biogenic sulphide for cyanide recycle and copper recovery in gold–copper ore processing

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Minerals Engineering 21 (2008) 509–517

Biogenic sulphide for cyanide recycle and copper recoveryin gold–copper ore processing

Mike Adams a,*, Rick Lawrence b, Mike Bratty b

a BioteQ Water (Australia) Pty Ltd., P.O. Box 374, Guildford, WA 6935, Australiab BioteQ Environmental Technologies Inc., 1700-355 Burrard Street, Vancouver, Canada V6C 2G8

Received 20 October 2007; accepted 1 February 2008Available online 20 March 2008

Abstract

Many complex-ore gold deposits with cyanide-soluble copper minerals such as chalcocite, covellite, bornite, cuprite, malachite andazurite are now being considered for processing. In some cases the ore is not currently being mined largely due to the various issues asso-ciated with the cyanide-soluble copper minerals in the ore, which results in low gold recoveries and high cyanide consumption and detox-ification costs. Moreover, the presence of copper in tailings supernatant tends to stabilize weak-acid dissociable cyanide in a form toxic towildlife.

In the SART process (sulphidization–acidification–recycle–thickening), the cyanide associated with the copper cyanide complexes isreleased by NaHS dosing to precipitate copper and convert cyanide to HCN gas under weakly acidic conditions, allowing it to be recy-cled back to the leach process as free cyanide. Copper is also recovered as a valuable, high-grade (�70% Cu) Cu2S by-product. Cyaniderecycling allows the leach circuit to be operated at higher cyanide levels, maximizing leach efficiency and minimizing copper deportmentto gold electrowinning. The first commercial plant is at Telfer in Western Australia.

Biogenically-produced hydrogen sulphide can be used to replace chemical sulphide at significantly lower cost and with the addedadvantage of lowering the acid demand substantially. While the capital cost of a biogenic SART plant will be higher than a cyanidedetoxification plant, this will be more than offset by the revenues generated by copper recovery and the savings realized by cyaniderecycle, allowing for a short payback time on the incremental capital.

BioteQ is currently working with Columbia Metals Corporation Ltd., Canada, to apply its sulphide generating and precipitation tech-nology in a SART circuit at both of Columbia’s gold projects (La Jojoba and Lluvia de Oro) in Northern Mexico. Processing of oresfrom both mines has presented challenges resulting from the presence of cyanide-soluble copper. This paper reviews the BioteQ biogenicSART process technology, economics and commercial status as well as environmental and operating outcomes.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Gold ores; Tailings disposal; Cyanidation; Biotechnology; Recycling

1. Introduction

As more complex gold orebodies become exploitationtargets, an increasing number of gold deposits with cya-nide-soluble copper minerals such as chalcocite, covellite,bornite, cuprite, malachite and azurite are being consideredfor processing. In many of these cases the ore is not cur-rently being mined, largely owing to the metallurgical chal-

0892-6875/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2008.02.001

* Corresponding author. Tel./fax: +61 8 9279 4061.E-mail address: [email protected] (M. Adams).

lenges, high costs and environmental issues associated withthe cyanide-soluble copper minerals in the ore (Sceresini,2005). Leachable copper can result in low gold recoveriesas well as high cyanide consumption and consequent detox-ification costs. Moreover, the presence of copper in tailingssupernatant tends to stabilize weak-acid dissociable cya-nide in a form toxic to wildlife yet less amenable to naturaldegradation processes (Adams, 2000).

In concert with these trends is the increasing focus onsound environmental stewardship in mining operations,particularly pertaining to responsible cyanide use. The

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Table 1Major copper minerals and their cyanide leaching characteristics (afterAdams, 2000)

Mineral Composition Leaching rate in cyanide

Oxide minerals

Azurite 2CuCO3 � Cu(OH)2 FastMalachite CuCO3 � Cu(OH)2 FastCuprite Cu2O FastTenorite CuO Fast

Sulphide minerals

Chalcocite Cu2S FastCovellite CuS FastChalcopyrite CuFeS2 Very slowCubanite CuFe2S3 SlowBornite FeS � 2Cu2S � CuS ModerateEnargite 3CuS � As2S5 SlowTetrahedrite 4Cu2S � Sb2S3 Slow

Other minerals

Valeriite Cu2Fe4S7 SlowNative copper Cu FastChrysocolla Cu2SiO5(OH)5 � nH2O Slow

510 M. Adams et al. / Minerals Engineering 21 (2008) 509–517

world’s major mining companies have already signed theircommitment to the International Cyanide ManagementCode, which currently prescribes a weak-acid dissociable(WAD) cyanide level not exceeding 50 mg/L at tailings stor-age facility (TSF) spigot, amongst other requirements(reviewed recently by Gibbons, 2005). For this reason goldmining companies are considering their options for thedestruction or recovery of tailings cyanide to meet thistarget.

