SGS MINERALS SERVICES TECHNICAL PAPER 2003-02 2003

THE ECONOMIC AND ENVIRONMENTAL CASE FOR RECOVERING CYANIDE FROM GOLD PLANT TAILINGS DR. CHRIS A. FLEMING, PH. D. –– SGS

ABSTRACT The ability to recover cyanide from gold plant tailings has been known almost as long as the cyanidation process has been practiced, but there has been little incentive to process tailings in this way in the past. With the development of technologies that make it possible to recover and recycle free or complexed cyanide directly from gold plant slurry tailings, which indicates, on paper at least, significant cost savings compared to cyanide destruction, coupled with the introduction of legislation to seriously limit the discharge of cyanide to the environment in most gold-producing regions of the world, many companies are evaluating the cyanide recovery alternative.

There is arguably an even more compelling reason for mining companies to consider cyanide recycling. With the growing storm of negative public opinion that the use of cyanide in the mining industry is attracting – following several highly publicized spills over the last couple of years – the time has come for the gold industry to demonstrate environmental diligence and stewardship in the use of this commodity that is so vital for their industry. There is no doubt that the widespread implementation of cyanide recycling will reduce the impact of the cyanidation process on the environment, both by reducing the risk of spills (with less cyanide being transported from manufacturing plants to gold mines), and by reducing the loading of toxic and nontoxic metals and ions in the tailings.

This paper briefly discusses the pros and cons of the different processes that are available for the treatment of gold plant tailings to regenerate and recovery cyanide for recycling, and presents a technoeconomic argument for the incorporation of this technology into many gold plant flowsheets today.

INTRODUCTION of the general public for the gold form in which it exists in gold plants, mining industry, due to several highly to hydrocyanic gas, HCN. The negative publicized cyanide spills in recent years perception of this highly toxic gas, Interest in the recovery of cyanide (breaches of tailings dams, accidents and concerns about the ability of the from gold and silver plant tailings has involving trucks carrying cyanide to gold mining industry to handle it safely in heightened in recent years. This plants, etc.). a processing plant, has probably been interest has been spurred by several the single largest impediment to the factors: The cost of recovering and recycling implementation of cyanide recovery • Increasingly stringent regulations cyanide from tailings will generally throughout the gold producing world, technology in the gold industry. In this be lower than the cost of purchasing governing discharge limits for free and regard, the mining industry can gain new cyanide. When this cost benefit total cyanide to tailings ponds and the confidence and experience from the is added to the lower cost of tailings environment. chemical industry, which manufactures detoxification, a strong case can • The increasing cost of active chemical sodium and calcium cyanide for the gold frequently be made for cyanide recovery. treatment versus passive (natural industry, and handles large quantities of degradation) treatment of tailing, to HCN gas routinely and safely. In most cases, cyanide is recycled by bring them in line with environmental converting it from the free or complexed regulations. • The worldwide trend to processing of more complex gold ore bodies, which is usually accompanied by higher levels of cyanide consumptions and greater concentrations of cyanide (particularly the copper cyanide complex) in tailings. • The growing negative perception SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 2

