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Effect of Activated Carbon Addition on theConventional and Electrochemical Bioleaching ofChalcopyrite ConcentratesAli Ahmadi a d , Mohammad Ranjbar b & Mahin Schaffie ca Department of Mining Engineering, Isfahan University of Technology, Isfahan, Iranb Department of Mining Engineering, Shahid Bahonar University of Kerman, Kerman, Iranc Department of Chemical Engineering, Shahid Bahonar University of Kerman, Kerman, Irand Mineral Bioprocessing Research Group (MBRG), Biotechnology and Bioengineering ResearchInstitute, Isfahan University of Technology, Isfahan, IranAccepted author version posted online: 15 May 2012.Published online: 10 Jan 2013.

To cite this article: Ali Ahmadi , Mohammad Ranjbar & Mahin Schaffie (2013) Effect of Activated Carbon Addition on theConventional and Electrochemical Bioleaching of Chalcopyrite Concentrates, Geomicrobiology Journal, 30:3, 237-244, DOI:10.1080/01490451.2012.665152

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Geomicrobiology Journal (2013) 30, 237–244Copyright C© Taylor & Francis Group, LLCISSN: 0149-0451 print / 1521-0529 onlineDOI: 10.1080/01490451.2012.665152

Effect of Activated Carbon Addition on the Conventionaland Electrochemical Bioleaching of ChalcopyriteConcentrates

ALI AHMADI1,4∗, MOHAMMAD RANJBAR2, and MAHIN SCHAFFIE3

1Department of Mining Engineering, Isfahan University of Technology, Isfahan, Iran2Department of Mining Engineering, Shahid Bahonar University of Kerman, Kerman, Iran3Department of Chemical Engineering, Shahid Bahonar University of Kerman, Kerman, Iran4Mineral Bioprocessing Research Group (MBRG), Biotechnology and Bioengineering Research Institute, Isfahan University ofTechnology, Isfahan, Iran

Received September 2011, Accepted January 2012

The effect of activated carbon addition on the rate and efficiency of copper mobilization from Sarcheshmeh chalcopyrite concentratewas studied in the presence and absence of a mixed culture of moderately thermophilic microorganisms. Conventional leaching ata 10% (w/v) pulp density in 500-ml Erlenmeyer flasks on a rotary shaker at 150 rpm, and electrochemical bioleaching in a stirredbioreactor at an ORP (oxidation-reduction potential) range of 400 to 430 mV measured against a Ag/AgCl reference electrode.The bioreactor contained ore concentrate at a pulp density of 20%, which was stirred at 600 rpm. All experiments were conductedin the presence and absence of 3 g/L activated carbon, at initial pH 1.5, temperature 50◦C, in Norris’s nutrient medium with anaddition of 0.02% (w/v) yeast extract. The results showed that the addition of activated carbon increased the rate and yield ofcopper extraction from the concentrate especially in the presence of bacteria. Final recovery after 20 days was 52% and 44% in theshake flask experiments with and without carbon addition, respectively. Enhanced rates of copper mobilization were achieved in theelectrochemical bioleaching experiments in which copper was leached selectively relative to iron. Final copper recovery after 10 dayswas 85% and 77% in the presence and absence of activated carbon, respectively. The positive effect of activated carbon on copperextraction could be related to the galvanic interaction between the inert carbon as cathode and chalcopyrite as anode. The bacterialelimination of sulfur produced on the sulfide minerals during chemical leaching is assumed to intensify the galvanic interaction.It seems that maintaining the ORP at a low potential and efficient mixing improves the bacterial and chemical subsystems in theelectro-bioreactor that accelerates the rate of copper mobilization from the concentrate.

Keywords: bioleaching, activated carbon, chalcopyrite, galvanic interaction, electrochemistry

Introduction

Until now, the predominant process for extracting copper fromchalcopyrite flotation concentrates has involved pyrometal-lurgy. However, this process has some major problems in-cluding the emission of SO2 and an inability to treat com-plex concentrates (Dimitrijevic et al. 2009). These problemswill become magnified in the future, because the resources ofhigh-grade copper ores in the world are becoming more andmore scarce making the processing of more complex ores nec-essary (Rubio and Garcia 2002). During the past four decades,researchers at universities and in industry have paid increas-

The National Iranian Copper Industries Company (NICICO) isacknowledged for financial support of a part of this work.∗Address correspondence to Ali Ahmadi, Department of MiningEngineering, Isfahan University of Technology, Isfahan, Iran;Email: a.ahmadi@cc.iut.ac.ir

ing attention to hydrometallurgical processes as an alternativetreatment of refractory concentrates. These methods have beenmainly focused on leaching, because the recovery of copperfrom solutions is an efficient and common process (Elamariet al. 2000).