This paper will discuss a new application of BioteQ’sBioSulphide� technology involving biogenic sulphide gen-eration and its techno-economically effective use in thetreatment of copper cyanide solutions for copper recoveryand cyanide regeneration.

2. Gold–copper ore processing

The processing of ores containing both gold and copperpresents a variety of challenges to the operator, dependentupon the relative grades. Processing of gold–copper ores(higher gold values than copper values), which is the focusof this paper, typically follows the simple route of whole-ore cyanidation with recovery by carbon-in-pulp (CIP) orcarbon-in-leach (CIL) or conventional heap-leaching thatemploys carbon for gold recovery from the pregnant solu-tion. Copper–gold ore processing (higher copper valuesthan gold values) generally involves flotation and smelting.In the latter case, gold credits are obtained from the smelt-ing of the copper concentrate, and if gold reports to the flo-tation tailings, they may also require cyanidation. Anotable and relevant exception to this generalization is thatof Newcrest Mining’s Telfer plant, where pyrite-associatedgold is recovered by second-stage flotation (after chalcopy-rite concentrate production for copper smelting) and thepyrite concentrate is subjected to cyanidation. Cyanideand copper are recovered by conventional SART, whichemploys Na2S as the sulphide source, in the most recentcommercial application of this process.

Copper is chiefly introduced into gold cyanidation leachcircuits when small amounts of cyanide-soluble copperminerals are present in the ore; hence an understandingof ore mineralogy is critical to improving process manage-ment in these cases.

2.1. Gold–copper ore mineralogy

Deposits containing gold and copper are mainly associ-ated with Proterozoic and Phamerozoic rocks, primarily inAustralasia and South America, although there are isolateddeposits scattered throughout the world, some with sub-economic ‘nuisance’ copper. Several large deposits havebeen found in Indonesia (Carlisle and Mitchell, 1994), con-taining significant resources of gold and copper mineralizedin porphyry, skarn and enargite.

Association of gold exclusively with copper minerals israre, as there is often some pyrite and other sulphide min-erals present. Gold is not usually encapsulated or submi-

croscopic in copper minerals, unlike its occurrence inpyrite and arsenopyrite. Major copper minerals are listedin Table 1, along with their cyanide leaching characteris-tics. Chalcopyrite (CuFeS2) is the most abundant coppermineral. Chalcocite (Cu2S) and covellite (CuS) are formedby the alteration of primary copper sulphide ores such asporphyries. Bornite (Cu5FeS4) is generally a minor compo-nent of copper–gold ores.

2.2. Gold–copper ore processing

This topic has been thoroughly reviewed, most recentlyby Jay (2001) and Sceresini (2005); hence the focus here ison the main challenges presented by copper contaminationof gold cyanidation circuits.

2.2.1. Leaching

The major challenges to the processing of gold–copperores using cyanidation is that of the high cyanide consump-tions that are typically experienced, along with effectivecontrol of the leach, particularly when there is variable cya-nide-soluble copper in the ore.

The gold leaching reaction in cyanide solution followsthe electrochemical half-reactions:

Anodic : Auþ 2CN� ¼ AuðCNÞ�2 þ e ð1ÞCathodic : O2 þ 4Hþ þ 4e ¼ 2H2O ð2Þ

Cyanide also forms series of complexes with several metals,including copper:

Cuþ þ CN� ¼ CuCN ð3ÞCuCNþ CN� ¼ CuðCNÞ�2 ð4ÞCuðCNÞ�2 þ CN� ¼ CuðCNÞ2�3 ð5ÞCuðCNÞ2�3 þ CN� ¼ CuðCNÞ3�4 ð6Þ

100

75

50

25

00

[CN free] (mg NaCN/L) 150 300 450 600

Cu(CN)32—

Cu(CN)2—

Dep

ortm

ent (

% o

f Cu)

Cu(CN)43—

Fig. 2. Influence of free cyanide levels on copper–cyanide species at pH10.5 and 40 mg/L Cu (after Adams, 2000).

