2 Volume 1 m3 = 35.3145 ft3 = 264.17 gallons 1 ft3 = 7.4805 gallons 1 gallon = 3.7854 liters 1 acre-foot = 43,560 ft3 = 1,233.5 m3 Mass 1 tonne (t) = 1,000 kg = 2,204.62 lbs 1 ton = 2,000 lbs = 907.19 kg 1 tonne (t) = 1.102 tons 1 kg = 2.20462 lbs 1 lb = 453.593 g 1 Troy ounce (ozTR) = 31.10 g Pressure 1 atm = 14.696 psi = 1.01325 bars = 760 mm Hg = 1.01325 × 105 Pa 1 kPa = 0.145 psi Flow 1 gpm = 0.227 m3/hr 1 m3/hr = 4.403 gpm 1 L/min = 0.264 gpm 1 ft3/sec (CFS) = 448.8 gpm Concentration 1 mg/L = 1 g/m3 = 0.062 lbs/ft3 1 g/tonne (g/t) = 0.0292 ozTR/ton Temperature o F = [1.8 × oC] + 32 o C = [oF – 32] ÷ 1.8

Table of Contents

Table of Contents CHAPTER 1

Chemistry Of Free And Complexed Cyanide

1.1 1.2 1.3 1.4

Introduction ................................................................................................................... 1 Gold Dissolution ............................................................................................................ 1 Gold Recovery From Solution ....................................................................................... 5 Solution Chemistry of Cyanide and its Metal Complexes .............................................. 6 1.4.1 Introduction........................................................................................................... 6 1.4.2 Cyanide Bonding .................................................................................................. 6 1.4.3 Free Cyanide ......................................................................................................... 7 1.4.4 Simple Cyanide Compounds................................................................................. 8 1.4.5 Weak and Moderately Strong Cyanide Complexes ............................................. 12 1.5 Solution Chemistry of the Iron Cyanide....................................................................... 16 1.6 Cyanide Related Compounds....................................................................................... 22 1.6.1 Introduction......................................................................................................... 22 1.6.2 Thiocyanate......................................................................................................... 22 1.6.3 Cyanate ............................................................................................................... 22 1.6.4 Ammonia ............................................................................................................ 23 1.6.5 Nitrate ................................................................................................................. 23 1.7 Process Solution Chemistry ......................................................................................... 23 1.7.1 Introduction......................................................................................................... 23 1.7.2 Iron Cyanides ...................................................................................................... 25 1.7.3 Copper Bearing Ores........................................................................................... 25 1.7.4 Silver Ores .......................................................................................................... 26 1.8 References ................................................................................................................... 26 1.9 Bibliography ................................................................................................................ 27

CHAPTER 2

Analysis of Cyanide

2.1 Introduction ................................................................................................................. 29 2.2 Analytical Procedures for Cyanides ............................................................................. 30 2.2.1 Introduction......................................................................................................... 30 2.2.2 Total Cyanide by Distillation .............................................................................. 32 2.2.3 Cyanide Amenable to Chlorination ..................................................................... 39 2.2.4 WAD Cyanide by Distillation ............................................................................. 40 2.2.5 WAD Cyanide by the Picric Acid Method .......................................................... 40 2.2.6 WAD Cyanide by the Zinc Dust and Ammonia Method..................................... 42 2.2.7 Free Cyanide by Titration with Silver Nitrate ..................................................... 42 2.2.8 Free Cyanide by Ion Selective Electrode............................................................. 43 2.2.9 Cyanide by Ion Chromatograph .......................................................................... 44 2.2.10 Cyanide by Automated Ligand Exchange ........................................................... 44

i

Table of Contents

Chemistry and Treatment of Cyanidation Wastes

Table of Contents (Continued) 2.2.11 Analysis Of Cyanide In Solids ............................................................................ 47 2.2.12 On-Line Process Monitoring For Cyanide .......................................................... 47 2.2.13 Cyanide Detection And Quantitation Limits ....................................................... 47 2.2.14 Thiocyanate Analysis .......................................................................................... 49 2.2.15 Cyanate Analysis................................................................................................. 49 2.2.16 Ammonia, Nitrite and Nitrate Analyses .............................................................. 49 2.3 Analytical Interferences ............................................................................................... 50 2.3.1 Introduction......................................................................................................... 50 2.3.2 Oxidizing Agents ................................................................................................ 50 2.3.3 Sulphides............................................................................................................. 51 2.3.4 Thiocyanate......................................................................................................... 51 2.3.5 Nitrite and Nitrate ............................................................................................... 52 2.3.6 Carbonates .......................................................................................................... 52 2.3.7 Thiosulphates, Sulphites and Other Sulphur Compounds.................................... 52 2.3.8 Metals ................................................................................................................. 53 2.3.9 Effects of Interferences on Cyanide Analyses ..................................................... 53 2.4 Preservation and Storage of Samples ........................................................................... 54 2.5 Calculation of a Cyanide Mass Balance....................................................................... 59 2.6 Comparative Analyses of Solutions ............................................................................. 61 2.7 Low Levels of Cyanide in the Environment................................................................. 65 2.7.1 Natural Sources of Cyanide................................................................................. 66 2.7.2 Anthropogenic Sources of Cyanide..................................................................... 67 2.7.3 Monitoring of Surface and Ground Waters ......................................................... 68 2.8 References ................................................................................................................... 68 2.9 Bibliography ................................................................................................................ 71

CHAPTER 3

Environmental Geochemistry and Fate of Cyanide

3.1 Introduction ................................................................................................................. 73 3.2 Environmental Geochemistry of Cyanide .................................................................... 74 3.2.1 Environmental Fate: The Cyanide Cycle............................................................. 74 3.2.2 Cyanide Geochemistry ........................................................................................ 74 3.3 Attenuation Mechanisms of Cyanide ........................................................................... 77 3.3.1 Complexation (Chelation) ................................................................................... 77 3.3.2 Cyanide Complex Precipitation........................................................................... 78 3.3.3 Adsorption .......................................................................................................... 78 3.3.4 Oxidation to Cyanate .......................................................................................... 79 3.3.5 Volatilisation....................................................................................................... 80 3.3.6 Bioattenuation ..................................................................................................... 82 3.3.7 Formation of Thiocyanate ................................................................................... 82

Mudder, Botz & Smith

ii

Table of Contents

Table of Contents (Continued) 3.3.8 Hydrolysis/Saponification of Free Cyanide......................................................... 83 3.3.9 Summary............................................................................................................. 83 3.4 Natural Cyanide Attenuation in Surface Ponds ............................................................ 83 3.5 Cyanide Geochemistry in Tailings ............................................................................... 92 3.5.1 Introduction......................................................................................................... 92 3.5.2 Surface Effects in Tailings Ponds........................................................................ 94 3.5.3 Reactions in the Tailings Mass............................................................................ 94 3.5.4 Overall Effects in Tailings .................................................................................. 99 3.6 Cyanide Geochemistry of the Heap Leach ................................................................... 99 3.6.1 Introduction......................................................................................................... 99 3.6.2 Hydrolysis and Volatilisation............................................................................ 102 3.6.3 Oxidation of Free Cyanide ................................................................................ 103 3.6.4 Hydrolysis/Saponification of Free Cyanide....................................................... 103 3.6.5 Aerobic Bioattenuation of Cyanide ................................................................... 104 3.6.6 Simple Cyanide Compounds............................................................................. 104 3.6.7 Metal Cyanide Complexes ................................................................................ 104 3.6.8 Anaerobic Bioattenuation of Cyanide ............................................................... 106 3.6.9 Field Data.......................................................................................................... 106 3.7 Cyanide Geochemistry in Soils .................................................................................. 110 3.7.1 Introduction....................................................................................................... 110 3.7.2 Cyanide Volatilisation from Soils ..................................................................... 110 3.7.3 Biological Attenuation in Soils ......................................................................... 114 3.7.4 Adsorption and Precipitation in Soils................................................................ 114 3.7.5 Hydrolysis/Saponification in Soils .................................................................... 115 3.7.6 Field Data.......................................................................................................... 115 3.7.7 Applications ...................................................................................................... 119 3.8 Cyanide Geochemistry in Groundwater ..................................................................... 121 3.8.1 Attenuation Mechanisms................................................................................... 121 3.8.2 Field Data.......................................................................................................... 122 3.9 Cyanide in the Atmosphere........................................................................................ 123 3.10 References ................................................................................................................. 124 3.11 Bibliography .............................................................................................................. 129

CHAPTER 4 4.1 4.2 4.3 4.4

Toxicity of Cyanide and Related Compounds

Introduction ............................................................................................................... 133 General Aspects of Toxicology.................................................................................. 133 Sources and Uses of Cyanide..................................................................................... 135 Toxicology of Cyanide .............................................................................................. 136 4.4.1 Physiology of Cyanide Toxicity........................................................................ 136

iii

Table of Contents

Chemistry and Treatment of Cyanidation Wastes

Table of Contents (Continued) 4.4.2 Detoxification Mechanisms............................................................................... 137 4.4.3 Cyanide Antidotes............................................................................................. 139 4.5 Cyanide Toxicity to Aquatic Organisms .................................................................... 139 4.5.1 Toxicity of Free Cyanide to Aquatic Organisms ............................................... 139 4.5.2 Toxicity of WAD Cyanide to Aquatic Organisms............................................. 142 4.5.3 Toxicity of Iron Cyanides to Aquatic Organisms .............................................. 146 4.6 Cyanide Toxicity to Birds .......................................................................................... 155 4.6.1 The Most Sensitive Species to Oral Exposure................................................... 155 4.6.2 Summary of Bird Mortalities at U.S. Mines...................................................... 158 4.7 Cyanide Toxicity to Mammals................................................................................... 164 4.8 Toxicity of Cyanide Related Compounds .................................................................. 168 4.8.1 Introduction....................................................................................................... 168 4.8.2 Thiocyanate....................................................................................................... 168 4.8.3 Cyanate ............................................................................................................. 173 4.8.4 Ammonia .......................................................................................................... 178 4.8.5 Nitrate ............................................................................................................... 182 4.9 Toxicity of Actual Mine Waters................................................................................. 183 4.10 References ................................................................................................................. 187 4.11 Bibliography .............................................................................................................. 193

CHAPTER 5

Water Management and Discharge Strategies

5.1 Introduction ............................................................................................................... 199 5.2 Water Management Plans .......................................................................................... 199 5.3 Water Balances .......................................................................................................... 204 5.3.1 Water Balance Components .............................................................................. 206 5.3.2 Water Balance Assessment ............................................................................... 206 5.3.3 Water Quality Assessment ................................................................................ 215 5.4 Cyanide Management Plan and Mass Balance........................................................... 218 5.5 Development of Effluent Design Goals ..................................................................... 223 5.5.1 Beneficial Use Classifications........................................................................... 223 5.5.2 The Use Attainability Analysis or Risk Assessment ......................................... 224 5.5.3 Effluent Design Goals ....................................................................................... 226 5.6 Screening and Selection of Water Treatment Processes............................................. 227 5.7 Effluent Discharge Strategies..................................................................................... 229 5.8 Example Water Management Approaches ................................................................. 233 5.9 References ................................................................................................................. 236 5.10 Bibliography .............................................................................................................. 236