During the past three decades, bioleaching has been ap-plied successfully to the extraction of copper from secondarycopper sulfides such as chalcocite and covellite as well as theoxidation of refractory gold ores in both bioheaps and stirredtank reactors (van Niekerk 2009; Watling 2006). However,in the case of chalcopyrite ores and concentrates, the useful-ness of bioleaching remains to be explored (Olson et al. 2003;Ranjbar et al. 2007; van-Aswegen et al. 2007; Watling 2006).The main impediment to achieving efficient copper extrac-tion from these refractory ores is a slow dissolution rate of theminerals even in very acidic solutions (Watling 2006). This hasbeen attributed to surface passivity of chalcopyrite by a con-troversial inhibiting layer at high redox potentials, which act asa diffusion barrier for leaching agents and reaction products

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238 Ahmadi et al.

(Ahmadi et al. 2010a, 2011; Cordoba et al. 2008; Hiroyoshiet al. 2001, 2008; Pinches et al. 2001; Third et al. 2002).

To enhance the rate and extent of leaching at ambient pres-sure and a temperature below the boiling point of water, andto overcome the inhibitory effect of passivating layers, numer-ous investigations such as adding some catalysts like silverion and activated carbon have been undertaken in the pastthree decades. Silver is very expensive and difficult to recoverafter the leaching is completed. Activated carbon is an elec-trical conductor and forms a galvanic couple in contact withsemiconducting metal sulfides such as chalcopyrite.

When the mineral is in contact with carbon, it acts as ananode and is therefore dissolved preferentially, yet activatedcarbon is cathodically protected. This mechanism has beenconfirmed by Nakazawa et al. (1998). Wan et al. (1984) re-ported that the rate of chalcopyrite dissolution by ferric sul-fate could be enhanced in the presence of conductive carbonparticles. They suggested that carbon particles decrease theresistivity of the passive layer leading to an increase in theleaching rate. Zhang and Gu (2007) also reported that theaddition of activated carbon could greatly accelerate the rateand efficiency of copper mobilization in the bioleaching oflow-grade primary copper sulfide ores using Acidithiobacillusferrooxiduns and Acidithiobacillus thiooxidans.

In this study, the addition of activated carbon on the chem-ical and bacterial leaching of chalcopyrite concentrate is in-vestigated in the absence and presence of moderately ther-mophilic bacteria, respectively. In addition, because until nowit has been unclear what the effect of activated carbon on theelectrochemical bioleaching of chalcopyrite would be at anoptimal oxidation-reduction potential (ORP) of the solution,we examined its influence on the solubilization of copper andiron from the concentrate at high pulp densities.

Materials and Methods

Materials

MineralsA copper flotation concentrate from Sarcheshmeh CopperMine (Kerman province, Iran) was used in experiments. Chem-ical analysis of the concentrate by X-ray fluorescence (XRF)is shown in Table 1. Mineralogical analysis performed by opti-cal microscopy (model: Leica DLMP) of a polished specimenfrom a representative concentrate showed that it contained44.0% chalcopyrite (CuFeS2), 24.0% pyrite (FeS2), 6.9%, cov-ellite (CuS), 5.8% chalcocite (CuS2), 0.9% bornite (Cu5FeS4),13.6% non-metallic minerals and 4.8% copper oxide minerals.Approximately 80% of the particles in the concentrate were76 microns or smaller in size. Some of the experiments weredone in the presence of activated carbon (Merck Company)

(3 g/L), which was of analytical grade, 80% of the carbon par-ticles being 38 µm or smaller. To investigate the adsorption ofmetal ions on activated carbon, an additional shake flask testwas carried out at pH 1.5 and 50◦C, in a 500-ml Erlenmeyercontaining 200 ml of Norris nutrient medium and 1 g/L eachof copper and iron (the molar ratio of Fe(III) to Fe(II) was1). The results showed that about 1.6% copper and 2.2% ironwas removed from the solution phase after 2 days. Hence, dur-ing this research, the adsorption of dissolved metals on theactivated carbon was ignored.

MicroorganismsA mixed culture of moderately thermophilic iron- and sul-fur oxidizing bacteria mainly containing Acidithiobacillus cal-dus, Solfobacillus and Thermosulfidooxidans obtained fromSarcheshmeh Copper Mine (originally from South Africa) wasused as inoculum in the bioleaching tests. Experiments werecarried out at initial pH 1.8, 50◦C in Norris nutient medium(Norris and Barr 1985) modified without iron with the fol-lowing composition: (in g/L) NH4SO4, 0.4; K2HPO4, 0.4;MgSO4·7H2O, 0.5. These conditions were found to be opti-mal in a previous study with these organisms (Ahmadi et al.2010b). Free cells in solution were counted by direct countingusing a Thoma chamber of 0.1 mm depth and 0.0025 mm2 areawith an optical microscope (×1500, model: Zeiss-Axioskop40).

Iron-oxidizing and sulfur-oxidizing microorganisms wereconsidered to be active as the ORP rose and pH decreased, re-spectively, during the culturing. During the biotic experiments,microbial growth was periodically verified by observation un-der an optical microscope.