M. Adams et al. / Minerals Engineering 21 (2008) 509–517 511

The pH-potential diagram for the copper–cyanide–watersystem is illustrated in Fig. 1 (the area contained withinthe dotted lines corresponds to the region of stability ofwater). The CuðCNÞ2�3 complex is the most predominantunder typical leach conditions of pH value around 10.5with some excess free cyanide. Shown are conditionsapproximating those encountered in practice. The effectof free cyanide on the relative proportion of copper–cya-nide complexes in typical tailings solutions is given inFig. 2.

Sufficient free cyanide must be present for effective goldleaching, and large amounts of cyanide-soluble copper canrapidly decrease the available cyanide far below the levelsrequired, resulting in unleached gold losses in tailings solidsor heap-leach residues. Moreover, there is evidence (Adamset al., 1996) that under these cyanide-deficient conditions,soluble gold is adsorbed back on to copper minerals suchas chalcopyrite and chalcocite, by reduction on to surfacedefect sites, again resulting in loss to tailings.

A general operating principle for the leach is to maintaina molar ratio of added CN:Cu of no less than 4, and pref-erably over 5. The use of cyanide autoanalyser or at mini-mum, more frequent cyanide titrations, is often required toachieve this. In cases where there is a large amount of cya-nide-soluble copper, such as an intensive leach, copper lev-els of �1000 mg/L have been observed, requiring highcyanide additions.

Adsorption of copper cyanide complexes on to activatedcarbon in the adsorption stage for gold recovery followsthe trend:

CuðCNÞ�2 > CuðCNÞ2�3 > CuðCNÞ3�4

The predominance of the CuðCNÞ�2 complex under cya-nide-deficient conditions is yet another reason to maintainexcess free cyanide in the leach, to avoid significant copper

2.0

Eh

1.0

0

—1.0

—2.00

pH4 8 12 16

CuCu2O

Cu(CN)32—

CuO

Cu2O

Cu2+

Cu(CN)2—

CuO22—

Fig. 1. pH-potential diagram for the copper–cyanide–water system(6.4 mg/L Cu, 26 mg/L CN; after Osseo-Asare et al., 1984).

loading, which may push gold off the carbon, elevating sol-uble gold losses. Moreover, excess copper in the eluate re-duces electrowinning cell efficiency and contaminates thedore bullion.

Measures such as selective mining are often not applica-ble, given that gold and copper mineralization is often con-current in these ores. Chemical means of removing reactivecopper, such as acid leaching, are generally not economic.Alternatives include the addition of ammonia to the cya-nide leach to suppress copper leaching (Hunt, 1901; Muiret al., 1995; Drok and Ritchie, 1997); this option has onlyseen minor application (Butcher, 1995) and is reported tobe difficult to control in practice. Copper mineral dissolu-tion can be suppressed by means of surfactants such asfatty alkyl amines (Bennett et al., 1991), which passivatethe copper mineral surfaces, thus reducing copper dissolu-tion. There may also be a reduction in gold leaching effi-ciency, however.

2.2.2. Tailings detoxification

The leach operating strategy described above results inhigher than average tailings cyanide levels when comparedwith low-copper leachates. Moreover, the CuðCNÞ2�3 com-plex, which predominates once the excess ionic cyanide lev-els have been lowered by HCN volatilization and cyanidehydrolysis, is quite stable under typical tailings conditions(pH 7–10). However, on ingestion of tailings supernatantor runoff water by avian or mammalian wildlife, the acidicconditions in the stomach result in rapid release of free cya-nide, and death:

CuðCNÞ2�3 þ 2Hþ ! 2HCNþ CuCN # ð7ÞTherefore, process plants that produce cyanide tailingsfrom gold–copper ore processing typically need to incorpo-rate some form of cyanide detoxification or recycling


circuit, and this adds to both the capital and operatingcosts of the plant. The most common detoxification circuitsinvolve some form of oxidation, such as the popular SO2/Air process as well as the Caro’s acid (H2SO5) process.Depending on the amount of copper and cyanide presentin the tailings water, reagent costs for these processes canbe prohibitive, and in these cases some form of cyaniderecovery or recycling can be contemplated.

