Minerals Engineering 34 (2012) 11–18

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Minerals Engineering

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Catalytic effect of pyrite on the leaching of chalcopyrite concentrates inchemical, biological and electrobiochemical systems

Ali Ahmadi a,d,⇑, Mohammad Ranjbar b, Mahin Schaffie c

a Department of Mining Engineering, Isfahan University of Technology, Isfahan 84156-83111, 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 Research Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 November 2011Accepted 22 March 2012Available online 15 May 2012

Keywords:Electro-bioleachingGalvanic interactionsChalcopyritePyriteModerate thermophiles

0892-6875/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.mineng.2012.03.022

⇑ Corresponding author at: Department of Mining Enof Technology, Isfahan 84156-83111, Iran. Tel.: +983912776.

E-mail address: a.ahmadi@cc.iut.ac.ir (A. Ahmadi).

The main objective of this research was to evaluate the potential of electrochemical bioleaching to extractcopper from pyritic chalcopyrite concentrates. Bacterial and chemical (uninoculated) shake flask leachingof Sarcheshmeh copper concentrate at 15% (w/v) pulp density, 150 rpm and stirred tank electrochemicalbioleaching of the concentrate at ORP (oxidation reduction potential) ranging from 400 to 430 mV (vs. Ag/AgCl), 20% pulp density and 600 rpm were conducted with and without pyrite addition. A mixed cultureof moderately thermophilic microorganisms was used in all bioleaching experiments at an initial pH of1.5, 50 �C, Norris nutrient medium and 0.02% (w/w) yeast extract addition. The results of leaching exper-iments in shake flasks showed that the addition of pyrite to the concentrate significantly increased theefficiency of copper extraction especially in the presence of microorganisms. In electrochemical bioleach-ing process, both the rate and extent of copper extraction were selectively (with respect to iron)enhanced in the pyritic copper concentrate in which about 90% copper recovery was achieved fromthe concentrate after 10 days. Analyses of optical microscopy and SEM/EDS revealed that pyrite remainedunaffected in the electro-biochemical system while chalcopyrite was preferentially dissolved. It can alsobe concluded that at low levels of solution ORP, pyrite remains inert, which acts as a cathode site relativeto chalcopyrite and other copper sulfide minerals (galvanic interaction) leading to enhance the anodicdissolution of the copper bearing minerals. Electrochemical system regulates the ratio of ferric to ferrousiron at an optimum level where the dissolution rate of chalcopyrite is maximum. Sulfur oxidizer micro-organisms intensify the galvanic interactions and the rate of electron transfer among sulfides by remov-ing the insulating sulfur product.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Over the last four decades, bioleaching has been considered asone of the most promising alternatives to extract copper fromflotation concentrates due to its environmental and technicaladvantages. It involves the use of iron- and sulfur-oxidizingmicroorganisms to catalyze the oxidation of metal sulfides in asulfate medium. Among copper sulfides, chalcopyrite which isthe most abundant copper bearing mineral is the most refractorymineral to bioleach.

Pyrite usually is one of the most abundant phases in dirty copperconcentrates especially in the concentrates obtained from bulk flo-tation of porphyry deposits. It has an adverse effect on the efficiencyof pyrometallurgical processes, so it must be separated from copper

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gineering, Isfahan University311 3915113; fax: +98 311

bearing sulfides which is usually performed by flotation method.On the other hand, lots of studies (Berry et al., 1978; Mehta andMurr, 1983; Tshilombo, 2004; Dixon et al., 2007; Majuste et al.,2012) have reported that in leaching processes pyrite has a positivegalvanic effect on the dissolution rate of copper sulfides especiallychalcopyrite. Galvanic interactions play an important role in theleaching of conducting or semi-conducting minerals by accelerat-ing or retarding the dissolution of the minerals in aqueous solu-tions. Berry et al. (1978) investigated the galvanic interactions ofchalcopyrite and pyrite and found that when pyrite and chalcopy-rite were in contact, the resulting galvanic interaction caused thechalcopyrite to be corroded more rapidly than the pyrite, whichwas effectively protected. Mehta and Murr (1983) also reportedthat the rate of copper dissolution from chalcopyrite increases withthe addition of pyrite especially in the presence of bacteria. Theyreported that the increase is as a result of galvanic interactions.

A number of recent studies have reported that solution ORP(oxidation reduction potential) is one of the main parametersgoverning the chemical and biological leaching rate of chalcopy-

Fig. 1. Particle size distribution of the representative sample of Sarcheshmehcopper concentrate.

