Improved gold and silver extraction from a refractory antimony ore by pretreatment

Improved gold and silver extraction from a refractory antimony ore by pretreatment with alkaline sulphide leachOktay Celep,İbrahim Alp ⁎,Hac ıDeveciDepartment of Mining Engineering,Karadeniz Technical University,61080,Trabzon,Turkeya b s t r a c ta r t i c l e i n f o Article history:Received 18February 2010Received in revised form 13October 2010Accepted 14October 2010Available online 21October 2010Keywords:Gold SilverCyanidation AntimonyRefractory oreAlkaline sulphide leachingThe pretreatment of an antimonial refractory gold and silver ore was investigated using alkaline sulphide leaching.The ore assayed 20g/t Au and 220g/t Ag and contained predominantly quartz/clay and barite,and to a lesser extent,sulphides such as pyrite,stibnite,sphalerite,zinkenite (Pb 9Sb 22S 42)and andorite (Sb 3PbAgS 6).The latter mineral was identi fied to be the most important sulphide phase for the occurrence of gold and silver.Cyanide leaching of the ore consistently resulted in low extraction of gold (49%)and silver (18%)con firming the refractory nature of the ore.Alkaline sulphide treatment of the ore under suitable conditions was shown to leach out up to 85%Sb,which remarkably improved the extraction of silver from b 18%up to 90%Ag in the subsequent cyanidation step.Gold extraction was also enhanced by 20–30%.These findings suggest that alkaline Na 2S leaching can be suitably used as a pretreatment method prior to the conventional cyanidation for the refractory Sb-bearing ores.©2010Elsevier B.V.All rights reserved.1.IntroductionGold-bearing ores can be classi fied as either free milling or refractory based on their metallurgical response to cyanide leaching (Adams,2005).While high gold recoveries (N 90%)from free milling ores can be readily achieved,refractory gold ores are often characterized by the low gold extractions (b 80%)by conventional cyanide leaching (Gupta and Mukherjee,1990).The refractoriness of gold ores can result primarily from the inherent mineralogical features such as having locked gold within pyrite/arsenopyrite,gold as tellurides or stibnides,or reactive gangue mineralogy including preg-robbing carbonaceous constituents (La Brooy et al.,1994).Pretreatment of refractory ores/concentrates prior to cyanide leaching is therefore required to render the contained gold and silver readily amenable to extraction.Roasting,pressure oxidation,biooxidation and,to a limited extent,ultra fine grinding are used as pretreatment methods in practice,but their effectiveness depends largely on the nature of the refractoriness (Gunyanga et al.,1999;Iglesias and Carranza,1994;Corrans and Angove,1991).Silver-bearing sul fides including proustite,pyrargyrite,tennantite and tetrahedrite (Table 1)are also refractory in character leading to very poor silver extractions (often ≤10%)in cyanide leaching (Baláž,2000).Alkaline sulphide leaching of tetrahedrite (Balážet al.,1998,2003),stibnite (Ubaldini et al.,2000),enargite (Balážet al.,2000;Curreli et al.,2009)and jamesonite (Balážand Achimovi čová,2006)has proved to be a suitable process for the pretreatment of these minerals to make silver available for subsequent cyanide leaching or to remove hazardous/penalty elements such as arsenic and antimony from ores/concentrates (Table 1).Ubaldini et al.(2000)also noted the bene ficial effect of the temperature on the dissolution of stibnite with this system.The decomposition of some arsenical and antimonial sulphides in alkaline sulphide leaching systems can be given by the following reactions (Baláž,2000;Curreli et al.,2009).Cu 14Sb 4S 13s ðÞ+2S 2−→7Cu 2S s ðÞ+4SbS −2ð1ÞCu 3AsS 4s ðÞ+2S 2−→2Cu 1:5S s ðÞ+AsS 3−4ð2ÞSb 2S 3s ðÞ+2S 2−→Sb 2S 4−5ð3ÞSbS −2+S2−→SbS 3−3ð4ÞPrevious studies on an antimonial gold/silver ore from Akoluk,Turkey have indicated that the ore is refractory in character,presumably linked with the occurrence of the sulphides including zinkenite (Pb 9Sb 22S 42)and andorite (Sb 3PbAgS 6)as the main silver-bearing component in the ore (and gold to a lesser extent)(Celep et al.,2006,2009).Diagnostic leaching tests suggested that the