Chemical and electrochemical basis of bioleaching processes

13

Chemical and electrochemical basis of bioleaching processes G. S.Hansforda and T. Vargasb aGold Fields Mineral Bioprocessing Laboratory, Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa, E-mail: [email protected] bDepartment of Chemical and Metallurgical Engineering, University of Chile, P.O.Box 2777, Santiago, Chile, E-mail: [email protected] The bioleaching of sulfide minerals involves electrochemical and chemical reactions of the mineral with the leach liquor and the extra-cellular polysaccharide layers on the microorganisms. The microorganisms derive energy by oxidising the sulfur moiety and ferrous iron, which can be interpreted using electrochemistry and chemiosmotic theory. Recently significant advances have been made in understanding the mechanism by which the bioleaching of sulfide minerals occurs. Kinetic models based on the proposed mechanism are being used successfully to predict the performance of continuous bioleach reactors. The measurement of oxygen and carbon dioxide consumption rates together with the measurement of redox potentials, has led to this further elucidation of the mechanism of bioleaching of sulfide minerals and enabled the kinetics of the sub-processes involved to be determined separately. It has been shown that bioleaching involves at least three important sub-processes, viz., attack of the sulfide mineral, microbial oxidation of ferrous iron and some sulfur moiety. The overall process occurs via one of two pathways depending on the nature of the sulfide mineral, a pathway via thiosulfate resulting in sulfate being formed or a polythionate pathway resulting in the formation of elemental sulfur. For the ease of pyrite, the primary attack of the sulfide mineral is a chemical ferric leach producing ferrous iron. The role of the bacteria is to re-oxidise the ferrous iron back to the ferric form and maintain a high redox potential as well as oxidising the elemental sulfur that is formed in some cases. The first two sub-processes of chemical ferric reaction with the mineral and bacterial oxidation of the ferrous iron are linked by the redox potential. The sub-processes are in equilibrium when the rate of iron turnover between the mineral and the bacteria is balanced. Rate equations based on redox potential or ferric/ferrous-iron ratio have been used to describe the kinetics of these sub-processes. The kinetics have been described as functions of the ferric/ferrous-iron ratio or redox potential which enables the interactions of the two sub-processes to be linked at a particular redox potential through the rate of ferrous iron turn-over. The use of these models in predicting bioleach behaviour for pyrite presented and discussed. The model is able to predict which bacterial species will predominate at a particular redox potential in the presence of a particular mineral, and which mineral will be preferentially leached. The leach rate and steady state redox potential can be predicted from the bacterial to mineral ratio. The implications of this model on bioleach reactor design and operation are discussed. Research on the chemistry and electrochemistry of the ferric leaching of sulfide minerals and an electrochemical mechanism for ferrous iron oxidation based on chemiosmotic theory will be presented and reviewed.

14 1. INTRODUCTION The biooxidation of sulfide minerals is now an established industrial technology for the pretreatment of refractory, arsenical gold ores and for the bioleaching of copper with processes for the bioleaching of cobalt and nickel under development. There is also a considerable industrial interest in the use of moderate thermophiles and thermophiles for biooxidation and indeed these microorganisms occur in the hotter regions of bioleach heaps. The development and optimization of these processes requires an understanding of the mechanism and kinetics of the microbial attack of sulfide minerals. Although the biooxidation of mineral sulfides has been investigated for many years, there is still not a generally accepted mechanism and the kinetics are not yet defined in terms of rate equations which can be used to predict the performance of the bioreactors used for biooxidation. It is the purpose of this paper to review recent research that is leading to an improved understanding of the chemical, electrochemical and biological basis and kinetics of biooxidation of sulfide minerals. 2. MECHANISMS OF BIOLEACHING There is growing agreement that the biooxidation of sulfide minerals involves a primary acidic or oxidative ferric reaction with the mineral producing as products ferrous iron and some sulfur compound, S(?) MS x

4Fe 2+ +

+

Fe3+

0 2 +

s(?) +o2

4H +

chemical

>

~~

~o~., > sol-

M x+ +

Fe 2+ + S(?)