12 A. Ahmadi et al. / Minerals Engineering 34 (2012) 11–18

rite. In this regard, several authors (Hiroyoshi et al., 1997, 2001,2008; Pinches et al., 2001; Third et al., 2002; Sandstrom et al.,2005; Cordoba et al., 2008a, 2008b; Gericke et al., 2010; Ahmadiet al., 2010a, 2011) have stated that during the chemical leachingof chalcopyrite in ferric sulfate media, both the rate and yield ofcopper dissolution is at a maximum in a narrow range of solutionORP around 400–450 mV (Pt. vs. Ag/AgCl), whereas at potentialsabove this range, the surface passivation of the mineral could oc-cur. Moreover, ferric iron is an inhibitor for bacterial growth andactivity; hence increasing ORP has a negative effect on the bacterialsubsystem. Moreover, pyrite seems to leach more slowly than chal-copyrite at low ORP levels. This leads to the decrease of iron con-centration in the solution phase which is beneficial for metalrecovery processes. On the other hand, when pyrite remains inthe pulp (as a result of slow dissolution rate), it acts as a catalystto precede galvanic reactions. This is useful for increasing the rateof anodic reactions such as chalcopyrite oxidation. Peters (1986)reported that pyrite can transfer the electrons gathered from theless noble minerals to molecular oxygen faster than other sulfideminerals. At ORPs lower than 550 mV, the dissolution rate of pyriteis slow due to its high rest potential (630 mV vs. SHE at standardconditions) but the rate of oxygen reduction on its surface is fast(Peters, 1977). Petersen and Dixon (2006) described a mechanismat which the bioleaching of chalcopyrite is preferentially acceler-ated at low ORP values resulted from high temperatures while athigher ORPs pyrite and covellite is leached much faster thanchalcopyrite.

The authors have investigated the capability of electrochemicalbioleaching to extract copper from high grade chalcopyrite concen-trates (Ahmadi et al., 2010a, 2011) and found that at low levels ofsolution ORP, this new method has a good efficiency to leach suchrefractory concentrates. However, until now, the influence of pyriteon the electrochemical bioleaching of copper concentrates has notbeen investigated at low ORP values. Hence, in this research, theaddition of this mineral on the rate and extent of copper and ironmobilization from the Sarcheshmeh copper concentrate was exam-ined using conventional and electrochemical bioleaching pro-cesses. The results indicate that the new method has a goodapplication potential to leach pyritic copper concentrates whichis promising to treat bulk flotation concentrates.

2. Materials and methods

2.1. Minerals

Flotation copper concentrate obtained from Sarcheshmeh Cop-per Mine (Kerman, Iran) was used throughout experiments. Chem-ical analysis by X-ray fluorescence (XRF) showed that theconcentrate included 27.5% Cu, 23.0% Fe, 14.8% S and 3.9% Si. Min-eralogical analysis performed by optical microscopy (model: LeicaDLMP) of a polished specimen from a representative concentrateshowed that it contained 44.1% chalcopyrite (CuFeS2), 24.0% pyrite(FeS2), 6.9% covellite (CuS), 5.8% chalcocite (CuS2), 0.9% bornite(Cu5FeS4), 13.6% non-metallic minerals and 4.8% copper oxide min-erals. The analysis of particle size by wet sieving and cyclosizershowed that the concentrate had a particle size of approximately80% passing 76 lm (Fig. 1).

Pure crystalline samples of pyrite obtained from SarcheshmehCopper Mine were used in the pyrite added experiments. Thesesamples were pulverized in an agate mortar to less than 75 lm.

2.2. Microorganisms

A mixed culture of moderately thermophilic iron- and sulfuroxidizing bacteria mainly containing Acidithiobacillus caldus, Solfo-

bacillus and Thermosulfidooxidans obtained from Sarcheshmeh Bio-hydrometallurgy Laboratory (originally from South Africa) wasused as inoculum in the bioleaching tests. Experiments were car-ried out at initial pH 1.8, 50 �C and Norris nutient medium (Norrisand Barr, 1985) modified without iron with the following compo-sition: 0.4 g/l (NH4)2SO4, 0.4 g/l K2HPO4, 0.5 g/l MgSO4�7H2O. Theseconditions were found to be optimal in a previous study with theseorganisms (Ahmadi et al., 2010b) Free cells in solution werecounted by direct counting using a Thoma chamber of 0.1 mmdepth and 0.0025 mm2 area with an optical microscope (�1500,model: Zeiss-Axioskop 40).

