Review

Alternatives to Cyanide in the Gold Mining Industry:What Prospects for the Future?

Gavin HilsonA and A.J MonhemiusB*

A School of City and Regional Planning, Cardiff University,Glamorgan Building, King Edward VII Avenue, Cardiff CF10 3WA, Wales, UK

B Department of Earth Science and Engineering, Royal School of Mines, Imperial College, London, SW7 2BP

_________________________________________________________________________

Abstract

This paper reviews a series of alternative lixiviant systemsfor the recovery of gold from ores and concentrates. For over100 years, cyanide has been the leach reagent of choice ingold mining because of its high gold recoveries, robustnessand relatively low costs. The environmental damagesresulting from its mismanagement, however, have initiatedwidespread research aimed at identifying and developing lesstoxic leaching agents. The most widely researched alternativelixiviants for gold ores are examined in this paper, but itis evident that none of them has yet made any significantinroad into the dominant position of cyanide as the reagentof choice at the vast majority of gold mines worldwide..

Keywords: cyanide; gold; alternatives; thiourea;thiosulphate; thiocyanate_________________________________________________________________________

Introduction

* Corresponding author Email: j.monhemius@imperial.ac.uk 1

The debate surrounding the use of cyanide in the miningindustry has fuelled considerable investigation into thedevelopment of more environmentally benign alternatives. Forover 100 years, cyanide has featured prominently as a leachreagent at gold mines because of its high efficiency andrelatively low cost. Although Hagen first discovered thatgold was soluble in cyanide in 1806, it was not until thework of John Stewart MacArthur in the 1880s that the value ofcyanide as a leach reagent for gold ore treatment was finallyrealized. A chemical metallurgist, MacArthur was grantedBritish Patent No. 14174, Process of Obtaining Gold and Silver fromOres, in 1887. This technology, called “cyanidation”,received immediate industrial success in New Zealand in 1889and South Africa in 1890. Gold worth US$14 million wasrecovered by cyanidation over the first five-year period ofoperation of the process [1], which eventually replacedchlorination processing outright. By 1907, global annualproduction of gold had doubled because of an increased use ofcyanide; it continues to this day to be the primary leachingmethod in the gold mining industry [2].

Today, over one million metric tonnes of cyanide –representing 80% of total production – are used annually bythe chemical industry for the production of organic chemicalssuch as nitrile, nylon, and acrylic plastics. The remaining20% is used to manufacture sodium cyanide, a solid form ofcyanide, of which 90% (i.e. 18% of total production) is usedin mining operations around the world for gold recoverypurposes [3]. Typically, dilute solutions of sodium cyanide(NaCN), normally in the range of 0.01- 0.05% cyanide (100 to500 parts per million), are used in tank leaching and heapleaching processes. Cyanide ions (CN-), are the activeingredients in the gold leaching process, which dissolve thegold contained in the ore by complexation (see below),forming a “pregnant solution” from which gold is laterextracted.

4Au + 8CN- + O2 + 2H2O = 4Au(CN)2- + 4OH-

A series of recent environmental accidents at various goldmines around the world, however, has precipitated widespreadconcern over the use of cyanide as a leach reagent. In themajority of these cases, cyanide from processing operationshas entered the environment either by leakage through tearsand or punctures in protective heap leach liners, or by

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spillage from overflowing solution ponds or tailings storageareas, [4]. For example, in Guyana, some 2.9 million m3 ofcyanide-laced tailings and wastewater was released by acollapse of the tailings dam at the Omai mine in Guyana onthe night of August 19th 1995, polluting the Omai River. Adisaster similar in scale occurred on January 30th 2000 inBaia Mare, Romania, where the tailings dam of Aurul S.A. wasbreached, resulting in the discharge of wastewater containingas much as 100 tonnes of cyanide that eventually reached theDanube river. (See Table 1 for additional examples ofcyanide spills and resulting impacts).