In abiotic tests, the medium was sterilized with 2% (v/v)bactericide (2% (w/w) thymol in ethanol) added to preventmicrobial growth. All reagents used in this study were of ana-lytical grade. Distilled water was used in all experiments.

Shake Flask Experiments

Flask experiments were carried out in 500 ml-Erlenmeyerflasks containing 200 ml of suspension of ore concentrateat a pulp density of 10% (w/v) in Norris’s medium supple-mented with 0.02% (w/v) yeast extract and having an initialpH of 1.5. Each flask was inoculated with a bacterial solution(20% v/v) and then incubated at 50◦C on a rotary shaker at150 rpm. To inoculate bacteria to a fresh medium, the bac-terial solution was added to an Erlenmeyer flask containingthe required fresh nutrient solution (at the desired pH). Then,the desired amount of concentrate and 3 g/L activated car-bon when nessecary, was carefully weighed and added to theflask. After mixing the resulting slurry, the pH was regulatedand the ORP was recorded. Bacterial solution consisted of anactive cell suspension of 3 × 108 cells/mL, which had been

Table 1. Chemical analysis of the copper sulfide concentrate from Sarcheshmeh Copper mine

Elements Cu Fe S Si Al Zn Mg K

Contents (Wt. %) 27.50 23.03 14.82 3.87 1.45 0.99 0.40 0.24

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Carbon and Bioleaching of Chalcopyrite Concentrates 239

previously adapted to a 10% pulp density of the ore. The ironconcentration (mostly ferric) of the liquid phase at the startof a bioleaching experiment was 1.32 g/L. During an exper-iment, the increase in iron concentration resulted from themobilization of iron from the ore concentrate.

Stirred Tank Electrobioreactor Experiments

To conduct electrobiochemical leaching experiments, a 2-literglass stirred reactor (working volume = 1.3 l) was used ina batch mode (Figure 1). It contained three separate com-partments of which two were used for fitting the workingand the counter electrodes, respectively. The third compart-ment was used for fitting the reference electrode. The reactorwas equipped with 4 perpendicular baffles. A 3-blade pitched-blade (axial) impeller (diameter = 5 cm) was chosen to pro-vide good solid suspension and generate low shear. The im-peller was rotated at 600 rpm with a Heidolf overhead stirrer.The desired temperature was maintained by circulating waterfrom a constant temperature bath through the double-walljacket. To minimize evaporation from the reactor compart-ments, a double-wall condenser was used. Air was supplied at1 vvm through a ring sparger under the impeller. A reticulated

Poten�ostat/Galvanostat

10

4 11

5

86

3

9

14

2 13

15

7

12

1

Fig. 1. Schematic presentation of the electrochemical bioreactor.1) Potentiostat 2) Reference electrode, 3) Outlet of circulatingwater, 4) Working electrode, 5) Baffle, 6) Sparger, 7) Outlet valve,8) Impeller, 9) Inlet of circulating water, 10) External wall, 11)Counter electrode, 12) Cathode compartment, 12) Outlet of feed,13) Anode compartment, 14) Reflux condenser, 15) Mechanicalstirrer (color figure available online).

titanium-platinum working electrode (15 cm × 9 cm × 0.1 cm,4 pixels per inch) was immersed into the cathodic compart-ment. A platinum foil as a counter electrode was put into theanodic compartment filled with nutrient medium. The cathodechamber was separated from the anode chamber by a frittedglass. The reference electrode consisted of Ag/AgCl and wasconnected to the electrolyte in the main chamber through aLuggin-capillary, which ended close to the working electrode.

The experiments were carried out at an initial pH of 1.5 anda temperature of 50◦C in Norris nutrient medium that hadbeen supplemented with 0.02% yeast extract and containedore concentrate at a pulp density of 20% (w/v). The reactionmixture was stirred at a rate of 600 rpm. The inoculum hadbeen adapted to a 20% pulp density of ore concentrate. It wasintroduced into the medium in the reactor as a cell suspensionat 20% (v/v) such that the initial cell concentration in the liq-uid phase of the inoculum was 5×108 per ml. To maintain theORP in the desired range (400–430 mV) during electrochem-ical bioleaching, the potential of the working electrode wascontrolled with respect to the Ag/AgCl reference electrodewith a Solartron Sl 1287 potentiostat. It should be pointedout that base on the previous work (Ahmadi et al. 2011), therate of copper mobilization from the chalcopyritic concentrateis at a maximum level in this range of solution ORP.

