SGS MINERALS SERVICES – T3 SGS 018 05-2005

CYANIDE DESTRUCTION

CYANIDE DETOXIFICATION SGS has extensive experience with the Advantages methods that are most commonly used •• Relatively simple to operate Most gold plants around the world are or promoted, and can determine the •• Strong oxidant required by law to destroy cyanide and advantages and disadvantages of each for •• Can oxidize thiocyanate (reaction 3) metal cyanide complexes in their tailings different applications. •• Can oxidize cyanate to N and CO prior to discharge from the metallurgical 2 2 •• Chlorination (reaction 4 in a second stage), site into the natural environment. Many •• Hydrogen Peroxide avoiding the hydrolysis of cyanate to plants destroy the cyanide in a contained •• SO /Air undesirable ammonia (reaction 6) area within the metallurgical site, so as 2 •• Ferrous Sulphate Complexation •• No copper catalyst required to minimize the exposure of wildlife, •• Ozonation •• Fast kinetics particularly birds, to these toxic solutions. •• Caro’s Acid •• Can oxidize CN-WAD efficiently (hence CN-WAD is also called “CN WHAT INFLUENCES amenable to chlorination”) leaving COMPARISON OF low toxic iron cyanide in solution PROCESS SELECTION? COMMON DETOXIFICATION Disadvantages There are a number of technologies METHODS •• Non-selective, leading to high reagent available for destroying cyanide and its consumption metal complexes, and the choice of the ALKALINE CHLORINATION •• Requires high pH (pH 11) to ensure best method for a particular application is rapid and complete hydrolysis of not simple. Complexity arises because: Reagents highly toxic cyanogen chloride gas Cl + 2NaOH → NaOCl + NaCl + H O (1) •• environmental regulations vary with 2 2 (reaction 8) respect to allowable concentration Reactions •• As for other methods, ferrocyanide limits in the treated tailings NaOCl + CN-WAD → CNO- + NaCl (2) is not destroyed, but only partially •• the various detoxification techniques oxidized to ferricyanide (reaction 5). 4NaOCl + SCN- + 2OH- → CNO- + SO 2- + vary widely in their ability to eliminate 4 Requires an additional stage and 4NaCl + H O (3) certain species 2 reagent to remove iron cyanide (if •• the chemical composition of the - + 3NaOCl + 2CNO + 2H → N2 + 2CO2 + required) as a base metal iron cyanide tailings from each plant is unique 3NaCl +H2O (4) precipitate (such as in reaction 9) •• reagent consumption and cost varies 4- 3- •• May leave residual chlorine in 2Fe(CN)6 + NaOCl + 2H+ → 2Fe(CN)6 from one country and location to the solution, thereby requiring a polishing + NaCl + H2O (5) next pond or aeration stage for removal - + CNO + H + H2O → CO2 + NH3 (6)

COST EFFECTIVENESS IS - + + CRITICAL NaOCl + CN + 2H → CNCl + Na + H2O (7) The cost of treating cyanide tailings CNCl + 2OH- → CNO- + Cl- + H O (8) can be a significant percentage of 2 3- 2 total operating costs, and unlike other 6FeSO4 + 4Fe(CN)6 → 2Fe3(Fe(CN)6) + 2- operating costs, yields no economic 6SO4 (9) “return”. It is therefore very important, for both regulatory and economic reasons, to select the correct process, and then optimize the operating conditions to minimize reagent dosages. This requires a good knowledge of local regulations as well as skillful laboratory test work and piloting. SGS MINERALS SERVICES – T3 SGS 018 2

HYDROGEN PEROXIDE SO2/AIR FERROUS SULPHATE COMPLEXATION Reagents Reagents Reagent

2+ 2+ H2O2, Cu catalyst SO2 + O2 + H2O → H2SO5, Cu FeSO4 Reactions catalyst (12) Reactions

- 2+ 4- H2O2 + CN-WAD → CNO + H2O (10) Reactions Fe + 6CN-WAD → Fe(CN)6 (15)