> 4Fe ~ + 2H20

(1)

(2) (3)

It is this ferrous iron and sulfur compounds, S(?), that form the substrates for microbial growth. The ferric-ferrous iron turnover may very well take place within the extracellular polysaccharide (EPS) layer excreted by the microorganisms attached to the mineral surface. The primary sulfur product formed depends on which sulfide mineral is being leached and is subsequently either chemically or biologically transformed to either elemental sulfur or sulfate. There is also evidence that this reaction of the sulfur product might also take place in the EPS layer. The mechanism proposed above is supported by the findings of Sand et al.[1] who described the bioleaching of pyrite to involve a chemical leaching step involving a reaction of pyrite with ferric hexahydrate to form ferrous hexahydrate and thiosulfate. This reacts further via tetrathionate, sulfane monosulfonic acid and trithionate in a cycle to produce sulfate and small amounts of elemental sulfur. The ferrous iron is re-oxidized to the ferric form by Thiobacillusferrooxidans or Leptospirillum ferrooxidans. This mechanism applies to pyrite, FeS2, molybdenite, MoS2, and wolframite, WS2. This is because of the electronic structure of pyrite and the other two sulfides [2]. The ferric species reacts with the iron in the pyrite leading to dissolution of ferrous iron while water reacts with the sulfur to produce thiosulfate as the primary dissolved sulfur species. This is further oxidized by ferric iron to produce sulfate. Lowson et al.,[3] using lSO-labelled 02 and water showed that the oxygens from

15 water are incorporated in the sulfate while labelled 02 in air was found in H20. Schippers and Sand [4] have named this the "thiosulfate" mechanism and have shown that it applies to both chemical and bioleaching of pyrite. For the ease of pyrite the thiosulfate probably reacts rapidly with ferric iron particularly at high redox potentials through to sulfate and is unlikely to be available as a substrate for microorganisms. For all other sulfide minerals which are acid-soluble to some extent, the reaction with ferric iron yields elemental sulfur [4]. This proceeds by the formation of intermediary polysulfides [5]. This is supported by the findings of Hackl et al.,[6], who detected polysulfides on the surface of chalcopyrite. Schippers and Sand [4] have presented these two mechanisms in the following simplified equations: The thiosulfate mechanism (FeS2, MoS2, WS2): $2O2- + 8Fe 3+ +

5H20

-->

2SO24-

+ 8 F e 2+ +

10H

(4)

In this mechanism Schippers et al.,[7] have reported the formation of a small amount of sulfur in their chemical and biological leaching experiments on pyrite. The polysulfide mechanism (ZnS, CuFeS2, FeAsS etc) MS + Fe 3+ + H §

0 . 5 H 2 S . + F e 3+

__>

M 2+ + 0 . 5 H 2 S ~ + F e 2+

--> 0.125S~ + Fe 2+ + H +

0.125S~+1.502+H20

--> SO4 - + 2 H §

(5) (6) (7)

These are shown in Figure 1 using the diagram presented by Schippers and Sand [4]. Then both mechanisms, the thiosulfate and the polysulfide can produce elemental sulfur. But it is interesting to note that according to the thiosulfate mechanism any sulfur formed on the sulfide surface (as has been observed in the bioleaching of pyrite for instance) is in fact explained only as precipitated from solution. Morphology of precipitated sulfur should be different from that of residual sulfur left after metal ions leaching from sulfide. In leaching chalcopyrite, Dutrizac [8] observed the formation of sulfur globules, which could only be explained as formed by precipitation Therefore sulfide minerals are either attacked by ferric ions or, ferric ions and protons, and the role of the microorganisms is to regenerate the ferric iron and maintain a sufficiently high redox potential for the reaction to proceed and to oxidize the sulfur product and maintain a low pH. In other words their role is to supply ferric iron and protons consumed by the leach reactions. These are most probably carried out in the extracellular polysaccharide layer surrounding the microorganisms attached to the mineral surface [9]. Crundwell [10] has also shown the growth of a monolayer of Thiobacillus ferrooxidans on pyrite within a such a biofilm composed of extracellular polysaccharide.