Mudder, Botz & Smith

iv

Table of Contents

Table of Contents (Continued) CHAPTER 6

Treatment and Recovery of Cyanide

6.1 6.2 6.3 6.4

Introduction ............................................................................................................... 239 Laboratory and Pilot Plant Investigations .................................................................. 240 Activated Carbon Adsorption .................................................................................... 242 Alkaline Chlorination................................................................................................. 246 6.4.1 Introduction....................................................................................................... 246 6.4.2 Process Chemistry............................................................................................. 246 6.4.3 Performance ...................................................................................................... 248 6.4.4 Operating Costs................................................................................................. 258 6.4.5 Toxicity of Alkaline Chlorination Effluents...................................................... 260 6.5 Active and Passive Biological Treatment................................................................... 263 6.5.1 Introduction....................................................................................................... 263 6.5.2 Process Chemistry............................................................................................. 265 6.5.3 Homestake Mine Biological Treatment Process................................................ 267 6.5.4 Nickel Plate Mine Biological Treatment Process .............................................. 273 6.5.5 Santa Fe Mine Passive Biological Treatment Process ....................................... 275 6.5.6 Summary........................................................................................................... 279 6.6 Caro’s Acid................................................................................................................ 282 6.6.1 Introduction....................................................................................................... 282 6.6.2 Process Chemistry............................................................................................. 282 6.6.3 Performance ...................................................................................................... 283 6.7 Cyanide Recovery by Tailings Washing .................................................................... 287 6.7.1 Introduction....................................................................................................... 287 6.7.2 Performance ...................................................................................................... 287 6.8 Cyanide Recovery by Stripping and Absorption ........................................................ 292 6.8.1 Introduction....................................................................................................... 292 6.8.2 Process Chemistry............................................................................................. 294 6.8.3 Performance ...................................................................................................... 298 6.8.4 Economics of Cyanide Recovery ...................................................................... 304 6.8.5 The Future of Cyanide Recovery....................................................................... 305 6.9 Hydrogen Peroxide .................................................................................................... 307 6.9.1 Introduction....................................................................................................... 307 6.9.2 Process Chemistry............................................................................................. 307 6.9.3 Performance ...................................................................................................... 308 6.10 Natural Attenuation.................................................................................................... 315 6.10.1 Introduction....................................................................................................... 315 6.10.2 Process Chemistry............................................................................................. 317 6.10.3 Performance ...................................................................................................... 320 6.11 Precipitation of Cyanide............................................................................................. 323 6.11.1 Introduction....................................................................................................... 323

v

Table of Contents

Chemistry and Treatment of Cyanidation Wastes

Table of Contents (Continued) 6.11.2 Performance ...................................................................................................... 323 6.12 Sulphur Dioxide and Air ............................................................................................ 327 6.12.1 Introduction....................................................................................................... 327 6.12.2 Process Chemistry............................................................................................. 327 6.12.3 Performance ...................................................................................................... 329 6.13 Other Treatment Processes......................................................................................... 334 6.13.1 Introduction....................................................................................................... 334 6.13.2 Ion Exchange .................................................................................................... 334 6.13.3 Ozone Oxidation ............................................................................................... 335 6.13.4 Reverse Osmosis ............................................................................................... 335 6.14 Removal of Metals and Cyanide Related Compounds ............................................... 336 6.14.1 Introduction....................................................................................................... 336 6.14.2 Metals Removal ................................................................................................ 337 6.14.3 Removal of Cyanide Related Compounds......................................................... 350 6.14.3.1 Cyanate Removal .................................................................................. 350 6.14.3.2 Thiocyanate Removal............................................................................ 351 6.14.3.3 Ammonia Removal ............................................................................... 352 6.14.3.4 Nitrate Removal .................................................................................... 354 6.15 Summary.................................................................................................................... 355 6.16 References ................................................................................................................. 356 6.17 Bibliography .............................................................................................................. 363 Index................................................................................................................................... 369

LIST OF TABLES 1.1

Minerals Associated with Gold in Sulphide Ores.............................................................. 3

1.2

Solubility of Metal Sulphide Minerals in Cyanide Solutions ............................................ 4

1.3

Classification of Cyanide and Cyanide Compounds in Cyanidation Solutions on the Basis of Stability .................................................................................... 9

1.4

Relative Concentrations of Hydrocyanic Acid and Cyanide Ion in a 10-4 Molar Solution as a Function of pH ................................................................................ 11

1.5

Stability Constants of Metal Cyanide Complexes ........................................................... 14

1.6

Free Cyanide Concentration Released at Various Metal-Cyanide Complex Concentrations ................................................................................................................ 15

1.7

Solubilities of Ferrocyanides and Ferricyanides.............................................................. 18

1.8

Solubilities of Complex Ferrocyanide and Ferricyanide Salts......................................... 19

Mudder, Botz & Smith

vi

Table of Contents

Table of Contents (Continued) LIST OF TABLES CONTINUED 1.9

Effect of Initial pH on Ferrocyanide Solubility............................................................... 19

1.10

Solubility of Hexacyanoferrates (II) and (III) Separately and in Mixtures ...................... 21

1.11

Range of Cyanidation Solution Chemistries ................................................................... 24

1.12

Listing of Several Iron-Cyanide Complexes ................................................................... 24

2.1

Comparison of Total Cyanide Analyses by Autoanalyser (USEPA Method 9012) and Manual (USEPA Method 9010) Methods ...................................................... 35

2.2

Comparison of Autoanalyser and Non-Autoanalyser Analyses for Total Cyanide in a Mining Effluent.......................................................................................... 36

2.3

Comparative Evaluation of Total Cyanide Methods Using Different Acids with a Synthetic Metal Cyanide Solution Containing Thiocyanate ................................. 37

2.4

Thiocyanate Additions to Cyanide Standards Analysed by WAD and Total Cyanide Distillations Using Sulphuric Acid ................................................................... 38

2.5

Comparison of Leach and Rinse Solution WAD Cyanide Analyses ............................... 41

2.6

Comparison of WAD, CAC and OIA-1677 Analytical Methods .................................... 46

2.7

Summary of Method Detection Limits and Practical Quantitation Limits for Cyanide Analyses...................................................................................................... 48

2.8

Comparison of WAD Cyanide Values from Preserved and Unpreserved Samples........................................................................................................................... 57

2.9

Effects of the Use of Preservatives for Oxidants and Sulphides on Cyanide Analyses.......................................................................................................................... 58

2.10

Example Cyanide Mass Balance Calculation.................................................................. 60

2.11

Summary of International Comparative Study of Cyanide and Thiocyanate Analyses.......................................................................................................................... 62

2.12

Summary of Cyanide Analytical Results for a USA Mine Site ....................................... 64

3.1

Cyanide Profile in Interstitial Water in a Decommissioned Tailings Impoundment in South Africa......................................................................................... 95

3.2

Chemistry of Oxidised, Intermediate and Reduced Zones in Tailings Impoundments (Mean Values) in South Africa............................................................... 95

3.3

Cyanide Chemistry of "Mixed" Tailings and Flotation Tailings Solution in North America ................................................................................................................ 96

3.4

Reaction Products Chemistry of "Mixed" Tailings in North America............................. 97

3.5

Time/Cyanide Concentration Dependency of Copper and Cobalt Cyanide in Tailings Interstitial Pore Solution in North America................................................... 97 vii

Table of Contents

Chemistry and Treatment of Cyanidation Wastes

Table of Contents (Continued) LIST OF TABLES CONTINUED 3.6

Cyanide Decay in Tailings Decant Solution in New Zealand ......................................... 98

3.7

Cyanide Decay in Tailings Pore Solution in New Zealand ............................................. 98

3.8

Typical Cyanide Reactions in a Heap Leach Environment ........................................... 101

3.9

Relative Stability of Metal Cyanide Compounds in Water............................................ 105

3.10

First Pore Volume Effluent pH and Cyanide Concentration from Leach Pad Neutralisation Testing............................................................................................ 109

3.11

Comparison of Total Cyanide Levels with Groundwater Composition in South Africa.................................................................................................................. 116

3.12

Attenuation Calculation Summary ................................................................................ 118

3.13

Partition Coefficient Values for Tailings Attenuation Tests.......................................... 120

4.1

Effects of Free Cyanide on Aquatic Organisms ............................................................ 140

4.2

Relative Toxicities to Fish of Metal Cyanide Compounds ............................................ 147

4.3

Ferrocyanide Toxicity Test Summary and 96-Hour LC50 Values Obtained .................. 152

4.4

Ferricyanide Toxicity Test Summary and 96-Hour LC50 Values Obtained ................... 153

4.5

Summary of Iron Complexed Cyanide Chronic Toxicity Test Conditions.................... 154

4.6

Effects of Free Cyanide on Selected Species of Birds................................................... 156

4.7

Wildlife Mortalities Reported by Nevada Mine Operators............................................ 162

4.8

WAD Cyanide in Tailings Ponds .................................................................................. 163

4.9

Cyanide Toxicity to Mammals...................................................................................... 165

4.10

Background Cyanide Concentrations in Selected Plants ............................................... 167

4.11

Toxicological Data for Cyanide Related Compounds ................................................... 169

4.12

Toxicity of Thiocyanate to Fish .................................................................................... 170

4.13

Toxicity of Thiocyanate to Fish, Effect of pH, Hardness and Temperature .................. 171

4.14

Toxicity of Thiocyanate to Fish, Effect of Hardness at pH 8.0 and Temperature 12°C......................................................................................................... 172

4.15

Chronic Thiocyanate Toxicity Studies for Rainbow Trout............................................ 174

4.16

Chronic Thiocyanate Toxicity Studies for Fathead Minnows ....................................... 174

4.17

Toxicity of Cyanate to Fish, Effect of pH and Temperature ......................................... 175

Mudder, Botz & Smith

viii

Table of Contents

Table of Contents (Continued) LIST OF TABLES CONTINUED 4.18

Toxicity of Cyanate to Fish, Effect of Hardness at pH 8.0 and Temperature 12°C.............................................................................................................................. 176

4.19

Effect of Hardness on Cyanate Toxicity ....................................................................... 176

4.20

Toxicity of Cyanate and Thiocyanate Mixtures ............................................................ 177

4.21

Percent NH3 in Aqueous Ammonia Solutions for 0°C to 30°C and pH 6 to 10.............................................................................................................................. 180

4.22

Toxicity of Un-Ionised Ammonia to Fish Species ........................................................ 181

4.23

Toxicity of Various Gold Mill Effluents to Rainbow Trout Fry.................................... 184

4.24

Water Quality Associated with the Homestake Lead, South Dakota, USA Mine Water Treatment Plant......................................................................................... 185

4.25

Water Quality Associated with the Golden Cross, New Zealand Mine Water Treatment Plant .................................................................................................. 186

5.1

Options for Water Management.................................................................................... 202

5.2

Potential Water Sources at a Mining Operation ............................................................ 210

5.3

Approaches to Estimating Water Volumes at a Mining Operation................................ 212

5.4

Approximate Chemical Composition Ranges in Cyanidation Solutions ....................... 217

5.5

Cyanide Solutions and Methods for Cyanide Management........................................... 221

5.6

Components of a Cyanide Balance ............................................................................... 222

6.1

Advantages and Disadvantages of Activated Carbon Water Treatment ........................ 245

6.2

Operating Parameters for Full-Scale Alkaline Chlorination Operations........................ 255