2+ 4- - 2+ 3+ 2Cu + Fe(CN) 6 → Cu2Fe(CN)6 (11) SO2 + O2 + H2O + CN-WAD → CNO + Fe + ½ O2 + H2O → 2Fe +

- + H2SO4 (13) - CNO + H + H2O → CO2 + NH3 (6) 2OH (16) - Na2S2O5 + 2O2 + H2O + 2CN WAD → 3+ + Advantages: Fe + 3H2O → Fe(OH)3 + 3H (17) - 2 CNO + 2NaHSO4 (14) •• Relatively simple to operate 2Fe + Fe(CN) 4- → Fe Fe(CN) (18) 2+ 4- 2+ 6 2 6 •• No production of toxic gases Cu + Fe(CN) 6 → Cu2Fe(CN)6 (11) 4Fe3+ + 3Fe(CN) 4- → Fe (Fe(CN) )3 (19) •• More selective toward CN-WAD - + 6 4 6 CNO + H + H2O → CO2 + NH3 than chlorination (cyanate is not (6) Advantages oxidized and only a small portion of Advantages •• Low reagent cost thiocyanate is oxidized) •• Relatively simple to operate •• Ferrocyanide is precipitated as a base •• Low reagent cost •• CN-WAD is converted to metal ferrocyanide complex (reaction •• More selective than chlorination ferrocyanide, which is precipitated 11). It may be necessary to add more and hydrogen peroxide toward CN- as green ferro ferrocyanide at neutral base metal than the copper needed WAD (cyanate is not oxidized and a pH (reaction 18) and/or prussian to catalyze the oxidation reaction. only small portion of thiocyanate is blue, ferri ferrocyanide at lower pH Ferrous sulphate will be cheapest. oxidized) (reaction 19) •• Can oxidize CN-WAD efficiently •• Sulphite salts such as sodium •• Can sequester CN-WAD efficiently metabisulphite or sodium sulphite Disadvantages: •• Does not oxidize cyanide to cyanate can be used (reaction 14) and, therefore, does not produce •• Require copper in solution as a •• Ferrocyanide is precipitated as a base ammonia catalyst (20 mg/L or higher) metal ferrocyanide complex (reaction •• Tends to leave some residual copper 11). It may be necessary to add more Disadvantages: in solution base metal than the copper needed •• Difficult to achieve <5 mg/L •• Can react with sulphides in the solid to catalyze the oxidation reaction. CNT because of residual, soluble phase, which results in high reagent Zinc sulphate is preferred. ferrocyanide consumption •• Can oxidize CN-WAD efficiently •• Requires longer retention time than •• High unit cost for the reagent chemical oxidation methods Disadvantages: •• Cyanate hydrolyzes to undesirable •• May require aeration to convert Fe2+ ammonia (reaction 6) •• Quite difficult to operate to Fe3+ •• Ferrocyanide is not destroyed, •• Requires copper in solution as a •• Does not oxidize thiocyanate but precipitates as a base metal catalyst (>30 mg/L Cu) •• Cyanide is not destroyed, but ferrocyanide complex. The precipitate •• Tends to leave some residual copper precipitates as mixed iron can redissolve at basic pH (pH>9) in solution ferrocyanides. The precipitates and release ferrocyanide back to •• Requires longer retention time than can redissolve at basic pH (pH>9), solution alkaline chlorination and hydrogen releasing ferrocyanide back to •• Copper may redissolve in a high peroxide solution chloride environment •• Requires very good dispersion of air

and SO2 gas (vigorous mixing) •• Cyanate hydrolyzes to undesirable ammonia (reaction 6) •• Ferrocyanide is not destroyed, but precipitates as a base metal ferrocyanide complex. The precipitate can redissolve at basic pH (pH>9) and release ferrocyanide back into solution •• Copper may redissolve in high chloride environment SGS MINERALS SERVICES – T3 SGS 018 3

CARO’S ACID OZONATION CONTACT INFORMATION Reagents Reagent Email us at [email protected] www.sgs.com/mining H2O2 + H2SO4 → H2SO5 + H2O (20 O3 Reactions Reactions - - 7CN-WAD + O → CNO- + O H2SO5 + CN WAD → H2SO4 + CNO (21) 3 2

- - - (23) SCN + 4H2SO5 + 2OH → CNO + 4H2SO4 2- SCN- + 2O + 2OH- → CNO- + SO 2- + + SO4 + H2O (22) 3 4 - + O + H O (24) CNO + H + H2O → CO2 + NH3 2 2 (6) - + CNO + H + H2O → CO2 + H3 (6) Advantages Advantages: •• Strong oxidant, fast kinetics •• Strong oxidant, fast kinetics •• Does not require copper catalyst •• Does not require copper catalyst •• No production of toxic gas •• Can oxidize thiocyanate •• Can oxidize part of the thiocyanate •• Can oxidize CN-WAD efficiently •• Can remove ferrocyanide as a base Disadvantages: metal iron cyanide precipitate •• Can oxidize CN-WAD efficiently •• Non selective. Reacts more readily

Disadvantages: (than hydrogen peroxide or SO2/air) with sulphides in solids, leading to •• Less selective than the alternative high reagent requirement processes. Reacts more readily (than •• May react with the solid phase and hydrogen peroxide and SO /air) with 2 release undesirable species such as sulphides in solids, leading to high arsenic into solution reagent requirement •• High reagent cost •• High reagent cost •• Can release toxic O gas, requiring •• More difficult to handle than peroxide 3 scrubbing because of fairly large amount of heat •• Requires O resistant equipment generated during mixing peroxide 3 •• May need to add copper, iron or zinc with concentrated sulphuric acid salts to precipitate iron cyanide •• May need to add copper, iron or zinc •• Tends to leave some residual copper salts to precipitate iron cyanide in solution •• Tends to leave some residual copper •• Cyanate hydrolyzes to undesirable in solution ammonia (reaction 6). •• Copper and/or zinc may redissolve in •• Ferrocyanide is not destroyed, but high chloride environment may precipitate as a base metal iron •• Cyanate hydrolyzes to undesirable cyanide complex. The precipitate ammonia (reaction 6) can redissolve at basic pH (pH>9) •• Ferrocyanide is not destroyed, but and release ferrocyanide back into may precipitate as a base metal iron solution. cyanide complex. The precipitate can redissolve at basic pH (pH>9) and release ferrocyanide back into solution © 2005 SGS Société Générale de Surveillance SA – All rights reserved