International Journal of Mineral Processing 132 (2014) 1–7

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International Journal of Mineral Processing

j ourna l homepage: www.e lsev ie r .com/ locate / i jm inpro

Bacterial and chemical leaching of chalcopyrite concentrates as affectedby the redox potential and ferric/ferrous iron ratio at 22 °C

Denise Bevilaqua a,b,⁎, Heidi Lahti-Tommila a, Oswaldo Garcia Jr. a,b,1, Jaakko A. Puhakka a, Olli H. Tuovinen a,c

a Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101 Tampere, Finlandb Institute of Chemistry, UNESP, Univ. Estadual Paulista, Araraquara, SP CEP 14.901-970, Brazilc Department of Microbiology, Ohio State University, 484 West 12th Avenue, Columbus, OH 43210, USA

⁎ Corresponding author at: Departamento de BioquInstituto de Química de Araraquara, Universidade Estadu14.901-970, Brazil. Tel.: +55 16 3301 9677; fax: +55 16

E-mail address: denise@iq.unesp.br (D. Bevilaqua).1 Deceased (September, 2010).

http://dx.doi.org/10.1016/j.minpro.2014.08.0080301-7516/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 February 2014Accepted 22 August 2014Available online 29 August 2014

Keywords:AcidithiobacillusBioleachingChalcopyriteIron oxidationPassivation

Biological oxidation of chalcopyrite by acidophilic and mesophilic bacteria becomes hindered over time due topassivation of the surface, making the mineral relatively recalcitrant to beneficiation in bioleaching processes.Several approaches have been discussed in the literature to overcome the passivation effect. The purpose ofthis work was to assess the effect of redox potential and Fe3+/Fe2+ ratios on the bioleaching of chalcopyritein shake flasks. Twomixed cultures, three pure cultures of acidophiles, and two samples of chalcopyrite concen-trates were used for this study. The initial redox levels between 350 and 600 mV were adjusted with Fe2+ andFe3+ ratios, Σ200 mM and Σ300 mM Fe, at 0.33 and 2.5% pulp densities, respectively. In general, the differencesin copper leaching were relatively minor among the test cultures and between the chemical controls andinoculated cultures. Copper dissolution increased linearly when the redox potential was in the 320–370 mVrange, and this involved the inhibition of bacterial iron oxidation by the chalcopyrite sample.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Chalcopyrite is known for its refractory properties in hydrometallurgyand biomining (Debernardi and Carlesi, 2013). This primary Cu-sulfidehas a wide band gap (0.6 eV) between the filled valence and empty con-duction bands and high lattice energy of 17,500 kJ, which both contributeto its refractory nature. In their review on chalcopyrite leaching,Debernardi and Carlesi (2013) focused on the semiconductor propertiesof chalcopyrite. The charge excess created during the oxidation ofchalcopyrite is distributed within a thin surface space charge region,and this changes the energy level of conduction and valence bandsand generates a decrease in the potential that hinders the dissolutionprocess. Therefore, in addition to secondary Fe(III) precipitates,intermediates of chalcopyrite oxidation accumulate on the surfacelayer and they include a variety of sulfur compounds (Debernardi andCarlesi, 2013; Majuste et al., 2013). The practical outcome is the incom-plete and slow oxidation of chalcopyrite in the presence of chemical orbiological oxidants, leading to intermediate oxidation products insurface layers in accord with the shrinking core model (Sohn, 1979;Wadsworth, 1979).

ímica e Tecnologia Química,al Paulista, Araraquara, SP CEP3322 2308.

The compounds in the surface layers are comprised of non-stoichiometric or Fe-deficient sulfides (e.g., covellite and chalcocite),elemental S or polysulfur, and Fe(III)-hydroxysulfate precipitates(mostly jarosite types). These compounds can slow down the diffusionand attenuate the fluxes of reactants and products on mineral surface,causing a passivation effect (Hackl et al., 1995). To overcome theselimitations, several experimental efforts have been made to modifythe conditions for chalcopyrite bioleaching. Examples of enhancementof chalcopyrite bioleaching include elevated temperatures in thethermophilic range to increase the rates of Cu dissolution, addition ofchloride to increase the porosity and modify the composition of thepassivation layer, low pH to alleviate jarosite precipitation, reducedmass transfer of oxygen to control jarosite precipitation, and silvercatalysis to set up galvanic cells on chalcopyrite surface (e.g., Sandströmet al., 2005; Watling, 2006; Klauber, 2008; Vilcáez et al., 2008; Qinet al., 2013). The composition of acidophilic bacteria or archaea,S-oxidizing acidophiles especially, has also been shown to affect thebioleaching of chalcopyrite, but with variable success (e.g., Vilcáezet al., 2008).