Copper reports to the tailings in the form of hydroxides,CuCN and via the precipitation of the highly insolubleCu2Fe(CN)6 compounds, provided iron is present:

2Cu2þ þ FeðCNÞ4�6 ! Cu2FeðCNÞ6 # ð8Þ

This reaction occurs on the surface of copper minerals, andhas been found to have an important role to play duringcyanide detoxification using various processes (Adamsand Kyle, 2000).

2.2.3. Cyanide recovery processes

Several ion-exchange resin-based processes (Goldblatt,1956, 1959; Fleming et al., 1995) and activated carbon-based processes (Adams, 1994) have been developed andtested over the past 50 years for the recovery of free or cop-per-bound cyanide from solution. While these processesmay be proven useful for specific cases, they have yet tohave enjoyed commercial application to gold plant tailings.Moreover, rapid saturation of the adsorbent is likely tobecome an issue where copper and cyanide levels are high,resulting in high adsorbent turnovers and inventories.Adsorbent-based processes may be economic in caseswhere solid–liquid separation costs or operability are anissue. It should be noted that tailings thickeners are oftenused in gold circuits.

The AVR (acidification–volatilization–regeneration)process involving the addition of acid to pH �2.5 withstripping and subsequent recovery of HCN by scrubbinghas seen some commercial application (Riveros et al.,1993), most recently in the pH �4–5 variant (Stevensonet al., 1998). Copper is not recovered for resale in this pro-cess however. Moreover, copper that is precipitated in theform of CuCN may potentially be remobilized on contactwith spikes of elevated cyanide during process upsets, forexample.

The titratable cyanide component (commonly deter-mined by silver-nitrate titration and approximated as theCN�, HCN, zinc-bound cyanide and the fourth cyanideligand from CuðCNÞ3�4 ) is easily recoverable by volatiliza-tion of the produced HCN(aq) at lowered pH value:

CN� þHþ ¼ HCN ð9ÞIn the presence of copper, insoluble CuCN is produced viaReaction (7), resulting in the loss of the copper as well assubstantial cyanide to tailings solids.

In the presence of both copper and iron, a mixed-metalinsoluble compound is produced, via Eq. (8) under aeratedor oxidizing conditions, forming Cu2Fe(CN)6, and via Eq.(10) under reducing conditions, producing Cu4Fe(CN)6:

4CuðCNÞ2�3 þ FeðCNÞ4�6 þ 12Hþ ! Cu4FeðCNÞ6 #þ 12HCN ð10Þ

In these cases, not only is the copper lost to tailings, butalso 6 moles of cyanide for every mole of iron present inthe leachate.

When thiocyanate is present, as is often the case whenleaching reactive sulphide-bearing ores or bacterial oxida-tion residues, for example, insoluble CuSCN may also beresponsible for copper precipitation in the AVR processat pH �2 after cyanide levels have dropped (CuSCN solu-bility product Ksp = 1.8 � 10�13; higher than that ofCuCN, with Ksp = 3.0 � 10�20):

CuðCNÞ2�3 þ SCN� þ 3Hþ ! CuSCN # þ3HCN ð11Þ

Addition of sulphide ions to the acidified cyanide solutionresults in the precipitation of cuprous sulphide (syntheticchalcocite), which is favoured because of its extremelylow solubility (Ksp = 2.3 � 10�48):

2CuðCNÞ2�3 þ S2� þ 6Hþ ! Cu2S # þ6HCN ð12Þ

This reaction is irreversible at pH values below four andtakes place quantitatively with stoichiometric additions ofsulphide ions and acid. Eq. (12) forms the chemical basisof both the SART process (MacPhail et al., 1998), whichis described in more detail below, and the similar MNRprocess (Potter et al., 1986). The optimum pH for libera-tion of cyanide from tailings solution by sulphide dosingwill be case-specific and dependent on the cyanide specia-tion in solution.

