International Biodeterioration & Biodegradation 104 (2015) 194e200
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International Biodeterioration & Biodegradation
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Silver-catalyzed bioleaching of copper, molybdenum and rheniumfrom a chalcopyriteemolybdenite concentrate
Hadi Abdollahi a, *, Mohammad Noaparast a, Sied Ziaedin Shafaei a, Zahra Manafi b,Jesús A. Mu~noz c, Olli H. Tuovinen d
a School of Mining, College of Engineering, University of Tehran, Tehran 1439957131, Iranb Sarcheshmeh Copper Complex, Research and Development Centre, Rafsanjan 7731643181, Iranc Department of Materials Science and Metallurgy, Faculty of Chemical Sciences, Complutense University of Madrid, ES-28340 Madrid, Spaind Department of Microbiology, Ohio State University, Columbus, OH 43210, USA
a r t i c l e i n f o
Article history:Received 14 December 2014Received in revised form8 May 2015Accepted 8 May 2015Available online xxx
This study evaluated silver-catalyzed bioleaching to solubilize copper from a chalcopyrite-bearingmolybdenite concentrate with mesophilic (32 �C) and moderately thermophilic (45 �C) microbial con-sortia. The Cu content of the concentrate was 0.98% (wt/wt), with chalcopyrite as the major Cu-bearingmineral. The molybdenite phase (53.8% Mo) also contained 0.055% rhenium. The addition of Agþ as silvernitrate greatly enhanced copper dissolution under bioleaching conditions. Overall, >93% Cu was leachedin the bioleaching experiments involving the addition of up to 200 mg l�1 Ag, contrasted by 53% Cusolubilization in the absence of silver. The highest relative yields (>99%) were obtained under moderatelythermophilic conditions. Different pulp densities (3e9%) and initial pH values (1.5e1.9) were also testedin the bioleaching experiments but their effects were masked by enhancement of copper leaching due tosilver catalysis. Molybdenum and rhenium responded comparably to changes in bioleaching conditions,suggesting that Re was located in the molybdenite matrix. Their relative dissolution was <1% Mo and <6%Re, and only a minor enhancement of their dissolution was observed upon Ag addition. Compared withthe bioleaching with mesophiles at 32 �C, the dissolution of Mo and Re was lower in the 45 �C experi-ments with moderate thermophiles. Because the corresponding yields of Cu dissolution increased withthe temperature, this difference suggested early passivation of molybdenite, Mo precipitation, or asuppression of molybdenite oxidation capacity of the moderately thermophilic culture at the highertemperature.
Cu- and Fe-sulfides are typical impurities inmolybdenite (MoS2)concentrates and are difficult to remove adequately by conven-tional physical and chemical separation techniques. In principle,Cu- and Fe-sulfide minerals can be oxidized in bioleaching pro-cesses but concurrent solubilization of molybdenite may also occur.The leaching of mixed sulfide minerals involves galvanic in-teractions. In the electrochemical series of semiconductive sulfideminerals, molybdenite has a higher electrode potential than chal-copyrite (CuFeS2). If only controlled by galvanic coupling,
molybdenite in contact with chalcopyrite acts as a cathode and isprotected from oxidative leaching while chalcopyrite is anodicallydissolved. However, many other factors interplay in the bioleachingprocess. The bioleaching of sulfide minerals involves oxidants suchas Fe3þ and iron- and sulfur-oxidizing, acidophilic prokaryotes.Bioleaching of molybdenite in acid solutions leads to the formationof the molybdate ion (Romano et al., 2001a,b,c).
The bioleaching of molybdenite is mediated by ferric iron. Therole of acidophilic prokaryotes in this process is to oxidize ferrousiron and sulfur compounds and help maintain high redox poten-tial in favor of molybdenite oxidation as determined by the Fe3þ/Fe2þ ratio. Mesophilic bioleaching of molybdenite was testedalready in the mid-1950's with undefined iron- and pyrite-
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oxidizing bacterial cultures (Bryner and Anderson, 1957; Bhappuet al., 1965). Brierley (1974) published results for the bioleachingof a molybdenite sample with a thermophilic archaeon, an Acid-ianus sp., tested at 60 �C. In general, molybdenite dissolution inbacterial or archaeal cultures was clearly higher than in abioticcontrols. Molybdenite is considered to be relatively recalcitrant inbioleaching systems (Lasheen et al., 2015) and the reported yieldsand rates have not been sufficient to warrant further developmentfor application.