16

Figure 1 : Sand & Schippers [4] Thiosulfate and polysulfide mechanisms for bacterial leaching of sulphide minerals These mechanisms provide the explanation for many observations reported. The fact that the purely ferrous iron oxidizer, Leptospirillum ferrooxidans has been found to predominate in the bioleaching of pyrite [11, 12, 13, 14]. The finding of Rawlings [15] using 16S RNA that in continuous reactors, bioleaching a pyrite arsenopyrite concentrate, Leptospirillum ferrooxidans a purely iron-oxidizer and Thiobacillus caldus a sulfur-oxidizer were found and no Thiobacillusferrooxidans was detected. Rojas et al.[ 16, 17] have observed colloidal sulfur in the extraceUular polysaccharide of Thiobacillus ferrooxidans grown on pyrite which they claimed to contain particles of colloidal sulfur. The black particles of precipitated silver sulfide observed are believed to be formed by the reaction of the silver chloride with polythionates formed in the thiosulfate mechanism in the extracellular polysaccharide. In developing a kinetic model for the bioleaching of sulfide minerals, rate equations for the individual sub-processes must be derived. These require an understanding of the mechanisms. From the above, it can be seen that the mineralogy of the sulfide and its structure are important in determining which of the chemical leach mechanisms will apply. A chemiosmotic mechanism for the oxidation of ferrous iron by Thiobacillus ferrooxidans was proposed by Ingledew [18] and it is reasonable to believe that a similar mechanism will apply to all mesophilic and thermophilic ferrous iron oxidizers which operate at low pH. Many researchers have chosen to use various forms of the Michaelis-Menten equation with ferrous substrate, ferric product and biomass inhibition terms [19]. Huberts [20] has derived rate equations based on Ingledew's chemiosmotic theory and this work has been extended by Crundwell et al., [21 ]. Boon et al.,[ 13], Hansford [22] and van Scherpenzeel et al., [23] have simplified the product inhibition proposed by Jones and Kelly [24] to include dependence on the ferric/ferrous-iron ratio. They suggest that the ferric/ferrous iron ratio or redox potential is the dominant factor in governing ferrous iron oxidation kinetics in agreement with the electrochemical mechanism proposed by Crundwell et al.[21]. Although there is work published on the oxidation of elemental sulfur and thiosulphate and tetrathionate by Thiobacillus thiooxidans, there is not a well established rate equation.

17 3. A T W O S U B - P R O C E S S M E C H A N I S M F O R T H E B I O L E A C H I N G O F P Y R I T E

Using off-gas analysis of carbon dioxide and oxygen to measure the rate of bacterial growth and pyrite oxidation in fed-batch cultures of Leptospirillum-like bacteria Boon et al., [13] obtained evidence for a two stage mechanism of pyrite bioleaching in which the primary attack of the pyrite was a chemical reaction by ferric iron and the role of the bacteria was to oxidised the ferrous iron produced back to the ferric form thus maintaining a high redox potential. These concepts have been extended to show that the two sub-processes are linked by the ferric-ferrous iron turnover between the mineral and the bacteria and the system attains pseudo-steady state when these are equal at a particular mineral surface area and bacterial concentration [22]. In this mechanism the two sub-processes of bacterial ferrous iron oxidation and chemical ferric leaching of the sulfide mineral are linked at pseudo steady state by equating the rate of ferrous iron production from the chemical ferric leach reaction to the rate of consumption of ferrous iron consumption by the bacteria [22]. In order to do this the rate equations for the two sub-processes, ferrous iron production from the chemical ferric leach step and ferrous iron oxidation to the ferric form by the microorgansims as: +w+