6.3

Performance Data for Full-Scale Alkaline Chlorination of Gold Mill Effluents ....................................................................................................................... 256

6.4

Giant Yellowknife Alkaline Chlorination Performance Data........................................ 257

6.5

Typical Analyses at Grey Eagle Treatment Plant.......................................................... 259

6.6

Summary of Alkaline Chlorination Reagent Consumptions.......................................... 259

6.7

Summary of Alkaline Chlorination Reagents Costs ...................................................... 261

6.8

Advantages and Disadvantages of the Alkaline Chlorination Process........................... 262

6.9

Homestake Mine Water Treatment Plant Performance ................................................. 272

6.10

Performance of the Nickel Plate Mine Biological Water Treatment Plant .................... 276

6.11

Reagent Usages in the Nickel Plate Mine Biological Water Treatment Plant in 1999................................................................................................................. 277

ix

Table of Contents

Chemistry and Treatment of Cyanidation Wastes

Table of Contents (Continued) LIST OF TABLES CONTINUED 6.12

Passive Biological Treatment of Leach Pad Draindown Solution in Laboratory Columns ..................................................................................................... 280

6.13

Advantages and Disadvantages of Biological Water Treatment.................................... 281

6.14

Slurry Treatment Results Using Caro’s Acid ................................................................ 285

6.15

Clear Solution Treatment Results Using Caro’s Acid ................................................... 285

6.16

Advantages and Disadvantages of the Caro’s Acid Process.......................................... 286

6.17

Advantages and Disadvantages of Cyanide Recovery by Tailings Washing................. 291

6.18

Performance of Cyanide Recovery in Treating Tailings Impoundment Decant Solution ............................................................................................................ 299

6.19

Results of CANMET Laboratory Cyanide Recovery Studies ....................................... 300

6.20

Pilot Cyanide Recovery Testwork Using Stripping Towers .......................................... 301

6.21

Advantages and Disadvantages of Cyanide Recovery by Stripping and Absorption .................................................................................................................... 306

6.22

Detoxification Results from Three Plants Using the Hydrogen Peroxide Process to Treat Gold Mill Effluents............................................................................. 312

6.23

Advantages and Disadvantages of the Hydrogen Peroxide Process .............................. 314

6.24

Examples of Natural Cyanide Attenuation in Tailings Impoundments in Australia ....................................................................................................................... 316

6.25

Effect of Temperature on the Rate Constants for the Attenuation of Metal Cyanide Complexes at pH 7.0....................................................................................... 319

6.26

Effluent Quality of Canadian Gold Mines Applying Batch Natural Cyanide Attenuation Systems ....................................................................................... 319

6.27

Chemistry of Tailings Impoundment Decant Solution Before and After Natural Attenuation....................................................................................................... 322

6.28

Precipitation of Free Cyanide Using Ferrous Sulphide at the Con Mine....................... 325

6.29

Laboratory Test Results of Cyanide Precipitation Using Ferrous Sulphate................... 326

6.30

Oxidation of Cyanide in Tailings Slurry Using the INCO SO2/Air Process .................. 332

6.31

Oxidation of Cyanide in Solutions Using the INCO SO2/Air Process........................... 332

6.32

Oxidation of Cyanide in Electroplating Wastes Using the INCO SO2/Air Process.......................................................................................................................... 332

6.33

Advantages and Disadvantages of the INCO SO2/Air Process...................................... 333

Mudder, Botz & Smith

x

Table of Contents

Table of Contents (Continued) LIST OF TABLES CONTINUED 6.34

Waste Treatment Options and Performance Data Summary for Antimony and Arsenic Removal.................................................................................................... 341

6.35

Waste Treatment Options and Performance Data Summary for Beryllium and Cadmium Removal................................................................................................. 342

6.36

Water Treatment Options and Performance Data Summary for Copper Removal........................................................................................................................ 342

6.37

Water Treatment Options and Performance Data Summary for Chromium Removal........................................................................................................................ 343

6.38

Water Treatment Options and Performance Data Summary for Lead Removal........................................................................................................................ 344

6.39

Water Treatment Options and Performance Data Summary for Mercury Removal........................................................................................................................ 345

6.40

Water Treatment Options and Performance Data Summary for Nickel Removal........................................................................................................................ 345

6.41

Water Treatment Options and Performance Data Summary for Selenium and Thallium Removal.................................................................................................. 346

6.42

Water Treatment Options and Performance Data Summary for Zinc Removal........................................................................................................................ 347

6.43

Long-Term Effluent Concentrations Achievable with Several Water Treatment Processes...................................................................................................... 348

6.44

Summary of the Performance of Acid Drainage Treatment Systems ............................ 349

LIST OF FIGURES 1.1

Relationship Between HCH and CN- with pH (25°C)..................................................... 10

2.1

Compounds Included in Total, WAD and Free Cyanide Analyses.................................. 31

2.2

Typical Cyanide Distillation Apparatus .......................................................................... 33

3.1

The Cyanide Cycle.......................................................................................................... 75

3.2

Interrelationships in Cyanide Chemistry......................................................................... 76

3.3

Eh-pH Diagram for the Free Cyanide-Water System at 25°C ......................................... 81

3.4

Natural Attenuation of Cyanide, Cyanate and Thiocyanate ............................................ 85

xi

Table of Contents

Chemistry and Treatment of Cyanidation Wastes

Table of Contents (Continued) LIST OF FIGURES CONTINUED 3.5

Observed and Predicted Attenuation of Cyanide in a Shallow Pond Operated in a Batch Mode in Canada.............................................................................. 87

3.6

Observed and Predicted Attenuation of Cyanide in a Deep Pond Operated in a Batch Mode in Canada ............................................................................................. 88

3.7

Observed and Predicted Attenuation of Cyanide in a Pilot Scale System Operated in a Continuous-Fill and Batch-Discharge Mode in Canada ............................ 89

3.8

Predicted and Actual Cyanide Attenuation in a USA Tailings Impoundment.................................................................................................................. 91

3.9

Flow Schematic of Tailings Disposal at a Gold/Uranium Mine and Possible Types of Chemical Reactions in the System ..................................................... 93

3.10

Prevailing Geochemical Conditions and Typical Cyanide Reactions in the Abandoned Heap Leach Environment .......................................................................... 100

3.11

Cyanide Reduction Data Illustrating Delayed Release of Cyanide from Rinsing a Heap Leach Pad ............................................................................................ 108

3.12

Unsaturated Soil Model Showing Soil Particles, Water, Gas and Mass Transport/Retardation Mechanisms .............................................................................. 111

3.13

Summary of Head Space Analyses for Cyanide in Saturated and Unsaturated Soil Test Columns..................................................................................... 113

4.1

Spectral Energy Distribution of Natural Sunlight and Spectralite Bulbs....................... 151

4.2

WAD Cyanide Concentration – Mortality Relationship for the Mallard Duck ............................................................................................................................. 159

4.3

Summary of Cyanide Toxicity to Mallard Ducks.......................................................... 160

4.4

Percentage of Total Ammonia as NH3 as a Function of pH and Temperature.................................................................................................................. 179

5.1

Components of a Mine Site Water Management Plan................................................... 201

5.2

Water Balance Schematic Showing Most of the Water Balance Components .................................................................................................................. 205

5.3

Example Water Balance Schematic for a Gold Mine in a Wet Climate ........................ 207

5.4

Example Water Balance Schematic for a Gold Heap Leaching Operation in a Wet Climate ........................................................................................................... 208

5.5

Heap Leaching Operations Pond Design Considerations .............................................. 209

5.6

Example Stream Flow Hydrograph............................................................................... 231

5.7

Schematic of an Effluent Discharge Diffuser................................................................ 232

Mudder, Botz & Smith

xii

Table of Contents

Table of Contents (Continued) LIST OF FIGURES CONTINUED 5.8

Example Water Management Approach for a Milling Operation.................................. 234

5.9

Flowsheet Illustrating the Life-Cycle Management of Water at a Heap Leaching Operation....................................................................................................... 235

6.1

Example Schematic of an Activated Carbon Treatment Plant....................................... 244

6.2

Redox and TRC Response Curve During Titration of Gold Mill Barren Solution with Standard Hypochlorite Solution.............................................................. 249

6.3

Mosquito Creek Mine - Alkaline Chlorination Flowsheet............................................. 250

6.4

Baker Mine - Alkaline Chlorination Flowsheet............................................................. 251

6.5

Carolin Mine - Alkaline Chlorination of Tailings Pond Reclaim Water........................ 252

6.6

Giant Yellowknife - Alkaline Chlorination and Arsenic Removal Flowsheet...................................................................................................................... 253

6.7

A Schematic of an Alkaline Chlorination Process ........................................................ 254

6.8

The Homestake Mine Biological Water Treatment Process .......................................... 269

6.9

Rotating Biological Contractor Plant for the Removal of Cyanide and Thiocyanate at the Homestake Mine............................................................................. 270

6.10

Nickel Plate Mine Biological Water Treatment Plant ................................................... 274

6.11

Schematic of the Santa Fe Mine Passive Biological Treatment System ........................ 278

6.12

Basic Flow Diagram for a Caro’s Acid Treatment System ........................................... 284

6.13

Flow Diagram of Cyanide Recovery by Tailings Washing ........................................... 289

6.14

Performance of Cyanide Recovery by CCD Tailings Washing..................................... 290

6.15

General Schematic of Cyanide Recovery by Stripping and Absorption ........................ 296

6.16

Flow Diagram of the Golden Cross Cyanide Recovery Plant ....................................... 302

6.17

Flow Diagram of the DeLamar Cyanide Recovery Plant .............................................. 303

6.18

Flowsheet for the Hydrogen Peroxide Treatment Process at the Ok Tedi Mine.............................................................................................................................. 310

6.19

Teck-Corona Hydrogen Peroxide Treatment System .................................................... 311

6.20

Simulation of Natural Cyanide Attenuation .................................................................. 322

6.21

Flowsheet for a Two-Stage INCO Cyanide Destruction Process .................................. 330

6.22

The Effects of Copper Concentration and pH on the Performance of the INCO Cyanide Destruction Process .............................................................................. 331

xiii

Table of Contents

CHAPTER ONE

Chemistry of Free and Complexed Cyanide 1.1

INTRODUCTION

The cyanidation process for the extraction of gold and silver from ore has been employed since 1898 when it was first used in New Zealand and Africa. It is a very efficient process capable of extracting gold in amounts as small as 0.25% of an ounce from a ton of rock with over 90% efficiency. The recovery of gold once in solution is equally efficient. The chemistry of cyanide solutions is quite complex and it is this complexity which is responsible for its ability to dissolve gold and silver. However, cyanide forms complexes with other metals, such as mercury, zinc, copper, iron and nickel, which partially accounts for the consumption of cyanide in gold extraction circuits, generates waters that may be difficult to treat and complicates the analysis of cyanide solutions. The principal reasons for the prominent place of cyanide in gold ore processing include its wide availability, its efficiency at extracting gold and silver, its relatively rapid extraction kinetics and the strength and solubility of its gold cyanide complex. This chapter provides background information regarding the cyanidation process and the chemical reactions associated with gold dissolution and recovery. The purpose of this chapter is to establish a basis for the nature and type of cyanide-bearing solutions which may require treatment prior to discharge from a mining operation.