Iron- and sulfur oxidizing microorganisms were considered tobe active as the ORP increased and pH decreased, respectively. Itshould be noted that since it was not injected any oxidant intothe leaching solutions, it was supposed that the main reactionsto oxidize ferrous iron to ferric iron and sulfur to sulfate are cata-lyzed by iron oxidizing and sulfur oxidizing bacteria, respectively.So, the activity of these bacteria in the solutions was monitored byits effect on ORP and pH. Moreover during the biotic experiments,microbial growth was periodically verified by observation under anoptical microscope.

In abiotic tests, the medium was sterilized with 2% (v/v) bacte-ricide (2% (w/w) thymol in ethanol) added to prevent microbialgrowth. All reagents used in this study were of analytical grade.Distilled water was used in all experiments.

2.3. Shake flask experiments

Shake flask experiments were carried out in 500 ml-Erlenmeyerflasks containing 200 ml of suspension of ore concentrate at a pulpdensity of 15% (w/v) in Norris’s medium supplemented with 0.02%(w/v) yeast extract and having an initial pH of 1.5. Each flask wasinoculated with a bacterial suspension (20% v/v) and then incu-bated at 50 �C and 150 rpm on a rotary shaker. To inoculate bacte-ria to a fresh medium, the bacterial solution was added to anErlenmeyer flask containing the required fresh nutrient solution(at the desired pH). Then, the desired amount of concentrate andpyrite when nessecary, was carefully weighed and added to theflask. After mixing the resulting slurry, the pH was regulated andthe ORP was recorded. These biotic experiments were inoculatedwith an active culture (as solution) with the cell density of about3 � 108 cells/ml, which had been previously adapted to a 15% pulpdensity of the concentrate. The initial concentration of iron (mostlyferric iron) which was originated from the inoculated solution was1.92 g/l. During the experiments, the diference between the con-centration of each ion in the solution and its initial concentrationwas accounted as what was leached from the concentrate.

Evaporation loss was measured by weighting flaks and then wascompensated by adding distilled water to the slurry beforesampling.

Table 1Summary of experimental conditions for copper leaching from the chalcopyrite concentrate (nutrient Norris medium, 50 �C and initial pH 1.5).

Parameter mode of experiment Pulp density (%, w/v) Pulp volume (l) Bacteria (moderate thermophiles) Pyrite: concentrate (mass ratio) ORP control

Shake flask experiments 15 0.2 Yes 0:3 No15 0.2 No 0:3 No15 0.2 Yes 1:3 No15 0.2 Yes 1:3 No

Reactor experiments 20 1.3 Yes 0:1 Yes20 1.3 Yes 1:1 Yes

A. Ahmadi et al. / Minerals Engineering 34 (2012) 11–18 13

Operational conditions of leaching experiments from the con-centrate are presented in Table 1.

Potentiostat/Galvanostat

10 4

11

5

86

3

9

14

213

15

7

12

1

Fig. 2. Schematic presentation of the electrochemical bioreactor.

2.4. Stirred tank leaching experiments

Tank leaching experiments were carried out in a 2-L stirred tankelectrobioreactor (working volume = 1.3 l) in a batch wise (Fig. 2).It contained three separate compartments of which two were usedfor fitting the working and the counter electrodes, respectively. Thethird compartment was used for fitting the reference electrode.The reactor was equipped with four perpendicular baffles. A 3-blade pitched-blade (axial) impeller (diameter = 5 cm) was chosento provide good solid suspension and generate low shear. Theimpeller was rotated at 600 rpm with a Heidolf overhead stirrer.The high stirring rate was needed for maintaining a homogeneoussuspension and increasing the rate of mass transfer (especially ofoxygen and carbon dioxide from the gas phase). The desired tem-perature was maintained by circulating water from a constanttemperature bath through the double wall jacket. To minimizeevaporation, a double-wall condenser was used; however the vol-ume of the evaporated liquid (not returned by condenser) was dai-ly replaced by adding acidified distilled water (pH = 1.5). Air wassupplied at 1 vvm (Volume per Volume per Minute) through a ringsparger under the impeller. A reticulated titanium-platinum work-ing electrode (15 cm � 9 cm � 0.1 cm, 4 pixels per inch) was im-mersed into the cathodic compartment. A platinum foil as acounter electrode was put into the anodic compartment filled withnutrient medium. The cathode chamber was separated from theanode by a fritted glass. The reference electrode was Ag/AgCl con-nected to the electrolyte in the main chamber through a Luggin-capillary, which ended close to the working electrode (Fig. 2).

The experiments were carried out at pulp density 20% (w/v), ini-tial pH 1.5, 50 �C, stirring rate 600 rpm and Norris nutrient med-ium with 0.02% (w/w) yeast extract addition.