Previous bioleaching studies (Third et al., 2002; Gericke et al., 2010)have shown that the passivation of chalcopyrite occurs at high redoxpotentials of leach solutions. The redox potential of leach solutions isdominated by Fe3+/Fe2+ ratios. The solution redox potential range of400–450 mV (Pt vs. Ag/AgCl; 698 mV vs. SHE) has been suggested tobe the threshold, above which the oxidative dissolution of chalcopyritebecomes gradually hindered due to the passivation effect (Córdobaet al., 2008a). Preliminary evidence for the low redox potential effect

Table 1Substrates and initial pH values of liquid media for culture maintenance.

Culture Fe2+ (mM) S0 (g/l) Initial pH Trace metals

L. ferriphilum 80 0 1.4 +A. caldus 0 0.1 1.8 +A. ferrooxidans 120 0 1.8 −TM enrichment 80 1.0 1.8 +

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in chalcopyrite bioleaching was reported by Ahonen and Tuovinen(1993), based on suppressed aeration to minimize ferrous iron oxida-tion. At 450 mV the activities of ferrous and ferric ions are nearlyequal under equilibrium conditions, and experimental data are closeto this theoretical value (Nemati and Webb, 1997). Córdoba et al.(2008b) studied the effect of redox potential on the chemical leachingof chalcopyrite at 35 °C and 68 °C. At 35 °C, Cu dissolution was lessthan 3% in all cases. At 68 °C, almost 90% Cu was extracted at 300 mVand 400 mV in 6 days, contrasted by 30% at 500 and 600 mV evenafter 13 days. Ferric iron precipitation and Fe2+ released concurrentlyfrom chalcopyrite accounted for the initially low redox potential.Controlled redox potential in the bioleaching of chalcopyrite was alsoevaluated by Córdoba et al. (2008c). They reported that redox potentialincreased relatively rapidly to themaximumvalue (650mV) in bacterialcultures and improved copper dissolution while the chemical leachingwas unaffected under these conditions.

Hiroyoshi et al. (1999) described a casewhere iron-oxidizing bacteriainhibited the dissolution of chalcopyrite. The extent of Cu leaching in thepresence of 40 mM Fe2+ was two-fold higher without bacteria as com-pared to the corresponding Acidithiobacillus ferrooxidans-inoculatedbioleaching. This difference may be related to the lower redox potentialin the absence of A. ferrooxidans. Hiroyoshi et al. (2000) proposed amodel for Fe2+-promoted chalcopyrite leaching, where the conditionslead to the formation of chalcocite (Cu2S), which is relatively readilyoxidized by dissolved O2 or Fe3+. Chemical reducing agents such asSO2 may be used to suppress redox potential, but they are readilyoxidized chemically by dissolved O2 or Fe3+ in the solution. Whenthe concentration of Fe2+ and Cu2+ is too low for these reactionsto take place, the dissolution occurs primarily by Fe3+ oxidationand the resulting copper extraction rate is low (Hiroyoshi et al., 2000,2001).

Similar results have been reported by Third et al. (2000), whostudied the inhibition or stimulation of iron-oxidizing bacteria in thebioleaching of chalcopyrite. They concluded that formation of ferriciron due to bacterial oxidation of ferrous iron suppressed chalcopyritedissolution. In further experiments, Third et al. (2002) controlled theredox potential at 380 mV by oxygen limitation. The enhancementwith 0.1 M ferrous iron and the suppression with 0.1 M ferric iron indi-cated that low redox potential was favorable to chalcopyrite leaching.These results showed that copper dissolution from chalcopyrite washindered when bacteria started to oxidize ferrous iron. With controlledredox potential, bacterial oxidation of ferrous iron was limited in orderto prevent ferric iron accumulation but still provided sufficient ferriciron for chalcopyrite oxidation, increasing copper dissolution two-fold.Third et al. (2002) also showed that, when the redox potential wascontrolled, the bacterial leaching of copper was higher than in the cor-responding abiotic control, and the onset of passivation was delayedbut not alleviated. Thus the dissolution of Cu from chalcopyrite is clearlyenhanced at low redox potential values (b450–500 mV Ag/AgCl).