3. Biosulphide� commercial applications

3.1. BioSulphide� process flowsheets

Sulphide reagents are widely used in mineral processing,hydrometallurgical operations and water treatment. Theprecipitation of metals using sulphide is fast, efficient andcan produce barren solutions and effluents with very lowconcentrations of residual metals. BioteQ’s BioSulphide�

technology for the generation of biogenic sulphide reagentby reduction of elemental sulphur at a lower cost than thechemical sulphide reagents has been described elsewhere(Lawrence et al., 2005, 2007). In some smaller BioteQplants, currently operating or under construction, NaHSis used for selective metal precipitation and recovery in aprocess known as ChemSulphide�.

A typical flowsheet utilizing the biogenic generation ofsulphide reagent as hydrogen sulphide to precipitate selec-tively metals contained in the feed water is shown in Fig. 3.

Hydrogen sulphide is generated by the reduction of ele-mental sulphur, or other sulphur source, in the presenceof an electron donor such as acetic acid in an anaerobic bio-reactor. The gas is passed to an anaerobic agitated contac-tor where conditions are controlled to selectively precipitatethe metal to be recovered as a sulphide. The high-grademetal sulphide precipitate is then recovered by conventional

Sulphur

Reagents

Bioreactor

Contactor

Clarifier

FilterFeed water

Treated water

Metal sulphideto smelter

H2S

Fig. 3. Simple flowsheet schematic of the BioSulphide� process.

Copper Productto Smelter (~40% Cu)

BioSulphide® Plant

Free acid

#7 Stockpile

Fig. 4. Simplified flowsheet showing the use of biogenic sulphide forcopper recovery at Bisbee.


clarification and filtration to produce a filter cake which canbe shipped to a smelter. The feed water does not passthrough the bioreactor, which can be operated under idealconditions at all times. The operation of the bioreactor isnot subject to process upsets due to changes in water chem-istry and flow, unlike other biological processes which actdirectly on the water to be treated.

The main advantages of using biological H2S generationinclude:

� Low cost of sulphide compared to the cost of Na2S,NaHS, or H2S.� Minimal hazards and increased safety mainly due to the

low system pressure and low inventory of H2S, avoidingthe need for special environmental permitting and per-sonnel for sulphide reagent storage.� Low capital cost mainly due to the ambient temperature

and pressure in bioreactors that are designed as conven-tional stirred tanks compared to pressure vessels withexpensive agitator seals.� Easy to scale-up and down over a wide range of H2S

production capacities.

If more than one metal is to be recovered, a single bio-reactor can provide sulphide reagent to separate contactor-dewatering circuits. It is possible, for example, to produceseparate and high grade sulphide concentrates of Cu, Zn,Ni–Co and Mn.

3.2. BioSulphide� process plants and projects

Since 2001, BioteQ has constructed and operated threecommercial operations utilizing sulphide generation andprecipitation technology (upstream of a lime plant at theCaribou Mine in New Brunswick for zinc–lead–copper–cadmium; copper recovery from stockpile drainage at theCopper Queen Mine in Bisbee, Arizona; and treatment ofnickel-containing mine drainage at the Raglan Mine, inNorthern Quebec). Four new plants are currently underconstruction in 2007, with a further four in the designphase for planned construction in 2008.

BioteQ’s first copper recovery plant was constructed andis operated in joint venture with Phelps Dodge at the Cop-per Queen Mine, in Bisbee, Arizona. The success of the

plant has opened up numerous opportunities to use bio-genic sulphide for the recovery of metals including copper,nickel, zinc and cobalt at a number of sites around theworld. The recovery of copper at Bisbee has demonstratedthat the technology has a niche for profitable operation forthe treatment of solutions too low grade for economicapplication of solvent extraction-electrowinning (SX-EW)and with a significantly lower capital investment.

The Bisbee plant was commissioned in 2004 and wasdesigned to recover up to 3 million pounds (1360 tonnes)per year of copper from the drainage of a large low-gradestockpile with flows of up to 10,900 m3 per day. The Bio-Sulphide� reactors have thus proven to be robust and flex-ible for industrial use. Fig. 4 shows a simplified flowsheet ofthe operation.

Precipitation of copper from the drainage is rapid andefficient, with copper recoveries consistently greater than99.5% from the feed solution containing copper in the range220–360 mg/L at a flow currently in the range of 8000–9000 m3 per day. The copper sulphide product is thickenedand dewatered using conventional equipment, with theplant effluent containing free acidity returned to the stock-pile. The filtered concentrate, containing typically 40–45%copper, is shipped to the Phelps Dodge-owned smelter inMiami, Arizona for processing to metal. Payback on theUS$3.2 million plant is expected to be less than 3 years.

