Pergamon 0892-6875(95) 00059-3

Minerals Engineering, Vol. 8, No. 9, pp. 949-965. 1995 Copyright © 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0892-6875/95 $9.50+0,00

INFLUENCE OF METALLIC IONS IN THE BIOLEACHING OF CHALCOPYRITE BY SULFOLOBUS BC: EXPERIMENTS USING

PNEUMATICALLY STIRRED REACTORS AND MASSIVE SAMPLES

J. L. MIER, A. BALLESTER, M. L. BL/i.ZQUEZ, F. GONZ/i, LEZ and J. A. MUlqOZ

Departamento de Ciencia de Materiales e Ingenierfa Metahirgica, Facultad de Ciencias Qufmicas, Universidad Complutense, 28040 Madrid, Spain

(Received 1 February 1995; accepted 11 May 1995)

ABSTRACT

The influence of several metal ions on the bioleaching of a copper concentrate by Sulfolobus BC was studied using pneumatically stirred columns. Ag, As, Bi, Co, Hg, Mo and Ru were tested, and 10 g metal/kg concentrate were added, except in the case of Mo and Ag where 4 g Mo/kg concentrate and I g Ag/kg concentrate were used. Bismuth was the only metal to enhance the initial dissolution rate of copper, and bring about a greater final extraction than was obtained in the culture with no cation added.

In addition, experiments were carried out with massive samples of two types of chalcopyrite in order to study the interaction of silver and bismuth with the mineral. The mechanism of action of each cation was different: silver reacted with the surface of the chalcopyrite to form a layer of Ag2S. This layer caused the passivation of the mineral in places with a deposit and promoted the anodic behavior of the surface not covered. On the other hand, bismuth acted in solution and prevented the formation of phosphate complexes of Fe 3+ by precipitating as bismuth phosphate so increasing the oxidizing potential of the Fe3+/Fe 2+ couple.

Keywords Bioleaching, chalcopyrite, Sulfolobus BC, metal ions, silver, bismuth.

INTRODUCTION

Copper principally occurs in the lithosphere as sulphides and oxides, with chalcopyrite being one of the most important from a commercial point of view. This mineral is closely related to bornite (Cu5FeS4) and cubanite (CuFe2S3), often appearing in high temperature type metallic seams. Chalcopyrite is associated with pyrite, pyrrhotite, sphalerite, galena, quartz, calcite, dolomite, and several other copper minerals. It is very resistant to chemical attack by reagents traditionally used in hydrometallurgy. Very strong oxidizing conditions are necessary for any appreciable solubilization.

The use of microorganisms is of increasing importance as an alternative to conventional hydrometallurgy. Microorganisms have already been successfully used in the extraction of copper from low grade ores. Nevertheless, the process is very slow and very dilute solutions are obtained from which it is difficult to separate the metal by electrolysis. Several studies [1-6] have recently shown an interest in using thermophilic microorganisms which grow at 50-100°C, especially bacteria of the genus Sulfolobus.

949

950 J.L. MIER et al.

However, there is a decrease in the solubility of 0 2 and CO 2 in the bioleaching medium due to the high temperatures used, which can limit the growth of the thermophilic microorganisms. The use of pneumatically stirred columns enhances the transfer of matter by diminishing the thickness of the limiting layer of diffusion between solution and solid [7,8]. Furthermore, the CO2/air current is of use as a stirring system, and because of this the application of this type of reactor in bioleaching seems advisable. However, pneumatic columns cannot operate with high pulp densities because they need a high rate of gas flowing through the system to maintain the mineral in suspension. In addition Sulfolobus only tolerates relatively low pulp densities. Norris [9] suggested that SulJblobus is extremely sensitive to the presence of mineral in pneumatically stirred reactors, because its growth is inhibited by 15% (volume/weight) of pyrite. For these reasons, it is necessary to adapt such microorganisms to increasing pulp densities.