The composition andproperties of passivation layers on chalcopyritegreatly vary with the bioleaching and chemical leaching conditions. Re-gardless of the acid and the oxidant, the passivation typically does notcompletely hinder copper dissolution at ambient temperatures withinthe time courses reported in the literature (e.g., Pan et al., 2012;Harmer et al., 2006; Acero et al., 2007). Tshilombo et al. (2002) reportedthat a thermophilic temperature (65 °C) prevented the formation of thepassivation layer in studies of anodic polarization of chalcopyrite,whereas passivation was severe at 25 °C. This could be interpreted toindicate that chalcopyrite passivation layers formed at thermophilictemperatures are less stable or have faster turnover than those formedat mesophilic temperatures. The underlying factor for passivation inanodic polarization experiments (Tshilombo et al., 2002) was the slowkinetics of ferric iron reduction on CuFeS2 surface. In a practical context,heap leaching of Cu-sulfides involves changes in the outside and insidetemperatures during the active bioleaching phase and their effects onpassivation phenomena have not been characterized.

Gu et al. (2013) investigated the initial reactions in the bioleachingof chalcopyrite by Leptospirillum ferriphilum under low redox potential(b450 mV vs. SHE). They concluded that chalcocite (Cu2S) was formedunder reducing conditions by the reaction of ferrous iron and CuFeS2within the first days of bioleaching of contact with the bioleachingsolution. This secondary solid phase is leached relatively rapidly micro-biologically and chemically. Thus the bioleaching of chalcopyrite wasinitially fast but gradually declined. After 21 days of bioleaching, poroussulfur and jarosite were detected loosely associated on the mineralsurface.

The present study was initiated to assess low redox potentialbioleaching of two chalcopyrite concentrate samples using differenttest cultures under similar conditions. The redox potential conditionswere initially adjusted with Fe3+/Fe2+ ratios. These experiments weredesigned as a screening study at relatively low pulp densities to seekoptimal conditions for the dissolution of Cu from chalcopyrite.

2. Materials and methods

2.1. Bacteria and growth conditions

Pure and mixed cultures of acidophilic bacterial used in thisstudy were (i) a mesophilic consortium designated TM, comprisingA. ferrooxidans, Acidithiobacillus thiooxidans, Acidithiobacillus caldus andLeptospirillum ferrooxidans; originally enriched from the TalvivaaraMine (Finland) pilot plant water samples (Halinen et al., 2009, 2012);and (ii) pure cultures of L. ferriphilum (DSMZ14647, Deutsche Sammlungvon Mikroorganismen und Zellculturen GmbH, Braunschweig,Germany), A. caldus (from M. Dopson, Linnaeus University, Kalmar,Sweden), and A. ferrooxidans LR (Garcia, 1991). Of these test cultures,all but A. thiooxidans and A. caldus can oxidize ferrous iron. The cultureswere grown in mineral salt media (MSM) which contained (per liter)0.5 g each of (NH4)2SO4, K2HPO4, and MgSO4 7H2O as well as tracemetals. The trace metal solution was formulated according toKaksonen et al. (2011). The substrates (either Fe2+ or S0) and the initialpH values for each culture are listed in Table 1. Fe2+ was added asferrous sulfate which was sterilized by membrane filtration. Elementalsulfur was dry sterilized at 150 °C for 24 h. Cultures were transferredin fresh media with 10% (vol/vol) inocula. A. ferrooxidans LR waspreviously adapted to grow with 2.5% (wt/wt) of chalcopyrite.

2.2. Chalcopyrite samples

Samples of chalcopyrite concentrates for this study were providedby Boliden AB, Sweden (sample A) and Vale S.A., Brazil (sample B).Sample A contained 29.3% Cu, 28.6% Fe, 0.33% Zn, 0.16% Pb, 0.017% Biand 32% S. The concentrate comprised 77.4% chalcopyrite, 8.4% pyrite,0.8% pyrrhotite, 0.1% sphalerite, and 13.2% silicate gangue. Silverwas not detected in either sample. Chalcopyrite was the only Cu-containing phase. Sample A was ground to 100% −120 mesh. SampleB contained 23% Cu, 27.3% Fe, 22.9% S, 3.2% Si and 7.0% Ca and comprisedchalcopyrite as the only sulfide phase andminor amounts of quartz andCa-apatite. The sample was ground to 100% −200 mesh.

Table 2Proportions of ferric and ferrous iron (as sulfates) and the corresponding initial redoxlevels for experiments with sample A.