To sterilize the electroreactor before the abiotic test, it wasoperated for 2 h at 85◦C and 800 rpm in a solution of 10%(v/v) bactericide (2% (v/v) thymol in ethanole) and 5% (v/v)HCl. In the uninoculated control tests, 50 mL of 2% thymolin ethanol was added instead of the bacterial inoculum.Thestrility was checked during experiments by observing the solu-tion under the microscope and recording ORP changes. Whenthe ORP increased, it was assumed that the bacteria had be-come active. In these cases, a 2% (v/v) thymol solution wasadded into the medium to compensate for evaporation in-stead of only distilled water. It should be noted that the initialsolutions of the electrochemical bioleaching for experimentswith and without carbon addition contained contained 2.46and 3.11 g iron per liter (mostly ferric iron), respectively. Thesource of the iron was the inoculum.

Analyses

Periodically, a 10 ml-sample of slurry from a reactor was fil-tered through a Whatman No. 41 cellulose filter paper andthe filtrate analyzed for copper and iron by atomic absorp-tion spectrometry (AAS) (model: Varian 240). The remainingsolids were returned to the reactor. The volume of of the slurryin the reactor was adjusted 20 min before sampling. After sam-pling, a solution of Norris nutrient medium whose volume wasequal to that of the filtrate was added to the reaction mixture.

The pH and ORP values in the leach solutions were mea-sured with a pH meter (model: Jenway 3540) outfitted with aPt electrode and a Ag/AgCl reference electrode (+207 mV vs.SHE at 25◦C). All ORP values are reported with respect tothis reference electrode. During the experiments, the pH waslowered to 1.5 by the addition of 6M H2SO4 when necessary.Evaporation loss was compensated by adding distilled waterto the slurry before sampling.

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240 Ahmadi et al.

Results and Discussion

Shake Flask Leaching Tests

Shake flask experiments were carried out in the presence andabsence of the mixed culture of moderately thermophile bac-teria with and without carbon addition. The changes in ORPand pH during leaching experiments are shown in Figures 2and 3. It can be seen in Figure 2 that ORP during bioleach-ing in the absence of added activated carbon increased rapidlyinitially and then remained at a high level about 540 mV. How-ever, during bioleaching in the presence of activated carbon,the ORP increased more slowly than in the experiment with-out activated carbon addition, but the final ORPs were verysimilar. The ORP measurements in the medium with addedactivated carbon in the first 8 days were probably lower thanthe readings in the medium without added activated carbon(Figure 2) because of an inhibitory effect of the activated car-bon on the activity of the bacteria. This negative effect couldhave been a result of higher viscosity of the medium due to theadded activated carbon, which resulted in a decrease in oxygentransfer rate from bubbles to the bacteria. In the uninoculatedexperiments, the ORP of both media (with and without car-bon) remained constants at low values about 320 mV.

In the uninoculated or inoculated reaction mixture contain-ing the copper concentrate, the ORP or Eh (ORP with respectto SHE) is a measure of the oxidizing or reducing tendency,determined in the liquid phase, and depends on the ratio ofthe concentrations of ferric to ferrous ion. It is described by

Fig. 2. Changes in ORP during the chemical and bacterial leach-ing of the copper ore concentrate with and without activatedcarbon addition (3 g/l) at 10% pulp density, initial pH 1.5, 50◦Cand Norris nutrient medium supplemented with 0.02% yeast ex-tract (CL = chemical leaching in the absence of carbon; CL+= chemical leaching in the presence of carbon; BL = conven-tional bioleaching in the absence of carbon; BL+ = conventionalbioleaching in the presence of carbon).

1.0

1.5

2.0

2.5

3.0

3.5

0 5 10 15 20 25

pH

Leaching time (days)

BL BL+

CL CL+

Fig. 3. Changes in pH during the chemical and bacterial leachingof the copper ore concentrate with and without activated carbonaddition (3 g/l) at 10% pulp density, initial pH 1.5, 50◦C and Nor-ris nutrient medium supplemented with 0.02% yeast extract (CL= chemical leaching in the absence of carbon; CL+ = chemicalleaching in the presence of carbon; BL = conventional bioleach-ing in the absence of carbon; BL+ = conventional bioleaching inthe presence of carbon).

the Nernst equation (Equation 1) (Rossi 1990).

Eh = Eho + R . Tn . F

ln

[Fe3+]

[Fe2+] (1)

where Eh is the solution potential in mV, Eh◦ is the stan-dard equilibrium potential in mV, R is the gas constant whichis equal to 8.314 J/(K.mol), T is temperature in K, n is thecharge number, F is Faraday’s constant, which is equal to96,485 Coulombs/mol, [Fe3+] and [Fe2+] are the concentra-tions of the ferric and ferrous iron, respectively, in mol/l. Inbioleaching systems, acidophilic iron-oxidizing microorgan-isms oxidize ferrous iron to ferric iron rapidly (Equation 2)and consume protons. As a concequence, these bacteria in-crease both ORP and pH in the solution.

4Fe2+ + O2 + 4H+ Iron oxidizing acidophiles−−−−−−−−−−−−−→ 4Fe3+ + 2H2O (2)

On the other hand, acidophilic sulfur-oxidizing microor-ganisms reduce solution pH by converting elemental sulfur(Equation 3) and/or sulfide species to sulfate.

