Cyanide Recovery

SGS MINERALS SERVICES – T3 SGS 019 11-2013

CYANIDE RECOVERY

CYANIDE RECOVERY COST EFFECTIVENESS IS WHY RECYCLE CYANIDE? TECHNOLOGY CRITICAL

COMPARISON The cost of cyanide consumption LOWER COSTS Legislation in most gold mining regions of and destruction can be a significant •• Recycled cyanide is 2-3 times the world demands that gold plants meet percentage of total operating costs on cheaper than new cyanide. rigorous environmental standards with a gold plant, and unlike other operating •• Reduced detoxification costs. respect to cyanide. These requirements costs, yields no economic “return”. The •• Opportunity to sell by-products such can be met either by destroying cyanide choice of the best strategy for handling as copper sulphide. after a single pass through the gold plant cyanide in the tailings can therefore or by recovering cyanide from the tailings have a major influence on the economic LESS ENVIRONMENTAL RISK and recycling it to leaching. health of a project. The economic impact of cyanide is different in every •• Reduced loading of potentially Although cyanide recovery processes are operation, but it is possible to compare harmful salts in the tailings and not yet widely practiced, the technologies recycle water (e.g. CNS, CNO, NH cyanide destruction and cyanide recovery 3 are fairly simple, both from chemistry and costs for a fairly generic high cyanide- and metal cyanides). process engineering perspectives, and consuming case. •• Less new cyanide is needed, so they have the potential to yield significant there is less transported, stored and economic and environmental benefits. The capital cost of a cyanide recovery handled, reducing the risk of spills. plant could pay back in less than 2 years in many cases, particularly when high AVAILABLE TECHNOLOGIES cyanide consumption is associated with FOR CYANIDE RECOVERY copper mineralisation in the ore. As a broad generalisation, cyanide recovery There are several processes for will produce more favorable economics recovering cyanide from gold plant tailings than cyanide destruction in those cases that are available to the mining industry. where potentially recoverable cyanide •• Cyanisorb consumption is greater than 1 kg/t NaCN, (AVR: Acidify, volatilize and or exceeds 1000 tons NaCN per annum. reneutralize) •• SART BASE CASE ECONOMICS (1kg/t NaCN CONSUMED, 0.4 kg/t Cu LEACHED) (Sulphidize, acidify, recycle and (PRICES AS OF 2005) thicken) •• Hannah Cyanide destruction Cyanide recovery (Strong base resin extraction of free New NaCN $1.50 operating $0.60 cyanide and metal cyanide species) •• AuGMENT Destruction $1.00 capital $0.30 (Strong base resin extraction of $2.50 Subtotal $0.90 copper and cyanide). SGS metallurgical group has been Cu credit $0.40 involved in the development of all of TOTAL ($/t) $0.50 these processes and can determine the advantages and disadvantages of each. SGS MINERALS SERVICES – T3 SGS 019 2

