Pergamon 0892--6875(98)00122-8

Minerals Engineering, Vol. 12, No. !, pp. 93-106, 1999 © 1998 Elsevier Science Ltd

All rights reserved 0892-6875/99/$ - - see front matter

BIOLEACHING OF A SPANISH COMPLEX SULPHIDE ORE BULK CONCENTRATE

C. GOMEZ, M.L. BL.~,ZQUEZ and A. BALLESTER

Dpto. de Ciencia de Materiales e Ingenierfa Metaldrgica, Facultad de Ciencias Qufmicas, Universidad Complutense, 28040 Madrid, Spain. E-mail: cgcastro@eucmos.sim.ucm.es

(Received 28 November 1997; accepted 10 August 1998)

ABSTRACT

In the present work the applicability of bioleaching using a mixed culture of mesophilic microorganisms (Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirilum ferrooxidans) on a bulk concentrate of a Spanish complex sulphide ore was studied. The bulk concentrate mainly consisted of by chalcopyrite, sphalerite and pyrite. Effects of nutrient medium, stirring, pulp density, temperature and the addition of CO 2 (1% v/v) to the airflow were also studied. The highest leaching rates and recoveries were obtained with mechanically stirred reactors at 5% pulp density and 9K medium. However, by using 9K medium higher jarosite precipitation was observed. Results showed that the optimum temperature for copper bioleaching was 30°C, whereas zinc dissolution increased with a rise in the temperature. © 1998 Elsevier Science Ltd. All rights reserved

Keywords Sulphide ores; bioleaching; extracive metallurgy

INTRODUCTION

A complex sulphide is an association of chalcopyrite, sphalerite and galena disseminated in a pyritic matrix. As well as copper, zinc and lead as valuable metals, such deposits may contain significant quantities of silver, gold, arsenic, antimony, bismuth and mercury. Numerous deposits of these ores exist in the world (Australia, Canada, Portugal, Spain, etc). It is estimated that Spanish complex sulphides contain 70 % of the reserves of these valuable metals belonging to the European Union Countries; the Spanish reserves are approximately 250 million tons, which gives an idea of their economical importance [1].

Complex ores are often characterised by particularly fine intergrowth of the mineral values. For instance, chalcopyrite and sphalerite are frequently intergrown, micro-size grains of 10-20 lma being dispersed within the pyrite. Due to these specific mineralogical characteristics, it is necessary to finely grind and concentrate the ore prior to the solubilisation of the valuable metals [2]. To obtain separate concentrates (of Cu, Zn and Pb) by selective flotation involves high unit-cost, poor quality of the concentrates and relatively low overall recoveries. To obtain bulk concentrates by bulk flotation in which the non-ferrous sulphides float together and are separated from the pyrite, it is more economic if a hydrometallurgical process is applied [3].

One alternative for the treatment of bulk flotation concentrates is bacterial leaching [4]. Microbiological processes use the ability of chemolithotrophic autotrophic bacteria, such as those belonging to the genus

93

94 C. G6mez et al.

Thiobacillus, to oxidise metallic sulphides and elemental sulphur into water-soluble sulphates. Especially remarkable are the species Thiobacillus thiooxidans and Thiobacillusferrooxidans which oxidise inorganic sulphide and other reduced valency sulphur compounds to sulphate and sulphuric acid. Accordingly it can be written:

T.ferrooxidans, T.thiooxidans S ° + H20 + 3•202 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . > H2SO 4 (1)

However, the more important role in the bacterial leaching processes is played by Thiobacillusferrooxidans. These bacteria, apart from the above mentioned oxidation reaction (1), are able to oxidise Fe 2* to Fe 3+. The ferric ion is known to be an effective oxidising agent and it further contributes to the dissolution of sulphides.

In the past, bioleaching on a number of sulphide minerals, such as sphalerite, chalcopyrite, pyrite, molybdenite, etc. have been extensively studied [5,6]. However, little work on bioleaching of complex sulphides has been done [7,8]. Thus, the aim of this paper is to study the applicability of a bioleaching technique to a bulk concentrate of a Spanish complex sulphide, obtaining the optimal conditions for the dissolution of the metal values.