S◦ + H2O + 32

O2Sulfur oxidizing acidophiles−−−−−−−−−−−−−−→ SO2−

4 + 2H+ (3)

Figure 2 shows that after a transient increase in the first2 days, the pH values of both bioleaching tests fell to about1.3. The initial increase of pH is as a concequence of pre-dominating acid consuming reactions, i.e., copper oxides such

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Carbon and Bioleaching of Chalcopyrite Concentrates 241

as tenorite (CuO) (Equation 4), cuprite (Cu2O) (Equation 5)and copper carbonates such as azurite (Cu(OH)2.2Cu2CO3)(Equation 6) and malachite (Cu(OH)2.CuCO3) (Equation 7)as well as the gangue minerals which are easily dissolved bysulfuric acid at ambient conditions.

CuO + H2SO4 → CuSO4 + H2O (4)Cu2O + 0.5O2 + 2H2SO4 → 2CuSO4 + 2H2O (5)

Cu(OH)2.2CuCO3 + 3H2SO4 → 3CuSO4 + 2CO2 + 4H2O(6)

Cu(OH)2.CuCO3 + 2H2SO4 → 2CuSO4 + CO2 + 3H2O(7)

Subsequent decrease of pH is as a result of predominatingacid producing reactions such as activity of bacteria to pro-duce acid (Equation 3), the oxidation of pyrite by ferric ion(Equation 8), and the hydrolysis of ferric iron to form jarosite(Equation 9).

FeS2 + 8H2O + 14Fe3+ → 15Fe2+ + 2SO2−4 + 16H+ (8)

3Fe3+ + X+ + 2HSO−4 + 6H2O → XFe3(SO4)2(OH)6 + 8H+

(9)

where X+ = K+, Na+, NH+4 and H3O+

On the other hand, in the uninoculated experiments, thepH values increased to the leveles higher than 1.5 in the first7 days (brought back (readjusted) to 1.5 by H2SO4) and thenremained around 1.6 in the following days.

The results of copper and iron dissolution from the concen-trate are presented in Figures 4 and 5. These figures show thatthe addition of activated carbon increased the recovery of cop-per and iron in both biotic and abiotic media. The figures showthat 52% of copper and 19% of iron was leached in the exper-iment with carbon addition compared to 44% of copper and17% of iron in the experiment without carbon addition, after20 days of leaching. In both tests, the rate of copper extractionin the initial days was much higher than that in the followingdays. The high rate of Cu mobilization is mostly related tothe leaching of copper oxides by sulfuric acid (Equations 4 to7) and the oxidative leaching of secondary copper sulphides(Equations 10 and 11) with ferric iron present in the inoculum.According to mineralogical analysis, copper oxides, chalcocite(Cu2S) and covellite (CuS) contain about 9%, 20% and 15% ofthe copper present in the concentrate, respectively. It should benoted that the leaching of chalcocite in ferric sulfate solutionsproceeds in two distinct stages as follows (Petersen and Dixon2007):

Cu2S + 2Fe3+ → Cu2+ + 2Fe2+ + CuS (10)CuS + 2Fe3+ → Cu2+ + 2Fe2+ + S◦ (11)

The first stage (Equation 10) proceeds readily even at roomtemperature, whereas the produced covellite is rather recalci-trant and usually the second stage (Equation 11) acceleratesat high ratio of ferric iron to ferrous iron (Petersen and Dixon2007). Indeed, the first stage consists of a series of steps in

0

10

20

30

40

50

60

0 5 10 15 20 25

Cu

rec

ove

ry (

%)

Leaching time (days)

BL BL+

CL CL+

Fig. 4. Effect of the addition of activated carbon (3 g/l) on therecovery of copper during the chemical and bacterial leachingof the ore concentrate at 10% pulp density, initial pH 1.5, 50◦Cand Norris nutrient medium supplemented with 0.02% yeast ex-tract (CL = chemical leaching in the absence of carbon; CL+= chemical leaching in the presence of carbon; BL = conven-tional bioleaching in the absence of carbon; BL+ = conventionalbioleaching in the presence of carbon).

0

10

20

30

0 5 10 15 20 25

Fe

reco

very

(%

)

Leaching time (days)

BL BL+

CL CL+

Fig. 5. Effect of the addition of activated carbon (3 g/l) on therecovery of iron during the chemical and bacterial leaching of theore concentrate at 10% (w/v) pulp density, initial pH 1.5, 50◦Cand Norris nutrient medium supplemented with 0.02% yeast ex-tract (CL = chemical leaching in the absence of carbon; CL+= chemical leaching in the presence of carbon; BL = conven-tional bioleaching in the absence of carbon; BL+ = conventionalbioleaching in the presence of carbon).

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242 Ahmadi et al.

which the reaction products may or may not be observed ex-perimentally depending on the rate of transformation of thesuccessive phase (Bolorunduro 1999).