Molybdenite can be the host for rhenium, andminor amounts ofRe are solubilized during the bioleaching process (Askari Zamaniet al., 2005). Rhenium is associated with molybdenum as anisomorphic solid solution of ReS2 in MoS2 (Rouschias, 1974).Rhenium may also partially substitute for isovalent Mo in molyb-denite (Voudouris et al., 2009; da Silva et al., 2013).
Selective bioleaching of the Cu-sulfide fraction is one of thepotential pretreatments for upgrading Cu-containing molybde-nite concentrates. Chalcopyrite associated with host molybdeniteis a problem in this regard because of its refractory properties.The bioleaching of chalcopyrite is a relatively slow processcompared to secondary Cu-sulfides. Secondary solid phasesformed on the chalcopyrite surface contribute to the refractoryproperties of chalcopyrite. These secondary phases include theformation of elemental S and Fe-deficient, Cu-rich intermediatesulfides on chalcopyrite surface (Debernardi and Carlesi, 2013; Liet al., 2013; Yang et al., 2015). Secondary Cu-sulfides alter thesemiconductive properties of the mineral surface and contributeto surface passivation. The formation of jarosite type precipitateson the chalcopyrite surfaces is also believed to contribute topassivation.
Elevated temperature and catalytic ions have been tested inattempts to alleviate the passivation effect on chalcopyrite surfaces.The use of thermoacidophilic iron- and sulfur-oxidizing archaea athigh temperatures (Plumb et al., 2002, 2008; Norris et al., 2013)increases the rates of bioleaching and attenuates the passivation ofchalcopyrite surfaces. Among catalytic ions, silver is of special in-terest because it has been proven effective and selective inenhancing Cu dissolution through the biological oxidation ofchalcopyrite (Li et al., 2013). The positive catalytic effect of silverions on the bioleaching of chalcopyrite has been demonstrated innumerous bioleaching experiments (e.g., Mu~noz et al., 1998,2007a,b; C�ordoba et al., 2009; Feng et al., 2013). The mechanismof silver catalysis, as proposed by Miller et al. (1981) and Miller andPortillo (1981), is based on the reaction of Agþ to form Ag-sulfide onthe chalcopyrite surface, followed by Fe3þ-mediated oxidation:
CuFeS2 þ 4Agþ/Cu2þ þ Fe2þ þ 2Ag2S (2)
2Ag2Sþ 4Fe3þ/4Agþ þ 4Fe2þ þ 2S0 (3)
Silver recirculates between the solution and solid phase re-actions and the overall sum of the chalcopyrite reaction yieldselemental S:
CuFeS2 þ 4Fe3þ/Cu2þ þ 5Fe2þ þ 2S0 (4)
Elemental Ag can also form during the process (Mu~noz et al.,1979).
CuFeS2 þ 4Agþ/Cu2þ þ Fe2þ þ 4Agþ 2S0 (5)
The concentration of dissolved silver is affected by the miner-alogical composition of the ore and the formation of precipitatesduring the bioleaching. Ferric iron in the leach solution is subject tohydrolysis, leading to jarosite type Fe(III)-hydroxysulfates insulfate-rich solutions in the presence of monovalent cations. Silver
can precipitate as argentojarosite with an ideal formula of AgFe3(-SO4)2(OH)6 in the bioleaching environment (Ahonen and Tuovinen,1990; Sasaki et al., 1995; C�ordoba et al., 2009) and may contributeto its depletion from the solution phase. Ag-jarosite lacks the cat-alytic effect on chalcopyrite leaching.