Since the bioleaching temperature of thermophilic organisms is higher than that used with mesophilic organisms, there is an increase in the chemical leaching of chalcopyrite and a decrease in the viscosity of the nutrient medium. This results in lower energy requirements and better mixing, especially in large reactors. The thermophilic microorganisms of the genus Sulfolobus have been shown to be more effective than mesophilic organisms in attacking minerals which are resistant to oxidizing agents, such as chalcopyrite [ 10-12] and molybdenite [ 13]. They have also been used in the bioleaching of pyrite [ 14], pyrrhotite [ 14,15] arsenopyrite [16,17], complex sulphides [4] and, even, in the elimination of sulphur from coal [18].

Nevertheless, the process is still too slow to be economically feasible and studies to solve this problem are needed. Silver has been used successfully as a catalytic agent in dissolving chalcopyrite with mesophilic microorganisms [19-22]. It is logical to think of using silver to accelerate the bioleaching of mineral sulphides by thermophiles, although the high inhibition of Sulfolobus in the presence of silver might limit the application of this cation. For this reason other cations (Bi 3+, Hg 2+, etc,..) should be considered for use at high temperatures [23]. In this respect, it is important to study the interaction of some potentially toxic metals with the bacteria because they are inevitably dissolved from the mineral in which they are found, or they are used in trace quantities as catalytic agents.

The present work studies the effect of certain metal ions on the bioleaching of a copper concentrate by Sulfolobus BC in pneumatically stirred columns. From the results, the cation which most promoted the dissolution of chalcopyrite, bismuth, was chosen for further comparative experiments with silver using massive samples of mineral to determine the characteristics of the attack. The aim was to study in more detail the interaction of bismuth and silver with the surface of the mineral and the nature of the compounds formed. This study would have been difficult to carry out with powdered samples of small particle size and thus massive samples were used.

In addition, the influence of the composition and proportion of other mineral phases was studied. For this reason, a high temperature chalcopyrite was used, along with another chalcopyrite in which the secondary phases consisted only of a very small proportion (chalcopyrite).

MATERIALS AND METHODS

Raw materials

Copper concentrate. A copper concentrate was provided by Rio Tinto Minera S.A. (Huelva, Spain). Its chemical composition was (% weight): 19.2% Cu; 28.3% S; and 32.5% Fe. X-ray diffraction showed the principal phases to be chalcopyrite and, in smaller proportion, pyrite and silica. The particle size was 97% less than 74/am.

High tempera ture chalcopyrite. This mineral was also provided by Rio Tinto S.A. (Huelva, Spain). Its chemical composition (% weight) was: 22.3% Cu; 31.5% S; and 36.3% Fe. A detailed study of the phases was performed in two ways, one before the attack by bioleaching and the other after.

Bioleaching of chalcopyrite 951

a) Assessment of the pre-attack was done by Scanning Electron Microscopy (SEM) using a Jeol JSM- 25C electron microscope, and by Electron Dispersive (X-ray) Spectrometry (EDS) microanalysis using Link detectors coupled to a Jeol JSM-6400 and a Jeol S-840 electron microscope. By this means it was possible to identify a matrix of copper and iron sulphides and a minor phase of magnetite. The backscattered electron image (Figure 1) shows the sulphide matrix (white) and rounded grains (grey) of magnetite at the edge of a fissure.

b)

Fig.! Backscattered electron micrograph of high temperature chalcopyrite before attack. Grey phase is magnetite. White phase is chalcopyrite/cubanite matrix.

Post attack was assayed by SEM and EDS microanalysis as above. In this case quantitative analyses were carried out with standards of cubanite (23.4% Cu; 35.5% S and 41.2% Fe) and chalcopyrite (36.6% Cu; 33.6% S and 29.8% Fe). The matrix structure was revealed and the presence of a lenticular phase, which was very little altered (Figure 2). It corresponded to the chalcopyrite in cubic structures over a very attacked interstitial phase of cubanite. The structure of the mineral showed magmatic origin. Cubanite was separated from a solid solution of chalcopyrite at approximately 500°C. The mineral underwent rapid cooling so that it kept this structure at low temperatures.