=

- rw+

=

++~

(8)

[Fe z+] I+B [Fe3+]

a[FeS2] and: - rF~,+ Cx

qFe2+

lnllx

qFo,+ I+K [Fe3+] [Fe 2§]

(9)

where the quantity ~ is introduced to base the kinetics of the surface area of the sulfide mineral. In this way the concentration, size and surface roughness of the mineral can be taken into account. So that at a particular pyrite and bacterial concentration the pseudo steady state will be defined by: ~v~2+a [FeS 2]

=

rFo2+oh~ =

[Ve2+] I+B [Fe3 +]

-- rFo,+~

=

qvo~+ c x [Fe 3+] I+K [FEZ+]

(10)

where it is possible to express the ferric/ferrous-iron ratio in terms of the redox potential using the Nernst equation as:

[Fe 3+] [Fe 2+]

-

exp Eh-E~ RT

(11)

18 The rate of ferrous iron production from the ferric leach step and the rates of ferrous iron consumption by bacterial oxidation by Thiobacillus ferrooxidans and Leptospirillum ferrooxidans are shown as functions of redox potential in Figure 2. The point of intersection of the chemical and bacterial curves, defines the pseudo-steady state redox potential and the rate of ferrous iron turn-over, which can be related stoichiometrically to the rate of pyrite bioleaching. The intersection point depends on both the concentration of bacteria and active surface area concentration of the pyrite. In this way the model presented here can be related to those which are based on a bacteria-to-mineral ratio, Cx/[FeS2 ]. As the bacterial concentration and/or the surface area concentration change the redox potential and overall pyrite bioleaching rate will change accordingly. The point of intersection of the curves represents the pseudo-steady state giving the rate of ferrous iron turn-over and the redox potential. It can be seen that for the bioleaching of pyrite that the ferric leach curve intersects the bacterial ferrous oxidation curve of Leptospirillumferrooxidans at a higher rate of ferrous turnover than Thiobacillusferrooxidans and therefore be Leptospirillumferrooxidans will be dominant species. This has been confirmed by Rawlings [15] who has found that Leptospirillum ferrooxidzms predominates in the bioreactors of the Gencor BIOX ~ process treating an arsenopyrite-pyrite concentrate. Similar observations have been reported recently by Battaglia-Brunet et a1.,[25].

350 300

,..-w ,.~ 250

,

Thiobacillus ferroxidans

,

+-

200

g +

llll

9

150 . . . . . . . . . . . . . .

~ 100

ii I

50

600

,

,

650

700

,:,, 750

800

850

..... 900

950

I000

Eh [mV(SHE)] Figure 2 : Rate of ferrous iron production by ferric leaching of +53-75 lxm pyrite at 10g/l together with the rate of ferrous iron oxidation by Thiobacillusferrooxidans and Leptospirillumferrooxidans at 150 mg C/I and total iron concentration of 12g/l as functions of redox potential

19 According to this two step mechanism for sulfide mineral bioleaching it is possible to determine the kinetics of the chemical and bacterial sub-processes independently and then use the kinetic constants so derived to predict both the steady state and dynamic performance of bioleaeh systems. Recent work on the purely chemical ferric leaching of pyrite [26] has shown that the values for ~Fe2+m~x and B agree with those obtained from the data for the bioleaehing of pyrite [13]. The dependence of the bacterial kinetics on redox potential is consistent with the chemiosmotic theory [18], while the dependence of the ferric leach kinetics on redox potential is in accord with electrochemical theory. The measured ferric leach rate data do not level off as predicted by Equation 8 but appear to increase with increasing redox potential [26]. This is what would be expected if ferric leaching is assumed to be an electrochemical process predicted by the Butler-Volmer equation:

i

io

oxpl

Where the corrosion current can be related to the leach rate of the sulphide mineral as shown that an equation of the form [26]: -r~es2

=

ro(exp(oq3(E-Eo)-exp((1-~)13(E-Eo)))

(13)