Our previous study demonstrated that silver addition enhancesthe bioleaching of Cu from a chalcopyrite-containing molybdeniteconcentrate (Abdollahi et al., 2014). The present work expands theprevious study by examining the effect of several independentvariables, most importantly the concentration of silver, initial pH,pulp density, and mesophilic and moderately thermophilic micro-bial consortia on the bioleaching of chalcopyrite. This study is partof a research program that examines the feasibility of using thesilver catalysis combined with bioleaching technology as a pre-treatment to selectively remove Cu-sulfides from molybdeniteconcentrates. While positive silver catalysis in the bioleaching ofchalcopyrite has been demonstrated in numerous publications,much less is known of its potential effects on catalysis of Mo and Redissolution, which were the other mainmetals of interest in the Cu-containing concentrate. A range of Ag concentration was tested, upto 1000 mg Ag l�1, in an effort to characterize the catalyst effect onthe bioleaching of Cu, Mo, and Re.
2. Materials and methods
2.1. Molybdenite concentrate
The sample of a chalcopyrite-containing molybdenite concen-trate was provided by the Sarcheshmeh Copper Complex of Iran.The particle size distribution was 90% �38 mm. The concentratesample contained (wt/wt) 53.8% Mo, 1.56% Fe, 0.98% Cu, and 0.055%Re. Molybdenite was the major phase (89.8% of the mineralogicalcomposition), and minor phases were chalcopyrite (2.1%), pyrite(2.0%), and covellite (0.3%). As reported by Aminszadeh et al. (2011)for the Sarcheshmeh CueMo deposit, rheniumwas associated withthe molybdenite matrix but its further mineralogical analysis wasnot conducted. Optical microscopic examination indicated that theCu- and Fe-sulfide minerals were mostly liberated from themolybdenite phase in this size fraction. These observations werealso verified with EDAX-analyses, which were carried out withscanning electron microscopy and backscattered electron emission.The same bulk sample was used in our previous study of silvercatalysis (Abdollahi et al., 2014).
2.2. Microorganisms
Two mixed cultures of acidophilic microorganisms, detailed byAbdollahi et al. (2014), were used in the bioleaching experiments.The mesophilic consortium originated from mine drainage of theSarcheshmeh copper mine, Kerman Province. The consortium con-tained Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidansand Leptospirillum ferrooxidans. The consortium was cultured at32 �C. The moderately thermophilic consortium was obtained fromMintek SA, Johannesburg, South Africa and it contained Acid-ithiobacillus caldus, Leptospirillum ferriphilum, and Sulfobacillus andFerroplasma spp. This consortiumwas cultured at 45 �C. The cultureswere maintained with 10% chalcopyrite concentrate as an energysource in shake flasks at 150 rpm. The mesophilic consortium wascultured in 9 K mineral salts at pH 1.8 (per liter: 3.0 g (NH4)2SO4,0.5 gMgSO4$7H2O, 0.1 g KCl, 0.5 g K2HPO4, and 0.01 g Ca(NO3)2). Themoderately thermophilic consortium was maintained with chalco-pyrite in mineral salts medium that contained (per liter) 0.2 g eachof (NH4)2SO4, MgSO4$7H2O, and K2HPO4 at pH 1.5. Both mixedcultures were gradually adapted to grow with up to 10%
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molybdenite concentrate as the sole energy source for the bio-leaching experiments.
2.3. Bioleaching tests
The bioleaching experiments were carried out in shake flasks at150 rpm at 32 �C (mesophiles) or 45 �C (moderate thermophiles).The molybdenite suspensions in mineral salts media were condi-tioned for 24 h for acid demand, followed by adjustment withsulfuric acid to pH 1.6 before inoculation and addition of 10 g Fe2þ
(as ferrous sulfate) and 10 g S0 per liter. The experiments withmesophiles tested the effect of seven levels of Ag addition(0e1000 mg l�1 Ag) as AgNO3 on the bioleaching of Cu, Mo, and Refrom the molybdenite concentrate at initial pH 1.6, 3% pulp density,and with 15% inoculum previously grown with the molybdeniteconcentrate. The concentrations of silver in these tests wereintended to include a range of positive catalytic and inhibitory ef-fects. The experiments with the moderately thermophilic pro-karyotes involved 30 bioleaching tests with the initial pH (pH 1.5,1.7, 1.9), concentration of solids (3, 6, and 9% pulp densities), size ofinoculum (10, 15, and 20% v/v) from a molybdenite-grown culture,and addition of silver as Ag-nitrate (0, 23, and 46 mg l�1 Ag) asindependent variables. The results of the bioleaching experimentsat 32 �C and 45 �C, with S7 and S30 combinations of time courses,respectively, are presented mostly as pooled clusters in order todemonstrate the general trends. Silver catalysis over a range of Ag-concentrations including high levels of up to 1000 mg Ag l�1 areexamined for the bioleaching of Cu, Mo, and Re from the molyb-denite concentrate sample.