Table 1: Examples of recent cyanide-related mine accidents and resulting environmental impacts [6]

Company/Mine Location and date

Impacts

Galactic Resources Ltd.’s Summitville Mine,

Colorado, USA, 1992

Caused severe environmental problems along a 17-mile stretch of the Alamosa River

Pegasus Corporation’s Zortman-Landusky Mine

Montana, USA, 1997

Severe contamination of groundwater; substantial wildlifedeaths

Echo Bay’s McCoy/Cove GoldMine

Nevada, USA, 1989 and 1990

Eight cyanide leaks over a two year period released almost 900 lbs of cyanide into the environment

Kumtor Mine Kyrgyzstan, 1998

Almost two tons of sodium cyanidewas accidentally released into surface waters

Cambior Mining Company’s Omai Mine

Guyana, 1995 Released more than 860 million gallons of cyanide-laden tailingsinto a major river

Although cyanide is not regarded as a persistent toxin, it isnevertheless a deadly poison in high concentrations, posing aserious health threat to a wide range of ecological entities.In fact, the contention that cyanide is not persistent anddoes not give rise to chronic health or environmentalproblems [3] is somewhat misleading. As Moran [5] explains,the CN- ion, which is ubiquitous in nature, tends to reactreadily with innumerable chemical agents and molecules toform hundreds of different compounds, many of which arelethal to organisms. To determine the persistence andtoxicity of the chemical, chemists have traditionallydistinguished “weak” from “strong” cyanide complexes. Weak

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cyanide complexes, often referred to as “weak aciddissociable” or WAD cyanide, break down, releasing freecyanide ions, when the pH is lowered. WAD cyanideincludescyanide complexes of cadmium, copper, nickel, silver, andzinc, all of which can disassociate in acid conditions andproduce environmentally-significant quantities of toxiccyanide ions. Strong cyanide complexes are more stable thanWAD cyanide, thus degrading much more slowly under naturalconditions. These include cyanide complexes containing gold,cobalt, and iron, which are more stable in solution [3, 7].The toxicity of cyanide varies, with free cyanide being themost toxic, and cyanide complexes being less or non-toxic[8].

In excessive quantities, cyanide can be lethal to humans andto animal species because it binds to the iron-carryingenzymes required for cells to use oxygen, thus inhibitingbodily tissues from extracting oxygen from the blood (Table2). The body then rapidly exhibits symptoms of oxygenstarvation and suffocation. Symptoms of cyanide poisoninginclude irregular heartbeat, convulsions, chest pains andvomiting [7]. Many species of migratory birds can come togrief at gold mine heap leaching facilities, some dyingimmediately after consuming cyanide-containing water. Signsof acute poisoning to livestock, including initial muscletremors, salivation, lacrimation, defecation, urination andlaboured breathing, usually occur within 10 minutes; anddeath can follow quickly [4, 9-11]. In the aquaticenvironment, young, freshwater fish such as salmonoids areparticularly susceptible to cyanide poisoning.

It is against the background of these environmental impactsthat indigenous groups and representatives of the NGOcommunity in particular have lobbied for the mining industryto develop alternative, less toxic, leach reagents. Thepurpose of this article is to review the most widely-researched alternative leach reagents to cyanide. Severalsubstitutes have been proposed (Table 3) but generally, thenon-cyanide lixiviants, thiourea, thiocyanate andthiosulphate are regarded as being the most realisticsubstitutes. These, along with a number of other possiblealternatives, including halides and the recently-developed

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coal-oil agglomeration process, are reviewed individually inthe discussion that follows.

Table 2: Single dose toxicity of sodium cyanide (in mg NaCN kg-1 body weight), selected species [13]

Species Oral LD50 (95% confidence limits)

Mallard 2.7Human 3.0American kestrel 4.0Coyote 4.1Black vulture 4.8Lab rat 5.1Little brown bat 8.4Eastern screech owl 8.6Japanese quail 9.4Domestic chicken 21White footed mouse 28

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Table 3: Suggested alternatives to cyanide [14]