WHAT CAN AND CANNOT BE RECOVERED? the base metals precipitate from solution as sulphides (e.g. Cu2S or ZnS), leaving cyanide in solution as HCN gas. The FREE CYANIDE CYANATE metal sulphide precipitate is flocculated, thickened and filtered, to produce a Cyanide Complexes of Cyanide Complexes of saleable concentrate. Recycling of a 2+ 3+ Cu, Zn, Ni, Cd and Co Fe and Co portion of the thickener underflow back Thiocyanate* to the primary sulphidization/acidification reactor increase the particle size of the *Thiocyante can be oxidized to regenerate the contained cyanide units, but the cost of oxidati- precipitate and improve its thickening/ on exceeds the value of the recovered cyanide. filtration properties. The thickener overflow and the filtrate (containing COMPARISON OF CYANIDE Advantages HCN gas) are treated with lime slurry to RECOVERY METHODS •• Established and proven on a fairly raise the pH to about 10. This converts large scale. the HCN back to free cyanide, which is •• Can be applied to solutions or pulps, recycled to leach. CYANISORB (AVR) although pulps are difficult to treat The SART process is especially suited The Cyanisorb process uses the well- and suffer high cyanide losses. to the treatment of heap leach liquors known AVR technique (Acidification, Disadvantages that contain elevated concentrations of Volatilization and Reneutralization). base metals as WAD cyanide complexes. •• Only recovers free cyanide. Cyanide SART can also be used to treat the Gold plant tailings are first acidified with associated with Cu, Zn or Ni tailings from milling operations, such sulphuric acid to a pH of 6-7 to convert complexes must be recovered by as CIP, CIL or Merrill Crowe barrens. all the free cyanide to hydrocyanic acid. alternative means. However, because the SART process The tailings are then contacted with air, •• Difficult to strip the final 20 to 50 regenerates free cyanide at essentially in a counter-current flow, in a baffled mg/L of cyanide from the tailings. the same concentration as in the tailings, vertical column. The HCN is stripped from •• Stripping columns are large and the cyanide cannot be recycled without the tailings into the gaseous stream, and costly with respect to both capex and creating a water imbalance in the plant. the air/HCN stream is recycled through opex. This is particularly true in the The excess water will contain cyanide, a second column containing a strong case of pulp. and must be treated via a cyanide caustic solution. This solution absorbs •• Stripping columns have scaling destruction process prior to discharge. the HCN gas from the air by converting problems and require frequent it back to free cyanide ions. The air then maintenance. The SART process was developed by cycles back to the stripping column. Once •• Large volumes of gas flow through SGS’ metallurgical group in collaboration the caustic solution in the scrubbing the stripping and scrubbing columns. with Teckcominco Corporation. column has converted to a sufficiently This HCN/air mixture is potentially Advantages high cyanide strength, it is recycled to hazardous, and the operation must be •• Simple to engineer and operate. leach. carefully engineered and controlled •• No HCN volatilization, which means a The Cyanisorb process is the only to ensure there can never be gas safer working environment. commercially-practiced technology release. •• Valuable by-products can be for cyanide recovery. Michael Botz of recovered. Elbow Creek Engineering developed the SART (SULPHIDIZATION, ACIDIFICATION, •• All WAD cyanide is regenerated as technology so that it could be scaled RECYCLING AND THICKENING) free cyanide for recycle. up for commercial operation. About five plants have been built over the years, but The SART process was developed to only two are currently operating at mines treat solutions containing cyanide in the worldwide. Where plants have closed, form of weak acid dissociable (WAD) this has been due to cessation of gold metal complexes (primarily copper operations for one reason or another, not cyanide, but also zinc or nickel cyanide). because of technical problems with the During sulphidization and acidification, cyanide recovery part of the process. SGS the cyanide solution is treated with offers this technology via an AVR pilot sulphide ions and sulphuric acid to yield plant, designed and built by Michael Botz. a pH of 4 - 5. Under these conditions, SGS MINERALS SERVICES – T3 SGS 019 3

Disadvantages Disadvantages Disadvantages •• Can only treat solutions. Therefore, •• No known technical disadvantages. •• Operation is critically dependent on the process is not well suited to •• Not operated yet at commercial the rapid analysis and control of the the treatment of CIP or CIL tailings scale. cyanide to copper ratio, in adsorption, pulps, unless they are separated into elution and electrowinning. As a solution and solid streams prior to AUGMENT PROCESS result, the process will be fairly cyanide recovery. In this case, the difficult to monitor, control and The AuGMENT process uses a strong wash water introduced during solid/ operate consistently and efficiently. base resin to extract and concentrate liquid separation must be bled from •• Vulnerable to feed composition copper cyanide from gold plant tailings the process, and the cyanide in the changes. solutions or pulps. AuGMENT is bleed stream is ‘lost’. •• Not yet operated on a commercial particularly well suited to solutions scale. containing copper cyanide at a low CN:Cu HANNAH PROCESS ratio of ~3:1, although it can handle higher The Hannah process uses a strong base CN:Cu ratios by advancing resin more THEORETICAL resin to extract and concentrate free rapidly through the adsorption plant. CONSIDERATIONS cyanide and metal cyanide complexes Copper cyanide is partially eluted from To remove cyanide and cyanide-bearing from gold plant tailings solutions or pulps. the resin using a strong copper cyanide complexes from gold plant solutions, the The cyanide and base metals are eluted solution with a relatively high CN:Cu ratio tailings can either be treated directly, as from the resin into a small, concentrated (>4). The eluate is then treated in an in the Cyanisorb and SART processes, eluate stream, which is processed to electrowinning cell to plate copper metal or they can be processed with an ion remove metals as sulphide complexes, and regenerate cyanide. The oxidation exchange resin prior to recovery, as in and then recycled directly to leach after of cyanide in the electrowinning cell is the Hannah and AuGMENT processes. neutralization with lime. The eluted resin avoided by pouching the anodes in ion The Hannah and AuGMENT processes is regenerated with acid and recycled to exchange membrane bags. Caustic is upgrade the cyanide strength by the adsorption. pumped into the bags to neutralize the concentrating effect of loading onto The Hannah process is particularly well acid generated at the anode. A bleed the resin, while the Cyanisorb process suited to the treatment of solutions stream of electrolyte/eluate is acidified to upgrades the cyanide strength by the containing copper cyanide, but it can also precipitate and remove copper (as CuCN) AVR process. The SART process recycles handle free cyanide, other metal cyanide and regenerate free cyanide for recycle cyanide without upgrading. complexes, and even the thiocyanate to leaching. The partially eluted resin is anion. regenerated with acid and recycled to DIRECT TREATMENT OF CYANIDATION The Hannah Process was developed by adsorption. TAILINGS SGS’ metallurgical group and John A. The AuGMENT process was developed Thorpe of Thorpe Consulting. by SGS’ metallurgical group and DuPont Free Cyanide Advantages Corporation. If the cyanide is present in the tailings as free or weakly-complexed cyanide (pKa •• Process treats solutions or pulps with Advantages = 9.4), more than 99% of the cyanide is equal efficiency. •• Treats solutions or pulps with equal converted to HCN gas by simply lowering •• The efficiency of recovery of free efficiency. the pH of the tailings to about 7. The cyanide and WAD cyanide complexes •• The efficiency of cyanide recovery cyanide is then recovered by air-stripping in tailings is very high (95 to 99%). from free cyanide and copper cyanide of the HCN gas from the tailings into an •• No HCN volatilization, resulting in a complexes in tailings is very high (95 alkaline scrubber. safer working environment. to 99%). •• Relatively simple to engineer and •• No HCN volatilization, resulting in a CN- + H+ ↔HCN (1) operate. safer working environment. •• Valuable by-products can be •• Valuable by-products can be generated. generated. •• No water-balance problems. •• No water balance problems. SGS MINERALS SERVICES – T3 SGS 019 4

