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Bioleaching of a chalcopyrite concentrate with moderatethermophilic microorganisms in a continuous reactor system

L. Cancho, M.L. Blázquez ⁎, A. Ballester, F. González, J.A. Muñoz

Department of Materials Science and Metallurgical Engineering, Universidad Complutense, Madrid, Spain

Received 28 June 2006; received in revised form 14 December 2006; accepted 19 February 2007Available online 21 March 2007

Abstract

The metal extraction efficiency of bioleaching processes can be greatly improved by using stirred-tank reactors. However,owing to the high cost of acquiring and maintaining these, their use is restricted to the treatment of high-grade ores andconcentrates. Unlike gold, the copper industry is not far from achieving commercial implementation of stirred processes on anindustrial scale. Recent research has focused on the development of continuous bioleaching processes for the treatment of copperflotation concentrates.

The aim of the present work was to optimize a process for continuous bioleaching of chalcopyrite concentrates using moderatethermophilic microorganisms and silver ions. The best results were obtained using a series of three reactors under the followingexperimental conditions: 45 °C, 14 days residence time, 2 g Ag/kg of concentrate (silver deposition stage at 35 °C), stirring rateadjusted to 350 rpm, pH between 1.2 and 1.4 and redox potential between 400 and 500 mV vs. Ag/AgCl. The optimizedcontinuous bioleaching system was able to dissolve copper steadily at a concentration higher than 11 g/L.© 2007 Elsevier B.V. All rights reserved.

Keywords: Copper flotation concentrate; Continuous bioleaching; Moderate thermophilic microorganisms; Silver catalysis

1. Introduction

The aerated stirred reactor improves both the yieldand the efficiency of bioleaching processes. The highercosts associated with the manufacture and maintenanceof these vessels restrict their use to the treatment of high-grade ores and concentrates. Most of the existingcommercial plants that work with stirred bioreactorsrecover gold from arsenopyrite concentrates.

Unlike gold, the commercial implementation of thisprocess for the recovery of copper from concentrates isstill in its infancy. However, in the last few years several

technologies and processes have shown promisingresults on both laboratory and pilot scales.

The IBES (Indirect Bioleaching with Effects Sepa-ration) and BRISA (Biolixiviación Rápida Indirecta conSeparación de Acciones: Fast Indirect Bioleaching withActions Separation) processes have been developed atthe University of Seville (Seville, Spain) for thetreatment of chalcopyrite in the former case, andsecondary copper sulphides such as chalcocite andcovellite, in the latter (Carranza et al., 1997; Palenciaet al., 2002). In both processes, the silver-catalysedchemical stage (leaching) is performed separately fromthe biological stage (bioxidation), in such a way that thehigh ferric concentrations produced by bacteria in thefirst reactor can later be used to dissolve the mineral.

Hydrometallurgy 87 (2007) 100–111www.elsevier.com/locate/hydromet

⁎ Corresponding author. Tel.: +34 91 394 4339; fax: +34 91 394 4357.E-mail address: mlblazquez@quim.ucm.es (M.L. Blázquez).

0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2007.02.007

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In 1998, a one-year stirred-tank bioleaching pilot testconducted at the copper mines in Mt. Lyell, Tasmania(Australia) demonstrated the technical and commercialfeasibility of copper concentrate bioleaching withmoderate thermophilic microorganisms (Rhodes andDeeplaul, 1998; Brierley and Brierley, 1999).

In the framework of the European Union ResearchProgramme, a group of European partners (BRGM,France; University ofWarwick andMIRO, UK; Boliden,Sweden and Cognis, Germany) worked under the projectHIgh-temperature bacterial OXidation, HIOX® from1996 to 1999. The goal of this project was to put a newthermophilic microorganism for the bioleaching ofchalcopyrite concentrates to use by proposing aneconomical and environmentally friendly industrialprocess for the recovery of copper. Several continuouschalcopyrite bioleaching studies using extreme thermo-philic microorganisms have been conducted under thisproject. The results were very promising, with copperextraction exceeding 90% in 5 days residence and a pulpdensity of 12% (d'Hugues et al., 2002).