Initial redox (mV) Fe3+ (mM) Fe2+ (mM)

350 0 200420 60 140460 140 60600 200 0

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2.3. Redox potential experiments

The effect of redox potential on the bioleaching of sample A wastested with L. ferriphilum, A. caldus, TM culture, A. ferrooxidans, andA. ferrooxidans LR previously adapted to growwith chalcopyrite. Appro-priate abiotic controls were included in the experiments to account forthe chemical leaching. The experiments were monitored by measure-ment of pH, redox potential, dissolved Cu, Fe2+ and total dissolved Feat intervals. The experiments were conducted in 150 ml cultures in250 ml conical flasks with 0.33% Boliden chalcopyrite concentrate(sample A) and 5% inoculation. All flasks contained about 5 mM ofFe3+ carryover added with the inocula. The mineral sample was drysterilized in a boiling water bath for 10 min. Duplicate cultures wereincubated on a rotary shaker at 160 rpm and at 22 ± 2 °C.

The redox potential was adjusted to four different levels by addingfilter-sterilized stock solutions of ferrous sulfate and ferric sulfate(Table 2). The total Fe concentration was held constant at 200 mM.

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Fig. 1. Changes in redox potentials during chalcopyrite leaching of sample A. A) Chemical contropreviously adapted to grow with CuFeS2. Symbols: in green: ▼, 0 mM Fe3+ and 200 mM Fe2+;black: ■, 200 mM Fe3+ and 0 mM Fe2+.

The pH of the ferric sulfate stock solution was adjusted with KOH to1.4. The pH of mineral salts medium and ferrous sulfate stock solutionwas adjusted with H2SO4 to 1.4. The redox potential and pH levelswere not controlled during the time course.

Bioleaching experiments with A. ferrooxidans were also conductedwith 2.5% Vale chalcopyrite concentrate (sample B), employing themost oxidized (200 mM Fe3+) and the most reduced (200 mM Fe2+)redox conditions. Additional experiments were conducted withA. ferrooxidans cultures supplemented with 100 mM, 150 mM and300 mM Fe2+.

2.4. Analytical methods

The pH and redox potential (Ag|AgCl|KClsat reference electrode)were measured at intervals in culture samples. For chemical analyses,sampleswere centrifuged (10,000×g for 15min) and the concentrationof Fe2+ in the supernatant was measured with the o-phenanthrolinemethod (APHA, 2005). The concentrations of dissolved Fe and Cu weremeasured with atomic absorption spectrometry in HCl-preservedsamples.

Solid leach residues were washed with dilute H2SO4 (pH 1.6–2.0,adjusted to the same pH as the culture suspension) thrice before airdrying at 26 °C, followed by storage under Ar headspace. The mineral-ogical composition was analyzed by XRD (Siemens D-500) equippedwith a diffracted-beam monochromator and CuKα radiation. Sampleswere scanned from10 to 70° 2θ at 0.05° 2θ incrementswith 2 s countingtime.

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3. Results and discussion

3.1. Leaching of CuFeS2 (sample A)

The effect of redox potential on the bioleaching of the Bolidenchalcopyrite sample was examined at four initial redox potential levels,adjustedwith different ferric/ferrous iron ratios (ΣFe 200mM). Changesin redox potential, pH, Cu2+, Fe2+, and total dissolved Fe concentrationswere monitored over time. As a result of Fe2+ oxidation, the redoxpotential increased concomitantly with pH and iron precipitation.Changes in redox potentials over time are presented in Fig. 1. Thehighest Fe2+ concentration (200 mM Fe2+) inhibited iron oxidationby L. ferriphilum, seen as an extended lag phase. The redox potentiallevels in the chemical controls and A. caldus-inoculated flasks remainedrelatively constant. The decrease in the initial 600 mV redox potentialwas attributed to Fe2+ and sulfur constituents leached from chalcopy-rite, and the increase from 350 mV to 400 mV was due to chemicaloxidation of Fe2+.

The redox potentials in A. ferrooxidans cultures remained somewhatlower (~650 mV) than those in L. ferriphilum and TM cultures(~700mV), indicatingminor differences in ferrous and ferric iron ratios.A small increase in the dissolved Fe concentration due to iron releasefrom chalcopyrite was seen at the beginning and a small decrease dueto Fe precipitation was seen toward the end of the time course (datanot shown). At the highest redox potential (200 mM Fe3+/0 mMFe2+), all iron remained as Fe3+ in iron-oxidizing cultures, while the

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in black: ■, 200 mM Fe3+ and 0 mM Fe2+.

concentration of Fe2+ increased to 0.5 g/l (9 mM) in the chemicalcontrol as well as the A. caldus culture because of the lack of ironoxidation.