EXPERIMENTAL

Mineral

The bioleaching experiments were carried out with a bulk concentrate of a Spanish complex sulphide ore obtained by continuous flotation in a pilot plant. It was provided by Rio Tinto Minera, S.A. (Huelva, Spain). Its chemical and mineralogical compositions are given in Table 1. Chemical analysis was carried out by Atomic Absorption Spectrometry (AAS), except for arsenic which was determined by a colorimetric method. X-ray diffraction showed sphalerite (ZnS), chalcopyrite (CuFeS2) and pyrite (FeS2) to be the major phases in the bulk concentrate. To determine the mineralogical composition it has been assumed that iron is stochiometrically present in chalcopyrite and pyrite, and in substitutional positions in 5% of the zinc sites of the sphalerite lattice. The granulometric analyses were carded out in a cyclosizer and indicated that the particle size of the complex sulphide concentrate was 80% below 30 pm.

TABLE 1 Chemical and mineralogical composition of the bulk concentrate

Zn Pb Cu Fe

14.0 25.0

As

Chemical composition (%) 17.1 1.7 0.12

Sphalerite Galena Chalcopyrite Pyrite

2.0 40.4 24.3 27.7 Mineral phases (%)

Bacteria

A mixed culture of iron- and sulphur-oxidising microorganisms, whose origin was a Rio Tinto mine water, was used. Characterization of this culture showed that the main bacteria strains were Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans. The culture was originally enriched in shake flasks using a chalcopyrite concentrate (5% wt/vol.) as substrate. The conditions for bacterial growth in a 9K medium modified without iron [9] were as follows: 35°C, 160 rpm and pH 1.8. The

Bioleaching of a Spanish complex sulphide ore 95

bacterial culture were tested by assessing their acid production and microscopic cell counting in a Thoma's chamber. When the culture was in the exponential growth phase (around l0 s bacteria/ml) a sample was taken to inoculate a bioleaching experiment.

Bioleaehing experiments

In order to optimize the bioleaching process, nutrient medium, stirring, pulp density, temperature and CO 2 addition (1% v./v.) to the air flow were studied. When the influence of one of them was studied the rest were fixed at the conditions shown in Table 2.

TABLE 2 Composition of the used nutrient media

Variable Tested conditions Fixed conditions

Nutrient medium

Type of stirring I reactor

Pulp density

C02 addition

Temperature

Tap water, D1, D2 and 9K

medium

Mechanically stirred

reactors and orbital shake.

5, 10, 15 and 20% pulp

density (w/v solid/liquid).

With and without CO 2

addition to the air flow.

20, 30, 35 and 40 *C

160 rpm, 35 *C, 5% pulp

density, CO2 addition (1%

v./v.) to the air flow.

160 rpm, 35 *C, D1

medium, 5% pulp density.

160 rpm, 35"C, D1

medium, CO2 addition (1%

voL/vol.) to the air flow.

160 rpm, 35 *C, D1

medium, 5% pulp density.

160 rpm, 35 *C, D1

medium, 5% pulp density,

COz addition (1% v./v.) to

the air flow.

All experiments were carried out in mechanically stirred reactors with 500 ml hemispherical flasks, except when the influence of the type of reactor was studied. In this case, an orbital shaker with 250 ml shake flasks was used. Before inoculation, the concentrate was added to the nutrient medium and pH was adjusted to 1.8 by H2SO 4 addition. The compositions of the nutrient media are given in Table 3. After a leaching period of 2 hours, 5 ml of bacterial culture per 100 ml of solution were added to the reactor. Additionally, in order to compare the effect of the nutrient medium on metal solubilisations, a control test (sterile) was performed. In this case a solution of thymol (2 % wt) was added in order to kill the microorganisms.