If the cyanide is present in the tailings scaling problems, such as encountered ion exchange is based on the water- as free cyanide (pKa = 9.4), it is possible at DeLamar Mine in Idaho, USA and balance in the plant. If preconcentration to convert >99% of the cyanide to HCN more recently at the Cerro Vanguardia is unnecessary, or is achieved by ion gas by lowering the pH of the tailings to plant in Argentina (Radcliffe, 2000). exchange, circumventing volatilization about 7. • Operating costs are lower for cyanide results in a simpler, safer and lower-cost recovery plants treating solution than plant. CN- + H+ <-> HCN (1) those treating pulp. The main operating cost is the sulphuric acid consumed Most of the process options for cyanide If the cyanide is present as a metal in lowering the pH to the desired final recovery have been tested at SGS cyanocomplex (Cu,Zn,Ni,Co etc), the pH pH value. When treating solutions, this Lakefield Research over the last few must be reduced to more acidic values consumption is close to stoichiometric years, in the course of several laboratory (which vary with the strength of the (i.e. half a mole of sulphuric acid per and pilot plant investigations. Cyanide complex) to break down the complex mole of cyanide). When treating pulp, tailings solutions from several operating and produce HCN gas. The addition of the acid consumption can be 2 to 10 gold plants and new exploration sulphide ions enhances this reaction by times higher, depending on the final properties have been tested, and the forming metal sulphide precipitates. The pH required and the acid-consuming results of these investigations were chemistry of cyanide recovery from constituents in the ore. published recently (Fleming, 2001). The metal cyanide complexes has been • When the main source of cyanide discussion below is drawn from these reviewed recently by Fleming (2001). consumption is the reaction to form experiences. copper cyanide complexes in solution PRACTICAL CONSIDERATIONS (as is frequently the case), an added PROCESS ALTERNATIVES benefit of treating solution rather An important consideration when than pulp is the opportunity afforded DIRECT RECOVERY WITHOUT evaluating process options for the to easily recover and sell the copper PRECONCENTRATION, BY TAILINGS precipitate. The sale of these by- treatment of the gold or silver plant RECYCLING tailings for cyanide recovery, is whether products can have a significant positive In order to ensure good leaching kinetics to treat the tailings directly as a slurry, impact on cyanide recovery economics. and high overall gold recovery, it is or to first separate the solids and liquids always necessary to add more cyanide and treat only the liquid phase. In making Despite the many positive attributes during leaching than will be consumed in this decision, the following factors must of treating solutions rather than pulps, the leaching process. This excess cyanide be taken into consideration: the cost and efficiencies of solid/ reports to the tailings as uncomplexed • The cost of solid/liquid separation. liquid separation will render cyanide or free cyanide, and it is often possible If the solids and liquids have already recovery from solution unattractive in to recycle a portion of this cyanide at been separated (in Merrill Crowe or some cases. In these situations, resin minimal cost. A number of gold plants heap leaching, for example) this is not a in pulp processes offer the best solution around the world have adopted this factor. If they have not been separated, and examples of this technology are approach. the cost of solid/liquid separation discussed below. must be included as part of the cost The basic requirements to achieve this to recover cyanide, and this can be In fact, ion exchange technology objective are firstly to recycle solution significant, particularly for high-clay, should always be considered, even rather than slurry, and secondly, to slimy or viscous slurries. In most cases, when treating solutions for cyanide satisfy an overall water balance in the the washed solids will still have to be recovery, as the resin processes will plant. These requirements are met quite treated by a detoxification process usually upgrade the strength of the naturally in a heap leach operation, where to remove residual soluble cyanide cyanide solution 20 to 100 fold prior to the residual cyanide in the pregnant species. acidification/precipitation. This in turn will solution emerging from the bottom of • Cyanide recovery by direct acidification lower the overall capital cost and improve a heap is recovered by simply recycling of tailings, followed by volatilization safety in the final cyanide regeneration this solution (after gold and/or silver and reneutralization of the HCN gas steps of the process. In many cases, the recovery) back to the top of the heap. It (known as the AVR process) is very cyanide strength in the eluate from an is not so simple in a milling operation, much faster and more efficient from ion exchange process will be sufficiently but some of the residual free cyanide solution than from pulp, requiring lower high for direct recycling to leach, can be recovered by thickening the mill volumes of air flow per unit volume of circumventing the need for volatilization slurry prior to leaching, and again tailings treated and smaller equipment. of HCN (AVR). thickening the leach plant tailings prior to Capital costs per unit volume of tailings discharge. For example, if the feed and feed are therefore lower for an AVR The volatilization part of the AVR discharge from cyanidation/gold recovery plant treating solution than for one process is a relatively high capital cost can be thickened to 60% solids, and the treating pulp. In addition, attempts to component of the overall process. leach/gold recovery operation is implement the AVR process on tailings The decision on whether or not to conducted at 40% solids, the net result slurry have been dogged by severe preconcentrate by volatilization or is that about half of the free cyanide in SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 3

the tailings is recovered and recycled. This is depicted schematically in Figure 1.