For each reactor leaching test, 20% (v/v) of inoculum as slurry(solid plus liquid from a previous adopted culture with the celldensity of about 5 � 108 cells/ml (in the solution phase) was usedwith 80% fresh nutrient medium. The microorganisms were con-sidered to be active as the ORP rose during the culturing andmicrobial growth verified by observation under an optical micro-scope. To maintain the ORP in the desired range (400–430 mV)during electrochemical bioleaching, the potential of the workingelectrode was controlled with respect to the Ag/AgCl referenceelectrode with a Solartron Sl 1287 potentiostat. Since to maintainthe potential in the working chamber, electrochemical reactionsshould be the opposite of bacterial oxidation of ferrous iron to fer-ric iron, the applied potential was cathodic. It should be pointedout that base on the previous work (Ahmadi et al., 2011), the rateof copper dissolution from the chalcopyritic concentrate is at amaximum level in this range of solution ORP. It should be pointedout that the initial solutions of the electrochemical bioleaching forexperiments with and without pyrite addition contained 2.46 and3.11 g/l iron (mostly ferric iron), respectively. These iron leveleswere originated from the inoculum.

2.5. Analyses

Periodically, a 10 ml-sample of slurry from a reactor wasremoved and then centrifuged for 5 min at 2500 rpm (model:SIGMA 3-16) to separate the residual solids. The solution wasanalyzed for copper and iron by atomic absorption spectrometry(AAS) (model: Varian 240). The remaining solids were returnedto the reactor. The volume of slurry was adjusted 20 min beforesampling. After sampling, a solution of Norris nutrient mediumwhose volume was equal to that of the filtrate was added to thereaction mixture.

The pH and ORP values in the leach solutions were measuredwith a pH meter (model: Jenway 3540) and a Pt electrode inreference to an Ag/AgCl electrode (+207 mV vs. SHE at 25 �C).All ORP values are reported with respect to this reference elec-trode. During the experiments, the pH was reduced to 1.5 bythe addition of 6 M H2SO4 when were necessary. Evaporationloss was compensated by adding distilled water to the slurrybefore sampling. In the end of experiment, solids were filteredfrom leach solution samples through Whatman No. 41 filterpaper. The residues were washed with acidified distilled water(pH � 1.5) and dried at 60 �C (12 h) to send for analyzing copperand iron.

All reagents used in this study were of analytical grade.

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20 25

pH

Leaching time (days)

BL (Conc.)

BL (Conc.+Pyrite)CL (Conc.)

CL (Conc.+Pyrite)

Fig. 4. Variation of pH during the shake flask leaching of the copper concentrate inthe presence (BL) and absence (CL) of moderate thermophiles with and withoutpyrite addition (concentrate: pyrite mass ratio = 3:1) at 15% pulp density, 50 �C,initial pH 1.5, Norris nutrient medium supplemented with 2% (w/w) yeast extract.

14 A. Ahmadi et al. / Minerals Engineering 34 (2012) 11–18

3. Results and discussions

3.1. Leaching experiments in shake flasks

In order to investigate the influence of pyrite addition on thedissolution of copper from the chalcopyrite concentrate, shakeflask leaching experiments were carried out with and without pyr-ite addition. In the case of pyrite added tests, the concentrate was amixture of Sarcheshmeh flotation concentrate and pyrite by a massratio of 3:1. So, compared with the original sample, the mixturecontained a higher percentage of pyrite (49.3% vs. 24% by weight)and a lower percentage of chalcopyrite (30% vs. 45%).

Figs. 3 and 4 represent the changes in ORP and pH values duringleaching experiments under different experimental conditions.Fig. 3 shows that the addition of pyrite to the concentrate in thebiotic test shortened the lag phase of bacterial activity from 6 daysto 2 days. This result indicates that pyrite addition was favorablefor iron oxidizing bacteria. The flatness of ORP curve from day 5to day 11 may be due to the dissolution of pyrite and the releaseof ferrous iron into the solution. However; in both abiotic leachingtests, ORP remained constant around 330–365 mV during the pro-cess (Fig. 3). It should be pointed out that since acidophilic iron-oxidizing microorganisms convert ferrous iron to ferric iron andconsum protons according to Eq. (1), the activity of these bacteriaincreases both ORP and pH values in the solution.

4Fe2 þ O2 þ 4Hþ�������������!Iron oxidizing acidophiles

4Fe3þ þ 2H2O ð1Þ

On the other hand, acidophile sulfur oxidizing microorganisms re-duce solution pH by converting elemental sulfur Eq. (2) and/or sul-fide species to sulfate.