4. Biogenic sulphide for cyanide recycle and copper recovery

4.1. Sulphide-based processes for cyanide and copperrecovery

In the SART process, the cyanide associated with thecopper (and zinc, if present) cyanide complexes is released,allowing it to be recycled back to the leach process as freecyanide. Copper is also recovered as a valuable, high-grade(�70% Cu) Cu2S by-product. Cyanide recycling allowsthe leach circuit to be operated at higher cyanide levels,maximizing leach efficiency and minimizing copper deport-ment to gold electrowinning. The SART process uses


chemical sulphide ions from reagents such as NaHS, toprecipitate copper and convert cyanide to HCN gas, underweakly acidic conditions (pH 5).

The basic chemistry of the SART process, which wasdeveloped in the 1990s (MacPhail et al., 1998) is identicalto the MNR process, which was extensively tested in the1980s (Potter et al., 1986). The difference between thetwo processes is in the addition of a clarifier thickener tothe circuit in the SART process to improve physical han-dling of the precipitate. In the MNR process, the Cu2S pre-cipitate is pumped directly from the primary reactor to apressure filter as a low-density slurry, whereas in the SARTprocess, the volume of slurry reporting to the pressure filteris greatly decreased. Both the MNR and SART processeshave been piloted, and a full-scale SART plant was builtand successfully commissioned at Newcrest’s Telfer opera-tion in Western Australia in �2001. After a 5-year periodof care and maintenance, a new circuit was implementedand the Telfer plant was restarted, with recommissioningof the SART circuit, in 2006.

4.2. BioSulphide� process for cyanide and copper recovery

The SART process, as originally developed, uses chem-ical sulphide ions from reagents such as NaHS. Chemicalsulphide can be replaced by lower cost biogenically-pro-duced hydrogen sulphide, which has the added advantageof lowering the acid demand by one third for copper cya-nide treatment and half for zinc cyanide. A simplified pro-cess flowsheet in which biogenic sulphide replaces chemicalsulphide in the SART process is shown in Fig. 5.

The SART process is ideally suited to heap-leachoperations, where barren solution (after gold recovery) isrecycled directly to leach because cyanide is not pre-con-centrated prior to recycling. Application to copper cya-nide-rich tailings from a CIP or CIL plant requires priorsolid/liquid separation using either filtration or counter-current decantation (CCD). For maximum cyanide recov-ery, the wash solution used in CCD or filtration must below in cyanide, and the wash waters must also be treated,producing excess water with lower cyanide tenor, thatcan be directed to a heap-leach operation if present.

Cu2S

H2SBioreactor

AcidificationCopper-cyanideladen solution

H2SO4

Fig. 5. Simplified diagram showing the SART process for copper recovery andreplacing chemical sulphide.

Economic benefits of the SART process arise from therecycled cyanide, which reduces the cyanide additionrequirement, and the saleable high-grade copper sulphideproduct. While the process allows operation at increasedcyanide tenors, allowing for increased copper leachingand recovery, this comes at a cost when copper mineralsin an oxidized state (e.g., covellite, CuS) are leached, dueto the oxidation of one mole of cyanide per mole of copper(although the actual existence of the copper (II) cyanidecomplex remains in question):

Cu2þ þ 4CN� ! CuIIðCNÞ2�4 ð13Þ2CuIIðCNÞ2�4 þ 4OH� ! 2CuIðCNÞ�2þ 2CN� þ 2CNO� þ 2H2O ð14Þ