Pros • The process is very simple with relatively minor capital cost (two thickeners, larger leach and CIP tanks) and minimal operating costs. • The process does not require conversion of free cyanide to HCN gas. Cons • It is difficult to recover more than ~50% of the free cyanide in the tailings. The remaining tailings must still be treated by a detoxification process prior to discharge to the environment. • Without additional processing, the cyanide present in tailings as metal complexes is not recovered.

DIRECT RECOVERY BY THE SART PROCESS The SART process was developed by SGS Lakefield Research and Teck Corporation (MacPhail et al., 1998), Figure 1. Simplified flowsheet for direct recovery of free cyanide from gold plant tailings and is similar to the previous process, in that there is no pre-concentration or more. A schematic flowsheet is and ~20% S. There was always minimal of cyanide. SART should be applied shown in Figure 2. cyanide in the precipitate (<0.15%). in those situations where there is a • Ferrocyanide and thiocyanate are not significant concentration of copper (or The basic chemistry of the SART process precipitated at all, and build up in the zinc) as weak-acid-dissociable (WAD) is identical to the MNR process, which recirculating solution phase. They can cyanide complexes in the tailings (or was extensively tested in the 1980s be precipitated from a bleed stream recirculating heap leach liquor). Metals (Potter et al., 1986). The fundamental by lowering the pH to about 2 (without are precipitated from solution by difference between the two processes sulphide addition). addition of acid (typically to ~pH 4-5) lies in the physical handling of the • The copper precipitation reaction rate and sulphide ions (Fleming, 2001). The precipitate. In the MNR process, the is fast (<5 minutes) and the precipitate precipitate is recovered by thickening Cu2S precipitate is pumped directly as flocculates readily (with appropriate and filtration, and the liquor is neutralized a low-density slurry from the primary flocculant addition) and settles rapidly, and recycled to leach. The concentration reactor to a pressure filter. In the SART producing a fairly dense thickener of free cyanide in the recycle solution is process, the volume of slurry reporting underflow (~10% solids) and clear essentially the same as the combined to the pressure filter is decreased by up overflow. concentrations of free and WAD cyanide to 99%, greatly reducing filter plant costs in the feed to the SART process. and improving the safety aspects of this The pros and cons of the SART potentially hazardous unit operation. Both process are essentially the same as The solids density in the primary reactor the MNR and SART processes have been the direct recycling approach, with the will usually be very low (in the 0.05 to piloted, and a full scale SART plant has added advantage that revenue can be 0.5% range), and therefore it is beneficial recently been built and commissioned at generated from the sale of high-grade to thicken the precipitate prior to Newcrest’s Telfer operation in Western copper (or zinc) sulphide precipitates. filtration. This also affords the opportunity Australia. to pump a portion of the thickener DIRECT RECOVERY BY THE AVR PROCESS underflow back to the primary reactor, The experience gained during several The earliest experience in the mining where it can act as seed material for new laboratory programs and one pilot plant industry with cyanide recovery from precipitate formation. With appropriate SART campaign can be summarized as tailings was the AVR process, which recycling and flocculation, solids follows: was practiced at the Pachuca silver mine densities of 5-15% can be produced in • When the WAD cyanide complex in the in Mexico and at the Flin Flon Mine in the thickener underflow, which results in tailings is copper (which was generally Manitoba, Canada (Davis, 1946; Flin Flon the volumetric flowrate to the pressure the case in these projects), the SART Mill Staff, 1946) more than 50 years ago. filter being reduced by a factor of 100 process produces a precipitate of The process has been installed more

almost pure Cu2S, analyzing ~70% Cu recently at the Golden Cross Mine in SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 4

New Zealand, the DeLamar Mine in Idaho, USA, the Morro de Ouro Mine in Brazil and the Cerro Vanguardia Mine in Argentina, although only the two South American mines are currently operating.