Recent research in bioreactors has shown promisingresults on the bioleaching of copper flotation concen-trates with mesophilic microorganisms (Sadowski et al.,2003).

An excellent example of the increasing importance ofstirred-tank bioleaching was the creation of a newconsortium, Alliance Copper Limited (ACL), betweenCodelco and BHP Billiton (Batty and Rorke, 2006;Clark et al., 2006). This company pioneered bioleachingof copper concentrates in stirred tanks, a new techno-logical departure that led to changes in the practice ofthe mining industry. In 1997, the new company started apilot plant in Chuquicamata (Chile) which worked for4 years, although presently the continuity of the projectis being reconsidered.

Most existing commercial plants and planned plantsuse mesophilic or moderate thermophilic microorgan-isms with a temperature growth range between 40 and50 °C. One atypical case is the semi-commercialindustrial reactor (300 m3) designed by BHP Billitonin South Africa, which operates at temperatures inexcess of 60 °C and was built to determine the mostsuitable design criteria for the Alliance Copper Ltd. pilotplant in Chile. Another case is the process shared byMintek-BacTech and Peñoles in Mexico (Rawlingset al., 2003). They jointly developed a tank bioleachingprocess up to demonstration plant level at the Peñolesoperation. The project focused on a polymetallic(chalcopyrite, sphalerite, galena) concentrate containingprecious metals. Recoveries of 96–97% Cu, 99% Zn,98–99% Au and 40% Ag were achieved at a feed rate of

2.7 t/day. This plant is designed to operate at tem-peratures exceeding 60 °C and produce 500 kg/day ofcopper cathodes by a SX/EW process (Olson et al.,2003; van Staden et al., 2003).

In general, the principal objective of a bioleachingstudy is to overcome the main drawback of the processscale-up: the low kinetics. In the case of chalcopyrite,the studies indicate formation of a passivating film onthe surface during dissolution, which prevents directcontact between the solution and the mineral and limitsthe electrochemical transport required for the electro-chemical dissolution mechanism to act (Tshilombo andDixon, 2003; Sandström et al., 2005).

One of the most common solutions to this problem,and to the slow kinetics of bioleaching processes ingeneral, is to raise the temperature. However, the use ofextreme thermophilic microorganisms can present anumber of difficulties such as: lower oxygen solubilityin water; these microorganisms are less resistant to highmetal concentrations and to catalysts like silver, andthey present lower mechanical resistance than meso-philic microorganisms to attrition of their membrane.Moreover, leaching reactions are exothermic, raising thetemperature of stirred tanks to 40–50 °C, which is theoptimum range for the growth of moderate thermophilicmicroorganisms. In many cases, industry has preferredthese moderate thermophilic microorganisms becausethey are more resistant to higher pulp densities andhigher heavy metal concentrations than extreme thermo-philes (Okibe et al., 2003; Olson and Clark, 2004).Because large amounts of mineral are treated bybioleaching and bioxidation, it would be more profitableto operate in continuous mode in stirred-tank reactorswith higher volumetric productivity and lower mainte-nance costs. In view of these considerations and thekinetics of bacterial growth, continuous stirred reactorsare the best option for this kind of treatment.

The basis for selection and design of the most suitablereactor for a biomining process must be the physical,chemical and biological characteristics of the system.Special attention should be given to the complex natureof the pulp, which consists of an aqueous solution withcells in suspension and adhering to the mineral, and alsoa suspended solid and air bubbles (Gormely andBrannion, 1989; Acevedo, 2000).