Relating pyrite leach rate with redox potential gives a reasonable fit to experimentally measured ferric leach rates of pyrite, Figure 3, and that the redox potential and rate for pyrite bioleaching agrees with their chemical leach results, Figure 4, [26]. However the chemical ferric leach data reported in the literature were determined at lower redox potentials than occur in bioleaching. This is probably the reason why the incorrect conclusion was drawn that pyrite bioleaching occurs via a direct mechanism [27]. Preliminary work suggests that the ferric leach kinetics of arsenopyrite can also be described by a rate equation of this form [28]. This two step mechanism appears to able to explain why Leptospirillumferrooxidans is the dominant bacterial species in the bioleaching of pyrite and arsenopyrite [11, 15] and why arsenopyrite is preferentially bioleached ahead of pyrite [29]. This hypothesis and approach, while yet to be extensively tested and rigorously proved shows great potential of becoming a very useful way of modelling the kinetics of bioleaching with engineering and industrial applications. The hypothesis is consistent with the findings for the chemical ferric oxidation of pyrite [30, 31]. This proposed hypothesis has provided valuable evidence for the mechanism by which the bioleaching of sulphide minerals by Thiobacillusferrooxidans and Leptospirillum ferrooxidans takes place. It remains to develop kinetic models based on these postulates. The fate of the sulphur moiety in the bioleaching of sulphide minerals has not been included in any published kinetic models even though in the case of base metal bioleaching it is clearly an important factor. It may be that sulphur products are the cause of passivation of the mineral surface and the bacteria have been claimed to assist in removing this passivation layer. The two sub-process model for pyrite [22] has been used to successfully predict the performance of a laboratory scale reactor treating a pyrite concentrate [32]. Current work is testing the applicability to several sets of pilot plant data on refractory gold concentrates.

20

1.6E-5 [] Experimental data

1.4E-5 "2

[]

~ B u t l e r - V o l m e r model

1.2E-5 1.0E-5

~

8.0E-6

r~ o

,~. 6.0E-6 ~

4.0E-6 2.0E-6 O.OE+O 620

I

I

I

I

I

630

640

650

660

670

680

E ( m V vs Ag/AgCI)

Figure 3 " Pyrite ferric leach rate versus redox potential from experimental data and ButlerVolmer model prediction [26].

2.5E-5

- ' - ' F e m c leach ~,

2.0E-5

[] Bioleach

"-;' r~ '7

o

o

,...,.i

.~

[] []

1.55-5

O

g [] r~

o

n

W

1.0E-5

5.0E-6

DD

O.OE+O 500

550

600

650

700

750

E (mV vs Ag/AgCI)

Figure 4 " Comparison of pyrite bioleach rates [22] with chemical ferric leach rates [26] over the same range of redox potential

21 4. OTHER SULFIDE MINERALS The two other minerals of major industrial interest are arsenopyrite in refractory gold ores and chalcopyrite, which has proved difficult to bioleach. Both of these will be leached via the polysulfide pathway yielding sulfur and supporting the growth of both iron- and sulfuroxidizing microorganisms. Rawlings [15] has shown that Leptospirillum ferrooxidans and Thiobacillus caldus are the predominant species present in an arsenopyrite-pyrite bioleach system. Work on the ferric leaching of chalcopyrite has proposed several reasons for its passivation and slow kinetics [6, 8, 33 and 34]. These include the production of jarosite, sulfur or polysulfide layers on the surface of the leached chalcopyrite. As yet there is no one accepted mechanisms nor kinetics based on this. The role of the sulfur-oxidizing species or possible use of thermophilic microorganisms is not dear. In the case of chalcopyrite there is an alternative proposed mechanism which involves only an acid initial attack of the sulphide [8]:

CuFeS2 + 2H2SO4 --> CuSO4 + FeSO4 + H2S (aq) 2H2S + 2Fe2(SO4)3 --> 4FeSO4 + 2H2SO4 + 2S ~

(14) (15)