The AVR process involves acidification of the gold plant tailings with sulphuric acid, to lower the pH from ~10 to less than 7, usually to pH 3 to 5. The process can be applied to solutions or pulps, although solution treatment is preferred in most cases for the reasons outlined above. During acidification, free cyanide and weakly complexed cyanide (complexes of Zn, Cd, Ni, Cu) are converted to HCN, which is then volatilized by passing a vigorous stream of air bubbles through the tailings pulp or solution. This operation is performed in a tall tower, which is baffled with plates or packed with inert media to improve gas/gas Figure 2. SART process flowsheet contact. • The pH of the acidification process stripping tower. The air/HCN gas stream is scrubbed in must be reduced to <3 if the tailings • The process is not well suited to a caustic solution in a second reactor contain significant amounts of copper treatment of slurries because of (a) tower to convert the HCN back to free cyanide complexes. It is not possible to high acid consumption, (b) exceptionally cyanide ions for recycling. Scrubbing in recover any cyanide that is associated slow HCN stripping rates and (c) the a lime scrubber has been attempted, with ferrous or ferric cyano complexes. potential for severe scaling problems. but has not been installed at any of • Acid consumption is close to • In practice it is difficult to achieve low the operating plants, mainly owing to stoichiometric based on total cyanide values of residual cyanide in the treated concerns about scaling. It is possible in solution, when treating solutions. solution, and supplementary cyanide to build up the cyanide concentration in In one campaign where solutions and detoxification may be required. the scrubber to the solubility limit of the pulps from the same operation were sodium or calcium cyanide salt, although tested, acid consumption was 5 times INDIRECT RECOVERY WITH a practical limit of ~100 g/L CN is usually higher for pulp treatment. The rate PRECONCENTRATION BY ION EXCHANGE used. The process is described in detail of HCN stripping from pulp was also RESINS by Riveros et al. (1993). significantly slower than from solution. It is often inconvenient, costly or inefficient to recover cyanide directly The experience gained from three Pros from gold plant tailings. This is the case laboratory/pilot campaigns conducted • The AVR process has been studied if the tailings pulp is difficult to separate at SGS Lakefield Research, treating and practiced for many years and the into solid and liquid components (as widely divergent feed materials, can be practical and theoretical aspects are occurs with laterite, high-clay, slimy or summarized as follows: well understood. viscous ores) and also if the tailings • The rate of conversion of free and • It is the only process involving HCN solids are high acid-consumers. In these complexed cyanide to HCN, as well generation that has been applied situations, direct acidification of the as precipitation of metals, is rapid successfully in the mining industry, tailings results in high operating costs (<10 minutes), whereas the rate of with a wealth of experience gained (due to high acid consumption) and stripping of HCN from the solution to from several operating plants around inefficient cyanide and copper recovery the gas phase is slow (several hours for the world. (due to the loss of CuCN precipitate to complete stripping). The HCN stripping • In cases of high cyanide concentration the tailings). Air stripping of HCN from rate can be significantly enhanced by in the tailings, the process has lowered slurry is also inefficient, requiring long heating the tailings, although this cost the overall operating costs in those residence time in the stripping column would be prohibitive in most cases. plants where it has been installed. and high air flowrates. • Given sufficient residence time in the Cons HCN stripping reactor, the AVR process • Capital costs are higher than the Anion exchange resins can play a very is capable of producing final effluents alternatives, mainly due to the slow valuable role in these situations. The that are very low in cyanide and metals rate of HCN volatilization, and the (CN , Cu, Fe <1 mg/L). resin in pulp process is welldeveloped TOT resultant large size of the packed SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 5 in the mining industry for other applications (uranium, gold recovery), and presents the opportunity to recover cyanide directly from pulp tailings, circumventing solid/liquid separation processes. Moreover, conventional commercial strong-base resins are well- suited to this application.