An example of the potential economic benefit of a biogenicSART process was reported previously (Lawrence et al.,2007); using NaHS, the operating costs for reagents, powerand labour for treatment of a barren leach solution contain-ing 250 mg/L Cu and 310 mg/L WAD cyanide in a SARTcircuit can be estimated to be US$0.40/m3, using H2SO4,NaHS and Ca(OH)2 costs of $100/t, $1000/t and $250/t,respectively. Data from commercial BioSulphide� plantsfor sulphate waters demonstrates that costs of the sulphidereagent are further reduced when compared with the use ofNaHS. Moreover, acid consumptions are significantly re-duced via utilization of the additional proton in the H2S re-agent produced biogenically. Assuming recoveries of 95%cyanide and 99% copper, unit costs of NaCN and copperof $0.69/lb and $2.50/lb, respectively, and a net smelter re-turn of 85% for sale of the copper, revenues of $1.98/m3 canbe estimated. Half of this revenue stems from the sale ofcopper sulphide, illustrating the potential added value ofthe leachable copper in a gold ore. By comparison, the costof cyanide detoxification to treat a bleed stream using, forexample, the SO2/Air process ranges from $1.50/kg NaCNto $3.00/kg NaCN, depending on the method used and thepresence and concentration of other species in the leach li-quor. Under the most favourable circumstances, the costof cyanide detoxification in the above hypothetical examplewould be about $0.90/m3 of heap-leach liquor treated.Therefore, the true benefit of the SART process with

CaSO4.2H2O

Re-Neutralization Solution toheap leach

Ca(OH)2

recycle of cyanide-bearing solution to heap-leach, with biogenic sulphide


current reagent costs and commodity pricing is expected tobe in excess of $2.00/m3 of heap-leach liquor treated.

The capital cost of a SART plant will be higher than acyanide detoxification plant, but this will be more than off-set by the revenues generated by copper recovery and thesavings realized by cyanide recycle, allowing for a shortpayback time on the incremental capital.

Capital and operating costs of the BioSulphide� processare dependent on many site-specific factors but can begenerically compared to alternative sources of sulphideused widely in the mineral processing and chemical indus-tries such as:

(1) direct shipment of compressed H2S gas,(2) shipment of sulphide salts such as NaSH, Na2S and

Ca(HS)2, and(3) onsite production of H2S from hydrogen and molten

sulphur.

The direct shipment of H2S gas is normally not practicalfor safety and permitting reasons unless a source is locatedvery close to the point of use. Sulphide salts are widely usedbut may be constrained by operating costs due to fluctuat-ing commodity prices of the salt or of the contained alkalisuch as NaOH or lime. The cost of sulphide salt shipmentmay be constrained by transport costs for users located farfrom tide water or a rail link, particularly when the salt isdelivered as an aqueous solution. If the sulphide salt isshipped in flake form, then the energy cost of drying thechemical raises the unit cost and the stability of the solu-tion may be an additional concern. Finally, the use of sul-phide salts may be constrained by processing concerns,such as their use in copper precipitation circuits wherethe presence of sodium, or the incremental increase in thepH of the process at the point of use are not desirable. Inthe case of the SART process, the savings in acid costmay also be significant through the replacement of a basicsulphide salt with the acidic sulphide gas. These factors alltend to favour biogenic H2S over the three alternative sul-phide sources cited above.

When compared to other onsite production methods,such as a molten sulphur–hydrogen reactor, the BioSul-phide� process would be more favourable in terms of cap-ital cost due to the following factors:

1. The materials of construction used in a bioreactor forsulphide production may be carbon steel, whereas moreexpensive materials such as stainless steel or other corro-sion-resistant alloys are used for the molten sulphur–hydrogen process.

2. The bioreactor is held at near atmospheric pressure,compared to the molten sulphur process, which takesplace in pressure vessels, resulting in lower reaction ves-sel costs for biogenic H2S production.

3. The bioreactor is held at near ambient temperature(�30 �C) whereas the molten sulphur process takes placeat elevated temperature; hence, biogenic H2S production

is at a lower energy cost and uses simpler materials ofconstruction.

4. The bioreactor may be a single large stirred tank withrelatively simple low-cost controls, whereas the moltensulphur process is complex in comparison consisting ofa hydrogen-generating plant, a steam plant, and a reac-tor vessel.

5. In cases where the point of use is not accessible by a nat-ural gas line, then the cost of generating hydrogen forthe molten sulphur process either by electrolysis or usingliquid fuels may make the process cost prohibitive. Thebioreactor, in contrast, uses inexpensive and readilytransportable feeds including: granular sulphur, a wasteproduct from sour-gas processing, a carbonaceousliquid used by the microorganisms as an electron donor,and simple agricultural fertilizers.

4.3. New projects – La Jojoba and Lluvia de Oro in Mexico

BioteQ is currently working with Columbia Metals Cor-poration Ltd., Canada, to apply its sulphide generating andprecipitation technology in a SART circuit at both ofColumbia’s gold projects (La Jojoba and Lluvia de Oro)in Northern Mexico. Processing of ores from both mineshas presented challenges owing to the presence of cya-nide-soluble copper.