Reagent type Concentration range

pHrange

Basicchemistry

Researchlevel

Extent of commercialisation

Ammonia High 8-10 Simple Low Pilot tests, +100 degrees centigrade

Ammonia/cyanide Low 9-11 Simple Extensive Applied to Cu/Auores

Ammoniumthiosulphate

High 8.5-9.5

Complex Extensive Semi commercial

Slurry CN-electrolysis

Low 9-11 Simple Historical

Limited historical

Sodium sulphide High 8-10 Simple Low Geological interest only

Alpha-hydroxynitriles

Moderate 7-8 Fairlysimple

Fairlypopular

None

Malononitrile Moderate 8-9 Fairlycomplex

Low None

Alkali cyanoform Poorlydefined

9? Poorlydefined

Low None

Calcium cyanide Poorlydefined

9? Poorlydefined

Low none

CSTU(1) High 9-10 Complex Very low None. KMnO4 is oxidant

Alkalinepolysulphides

High 8-9 Poorlydefined

Low None

Biocatalyzedbiosulphate

Poorlydefined

8-10 Poorlydefined

Low None

Hypochlorite/chloride

Highchloride

6-6.5 Welldefined

Extensive Historical and modern

Bromocyanide High 6-7 Poorlydefined

Historical

Historical

Iodine High 3-10 Poorlydefined

Low None

Bisulphate/sulphur dioxide

High 4-5 Fairlysimple

Low None

Bacteria High 7-10 Fairlycomplex

Low,growing

None

Natural organicacids

High 5-6 Fairlycomplex

Low None

DMSO, DMF Poorlydefined

7 Poorlydefined

Very low None

Bromine/bromide High 1-3 Welldefined

Low Historical

Thiourea High 1-2 Welldefined

Fairlypopular

Some concentrates

Thiocyanate Low 1-3 Welldefined

Low None

Aqua regia High Below1

Welldefined

Low Analytical and refining

Acid ferricchloride

High Below1

Welldefined

Low Electrolytic Cu slimes

Ethylenethiourea

High 1-2 Poorlydefined

Very low None

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Haber process Poorlydefined

Proprietary

Oneentity

None

“Bio-D leachant” Poorlydefined

Proprietary

Oneentity

None

High tempchlorination

High 6-7 Simple historical

Historical

The Challenges to Developing Alternatives for Cyanide

The main challenge to developing substitutes for cyaniderests in ensuring that the particular metallurgical processesselected for the extraction of gold matches thecharacteristics of the ore. Alternative lixiviants orleaching agents should also be [12]:1) inexpensive and recyclable2) selective;3) non-toxic; and4) compatible with downstream recovery processes.

Box 1 highlights additional criteria which must be consideredfor gold lixiviants.

For over a century, sodium cyanide has been the predominantleach reagent for gold because of its excellent performancewith a great variety of ores and its relatively low cost[15]. Moreover, despite being a potentially deadly poison ifingested in high doses, cyanide does not give rise to chronichealth or environmental problems if present in lowconcentrations [3]. It is ubiquitous in nature, produced bya wide range of micro-organisms and over 2500 plant speciesas part of their normal metabolisms [7]. The lowconcentrations of cyanide used in gold milling and heapleaching are quickly degradable: cyanide is nowhere near asharmful as other naturally occurring toxins (e.g. tetanus,strychnine) because it is readily transformed by naturalphysical, chemical, and biological processes, into less ornon-toxic forms.

The primary challenge in devising a suitable substitute forcyanide in gold processing, therefore, lies in developing anequally effective and degradeable leach reagent, which is nota persistent environmental toxin. As gold cyanidation ratesare relatively slow, the industry has been searching for

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faster gold leaching reactions capable of facilitating highmetal recovery rates. Although there has been significantresearch undertaken to develop plausible substitutes, drivenmainly by economics, the list of effective options is limited,as most are addressed to refractory ores and are not amenableto simple cyanide leaching [15]. The discussion that followsreviews the most promising alternatives.

Box 1: Criteria which should be considered for goldlixiviants [16]

Economic Considerations Capital outlay Extraction costs Availability Detoxification/recycling costs

Toxicity Emissions Handling Environmental toxicity

Process applicability Limitations (e.g. ore type, selectivity, control,

separation, etc.) Recyclability Detoxifiability Feasibility at a large scale

Thiourea Leaching of Gold

Overview

The use of thiourea (NH2CSNH2) as a gold extraction agent hasshown considerable promise. In acidic conditions,thioureadissolves gold, forming a cationic complex;thereaction is rapid and gold extractions of up to 99% can beachieved [15]. The anodic reaction follows the equation:

Au + 2 CS(NH2)2 = Au(CS[NH2]2)2 + + e-

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The reagent, however, has to be used under relativelyrestricted conditions, as it is fairly stablethermodynamically in acid and neutral mediums, but decomposesrapidly in basic solutions [17]. Leaching is normallycarried out in the pH range of 1-2, and its successfulapplication is contingent upon careful optimisation andcontrol of pH, redox potential, thiourea concentration, andleaching time. The critical parameters for thiourea leachingare as follows [18]:

pH: 1.4 adjusted accordingly with H2SO4

Redox potential Max 250 mvMin 150 mv

Thiourea Conc. 1%Thiourea Consump. 2 kg/tLeach Time 10-15 minutes

It is claimed in a number of sources [15, 19-20] that thework of the Soviet scientists Plastin and Kozukhova [21-22]was the pioneering research in this field, but the potentialof thiourea for gold dissolution was recognized much earlierby Moir [17] who, in 1906, published a paper entitled“Thiocarbamide - a new solvent for gold” [23]. Moir reportedthe results of experiments in which he dissolved pieces ofgold leaf in thiourea (then called thiocarbamide) solutions.He showed that acid conditions were necessary and that thereaction was rapid in the presence of suitable oxidants. Atthat time, however, cyanide was already established as thereagent of choice for gold ore treatment and so Moir’sdiscovery lay neglected for over half a century. Thioureahas only received serious consideration as an alternative tocyanide in the past 20 years [12]. It is an attractiveoption for treating refractory ores and floatationconcentrates, although it is hoped that with furtherresearch, it can be adopted to treat lower grade ores [18].

The Thiourea Process and Potential Benefits

In a comprehensive review, Pyper and Hendrix [24] identifiedthe following about thiourea kinetics: The use of ferric ion in sulphuric acid is the most

effective system;

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The rate of leaching is contingent upon thiourea andoxidant concentrations;

The ferric ion ties up thiourea in iron-thioureacomplexes; and

The rate of gold dissolution is strongly determined by pH.

Generally, gold can be recovered from thiourea viacementation (using iron powder, or aluminium), or byabsorption on activated carbon, or by electrowinning, or bythe use of ion-exchange resins. Compared to cyanide,thiourea has certain advantages, including low sensitivity tobase metals (Pb, Cu, Zn, As); low sensitivity to residualsulphur in calcines; high gold recovery from pyrite andchalcopyrite concentrates; and satisfactory recovery of goldfrom carbonaceous (refractory) ores [15]. It also poses lessof a threat environmentally [25-26]. On its own, thioureahas a lower toxicity and higher rate of gold and silverdissolution than cyanide [27]. Moreover, if using cyanide totreat refractory sulphide gold-containing ore, which havebeen pre-oxidised to release the gold, the highly acidicsolutions produced must be neutralized prior to cyanidation,whereas thiourea can be used directly, since gold leachingoccurs in acidic solutions [28].

Despite the proven effectiveness of thiourea as a gold leachreagent [27,29-30], there are still few full-scale operationsin existence. Its commercial adoption has been hindered bythe following three factors [31]:(1) It is more expensive than cyanide;(2) Its consumption in gold processing is high; and(3) The gold recovery step requires more development.

Reagent consumption is exceedingly high because thiourea isreadily oxidized in solution, and as Prasad [18] implies, thehigh costs attached to leaching are likely due to thethiourea process still being in an infancy stage. Pastattempts to adopt this reagent has led to over-consumptionand increased production costs. There is also an emergingconcern about its environmental impact. Despite claims thatthiourea is less toxic than cyanide, it is a suspectedcarcinogen, and thus must be treated with caution [12].

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Experimental Work and Commercial Developments

The effectiveness of thiourea as a substitute for cyanide hasbeen verified through substantial experimental work. AsMonhemius [17] reports, in terms of its feasibility as aleach reagent, Shultz originally argued in his work that theaddition of sulphur dioxide during thiourea leaching preventsit from degrading chemically, thus resulting in lowerconsumptions of the reagent[32-33]. The author proposed thatthe bubbling of sulphur dioxide during the thiourea leachingof precious metals prevents decomposition of formamidinedisulphide , which can lead to thecoating of gold surfaces byfine adhesive sulphur, a decomposition product.