1.2

GOLD DISSOLUTION

Gold dissolution by cyanide (i.e. cyanidation) is believed to be a two-step process in which hydrogen peroxide is formed as an intermediate (Marsden and House, 1993): (1.1)

2Au + 4NaCN + 2H2O + O2 → 2NaAu(CN)2 + 2NaOH + H2O2

(1.2)

2Au + 4NaCN + H2O2 → 2NaAu(CN)2 + 2NaOH

The overall reaction known as Elsner's equation is as follows: (1.3)

4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH

1

Chemistry and Treatment of Cyanidation Wastes Relatively weak cyanide solutions can be used because of the strong complex formed between cyanide and gold. In the absence of other metal cyanide complexes, a 100 mg/L solution of NaCN (i.e. about 50 mg/L free cyanide) can provide the maximum rate and extent of gold dissolution. The reactions with metallic silver are analogous to the aforementioned reactions of cyanide with gold. However, the silver cyanide complex is weaker than the gold cyanide complex and stronger cyanide solutions and/or longer reaction times must be employed for its dissolution. Other conditions being equal, dissolution of an equivalent silver content requires about 10-fold the cyanide level needed for gold extraction. Under these conditions, gold can dissolve twice as rapidly as silver. Alloys of these two precious metals, known as electrum, dissolve at intermediate rates. The need for more aggressive conditions in the presence of silver can increase the attack on other minerals present in the ore, leading to increased cyanide consumption, decreased selectivity towards gold and silver and higher concentrations of other metals in solution. Elevated cyanide and metal-cyanide concentrations in solution can significantly increase the complexity and cost of recovering gold and silver. An economic evaluation of increased cyanide usage and silver recovery on the eventual water characteristics and treatment requirements should be considered during the preliminary design phase of a metallurgical flowsheet. In spite of the relative specificity of the gold-cyanide reaction, other metals and inorganic constituents react both with cyanide and to a certain extent with hydroxide, which must be present to maintain cyanide in its ionic form. Table 1.1 lists several of these constituents. Table 1.2 shows the extent to which zinc and copper, two primary ore constituents, are dissolved from sulphide minerals during cyanidation. The reactions with the iron minerals, pyrite and pyrrhotite, are of greater importance in many cases since these minerals are among the highest cyanide-consuming minerals in a gold ore. When pyrite and pyrrhotite-bearing ores are treated directly with cyanide solution, formation of thiocyanate occurs and is accelerated under conditions which combine partial or inadequate aeration with high alkalinity. The production of thiocyanate is highest for the free sulphur or pyrrhotite containing minerals. The formation of the ferrocyanide ion, Fe(CN)6-4, is often of greater concern from operational and environmental viewpoints. As is noted later in this chapter, conditions for its formation are more limited. It is formed relatively rapidly under conditions of low alkalinity and reduced aeration, particularly if pre-oxidation of the ore has led to the presence of ferrous ion. This is partially due to a drop in pH caused by incomplete precipitation of ferrous hydroxide. It should be noted that the conditions for formation of thiocyanate and ferrocyanide from iron sulphide minerals are to some extent mutually restrictive, since conditions chosen to minimize one may promote formation of the other. This is of particular importance from a water treatment viewpoint, since thiocyanate creates an oxidant demand, is potentially toxic itself and breaks down forming ammonia and nitrate, potentially causes of further concern.

Mudder, Botz & Smith

2

Chemistry of Free and Complexed Cyanide

TABLE 1.1

Element Iron

Minerals Associated with Gold in Sulphide Ores

Sulphides FeS Pyrrhotite FeS2 Pyrite Marcasite

Cobalt

Silver

Au Native Gold Au, Ag Electrum Ag Native Silver Ag, Au Electrum

Cu Native Copper

Lead Zinc Carbon

Tellurides

AuTe2 Krennerite Calaverite

AuSb2 Aurosibite

Ag2S Argentite (Pb,Ag)S Argentiferrous Galena HgS Cinnabar Cu2S Chalcocite CuS Covellite Cu5FeS4 Bornite CuFeS2 Chalcopyrite PbS Galena ZnS Sphalerite

Ag3AsS3 Proustite (Cu,Fe,Ag)As4S13 Argentiferrous Tennantite

Ag3SbS3 Pyragyrite (Cu,Fe,Ag)Sb4S13 Argentiferrous Tetrahedrite

Cu3AsS4 Enargite (Cu,Fe)As4S13 Tennantite

(Cu,Fe)Sb4S13 Tetrahedrite

Ag2Se Naumannite

Ag2Te Hessite

C Graphite C Amorphous C

Arsenic

Antimony Bismuth

Selenides

(Fe,Ni)9S8 Pentlandite

Mercury Copper

Antimonides

CoAsS Cobalite

Nickel Gold

Arsenides FeAsS Arsenopyrite

Bi Native Bismuth

AsS Realgar As2S3 Orpiment Sb2S3 Stibnite Bi2S3 Bismuthinite

Bi2Te2S Tetradymite

Source: Scott and Ingles, 1987

3

Chapter One

Chemistry and Treatment of Cyanidation Wastes

TABLE 1.2

Solubility of Metal Sulphide Minerals in Cyanide Solutions

Mineral Sphalerite Chalcocite Chalcopyrite Bornite Enargite Tetrahedrite Metallic Copper

Formula ZnS Cu2S CuFeS2 FeS – 2Cu2S - CuS 3 CuS - As2S5 4 Cu2S - Sb2S3 Cu

Source: Scott and Ingles, 1987

Notes: (1) 2.0 g/L NaCN solution at 45°C (2) 1.0 g/L NaCN solution at 23°C

Mudder, Botz & Smith

4

Percent Extraction of Metal 18.4 (1) 90.2 (2) 5.6 (Cu) (2) 70.0 (Cu) (2) 65.8 (Cu) (2) 21.9 (Cu) (2) 9.0 (2)

Chemistry of Free and Complexed Cyanide Iron sulphides are not the only minerals capable of generating thiocyanate in cyanide solutions. All sulphide minerals, except lead sulphide (galena), have this potential. The primary requirement is the formation of compounds containing labile sulphur atoms in solution. These include elemental sulphur micelles (S8) and alkali or alkaline earth polysulphides, thiosulphates and polythionates. Cyanide does not react directly with the sulphide ion, but the compounds mentioned above can all yield sulphide ions which arise from alkaline attack on the metal sulphide minerals. One method of minimizing thiocyanate formation is by promoting complete oxidation of sulphides to sulphates, which can be accomplished to some extent by aeration. Better results are possible if the addition of cyanide is postponed until the sulphide oxidation reactions are complete. If, at the same time, the pH of the pulp is maintained strongly basic, some reduction of ferrocyanide formation will occur. However, generation of elevated sulphates may pose additional treatment and environmental problems (e.g. the precipitation of gypsum).

1.3

GOLD RECOVERY FROM SOLUTION

There are two basic methods employed for the recovery of gold from solution; cementation (Merrill-Crowe process) and granular activated carbon adsorption. The zinc cementation (Precipitation) process is based upon an electrochemical reaction between metallic zinc powder and the gold-cyanide complex, in the absence of oxygen, to reduce the univalent gold ions to its free metal: (1.4)

Zn + 2NaAu(CN)2 → 2Au + Na2Zn(CN)4

The presence of nickel ions in solution promotes hydrogen evolution by lowering the hydrogen overvoltage of zinc, causing poor gold recovery and high zinc consumption. Lead ions yield metallic lead which increases the hydrogen overvoltage and provides surfaces for gold deposition. Any ferricyanide ions present are reduced to ferrocyanide. Zinc precipitates gold and silver nearly completely, along with a portion of the copper. Other components of the solution, with the exception of mercury, arsenic and antimony, are relatively unaffected. The granular activated carbon (GAC) adsorption process introduces no additional reagents in the gold and silver recovery stage as does the Zinc Cementation process. With the activated carbon process, gold and silver cyanide complexes are adsorbed onto active carbon sites, thereby removing gold and silver from solution. Stripping gold and silver from the loaded carbon is typically accomplished using a solution containing 0.1% NaCN and 1% NaOH at elevated temperatures (Zadra Process). The gold is usually recovered from the strip solution by electrowinning, so that a portion of the cyanide is recycled. The addition of zinc and the characteristics of the zinc precipitation solutions are eliminated using the granular activated carbon method. The use of activated carbon can decrease the concentration of undesirable metals in solution (e.g. mercury and copper) and increase the efficiency and ease of water treatment.

5

Chapter One

Chemistry and Treatment of Cyanidation Wastes In addition to cyanidation, cyanide is also used as a depressant in the flotation of base metal sulphide ores. During the flotation process, metal cyanide complexes may be generated (e.g. copper and iron), which require removal prior to discharge of these solutions into the environment. Although the levels of cyanide used are significantly lower than those used in cyanidation, the potential environmental issues relating to this secondary use of cyanide should be considered. The above discussion provides an introduction into the chemistry of cyanide usage and cyanidation in the mining industry. The information is useful in understanding the effect cyanide usage and ore geochemistry have on the chemistry of process solutions generated during metals recovery. The next section includes a discussion of the chemistry of the various forms of free and metal complexed cyanide.

1.4

SOLUTION CHEMISTRY OF CYANIDE AND ITS METAL COMPLEXES

1.4.1 Introduction The cyanide compounds present in gold mine or cyanidation solutions and effluents include free cyanide, the alkali earth salts and the metal cyanide complexes formed with gold, mercury, zinc, cadmium, silver, copper, nickel, iron and cobalt. These may be classified into five general categories, as shown in Table 1.3. The discussion of cyanide chemistry involves consideration of the following issues: • • • •

The nature of bonding in the cyanide radical The behaviour of hydrogen cyanide and "free" cyanide ions So-called simple cyanide compounds Cyanide complexation

The preceding elements of cyanide chemistry, together with cyanide reactions involving oxygen, sulphur species and biological processes, produce a variety of cyano-species and derivatives which are addressed in the following section. 1.4.2 Cyanide Bonding The complex nature of cyanide behaviour can be explained, at least in part, by the bonding in the cyanide radical. Baker (1984) has produced a discourse that examines the nature of cyanide and its chemistry, using these chemical factors as a basis for the general lack of adverse environmental impacts from cyanide.

Mudder, Botz & Smith

6

Chemistry of Free and Complexed Cyanide The cyanide ion is an anion which comprises one atom of carbon and one of nitrogen. It has one sigma bond, two pi bonds and two empty bonding orbitals. The first two orbitals in its structure are filled with the maximum number of electrons, while the other orbitals are empty. Because the "s" and "p" (1 + 2) orbitals are filled with electrons it behaves like a halogen (i.e. fluorine, chlorine, bromine and iodine). This means some of the properties of, say, NaCl will not be radically different from NaCN (sodium cyanide). However, its pseudo-halogen behaviour cannot explain the formation of cyanide metal complexes with transition series metals (i.e., Fe, Co, Ni, Cu and Zn). The empty anti-bonding orbitals on the cyanide ion can form bonds with the "d" orbitals (partially or wholly filled) of transition series metals. The contribution of an electron pair (either from the cyanide ion to the metal or vice versa) is known as "back bonding" and explains the stability of the cyanide-metal complexes. The cyanide ion also has a triple bond and triple bonds of this type can be broken easily and, hence, can be considered to be quite reactive. In summary, these factors taken together explain the complex behaviour of cyanide: •

Pseudo-halogen behaviour of the cyanide anion

•

Back bonding utilizing empty anti-bonding orbitals to explain stable complex formations

•

A triple bond which can be easily broken to explain its relatively rapid natural degradation

1.4.3 Free Cyanide Free cyanides are defined as the forms of molecular and ionic cyanide released into aqueous solution by the dissolution and dissociation (or ionisation) of cyanide compounds and complexes. Therefore, the term free cyanide is confined to two species, the cyanide ion (CN-) and hydrocyanic acid or hydrogen cyanide (HCN). The relative proportion of these two forms depends upon the pH of the system. Hydrocyanic acid is a relatively weak acid and its dissociation in aqueous solution into the cyanide anion is expressed by the following equation: (1.5)

HCN → H+ + CN-

At any particular pH and temperature, the relative amounts of each of these two free cyanide compounds present at equilibrium can be determined from the following expression: (1.6)

K=

[H + ][CN − ] = 2.03 x 10−10 , pKa = 9.31 (20o C) [HCN]

In Equation 1.6, the compounds in brackets are molar concentrations and K is the equilibrium constant. Figure 1.1 and Table 1.4 present this relationship in graphical and mathematical form. In natural waters with a pH below about 8.5, cyanide is present predominantly as the free acid (i.e., HCN).