Periodically, when the pH was higher than 1.8, it was adjusted to this value by acid addition. However, if the pH was lower than 1.8, a free evolution of this variable was allowed. Water losses by evaporation were compensated by the addition of distilled water. Each experiment was monitored, following the copper, zinc and total iron concentrations by AAS. Upon termination of the experiments, the leached residues were

96 C. G6mez e t al.

washed with sulphuric acid in water at pH 1.8, and analysed by X- ray diffraction in order to identify the products resulting during bioleaching.

TABLE 3 Tested variables for the bioleaching optimisation

Nutrient salt

(NH4)2S04

MgSO4"7H20

K2HP04

KCI

Ca(NO3)2-H20

9k medium

(g/L)

3.0

0.5

0.5

0.1

0.01

Norris' medium D1 D2

(g/L)[10] (g/L) (g/L)

0.2 0.06 0.01

0.2 0.06 0.01

0.2 0.02 0.01

0.02 0.01

RESULTS AND DISCUSSION

Influence of nutrient medium

In Figure 1 the influence of nutrient medium on the leaching of copper (Figure la), zinc (Figure lb) and iron (Figure lc) is shown. Five different media, 9K, Norris, D1, D2 and tap water were tested (Table 3). None of these media contained Fe 2+, thus preventing the precipitation of j arosites [ 10-12] and ensuring that the microorganisms obtained their energy from the sulphide minerals. As can be observed, the nutrient medium strongly influenced both the leaching rate and the copper, zinc and iron extractions. The following order can be established for the leaching rate (V) : V9K > Vnorris > VDI > VD2> V tap water" For "sterile" tests the metal recoveries remained practically zero at all times. These data have been omitted in the Figures for the sake of clarity.

It is noteworthy that our results are in agreement with the currently accepted electrochemical mechanism of explaining the bioleaching of metal sulphides. According to this, the following order of dissolution from the concentrate could be expected [13]: sphalerite>chalcopyrite>pyrite. Sphalerite is the most electrochemically active, while the chalcopyrite will behave anodically with respect to sphalerite and cathodicaily with respect to sphalerite. In all tested media (Figure 1), the above mentioned sequence is observed; i.e., after the 10th day, in the 9K medium, about 80 % Zn, 30 % Cu and 20 % Fe are dissolved.

Bioleaching of a Spanish complex sulphide ore 97

40 -] - - O - - g K m e d i u m _ , . , 1 ~ , 1 oo O_-O O_o. '"1 ... 20 J -a--v2 m.ium / t X l X ~ ~ I l - ° - = " r / / x I

10" ~ ~ J I Q ~ l

80 | I

0

50

40

~ 30

~ 2o

(d)

2.o 1.5

0 2 4 6 8 10 12 14 16 18 20 22

Time (days)

Fig. 1 Influence of nutrient medium in the leaching of copper (a), zinc (b) and iron (c) and evolution of pH (d).

The decrease in the leaching rate in the more diluted media (D 2 and tap water) is a consequence of a lower bacterial activity, as can be followed from the iron evolution (Figure lc). As is well known, a minimum concentration of basal salts is necessary to obtain a good bacterial activity and metals solubilization. As can be seen in Figure 1, the optimum medium of those investigated for the growth of microorganisms is the 9K medium. However, its main drawback is the high concentration of sulphate, ammonium and potasium ions, which produce the precipitation ofjarosites [12], which have been detected by X-ray analysis of the residues in the 9K medium in this work. No traces of jarosite were observed in other media.

A better bacterial activity in the 9K medium, compared with the other media, can be seen in Figure ld, which is seen in the pH evolution. In all cases, an initial increase in pH, attibuted to the chemical dissolution of the concentrate, was observed between 0 and 3 days. However, after the eighth day in the 9K medium, a strong decrease in the pH of the solution was observed. This decrease in pH is not produced in the other media, in which the pH remained almost constant between 2.0 and 2.3. Two reasons can

98 C. G6mez et al.

account for this decrease in pH, a great bacterial growth (Eq. 1), detected by microscopic observations, and a precipitation of jarosites according to:

3Fe 3+ + X + + 2HSO4- + 6H20 ~ XFe3(SO4)2(OH) 6 + 8H ÷ (2)

where X= K +, Na ÷, NH4 ÷ and H3 O÷.