S� þH2Oþ 32

O2 �������������!Sulfur oxidizing acidophiles

H2SO4 ð2Þ

Fig. 4 shows that the addition of pyrite on the bioleaching mediumsignificantly decreased the solution pH. Initially pH increased(brought back to 1.5 by acid) and then dropped to about 1.2 inthe following days. This low level of pH is mainly attributed tothe bacterial oxidation of sulfur (Eq. (2)), the leaching of pyrite(Eq. (3)) and the precipitation of jarosite (Eq. (4)).

FeS2 þ 8H2Oþ 14Fe3þ ! 15Fe2þ þ 2SO2�4 þ 16Hþ ð3Þ

3Fe3þ þ Xþ þ 2HSO�4 þ 6H2O! XFe3ðSO4Þ2ðOHÞ6 þ 8Hþ ð4Þ

where Xþ ¼ Kþ;Naþ;NHþ4 and H3Oþ.

300

350

400

450

500

550

0 5 10 15 20 25

OR

P (m

V, v

s. A

g/A

gCl )

Leaching time (days)

BL (Conc.)

BL (Conc.+Pyrite)

CL (Conc.)

CL (Conc.+Pyrite)

Fig. 3. Variation of ORP during the shake flask leaching of the copper concentrate inthe presence (BL) and absence (CL) of moderate thermophiles with and withoutpyrite addition (concentrate: pyrite mass ratio = 1:1) at 15% pulp density, 50 �C,initial pH 1.5, Norris nutrient medium supplemented with 2% (w/w) yeast extract.

While during abiotic leaching tests, the pH values increased tothe leveles higher than 1.5 in the first 12 days (readjusted to 1.5by H2SO4) and then remained around 1.4 in the following days tothe end of experiments. In these tests, pH values were generallyhigher than those in the biotic tests. The initial increase of pH isconsidered to be as a result of predominating acid consuming reac-tions such as the dissolution of copper oxides and gangue mineralswhich are easily leached by sulfuric acid at ambient conditions.However; the later gradual decrease is mainly attributed to the ter-minating of acid consuming reactions and the oxidation of H2Swhich produce proton according to Eqs. (5) and (6) and/or produceelemental sulfur according to Eqs. (6) and (7).

H2Sþ Cu2þ ! CuSþ 2Hþ ð5Þ

H2Sþ 2Fe3þ ! S� þ 2Fe2þ þ 2Hþ ð6Þ

H2Sþ 12

O2 ! S� þH2O ð7Þ

It should be stated that H2S can be formed during the acid leachingof copper sulfides according to Eqs. (8)–(10). The smell of rotteneggs during the process could be attributed to the production ofH2S through the following reactions:

Cu2Sþ 2Hþ ! Cu2þ þ Cu� þH2S ð8Þ

CuSþ 2Hþ ! Cu2þ þH2S ð9Þ

CuFeS2 þ 4Hþ ! Cu2þ þ Fe2þ þ 2H2S ð10Þ

Figs. 5 and 6 present the recovery of copper and iron from theconcentrate during bacterial and chemical (uninoculated) leachingprocesses. Fig. 5 shows that the addition of pyrite to the concen-trate significantly increased the extraction of copper in both bioticand abiotic systems. In the bioleaching experiments, copper wasextracted with a high initial rate and final recovery reached 39%and 45% in the low pyrite and high pyrite concentrates, respec-tively. This high rate of copper extraction is mostly related to theleaching of copper oxides by sulfuric acid and the oxidative leach-ing of secondary copper sulfides with ferric iron present in theinoculum. By comparing copper recovery profiles (Fig. 5) with theirrelevant ORP and pH profiles (Figs. 3 and 4, respectively), it can beseen that the high rate of copper recovery in the first 10 days coin-cides with the first increase and the flat regions of ORP curve whichis accompanied by an intense drop of solution pH. The flat region isconsidered to be as a consequence of bacterial dissolution of pyrite

0

10

20

30

40

50

0 5 10 15 20 25

Cu

reco

very

(%)

Leaching time (days)

BL (Conc.)BL (Conc.+Pyrite)

CL (Conc.)CL (Conc.+Pyrite)

Fig. 5. Recovery of copper during the shake flask leaching of the copper concentratein the presence (BL) and absence (CL) of moderate thermophiles with and withoutpyrite addition (concentrate: pyrite mass ratio = 3:1) at 15% pulp density, 50 �C,initial pH 1.5, Norris nutrient medium supplemented with 2% (w/w) yeast extract.