The pH values increased during chalcopyrite leaching, representinga net effect of acid consumption, iron oxidation, iron precipitation, andacid-producing oxidative reactions. At pH N1.8, iron precipitation wasapparent and pH decreased. With A. caldus, the pH decrease may bedue to carryover sulfur oxidation, but the pH value at 600 mV levelremained constant. The initial pH was affected by the pH of theinoculum, which was lower for the A. caldus and TM cultures than forA. ferrooxidans and L. ferriphilum. Chalcopyrite was most reactive inL. ferriphilum, TM and A. ferrooxidans cultures as pH values increasedover time from pH 1.5–1.6 range to pH 2.1. The pH change in thechemical control was within ΔpH 0.2. At the initial 600 mV the redoxpotential decreased and the reaction was slightly acid-producing(Fig. 2).

The dissolution of Cu is presented in Fig. 3. Because of the low pulpdensity (0.33%), the differences in Cu concentrations at the four redoxpotential levels were relatively minor. The yields of Cu in all inoculatedflaskswere higher than in the chemical controls. The highest concentra-tion of dissolved Cuwas achievedwith A. caldus and the TM enrichmentculture. At the lowest redox potential level the Cu yields withA. ferrooxidans were comparable to the controls, and the relative yieldswere b6% in all experiments. The solid phase transformations wereminor and secondary phases on CuFeS2 were not detected in experi-ments with sample A.

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Fig. 3. Changes in dissolved Cu concentrations during chalcopyrite leaching of sample A. A) Chemical control, B) A. caldus, C) TM enrichment, D) L. ferriphilum, E) A. ferrooxidans,and F) A. ferrooxidans previously adapted to grow with CuFeS2. Symbols: in green: ▼, 0 mM Fe3+ and 200 mM Fe2+; in blue: ▲, 60 mM Fe3+ and 140 mM Fe2+; in red: ●, 140 mMFe3+ and 60 mM Fe2+; in black: ■, 200 mM Fe3+ and 0 mM Fe2+.

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3.2. Leaching of CuFeS2 (sample B)

Sample B chalcopyrite concentrate was tested at 2.5% pulp densityunder two extreme redox potential conditions defined by 200 mMFe2+ and 200 mM Fe3+. These experiments were conducted underchemical leaching conditions and with A. ferrooxidans. With the initial200 mM Fe2+ the dissolution of Cu reached 22% in 17 d, whereas with200 mM Fe3+ the corresponding value was 8%. The extent of Cu disso-lution cannot be compared to sample A because sample B had a higherCu-content and was ground to a finer particle size.

The untreated chalcopyrite sample B also contained minor amountsof hydroxyl apatite and hornblende. XRD analysis of solid residues after39 d of contact time with bacteria and initial 200 mM Fe3+ revealedjarosite, gypsum (CaSO4·2H2O), and brushite (CaHPO4·2H2O) as newphases (data not shown). Brushite has been previously detected as asecondary solid phase formed during the bioleaching of an ore sample(collophanite) containing fluorapatite and carbonate-fluorapatite (Liet al., 2013).With 39 d of contact with the initial 200mM Fe2+, jarositeand gypsumwere absent and onlyminor brushite peakswere present inthe X-ray diffractogram. Neither elemental S nor Cu-monosulfides(Cu2S or CuS) were detected. All XRD data were in keeping withprevious XRD analyses of bioleached solids of this chalcopyrite sample(Bevilaqua et al., 2013).

Iron oxidation by A. ferrooxidans cultures with 2.5% chalcopyrite wasstrongly inhibited at 150mMand 300mMFe2+, whereaswith 100mMFe2+ A. ferrooxidans oxidized iron after 3 1/2 weeks of incubation.The oxidation increased the redox potential to 580 mV and the total

dissolved Fe concentration decreased by about 35% due to jarositeprecipitation (Fig. 4). After iron was oxidized and the redox potentialincreased, Cu dissolution started to decline, whereas it continuedmore linearly at 150 and 300 mM Fe2+ concentrations and in thechemical controls. Copper that was dissolved with 300 mM Fe2+

corresponded to 40% yield, with 150 mM Fe2+ 31% and 100 mM Fe2+

27%. The linear dissolution of copper at 300 mM Fe2+ concentrationsuggested that substantial passivation did not take place. The lineardissolution took place at 320–370 mV range (Fig. 4), clearly at lowredox potential which is not characteristic to iron-oxidizing cultures ofA. ferrooxidans. A. ferrooxidans had no major discernible role in copperleaching except in the case of the 100 mM Fe2+ concentration. Inthe absence of chalcopyrite, A. ferrooxidans oxidized 300 mM Fe2+ inmineral salts medium, suggesting that the inhibition was associatedwith 2.5% CuFeS2 rather than the 100–300 mM Fe2+ concentration.