decomposition of the sulphides could improve the extraction of gold and silver by about 29%and about 57%,respectively from the antimonial ore (Celep et al.,2009).Ultra fine grinding or roasting ofHydrometallurgy 105(2011)234–239⁎Corresponding author.Fax:+904623257405.E-mail address:ialp@.tr (İ.Alp).0304-386X/$–see front matter ©2010Elsevier B.V.All rights reserved.doi:10.1016/j.hydromet.2010.10.005Contents lists available at ScienceDirectHydrometallurgyj o u r n a l h o me p a g e :w w w.e l s ev i e r.c o m/l o c a t e /hyd ro m e tthe ore was reported to be ineffective to improve the cyanide leaching of gold and silver(Celep et al.,2009,2010).Despite the extensive alkaline sulphide leaching studies on many gold-and silver-bearing sulphides(Ubaldini et al.,2000;Ficeriováet al.,2002,2005a,b;Balážet al.,2003),to our knowledge,no detailed studies on ores/concentrates containing andorite and zinkenite have been reported.Therefore,this study was designed to evaluate alkaline sulphide leaching as a potential pretreatment process prior to cyanidation.Prior to the leaching tests,detailed mineralogical studies were performed to identify the presence and association of gold and silver within the ore.Because of the intimate association of gold and silver found with antimony minerals,the leaching behavior of antimony from the ore was also monitored during the alkaline sulphide leach tests.In the alkaline leaching tests,the effects of particle size,temperature and concentrations of Na2S and NaOH on the removal of antimony and the extraction of gold and silver in subsequent cyanide leaching were investigated.2.Experimental2.1.MaterialsAn antimonial refractory gold/silver ore sample obtained from Akoluk(Ordu-Turkey)was used in this work.The ore sample was crushed down to−4mm using jaw and roll crushers and riffled to obtain1kg representative sub-samples.These were then ground in a laboratory rod mill or stirred media mill to the desiredfineness(80% passing size,d80=15,10and5μm)prior to the leaching tests.The particle size analysis of grounded samples was performed by a Malvern Mastersizer laser particle size analyzer.The chemical composition of the ore sample(Table2)was determined by wet chemical analysis methods using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy)and NAA(Neutron Activation Analysis)after digestion in aqua regia.The sample was determined to be rich in silver containing220g/t Ag and 20g/t Au.The ore consisted predominantly of quartz/illite(52.2% SiO2)and barite(29.1%BaSO4)with pyrite,stibnite,sphalerite, zinkenite,and andorite being present as sulphide phases(Celep et al.,2006).2.2.Detailed mineralogical characterization of the oreEarlier studies(Celep et al.,2006,2009)have revealed that gold is present as particles ranging from1to88μm in size associated with sulphide minerals and quartz.Further detailed mineralogical analysis of the ore sample was performed to determine gold and silver-bearing phases.Hand-picked pieces of the ore were used to prepare the polished sections for mineralogical examination under an ore microscope(Leitz Wetzlar).Scanning electron microscopy(SEM) studies were also carried out using a FEI Quanta400MK2Model SEM equipped with the EDAX Genesis4XMI Model—a light element energy dispersive system to obtain backscattered electron micro-graphs(BSE).Elemental analysis of the grains was performed with wavelength dispersive(WDS)and energy dispersive(EDS)spectros-copy systems.WDS analyses were carried out by a JEOL JXA8900 electron probe X-ray microanalyzer(EPMA).This system hasfive wavelength dispersive spectrometers(WDS)operated at20kV with a probe current of20to30nA.The occurrence and association of gold and silver are illustrated in Fig.1where the sections were initially observed by ore microscopy (Fig.1a,b,and c)and the same areas were then analyzed under SEM-WDS(Fig.1d,and e)and SEM-EDS(Fig.1f)to confirm the phases. Table3shows the results of microprobe analyses of the phases within the numbered spots in Fig.1.The majority of the gold particles were observed to be smaller than 3μm in size(Fig.1).Andorite was determined to be the main gold-and silver-bearing component in