More recently, Abbruzzese et al. [34] carried out furtherlaboratory evaluations to assess the feasibility of thiourealeaching. Gold was recovered from leaching solutions byadsorption onto activated carbon; gold was eluted from thecarbon using water/alcohol mixtures, and the final recoveryof gold from the elution solutions was accomplished viaelectrowinning. The authors reported an overall gold yield of82% with a low consumption of reagents (based upon a goldextraction of 85% at room temperature, and a 99% recoveryafter the adsorption, desorption and electrowinning phases).More specific studies have been carried out such as that ofGonen et al. [35], who examined the effects of thiourea andoxidant type, pH, agitation time, and particle size on goldrecovery from Gumushane-Mastra ore in Turkey. It wasdetermined that with short leaching times and low pH values,the amount of oxidantand the consumption of thioureadecreased. Gold extractions of 75% were achieved using aparticle size of 150-mesh in a mixing time of five hours.

Miroslav et al. [36] examined the feasibility of thioureaextraction of gold from copper slimes in Kovohutny Krompachy,Slovenia. The authors reported that thiourea extracted some80% of gold from the residues. Orgui and Atalay [37] reportedsimilar results. In an attempt to extract gold from Kaymazgold ore (4gAu/t) using thiourea and iron (III) sulphate asan oxidizing agent, at room temperature, it was possible toobtain a gold extraction of 80% in six hours. The process hasalso proven to be a successful non-toxic solvent for gold inRussia [38].

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Thiourea leaches gold more rapidly than cyanide, and is lesstoxic; forms cationic complexes; and can be utilized on oresrefractory to cyanideand in heap and in situ leaching [39].However, as previously explained, research gaps and costshave hindered its wide-scale adoption as a leach reagent inthe gold mining industry. Costs can be exorbitant because ofthe quantity of reagent required. As Presad et al. [18]explain, the high consumption of thiourea and low rate ofdissolution is attributable to an inhibiting coating ofsulphur on the surface of the gold particles formed becauseof thiourea degradation. It has, however, been proposed thatadding sulphur dioxide to leaching solutions would controlthe redox couple between the thiourea and its firstdecomposition product, formamidine sulphide, thereby keepingthe formation of sulphur to an absolute minimum.

The most significant commercial development involvingthiourea made to date in the Western world has been at theNew England Antimony Mines in New South Wales, Australia,which was commissioned in 1982. As la Brooy [31] explains,because of thiourea’s tolerance of stibnite, the company wasable to recover 50-80% of the gold from a stibniteconcentrate using a thiourea leach, followed by goldrecovery onto activated carbon. More recently, Lacoste-Bouchet et al. [40] used thiourea to leach gold from orecontaining 0.4% Cu and 5.6g/t Au obtained from the Val d’OrMine in Quebec. Gold extractions of 89% were obtained usingthiourea at a rate of 3.9 g/t.

Thiocyanate

In 1906, White [41] first demonstrated that thiocyanatedissolved gold effectively in the presence of a suitableoxidizing agent; however work with this reagent remainedstagnant until Fleming re-opened the field in 1986 [42]. AsBarbosa and Monhemius explain [43], the leaching of gold withthiocyanate, if performed between pH 1 and 2, enables the useof iron (III) as an oxidizing agent. During the dissolutionof gold by iron (III)-thiocyanate, FeIII is reduced to FeII

while oxidizing SCN-, producing several intermediate species,including (SCN)3

- and (SCN)2, which can both oxidise andcomplex gold. In aqueous solutions, acidified thiocyanate candissolve gold to form both Au(I) and Au(II) complexes [44].

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In terms of relevant research, Barbosa-Filho and Monhemius[45] describe aspects of the chemistry and thermodynamics ofthiocyanate, and Broadhurst and du Perez [46] describe thespeciation of the gold-thiocyanate system. In subsequentexperimental work undertaken by Barbosa-Filho and Monhemius[47-8] withsolutions of FeIII-thiocyanate, which reduced fromFeIII to FeII with the oxidation of SCN-, it was shown that theauto-reduction reaction produced both (SNC)3

- and (SNC)2,which were fundamental in the mechanism of the thiocyanatedissolution of gold. The authors concluded that thiocyanateis an “effective lixiviant for gold in acidic conditions,yielding dissolution rates that are comparable to thoseobtained with thiourea while offering the advantage of muchgreater stability against oxidative decomposition”.Kholmogorov et al. [49] recovered over 95% of gold from weakacidic solutions, recovering thiocyanate complexes on carbonadsorbents and ion exchangers [49]. Further research wasperformed at a laboratory scale by Monhemius and Ball, whoused iodide-catalyzed thiocyanate solutions to leach goldores derived from the Dominican Republic [50]. Theseresearchers concluded that the thiocyanate system used gaveresults comparable to that of a cyanide system, and exhibitedconsiderably greater effectiveness than thiourea.