7

Chapter One

Chemistry and Treatment of Cyanidation Wastes The pH behaviour of cyanide is important in gold-cyanide extraction processes because at pH 10.5 or greater most of the free cyanide in process slurry water or heap leach interstitial fluid will be as the cyanide anion (CN-). Since the HCN content is low at this pH value, the capacity for cyanide loss by volatilisation is limited. In natural aqueous systems that have pH values in the range of about 5.0 to 8.5, the majority of free cyanide will be in the form of HCN and can be lost by volatilisation. Molecular HCN has a low boiling point and a high vapour pressure and can be lost from solution, particularly where there is a water/air interface. From a practical, economic, or safety point of view, the solution pH must be in excess of about 10.0 to limit HCN formation and loss from aqueous systems. 1.4.4 Simple Cyanide Compounds The simple cyanides can be defined as the salts of hydrocyanic acid (e.g. KCN and NaCN), which dissolve completely in aqueous solution producing free alkali earth cations and cyanide anions: (1.7)

NaCN → Na+ + CN-

(1.8)

Ca(CN)2 → Ca+2 + 2CN-

The CN- then reacts with water to form HCN in an alternative form of Equation (1.5): (1.9)

CN- + H2O → HCN + OH-

The simple cyanides are electrically neutral (the positive charges of the metal ion balance exactly with the negative charges of the cyanide ions) and are capable of existing in solid form. The simple cyanide compounds are water soluble and dissociate or ionise readily and completely to yield free cyanide (as defined above) and the metal ion. The most common industrially used cyanide solid is NaCN, though lesser quantities of KCN and Ca(CN)2 are also used.

Mudder, Botz & Smith

8

Chemistry of Free and Complexed Cyanide

TABLE 1.3

Classification of Cyanide and Cyanide Compounds in Cyanidation Solutions on the Basis of Stability

Classification 1. Free Cyanide 2. Simple Compounds Readily soluble neutral Insoluble salts 3. Weak Complexes 4. Moderately Strong Complexes 5. Strong Complexes

Compound CN-, HCN NaCN, KCN, Ca(CN)2, Hg(CN)2 Zn(CN)2, Cd(CN)2, CuCN, Ni(CN)2, AgCN Zn(CN)4-2 , Cd(CN)3-, Cd(CN)4-2 Cu(CN)2-, Cu(CN)3-2, Ni(CN)2-2, Ag(CN)2Fe(CN)6-4, Co(CN)6-4, Au(CN)2-, Fe(CN)6-3

Source: Scott and Ingles, 1987

9

Chapter One

Chemistry and Treatment of Cyanidation Wastes

FIGURE 1.1

Relationship Between HCN and CN- with pH (25oC)

100% 90%

Free Cyanide Present as HCN

80% 70% 60% CN-

HCN

50% 40% 30% 20% 10% 0% 6.0

7.0

8.0

9.0 pH

Mudder, Botz & Smith

10

10.0

11.0

12.0

Chemistry of Free and Complexed Cyanide

TABLE 1.4

Relative Concentrations of Hydrocyanic Acid and Cyanide Ion in a 10-4 Molar Solution as a function of pH

pH 6.0 6.6 7.0 7.5 7.8 8.0 9.0 10.0 11.0

Cyanide (as CN, mg/L) 0.002 0.007 0.02 0.06 0.11 0.17 1.09 2.28 2.56

11

Hydrocyanic Acid (as CN, mg/L) 2.598 2.593 2.58 2.54 2.49 2.43 1.51 0.32 0.04

Chapter One

Chemistry and Treatment of Cyanidation Wastes 1.4.5 Weak and Moderately Strong Cyanide Complexes The chemistry of the cyano-metal complex is more involved than that of the free cyanide compounds examined in the previous section. Huiatt et al. (1982) gives a description of alkalimetal cyanide complexes and their behaviour. However, not all complex cyanides are alkalimetal cyanide complexes and subsequent discussion must be seen in that context. The following description addresses the transition metal cyanide complexes as an illustration of cyanide-metal complex chemistry. Alkali-metal cyanides complexes have the general formula: AaM(CN)b where: A: M: a: b:

is the alkali metal cation (e.g. Na+ or K+) is usually a transition series metal (i.e. Fe, Cu, Co, Ni, Cr and Zn,) is the number of cations is the number of cyanide groups

For example, the formula of potassium ferricyanide would be K3Fe(CN)6: the ferric ion is trivalent, the potassium monovalent. Hence the number of potassium atoms is 3 and the number of cyanide groups is 6. When an alkali-metal cyanide dissolves in water, instead of release of the cyanide ion, as described with the free cyanide or the simple alkali cyanide compounds, the transition metal and cyanide remain combined as a complex radical anion. To illustrate using the compound K3Fe(CN)6: (1.10) K3Fe(CN)6 → 3K+ + Fe(CN)6-3 The metal cyanide complex anion stability is dependent upon the metal cation with which it is associated, the pH and the redox potential of its associated environment. Another aspect of metal cyanide complex chemistry is the formation of insoluble double-metal cyanide precipitates. This reaction has been used very successfully to remove free cyanide from solutions by the formation of ferric ferrocyanide, Fe4(Fe(CN)6)3 or other transition metal ferrocyanide precipitates. This cyanide removal mechanism is discussed in greater detail in Chapter 6. The weak and moderately strong cyanide complexes primarily include cyanide complexed with cadmium, copper, nickel, silver and zinc. These complexes form in a step-wise manner, with successively higher cyanide contents as the cyanide concentration of the solution is increased.

Mudder, Botz & Smith

12

Chemistry of Free and Complexed Cyanide For example, the step-wise formation of the copper-cyanide complexes occurs as follows: (1.11) CuCN + CN- → Cu(CN)2(1.12) Cu(CN)2- + CN- → Cu(CN)3-2 (1.13) Cu(CN)3-2 + CN- → Cu(CN)4-3 Depending on the concentration of cyanide, metals and the solution pH, the metal-cyanide complexes are present in solution in varying proportions. The stability of these cyanide compounds varies according to the metal ion involved, with zinc and cadmium forming the weakest complexes and iron and cobalt forming the strongest complexes. However, even these complex anions in some cases can dissociate to release the cyanide ion in the presence of UV radiation or very strong acids. The dissociation constants in order of metal complex stability are presented in Table 1.5. The rates of metal-cyanide complex dissociation and release of free cyanide are affected by several factors including the intensity of light, water temperature, pH, total dissolved solids and complex concentration. The liberation of free cyanide through photolysis is most pronounced for the iron-cyanide complexes. The dissociation constants can be utilized to calculate the concentration of free cyanide released when these complexes are present in solution. Table 1.6 presents the equilibrium concentrations of free cyanide produced at various complex concentrations dissolved in water at pH 7.0 and 25°C. The very low concentrations of free cyanide indicated in Table 1.6 demonstrate the stability of complexes under ambient conditions. In general, a decrease in pH and complex concentration increases the percentage of free cyanide produced. As a result, the breakdown of each metal cyanide complex increases with decreasing concentration. It is the stability of the specific complex which dictates its ease of treatment and relative toxicity. The most important parameter in determining the stability or extent of dissociation of these metal cyanide complexes is the solution pH. A thorough discussion of the chemistry of metal cyanide complexes can be found in Flynn and McGill (1995). The next section examines the chemistry of the iron cyanide complexes in more detail.

13

Chapter One

Chemistry and Treatment of Cyanidation Wastes

TABLE 1.5

Stability Constants of Metal Cyanide Complexes

Complex Ion

Stability Constant (1)

Zn(CN)3Cd(CN)4-2 Zn(CN)4-2 Ag(CN)2-1 Ag(CN)3-2 Cu(CN)2-1 Cu(CN)3-2 Cu(CN)4-2 Ni(CN)4-2 Cr(CN)6-3 Fe(CN)6-4 Au(CN)2-l Hg(CN)4-2 Fe(CN)6-3 Co(CN)6-3

1016.0 1017.9 1019.6 1020.5 1021.4 1021.7 1027.0 1028.5 1030.2 1033.0 1035.4 1037.0 1039.0 1042.3 1073.0

Source: Flynn and McGill, 1995

Mudder, Botz & Smith

14

Chemistry of Free and Complexed Cyanide

TABLE 1.6

Complex(1,2) [Hg(CN)4]-2 [Ag(CN)2]-1 [Cu(CN)3]-2 [Fe(CN)6]-3 [Fe(CN)6]-4 [Ni(CN)4]-2 [Cd(CN)4]-2 [Zn(CN)4]-2

Free Cyanide Concentration Released at Various Metal-Cyanide Complex Concentrations

1 mg/L 0.00002 0.00009 0.0003 0.0002 0.0012 0.135 (3) (3)

10 mg/L 0.00003 0.0002 0.00054 0.0032 0.0016 0.215 2.30 2.26

100mg/L 0.000045 0.0004 0.00097 0.0004 0.0022 0.340 3.64 3.59

1,000 mg/L 0.00007 0.0009 0.0017 0.0006 0.0031 0.539 5.77 5.68

100,000 mg/L 0.00018 0.0041 0.0054 0.0012 0.0061 1.324 14.49 14.28

Source: Caruso, 1975

Notes: (1) All values in mg/L. (2) Free cyanide levels calculated at pH 7.0 and 25oC. (3) Calculations indicate that at this dilution the two complexes are completely ionised.