The net result of these two reactions is the same as that of the direct ferric sulphate attack of the chalcopyrite: CuFeS2 + 2Fe2(SO4)3 --~ CuSO4 + 5FeSO4 + 2S ~

(16)

Although the leaching reaction can be explained equally well by either mechanism, Equations 14 and 15 or Equation 16, the morphological characteristics of surface sulfur indicates that at least some of the sulfur is produced through reactions 14 and 15. In fact, sulfur on leached chalcopyrite is present of isolated globules, which is difficult to explain unless the sulphur has been deposited from solution on pre-existing sulphur grains. 5. ELECTROCHEMICAL ASPECTS OF THE CATALYTIC ACTION OF ATTACHED BACTERIA In the bioleaching of sulfides usually an important fraction of bacteria are temporarily or permanently attached to the solid sulfur sustrate. These bacteria are in special situation because, apart from being able to oxidize ferrous ion in solution, they can utilize the sulfide solid substrate as an alternative energy source. An important discussion topic in bioleaching is related to the behaviour of attached bacteria and its catalytic influence on the leaching process. In normal aerobic conditions attached bacteria can utilize ferrous ion present in solution as an energetic substrate (reaction 2). In this case bacteria obtain energy for its growth from the transference of electrons from Fe+2 to 02. Attached bacteria can also obtain energy from direct oxidation of the reduced sulfur substrate, transfering electrons from the sulfide substrate to dissolved oxygen [35]. On the other hand, it has been shown that bacteria can grow in anerobic conditions out of oxidizing a solid sulfide substrate, such as elemental sulphur [36] or covellite [37], using ferric

22 iron as electron acceptor. In other words, Thiobacillus ferrooxidans can grow using the Fe+3/Fe+2 redox couple either as an electron donor or an electron acceptor, according to the relative potential value of other redox couples available in the system. It is interesting to note that in this case bacteria can get energy for their growth also when the electron acceptor and donor are both in the periplasmic zone. It has been suggested that sulphur compounds degradation in bioleaching is mediated by reduction of Fe+3 ions complexed in the extracellular polymeric iron (EPS) of attached bacteria [1]. There are also several indications that suggest that ferric reduction by bacteria attached to sulfide subtrates can still be a relevant sub-process in the bioleaching of sulfides in aerobic conditions [38]. This reaction probably also involves participation of extracellular polymeric iron (EPS). Accumulation of ferrrous ion in aerobic conditions does not seem likely, however, in view of the very high ferrous iron-oxidizing activity of these microorganisms [18]. Considering the relevance that the activity of complexed EPS iron has on the activity of attached bacteria, by applying some basic electrochemical concepts it is posible to postulate a simple mechanism to explain the behaviour of these bacteria under different bioleching conditions. Figure 5 shows the relative position of the electrochemical potential of main redox reactions involved in bioleaching and also schemes of possible passways for electron transfer in the periplasmic and cytoplasmic space. In this figure, Eox represents the redox potential of the O2/1-120 couple in the cytoplasmic space, which approaches + 0.82 V vs EHE in oxygenated solutions; Esox represents the redox potential of a kinetically significant reaction of oxidation of a reduced sulfur compound available to the attached bacteria; EEp represents the redox potential of the complexed Fe+J/Fe+2 in the EPS layer of attached bacteria.

Fe+2

EOX

EEp ESOX~

/ EOX

bl

~

Fe+2

b2

\

Fe+2

EEp

ESOX--

Figure 5. Redox potential of main redox reactions and schemes of electron flow in the bioleaching of sulfide by attached bacteria. Figure 5-a shows the case when EEp is minimized, which is obtained when most of the dissolved iron is present as ferrous ion. In this case the potential difference EEp and Eox is