Fig.2 Scanning electron micrograph showing high temperature chalcopyrite after leaching by Sulfolobus BC. Chalcopyrite/cubanite matrix reveals cubic structure while

magnetite (black phase) is not altered.

Chalcopyrite. The mineral came from Huar6n mines (Peru). The chemical composition was: 34.1% Cu; 28.7% Fe and 32.7% S. X-ray diffraction showed the predominant phase to be chalcopyrite. SEM and EDS

952 J.L. MIER et al.

microanalysis showed small quantities of silica, aluminum silicates and pyrite. A general aspect of the mineral before bioleaching attack can be observed in Figure 3, in which particles of pyrite with its characte- ristic cubic shape are in the chalcopyrite matrix.

Fig.3 Chalcopyrite before attack.

Microorganisms. Sulfolobus B C cultures came from waste deposits in Birch Coppice coal mine (Great Britain). The nutrient medium for growing the microorganisms was produced an aqueous solution of (NH4)SO 4 0.4 g/L; MgSO4.5H20 0.5 g/L; and K2HPO 4 0.2 g/L. The initial pH in all the experiments was 1.5 and was left to evolve freely. Temperature was fixed at 68°C.

Metals. The metals used and the corresponding salts are shown in Table 1. They were chosen according to bioleaching studies of complex sulphides and copper concentrates [23] and based on the work of Scott and Dyson [24] who evaluated the catalytic effect of certain metals on the abiotic oxidation of zinc sulphide under pressure.

TABLE 1 Metals studied

METAL COMPOUND

Ag Ag2SO 4

As NaHAsO4.7H20

Bi Bi(NO)3.5H20

Co COSO4.7H20

Hg HgSO 4

Mo MovO24(NH4)6.4H20

Ru RuC13

Experiments in pneumatically stirred columns

The experiments were performed in a cylindrical glass reactor, 47 cm high and 4.5 cm diameter, closed at the bottom (A) (Figure 4). Inside were a central glass tube (F) and a narrow tube (D), through which air containing 1% CO 2 was injected at the base of the column, producing stirring and gas mass transfer in a

Bioleaching of chalcopyrite 953

three-phase system. In the upper part there were two orifices where the gas supply tube (C) and a condenser to avoid evaporation (E) were connected. The latter orifice also served for taking samples. The reactor was heated by a water jacket (B) joined to a pump (M) which heated the water, as well as controlling temperature.

M

I I

E

B

"I.,

pt"~H Meter ]

Fig.4 Diagram of the pneumatically stirred column.

Three columns were used for each cation. Two corresponded to experiments with bacteria and contained at the outset 3 g chalcopyrite concentrate, 270 ml of nutrient medium and, in one only, the required amount of cation. The other contained no cation. After a period of chemical conditioning and when bacterial population was approximately 108 cells/ml, 30 ml ofSulfolobus BC culture grown on chalcopyrite were used for inoculation. In this way, a 1% pulp density (1% p.d.) was obtained. The third column was a sterile experiment (control) with 3 g of copper concentrate, 300 ml of nutrient medium (1% p.d.) and the corresponding quantity of cation. In addition, 2 ml of a 1% alcohol solution of thymol was introduced to keep the medium sterile.

After a certain time, the pulp density was increased in the three columns to 5% by adding an extra 12 g of copper concentrate. This time was determined by microscopical observation of the bacterial population and by relating the quantity of copper extracted from the concentrate with the bacterial activity [25], taking the bacterial growth up to 108 cells/ml and the stabilization of the copper concentration as an indicator for adding the concentrate.

The quantities of metal introduced in the columns were: 10 g metal/kg concentrate for As, Bi, Co, Hg and Ru; 4 g metal/kg concentrate for Mo; and 1 g metal/kg concentrate for Ag. When the pulp density was raised to 5% the correct quantity of metal was added to keep the metal/concentrate ratio constant.