The most common cyanide species in gold plant tailings are free cyanide anions (usually 100 – 500 mg/L) and the cyano complexes of copper, zinc, iron and sometimes nickel.

The free cyanide anion has a very low affinity for anion-exchange resins, and can only be extracted efficiently after all the metal cyanide complexes plus most other anions (especially thiocyanate) have been extracted. However, free cyanide can be efficiently extracted if it is pre-complexed with a metal ion such as zinc (or copper) prior to ion exchange, as discussed below.

The ability of anion exchange resins to Figure 3. The equilibrium loading of various metal cyanide complexes from a plant leach liquor onto a efficiently extract copper, zinc and strong base resin, A101Du (Fleming and Cromberge, 1984) nickel cyanide, as exemplified in the Figure 3. adsorption isotherms shown in Figure 3, presents an opportunity to recover these Zinc cyanide loads onto a strong base anion exchange resin via the following complexes when they are present in the stoichiometry: tailings of a gold plant.

2− 2− 2− 2− (® - N+R3)2 SO4 + Zn(CN)4 –> (® - N+R3)2Zn(CN)4 + SO4 (2) Nickel cyanide is uncommon in gold plant tailings, while cobalt (III) and iron where the symbol ® represents the resin matrix. The stoichiometry of equation 2 cyanides form such stable complexes shows that each resin functional group effectively loads two cyanide ions, so not that it is impossible to break them down only does the pre-complexation reaction convert cyanide from the poorly loading free without simultaneously destroying the cyanide form to the strongly loading zinc cyanide complex form, it also doubles the cyanide ion. Thus, these complex anions total loading capacity of the resin for cyanide ions. Resins are capable of loading to can be extracted from gold plant tailings their theoretical capacity (30-40 g/L zinc, 50-65 g/L cyanide) from quite dilute zinc by ion exchange, making the tailings cyanide solution (<100 mg/L Zn), and are able to produce very low levels of cyanide in amenable for direct discharge to the the final effluent (<1 mg/L). environment, but they do not yield their cyanide for recycling. To elute the zinc cyanide, the loaded resin is treated with sulphuric acid, which breaks down the zinc cyanide complex completely, producing zinc sulphate and HCN (a) Loading Enhancement by Zinc gas in solution. If the acid solution (100-150 g/L H2SO4) and the resin are moved Complexation countercurrent to one another, it is possible to produce a strong HCN solution (>5% Zinc cyanide is found naturally in many HCN) containing no excess acid (pH >5). gold leach circuits, particularly those employing Merrill Crowe for final gold 2− 2− (® - N + R3)2Zn(CN)4 + 2H2SO4 –> (® - N + R3)2SO4 + ZnSO4 + 4HCN (3) recovery. In addition, a zinc salt can be added to the leach tailings to pre- It is necessary to separate the ZnSO4 and HCN in the strip liquor, so that the zinc can complex all of the free cyanide as the be recycled to the pre-complexation reaction. This is done by AVR. The HCN gas is zinc cyanide WAD complex. Although volatilized from the regenerant solution in a stream of air, which is scrubbed in lime or this complex is the weakest of the WAD caustic solution to recover the cyanide as the usable free cyanide ion. Because of the 2− cyanide complexes (Zn(CN)4 , log β4 high concentration of HCN in the regenerant solution, the size of the stripping column = 17.4), it has a very strong affinity for will be relatively small compared to an AVR plant treating tailings solution directly. anion exchange resins, as shown in SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 6

2HCN + Ca(OH)2 –> Ca(CN)2 + H2O (4)

The barren regenerant solution can be recycled directly to the pre-complexation reactor, or it can be treated with lime or caustic to precipitate zinc hydroxide for sale, discard or recycling. A simplified flowsheet of this process is shown in Figure 4.

The only reagents theoretically consumed are sulphuric acid (stoichiometric consumption: 0.5 mole per mole cyanide recovered, or 1 kg

H2SO4/kg NaCN) and lime (stoichiometric consumption: 0.5 mole per mole cyanide, or 0.75 kg Ca(OH)2/kg NaCN).