Zinc, Copper, Nickel of equation (4), it can be seen that the the formation of cuprous sulphide (or If, however, the cyanide is present as a ferrocyanide molecule releases the third synthetic chalcocite) is favored because metal cyanocomplex, the pH must be molecule of CN from the copper tricyano of its extremely low solubility (Ksp = 2.3 reduced to more acidic values (which vary complex, so the presence of ferrocyanide x 10-48). The following reaction takes with the strength of the complex) to break results in increased recovery of cyanide place: from the copper cyanide species. The 2– down the complex and produce HCN gas. 2Cu(CN) 3 + S2- + 6H+ → Cu S + analogous zinc-iron double metal cyanide 2 The weak zinc cyanide complex (log β4 = complex, Zn Fe(CN) , is formed under 6HCN (6) 17.4), for example, dissociates completely 2 6 at about pH 5, producing zinc sulphate acidic conditions when zinc tetracyanide This reaction is irreversible at low pH (pH and 4 moles of HCN. Once again, cyanide and ferrocyanide are present in solution. <4) and takes place quantitatively with is recovered by stripping the HCN gas Copper and Thiocyanate stoichiometric additions of sulphide ions from the tailings in a stream of air. and acid. Reaction (6) forms the chemical 2– When both copper cyanide and 4 basis of the SART process. Zn(CN) + 2H2SO4 → ZnSO4 + 4HCN (2) thiocyanate anions are present in the Resin-Based Treatments of Cyanidation The copper cyanide complex does not cyanide leach solution, which frequently Tailings break down completely even in strong occurs during leaching of gold ores that acid solution, unless there is an oxidant contain reactive copper sulphide minerals, It is often inconvenient, costly or present in the solution. the following reaction takes place in inefficient to recover cyanide directly from addition to reactions gold plant tailings. This is the case if the In the absence of oxidant, the tricyano tailings pulp is difficult to separate into copper species (which is the most stable (1), (3) and (4): 2– solid and liquid components (as occurs copper complex under normal cyanidation 3 Cu(CN) + SCN- + 3H+ → CuSCN + with high clay, slimy, viscous ores) and conditions: log β3 = 28) decomposes to 3HCN (5) also if the tailings solids are high acid- CuCN precipitate plus two moles of HCN, The solubility product of the CuSCN consumers. In these situations, direct at pH values less than 3. Hence, a third of species (Ksp = 1.8 x 10-13) is higher than acidification of the tailings results in the potentially recoverable cyanide is lost that of the CuCN species (Ksp = 3x10- high operating costs (due to high acid to the precipitate. 2– 20), which means reaction (3) should consumption) and inefficient cyanide and 3 Cu(CN) + 2H+ → CuCN + 2HCN (3) be favored over reaction (5). However, copper recovery (due to the loss of CuCN The nickel tetracyanide complex (log β4 = at low pH values (pH ≤2) and low HCN precipitate to the tailings). Air stripping 30) also requires a pH value of less than concentrations (as in the AVR process), of HCN from slurry is also inefficient, 3 to dissociate and liberate HCN gas, in a reaction (5) is favored over reaction (3). requiring a relatively long residence reaction equivalent to equation (2). There are several important advantages time in the stripping column and high air of a process utilizing the reaction (5) flowrates. Iron and Cobalt chemistry over that shown in reactions (3) Anion exchange resins can play a very The very strong ferrocyanide (log β6 and (4): valuable role in these situations. The = 35), ferricyanide (log β6 = 44) and •• All three molecules of CN per resin in pulp process is a well-developed, cobalticyanide complexes (log β6 = 64) do molecule of copper are released industrial process, which presents the not break down at all, even in strong acid for recovery and recycling, versus opportunity to recover cyanide directly solutions, without destroying the cyanide. two molecules of CN in the CuCN from pulp tailings, circumventing solid/ Copper and Ferrocyanide process. liquid separation processes. Moreover, In situations where both ferrocyanide and •• The precipitate contains no cyanide, conventional, commercial strong-base cuprous cyanide are present in the same and thus is more acceptable as resins are well-suited to this application. cyanide leach liquor (which frequently smelter feed. The most common cyanide species occurs with high cyanide-consuming •• There is a bleed for thiocyanate. in gold plant tailings are free cyanide ores), treatment of the solution with Otherwise, this reports to the anions (usually 100-500 mg/L) and acid to pH <4 produces a double metal final tailings and could become an the cyano complexes of copper, zinc, environmental issue. cyanide precipitate such as Cu2Fe(CN)6 or iron and sometimes nickel. The free