Thiosulphate Leaching

Thiosulphate (S2O32-), a chemical used widely in photography

and in the pharmaceutical industries, has also been proposedas a substitute for cyanide. As Yannopoulis [23] explains,gold dissolves slowly in alkaline thiosulphate. The rate ofdissolution is influenced by the concentrations ofthiosulphate and dissolved oxygen, the process temperature,and can be enhanced by adding copper ions [44]. Gold forms astable anionic complex with thiosulphate according to thefollowing reaction:

2Au + 0.5 O2 + 4S2O32- + H2O 2Au (S2O3) 2

3- + 2OH-

Although the gold thiosulphate complex is fairly stable onceit has formed, alkaline conditions are necessary to preventthiosulphate decomposition by acid [31]. As Aylmore [51]explains, the thiosulphate leaching process is catalysed by

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copper and has several advantages over the conventionalcyanidation process, in spite of its slow rate. Acceptablegold leaching rates using thiosulphate are achieved in thepresence of ammonia, together with copper [52]. In recentyearsthe reagent has been considered seriously as a potentialsubstitute for cyanide because it generally causes fewerenvironmental impacts: thiosulphate leaching exhibits lessinterference from foreign cations and poses fewer pollutionconcerns [53-54].

The use of thiosulphate as a leach reagent in gold processingwas first proposed during the early-1900s. As Aylmore andMuir [55] explain, in a procedure known as the “Von PateraProcess”, gold and silver ores, first subjected to achloridizing roast, were leached with thiosulphate. BeforeWorld War II, silver-rich sulphide ores in South America andthe ore at Mexico’s LeColorado Mine were leached commerciallywith thiosulphate following a chloridizing roast [56-57].Improvements were made to the procedure during the 1980s,most notably through the addition of copper ions andstabilization of thiosulphate by addition of SO2 orbisulphite [18].

Several promising experimental investigations [58-60] havesince been undertaken. Most of this work has been carried outon complex ores containing copper, carbonaceous ores, or orescontaining high concentrations of lead, zinc or manganese.For example, Yen et al. [61] used a mildly refractory goldore containing pyrite and chalcopyrite to determine thefeasibility of thiosulphate as an alternative heap leachingtechnology. The authors report that a gold extraction of 72%was achieved over a period of 50 days. The US Bureau ofMines used statistical experimental methods to determinethiosulphate leaching feasibility. Gold extractions of up to62% were achieved from the leaching of low-grade carbonaceousores [62].

A number of the more recent research efforts have targetedthe development of pre-treatment strategies in order toimprove recovery with thiosulphate. Balaz et al. [63]investigated the use of a mechanically-activated flotationconcentrate from a complex copper-lead-zinc ore from BanskaHodrusa, Slovakia. Mechanical activation of the concentrate

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was achieved by ultra-fine grinding and proved to have abeneficial influence on the rate of extraction and recoveryof gold using ammonium thiosulphate in the presence of copperions. The authors report that 90% gold recovery was achievedin only a few minutes, compared to the 54% extractionachieved under equivalent conditions for untreatedconcentrate. Liu et al. [64] reported that sponge gold of99.92% purity was produced by ammonium thiosulphate leaching,although it was found that the addition of copper sulphatenot only accelerated the rate of gold leaching, but alsoincreased thiosulphate consumption. Panayotov [65] reportedsimilar results, indicating that a gold recovery of 91% canbe achieved using thiosulphate, if the concentrate is pre-oxidised. In commercial practice, thiosulphate has been usedsuccessfully by Newmont and Consolidated Empire Gold Inc. inheap leaching of gold ore.