A fairly basic operating cost comparison of recycling cyanide (from the free cyanide or zinc cyanide form) versus cyanide destruction is made in Table 1, from which it is apparent that recycling Figure 4. Simplified flowsheet for cyanide recovery from gold tailings slurry by ion exchange with zinc will be economically attractive in most pre-complexation cases. In addition, the various unit Table 1. Operating cost comparison: Cyanide recycling versus cyanide destruction operations needed for a cyanide recovery plant (by ion exchange) require simple Cost of Recycled Cyanide equipment, operated under ambient (per kg NaCN) $ / kg conditions. It is expected that the capital H2SO4 consumed at 1.2 kg H2SO4 per kg NaCN (20% cost will be relatively low, with a payback excess assumed) 0.14 – 0.20 time significantly less than the rest of Lime consumed at 0.9 kg per kg NaCN (20% excess the gold plant. assumed) 0.14 – 0.20 Resin and zinc losses, labour, power and maintenance Pros (range based on size of plant) 0.20 – 0.50 • The process can be applied to solutions Total or slurries, circumventing the need for $ 0.48 – 0.90 solid/liquid separation in the case of Cost of New Cyanide milling operations. (per kg NaCN) • It is possible to recover all the free and WAD cyanide in the gold plant tailings Purchase Price of New Cyanide 1.20 – 1.50

without upsetting the water balance Cost of cyanide destruction (quoted by Inco for SO2 in the plant, and without the need for process) 0.65 – 2.40 ## supplemental detoxification of the final Total $ 1.85 – 3.90 tailings. ## The wide range of costs for the Inco process takes into consideration variable CN- in the tailings (in • The AVR process is applied to a small, the range 100 to 800 mg/L CN) and the variable cost of different SO2 sources. Other CN destruction highly concentrated solution of HCN, processes are generally more expensive than the Inco process. and it is therefore potentially safer than must be carefully designed and engineered. direct AVR on tailings, and will be more efficient and rapid. The process should always be considered for the treatment of gold plant tailings Cons in which the recovered cyanide is present in the form of free cyanide or the zinc • No commercial installations have been cyanide complex. Non-recoverable forms of cyanide (ferrocyanide and thiocyanate, for built yet. example), do not load as well as zinc cyanide on the resin, and can be rejected to the • The process is more complex than tailings if desired. If the tailings contain significant amounts of WAD cyanide in the direct recovery from tailings (without form of the copper cyanide complex, the zinc process is not ideal, and the Hannah preconcentration of the cyanide). process described below should be considered. • Handling of concentrated HCN in a small AVR plant is potentially (b) Loading Enhancement by the Augment Process hazardous, and this part of the plant The Augment process was developed by Lakefield Research and the DuPont SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 7