Water lost by evaporation was compensated for by addingdistilled water. Periodically, pH and redox potential (Pt electrodewith an Ag/AgCl reference electrode) were measured and 5 mlsamples were removed from the cultures for analysis of soluble Cu,Mo, and Re with inductively coupled plasma-atomic emissionspectroscopy. The sample volumes were replaced with fresh min-eral salts solution.
3. Results and discussion
3.1. Mesophilic tests
The effect of silver on the mesophilic bioleaching of themolybdenite concentrate was tested in the range of 0e1000 mgAg l�1, added as AgNO3. Selective results are shown in Fig. 1 asexamples of changes over time in pH, redox potential, cell countsand the leaching of Cu, Mo, and Re with 0, 120, and 1000 mg l�1 Agaddition.
The initial redox potential was about 560 mV in all experi-mental conditions. In the absence of Ag addition, the redox po-tential increased and remained at 620e670 mV. At 120 mg Ag l�1,the redox potential declined and then transiently raised to 670mVrange, indicating an increase in the oxidation capacity of thesuspension (Fig. 1). The subsequent decrease to 500e550 mVrange suggested loss of oxidation capacity, perhaps due to inhi-bition of bacterial oxidation of the molybdenite/Fe2þ/S0 admix-ture. At the highest silver concentration, 1000 mg Ag l�1, thecontinual decrease in the redox potential suggested the inhibitionof the bioleaching.
In the absence of silver, the pH increased in the first five days,and then decreased in the following 25 days; the final pH stabilizedat 1.4e1.6 after 20 days of bioleaching. In the presence of0e400 mg Ag l�1, the initial cell counts generally decreased by lessthan one order of magnitude and were subsequently more or lesssteady for the rest of the time course (Fig. 1). An exception was theculture with 1000 mg Ag l�1, in which the cell numbers decreased
by two orders of magnitude especially toward the end of theexperiment. As this was a single culture the experimental validitycannot be proven without further analysis.
About 40% of Cu was dissolved in the mesophilic bioleachingfrom the molybdenite concentrate after 30 days without silveraddition. The bioleaching of copper increased with 60 mg l�1 Agaddition and reached 96e98% solubilization at120e400 mg Ag l�1 (Fig. 2). Copper recovery decreased by almost50% when silver concentration was increased to 1000 mg Ag l�1.This decrease in copper bioleaching indicated that the1000 mg l�1 silver concentration exceeded the level needed forthe catalytic effect. The toxic threshold level of silver in culturesolution of iron-oxidizing acidithiobacilli in the absence of sul-fide minerals is in the order of <1 mg Ag l�1 (Tuovinen et al.,1985). This level of sensitivity to silver is not applicable tochalcopyrite bioleaching systems because the silver catalyst cir-culates between the solid and solution phases (reactions 2e5),minimizing silver bioavailability and alleviating the toxicity. It isnot clear whether silver added at 1000 mg l�1 results in toxiclevels to the bacterial culture because it is in excess of the con-centration required for the catalyst effect. Since silver was addedas AgNO3, the toxicity of nitrate at 575 mg l�1, equaling theaddition of 1000 mg Ag l�1, should also be considered. Previouswork (Wang et al., 2013) has demonstrated that nitrate ion at thecomparable concentration has a medium inhibitory effect on ironoxidation by A. ferrooxidans. At lower AgNO3 concentrations thetoxicity of nitrate is insignificant. Similarly, medium inhibitoryresponses to nitrate (tested as KNO3) concentrations of600e1200 mg l�1 have been reported for several bioleachingmicroorganisms (Shiers et al., 2014).