0

5

10

15

20

0 5 10 15 20 25

Fe re

cove

ry (%

)

Leaching time (days)

BL (Conc.)BL (Conc.+Pyrite)

CL (Conc.)CL (Conc.+Pyrite)

Fig. 6. Recovery of iron during the shake flask leaching of the copper concentrate inthe presence (BL) and absence (CL) of moderate thermophiles with and withoutpyrite addition (concentrate: pyrite mass ratio = 3:1) at 15% pulp density, 50 �C,initial pH 1.5, Norris nutrient medium supplemented with 2% (w/w) yeast extract.

0.5

1.0

1.5

2.0

2.5

3.0

250

300

350

400

450

500

0 2 4 6 8 10 12

OR

P vs

. Ag/

AgC

l (m

V)

Leaching time (days)

ORP (Pyrite addition) ORP (no pyrite addition)

pH (Pyrite addition) pH (no pyrite addition)

pH

Fig. 7. Variation of ORP and pH as a function of leaching time for electrochemicalbioleaching in the presence of pyrite (concentrate: pyrite mass ratio = 1:1) at pulpdensity 20%, 50 �C, stirring rate 600 rpm, yeast extract addition 0.02% (w/v) andnutrient medium Norris.

0

20

40

60

80

100

0 2 4 6 8 10 12

Leaching time (days)

Cu recovery (Pyrite addition)Fe recovery (Pyrite addition)Cu recovery (no pyrite addition)Fe recovery (no pyritr addition)

Rec

over

y (%

)

Fig. 8. Recovery of copper and iron as a function of leaching time for electrochem-ical bioleaching in the presence of pyrite (concentrate: pyrite mass ratio = 1:1) atpulp density 20%, 50 �C, stirring rate 600 rpm, yeast extract addition 0.02% (w/v)and nutrient medium Norris.

A. Ahmadi et al. / Minerals Engineering 34 (2012) 11–18 15

by contact mechanism which releases ferrous iron to the solutionaccording to Eq. (3). The result of this experiment is in agreementwith that obtained by Sadowski et al. (2003) in which the additionof 3% pyrite to a copper concentrate increased the rate of bioleach-ing process. In control tests of chemical leaching, copper recoverywas insignificant and the final recovery reached 19% and 14% inhigh pyrite and low pyrite concentrates, respectively. The extrac-tion of copper from the concentrate in the chemical tests is mainlyattributed to the dissolution of copper oxides and to some extent tothe acid leaching of chalcocite and covellite minerals (Eqs. (8) and(9)). On the other hand, as Fig. 6 shows, in both chemical and bac-terial leaching tests, the extraction of iron was much lower thanthat of copper (Figs. 7 and 8) in which the final iron recovery variedfrom 5% in the chemical test of high pyrite concentrate to 15% inthe bacterial test of low pyrite concentrate. Low extraction of ironin the presence of pyrite is mainly attributed to the lower dissolu-tion of pyrite as one of the most iron-bearing minerals in the lowlevels of solution ORP. As described in our previous works (Ahmadiet al., 2010a, 2011), the lower extraction of iron in the bioleachingtests is ascribed to the removal of a part of dissolved iron as jaro-site precipitates.

It should be noted that when two electrically conducting sulfideminerals such as chalcopyrite and pyrite are in contact withtogether in a solution system, the mineral with the lower restpotential (chalcopyrite) is anodically dissolved while the one with

the higher rest potential (pyrite) is cathodically protected (Mehtaand Murr, 1983). In this case pyrite acts as a cathodic site for oxy-gen reduction. Natarajan (1990) measured the rest potential ofpyrite and chalcopyrite in the presence and absence of Acidithioba-cillus ferrooxidans bacteria and reported that the rest potential ofpyrite is higher than chalcopyrite in all conditions (Table 2). Itcan also be seen that the presence of bacteria in the medium shiftsthe rest potential in a positive direction.

The higher extraction of copper in the pyritic sample could berelated to the positive effect of galvanic interactions among pyriteand copper sulfides.

3.2. (Electro)-bioleaching experiments in a stirred reactor

To investigate the effect of pyrite on the efficiency ofelectrochemical bioleaching of the copper concentrate, electro-bioleaching experiments were carried out with and without pyriteaddition. In the case of pyrite added concentrate, the mineral feedwas a mixture of the original concentrate and pyrite by a massratio of 1:1. The mixture contained a higher percentage of pyrite(66.2% vs. 24%) and lower percentage of chalcopyrite (22.5% vs.45%). This amount of pyrite is often present in primary copper sul-fide ores and can be achieved by bulk flotation of sulfide minerals.This has added advantage of minimizing copper losses in theflotation circuit and reducing the consumption of flotation re-agents (Dixon et al., 2007).

Table 2Rest potential of pyrite and chalcopyrite in 0.9 K nutrient medium (Natarajan, 1990).