Influence of the type of reactor/stirring

To study the effect of stirring two types of reactors were used: mechanically stirred reactors and shake flasks. In Figure 2, copper (Figure 2a), zinc (Figure 2b) and iron (Figure 2c) dissolution for both reactors can be observed. The highest rates and yields were obtained with mechanically stirred reactors.

40

30

20

0 10

0

80

60

40

2O

0 50

4O

~ 30

,? 20

10

0

- -A-- Mechanical stirring ~ A . . . . . . A _ . . . _ A ~ A - -D-- Shake flasksA..~..A

(a)

d .-----El

~ ~ , I , t , I , t . I . I . I . I ,

A - - - ' - - - A " ~ ' ' A A (b)

o/ - ~~._~_~/o ~ °

J=.._,_~~,. ,---:. ~ , . , . , . , . , . , . , 0 2 4 6 8 10 12 14 16 18 20 22

Time (days)

Fig.2 Influence of type of reactor/stirring on the leaching of copper Cu (a), zinc (b) and iron (c). Sterile recoveries were always lower than 1% for all metals.

A kinetic study was carried out in order to determine the rate-controlling step of the bioleaching reaction. Thus. the obtained data were fitted according to the shrinking core product layer model for heterogeneous

Bioleaching of a Spanish complex sulphide ore 99

reactions [14]. As already mentioned, during bioleaching both sulphur (produced in the oxidation reaction of mineral sulphides) and jarosites (equation 2) were formed as two insoluble products, which form a product layer over the mineral sulphides. In such conditions, if the leaching rate was controlled by diffusion through these products, the following equation rate would apply [14]:

2

1 - 2 R - ( l - R ) 3'-- Kpt (3) 3

where R=fraction reacted, t=time (day) and Kp=parabolic rate constant (day-l).

However, if the chemical reaction was the controlling step of the reaction the following rate equation would apply [15] :

1

1- (1- R) 3 : kt (4)

Figure 3 shows that copper and zinc leaching follow a mathematical law such as (4) for data in the bacterial exponential growth. That is to say, the chemical reaction is the controlling step of the reaction and not the diffusion through the layer. The K values for copper and zinc calculated from Figure 3 are shown in Table 4. As can be seen, the rate of zinc dissolution is faster than the rate of copper dissolution and better kinetics were obtained in mechanically stirred reactors.

0.25

fie I

%.,...

" v

0.20 I ' ~ ~// zx Mechanical stirring

0.15

0.10

0.05

0.00

0.10

0.08

0.06

0.04

0.02

0.00

1 . -

n," I

"7"

I = I i I t I i I

/ I = I = I = I I I

100 150 200 250 300

Time (h)

Fig.3 Plots of 1 - ( l -R) v3 versus bioleaching time for copper (a) and Zn (b) of the bulk concentrate.

100

TABLE 4

C. G6mez et al.

Values of the kinetic constants (h -1) for the copper and zinc bioleaching for shake flasks and mechanically stirred reactor

Zn

Cu

Shake flasks

11.7 " 10 "4

2.9" 10 "4

Mechanically stirred reactors

26.9- 104

4.2 "10 "4

Influence of CO z addition

It is well known that autotrophic microorganisms, such as Thiobacillusferrooxidans need a minimum CO 2 concentration [16], since they obtain their cellular carbon from this compound. Figure 4 shows the influence of the CO 2 addition (1% v/v) on the air flow in the leaching of copper (Figure 4a), zinc (Figure 4b) and iron (Figure 4c) in mechanically stirred reactors. It can be seen that the leaching rates and extractions of these metals were similar, irrespective of the CO 2 addition. These results are in agreement with those of Bruynesteyn who affirmed that the carbon content in the atmosphere can be enough to cover the nutritional requirements of microorganisms [17].