Yet another alternative in chalcopyrite bioleaching isthe use of silver as a chemical catalyst. Copper recoveryfrom chalcopyrite has been improved through the use ofsilver as a catalyst in both chemical and biologicalleaching systems, and the reaction mechanisms (1)–(2)involved are apparently identical in both cases: in thepresence and the absence of bacteria (Ballester et al.,

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1990; Gómez et al., 1999). Silver interacts withchalcopyrite as follows:

CuFeS2 þ 4Agþ→2Ag2Sðchalcopyrite surfaceÞþ Cu2þ þ Fe2þ

ð1Þ

Silver sulphide is formed on the mineral surface bymeans of a chemical reaction involving an interchangebetween the silver and the copper and iron from thechalcopyrite lattice. The silver sulphide dissolves in thepresence of an oxidizing agent such as ferric ion:

Ag2S þ 2Fe3þ→2Agþ þ 2Fe2þ þ S0 ð2Þ

In bioleaching, the bacteria play an indirect role inthe process, facilitating the oxidation of Fe2+ to Fe3+ anddepolarizing the cathodic half-reaction in accordancewith:

2 FeSO4 þ 0:5O2 þ H2SO4→Fe2ðSO4Þ3 þ H2O ð3ÞReaction (3), plays an important role in the process.

When the bacterial activity decreases, the ferrous ion isnot oxidized to ferric ion and the dissolution ofchalcopyrite is halted or continues at a very slow rate.

The regeneration of Ag+ gives rise to a cyclic processthat increases the rate of copper dissolution. The silvereffect is enhanced in the presence of iron and sulphuroxidizing microorganisms. On the one hand, bacteriamaintain a favourable Fe3+/Fe2+ ratio, contributing tooxidation and, on the other hand, microorganismsoxidize the elemental sulphur layer produced on thechalcopyrite surface in reaction (2), preventing chalco-pyrite passivation:

S0 þ H2Oþ 3=2O2 YMicroorganism

H2SO4 ð4Þ

The reaction of silver with mineral sulphides otherthan chalcopyrite has also been considered in the bio-leaching process. Ahonen and Tuovinen (1990) reportedthat the leaching of zinc from sphalerite and iron frompyrite in a complex sulphide ore was inhibited by silveraddition. Similarly, Nakazawa et al., using a flotationconcentrate containing chalcopyrite and pentlandite,observed catalysis of silver on the bioleaching ofchalcopyrite but not of pentlandite, with increasinginitial silver concentrations (Nakazawa et al., 1993).

The aim of the present work was to optimize aprocess for continuous bioleaching of chalcopyriteconcentrates with moderate thermophilic microorgan-isms and silver. To that end, different variables werestudied (residence time, silver concentration, stirring,redox potential and temperature) to obtain copper con-centrate solutions.

2. Materials and methods

2.1. Mineral concentrate

A differential flotation chalcopyrite concentrate,named RT, was used (for origin and chemical compo-sition see Table 1).

X-ray diffraction of mineral RT showed the presenceof chalcopyrite (CuFeS2) as the main phase, and pyrite(FeS2), sphalerite (ZnS) and silica (SiO2) as minorityphases.

2.2. Bacterial culture

The culture was one of moderate thermophilic mi-croorganisms (named TMRT) grown from acid minedrainage from Río Tinto (Huelva, Spain). The culturewas composed mainly of S-oxidizing bacteria withability to oxidize different sulphide minerals, especiallycopper sulphides. An important feature of the culturewas that its heavy metal resistance at low pH was higherthan that of other moderate thermophiles described inthe literature. Moreover, the culture also containeddifferent Fe-oxidizing microorganisms, but in smallerproportions (Gómez et al., 1999). Particularly importantin this culture was the moderate thermophilic microor-ganism named TMRT, isolated by the ExtractiveMetallurgy research group at the Complutense Univer-sity of Madrid (Gómez et al., 1997). This is an aerobicbacterium, strictly chemolithoautotrophic, with theability to oxidize reduced sulphur compounds.