Periodically, 5 ml samples were taken from each column for copper and iron analysis in solution without stopping the experiment. Solid was settled by centrifugation for 15 minutes and 1 ml of the liquid

954 J, L. MIER et al.

supernatant was taken, to which 1 ml of 3.64 N H4SO 4 was added to avoid the precipitation of Fe 3+ compounds. Copper and iron concentrations of these samples were analyzed by atomic absorption spectroscopy (AAS). Another lml of the supernatant was taken for determination of Fe 2+ concentration by volumetry with a solution of cerium sulphate (5 mM Ce) in 0.5 N sulphuric acid using a titroprocessor.

Experiments with massive samples

These experiments were carried out to study the influence of the addition of Ag + and Bi 3+ on the bioleaching of chalcopyrite by Sulfolobus BC. The massive samples were polished and coated with graphite for SEM observations before the attack. The graphite was then carefully removed and the samples were introduced into 250 ml conical flasks with 90 ml of nutrient medium at pH 1.5 and the corresponding cation (0.01 g cation/L). No cation was added to the control experiments. The flasks were placed in orbital incubators at 68°C and shaken at 100 rpm. After chemical conditioning, 10 mi of Sulfolobus BC solution, grown on copper concentrate, were added. A second inoculation was performed after 20 days to re-enforce the attack of the massive samples. The experiments lasted 40 days. Sterile experiments were performed in the same conditions, except that 1 ml of 1% alcohol solution of thymol was introduced to prevent bacterial growth.

When the experiments were stopped, the massive samples were washed gently with distilled water to remove remaining solution, but not the precipitates formed which firmly stuck to the mineral surface. After washing and drying, the massive samples were coated with graphite to observe the morphology of the attack by SEM as well as to determine the nature of the precipitates and the chemical alterations on the surface by EDS microanalysis.

In addition, the rest potentials of high temperature chalcopyrite, chaicopyrite and a sample of magnetite from Badajoz (Spain) were measured to reveal the electrochemical behavior of the principal phases. For this, a Bank Electronik LB-81-M potentiostat was used. The potential measurements were accomplished with a working electrode made from a sample of mineral packed in a teflon support in contact with a copper thread; the electrode was introduced in a cell containing 120 ml of nutrient medium at pH 1.5 and 68°C. The heat was provided by a water jacket around the cell and connected to a thermostatic bath. The potential of the electrode was controlled by an Ag/AgC1 reference electrode (+207 mV SHE) situated at the end of a Lugging capillary with a saturated solution of KCI. The measurements were taken 30 minutes after introducing the working electrode into the medium to give time for the potential signal to stabilize.

RESULTS AND DISCUSSION

One of the immediate consequences of adding metallic ions to the solution was the different bacterial activity in the columns with 1% pulp density. This influenced the time at which the extra copper concentrate had to be added to reach 5% pulp density. In those experiments where bacterial activity was totally inhibited at 1% pulp density no concentrate was added (Table 2), as in the cases of molybdenum and silver.

TABLE 2 Mineral addition times (h.) up to 5 % pulp density in pneumatically stirred reactor tests. There was no mineral addition in tests with silver and molybdenum.

With cation

Reference

II As I I Bi I Co I"g I MoL "u 70 70 70 94 150

70 70 70 70 70 70 70

Bioleaching of chalcopyrite 955

Thus, the metals studied can be divided into the following four groups:

a) Bismuth. This was the only experiment in which the dissolution of copper (final concentration of 7 g Cu/L) exceeded that of the reference (6 g Cu/L) (Figure 5A). Furthermore, the dissolution rate increased immediately when concentrate was added. However, the Fe2+/Fe 3+ curves (Figure 6A) show that the presence of bismuth did not lead to an increase in bacterial activity.

Copper leached (g/L)

8 [ . . . . Reference

With Bi / ~ F ,

6 ~- Ref. Sterile /~.'J'"

Bi Sterile ~ / , , , /

4. A B ,"

2

0 50 100 160 2 0 0

Time (h.)

A

260

C o p p e r l e a c h e d ( g / L )

8 . . . . Reference

Wi th As

~" Ref. Sterile

As Sterile

B

. ,m - - _ l . . . -I

A B ,' / "

50 100 150 200 250

Time (h.)

6

4

1 = 0

Copper leached (g/L)

-'- Reference

With Co . . . . . .