Cu4Fe(CN)6. 2– 4– Copper and Sulphide cyanide anion has a very low affinity for 3 6 4Cu(CN) + Fe(CN) + 12H+ → As an alternative to reactions (4) and (5), anion-exchange resins, and can only be extracted efficiently after all the metal Cu4Fe(CN)6 + 12HCN (4) for situations where there is insufficient cyanide complexes plus most other The Cu Fe(CN) species is formed under ferrocyanide or thiocyanate in solution 4 6 anions (especially thiocyanate) have been oxygen-starved conditions in solution, (Cu>Fe, SCN), sulphide ions can be extracted. However, free cyanide can be whereas Cu Fe(CN) is formed in a well- added to the cyanide leach solution to 2 6 efficiently extracted if it is pre-complexed aerated solution. From the stoichiometry achieve the same objective. In this case, with a metal ion such as zinc (or copper) SGS MINERALS SERVICES – T3 SGS 019 5 prior to ion exchange, as discussed below. However, cyanide recovery via an ion or recycling. exchange process will produce superior The ability of anion exchange resins to ZnSO4 + Ca(OH)2 → Zn(OH)2 + CaSO4 efficiently extract copper, zinc and nickel economics in most cases. (10) cyanide, as exemplified in the adsorption Zinc cyanide loads onto a strong base This process chemistry is also very readily isotherms shown below, presents an anion exchange resin via the following applied to free cyanide recovery. The opportunity to recover these complexes stoichiometry: zinc hydroxide recovered by neutralization when they are present in the tailings of a + 2– 2– N 4 4 of the barren regenerant (equation 10) gold plant. (® - R3)2 SO + Zn(CN) → + 2– 2– can be recycled back to the feed to the Nickel cyanide is uncommon in gold plant (® - N R3)2Zn(CN)4 + SO4 (7) ion exchange plant (i.e. the gold plant tailings, while cobalt and iron cyanides where the symbol ® represents the resin barrens), where it will readily react with form such stable complexes that it is matrix. The process is very efficient free cyanide ions to produce the zinc impossible to break them down without cyanide complex: because of the great affinity of strong 2– simultaneously destroying the cyanide 4 base resins for the zinc cyanide complex Zn(OH)2 + 4CN- → Zn(CN) + 2OH- (11) ion. Thus, these complex anions can (Figure). Resins are capable of loading be efficiently extracted from gold plant The process chemistry then follows to capacity (30-40 g/L zinc, 50-65 g/L equations 8 to 11, and the economics are tailings by ion exchange, making the cyanide) from quite dilute zinc cyanide tailings amenable for direct discharge to the same, as zinc is not consumed in the solution (<100 mg/L Zn), and are able process. the environment, but they do not yield to produce very low levels of cyanide in their cyanide for recycling. the final effluent (<1 mg/L). It should be The only reagents consumed are Zinc Cyanide and Free Cyanide Recovery apparent from the stoichiometry shown sulphuric acid (0.5 mole per mole cyanide recovered, or 1 kg H SO /kg NaCN) and in equation (7) that each resin functional 2 4 Zinc cyanide is found in many gold leach lime (0.5 mole per mole cyanide, or 0.75 circuits, particularly those employing group carries two cyanide ions, versus kg Ca(OH)2/kg NaCN). Merrill Crowe for final gold recovery. only one CN per functional group if Although zinc cyanide is a weak complex cyanide were loading as the free cyanide Copper Cyanide Recovery ion. (log β4 = 17.4) and yields its cyanide ions Ion exchange resins offer a unique to the stronger aurocyanide complex (log To elute the zinc cyanide, the loaded opportunity to not only recycle the β2 = 38.3) during gold leaching, the build resin is treated with sulphuric acid, which cyanide associated with the copper up of zinc must be controlled to minimize breaks down the zinc cyanide complex complex, but to also generate revenue via the total cyanide concentration in the completely, producing zinc sulphate and the sale of the copper itself. This revenue circuit. HCN gas in solution. If the acid solution will, in many cases, offset the operating (100-150 g/L H SO ) and the resin are costs of the cyanide recovery plant, This is usually done by taking a bleed 2 4 stream to a cyanide destruction plant. moved countercurrent to one another, it is and the technology has the potential to possible to produce a strong HCN solution transform uneconomic ore bodies into (>5% HCN) containing no excess acid (pH exciting opportunities. >5). + 2– N 4 (® - R3)2Zn(CN) + 2H2SO4 → + 2– N 4 (® - R3)2SO + ZnSO4 + 4HCN (8) The HCN gas is volatilized from the regenerant solution in a stream of air, which is scrubbed in lime or caustic solution to recover the cyanide as the usable free cyanide ion. Because of the high concentration of HCN in the regenerant solution, the size of the stripping column will be relatively small compared to an AVR plant treating tailings solution directly. Figure 2: The equilibrium loading of various 2HCN + Ca(OH)2 Ca(CN)2 + H2O (9) metal cyanide complexes from a plant leach The barren regenerant solution can liquor onto a strong-base resin, A101Du. also be treated with lime or caustic to precipitate zinc hydroxide for sale, discard SGS MINERALS SERVICES – T3 SGS 019 6