The culture was grown in an orbital shaker at 45 °C,pH 1.5 and 150 rpm in Norris medium, modified withoutiron (Norris et al., 1986), with the following chemicalcomposition: 0.4 g/L MgSO47H2O; 0.2 g/L (NH4)2SO4

and 0.1 g/L K2HPO4. The RT concentrate was used asenergy substrate at a pulp density of 5% w/v. The culturewas also adapted to different amounts of silver (0.3, 0.6,

Table 1Origin, chemical composition and particle size of RT concentrate

Origin Cu % Fe % Zn % Pb % S % Particle size

Río Tinto (Huelva, Spain) 22.44 31.20 2.70 0.02 38.79 b105 μm (95%)

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1 and 2 g Ag/kg concentrate) under different experi-mental conditions. In the silver-catalysed bioleachingtests, the appropriate amount of Ag+ was added to thepulp before adding the bacterial inoculum.

2.3. Continuous bioleaching set-up

Fig. 1 shows the continuous bioleaching set-up used.This consists of a feeding tank (A), a peristaltic pump(B) to supply fresh pulp to the first of three mechanicallystirred (M) reactors in cascade (R-1, R-2, R-3), and acollector for the final product (P). The pulp wastransferred by overflow from tank to tank.

In the first three continuous bioleaching experiments,all the tanks including the feeding tank had an effectivecapacity of 2.4 L. The reactors were thermostatted andclosed to avoid evaporation of the solution. Temperaturewas controlled with a thermocouple (T). The air supplyfor bacterial growth (500 mL/min) was controlled bydifferential water manometers (D) and injected into thereactors through glass capillaries (O). Flasks (F) wereplaced between both devices to prevent the backward ofthe liquid. The last test was performed in an improvedsystem. The improvement chiefly affected the stirringsystem, in which baffles were used and a flow of air wassupplied from the base of the reactor. Additionally, the

first reactor (6 L) was twice the volume of the second andthird reactors (3 L) in order to increase the residence timeand to encourage adaptation of the microorganisms.

2.4. Experimental procedure

All the continuous experiments were preceded by adiscontinuous stage designed to adapt the culture to thetest conditions. Once each reactor was inoculated, metalconcentration in solution (Cu, Fe, Zn), pH, redoxpotential and bacterial population were measured. Thecontinuous feed was started once the optimum culturegrowth was attained. The pulp density was 5% in all tests.

Evaporation was restored periodically with distilledwater, pH controlled and adjusted (with sulphuric acid20% v/v) to below 1.5; redox potential was measured (vs.Ag/AgCl) and a sample was removed to determine i) thenumber of cells in the liquid phase using a Thomacounting chamber (depth 0.1 mm) with a phase-contrastmicroscope, and ii) iron and copper concentration usingatomic absorption spectrophotometry.

The end of the experiment in each reactor wasreached when the bacterial population and the amount ofdissolved metal stabilized. The overall yield wasdetermined by mass balance of the solid residues andthe feeding solutions, in each reactor and in the

Fig. 1. Continuous bioleaching set-up.

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collector. This balance is expressed by the followingequation:

MetalFed ¼ MetalR�1 þMetalR�2 þMetalR�3 þ ΣMetalPi

The main errors associated with this balance are lossof solids during filtration and loss of liquid in the cakes.That error was calculated as a function of the differencebetween the two terms of the equality.

3. Results and discussion

The variation of the concentration of metals (g/L), pHand redox potential in the tank reactors (R-1, R-2 and R-3), in the feeding tank (A) and in the collector (P) isshown for each test. The results depicted in the initialportion of each figure (Figs. 2–9) illustrate the batch(discontinuous) phase of the experiment.

At the beginning of the continuous tests, the metalconcentration in solution and the cell populationdiminished. This indicates that the gradual increase ofmetal concentration observed during the previousdiscontinuous stage was a result of bacterial action.However, once the continuous test started, the feeding offresh pulp into reactor R-1 and the output of concentratedsolution from reactor R-3 produced a diluting effect,reducing the metal concentration in each reactor.