E]- Ref. Sterile f " ;,~. .~j~. , -

Co Sterile ~ / ~ "

60 100 160 2 0 0

T ime (h . )

C

2 5 0 2 5 0

Copper leached (g/L}

D . . . . Reference - + - W i t h Hg

6 -El- Ref. Sterile ""

Hg Sterile pc" . . '¢ _ ~

4 A / '

2

O 60 100 150 200

Time (h.)

Fig.5 Evolution of copper concentration in pneumatically stirred reactors tests in the presence of: (A) bismuth, (B) arsenic, (C) cobalt, (D) mercury. Lines A and B represent the mineral addition up to 5% p.d. in the reference and the culture with metal respectively. Mineral addition in innoculated tests and

sterile controls was carried out at the same time for each cation.

956 J .L. MIER e t al.

Copper leached (g/L) 8

.=

i []- Ref. Sterile ~.~

¢ 0 50 100 150 200 250

. . . . Reference

With Ru

Ru Ster i le

Time (h.)

E

300

Copper leached (g/L) 8

4

. . . . Reference

~ - - W i t h M o

~ - Ref. Steri le

M o Steri le

J_ w

U

m

P

P

L 0 50 100 160 200

Time (h.)

F

1

250

Copper leached (g/L) 8

o

Reference

i W i t h Ag

[ ] Ref. Steri le

Ag Ster i le

='

G

50 100 150 200 250

Time (h.)

Fig.5 (continued) Evolution of copper concentration in pneumatically stirled reactors tests in the presence of: (E) ruthenium, (F) molybdenum and (G) silver. Lines A and B represent the mineral addition up to 5% p.d. in the reference and the culture with metal respectively. Mineral addition in innoculated tests and

sterile controls was carried out at the same time for each cation.

b) Arsen ic and cobal t . The results obtained with these metals are similar. With both metals the dissolution rate and final extraction of copper were slightly below those of the reference (Figures 5B and C). The Fe2+/Fe 3+ curves (Figures 6B and C) indicate a lower oxidation rate in the arsenic experiment, since there was a delay in the beginning of the maximum rate of Fe 2+ oxidation, related with a lag period of one day (see copper dissolution curve in Figure 5B). There are numerous studies which mention the toxic effect of arsenic on bacterial growth due to the development of arsenopyrite bioleaching processes [16,26-27]. Among others, Ngubane and Baecker [16] showed that the growth of Sulfolobus BC on pyrite is completely inhibited by the addition of 10 mM arsenate (0.7 g As/L), seven times the quantity added in the present experiment.

Bioleaching of chalcopyrite 957

F e r r o u s / T o t a l Iron

0 i ?'~

0,6 ̧

0,4

0,2

0 0 50

-~- Reference

-q-- With Bi

/,

100 150 200 Time (h.)

A

250

Ferrous/Total Iron 1

0,8

0,6

0,4

0,2

0 0 50

~ ~ ~ Reference

--I-- With As

100 150 200 Time (h.)

B

250

0,8

0,6

0,4

0,2

F e r r o u a / T o t a l Iron

' k ~ - + - Reference

f 1 I With Co

~,AB

- - ~ t i

50 100 150 200 Time (h.)

C

250

0,8

0,8

0.4

0,2

0 0

F e r r o u s / T o t a l Iron

.. . . Reference

, -'~--- With Hg

, B ÷ I I L ~ L . - t - L I

50 100 160 200 260 300 Time (h.)