It also seems plausible that at 1000 mg Ag l�1 the thickness orthe composition of the Ag-containing layer formed on chalcopyritesurface may retard the dissolution process. This layer may alsoinvolve Ag sequestration as argentojarosite (Ahonen and Tuovinen,1990; Palencia et al., 1998; Mu~noz et al., 2007a,b). The formationand the amount of Ag-jarosite was not verified in the present study.In addition to argentojarosite, secondary phases in this systeminevitably involve the formation of a solid solution of hydronianjarosite, possibly in admixture with K-jarosite and NH4-jarosite, allof which are difficult to differentiate by X-ray diffraction analysisamidst solid leach residues. Confirmation of argentojarosite in re-sidual solids would require chemical analysis of the jarositefraction.
The silver catalyst effect at 68 mg Ag l�1 has been reported toresult in the formation of Fe-deficient Cu-sulfides (C�ordoba et al.,2009), which can obstruct the galvanic contact between chalco-pyrite and silver. Silver does not have the catalyst effect on thebioleaching of secondary Cu-sulfides. The formation of a minoramount of chlorargyrite (AgCl) is also possible due to the pres-ence of 0.1 g KCl l�1 (46 mg Cl� l�1) in some mineral saltsformulation such as 9 K used for mesophiles in the present study.While it is clear that 1000 mg Ag l�1 is unrealistically high forany practical application, its effects highlight the complexinterplay involving microorganisms, mineral surfaces and cata-lyst effects in the bioleaching of chalcopyrite from molybdeniteconcentrate sample. The uncertainty of data interpretation andmechanistic interpretation of Ag effects cannot be resolvedwithout further study.
The bioleaching of Mo and Re responded similarly to silveraddition, with about 1%Mo and 6% Re dissolution in the presence of120mg Ag l�1 Ag, and at 1000mg l�1 Ag suppressed the dissolution(Fig. 3). The toxicity of molybdenum to microorganisms during thebioleaching of chalcopyrite and molybdenite has not been un-equivocally defined and contrasting results on Mo toxicity andleaching yields have been reported in the literature. For example,
Fig. 1. Changes in (A) pH, (B) redox potential, (C) cell counts �10�7 (semilogarithmic scale), and leaching of (D) Cu, (E) Mo, and (F) Re in selected mesophilic experiments with andwithout silver addition. Symbols: >, no Ag; B, 120 mg Ag l�1 (4.00 g kg�1 solids); △, 1000 mg Ag l�1 (33.3 g Ag kg�1 solids). Initial pH 1.6, 3% pulp density, 15% inoculum. 100% Cudissolution ¼ 294 mg Cu l�1; 2% molybdenite dissolution ¼ 323 mg Mo l�1; 10% rhenium dissolution ¼ 165 mg Re l�1.
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Nasernejad et al. (1999) reported that dissolved Mo increased to2 g l�1 during the bioleaching of molybdenite in A. ferrooxidanscultures previously adapted toMo. In bioleaching experiments withA. ferrooxidans and a mixed chalcopyrite-molybdenite concentrate,dissolved Mo concentrations were generally below 1% yields(<200 mg l�1) regardless of the pulp density, mineral salts mediumand energy source (Fe2þ, FeS2, or S0) conditions (Askari Zamaniet al., 2006). A microbial consortium dominated by Leptospirillumferrooxidans was tested by Olson and Clark (2008) for the bio-leaching of molybdenite. Dissolved Mo concentrations reached upto 4.4 g Mo l�1 during the bioleaching, but in separate tests, in theabsence of the concentrate, bacterial oxidation of Fe2þ was inhibi-ted at 97 mg Mo l�1. Similarly, Pistaccio et al. (1994) and Pradhanet al. (2009) reported Mo toxicity to A. ferrooxidans at concentra-tions >96 mg Mo l�1 and >40 mg Mo l�1, respectively. Karavaikoet al. (1989) reported that iron oxidation by A. ferrooxidans wasinhibited in the range of 24e50 mg Mo l�1. Toxicity was alleviatedby increasing the amount of inoculum from a spent culture orincreasing the concentration of ferric iron. Molybdenum resistance
to 120 mgMo l�1 was reported by Yong et al. (1997) for iron-grownA. ferrooxidans. Askari Zamani et al. (2005) determined thatfollowing adaptation to molybdenite A. ferrooxidans could grow inthe presence of 250 mg Mo l�1 if pyrite was added as the substrate.A comparable threshold level of 248 mg Mo l�1 was reported foriron-oxidizing A. ferrooxidans by Venkatakrishnan and Chandra(1979). Thus, while pure culture studies of iron-oxidizing acido-philes have yielded results for toxicity thresholds in the range of40e250 mg Mo l�1, contrasting data have been reported for mo-lybdenum toxicity in bioleaching situations, possibly because ofmolybdate complexation with ferric iron which alleviated theinhibition.