30. ---O-W~out CO= (a) --El-- W'dll CO 2 _ j O ~ ( : ~ ^ m

,..., 2 0 ' - -X - - Sterile (with COz) / ~ . . . > ' ~

0 Ill . ~ X , , . ~::~--:l<-TN . - - ;~ r -~<7- .x , ¥-T--:-.X, I

so (b) j -Y° g 4o

20

0 . r ' ~ ; - - T - ; - - ~ < : ~ - - ~ : - - : - : ~ . - - : - - r - - , x - r -¥ , ¥ - r - r X , .

15 . . . . ~ O o j . . ~ O (C)

, - . 10 i ~ ~ ~ ~ . , . x 6

- r - .X , ,xT--;- ,x,

0 2 4 6 8 10 12 14 16 18 20 22

Time (days)

Fig.4 Influence of the CO 2 addition on the copper (a), zinc (b) and iron (c) bioleaching.

Bioleaching of a Spanish complex sulphide ore 101

Influence o f pulp density

The influence of pulp density, ranging from 5 to 20%, is shown in Figure 5. A significant decrease in both bioleaching rate and extractions of copper (Figure 5a), zinc (Figure 5b) and iron (Figure 5c) were observed when the pulp density was increased, being maximum for 5% pulp density. The decrease in the percentage of metal dissolution when the pulp density is increased would be due to a lower availability of oxygen and CO2, which are essential nutrients for ThiobaciUus ferrooxidans growth. Our results agree with those observed by Chaudhury and coworkers on Cu-Pb-Zn complex sulphides [18] and zinc concentrates [19].

3O • - o - s ~ p . d . ( a )

25 - - o - 10% p.d. j . . . C 3 ~ E 3 [ ] • - - A - - 15% p.d. J 3 " " -

2 0 - - - 0 - - 20% p.d.

15 " - x - Sterile (5% p.d.) . . ~

,-, - / : . . 7 . , ~ °~° (b) / u = _ O _ . . . 9 1 ~ . _ _ _ A _ _ A

• / . . . .O ~ ^ . - - - - ~ " - ' =

0- ~ ,s,=~IC--~-T-T- .x-T-xTx~-. >p-:-~.

'°i I 40 ~9.~-~- ~- -A 2 0

0 - ~ ~ ~

1o. I 5 " ~ [ : f . . . - O ~ . . . . - A - - A - - - - - A - - A I

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (days)

Fig.5 Influence of pulp density on the copper (a), zinc (b) and iron (c) bioleaching.

In order to relate the pulp density to the metal extraction, which directly depends on the bacterial concentration, a kinetic study has been performed. According to the model proposed by Michaelis-Menten [20] the dissolution rate is given by the following equation:

102 C. G6mez e t al.

V . S V -- m ( 6 )

(Ks+ S)

where V is the extraction rate of metals, V m is the maximum metal extraction rate, S is the pulp density and K s is the Michaelis' constant. This constant gives an idea about the affinity of the bacteria to the mineral substrate. In Figure 6 plots of 1/V versus I/S for copper and zinc dissolutions are shown. From these representations the kinetic parameters V m and KS for copper and zinc dissolutions can be obtained. Their values are summarised in Table 5. These results confirmed a better zinc than copper dissolution from the bulk concentrate, because Vm)zn is four times higher than Vm)cu. The higher value of KS)Cu than Ks)zn might indicate a preference of the microorganisms for the copper from the concentrate.

400 0 Cu

350

300 =

~-~ 250-

"' 200-

150- , , ¢ - -

100 -

50-

O - ' ' , I , I , I ,

0.000 0.005 0.010 0.015 0.020 0.025

1/S (g.L) 1

Fig.6 Plot of Michaelis-Menten for the copper and zinc dissolutions.