Corporation (Fleming et al., 1995) to (c) Loading Enhancement by the Hannah 20:1 to 100:1. recover copper and cyanide in gold plant Process • Total volumetric resin inventory in the tailings, by an ion-exchange resin based The Hannah process (patents pending) plant (loading, elution and regeneration process. was also developed at Lakefield is expected to be in the range 0.5 to 2 The chemistry involved in the various unit Research, in collaboration with John A. times the volume of tailings solution operations of the Augment process was Thorpe. Like the Augment process, it processed per hour. described recently by Fleming (2001). was developed with the treatment of • Resin elution is done at ambient copper cyanide-bearing gold plant tailings temperature, and the hold up time is The process was extensively tested at in mind, but is able to efficiently recover about 1 hour. Lakefield Research in the 1990s, and it free cyanide and the cyanide associated • Resin regeneration is preferably done was demonstrated that copper cyanide with other WAD complexes as well. in columns, in a residence time of and free cyanide could be extracted from The process is based on extraction with ~2 hours. The regenerant is alkalized gold plant tailings to low barren solution anion exchange resins, so can also be to produce a concentrated Ca(CN)2 values (<10 ppm), in a continuous applied to solution or slurry tailings. solution for direct recycling to leach. process that produced concentrated cyanide solution (~100 g/L NaCN Whilst the details of the process cannot The Hannah process is expected to equivalent) and ~99.9% copper cathodes be published at this stage, since the produce similar favourable economics as products. Consumption of acid and application for patents is still under as depicted in Table 1 above, and is base were almost stoichiometric (~10% review, the following brief outline can currently being evaluated by two mining excess), as in the zinc cyanide (or free be used to size major equipment and to companies. cyanide) case above. The economics calculate reagent consumptions: of this process could, in most cases, • The process uses conventional, large- Pros produce slightly more favourable bead, strong base resins. • The process enjoys the same economics of cyanide recovery versus • The main purpose of the process is to advantages as the Augment process, cyanide destruction (because of the recover cyanide, but it can also recover with the added advantage of being additional revenue from copper sales) copper, zinc and thiocyanate (as a simple to understand, optimize and than those depicted in Table 1, for the concentrated solution or as a CuSCN operate. zinc pre-complexation process. precipitate). If desired, thiocyanate can • Unlike the zinc enhancement process, be rejected by the process, and report the Hannah process is able to treat The Augment marketing company has to the tailings. tailings that contain both copper and recently been dissolved, and the rights to • The optimum cyanide removal zinc as WAD cyanide complexes, and this technology are held by the University efficiency will vary with pulp produce separate ZnS and Cu2S by- of Reno in Nevada, USA. composition, environmental regulations products. and local costs, but will normally be Cons Pros >90%. • No commercial installations have been • The same advantages as the process • The preferred reagents for the process built yet. involving enhancement by zinc are: - complexation, with the added benefit Consumption (mole/mole CN ENVIRONMENTAL, SOCIAL, of revenues from the sale of cathode recovered) HEALTH AND SAFETY BENEFITS copper. Reagent Low High OF CYANIDE RECYCLING Cons CN:Cu(1) CN:Cu(2) • No commercial installations have been Ca(OH) 1.0 0.6 The discussion above has centered on built yet. 2 the technical aspects of cyanide recovery • The process is more complex than H SO 0.8 0.6 2 4 and the economic benefits that should zinc cyanide enhancement. During NaSH 0.2 <0.05 flow from the implementation of this piloting at Lakefield Research, it was where (1) CN:Cu <5:1, and (2) CN:Cu technology. As shown, the favourable shown that the cyanide to copper >10:1. economics alone should persuade gold ratio is a key parameter that must • Resin flow will be 0.6 to 1.2 litres of mining companies to seriously examine be closely monitored and, in some resin per mole of cyanide recovered cyanide recovery versus cyanide cases, controlled, through all the (cyanide equivalent loading on resin = destruction in their mills. unit operations of loading, elution, 22 to 44 g/L). regeneration and electrowinning. The • Resin phase retention times in the There is, today, an even more compelling requirement for continuous, on-line loading tanks will be 1 to 2 hours. Two reason for gold mining company analysis of copper and cyanide in to four loading tanks in series will be executives to seriously evaluate the solution creates analytical challenges, normal. option of recovering and recycling and it is anticipated that the efficient • The volumetric ratio of tailings solution cyanide from gold plant tailings, and operation