In the continuous experiments, the optimum resi-dence time was determined by the stabilization of metalconcentration in solution in the zone of maximumbacterial growth. When these curves trended upwardsthe residence time would be shorter, and when theytrended downwards the time would be longer. The goalwas to achieve a constant yield for better control of thesystem.

The duration of continuous experiments should be atleast twice the residence time of a discontinuousprocess, since the system requires a time equal to theresidence time to evacuate all metals accumulatedduring the discontinuous stage. In fact it is during thesecond stage that the system functions in continuousmode according to the experimental conditions.

3.1. Preliminary test

The first continuous test was performed at 45 °C,with 20 days residence time (across the three reactors)and without silver. The variation of iron, copper and zincin solution is shown in Fig. 2.

The system achieved the highest yield in reactor R-1during the discontinuous stage, with dissolution of26.6% Fe, 48.2% Cu and 44.4% Zn in the mineral.

However, the lowest metal concentration in solution wasachieved in the first reactor during the continuous stage.The dissolution of copper and iron in reactor R-3 waspractically double than in reactor R-2, and in the latter itwas double than in R-1. Such differences are to beexpected since there is an increase of metal concentra-tion in solution as the pulp passes through the reactors incascade, so that the mineral reaching each reactor issubject to heavier attack and hence is free of surfaceproducts (flotation reagents, oxidized phases) which canbe harmful to bacterial activity. For that reason, themicroorganisms in reactor R-3 were supplied with amore readily soluble substrate which substantiallyincreased the dissolution yield.

Fig. 2. Preliminary test: 45 °C and 20 days residence time. Variationsof the concentration of iron (A), copper (B) and zinc (C).

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pyFig. 2A shows the variation of iron in solution. The

drop in concentration caused by dilution at the outset ofthe continuous process was followed by stabilization ofthe concentration after approximately 10 days, whichclearly indicates that the residence time employed wasappropriate for this metal. The lack of sharp fluctuationsin the concentration curve of this element is indicative ofweak precipitation of jarosites. Analysis of residues byX-ray diffraction confirmed this point, since theproportion of jarosites was small.

The variation of copper concentration (Fig. 2B)reached a steady state later than iron, practically at theend of the experiment, with copper dissolutions of 4 g/L.An overall copper yield of 12% was achieved.

The dissolution rate of zinc remained practicallyunchanged during both discontinuous and continuousstages, as sphalerite is the most soluble mineral in theconcentrate (Fig. 2C).

The bacterial concentration in solution during thecontinuous experiment was stable, attaining highconcentrations, above 108 cells/mL, in all the reactors(Fig. 3A). Although there was also bacterial growth inthe feeding tank, the concentration was much lower thanin the reactors at all times. This bacterial activity raisedthe redox potential to 550 mV (vs. Ag/AgCl) (Fig. 3B)

and maintained the pH between 1.5 and 1 without acidaddition, through the regeneration of acid by bacteria.Only in the feeding tank and in reactor R-1 were thevalues higher.

These results indicate that in spite of the longresidence time tested, the dissolution yield achieved wasstill low. The next step was therefore to catalyse theprocess with silver.

3.2. Continuous bioleaching test with silver at 45 °C

A new continuous bioleaching experiment wasperformed at 45 °C with 15 days residence time (across

Fig. 4. Continuous bioleaching with silver at 45 °C: 15 days residencetime and 0.3 g Ag/kg of concentrate. Variations of the concentration ofiron (A), copper (B) and zinc (C).

Fig. 3. Preliminary test: 45 °C and 20 days residence time. Variationsof the bacterial population (A) and the redox potential (B).

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pythe three reactors) using 0.3 g Ag/kg of concentrate,fixed on the mineral surface at 35 °C for 24 h. Thisamount was chosen on the basis of previous studieswhich demonstrated that lower amounts were insuffi-cient to catalyse chalcopyrite dissolution, whereasamounts higher than 0.3 g Ag/kg of concentrate didnot improve the mineral dissolution (Blázquez et al.,1999). In addition, these studies showed that the mostfavourable temperature to fix silver on the chalcopyritesurface was 35 °C (Cancho et al., 2004).