The bioleach profiles can be divided into a number of dis-tinct phases. Initially, for both tests, approximately 24% ofcopper is leached rapidly within the first two days; this dis-solution is associated with the dissolution of the oxides andcarbonates of copper (Equations 4 to 7) and the first stageleaching of chalcocite (Equation 10), which proceed fast inthe presence of initial acid and ferric iron present in the inoc-ulated solution (∼1.3 g/L). This phase is followed by anotherlinear increase to about 51.3% and 44.5%, between days 2and 15 in the presence and absence of added activated carbon,respectively.

This phase is probably associated with the leaching of chal-copyrite and covellite (natural mineral and the product ofchalcocite leaching in stage 1). This second phase is followedby a slowing of the copper mobilization in the remaining daysto the end of the experiment. This parabolic behavior is proba-bly caused by the passivation of chalcopyrite as a consequenceof high solution ORP in the last 5 days. These figures also showthat the addition of activated carbon increased the extractionof both iron and copper from the concentrate in the uninocu-lated experiments in which 15% of copper and 8% of iron wasextracted compared to 11% of copper and 5.5% of iron in theexperiment without carbon addition (after 20 days).

Comparisons of the results indicate that the positive effectof activated carbon addition on the rate of copper extractionwas more significant when bacteria were used. This could havebeen as a result of galvanic interactions between activated car-bon and metallic sulfide minerals. The bacterial elimination ofsulfur produced on the sulfide minerals during their chemicalleaching step is supposed to be the reason for enhancing thegalvanic interaction between different sulfide phases in thepresence of bacteria (indirect and/or contact mechanisms de-scribed by Tributsch (2001) is assumed).

Electrochemical Leaching Tests

Changes in pH during chemical, bacterial and electrobiochem-ical leaching processes of the concentrate are presented pro-cesses of the concentrate are presented in Figure 6. Electro-chemical bioleaching teste were conducted in the presence andabsence of 3 g/L activated carbon. The pH values rose duringthe first days of leaching, which was due to acid consumptionby the reactions of copper bearing and gangue minerals, whilethe subsequent decrease of pH was mainly due to the bacterialacid-producing activity (Equation 3), the oxidation of pyriteby ferric ion (Equation 8) and the hydrolysis of ferric iron toform jarosite (Equation 9).

Figure 7 shows the changes in ORP during leaching underdifferent reaction conditions. It can be seen that in the conven-tional bioleaching test, ORP increased from 390 to 540 mV(after day 7) with the lag phase of bacterial growth beingshorter than 1 day. The ORP of the liquid phase during theelectro-bioleaching process was electrochemically controlledwithin a window of optimal potential of 400–430 mV. Therapid increase of ORP in the bioleaching test to relatively high

1.0

1.5

2.0

2.5

3.0

3.5

0 2 4 6 8 10 12

pH

Leaching time(days)

CL BL

EBL EBL+

Fig. 6. Changes in pH during different leaching processes at pulpdensity 20%, initial pH 1.5, stirring rate 600 rpm, 50◦C and Nor-ris nutrient medium supplemented with 0.02% yeast extract (CL= chemical leaching; BL = conventional bioleaching; EBL =electrochemical bioleaching in the absence of carbon; EBL+ =electrochemical bioleaching in the presence of carbon (3 g/l)).

levels (Figure 7) and the pH decrease (Figure 6) indicate thatthe activity of the bacteria was very favorable under the con-ditions studied (high pulp density and high stirring rate).

Figures 8 and 9 summarize, respectively, results on per-cent copper and iron recoveries from the ore concentrate

250

300

350

400

450

500

550

0 2 4 6 8 10 12

OR

P (

mV

vs.

Ag

/Ag

Cl)

Leaching time (days)

CL BLEBL EBL+

Fig. 7. Changes in ORP during different leaching processes at pulpdensity 20%, initial pH 1.5, stirring rate 600 rpm, 50◦C, Norrisnutrient medium and supplemented with 0.02% yeast extract (CL= chemical leaching; BL = conventional bioleaching; EBL =electrochemical bioleaching in the absence of carbon; EBL+ =electrochemical bioleaching in the presence of carbon (3 g/l)).