TABLE 5 Values of maximum rate of dissolution, Vm, and kinetic constant, Ks, for copper and zinc bioleaching

Vm (mg'L'l"h "1) Ks (g.L "1)

Zn 29.7 36.7

Cu 7.3 86.9

Bioleaching of a Spanish complex sulphide ore 103

Influence of temperature

Figure 7 shows the influence of temperature on the rate of copper (Figure 7a), zinc (Figure 7b) and iron (Figure 7c) dissolution. A different behaviour in copper and zinc dissolution can be observed. The rate of zinc leaching increased with a rise in temperature, whereas the copper leaching rate reached its optimum at 30°C and strongly decreased at 20°C and 40°C. These results could be justified on the basis of two different mechanisms for copper and zinc dissolutions. Copper could be mainly dissolved by a microbial mechanism, whose maximum activity is reached at temperatures around 30°C, as can be observed in Figure 7c. However, the behaviour observed for zinc could be associated to a chemical dissolution mechanism, obtaining better extractions when the temperature is increased.

30 q o o (a) / O - - - - O

1 ~ o - O " ^___._..~--zx----A I

1 I .0 ¢~10q /,//'~ ~ 6_o~o_o o ~ O / O , 0 o ~

80

60

40

20 [] 0 - - - - - - - -0

0. -r

] (c) I 15 ] ~o -o'°" ./~-'~- I

"- - - ° I 0 . ~ x - - x - - x ~

0 2 4 6 8 10 12 14 .16 18 20 22

Time (days)

Fig.7 Influence of the temperature on the copper(a), zinc (b), iron (c) bioleaching.

The Arrenhius plot is a very useful tool for elucidating the mechanism involved and explicitly expressing the kinetics of a given process [21]. An attempt has been made to calculate the activation energy for zinc and copper dissolutions using the Arrhenius equation:

K-- A ' e -E (7) "RT

where K is the rate constant, A is the frequency factor, T is the absolute temperature, E a is the activation energy and R is the gas constant.

104 C. G6mez et al.

The rate constant, K, was determined assuming that the kinetics of dissolution were controlled by the chemical reaction. As can be observed, in the Arrhenius plot (Figure 8), In(K) for the zinc dissolution shows a linear dependence with l/T, while this dependence is parabolic for the copper dissolution. These results suggest greater importance of the microbial processes in copper bioleaching. However, in the zinc dissolution a chemical leaching of ferric ion produced could be presumed. From Figure 8 an activation energy of 110 KJmol -~ has been obtained for the zinc dissolution.

-6.0

a Cu -6.5. t, Zn

-7.0.

-7.5

~ -6,0 • J ' ' - ' - - " ~ - C .-I

- 8 .5

-g.0-

- 1 0 . 0 I I i I ,

3 . 1 5 x 1 0 °3 3 . 2 5 x 1 0 .3 3 . 3 5 x 1 0 .3 3 . 4 5 x 1 0 .3

I/T(K -I)

Fig.8 Arrhnenius plot to measure activation energy for copper and zinc leaching. K is given in h-L

CONCLUSIONS

On the basis of the preceeding results the following conclusions can be made:

1. The best leaching rates and recoveries were obtained with 9K medium. However, its use produced much jarosite precipitation.

2. Copper and zinc dissolutions conform to a shrinkage core-chemical controlled reaction. Faster kinetics were obtained in mechanically stirred reactors.

3. Copper and zinc dissolutions fit the kinetic model of Michaelis-Menten in the range pulp density 5 to 20%, the optimum being was 5%.

4. The optimum temperature for copper bioleaching was 30°C, while zinc bioleaching leaching rate increases with a rise in temperature.

5. Kinetic studies suggest that copper dissolution ocurred by a microbial mechanism. However the zinc dissolution could be associated to a chemical reaction.

Bioleaching of a Spanish complex sulphide ore 105

ACKNOWLEDGEMENTS

The authors wish to express sincere gratitude to the Comision Interministerial de Ciencia y Tecnologfa (Spain) and to the Commission of the European Communities for funding this research through projects PBT87-0032-C0201 and MA2M-CT90-0028 (SMA), respectively.

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2.

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4.

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6.

7.

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11.

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15. 16.

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18.

19.

20. 21.