D

350

Fig.6 Evolution of Fe2+/Fe 3+ ratio in pneumatically stirred reactor tests in the presence of: (A) bismuth, (B) arsenic, (C) cobalt, (D) mercury. Lines A and B represent the mineral

addition up to 5% p.d. in the reference and the culture with metal respectively.

c) Mercury a n d r u t h e n i u m . The presence of these metals partially inhibited bacterial growth with l%p.d., which delayed the addition of concentrate to reach 5% p.d. In this second step, the slow bacterial growth is reflected by lower copper dissolution (Figures 5D and E), particularly so with the ruthenium experiment where only 3.0 g Cu/L was recovered. The curves for the variation in Fe2+/Fe 3+ ratio (Figures 6D and E) show the delay in bacterial activity in the presence of these two metals.

d) Molybdenum and silver. (Figures 5F and G, 6F and G). The effect of these cations on the cultures grown in the columns was total inhibition with 1% p.d. Because of this no extra concentrate was added to reach 5% p.d. Consequently, the oxidation of Fe 2+ was chemical (Figures

958 J.L. MIER et al.

6F and G). These results were in accordance with the extremely toxic effect of these cations on Sulfolobus BC as described by Norris [28], who obtained a moderate inhibition of bacterial growth with 0.1 HM Ag and 1 mM Mo. However these results contrast with the work of Brierley [13] and Brierley and Murr [29] who reported that Sulfolobus can grow in media containing 750 ppm Mo.

Ferrous/Total Iron 1

0.8 ,~,,,, ' ~ ,

0 , 6 "

0,20,4 ',,,,,, ,, l i " , , ',

L I ,

0 60 100

-~- Reference --t-- With Ru

\

1,

150 200 250 Time (h.)

E

300

Ferrous/Total Iron

I

o,81..'!, ' i: i

O, 6

~i,~ ~,

0,4 ,' ", A j

0 50

-~- Reference ~- - With Mo

F \

100 150 200 250 Time (h.)

Ferrous/Total Iron 1

0,8 ~ ~

0 , 6 , ,

0 , 4 ',

0 50 100 160 Time (h.)

- ~- Reference -q-- With Ag

: G i , ,P

= 1 I

200 250

Fig.6 (continued) Evolution of Fe2+/Fe 3+ ratio in pneumatically stirred reactor tests in the presence of: (E) ruthenium, (F) molybdenum and (G) silver. Lines A and B represent the mineral addition up to 5%

p.d. in the reference and the culture with metal respectively.

Bioleaching of chalcopyrite 959

Massive samples

High temperature chalcopyrite.

There was a generalized attack on the cubanite/chalcopyrite matrix in the massive sample bioleached by Sulfolobus BC in the absence of cations. It was more intense on the interstitial (cubanite) than on the laminar phase (chalcopyrite) (Figure 2). The dissolution of this matrix was particularly noticeable at the grain boundaries surrounding the magnetite.

The magnetite remained unaltered due to the existence of a galvanic couple magnetite/matrix, in which magnetite behaved cathodically. This was confirmed by the fact that chalcopyrite has a lower rest potential than magnetite. Table 3 shows rest potentials obtained for the high temperature chalcopyrite, chalcopyrite and magnetite, contrasted with those obtained by Natarajan [30] for chalcopyrite and cubanite/chalcopyrite in a 9K medium at 35°C with Thiobacillusferrooxidans. The presence of cubanite in chalcopyrite minerals lowers the rest potential.

TABLE 3 Rest potentials of the massive mineral samples in the medium described on page 6 at pH 1.5 and 68°C; and (*) according to Natarajan (26), obtained in 9K medium

and in the presence of Thiobacillus ferrooxidans.

MINERAL E R, mV (SHE)

High temperature chalcopyrite +490

Chalcopyrite +510

Magnetite +550

Chalcopyrite--Cubanite +486 to +536 (at pH 2.5) (*)

Chalcopyrite +548 (at pH 2.2) (*)

In the presence of Ag + the chemical attack on the high temperature chalcopyrite in a sterile medium led to Ag2S precipitation on the mineral surface according to the following reaction:

CuFeS 2 + 4 Ag + > 2 Ag2S + Fe 2+ + C u 2+ (1)

Presumably a similar reaction takes place between silver and cubanite, although this has still to be demonstrated.