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Carbon and Bioleaching of Chalcopyrite Concentrates 243

0

20

40

60

80

100

0 2 4 6 8 10 12

Cu

rec

ove

ry (

%)

Leaching time (days)

CL BL

EBL EBL+

Fig. 8. Recovery of copper as a function of leaching time in thepresence and absence of activated carbon (3 g/l) at pulp den-sity 20% (w/v), 50◦C, stirring rate 600 rpm and Norris nutri-ent medium supplemented with 0.02% (w/v) yeast extract (CL= chemical leaching; BL = conventional bioleaching; EBL =electrochemical bioleaching in the absence of carbon; EBL+ =electrochemical bioleaching in the presence of carbon (3 g/l)).

under different reaction conditions. Figure 8 shows that cop-per was electrobioleached progressively without leveling off.After 10 days, 85% of the copper was mobilized from the oreconcentrate in the presence of added carbon (EBL+) com-

0

10

20

30

0 2 4 6 8 10 12

Fe

reco

very

(%

)

Leaching time (days)

CL BL

EBL EBL+

Fig. 9. Recovery of iron as a function of leaching time in thepresence and absence of activated carbon at pulp density 20%(w/v), 50◦C, stirring rate 600 rpm and Norris nutrient mediumsupplemented with 0.02% (w/v) yeast extract (CL = chemicalleaching; BL = conventional bioleaching; EBL = electrochemicalbioleaching in the absence of carbon; EBL+ = electrochemicalbioleaching in the presence of carbon (3 g/l)).

pared to 77% in the absence of added carbon (EBL). In re-spect to iron, 22% was mobilized after 10 days in the presenceof added carbon (EBL+), whereas 28% was mobilized in theabsence of added iron (EBL). The increased copper recoveryin the presence of added carbon was the result of galvanicinteractions between activated carbon and copper sulfides es-pecially chalcopyrite.

It should be explained that carbon is a conductor with a restpotential that is much higher than the sulfide minerals, so thatwhen it comes in contact with them in the leaching medium,a galvanic couple is established. As a consequence, the sulfidemineral reacts at an enhanced rate while the carbon is cathod-ically protected. Under this condition, ferric and/or oxygenare reduced on the surface of both the carbon and the sulfidemineral instead of merely on the sulfide mineral in the absenceof the carbon. Thus, the cathodic reaction area increased.At the same time, the anodic current density is increased. Inaddition to this, carbon may also act as a charge carrier whichenhances the electroreduction of chalcopyrite. In this case thecarbon particle acts as a slurry electrode, which accepts chargeby contacting the cathode electrode. The carbon then trans-fers its electrical charge to a semiconducting mineral such aschalcopyrite, promoting its reduction.

Kastening et al. (1997) described the characteristics ofslurry electrode systems and pointed out that in a suspen-sion of particles with large double-layer capacity, such as ac-tivated carbon, in an electrolyte solution, particles transfercharge from an electrochemical cell to an external reactor,where a substance is oxidized or reduced, while the parti-cles are recharged in the cell. Wan et al. (1984) showed thatthe dissolution rate of chalcopyrite in the ferric sulfate leach-ing was enhanced by the formation of chalcopyrite/carbonaggregates.

They suggested that the conductive carbon particles changethe conductivity of the reaction product layer, such a phe-nomenon accounting for the increase in the rate of leaching inleaching rate. Nakazawa et al. (1998) reported that the addi-tion of activated carbon accelerates the dissolution of copperfrom the chalcopyrite concentrate in both abiotic and bioticmedia inoculated with At. ferrooxidans. They attributed thisincrease to galvanic interactions between carbon and chal-copyrite. About 50% and 13% of copper were dissolved inthe conventional bioleaching and chemical bioleaching exper-iments, respectively (Figure 8).

In all processes the mobilization rates and final recover-ies of iron were significantly lower than those of copper. Therapid mobilization of copper from copper oxides, covellite andchalcocite, which represented about 40% of the total copperin the feed material and accounted for the low initial Fe:Curatio in solution during the first days of leaching under thedifferent conditions. Thereafter, the divergence in the Fe:Curatios under the different conditions requires different expla-nations. The Fe:Cu ratio observed with chemical leaching isexplained by the low reactivity of pyrite and chalcopyrite, themain iron-bearing phases in the concentrate. The low ironrecovery in solution during conventional bioleaching can beexplained by the precipitation of a part of the mobilized ironas ferric hydroxyl sulfates such as jarosite. The low iron recov-ery in electrochemical bioleaching could be the result of low

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244 Ahmadi et al.

solubility of pyrite, as one of the main iron bearing phases, atlow solution ORPs.

Conclusions

It is concluded that the addition of activated carbon enhancesthe efficiency of conventional and electrochemical bioleach-ing of copper from the chalcopyrite concentrate presumablydue to the positive effect of galvanic interactions betweenthe inert carbon as the cathodic site and the metallic cop-per sulfides as anodic sites. It was also found that in thebacterial tests, the addition of carbon enhanced the rate ofbioleaching more than the rate of chemical leaching. Thismay be a result of the bacterial elimination of sulfur producedon the sulfide minerals during their chemical leaching whichcan intensify the galvanic interaction among different sulfidephases.

References

Ahmadi A, Schaffie M, Manafi Z, Ranjbar M. 2010a. Electrochemicalbioleaching ofhigh grade chalcopyrite flotation concentrate in astirred tank reactor. Hydrometallurgy 104:99–105.

Ahmadi A, Ranjbar M, Schaffie M, Manafi, Z. 2010b. Optimizationof copper concentrate bioleaching by a mixed culture of moder-ately thermophilic bacteria. Proceedings of Copper 2010 Confer-ence, GDMB, Hamburg, 2575–2589.