15

Chapter One

Chemistry and Treatment of Cyanidation Wastes 1.5

SOLUTION CHEMISTRY OF THE IRON CYANIDES

From an environmental viewpoint the iron cyanides (i.e. hexacyanoferrates) require special attention due to their stability in the absence of light and their tendency to dissociate in its presence. Considerable controversy has evolved concerning the relative toxicity of the iron cyanides due to photolysis. Although these complexes resist natural degradation, they are nonetheless capable of releasing toxic levels of hydrogen cyanide when exposed to intense ultraviolet radiation. The hexacyanoferrates undergo a much broader range of reactions than do the other metal cyanide complexes and their solution chemistry has been studied more thoroughly. Ferrocyanide and ferricyanide both form stable salts with other metals without undergoing exchange of the cyanide ligand. Similarly, ferrocyanide is readily and reversibly oxidized to ferricyanide although the cyanide content remains unaffected. Since most methods of cyanide removal depend on oxidation processes, the removal of hexacyanoferrates from an effluent requires consideration of other aspects of its chemical behaviour (e.g. chemical precipitation). Cyanide reacts with iron to form stable octahedral complexes including hexacyanoferrate (II) or ferrocyanide, in which the iron is in the reduced state with a valence of +2. Ferrocyanide, which is the usual form in solution at ambient redox potentials, is readily oxidized to ferricyanide or hexacyanoferrate (III). In this case iron is present in the oxidized ferric form with a valance of +3. Hexacyanoferrate (II) can be formed by addition of a soluble ferrous salt or freshly prepared ferrous hydroxide to a solution containing free cyanide. In practice, the reaction appears to be limited to a pH below about 9.0. There is evidence that dissociation of the complex occurs rapidly above this pH. It is much slower than the reactions with copper and nickel, and approximately the same as that of zinc. Only the free cyanide precipitates with ferrous addition, a process which requires 15 to 60 minutes, depending upon solution pH and the cyanide and ferrous levels. The addition of excess ferrous iron increases the amount of complex formed. There is some evidence that a large excess of ferrous, coupled with a pH below 4.0, would precipitate other metal cyanide complexes as well. In spite of its larger stability constant, ferrous iron will not displace zinc, copper or nickel from their cyanide complexes. Hexacyanoferrate (III) cannot be formed directly in solution from ferric iron and cyanide, probably due to the greater insolubility of ferric hydroxide. Its formation is primarily the result of the oxidation of hexacyanoferrate (II). The hexacyanoferrates are classified as "inert" complexes, in that their chemical stability results from extremely slow rates of dissociation and relatively low toxicity. Although the precipitated iron cyanides present in mining solutions and wastes are mainly in the mixed ferro- and/or ferriforms, other relatively insoluble metal iron cyanide compounds do exist. A compilation of the solubilities of various iron cyanide complexes is shown in Tables 1.7 and 1.8.

Mudder, Botz & Smith

16

Chemistry of Free and Complexed Cyanide In the presence of ultraviolet light, photolysis and hydrolysis of iron-cyanides occurs, in which a water molecule displaces one of the cyanide moieties in the complex. On prolonged exposure, hexacyanoferrate (II) and (III) have been shown to release up to 85% and 49% percent of their cyanide content, respectively (Broderius and Smith, 1980). However, the experiments involved closed systems and very high levels of ultraviolet radiation when compared to natural systems. Boiling with phosphoric acid and/or certain catalysts (mercuric or magnesium compounds), or with strong complexing agents (e.g. EDTA or tartaric acid), results in the decomposition of the hexacyanoferrates and liberation of hydrocyanic acid. The hexacyanoferric (II) and (III) acids are relatively strong, (i.e. their hydrogen ions are easily dissociated to liberate the anions), a property which is attributed to the coordination of the cyanide group with a resultant weakening of the hydrogen linkage. The dissociation constants of the acids must be taken into account in calculating solubilities and concentrations of complex ions since they affect the concentration of the anion available for these reactions. As a result, changes in pH affect the solubility and stability relationships of the metal ferrocyanide compounds. The hexacyanoferrate (II) and (III) salts are formed by the reactions of the hexacyanoferrate ions and the corresponding cation. In the case of ferrocyanide, if alkali earth metals ions are present, the resulting precipitate will usually contain the alkali as well, often as a double salt. Ferricyanides are less subject to this phenomenon. The alkali and alkaline earth hexacyanoferrates are all soluble in water, except for barium hexacyanoferrate (II) which is only moderately soluble. All the alkali and alkaline earth salts of hexacyanoferrate (II) are insoluble in alcohol. The heavy metal salts of hexacyanoferrate (II) are insoluble and precipitate in water. Because the corresponding acid is fairly highly dissociated, the solubility of these precipitates, in the absence of metal-complexing ligands, is not greatly affected by changes in pH over the range pH 2 to pH 11 as shown in Table 1.9. In the case of those metals that form strong cyanide or amine complexes such as cadmium, copper, nickel and silver, the precipitates either dissolve or fail to form in solutions that contain excess cyanide ions or free ammonia. However, the complexes can again precipitate if the pH is lowered to a point where the concentrations of these ligands are insufficient to maintain the metal complex.

17

Chapter One

Chemistry and Treatment of Cyanidation Wastes

TABLE 1.7

Name Ammonium Ferricyanide Ammonium Ferrocyanide Barium Ferrocyanide Cadmium Ferrocyanide Calcium Ferrocyanide Cobalt Ferrocyanide Copper (I) Ferricyanide Copper (II) Ferricyanide Copper (II) Ferrocyanide Iron (II) Ferricyanide Iron (III) Ferricyanide Iron (II) Ferrocyanide Iron (III) Ferrocyanide Lead Ferricyanide Magnesium Ferrocyanide Manganese (II) Ferrocyanide Nickel Ferrocyanide Potassium Ferricyanide Potassium Ferrocyanide Silver Ferricyanide Silver Ferrocyanide Sodium Ferricyanide Sodium Ferrocyanide Strontium Ferrocyanide Thallium Ferrocyanide Tin (II) Ferrocyanide Tin (IV) Ferrocyanide Zinc Ferrocyanide

Solubilities of Ferrocyanides and Ferricyanides

Formula (NH4)3Fe(CN)4 (NH3)5Fe(CN)6 • 3 H2O Ba2Fe(CN)6 • 6 H2O Cd2Fe(CN)6 • X H2O Ca2Fe(CN)6 • 12 H2O Co2Fe(CN)6 • X H2O Cu3Fe(CN)6 Cu3(Fe(CN)6)2 • 14 H2O Cu2Fe(CN)6 • X H2O Fe3(Fe(CN)6)2 FeFe(CN)6 Fe2Fe(CN)6 Fe4(Fe(CN)6)3 Pb3(Fe(CN)6)2 • 5 H2O Mg2Fe(CN)6 • 12 H2O Mn2Fe(CN)6 • 7 H2O Ni2Fe(CN)6 • X H2O K3Fe(CN)6 K4Fe(CN)6 • 3 H2O Ag3Fe(CN)6 Ag4Fe(CN)6 • H2O Na3Fe(CN)6 • H2O Na4Fe(CN)6 • 10 H2O Sr2Fe(CN)6 • 15 H2O Th4Fe(CN)6 • 2 H2O Sn2Fe(CN)6 SnFe(CN)6 Zn2Fe(CN)6

Source: Huiatt et al., 1982

Mudder, Botz & Smith

18

Solubility g/L (°C) very soluble soluble 1.7g (15°) insoluble 868g (25°) insoluble insoluble insoluble insoluble insoluble -insoluble insoluble slightly soluble 330g insoluble insoluble 330g (4+°) 278g (12°) 0.00066 (20°) insoluble 189g (0°) 318.5g (20°) 500g 3.7g (18°) insoluble insoluble insoluble

Chemistry of Free and Complexed Cyanide

TABLE 1.8

Solubilities of Complex Ferrocyanide and Ferricyanide Salts

Compound

Solubility

Fe4 (Fe(CN)6)3

25 x 10-5

Zn2 Fe(CN)6

260 x 10-5

Zn3 (Fe(CN)6)2

2.2 x 10-5

Source: Huiatt et al., 1982

TABLE 1.9

Initial pH

Cadmium

2.0 6.0 8.0 11.0

3.0 3.1 3.1 2.4

Effect of Initial pH on Ferrocyanide Solubility Solubility of Fe(CN)6-4 (mg/L) Copper Iron Manganese (Ferrous) 2.4 4.5 3.7 2.5 3.5 3.7 1.3 3.0 3.2 1.3 2.3 2.8

Zinc 3.3 1.5 1.8 1.9

Source: Hendrickson and Daignault, 1973

19

Chapter One

Chemistry and Treatment of Cyanidation Wastes The heavy metal salts of hexacyanoferrate (III) are sparingly soluble but, nevertheless, are considerably more so than their ferrocyanide analogues. Table 1.10 compares their solubilities as well as those of mixtures of the two. As a cautionary note, while the data in Tables 1.9 and 1.10 are useful for comparative purposes, they should not be considered as absolute values, since the authors provided limited information on the test method or the analytical procedure used and indicated that there could have been a small positive interference during analysis. Hexacyanoferrate (II) and (III) form an oxidation-reduction couple: (1.14) Fe(CN)6-3 + e- → Fe(CN)6-4 Although the reaction does not itself involve the hydrogen ion, it is nevertheless pH dependent due to the difference in dissociation of the corresponding acids and in the relative proportion of the two free ions present at pH values below 7. As a result, hexacyanoferrate (II) is more easily oxidized in neutral than in acid solutions. Hexacyanoferrate (III) is reported to be reduced to hexacyanoferrate (II) by cyanide and would be readily reduced during the Merrill-Crowe precipitation step used for gold recovery (Williams, 1915). Hexacyanoferrate (II) is not oxidized by air in neutral or alkaline solutions in the absence of light or catalysts. It is oxidized by hydrogen peroxide in acid solution but not in alkaline solution. Chlorine, hypochlorite and ozone all yield hexcyanoferrate (III). Where these reagents have been employed for cyanide destruction, the addition of a suitable reducing agent would promote its reduction and removal. The pH must be adjusted into the range of about 3.0 to 7.5 for the reaction. Suitable reducing agents include Na2SO3, Na2S2O3, Na2S2O5, hydroxylamine hydrochloride and hydrazine. Ferrocyanide binds readily to both weak and strong base anion exchange resins because of its high negative charge. The reaction is highly selective for ferrocyanide over other anions including cyanide. Weak base resins can be regenerated using 1% to 10% sodium hydroxide solutions. The bond between ferrocyanide and the usual strong base resins (e.g. IRA-400) is so strong that regeneration is very difficult. However, one resin (IRA-958) has been specifically designed for this purpose and is regenerated with sodium chloride solution.