3.3. Trends in chalcopyrite bioleaching and redox potential

With sample A, copper dissolution increased at 50% iron oxidation,but stopped when all iron was oxidized. The redox potential was430 mV when about 50% of iron was oxidized and it increased up to580 mV. These findings are consistent with the results of Third et al.(2002) and Kametani and Aoki (1985), who reported that the leachingrate increased until the redox potential reached the critical potential(450 mV), and the rate decreased above this value. Similar trendswere reported by Gericke et al. (2010) for a chalcopyrite–pyrite sample.This phenomenon could be seen in almost all experiments in the

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Fig. 4. Changes in total dissolved Fe (A), Fe2+ (B), pH (C), redox potential (D), and dissolved Cu (E) during chalcopyrite leaching (sample B) in the presence of 100, 150 or 300mM Fe2+ inA. ferrooxidans cultures (black) and in the corresponding chemical controls (red).

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present work. The importance of the role of efficient sulfur-oxidizingacidophiles in chalcopyrite bioleaching has been brought up in severalreports (e.g., Zhou et al., 2007; Feng et al., 2012; Xia et al., 2013).

The highest copper yields were obtained at the highest initialredox potential of 600 mV for sample A (Figs. 1 and 3). The results arecomparable to the data of Córdoba et al. (2008c), which were basedon 0.5%pulp density and 90mMdissolved Fe. They reported that copperdissolution was higher at initially high redox potential level and betterat 9 mM dissolved Fe as opposed to 90 mM dissolved Fe. Their datasuggested that the reason for low Cu recovery at high Fe3+ concentra-tions was excessive jarosite precipitation. Jarosite formation mayembed cells in the precipitate and cover reactive surface and, therefore,hinder fluxes of reactants and products of dissolution. At low pulpdensity and high iron concentration this problem may be pronounced.

Third et al. (2000) reported that chalcopyrite was preferentiallyleached in the presence of 100 mM Fe2+ as compared to 100 mMFe3+. Third et al. (2002) controlled the iron oxidation rate, and hence

the redox potential, by oxygen limitation and reported enhancedbioleaching as compared to the chemical leaching. In the present study,chalcopyrite (sample B) was leached with Fe2+ concentrations of 100,150 and 300 mM Fe2+ but the differences between A. ferrooxidanscultures and chemical controls were minor. No bacterial activity wasobserved with 150 and 300 mM Fe2+, but more Cu was dissolved athigher Fe2+ concentration concurrent with a low redox potential(b370 mV). In this study, iron oxidation increased the redox potential,pH, and iron precipitation, and this effect was seen as slightly lowercopper yields in A. ferrooxidans and L. ferriphilum cultures. In general,however, the results indicated only minor differences in copperleaching between chemical controls and bacterial cultures.

4. Conclusions

Elimination or partial alleviation of chalcopyrite surface passivationwith low redox potential bioleaching was sought in this study. Redox

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potentials were adjusted in the range of 350 and 600 mV with Fe3+/Fe2+ ratios, and acidophilic iron- and sulfur-oxidizers and chalcopyriteconcentrate samples from two sources were tested in this study. Therelative differences in the performance of the test cultures were mostlysubtle, and differences between inoculated cultures and chemical con-trols were minor. The leaching of chalcopyrite yielded the best resultsin the presence of 150 and 300 mM Fe2+ regardless of A. ferrooxidans.Under these conditions, bacterial oxidation of Fe2+ was stronglyinhibited and low redox potential could, therefore, be maintained.

Acknowledgments

This research was funded by the Finnish Funding Agency forTechnology and Innovation (Finland Distinguished Professor Program,402/06). Additional support in Brazil for this study was received fromConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq;AVB, 305890/2010-7), Fundação de Amparo à Pesquisa do Estado deSão Paulo (FAPESP; DB, 19868-5/2011), and Vale S.A. (Brazil).

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Acero, P., Cama, J., Ayora, C., 2007. Kinetics of chalcopyrite dissolution at pH3. Eur. J.Mineral.19, 173–182.

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