EDS analysis provided signals for both silver and sulphur, revealing the formation of Ag2S on the whole surface. The massive sample showed no discernable alteration of the cubanite/chalcopyrite matrix, although no grains of magnetite could be found after the attack (Figure 7). The Ag2S layer passivated the mineral matrix in such a way that the electrochemical behavior was inverted and magnetite acted anodically in the reaction medium. Previous studies [4] revealed rest potential values of 550 mV to 600 mV (SHE) when massive samples of Transvaal chalcopyrite were attacked with 0.02 g Ag/L. The same medium was used at 68°C and passivation of the mineral ocurred. This would explain the electrochemical inversion observed in the attack on high temperature chalcopyrite using silver.

The precipitate of Ag2S did not cover the whole matrix when high temperature chalcopyrite was attacked by Sulfolobus BC and Ag + (Figure 8). The redissolution of the precipitate may be due to bacterial activity since bacteria metabolically oxidize Fe 2+ to Fe 3+ and this cation acts as leaching agent on the Ag2S [31]

Ag2S + 2 F e 3+ > 2 A g + + 2 F e 2 + + S O (2)

960 J.L. MIER et al.

Thus, Ag + reacts with chalcopyrite to regenerate Ag2S and Fe 2+. Fe 3+ also attacks chalcopyrite according to the reaction:

CuFeS 2 + 4 Fe 3+ -> Cu 2+ + 5 Fe 2+ + 2 S o (3)

The oxidations of ferrous iron and elemental sulphur to sulphate by Sulfolobus BC displace the equilibrium of reactions [2] and [3] to the right. Experiments in which Ag + is used as catalytic agent in the bioleaching of chalcopyrite by Sulfolobus BC showed an increase of Ag + concentration in solution coinciding with bacterial growth [6].

Fig.7 Sterile test with silver. Micrograph showing cavities which contained magnetite (anodic zone) before attack. Matrix is passivated by Ag2S precipitate (cathodic zone).

Fig.8 Sample of high temperature chalcopyrite leached by Sulfolobus BC and in the presence of silver. Ag2S dissolution can be observed. Matrix zones in contact with medium sustained the attack.

In the sterile experiment with bismuth, high temperature chalcopyrite showed hardly any sign of attack although there was a fine precipitate which made it difficult to see the mineral surface (Figure 9). EDS microanalysis of magnetite showed the presence of bismuth and phosphorus peaks. The precipitate was analyzed by X-ray diffraction. Bismuth phosphates (BiPO 4 and BisPO10 ) were identified. The formation of the latter compounds might be of special importance for the oxidizing conditions of the medium, which are basically determined by the Fe3+/Fe 2+ ratio. According to Hoffmann et al. [32] and Vuorinen and Tuovinen [33], Fe 3+ might form complexes of varying stoichiometry with phosphate in the bioleaching medium. At pH 1.5 phosphate exists mostly as H3PO 4. It has been shown that the oxidizing potential of the Fe3+/Fe 2+ couple decreases in solutions of phosphoric acid due to the formation of Fe 3+ phosphate complexes and the precipitation of FePO 4 [34].

Bioleaching of chalcopyrite 961

Fig.9 High temperature chalcopyrite. Bismuth phosphate precipitate (on the left of the micrograph) in sterile test with bismuth.

The presence of bismuth in the bioleaching medium would prevent the formation of phosphate compounds of Fe 3+ by the precipitation of bismuth phosphate which has a small solubility product (Ks=[Bi3+][PO43- -]=1.3.10-23; [35]. In this way, there would be a higher concentration of free Fe 3+ and, therefore, more effective oxidizing agent. Previous experiments have shown that the oxidizing potential of the medium increased 54 mV when 0.1 g Bi3+/L were added to the abiotic solutions containing 0.1 g Fe3+/L at 68°C and pH 1.5 [6]. Thus, the action mechanisms of Ag + and Bi 3÷ are different. The former attacks the chalcopyrite surface and renders the mineral inactive at sites with Ag2S deposit, favoring the anodic behavior of the uncovered surface, while the latter avoids the formation of Fe 3+ complexes in solution which lower the oxidizing potential of the Fe3+/Fe 2+ couple.