The principal problem with thiosulphate leaching is the highconsumption of reagent during extraction [54]. Moreover, theprocess is generally slow, although acceptable leaching ratescan be achieved in the presence of ammonia using copper (II)as an oxidant [51, 53]. Nevertheless, high reagentconsumption renders most thiosulphate systems uneconomicaloverall, in spite of their potential environmental benefits.At present, no simple and affordable method for recoveringgold from thiosulphate leach solutions exists [10].

Coal-oil gold agglomeration

The coal-oil gold agglomeration (CGA) method has beenidentified as an alternative to cyanide for large-scaleoperations. More recently, it has also been promoted as analternative to mercury amalgamation in the small-scale(artisanal) gold mining industry. Although the ability ofoil to recover gold was known in the early-1900s, as Calvezet al. [66] explain, it was not until BP Australia patentedthe CGA process in 1986 that it began gaining recognition inthe public domain as an alternative to cyanide in the goldmining industry. Batmen Engineering International Ltd. andCharlton Mineral Associates Pty Ltd. have also patentedvariations of the CGA method.

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As described by Kotze and Petersen [67], the CGA process isbased upon the recovery of hydrophobic/oleophilic free goldparticles from ore slurries; gold grains ranging from one to100 microns can be recovered with ease. Oil serves as abridge between coal and gold particles, whilst coal acts as acarrier and facilitates effective separation of the gold-bearing agglomerates. The CGA method has proved to beeffective if used to process liberated/free gold particles,such as those contained within alluvial deposits and someprocess tailings.

Considerable experimental work has been undertaken whichsuggests that CGA can be an effective gold ore processingalternative. For example, findings by Bellamy et al. [68],who tested ore samples collected from a New Zealand beach,show that the CGA method is a quicker, cleaner and moreeffective method than convention gold processing techniques,including cyanidation. Bonney [69] conducted laboratoryanalysis on mixtures of silica and gold powder and pilottests on gold tailings; the laboratory results showing thatunder optimum conditions, gold recoveries of 95% could beachieved, whilst pilot test results gave recoveries of. 62-75%

Additional experimental work has been carried out to furthervalidate the CGA method. For example, Marciano et al. [70]used diesel oil, kerosene and vegetable oil as agglomeratingliquids to process gold-containing materials, achieving 90%gold recovery. Calvez et al. [66] performed tests usingsynthetic goldbearing samples to evaluate the underlyingprinciples by which gold is recovered by coal-oilagglomerates, including oil-coal ratios, agglomerate-oreratios, pH and coal particle size. The results show thatgold recovery increases with decreasing coal particle size,an increased agglomerate-ore ratio, and a decreased oil-coalratio. Moses and Petersen [71] also performed batch tests onsynthetic (7gAu/t) ore, demonstrating that increased oilviscosity led to the formation of stronger agglomerates andhence, greater gold recovery. Kotze and Petersen [67] alsoachieved high gold recoveries (85%) in batch tests onartificial gold slurry mixtures.

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Although promising experimental work has been carried outusing the CGA method, it can only be used to process freegold particles. It therefore has limited potential at thelarge-scale, but as explained previously, it could prove tobe a viable alternative to amalgamation in small-scalemining.

Halides

The use of halide (fluorine, chlorine, bromine, iodine andastatine) systems for gold dissolution pre-dates cyanidation[31]. With the exceptions of fluorine and astatine, allhalogens have been tested and/or used for the extraction ofgold [23]. Prior to the inception of cyanide, chlorine wasextensively used as a leaching reagent for gold recovery fromcalcines, ores and concentrates. Chlorine can be generated inslurries and solutions via electrolysis of NaCl solution orby addition of MnO2 to hydrochloric acid [15]:

MnO2 + 4HCl MnCl 2 + 2H2O +Cl2

Gold is leached rapidly by chlorine at low pH:

2Au + 3Cl 2 2AuCl3

Chlorination rates are favoured by low pH, high chloride andchlorine levels, increased temperatures, and high ore surfaceareas [15, 31]. Although gold dissolution with chlorine issubstantially faster than with cyanide, low concentrations ofsulphides or other reactive components in the ore can makereagent consumption excessive and can reduce AuCl4

- back tometallic gold.