The enhancing effect of silver addition on the bioleaching of Moand Re between 120 and 400 mg l�1 Ag was minimal (Fig. 3),suggesting that Ag catalysis was selective for chalcopyrite bio-leaching and did not appreciably enhance the dissolution of thehost molybdenite matrix. The dissolution of Mo and Re followed asimilar response to experimental conditions (Fig. 4). The data in thescatter plot (Fig. 4) could be described with a linear regression
Fig. 2. Effect of silver (added as AgNO3) on the bioleaching yields of Cu with themesophilic consortium after 30 days of contact time (initial pH 1.6, 3% pulp density,15% inoculum). 100% Cu leaching ¼ 294 mg Cu l�1.
Fig. 4. Scatter plot of the relative dissolution of Re and Mo in the mesophilic bio-leaching experiments (32 �C). All time course data were used to construct the plot. TheR2 value is for data fit in the linear regression model.
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model (R2¼ 0.912). Based on Eh-pH diagrams, dissolved molybde-num occurs as MoO4
2� (molybdate) and rhenium as ReO4�
(perrhenate) as their oxyanion chemistry is similar in acid solutions(Brookings, 1988; Icenhower et al., 2008).
B
Fig. 3. Relative yields of (A) molybdenum and (B) rhenium dissolution after 30 days ofcontact time in mesophilic bioleaching experiments.
Copper leaching at 45 �C by the moderately thermophilic con-sortium was tested in matrix of pulp density (3, 6, and 9%) andsilver addition (0, 23 and 46 mg l�1 Ag) as independent variables.These additions equal 0.77 and 1.53 g Ag kg�1 solids at 3% pulpdensity, 0.38 and 1.00 g Ag kg�1 solids at 6% pulp density, and 0.26and 0.51 g Ag kg�1 solids at 9% pulp density. The size of the inoc-ulum (10, 15 and 20% v/v) and initial pH (1.5, 1.7, and 1.9) were alsovaried in these experiments. However, the effects of inoculum sizeand initial pH were negligible (data not shown) and masked bypulp density and silver addition. Regardless of the initial pH, acidconsumption took place initially followed by minor changes for upto 25 days.
The solubilization of copper plateaued after 12 days of contacttime with <5% change for up to 30 days. Silver-catalyzed leachingwas relatively fast as about 90% of the final yield was obtained inthe initial five days. Fig. 5 presents the relative solubilization of Cuand Mo as pooled averages from experiments at three Ag concen-trations (0, 23 and 46 mg l�1), three initial pH values (1.5, 1.7, and1.9), and three pulp densities (3, 6 and 9%). Silver addition had anoverriding effect on copper bioleaching, with the yields reaching>98e99% Cu dissolution. In the absence of silver, copper yieldswere 40e55%, the highest being at 3% pulp densities. The concen-trations of added silver were lower than in the mesophilic experi-ments, but the Ag-catalyzed Cu dissolution in the moderatelythermophilic cultures was substantial, demonstrating theenhancing effect of higher temperature in combination with Agcatalysis. The effect of Ag-catalysis on the bioleaching of Cu wascomparable at 32 and 45 �C. In general, these results are compatiblewith the positive effect of silver on the rate of chalcopyrite leachingas reported in the literature (Watling, 2006; Li et al., 2013).