The dominant copper cyanide species out on copper cyanide that is loaded onto exchange resin, by essentially maintaining in cyanide leach solutions is the tricyano an anion exchange resin, the two moles the concentration of copper in the resin at complex, which has a great affinity of cyanide are liberated, but the CuCN steady state. However, these processes for strong base resins. The loading precipitate remains trapped in the resin were unable to handle copper cyanide mechanism is the same as that shown pores, and net copper elution efficiency in the feed, as they did not incorporate above for the zinc cyanide anion: is zero. a cost-effective method of eluting + 2– 2– + 2– N 4 3 N 3 copper from the resin. This problem has (® - R3)2SO + Cu(CN) → (® - R3)2Cu(CN) + H2SO4 → + 2– + 2– 2– now been overcome in the AuGMENT N 4 (® - N R3)2Cu(CN)3 + SO4 (12) (® - R3)2SO (CuCN) + 2HCN (13) process, wherein copper cyanide is and the capacity of a typical, fully-loaded This fairly unique chemistry has been eluted from the resin with a strong copper resin for copper (30-40 g/L) and cyanide turned to advantage in the AuGMENT cyanide solution at a relatively high CN:Cu (40-50 g/L) is also similar. process, wherein CuCN is intentionally molar ratio (~4:1). Elution is coupled with precipitated in the pores of a conventional electrolysis, and the AuGMENT process However, when an acid-stripping reaction strong base resin and, in this form, produces cathode copper. analogous to that shown in equation (8) produces a regenerated resin that is an is carried out, the copper remains in the efficient adsorbent for free cyanide ions resin phase. The reason for this is the and soluble copper cyanide. fact that the stable precipitate CuCN is formed (equation 3), and only two Similar processes had been developed moles of cyanide are released per mole previously to recover free cyanide with of copper. When this reaction is carried CuCN precipitated in the pores of an anion