23 maximized and most of the bacterial activity is linked to ferrous ion oxidation. On the other hand, the potential gap between Esox and EEp is minimized and reduction of ferric by transfer of electrons from the sulfide substrate should be negligible. Direct transfer of electrons from the sulfide substrate to oxygen without mediation of EPD iron is also considered to be negligible [1]. A very different situation is obtained when most of the dissolved iron has been converted into ferric ion, situation which is pictured in Figures 5-b 1, b2. Here EEp is much more positive with respect to Esox and there are good electrokinetics conditions to reduce polymeric ferric ion by transfering electrons from the available surface sulfur compound. If there is good supply of oxygen to the cytoplasmic space of the cell, the polymeric ferrous ion so formed should be continuously transformed into ferric ion enabling further oxidation of the sulfide substrate (Figurre 5-b 1). If, on the other hand, there is not adequate supply of oxygen to the attached cells, as in the case of formation of well structured biofilms [39], the oxidation of surface sulfur compounds can continue only through the supply of ferric iron from the bulk of the solution (Figure 5-b2). According to the proposed reaction patterns in Figure 5 the behaviour of attached bacteria is very dependent on the Fe+3/Fe+2 ratio in the exopolymer membrane, which itself depends on the redox potential in solution and the concentration of dissolved iron. For instance, it has been observed that when Thiobacillusferrooxidans is grown on sulfur in the presence of important concentrations of ferrous ion, the process of sulphur oxidation starts only after most of the ferrous iron has been converted into the ferric form [40]. Coincident with the reaction patterns proposed in Figure 5 the process of sulphate formation started only when the redox in solution was very high, and was paralleled by the simultaneous reduction of ferric ion. In other work [41], sulphur prills previously colonized by ThiobaciUusferrooxidans were biooxidized in three different acid basal solutions: iron-free, with pure ferrous ion, with pure ferric ion. In every case sulfur dissolution consistently started only when the redox potential in solution was over + 0.7 V vs EHE. Reaction schemes in Figure 5 can also help to explain the strong influence of the mineral sulfide polarization on the surface pitting by attached Thiobacillusferrooxidans [42]. Pyrite electrodes le~ in open circuit exposed to attached bacteria in inoculated iron-free acid medium did not show any signs of surface pitting even after 22 days. Dissolved iron concentration reached only a few ppm during this period and the pyrite potential remained at about 0.5 V vs EHE. On the contrary, when pyrite electrodes exposed to attached bacteria were potentiostatically maintained at +0.8 V vs EHE, a severe surface pitting was observed in the same leaching period. Results from a third experiment in which pyrite electrodes were potentiostatically maintained at + 0.8 V vs EHE, but in abiotic conditions, did not show surface pitting either, which indicated that pitting was not linked to induced anodic dissolution of the sulfide. A second experimental series conducted with particulated pyrite electrodes where the dissolution kinetics was characterized from the monitoring of dissolved iron, confirmed the trends observed previously with massive electrodes. Iron dissolution after 72 hrs in inoculated pyrite polarized to + 0.8 V vs EHE was 3 times larger than that reached with inoculated pyrite at +0.6 V vs EHE and with pyrite polarized to + 0.8 V vs EHE in sterile medium, respectively [41]. Experiments under controlled potential enabled to induce in pyrite a potential of +0.8 V vs. EHE, a polarization which corresponds roughly to that induced in a sulfide particle inmersed in a leaching solution containing 1 g/1 of dissolved iron converted to ferric ion. The different

24 attack obtained when this potential was applied in abiotic and inoculated conditions clearly demonstrate that in this sulfide bacterial action can not be simply replaced by the chemical (or electrochemical) reproduction of the high Fe§ § ratio maintained by the bacterial oxidation of ferrous ion. In other words, indirect bacterial action on its own does not account for all the catalytic effects involved in the presence of bacteria. What happened is that in the inoculated case the bacterial oxidation of ferrous ion in the bulk of the solution help to increase Fe§ § ratio in the EPD iron, which we postulate can eventually triggers an additional type of action in the attached bacteria. However, in the case of pyrite it is very likely that the net impact of additional activity of attached bacteria may be relevant only if the conditions in the bioreactor are adequate for bacteria to maintain dissolved iron fully converted into ferric ion.