the ore(Fig.1and Table3).Gold particles containing silver also occurred associated with quartz and as inclusions within the minerals such as andorite(Fig.1and Table3). Framboidal pyrite consisted of concentric zones having antimony concentrations accompanying silver(Fig.1b,e and Table3).2.3.Leaching testsIn these tests,the effects of concentrations of sodium sulfide(0.5–4.1mol/L Na2S)and sodium hydroxide(1–3.75mol/L NaOH),tem-perature(20–70°C)and particle size(d80:5–15μm)were studied (Table4).The ground samples(d80:≤15μm)were leached in a1-L glass vessel,which was mechanically stirred at750rpm and placed in a water bath to control the leaching temperature within±2°C.A200-ml leach solution(Na2S+NaOH)was added to the vessel,followed by a70g ore sample.After a leaching period of150min solid and liquid phases were separated byfiltration and thefiltrates were analyzed for antimony,gold and silver.The residues were dried,and also analyzed for gold,silver and antimony after acid digestion to establish a mass balance.The residues were then subjected to cyanide leaching so as to assess the effectiveness of alkaline sulphide leaching pretreatment for the extraction of gold and silver,and its correlation with the removal of antimony—based upon the residue analysis.All cyanide leaching tests(24h)were performed in a1-L glass reactor equipped with a pitched-blade turbine impeller rotating at 750rpm.A200-ml leach solution(1.5g/L NaCN)was added to the vessel.The reactor was aerated at0.3L/min.Table4presents the conditions for cyanide leaching tests.In all tests,10-ml samples were taken from the leach pulp at pre-determined time intervals and then, centrifuged to obtain clear aliquots for the determination of Au,Ag and free cyanide in the solution.Free CN−concentration was determined by titration with a silver nitrate solution(0.02mol/L)Table1Antimony–arsenic sulphide minerals treated by alkaline sulphide leaching. Minerals Formula Process ReferencesTetrahedrite Cu12Sb4S13Sb removal Balážet al.,1998 Tennantite Cu12As4S13As removal Balážet al.,2003Stibnite Sb2S3Sb removal Ubaldini et al.,2000 Proustite Ag3AsS3As removal/AgrecoveryBaláž,2000Pyrargyrite Ag3SbS3Sb removal/AgrecoveryBaláž,2000Jamesonite FePb4Sb6S12Sb removal Balážand Achimovičová,2006Enargite Cu3AsS4As removal/CurecoveryCurreli et al.,2009Andorite Sb3PbAgS6––Zinkenite Pb9Sb22S42––Table2Chemical analysis of the ore samples.Compound Content(%)Element Content(g/ton) SiO252.2Au20.2BaSO429.1Ag220Al2O3 4.7Cu400Fe2O3 1.3As262Sb 1.6Cd62.7Zn 1.5Zr40.7Pb0.4Ni 6.0Sr0.3Tot.S 6.9LOI a 4.6Tot.C500a L.O.I.=Loss on ignition.235O.Celep et al./Hydrometallurgy105(2011)234–239using p -dimethylamino-benzal-rhodanine (0.02%w/w in acetone)as the indicator.The consumption of cyanide was recorded.The concentration of free cyanide was maintained at 1.5g/L over the leaching period by the addition of a 5%NaCN solution.On the termination of cyanide leaching tests,the residues were digested in acid (HCl,HNO 3,HClO 4and HF)to determine the undissolved metal content.Analysis of gold,silver and antimony from the solutions was carried out using an atomic adsorption spectrometer (AAS-Perkin Elmer AAnalyst 200).The extraction of metals was calculated based on the metal content of leaching residues.3.Results and discussion3.1.Cyanidation without pretreatmentCyanidation tests revealed that the extraction of gold and silver from the ore was low (49%Au and 18%Ag)even at a fineness of grind of d 80:−15μm (Fig.2),con firming the earlier findings (Celep et al.,2009).The extraction of gold and silver occurred most extensively over an initial period of 1–3h.Thereafter,the dissolution of gold appeared to be arrested while a signi ficant decrease in theextractionFig.1.(a,b,and c)Presence and association of gold and silver in ore under ore microscopy (Au:Gold;Py:Pyrite;Qz:Quartz;And:Andorite;and Sb –Ag-S:Sb –Ag sulphide)and scanning electron microscopy (SEM)equipped with wavelength dispersive spectrometers (WDS):(d)Au disseminates within the andorite mineral (1,2,3,and 4:andorite;5,6,and 7:Au particles);(e)Au particle in the quartz matrix with framboidal pyrite (1:Au,2,3,and 4:pyrite);(f)Sb –Ag sulphide minerals within quartz (1,and 2:Sb –Ag sulphide;3:quartz).236O.Celep et al./Hydrometallurgy 105(2011)234–239of silver was observed.Zhang et al.