Mineral Rest potential, ESCE (mV)

Uninoculated Inoculated ironfree medium

Inoculated ironcontaining medium

Pyrite 300 340 450Chalcopyrite 200 240 310

Fig. 9. Mineralogical image of the leached copper concentrate after electrochemicalbioleaching at 20% pulp density (Cc = chalcocite; Ccp = chalcopyrite; Py = pyrite).

Pyrite

Chalcopyrite

50 µm

Fig. 10. SEM microphotographs of the solid residues of electrochemical bioleachingwith moderately thermophile microorganisms.

16 A. Ahmadi et al. / Minerals Engineering 34 (2012) 11–18

The changes in solution ORP and pH with time are presented inFig. 7. It shows that ORP was controlled between 400 and 430 mVduring the electro-bioleaching processes. After a transient increasein the first day, pH values decreased to around 1.3 and remainedaround it in the following days to the end of the experiments.

Fig. 8 represents the recovery of copper and iron during electro-bioleaching experiments. It shows that the initial rate of copperdissolution in the pyrite added medium was very high in whichafter the first day of the experiment 43.7% of copper was mobilizedinto the solution. Copper was then dissolved by a lower rate andreached 87% in day 10. By comparing these results with those ob-tained in the experiment without pyrite addition (Fig. 8), it can beconcluded that the addition of pyrite significantly increased therate and extent of copper dissolution (about 10%). On the otherhand, the extraction of iron remained at a low level and did notever exceed 25%. The low extraction of iron and high extractionof copper implies that the mobilized iron has mainly originatedfrom chalcopyrite and the leaching of pyrite was supposed to beminor. The average microbial concentrations of solution phasefor pyrite added and no pyrite added tests were enumerated8.8 � 108 and 9.1 � 108 cells/ml, respectively. This indicates thatbacterial growth was very good especially in the case of pyriteadded test. Figs. 9 and 10 show the mineralogical (by an opticalpolarizan microscope) and SEM/EDS analyses of the solid residues,respectively. It can be seen that pyrite particles have been re-mained unaffected in the medium while chalcopyrite was prefer-entially dissolved. Preferential leaching of chalcopyrite withrespect to pyrite can be resulted from the metal extraction curves(Fig. 8) in which Cu:Fe extraction ratio is around 4. It can be con-cluded that the mobilized iron is mainly comes from chalcopyriterather than pyrite. The figures show the image of leaching residueafter electrobioleaching process with pyrite addition. From thefigures, it can be seen that the shape and morphology of the parti-cles are different. Pyrite particles seem to be more smooth and

unaffected, while chalcopyrite particles are irregular and seem tohave been leached. However, covering chalcopyrite by chalcocitewhich was also observed in our previous work (Ahmadi et al.,2011) is justified by the mechanism proposed by Hiroyoshi et al.(2001) in which at the potential range of 400–450 mV, chalcopyriteis reduced to intermediate chalcocite by ferrous iron and the chal-cocite is oxidized by ferric iron. It can be concluded that at such alow level of solution ORP, the rate of pyrite leaching is slower thanthat at a high ORP level which is usually resulted in the conven-tional bioleaching. In such a condition, as a result of galvanic inter-actions, pyrite acts as a cathodic site (relative to chalcopyrite andother sulfide minerals) and remains inert in the system. The mainrole of electrochemical system is to regulate electrically the ratio offerric to ferrous iron at a level in which bacterial and chemical sub-systems are improved and so dissolution rate of copper would bemaximal. The mechanism in which these improvements areachieved was described in our previous work (Ahmadi et al.,2011). Sulfur oxidizer microorganisms intensify the galvanic inter-actions and the rate of electron transfer among sulfides by remov-ing the insulating sulfur product formed during chemical leachingprocess. In Fig. 11, a combined galvanical–electrochemical–bacte-rial model is illustrated to present the mechanism in which differ-ent sub-systems of electro-bioleaching process proceeds. Itdescribes the main reactions occurred in the system i.e. anodic oxi-dation of chalcopyrite in contact with pyrite as a result of galvaniceffect, electro-reduction of chalcopyrite, electrochemical reductionof ferric iron and biological oxidation of ferrous iron and sulfur.

It should be mentioned that in conventional bioleaching of cop-per ores and concentrates, a high amount of pyrite leads to in-creased levels of dissolved iron (mostly ferric iron) whichprovides conditions for jarosite precipitation (Eq. (4)). In addition,because of the adverse effect of iron ions in metal recovery pro-cesses, the high level of generated iron needs to be neutralized be-fore the recovery processes. Furthermore, usually pyrite has a closeintergrowth with copper minerals in copper sulfide ores. This is ledto the production of low grade and dirty concentrates by flotationprocess which their later pyrometallurgical treatment is costly.While in electrochemical bioleaching process, pyrite remains inertand its dissolution is controlled at a low level which is useful from

Fig. 11. Model illustrating bacterial, electrochemical and galvanic reactions in the electrobioreactor.