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Carranza, F., Las piritas espafiolas: un recurso natural poco explotado. Qufmicos del Sur. 4. Julio, 13 (1985). Barbery, G., Fletcher, A.W., Chem, C. and Sirois, L.L., Exploitation of complex sulphide deposist: a review of processing options from ore to metal. In: Conference. The Institution of Mining and Metallurgy. Roma : 135 (1980). Ortega, A. and Bonilla, A., Flotaci6n de sulfuros complejos de matriz piritica. Estudio de posibilidades de tratamiento de sus concentrados. In: Anales del III Congreso Nacional de Metal~r- gia. Santiago de Chile, I l l B: 280 (1983). Carta, M, Ghiani, M. and Rossi, G., Beneficiation of a Complex Sulphide Ore by an Integrated Process of Flotation and Bioleaching. In: Procc. Complex Sulphide Ore Conference. Rome. Institution of Mining and Metallurgy. London, 178 (1980). Rossi, G., Biohydrometallurgy. 494 McGraw-Hill (1990). Groudev, S.N., Oxidation of zinc sulphides by Thiobacillus ferrooxidans and Thiobacillus thiooxidans. Comptes Rendus de l' Acad~mie Bulgare de Sciencies, 36, 1, 103 (1983). Chaudhury, G.R., Jena, K.N. and Das, R.P., Bacterial leaching of Ambamata complex sulphide ore. Proc. Indian Natn. Sci. Acad., 50A, 5, 410 (1984). Frenay, J., La lixiviation bact6rienne: outil pour la valorisation des minerais complexes ou r6fractaires. Industrie Minerale. Mines et Carri~res. Les Tecniques, 71. Abril: 49-55 (1989). Silverman, M.P. and Lundgren, D.C., Studies on the chemoautotrophic iron bacterium Ferrooxidans. An improved medium and a harvesting procedure for securing high cell yields. J. Bacteriol., 77, 642 (1959). Lazaroff, W., Sigal, W. and Wasserman, A., Iron oxidation and precipitation of ferric hydroxysul- fates by resting Thiobacillus ferrooxidans cells. Appl. and Environmental Microbiology, Apr. 924 (1982). Ivarson, K.C., Ross G.J., and Miles, N.M., The microbiological formation of basic ferric sulphates II. Crystallization in presence of potassium, ammonium, and sodium salts. Soil Sci. Soc. Am. J., 43, 908 (1979). Grishin, S.I., Bigham, J.M. and Tuovinen, O.H., Characterization ofjarosite formed upon bacterial. Oxidation of ferrous sulfate in a packed-bed reactor. Applied and Environmetal Microbiology, Dec., 3101 (1989). Natarajan, K.A., Electrobioleaching of base metal sulfides. Metallurgical Transactions B, 23B, 5 (1992). Sohn, B.Y. and Wadsworth, M.E., Rate Processes of Extractive Metallurgy. p.141 Plenum Press, Nueva York, (1979). Habashi, F., Principles of Extractive Metallurgy. p. 111.Gordon and Breach, New York, (1969). Ingledew, W.J., Thiobacillus ferrooxidans. The bioenergetics of an acidophilic chemolithotroph, Biochim. Biophys. Acta, 683, 89 (1982). Bruynesteyn, A., The biological aspects of heap and in-place leaching of Uranium ores. p. 59. Sixth Annual uranium seminar. SME-AIME. New York (1983). Chaudhury, G.R. and Das, R.P., Bacterial Leaching--Complex Sulphides of Copper, Lead and Zinc. International Journal of Mineral Processing, 21, 57 (1987). Chaudhury, G.R., Sukla, L.B. and Das, R.P., Kinetics of Bio-Chemical Leaching of Sphalerite Concentrate. Metallurgical Transactions B, 1611, 667 (1985). Diaz, M. and Roig, A., p.1159. Qu(mica Fisica vol.2. Alhambra, Madrid (1980). Rossi, G., Biohydrometallurgy. p. 192, McGraw-Hill (1990).

106 C. G6mez e t al.

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