Mudder, Botz & Smith

20

Chemistry of Free and Complexed Cyanide

TABLE 1.10 Solubility of Hexacyanoferrates (II) and (III) Separately and in Mixtures

Solubility of Metal Iron-Cyanide Complexes (mg/L) Ferrous Hexacyanoferrate Manganese Ratio Fe(CN)6 Cadmium Copper Iron Compound Ferrocyanide 100 750 2.9 4.5 2.9 2.6 Ferricyanide 0 0 ----Ferrocyanide 75 562.5 2.0 3.6 2.0 7.0 Ferricyanide 25 187.5 3.2 0.8 1.0 53 Ferrocyanide 50 375 2.2 10.2 2.6 12.8 Ferricyanide 50 375 5.0 7.0 0.5 194 Ferrocyanide 25 187.5 2.0 13.4 3.3 31.5 Ferricyanide 75 562.5 2.8 5.7 1.2 190 Ferrocyanide 0 0 ----Ferricyanide 100 750 327 307 74 300

Zinc 2.3 -4.8 45 10.2 55 4.1 47 -256

Source: Hendrickson and Daignault, 1973

21

Chapter One

Chemistry and Treatment of Cyanidation Wastes 1.6

CYANIDE RELATED COMPOUNDS

1.6.1 Introduction As a result of cyanidation, natural attenuation or water treatment, a variety of cyanide related compounds are formed in solution including thiocyanate, cyanate, ammonia and nitrate. These compounds are important from both toxicological and treatment standpoints. More laboratory and field evidence is accumulating indicating that these compounds, once thought relatively unimportant, must be considered during the design of effective and efficient water treatment facilities. 1.6.2 Thiocyanate The presence of thiocyanate (i.e. SCN-) in process solutions and effluents results from the reaction of cyanide with labile sulphur atoms, either during pre-aeration or during leaching. The labile sulphur may originate directly from the attack of lime or cyanide on pyrrhotite, or be formed by the air-oxidation of sulphide ions released by dissolution of the readily soluble metal sulphide minerals. The dissociation constant for thiocyanic acid (i.e. HSCN) is near zero and exists predominantly as the anion at pH values above about 2.0. Thiocyanate is chemically quite different from cyanide, exhibiting a lower toxicity and a somewhat lower tendency to form soluble metal complexes. The thiocyanate ion and cyanide are classified chemically as pseudohalogens (i.e., they have properties similar to chloride, bromide and iodide) and can form insoluble ionic salts with silver, mercury, lead, copper and zinc. Thiocyanate is chemically and biologically degradable, with the metabolic by-products being ammonia, carbonate and sulphate. Since thiocyanate is more readily oxidisable than cyanide in certain instances, there are concerns regarding its removal during treatment. The primary environmental concerns associated with thiocyanate include its toxicity and its breakdown products (cyanate, ammonia and nitrate), which may be toxic if present in sufficient levels. 1.6.3 Cyanate Many oxidants, including chlorine, ozone, oxygen and hydrogen peroxide, convert cyanide to cyanate (i.e. OCN-). The mechanism for the dissolution of gold involves formation of hydrogen peroxide as part of the initial step and it is possible that the cyanate present in the cyanide leach solutions arises as a result of peroxide attack on cyanide. Cyanate is normally present in cyanidation solutions, but does not tend to accumulate due to its hydrolysis to ammonia. Cyanate is also chemically quite different from cyanide, exhibiting a lower toxicity and a much lower tendency to form soluble metal complexes. The toxicity of cyanate and its breakdown products are important and are discussed in Chapter 5.

Mudder, Botz & Smith

22

Chemistry of Free and Complexed Cyanide 1.6.4 Ammonia The two sources at mining operations that are responsible for the majority of ammonia present in waters are from residual blasting agents (i.e. ammonium nitrate fuel oil mixture) and from the hydrolysis of cyanate. Ammonia may also result from the stripping of loaded carbon where relatively hot cyanide solutions partially oxidize to generate ammonia. The hydrolysis of cyanate to ammonia occurs at a relatively slow rate in alkaline solution, but may proceed rapidly at a pH of less than about 6.0 or at elevated temperatures. Free ammonia forms soluble amine complexes with many heavy metals, including copper, nickel, silver and zinc. The hydrolysis constant for the free ammonia - ammonium ion equilibrium is 1.86 x 10-10 at 10°C (pKa = 9.73). As a result, the presence of ammonia in effluents can inhibit the precipitation of these metals at pH values above 9.0, the pH range generally chosen for precipitation of metal hydroxides. The toxicity of ammonia is discussed in detail in Chapter 5. 1.6.5 Nitrate Nitrate (i.e. NO3-) is the predominant end-product resulting from the oxidation of cyanide and is formed from the oxidation of ammonia through biological and chemical reactions. Nitrate is a relatively non-toxic compound and is stable in the environment under a wide variety of natural conditions. The primary concern with nitrate is with drinking waters where elevated concentrations can be harmful to humans, particularly young children and infants. In addition, nitrate is a biological nutrient and in some cases can lead to accelerated algae growth in waters, thereby consuming dissolved oxygen and potentially impairing the ability of fish to survive. Therefore, it is important that nitrate concentrations in untreated and treated waters be considered, though this topic is left to the large number of texts dealing specifically with nitrogen control.

1.7

PROCESS SOLUTION CHEMISTRY

1.7.1 Introduction Process solutions resulting from the cyanidation of gold and silver ores can be quite complex, containing various levels of a number of cyanide and related compounds. Formation of these compounds primarily takes place in cyanidation circuits, though their formation and build-up can be complicated by the recirculation of process solution which typically takes place at mining operations. An evaluation of projected long-term cyanidation solution chemistry is critical during upfront metallurgical testwork, particularly since laboratory testwork often cannot mimic the solution recirculation that will take place on the full-scale. The range of leach solution chemistries possible in cyanidation circuits is reflected in data presented by IEC (1979) as summarized in Table 1.11.

23

Chapter One

Chemistry and Treatment of Cyanidation Wastes

TABLE 1.11 Range of Cyanidation Solution Chemistries

Compound

Concentration Range (mg/L) 27 to 650 3 to 275 0.1 to 10 0.1 to 36 1 to 237 16 to 510

Total Cyanide Copper Nickel Iron Zinc Thiocyanate Source: IEC, 1979

TABLE 1.12 Listing of Several Iron-Cyanide Complexes

Iron-Cyanide Complex Fe(CN)6-3 Fe(CN)6-4 Fe(CN)5-3 Fe(CN)5-2 [Fe(CN)5 • H2O]-3 [Fe(CN)5 • H2O]-2 Fe2(CN)6 (aq) H2Fe(CN)6-2 HFe(CN)6-3

Mudder, Botz & Smith

24

Chemistry of Free and Complexed Cyanide In addition to compounds listed in Table 1.11, other cyanide related compounds are frequently encountered in leach solutions, including cyanate, ammonia and nitrate. It is critical that persons involved in the analysis of cyanide be familiar with the interactions of cyanide with metals in solution, particularly for environmental monitoring samples. The following sections provide brief descriptions of a few process solution issues frequently encountered at mine sites. 1.7.2 Iron Cyanides While there is generally a good understanding of the chemistry of the iron-cyanide complexes, iron-cyanide chemistry is nonetheless complicated. The most commonly encountered ironcyanide complexes are the two formed with ferrous and ferric iron, Fe(CN)6-4 and Fe(CN)6-4. However, as indicated in Table 1.12 there are a number of other complexes that may be present, though usually these are present at low levels. The presence of these other complexes becomes important when cyanide solutions are exposed to UV irradiation (e.g. sunlight) where the complexes undergo photocatalytic degradation, resulting in the formation of several intermediate iron-cyanide complexes. These complexes vary in stability and some may be detected in analytical procedures not intended for their detection. This is important with environmental monitoring where low levels of cyanide would be significantly influenced by the presence of iron-cyanide complexes or where these complexes may interfere with an analytical procedure. 1.7.3 Copper Bearing Ores The presence of nuisance copper in cyanidation circuits is of major concern because of its impact on cyanide consumption, gold recovery and tailings treatment. Cyanide-soluble copper is present in many gold and silver ores, and resultant concentrations of copper in process solutions can range from only a few mg/L up to several hundred mg/L. When present in cyanide leach solutions, copper is predominantly present as a mix of three forms: Cu(CN)2-, Cu(CN)3-2 and Cu(CN)4-3. As the cyanide to copper molar ratio (CN:Cu) in solution drops to less than about 3:1, there begins to be less free cyanide in solution to leach gold and silver. This can lead to reduced gold and silver recovery, which is oftentimes countered with an increase in the leach solution cyanide concentration. This in turn, may result in increased leaching of copper and a further consumption of cyanide. In some cases, this circular response leads to very high levels of cyanide in solution, most of which is bound as a copper-cyanide complex and unavailable for gold and silver leaching. The implications of this situation on tailings treatment processes must be considered during the laboratory metallurgical testwork stage. In some cases, full-scale plants may require a treatment system for copper removal, either from the plant tailings or from a leach solution bleed stream.

25

Chapter One

Chemistry and Treatment of Cyanidation Wastes 1.7.4 Silver Ores Silver ores require relatively high levels of cyanide for leaching, commonly at levels two to ten times that required for an equivalent gold ore. However, with higher levels of cyanide, other metals present in an ore that may not normally leach could begin to leach along with silver. Aside from complications this poses to silver recovery processes, this can lead to complicated and expensive downstream water treatment processes. Waters resulting from silver leaching can contain elevated concentrations of both cyanide and metals that are not characteristic of gold leaching solutions. These other metals may include cadmium, cobalt, chromium, copper, mercury, silver, thallium and selenium, all of which are known to form complexes with cyanide. In the design of a silver cyanidation circuit, the cost of water treatment must be considered along with the economics of silver recovery versus leach solution cyanide levels.

1.8

REFERENCES

Baker, D.J., "Cyanide in the Environment", Unpublished Data, 1984. Broderius, S. and Smith, L., USEPA, Grant No. R805291, 1980. Caruso, S.C., "The Chemistry of Cyanide Compounds and Their Behavior in the Aquatic Environment", Carnegie Mellon Institute of Research, June, 1975. Flynn, C.M. and McGill, S.L., Cyanide Chemistry – Precious Metals Processing and Waste Treatment, U.S. Bureau of Mines, NTIS Publication PB96-117841, 1995. Hendrickson T.N. and Daignault, L.G., "Treatment of Complex Cyanides for Reuse and Disposal", Report No. EPA-R2-73-269, USEPA, 1973. Huiatt, J., Kerrigan, J., Olson, F., and Potter, G., Proceedings of a Cyanide Workshop, Cyanide from Mineral Processing, U.S. Bureau of Mines, Salt Lake City, Utah, February 2-3, 1982. IEC, “Factors Affecting Natural Degradation of Free and Metal-Complexed Cyanides from Gold Milling Effluents”, International Environmental Consultants Ltd., Toronto, Ontario, 1979. Marsden, J. and House, I., The Chemistry of Gold Extraction, Ellis Horwood Limited, Hertfordshire, United Kingdom, 1993. Scott, J. and Ingles, J., State-of-the-Art Processes for the Treatment of Gold Mill Effluents, Mining, Mineral, and Metallurgical Processes Division, Industrial Programs Branch, Environment Canada, Ottawa, Ontario, March, 1987. Williams, H.E., Cyanide Compounds, E. Arnold, London, 1915.

Mudder, Botz & Smith

26

Chemistry of Free and Complexed Cyanide 1.9

BIBLIOGRAPHY

American Cyanamid Co., The Chemistry of the Ferrocyanides, Vol. VII, Cyanamid's Nitrogen Chemicals Digest, American Cyanamid Co., New York, New York, 1953. Adamson, R.J., "The Chemistry of the Extraction of Gold from its Ores", Gold Metallurgy in South Africa, Cape and Transvaal Printers, Ltd., Cape Town, South Africa, 1972. Brickell, R.H., C-I-L Inc., "Chemistry of Cyanide Solutions", January, 1981. Dorr, J. and Bosqui, F., Cyanidation and Concentration of Gold and Silver Ores, McGraw Hill Book Co., 2nd Edition, 1950. Ecological Analysts Inc., "Cyanide: an Overview and Analysis of the Literature on the Chemistry, Rate, Toxicity, and Detection in Surface Waters", prepared for the Inter-Industry Cyanide Group by Ecological Analysts Inc., Towson, Maryland 2124, 1979. Engineering Science Inc., 'Background Information on Cyanide in the Mining Environment", Amax of Canada Ltd., Vancouver, British Columbia, 1980. Finklestein, N.P., "The Chemistry of the Extraction of Gold from its Ores", in: Gold Metallurgy in South Africa, Adamson R.J. (ed.), Chamber of Mines of South Africa, Johannesburg, South Africa, Chapter 10, 1972. Hedley, N. and Tabachnik, H., "Chemistry of Cyanidation", Mineral Dressing Note No. 23, American Cyanamid Co., New York, New York, 1958. Heinen, H.J., Peterson, D.G., and Lindstrom, R.E., "Processing Gold Ores Using Heap LeachCarbon Adsorption Methods", U.S. Bureau of Mines, Information Circular 8770, 1978. Moggi, L., Bolleta, F., Balzani, V., and Scandola, F., "Photochemistry of Coordination Compounds - XIV, Cyanide Complexes", J. Inorganic Nucl. Chemistry, 28, pp.2597,1966. Sillen, L.G., Stability Constants of Metal-Ion Complexes, The Chemical Society, London, England, 1964. Tchobanoglous, G. and Burton, F.L., Water Engineering, Treatment, Disposal, and Reuse, Metcalf & Eddy, Inc., 3rd Edition, McGraw-Hill, Inc., New York, 1991.