A. Ahmadi et al. / Minerals Engineering 34 (2012) 11–18 17

the galvanic interactions point of view. This is led to preferentialdissolution of copper over iron from the concentrate. Such a leach-ing behavior is of practical significance in the selective dissolutionof copper from bulk flotation or pyrite bearing concentrates. Insuch a condition, as a result of low level of dissolved iron, thefollowing processes of metal recovery i.e. solvent extraction andelectrowining are performed more efficient than those in the con-venient process of bioleaching where due to their high solutionORP, pyrite is preferentially dissolved respect to chalcopyrite. Meh-ta and Murr (1983) observed that when pyrite, chalcopyrite, chal-cocite and covellite are in contact with each other, as a result oftheir different rest potentials, the rate of covellite dissolutionwould be the fastest of all (anodic corrosion), followed by chalco-cite and chalcopyrite, while the rate of pyrite dissolution wouldbe the lowest of all (cathodic protection). Tshilombo (2004) foundthat in the absence of pyrite, the anodic dissolution of chalcopyriteis coupled to the reduction of ferric and/or oxygen, which must oc-cur exclusively on the chalcopyrite surface. While, when the chal-copyrite is in contact with pyrite, the reduction of ferric and/oroxygen takes place on the surfaces of both pyrite and chalcopyrite.Tshilombo reported that the rate of reduction of ferric is greater onthe pyrite surface than on the chalcopyrite surface. At high levels ofORP, pyrite is also dissolved; hence an anodic site forms on its sur-face and the cathodic areas decrease. Tshilombo (2004) reportedthat at high levels of ORP, the galvanic system would have a largeranode surface and so, the total anodic current density originatedfrom the galvanic interaction would be reduced. It should be notedthat the galvanic effect would be more significant at higher FeS2:-CuFeS2 ratios, because pyrite provides more cathodic area for thereduction process. It could also be expected that increasing pulpdensity enhances the contact between chalcopyrite and pyrite par-ticles which may increase the galvanic effect. On the other hand, ata constant pulp density, more pyrite in the feed, lower copper pre-sents in it, so the toxicity effect of dissolved copper ions for bacte-ria diminishes.

Furthermore, our previous studies (Ahmadi et al., 2010a, 2011)showed that in electrochemically controlled bioleaching systemsboth biological and chemical sub-systems are improved. In thissystem the formation of passive layers such as jarosites was pre-vented, so the growth and activity of bacteria enhanced and recal-citrant chalcopyrite was converted to more soluble copperminerals such as chalcocite and covellite.

4. Conclusions

The main conclusions obtained from this research work are asfollows:

– Results of leaching experiments in shake flasks showed that theaddition of pyrite to the chalcopyrite concentrate significantlyenhanced the rate and extent of copper extraction in both bioticand abiotic leaching tests.

– In electrochemical bioleaching process at 20% (w/v) pulp den-sity, when pyrite was added to the concentrate, about 90% cop-per recovery was achieved at the potential range of 400–430 mV (after 10 days). This amount of recovery was signifi-cantly (11% more) higher than that obtained in the electro-chemical bioleaching experiment without pyrite addition. Thisresult is industrially important, because pyrite is often presentin the primary copper sulfide ores and it could be achieved bybulk flotation with the advantages of minimizing copper lossesin the circuit and reducing the consumption of reagents.

– It can be concluded that at low levels of solution ORP, the rate ofpyrite dissolution was slow and it remained inert in the med-ium. In this condition, galvanic interactions was supposed tohave an important role on the process in which pyrite actedas a cathode site relative to chalcopyrite and also other coppersulfide minerals. While in conventional bioleaching, pyrite ispreferentially dissolved with respect to chalcopyrite and highamounts of pyrite dissolution is led to increase the levels of dis-solved acid and iron (mostly as ferric iron) which need to beneutralized before metal recovery processes.

– Electrochemical system regulates the ratio of ferric and ferrousiron to maximize the overall rate of chalcopyrite dissolution.

– From the results presented in this research, it can be concludedthat electrochemical bioleaching at low levels of ORP has a highapplication potential to selective dissolution of copper frompyritic flotation concentrates which are not suitable for conven-tional pyrometallurgical treatment.

Acknowledgments

Financial support from the National Iranian Copper IndustriesCompany (NICICO) during a part of this research is gratefullyacknowledged.

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