Several investigations have been undertaken to determine theeffectiveness of chlorine [72-76]. Historically, AuCl4

- wasrecovered from chloride leach solutions as metallic gold bynatural reduction on carbon or charcoal, after which the“loaded” carbon was burnt to yield the gold. Today, the goldchloride complex is recovered on activated carbon as metallicgold, which generally leads to the loss of gold to tailings,due to attrition of fine gold particles from the surface ofthe activated carbon particles. Chloride gold extraction is

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more difficult to apply than cyanide extraction, however, fora number of reasons [4]: special stainless steel and rubber-lined equipment is

required to withstand the highly corrosive acidic andoxidizing conditions;

chlorine gas is highly poisonous and must be containedboth to allow optimum utilization of the gas and to avoidany health risk; and

although chlorination was used for pre-treating refractoryand carbonaceous ores in several plants in the UnitedStates in the 1980s, few pilot or demonstration studieshave since been reported on the use of these systems [78-79].

Bromine was first identified as a potential solvent for goldin 1846; its rate of leaching gold is enhanced in thepresence of a protonic cation [15]. However, despite longbeing recognized as a powerful gold extractant, it is onlyrecently that its application has been seriously considered.Bromine offers a number of advantages, including rapidextraction, non-toxicity, and adaptability to a wide range ofpH values.

Although the high dissolution rates give it a definiteadvantage over cyanide, there is often high reagentconsumption with bromine use [18], and because it can combinewith other elemental species to form toxic compounds, thecosts of construction materials to withstand the severeconditions and resultant health impacts can be exorbitant[4]. However, as Tran et al. [72] explain, the bromide-bromine system is receiving increasing attention,particularly since the Great Lakes Corporation patented itsbromine-based gold leaching process, which is based upon aproprietary dibromo-dimethyl hydantoin compound.

Gold leaching rates in iodide solutions are also much fasterthan cyanidation. Despite the fact that gold iodide complexesare the most stable in aqueous solutions (among halogens),iodine continues to be under-utilised and under-researched asa gold lixiviant. This is largely because of its cost [15].

Presently, halide systems have limited potential assubstitutes for cyanide, which itself had replaced gold

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leaching agents such as chlorine in the first place. As LaBrooy [31] explains, halide systems are currently limited toanalytical applications, where the capacity for simultaneoussulphide destruction and speed of dissolution are moreimportant than reagent consumption. Moreover, gold halidecomplexes are generally unstable and critics remain scepticalabout their widespread use because of the significantchemical and process control required during processing tomaintain gold in solution [72]

Conclusions

This paper has reviewed the most promising alternative goldleaching agents to cyanide. Although considerable researchhas been undertaken at a laboratory scale, most, if not all,of the alternatives identified have proven to havelimitations that hinder their widespread adoption in the goldmining industry. Thiourea, for example, despite being aproven technology and suitable for refractory ores, isassociated with high detoxification costs, has limitedrecyclability, and features process parameters that aredifficult to control. Thiosulphate offers the sameadvantages, but is highly unstable and also has highdetoxification costs and limited recyclability. Thiocyanate,whilst being more stable than the former two reagents, isunder researched with considerable obstacles to be overcomebefore it could be considered as a viable alternative tocyanide. The CGA process, developed and patented by BP duringthe mid-1980s, is only suitable for processing free goldparticles, and is therefore more likely to become asubstitute for small-scale mercury amalgamation thancyanidation. Finally, halides, which have been utilized inthis context in the past, are difficult to handle andcontrol, and are also expensive.

It is concluded that, in spite of the vociferous oppositionto the use of cyanide in the gold mining industry by sectionsof the environmental movement, unless further research anddevelopment makes one or more of the alterative lixiviantseconomically competitive, cyanide will continue to be theonly practical leach reagent in large-scale gold extractionprocesses. It should be realised, however, that much of the

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opposition to the use of cyanide in the mining industry isbased on a relatively small number of well-publicisedpollution incidents, most of which were caused either by poordesign or faulty operation of gold extraction processes.Cyanide has been used for over a century at gold mines allover the world and the evidence is overwhelming that in well-designed and properly managed mining operations, cyanide canbe safely used as a gold leaching reagent without significantrisk to human life or the environment.

Acknowledgements

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