6. CONCLUSIONS It has been shown that bioleaching involves two pathways, the thiosulfate pathway resulting from the reaction of ferric ions with sulfides such as pyrite, molybdenite and wolframite, whose electronic structure makes them susceptible only to oxidative ferric attack, and the polysulfide pathway for those sulfides whose electronic structure makes them susceptible to acid and ferric attack. The thiosulfate pathway results in the dissolution of the metal, the production of ferrous iron and sulfate and can be carried out by microorganisms which can oxidize ferrous iron and need not have sulfur oxidizing capabilities. On the other hand, the polysulfide pathway produces both ferrous iron and elemental sulfur which can form the substrates for iron- and sulfur oxidizing microorganisms In the case of the thiosulfate pathway, the primary attack on the mineral is by ferrous iron and it has been shown that a kinetic model based on the rate equations for the chemical ferric leach step and the bacterial ferrous iron oxidation sub-process can be used to predict the kinetics of pyrite bioleaching. Work is in progress to extend this work to those sulfides which are leached via the polysulfide pathway where the oxidization of sulfur and the formation of passivation layers also need to be taken into account. Although much still needs to be done to fully understand the mechanism and kinetics of bioleaching, the framework for achieving this has been established. ACKNOWLEDGEMENTS Financial support from Billiton Process Research, the Gold Fields Foundation, and the South African Foundation for Research Development is gratefully acknowledged. REFERENCES 1. Sand,W., T.Gehrke, R.Hallman and A.Schippers, Appl.Environ.Microbiol., 43 (1996) 961. 2. Rawlings,D.E., H.Tributsch and G.S.Hansford, Microbiology, 145 (1999) 5. 3. Lowson,R.T., B.J.Reedy and J.Beattie, Chemistry in Australia, (8) (1993) 389. 4. Schippers,A. and W.Sand, Appl.Environ.Microbiol., 65 (1999) 319. 5. Steudel,R., Ind.Eng.Chem.Res., 35 (1996) 1417. 6. Hackl,R.P., D.B.Dreisinger, E.Peters and J.A.King, Hydrometallurgy, 39 (1995) 25. 7. Schippers, A., P.G.Josza and W.Sand, Appl.Environ.Microbiol., 62 (1996) 2424. 8. Dutrizac, J.E., Can.Metall.Quart., 28(4) (1989) 337. 9. Gehrke, T., J.Telegdi, D.Thierry and W.Sand, Appl.Environ.Microbiol., 64 (1998) 2743. 10. Crundwell,F.K., Minerals Engineering, 9(10) (1996) 1081.

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LIST of SYMBOLS Cx

Eh F [Fe2+] [Fe 3+] [FeS2] i io K n qFe2+ qFe2+max

R rFe2+ rFe2+,bact rFe2+,chem

T t (~FeS2 ~Fe2+ max Fe2+

concentration of bacteria redox potential - standard hydrogen electrode Faraday constant concentration of Fe 2+ concentration of Fe 3+ concentration of FeS2 corrosion current corrosion current kinetic constant number of electrons involved in reaction bacterial specific ferrous iron oxidation rate maximum bacterial specific ferrous iron oxidation rate, universal gas constant ferrous iron production rate bacterial ferrous iron production rate chemical ferrous iron production rate temperature time specific surface area of pyrite over potential area specific ferrous iron production rate maximum area specific ferrous iron utilisation rate

mol.1-1 mV C.mo1-1 mol.1-1 mol.1-1 mol.1-1 amps amps dimensionless molF e2+.(molC) -1.h -1 molF e2+.(molC) -1.h -1 kJ. mol "I.K -1 molFe2+.l-1 .h-1 molFe2+.1-1 .h-1 molFe2+.1-1 .h -1 K h m2.mo1-1 mV

molFe2+.m -2.h-1 molFe2+.m -2.h-1