The results of this test are shown in Fig. 4. Thehighest yield during the discontinuous stage wasachieved in reactor R-1 with dissolution of 34.8% Fe,36.8% Cu and 71.8% Zn after 40 days. Silver was onlyadded to the feeding tank, and so the discontinuousstage prior to experiment 2 was performed under thesame experimental conditions as the previous test.

Based on the catalytic effect of silver ions onchalcopyrite dissolution, it was decided to use a shorterresidence time (15 days) than in the preliminaryexperiment (20 days). Stabilization of the system wasreadily achieved during the continuous stage, indicatingthat the decision to shorten the residence time was theright one.

The copper dissolution rate increased in the threereactors during the continuous stage, but the differencesamong them diminished. In any case the overalldissolution yield of this metal was twice that (24.5%)achieved in the test without silver (Fig. 4B). The zinc insolution decreased, confirming that silver does notcatalyse dissolution of sphalerite, but converselydecreases its reactivity (Fig. 4C).

The bacterial population was high, even moreabundant than in the test without silver, indicating thatsilver did not affect culture growth and that the moderatethermophilic bacteria tolerated silver well (Fig. 5A).

Fig. 5. Continuous bioleaching with silver at 45 °C: 15 days residencetime and 0.3 g Ag/kg of concentrate. Variations of the bacterialpopulation (A) and the redox potential (B).

Fig. 6. Continuous bioleaching with silver at 40 °C: 0.3 g Ag/kg ofconcentrate and 15 days residence time. Variations of the concentrationof iron (A), copper (B) and zinc (C).

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pyThe redox potential in steady state was lower than in

the absence of silver. This is probably related to adecrease of the Fe (III)/Fe (II) ratio produced byconsumption of ferric ion during regeneration of thecatalyst according to reaction (2) (Fig. 5B).

3.3. Continuous bioleaching test with silver at 40 °C

Previous studies have shown that the catalytic effectof silver on chalcopyrite is strongly affected by thetemperature of fixation (Blázquez et al., 1999). Forinstance, dissolution of chalcopyrite increases substan-tially at a silver deposition temperature of 35 °C but nocatalytic effect is observed at 68 °C. On the basis ofthese results, a temperature of 40 °C, close to theoptimum for the silver action (35 °C), was tested. A 5 °Creduction of temperature should not be as detrimental tobacterial growth as exceeding the maximum growthtemperature; although it might affect the kinetics of theprocess, this would be offset by silver catalysis. A thirdexperiment was therefore performed at 40 °C, main-taining a residence time of 15 days (across the threereactors) with 0.3 g Ag/kg of concentrate fixed at 35 °C.

Fig. 6 shows the results of this test. The priordiscontinuous stage was also conducted in the presence

of silver. The highest yield was achieved in reactor R-2:31.8% Fe, 48.6% Cu and 51.8% Zn, after 20 days.These results indicated a higher rate of metaldissolution than in the corresponding discontinuousstage of the previous experiment. This was possiblydue to the presence of silver, since in the previousexperiment the silver was not added during the discon-tinuous stage, but to the feeding tank during thecontinuous stage.

During the continuous stage, a steady state of metaldissolution was achieved half way through the experi-ment. A higher iron concentration was reached, close to6 g/L (Fig. 6A). However, copper concentration decreasedslightly with respect to the previous test (Fig. 6B). Theoverall copper yield was 18.4%, an intermediate valuebetween the two previous tests. The pH remained between1.0 and 1.4 as in the previous tests.

The most important differences between this test andthe one carried out at 45 °C were: a) an increase in theamount of iron and zinc in solution, principally in thelast two reactors (Fig. 6A and C) and b) a decrease andstabilization of copper dissolution; c) an increase ofbacteria in suspension in all the reactors, double that ofthe two previous tests (Fig. 7A); and d) also highoxidizing potentials of 600 mV, mainly in the lastreactors (Fig. 7B). Under these conditions, chalcopyritepassivation took place.