(1997)reported a similar decrease in the extraction of silver after2h leaching despite high free cyanide levels of N50mmol/L.Based on the thermodynamic analysis,they suggested that the precipitation of Ag2S occurs in the presence of soluble sulphide since its oxidation is kinetically unfavourable under cyanide leaching conditions.It may be relevant to note that no such trend of decrease in the extraction of silver from the Akoluk ore was observed during the cyanide leaching of the roasted ore samples presumably due to the oxidative removal of sulphide(Celep et al., 2010).3.2.Antimony leaching during pretreatmentThe leaching behavior of antimony from the ore was evaluated during the alkaline sulphide leach tests performed under different conditions(Figs.3and4).At afixed level of2.05mol/L Na2S,the dissolution of antimony was observed to increase with increasing the concentration of NaOH in the range of1–3.75mol/L(Fig.3).This trend was also confirmed at a higher concentration of Na2S(4.1mol/L)in which the extraction of antimony was also consistently higher than that in2.05mol/L Na2S.The enhancing effect of increasing concentra-tions of both Na2S and NaOH on the release of antimony is also shown in Fig.4.This can be attributed to the decomposition of andorite, stibnite and zinkenite in the ore as represented by:2Sb3PbAgS6sðÞ+6S2−→Ag2S sðÞ+2PbS sðÞ+3Sb2S4−5:ð5ÞThe maintenance of alkaline conditions is required to minimise the hydrolysis of S2−(S2−+H2O=HS−+OH−)and hence to increase the effectiveness of sulphide leaching(Baláž,2000;Curreli et al., 2009).This is consistent with the beneficial effect of increasing the NaOH concentration on the release of antimony.Furthermore,the leaching of stibnite would also be expected to occur by NaOH (Ubaldini et al.,2000;Smincáková,2009).The influence of the temperature of Na2S–NaOH alkaline pretreat-ment(4.1mol/L Na2S+1.875mol/L NaOH,d80:15μm and2.5h)on the release of antimony is shown in Fig.5.A substantial dissolution of Sb(54%)was observed to occur even at20°C.However,increasing the temperature to50and70°C improved the release of antimony to ~71%and~84%,respectively.The effect of the particle size of the ore(d80:5–15μm)on the alkaline sulphide leaching process was also studied at70°C,4.1mol/L Na2S and1.875mol/L NaOH.Decreasing the particle size(d80)from 15to5μm did not produce a discernable effect on the solubilisation of Sb,which remained at84–85%(data not shown).During these alkaline sulphide leaching tests,the dissolution of gold,in particular,and other metals from the ore was also monitored. The extent of gold extraction was determined to be3–9%which tended to increase with increasing temperature and concentrations of Na2S and NaOH under the conditions tested.3.3.Cyanidation after treatmentFigs.6–8illustrate the effect of alkaline sulphide pretreatment of the ore under different conditions on the subsequent cyanide extraction of gold and silver.A remarkable69%increase in the extraction of silver was noted to occur when the ore was subjected to alkaline sulphide pretreatment prior to cyanidation(Fig.6).Similarly, gold extraction improved from49%to55–78%after the pretreatment. The highest extraction of silver(87%)was recorded to occur with 4.1mol/L Na2S and2.0M NaOH(Fig.6).The extraction of silver and gold tended to improve with increasing the concentration of Na2S at fixed concentrations of NaOH(Fig.7).It can be inferred from theseTable3Microprobe phase analyses from spots in Fig.1.Spot wt.