of an Augment process will flow to countercurrent resin flow that is the positive impact that this require a highly skilled workforce. will vary with the composition of the technology will have on the environment tailings, but will typically range from and on the perception of the gold SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 8 industry in the minds of the public, stewardship, Cyanide: Social, Industrial special interest groups, regulators and There are several processes available, and Economic Aspects (C.A. Young, L.G. governments. and the choice of the best technology Twidwell and C.G. Anderson, editors). will be based on factors such as: The Minerals, Metals and Metallurgical These less-tangible benefits of cyanide • The composition of the tailings. Society Annual Meeting, New Orleans, recovery can be summarized as follows: - • The form of the tailings (solution or pp 271-288. • The current ‘best’ practice is to oxidize pulp). cyanide in gold plant tailings to cyanate • The efficiency and cost of solid/liquid Flin Flon Mill Staff. (1946). Cyanide and (and sometimes ammonia), and separation. regeneration plant and practice at Flin precipitate heavy metals (plus strongly • The local environmental regulations Flon”, The Canadian Institute of Mining bound cyanide) in the tailings, before for final discharge of tailings and bleed and Metallurgy Transactions, Volume release into a tailings dam or aquifer. streams. XLIX, pp 130-142. Whilst this represents a significant • The availability of local expertise and improvement on the historical practice infrastructure. Goldblatt, E. (1956, 1959). Recovery of of releasing cyanide directly into tailings cyanide from waste cyanide solutions dams (and allowing air and sunlight In most cases, the cost of recycled by ion exchange. Ind. Eng. Chem., vol to slowly decompose the cyanide), cyanide will be less than one third the 48(12), p 2107 and vol 51(3), p 241. cyanide detoxification cannot be cost of new cyanide purchases, and considered a ‘clean’ technology. For capital investment to recover cyanide will MacPhail, P.K., Fleming, C.A., Sarbutt, example, salts such as sodium cyanate pay back in less than 2 years. K.W. (1998). Cyanide recovery by the and thiocyanate accumulate in aqueous ‘SART’ process for the Lobo- Marte solutions (and ultimately in the ground The implementation of this technology Project – Chile. Proceedings, Randol water) while heavy metals remain will reduce shipments of new cyanide to Gold Forum ’98, Denver, Colorado, pp in the tailings. Whilst those species mine metallurgical sites, reduce soluble 319-324. are less toxic than cyanide, cyanate salt loadings in tailings, and demonstrate represents a biological oxygen demand proactive environmental stewardship to Potter, G.M., Bergmann, A., Haidlen, U. and nitrogen loading in aquifers, while a public that has become increasingly (1986). Process of recovering copper ammonia is extremely toxic to aquatic hostile in recent years. and of optionally recovering silver and life. gold by leaching of oxide and sulphide - • Cyanide recycling reduces the need Cyanide recycle should be evaluated containing materials with water-soluble for new cyanide purchases, which in during project feasibility for all new cyanides. U.S. Patent No. 4,587,110. turn reduces the risk of cyanide spills gold operations, and should also be in transit to a mine site. In addition, the considered for retrofitting into many Radcliffe, P. (2000). Anglogold Corp., preferred reagent for transportation is existing plants. Personal Communication. solid sodium cyanide, which must be carefully handled during manufacture, REFERENCES Riveros, P.A., Molnar, R.E., McNamara, shipping and unloading at the mine site, V.M. (1993). Alternative technology to to avoid exposure of workers to toxic Davis, D. et al. (1946). Cyanide and decrease the environmental impact of dust. regeneration plant and practice at Flin gold milling. CIM Bull., pp 167-171. • Cyanide recovery technology will Flon. The Canadian Institute of Mining extend the life of mines by allowing and Metallurgy, Transactions, vol XLIX, pp mining companies to process lower 130-142. grade ores as well as ores that contain high cyanide-consuming minerals, Fleming, C.A., Cromberge, G. (1984). The such as copper sulphides and oxides. extraction of gold from cyanide solutions In several of the cyanide recovery by strong-and weak-base anion-exchange processes, copper is recovered and can resins. J. S. Afr. Inst. Min. Metall., vol generate extra revenue for the operator, 84(5), pp 125-137. instead of becoming the death knell of a mine, as is often the case today. Fleming, C.A., Grot, W., Thorpe, J.A. (1995). Hydrometallurgical extraction CONCLUSIONS process. U.S. Patent No. 5,411,575

Free and weak-acid dissociable cyanide Fleming, C.A. (2001). The case for are readily recoverable from gold and cyanide recovery from gold plant tailings silver plant tailings solutions or pulps. – positive economics plus environmental SGS MINERALS SERVICES TECHNICAL BULLETIN 2003-02 9

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