27

Chapter One

28

CHAPTER TWO

Analysis of Cyanides 2.1

INTRODUCTION

No book on cyanide chemistry and treatment would be complete without a chapter on analysis, since it is the chemical analysis that is critical in the control and monitoring of the treatment of cyanide-containing wastewater discharges. Yet, despite its critical importance, the reliable analysis of cyanide in mining-related solutions is frequently a source of concern and confusion to both operators and regulators around the world. The concern and confusion stems from the regulation of the various forms of cyanide using multiple analytical methods of varying reliability for different industries such as mining and electroplating. In the United States, the current status and applicability of methods of cyanide analysis to mining solutions remains in somewhat of a state of flux. Effluent discharge standards, which are based upon free cyanide, are often included in permits using the total cyanide analysis for compliance purposes. The magnitude of an effluent limitation is frequently below the practical quantitation limit (PQL) for the cyanide analysis in mining effluents, making monitoring and compliance difficult at best. This is despite acknowledgement by many professionals and a considerable database to indicate that there are problems with the accurate determination of cyanide when the analyses are not carefully conducted, particularly at levels below 1.0 mg/L. The United States Environmental Protection Agency (USEPA) has been re-evaluating all cyanide analytical methods by examining the fundamentals of these procedures. The agency is considering aspects such as interferences and applicability, with the objective of recommending appropriate and defensible procedures. These may include analysis of specific metal-complexes rather than the present procedures, which place cyanides into groups based on their overall similarities in behaviour. The process of re-evaluation to such a fundamental level will be time consuming and the present state of uncertainty will continue for some time. It is in this context that cyanide analytical methods are discussed in this chapter. Rather than being merely a reiteration of various excellent studies on the analysis of cyanide, for example Conn (1981), the chapter is focused on those methods currently considered either most applicable or most appropriate for mining effluents. This does not mean that the methods described are neither without their problems and interferences, nor are they universally applicable, but that these methods have found successful application in cases in which a cyanide containing solution is being treated and potentially discharged into the environment.

29

Chemistry and Treatment of Cyanidation Wastes Around the world, the trend in the mining industry has turned away from the traditional approach of regulating free cyanide using the overly conservative total cyanide analysis, in favour of employing the weak acid dissociable (WAD) cyanide analysis, which measures the “toxicologically important” forms.

2.2

ANALYTICAL PROCEDURES FOR CYANIDES

2.2.1 Introduction There are three main categories of cyanide as determined from analytical methods. These three categories are total, weak acid dissociable (WAD) and free cyanide. As indicated in Figure 2.1, total cyanide includes all the cyanide compounds present in solution, including cyanide complexed with metals, except for cobalt. In Australia, a number of mines have significant levels of cobalt in the ore with several mg/L of cobalt cyanide complexes in the process solutions, mainly adsorbed to particulates. Total cyanide analyses in these cases should include cyanide associated with cobalt. WAD cyanide includes the free and complexed forms of cyanide that can be liberated in a weakly acidic environment, including the weak to moderately strong metal-cyanide complexes. Free cyanide includes only the uncomplexed forms of cyanide, which are the cyanide anion and molecular hydrogen cyanide. For a given sample, the level of total cyanide is always greater than or equal to the WAD cyanide level, and the WAD cyanide level is always greater than or equal to the free cyanide level. Selection of an appropriate cyanide analytical method for a particular situation should involve the following considerations: Complete characterization of the solution to be analysed with particular emphasis on the species of cyanide present and potential interfering components. •

Knowledge of the basic chemistry of cyanides.

•

Awareness of the strengths and weaknesses of a specific analytical method for a given set of conditions and chemical matrix.

•

Understanding of the capabilities of equipment and operator expertise and experience.

•

Knowledge of the potential treatments to obviate or reduce the effect of interferences.

•

Recognition of the fact that treatments to eliminate interferences may introduce additional interferences.

•

Understanding of the particular regulatory requirements being applied in a given situation.

In the context of the above considerations, descriptions of several common analytical methods for cyanide and its related compounds are provided in the following sections.

Mudder, Botz & Smith

30

Analysis of Cyanide

FIGURE 2.1 Compounds Included in Total, WAD and Free Cyanide Analyses

Strong Metal-Cyanide Complexes of Fe

Total Cyanide

WAD Cyanide

Weak and Moderately Strong Metal-Cyanide Complexes of Ag, Cd, Cu, Hg, Ni and Zn Free Cyanide

31

-

CN HCN

Chapter Two

Chemistry and Treatment of Cyanidation Wastes 2.2.2 Total Cyanide by Distillation Procedures for total cyanide analyses are presented in Standard Methods (APHA, 1998), in USEPA Methods 9010 and 9012 (USEPA, 1986), in ASTM D2036 Method A (ASTM, 1991) and in ASTM D4374 (ASTM, 1997). The most commonly employed total cyanide analysis for compliance and monitoring purposes involves manual distillation under strongly acidic conditions and a slight vacuum, with the addition of magnesium chloride as a catalyst (APHA, 1998). Sulphuric acid is used to adjust the pH of the sample to less than 2.0 at an elevated temperature to facilitate dissociation of the strong iron-cyanide complexes. During distillation, the free and metal complexed forms of cyanide are converted to the volatile molecular HCN, which is then swept from the flask along with the air and recaptured in a separate caustic scrubber. The caustic solution containing the captured cyanide is then removed from the scrubber and subjected to a colourimetric analysis using pyridine, barbituric acid and chloramine-T as reagents. A reddish-blue colour is formed in the presence of cyanide, the intensity of which is related to cyanide concentration at a specific wavelength of visible light using a spectrophotometer. The entire procedure provides an indirect measurement of cyanide, unlike the direct analysis of a metal using atomic absorption spectrometry. Constituents other than cyanide can react with the reagents used in the procedure producing both positive and negative interferences, leading to observed cyanide levels in a sample that may be either higher or lower than the actual concentration. A schematic of a typical distillation apparatus used in the determination of total cyanide is shown on Figure 2.2. A similar apparatus is used in other cyanide analytical procedures, as discussed in subsequent sections of this chapter. The total cyanide method is subject to a number of common interferences, notably thiocyanates and sulphides. However, these interferences are treatable to a certain extent and the detection limits under favourable conditions are good when the analysis is performed carefully. The total cyanide method will recover all cyanide species with the exception of extremely stable metal cyanides like those of cobalt. It should be noted that thiocyanate and cyanate, although related to cyanide, are not cyanide compounds. Therefore, the total cyanide procedure, or any other cyanide analytical procedure, will not (i.e., should not) measure cyanate or thiocyanate. Separate analytical procedures are available for these compounds, as discussed later in this chapter. The traditional detection limit for total and WAD cyanide has been 0.02 mg/L, although some commercial laboratories are employing lower limits. In practice, the detection limit and practical quantitiation limit are often several fold higher due to matrix interferences.

Mudder, Botz & Smith

32

Analysis of Cyanide

FIGURE 2.2 Typical Cyanide Distillation Apparatus

33

Chapter Two

Chemistry and Treatment of Cyanidation Wastes From experience, if performed by skilled technicians, the manual distillation method for total cyanide (USEPA Method 9010) is more reliable than the automated USEPA 9012 procedure (USEPA, 1986). While the reliability of the autoanalyser version of the process is accepted by the USEPA, the autoanalyser method has been shown to give erroneous results on samples verified using the manual distillation. The USEPA did not establish a Best Achievable Technology (BAT) limitation for total cyanide, in part due to the problems associated with its analytical measurement, which the agency indicated was unreliable below 0.20 mg/L (USEPA, 1982). Table 2.1 gives comparative values for total cyanide determinations on effluents from a gold mining project in the United States over a six-month period. With anticipated values of total cyanide being in the 0.005 to 0.02 mg/L range, the manual distillation data are seen to be far more representative than the equivalent autoanalyser results. A second example is from a comparative study of three laboratories, two of which used a manual distillation method on samples of effluent from a heap leach project in the United States (Table 2.2). Here the cyanide values are two orders of magnitude higher than the first example, yet the autoanalyser data are still in error, being about three times the values seen in the manually determined samples. These two examples suggest that if it is proposed to use the autoanalyser technique for routine determination of total cyanide, a comparative study should be made with manual distillation data to ensure that the autoanalyser method is appropriate for the sample matrix being evaluated. In several Australian studies (AMIRA, 1991, 1997, 2000 and Schulz, 1992 and 1997) it was shown that the autoanalyser method based on ASTM D4374 but utilising the McLeod microstill produces results comparable to the manual methods (with skilled operators) and is the preferred method if instrumental costs is not a deterrent. Although removing operator variables, increasing sample turnaround time and the ability to conduct total and WAD cyanide analyses simultaneously provide a commercial advantage, it is essential to utilize appropriate quality control samples and to have a skilled operators familiar with cyanide chemistry and the instrumentation. In recent method evaluations at the Chemistry Centre (Schulz Unpublished Data, 2000) it was found that diffusion membranes used in place of the microstill may give unsatisfactory results due to poor reproducibility. In research conducted in the Homestake Mine analytical laboratory during the early 1980’s, it was noted that the total cyanide method using sulphuric acid in the presence of thiocyanate resulted in a positive error, probably due to the breakdown of thiocyanate promoted by the acid (Whitlock, Sharp and Mudder, 1981). As the thiocyanate concentration was increased in a synthetically prepared sample of cyanide, the magnitude of the error increased. These observations are shown in Tables 2.3 and 2.4. It was found that substitution of phosphoric acid for sulphuric acid minimized the interferences, although this practice has not yet been accepted on a standardized basis. Phosphoric acid has also been recommended for use with the automated total cyanide method. It should also be noted that the WAD cyanide method was found less susceptible to the thiocyanate interference than the total cyanide method, due largely to the strong acid and oxidizing conditions associated with the total cyanide analysis.

Mudder, Botz & Smith

34

Analysis of Cyanide

TABLE 2.1 Comparison of Total Cyanide Analyses by Autoanalyser (USEPA Method 9012) and Manual (USEPA Method 9010) Methods

Sample Number 1 2 3 4 5 6 7 8 9 10 11 12 13

Total Cyanide (Autoanalyser) 90 80 1,610 2,780 780 1,200 560 2,520 1,900 1,090 620 1,550 1,770

Total Cyanide (Manual) 14