Ahmadi A, Schaffie M, Petersen J, Schippers A, Ranjbar M. 2011. Con-ventional and electrochemical bioleaching of chalcopyrite concen-trates by moderately thermophile bacteria at high pulp density. Hy-drometallurgy 106(1–2):84–92.

Bolorunduro SA. 1999. Kinetics of leaching of chalcocite in acid fer-ric sulphate media: Chemical and bacterial leaching. M.Sc. Thesis,University of British Columbia, Canada.

Cordoba EM, Munoz JA, Blazquez ML, Gonzalez F, Ballester A. 2008.Leaching of chalcopyrite with ferric ion. Part II: Effect of redoxpotential. Hydrometallurgy 93:88–96.

Dimitrijevic M, Kostov A, Tasic V, Milosevic N, 2009. Influence ofpyrometallurgical copper production on the environment. J HazardMater 164:892–899.

Elamari K, Jdid EA, Blazy P, Bouhafid A, Meray ME, Akalay I.2000. Acid and oxidizing leaching of chalcopyrite concentrate byferric sulphate and fluorosilicic-nitric media. Separ Sci Technol35(13):2143–2158.

Hiroyoshi N, Miki H, Hirajima T, Tsunekawa M. 2001. Enhancementof chalcopyrite leaching by ferrous ions in acidic ferric sulfate solu-tions. Hydrometallurgy 60:185–197.

Hiroyoshi N, Kitagawa H, Tsunekawa M. 2008. Effect of solution com-position on the optimum redox potential for chalcopyrite leachingin sulfuric acid solutions. Hydrometallurgy 91:144–149.

Kastening B, Boinowitz T, Heins M. 1997. Design of a slurry electrodereactor system. J. Appl Electrochem 27:147–152.

Nakazawa H, Fujisawa H, Sato H. 1998. Effect of activated carbon onthe bioleaching of chalcopyrite concentrate. International Journalof Mineral Processing 55:87–94.

Norris PR, Barr DW. 1985. Growth and iron oxidation by acidophilicmoderate thermophiles. FEMS Microbiol Lett 28:221–224.

Olson GJ, Brierley JA, Brierley CL. 2003. Bioleaching review part B:Progress in bioleaching: applications of microbial processes by theminerals industries. Appl Microbiol Biotechnol 63:249–257.

Petersen J, David GD. 2007. Principles, mechanisms and dynamics ofchalcocite heap bioleaching. In: Donati ER, Sand W, editors. Mi-crobial Processing of Metal Sulfides, Chapter 10. Dordrecht, TheNetherlands: Springer, p193–218.

Pinches A, Myburgh PJ, Merwe C. 2001. Process for the rapid leaching ofchalcopyrite in the absence of catalysis. Patent No: U.S. 6,277,341B1.

Ranjbar M, Schaffie M, Pazouki M, Ghazi R, Akbary A, ZanddevakiliS, Seiedbaghery SA, Manafi Z. 2007. Application potential of bio-hydrometallurgyin the Iranian mining industry. Advan Mater Res20–21:38–41.

Rossi G. 1990. Biohydrometallurgy. Hamburg: McGraw-Hill.Rubio A, Garcia FJ. 2002. Bioleaching capacity of an extremely

thermophilic culture for chalcopyritic materials. Miner Eng15(9):689–694.

Third KA, Cord-Ruwisch R, Watling HR. 2002. Control of the redoxpotential by oxygen limitation improves bacterial leaching of chal-copyrite. Biotechnol Bioeng 78(4):433–441.

Tributsch H. 2001. Direct versus indirect bioleaching. Hydrometallurgy59:177–185.

van-Aswegen PC, Niekerk JV, Olivier W. 2007. The BIOXTMprocessfor the treatmentof refractory gold concentrates. In: RawlingsDE, Johnson DB, editors. Biomining. Berlin/Heidelberg: Springer-Verlag.

van Niekerk J. 2009. Recent advances in BIOX R© technology. Hy-drometallurgy Conference, Johannesburg, South Africa. February2009. The Southern African Institute of Mining and Metallurgy,p167–176.

Wan RY, Miller JD, Foley J, Pon S. 1984. Electrochemical features ofthe ferric sulfate leaching of CuFeS2-C aggregates. In: RichardsonPE, Srinivasan S, Woods R, editors. Electrochemistry in Mineraland Metal Processing. Pennington, NJ: Electrochemical Society.p391–416.

Watling HR. 2006. The bioleaching of sulfide minerals with emphasis oncopper sulphides - a review. Hydrometallurgy 84:81–108.

Zhang W-M, Gu, S-f. 2007. Catalytic effect of activated carbon on bi-oleaching of low-grade primary copper sulfide ores. Trans NonferrMet Soc China 17:1123–1127.

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