The Lluvia de Oro project will be the first of the twoprojects to go into production. The Lluvia de Oro mineis located approximately 13 km northwest of the town ofMagdalena de Kino, Sonora, Mexico. The low-grade goldore deposit, which contains a significant quantity of coppermineralization, has been previously worked by others, butlower gold prices, together with high cyanide consumptionand diminishing carbon adsorption capacity resulting fromthe build-up of copper in the leach solution, led to discon-tinuation of operations in the late 1990s. Columbia Metalsis planning to restart the exploitation of the deposit byopen-pit mining, crushing and cyanide heap-leaching fol-lowed by carbon adsorption in an ADR (adsorption–desorption–recovery) plant. Gold dore will be producedon site following carbon stripping and electrowinning. Itis the development of the SART process that now providesboth Columbia Metals and BioteQ the opportunity for suc-cessful operations at this site through the recovery of sale-able copper and the ability to recover cyanide from thebarren leach solution.

In the first phase of the project, a portion of the existingheap-leach pad will be re-crushed, placed on a new pad andsubjected to cyanide leaching. In the second phase of theproject, fresh ore will be mined and crushed prior to beingplaced on the pad. A schematic of the overall project flow-sheet is shown in Fig. 6.

Fig. 7 provides a more detailed schematic of the SARTprocess using biogenic sulphide in place of chemical sul-phide reagent. In the configuration shown in Figs. 6 and7, the feed to the SART plant will be taken from the

FilterThickener

H2SSulphur andReagents

Bioreactor

PrimaryReactor

H2SO4

NeutralizationReactor

Lime

Barren CyanideSolution with Cu

Scrubber

Cu2S product

GypsumThickener

CyanideRecycle

Cu2S Recycle

Thickener

H2S

PrimaryReactor

4

t

CyanideRecycle

Cu2S Recycle

Fig. 7. Flowsheet showing the use of biogenic sulphide to replace chemical sulphide in the SART process for cyanide recycle and copper recovery in gold–copper ore processing.

Heap Leach

ADRCarbonPlant

Cu2S tosmelter

SARTPlant

Barren Solution Pond

Gypsum toholding cells

Pregnant Solution Pond

EWFurnace

Au/Ag Dore

CN recycle

Heap Leach

ADRCarbonPlant

ter

SARTPlant

EWFurnace

Heap Leach

ADRCarbonPlant

r

SARTPlant

EWFurnace

Fig. 6. Overall process flowsheet showing gold production and application of SART process for copper recovery and cyanide recycle planned for theLluvia de Oro project.


discharge of the carbon ADR plant, with the solution fromthe SART plant returned to the heap-leach. However,depending on a number of process factors, the feed toSART could be taken from the pregnant solution pondand with the SART plant effluent passing to the carbonADR plant if it is shown that gold recovery can be opti-mized. The copper sulphide product will be recovered ina thickener – pressure filter circuit and will be shipped toa copper smelter in Mexico or the USA.

Gold production at the La Jojoba property, locatedapproximately 4 km away from Lluvia de Oro, is plannedto start at a later date. Processing of pregnant solutionfrom La Jojoba into the plant at Lluvia de Oro is a poten-tially feasible option.

5. Conclusions

The challenges surrounding the treatment of gold–cop-per ores are many and varied, including excessive cyanideconsumption, increased cyanide detoxification costs and

the environmental downside due to copper-stabilizedWAD cyanide in tailings dam supernatant solutions. Inthese cases, the gold operator is typically forced to inte-grate an expensive cyanide oxidation circuit on the tailingsstream. To a large extent, these challenges can be overcomeby inclusion of a biogenic SART circuit, where the excesscyanide both leaches more recoverable saleable copperand is recycled, and the tailings are rendered benign.Importantly, in an age where detoxification costs are oftenconsidered more disposable than routine maintenancecosts, a biogenic SART circuit can potentially almost com-pletely offset the cost of achieving tailings stream CyanideCode compliance in the case of gold–copper ores.

Acknowledgements

The authors wish to acknowledge the valuable discus-sions and prior paper contributions with David Kratochviland Chris Fleming, which most certainly enhanced thequality of this manuscript.


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