The relative dissolution of Mo and Re remained at <0.35% and<2%, respectively, in the moderately thermophilic experimentsunder all experimental conditions (Fig. 6). The low extent of Mo andRe dissolution demonstrated that the effect of silver addition andpulp density could not be discerned with these data (Figs. 5 and 6).Thus silver catalysis was inefficient for enhancing the dissolution ofMo from molybdenite in moderately thermophilic cultures.
In agreement with these low Mo recoveries, Rastegar et al.(2014) reported a maximum of 1.7% leaching of Mo from a Sarch-eshmeh molybdenite concentrate sample at 60 �C using thermo-philic Acidianus brierleyi with 0.3% pulp density. In the present
Fig. 5. Relative yields of Cu and Mo dissolution in the presence of silver addition (0, 23and 46 mg Ag l�1) and pulp density in the moderately thermophilic mixed culture after30 days contact time. The data are averages pooled from all 45 �C experimentsinvolving 10, 15 and 20% inoculum and initial pH 1.5, 1.7 and 1.9.
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study, the moderately thermophilic bioleaching of Mo and Re wasless responsive to Ag catalysis as compared to the mesophilestested at 3% pulp density. This cannot be attributed to the higherpulp density because the Ag-enhanced bioleaching of chalcopyritewas not adversely affected at the 3, 6 and 9% pulp densities used atmoderately thermophilic temperatures. The highest Mo concen-tration after 30 days of contact time was about 170 mg Ag l�1
(0.34%), lower than in the mesophilic experiments, and conse-quently also the concentration of Re was lower. For reference, Yuet al. (2015) reported that the concentrations of Mo increased to350e550 mg l�1 during the bioleaching of a molybdenite concen-trate sample tested with moderate thermophiles at 44 �C. Reasonsfor the suppressed molybdenite dissolution, such as limited solu-bility of molybdate, passivation of molybdenite surface, or molyb-denum toxicity to the moderately thermophilic consortium, needadditional investigation.
Fig. 6. Scatter plot of the relative dissolution of Re and Mo in the moderately ther-mophilic bioleaching experiments (45 �C). The plot was constructed with all data fromthe 30 days of contact time in experiments with 0, 23 and 46 mg Ag l�1; initial pH 1.5,1.7; 10, 15 and 20% inoculum; and 1.9, 3, 6 and 9% pulp density. The R2 value is for datafit in the linear regression model.
4. Conclusions
Silver catalyst effects involve dynamic reactions with both bio-logical and chemical components in the bioleaching of chalcopyriteand molybdenite. The amount of copper dissolved from themolybdenite concentrate was substantially improved by the addi-tion of silver as AgNO3. In all bioleaching experiments >93% Cu wassolubilized in the presence of up to 200 mg Ag l�1, contrasted by40% Cu dissolution without silver addition. The concentration ofsilver had the single most significant effect over pulp density,inoculum volume and initial pH. The relative dissolution of mo-lybdenum and rhenium remained low and both responded simi-larly to experimental conditions. The study demonstrated thatrhenium and molybdenum are refractory to bioleaching conditionseven with silver catalysis. The toxicity of Mo and Re to iron andsulfur oxidation by acidophilic prokaryotes is unclear. Further, it isnot understood whether the published Mo toxicity data are rele-vant to the bioleaching of molybdenite ores and concentrates andhow the silver catalysis impacts silver toxicity to microbes in thebioleaching of these sulfide minerals. The practical application ofsilver-catalyzed bioleaching process remains vague because ofproblems of Ag recovery and recycling. The role of iron warrantsdetailed analysis in Ag-catalyzed bioleaching of chalcopyrite inlight of its role as a redox shuttle and oxidant of sulfide mineralphases and its chemical and biological transformations affecting itssolubility and precipitate composition.
Acknowledgments
This work was supported by the National Iranian Copper In-dustry Co and Geological Survey of Iran.We are grateful to A. Amini,Head of Mineral Processing Division, Geological Survey of Iran, andS. Daneshpajouh, Head of Hydrometallurgy, Sarcheshmeh CopperComplex, for facilities and scientific and technical assistance. Wethank all members of the project team for their contributions to thework reported in this paper.
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