When high temperature chalcopyrite was attacked by Bi 3+ and Sulfolobus BC, the initial precipitate of bismuth was mostly dissolved. Previous work has shown that the concentration of bismuth in solution increases with time in the presence of bacteria [6]. The precipitation of bismuth phosphate does not lead to a decrease in the bacterial activity. The type of interaction between the bacteria and bismuth phosphate should be investigated, since the latter might be a source of phosphate for the microorganisms. A generalized attack was observed on all the matrix and the chalcopyrite/cubanite structure was clearly revealed. There was a substantial deterioration of the mineral as shown by the size and extent of the fissures (Figure 10). The chalcopyrite laminae hardly suffered any alteration and even overlapped the fissures occasionally, while the dissolution of cubanite was more pronounced.

Fig. 10 Sample of high temperature chalcopyrite leached by Sulfolobus BC in the presence of bismuth, showing chalcopyrite lametla in a cracky structure in which cubanite dissolution was preferential.

962 J.L. MIER et al.

Chalcopyrite

A less severe attack was to be expected because the matrix is formed exclusively of chalcopyrite without the multiple and extensive galvanic couples described in the high temperature chalcopyrite experiments. The less important presence of other phases (pyrite, silicates, etc,..) means that the anodic/cathodic surface ratio was very large and the dissolution of the mineral was not favored.

The high resistance of chalcopyrite to chemical and biological attack is shown in the massive sample leached by Sulfolobus BC without cation (Figure 11A), in which the surface was hardly altered and, where there was evidence of attack, it was homogeneous.

A B

C

Fig. 11 Chalcopyrite: (A) Leached by Sulfolobus BC. (B) Sterile test with bismuth. Bismuth phosphate precipitate can be observed on the left of the micrograph.

(C) AgzS precipitate in the sample leached by Sulfolobus BC in the presence of silver.

There was no clear alteration of the chalcopyrite surface in the case of the samples leached in the presence of bismuth. The sterile test resulted in a precipitate all over the chalcopyrite surface (Figure 11B). EDS analysis again showed peaks of bismuth and phosphorus..As in the high temperature chalcopyrite experiments this precipitate practically disappeared in the presence of microorganisms.

Bioleaching of chalcopyrite 963

In the experiment with Sulfolobus BC and Ag+ (Figure l lC), there was a Ag2S precipitate on the

chalcopyrite surface which appeared less compact than in the sterile experiment with Ag +. In this case, the interaction between precipitate and bacteria is not clear because there was not an extended dissolution of Ag2S, although it could be possible that the different morphology of the precipitate is related to the bacterial activity.

CONCLUSIONS

The addition of bismuth resulted in more copper output from copper concentrate than when the experiment was peformed in the absence of the cation. Mercury and ruthenium partially inhibited bacterial activity, while the presence of molybdenum and silver led to total inhibition.

In the experiments with massive samples, the existence of large quantities of cubanite and magnetite in high temperature chalcopyrite changed the electrochemical behavior of the mineral. Compared to this mineral, chalcopyrite was more resistant to bioleaching attack since the other phases represented only a minor part of the whole. In high temperature chalcopyrite, the cubanite/chalcopyrite matrix behaved anodically and was strongly altered by Sulfolobus BC bioleaching. Within the matrix, cubanite was more readily dissolved than chalcopyrite.

These experiments proved that silver and bismuth have different mechanisms of action in the bioleaching of chalcopyrite by Sulfolobus BC. Silver reacts with the chalcopyrite surface to form Ag2S. This deactivates the mineral where it is deposited and promotes the anodic behavior of the uncovered surface. Bismuth, on the other hand, prevents the formation of iron phosphate complexes in solution because of bismuth phosphate precipitation. Thus, the oxidizing potential of the Fe3+/Fe 2+ couple is increased. The dissolution of bismuth phosphate in the inoculated experiments suggests the existence of a possible precipitate-microorganism interaction that should be studied in future experiments.

ACKNOWLEDGMENTS

The authors would like to acknowlegde the financial support of the Comisi6n Interministerial de Ciencia y Tecnologfa (Spain) and the Commission of the European Communities. They would like to thank Dr. Paul R. Norris of Department of Biological Sciences (University of Warwick) for his kind co-operation.

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