The increase in the redox potential and free bacteriain suspension, with iron as the main oxidizablesubstrate, was probably the result of selection of Fe-oxidizing bacteria in the culture caused by a reduction intemperature. According to several authors (Olson andClark, 2004; Third et al., 2002), Leptospirillum is thedominant species in the bioleaching of gold concentratesat 40 °C and pH 1.6, with arsenopyrite and pyrite as themain minerals.

Bacterial selection of this microorganism occurswhen redox potential is high because it is lesssensitive to high concentrations of ferric ion. Thestrong Fe-oxidizing tendency of Leptospirillum wouldexplain the high redox potentials attained during theseexperiments.

A 5 °C reduction in temperature modified the pro-portion of Fe-oxidizing microorganisms in the mixedculture, raised the redox potential of the solution toabove 500 mV vs. Ag/AgCl causing chalcopyrite pas-sivation (Third et al., 2002), and favoured the dis-solution of pyrite and sphalerite.

This temperature-dependant bacterial selection couldnegatively affect the activity of S-oxidizing bacteria,which is inhibited in the presence of high concentrationsof ferric ion. These conditions promote passivation of

Fig. 7. Continuous bioleaching with silver at 40 °C: 0.3 g Ag/kg ofconcentrate and 15 days residence time. Variations of the bacterialpopulation (A) and the redox potential (B).

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the chalcopyrite surface and are unfavourable to itsdissolution.

From these results it seems clear that the control ofredox potential is extremely important for the system,and this variable would therefore be a good indicator ofthe effectiveness of chalcopyrite catalysis.

3.4. Improved continuous bioleaching system

The last continuous bioleaching test was performedunder the best conditions established during the

previous experimentation: in an improved system at45 °C. The main changes introduced were:

a) The stirring rate was optimized to avoid settlement ofthe solid. The use of a stirring rate in excess of400 rpm during experimentation produced a morehomogeneous pulp, but this affected bacterial growthnegatively by increasing cell attrition and reducingthe adherence of microorganisms to the mineral,which is necessary for oxidation of the sulphur thatforms on the chalcopyrite surface. A homogeneous

Fig. 8. Improved continuous bioleaching system. Variations of the concentration of iron (A), copper (B) and zinc (C).

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pypulp was achieved by improving the design of thereactors: placing baffles in the walls, using a screwpropeller capable of generating greater turbulence,and supplying air from the centre of the reactorbottom to give better gas distribution. As a result, thesolid was kept in suspension and mass transfer andoptimum conditions for the growth of bacteria wereachieved.

b) The volume of the first reactor was doubled toincrease the residence time, considering that there isconstant dilution in that reactor due to continuousinput of pulp.

c) The mineral was washed and silver was deposited onit at 35 °C to eliminate surface compounds (flotationreagents and oxidation products) that could interferewith the interaction between silver and chalcopyrite.

d) Recent studies have shown that the growth ofmoderatethermophilic microorganisms in continuous culturesand in the presence of silver favours their S-oxidizingability but inhibits their Fe-oxidizing capability(Cancho et al., 2005). However, the presence of Fe-oxidizing microorganisms in the mixed culture isimportant in order to regenerate the catalyst (reaction

(2)). The culture used in this experiment was a mixedone (Fe- and S-oxidizing bacteria) of moderatethermophilic microorganisms unadapted to silver.

During the test, different variables were modified tofind the best bioleaching conditions and to improve theoverall yield of the process.

Fig. 8 shows the variation in the concentrations ofiron, copper and zinc during the experiment. A silverconcentration of 0.3 g/kg of concentrate was used duringthe discontinuous stage. The highest dissolutions wereachieved in reactor R-1: 42% Fe, 60% Cu and 53% Zn.At the start of the continuous test, the concentration ofcatalyst was kept at 0.3 g Ag/kg of RT and the residencetime at 7 days.