%1234567891011121314Au84.888.285.584.8Ag 6.710.111.49.914.611.912.714.60.60.50.712.713.1Cu0.3 1.3 1.5 1.40b0.1b0.10.20.30.3Fe41.235.440.8Sb32.940.342.840.8 4.213.4 3.351.752.2Pb38.326.022.025.9 1.2 1.4 1.1As0.70.80.7Zn b0.10.4b0.1S20.221.822.421.948.537.248.434.234.3Si b0.1b0.1b0.145.4 O 3.699.3 2.152.7 Total98.499.5100.199.999.4100.298.399.4100.598.797.498.699.698.1Table4Experimental conditions for alkaline leaching and cyanidation of the ore.Parameter Alkaline leaching CyanidationNa2S concentration,mol/L0.5–4.1–NaOH concentration,mol/L1–3.75–NaCN concentration,g/l– 1.5Pulp density,(w/w),%2624.5Temperature,°C20–50–70±220±2 Leach time,hour 2.524 Agitation,rpm750750 Particle size,d80:micron5–10–155–10–15 Aeration,l/min–0.3pH(NaOH)Not measured10.5±0.3204060801006810121618Leaching time, hoursAuAg224Metalextraction,%Fig.2.Extraction of gold and silver from the as-received ore by cyanidation(1.5g/L NaCN)over a period of24h,particle size(d80)15μm).237O.Celep et al./Hydrometallurgy105(2011)234–239findings that alkaline sulphide pretreatment renders the gold and silver-bearing refractory phases amenable to cyanide leaching.In a similar manner,the extraction of silver in the cyanide leaching was enhanced from 66%to 87%with the increase in the temperature of the alkaline sulphide pretreatment using 4.1M Na 2S and 1.8M NaOH from 20°C to 70°C as illustrated in Fig.8.These findings support the conclusions that the extraction of silver can be correlated with the dissolution of silver-bearing antimony sulphides such as andorite.However,compared with Ag,only an 8%improvement in the extraction of gold was observed with this Na 2S/NaOH composition giving a maximum of only about 60%overall recovery (Fig.8).With this alkali sulphide composition,a decrease in the particle size of the ore from 15μto 5μprior to leaching did not produce any signi ficant effect on the extraction of gold (55–62%Au)and silver (87–91%Ag).4.ConclusionsDetailed mineralogical analysis of the ore revealed that gold occurs as alloyed with silver (~12–15%Ag)and mainly associated with andorite (Sb 3PbAgS 6)which is the most important antimonial silver phase.Alkaline sulphide leaching pretreatment was shown to signi ficantly improve the extraction of gold and silver,in particular,by cyanidation.The temperature and relative concentrations of Na 2S and NaOH in the leach were identi fied as the most important factorsA n t i m o n y r e m o v a l , %NaOH concentration, mol/LFig.3.The effect of the NaOH concentration on antimony removal (temperature:70°C;leach time:2.5h;particle size (d 80):15μm).A n t i m o n y r e m o v a l , %Na 2S concentration, mol/LFig.4.The effect of the Na 2S concentration on antimony removal (temperature:70°C;leach time:2.5h;particle size (d 80):15μm).A n t i m o n y r e m o v a l , %Temperature, οCFig.5.Effect of the temperature on antimony removal inalkaline sulphide pretreatment (particle size d 80:15μm;Na 2S:4.1mol/L;NaOH:1.8mol/L).Fig.6.Cyanidation of the pretreated residues (2.05and 4.1mol/L Na 2S)for gold and silver recovery (1.5g/L NaCN;pH 10.5;leach time:24h;particle size (d 80):15μm).M e t a l e x t r a c t i o n , %Na 2S concentration, mol/LFig 7.Cyanidation of the pretreated residues (2.5and 3.75mol/L NaOH)for gold and silver recovery (1.5g/L NaCN;pH 10.5;leach time:24h;particle size (d 80):15μm).238O.Celep et al./Hydrometallurgy 105(2011)234–239affecting the extraction of antimony and the subsequent cyanide extraction of silver and gold.The highest gold and silver extractions were 78%and 91%,respectively,corresponding to an increase of 29%and 74%compared to the untreated ore.The improvement in silver extraction could be attributed to the leaching of andorite since up to 89%Sb was extracted in the alkaline sulphide leach.These findings support other studies that indicate that alkaline Na 2S leaching is a suitable pretreatment method prior to conventional cyanidation for refractory antimonial gold and silver ores.AcknowledgementsSincere thanks and appreciation go to the Research Foundation of Karadeniz Technical University (Project No:2004.112.008.2)for financial support,to the General Directorate of the Mineral Research and Exploration of Turkey,to Prof.Dr.Do ğan Paktunçand Dr.Yves Thibault for support and to the Gürçelik Mining Trading Ind.Ltd.for kindly providing the ore samples.ReferencesAdams,M.D.,2005.Advances in gold ore processing.In:Adams,M.D.(Ed.),Developments in Mineral Processing 15.Elsevier,Amsterdam.Baláž,P.,2000.Extractive Metallurgy of Activated Minerals.Elsevier,Amsterdam.Baláž,P.,Achimovi čová,M.,2006.Selective leaching of antimony and arsenic frommechanically activated 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