The metallic dissolution decreased in all the reactorsat the beginning of the continuous process, indicatingthat the residence time was too short to achievestabilization of the system. Therefore, a new residencetime of 10.5 days was tried after day 60 of the test.However, the system did not respond as expected. Theredox potential rose since the bacteria had longer togenerate Fe (III), and the feeding tank produced less

Fig. 9. Improved continuous bioleaching system. Variations of the bacterial population (A) and the redox potential (B).

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dilution (Fig. 9B). This increase of the redox potential toabove 500 mV in the three reactors coincided with adecrease of copper in solution, which again confirmsboth the passivation of chalcopyrite at high potentialsand the absence of silver catalysis. The amount of silverion added could be insufficient if it partially reduces tometallic silver. Increasing the residence time to 14 days(after day 77 of the test) and the amount of silver to 1 gAg/kg of RT (after day 100 of the test) had a beneficialeffect on the system: The redox potential decreased andthe copper dissolution increased. Once the systemstabilized, after 143 days, further silver was added(2 g Ag/kg of RT) to improve the copper dissolution.After an initial reduction of copper and iron in solution,both rose until the system stabilized. The drop in metalsseems to be connected with the culture's adaptation tothe increased amount of silver.

The increase of Zn in solution was associated with anincrease of the redox potential, leading to passivation ofthe chalcopyrite. The consumption of Fe (III) during Zndissolution produced a reduction of the Fe (III)/Fe (II)ratio and hence of the redox potential, which againfavoured chalcopyrite dissolution.

The highest copper dissolution in the three reactorswas achieved with 2 g Ag/kg of concentrate, at a redoxpotential between 400 and 500 mV vs. Ag/AgCl and at apH lower than 1.5. The last two variables wereresponsible for the resulting stable and abundantbacterial population (Fig. 9A), which indicates that inthese conditions silver catalysis was efficient. Theconditions at the end of the experiment were the bestof all those tested: For the first time, the system achieveda concentration of copper in solution of 11 g/L, with anoverall copper yield of 66%.

4. Conclusions

These experiments demonstrate the effectiveness of amixed culture of moderate thermophilic microorganismsand silver for dissolution of chalcopyrite concentrates.The best results were obtained with three tanks in series at45 °C, a residence time of 14 days, a silver concentrationof 2 g/kg of concentrate fixed on the mineral surface at35 °C, a stirring rate of 350 rpm, a pH between 1.2 and1.4 and a redox potential between 400 and 500 mV.

The continuous bioleaching system produced coppersolutions with concentrations higher than 11 g/L and anoverall copper yield of 66%.

Dissolution in the first reactor increased when theresidence time was doubled with respect to the otherreactors, as microorganisms were able to adapt betterthanks to a constant supply of fresh pulp.

Silver substantially raised the rate of chalcopyritedissolution and copper extraction when the redoxpotential was lower than 500 mV vs. Ag/AgCl.

Reducing the temperature caused selection of Fe-oxidizing microorganisms and a consequent increase ofthe Fe (III)/Fe(II) ratio. High Fe (III) concentrationsinhibited the activity of S-oxidizing bacteria andfavoured passivation of chalcopyrite and dissolution ofpyrite and sphalerite.

The stirring rate was adjusted to optimize homoge-nization of the pulp and the presence of microorganisms.A stirring rate higher than 350 rpm reduced theeffectiveness of the culture due to increased cell attritionand poorer adherence of microorganisms to the mineral,which is necessary to oxidize the sulphur that forms onthe chalcopyrite surface.

Acknowledgements

The authors wish to express their gratitude to theComisión Interministerial de Ciencia y Tecnología(CICYT) of the Spanish government for funding thiswork. Furthermore, one of the authors (L. Cancho) wishesto express her gratitude to the Comunidad Autónoma deMadrid for the graduate grant that helped her to carry outthese research studies.

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