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Past, present and future o f biohydrometallurgy Henry L. Ehrlich Department of Biology, Rensselaer Polytechnic Institute, Troy, NY 12180-3590 USA

The history of bioleaching and its microbiololgical basis are summarized. A possible future developmental direction is indicated. 1. PAST When in 1676, Antonie van Leeuwenhoek first described what has been interpreted to have been bacteria in a peppercorn infusion, which he examined with his ingeniously fashioned simple microscope (1), tittle did he or those to whom he revealed his discovery suspect that other tiny creatures not unlike the ones he saw are able to extract metals from ore. Indeed, bacteria were not generally recognized as a unique group of organisms until Ferdinand Cohn did in 1875, who classified them with plants (2,3), and it was not until the 1960's that they were assigned to a special group of organisms that were distinct from plants and animals, the prokaryotes. This was based on their unique cell organization (4). What is more, it was not until 1977 that Carl Woese first recognized that the prokaryotic bacteria could be devided into two distinct phylogenetic groups, the eubacteria (now bacteria) and the archaebacteria (now archeota) (5). Both of these groups include members of special importance to biohydrometallurgy. In this historical context, it is noteworthy that bioleaching of copper from ores was practiced many centuries before the discovery of bacteria. It was a purely empirical process then and not recognized to have a biological connection. It appears to have been performed in China at least 100-200 years BCE and in Europe at least as far back as the second century CE (6). According to Hallberg and Rickard, more than 2 million tons of copper have been leached from the copper deposit of the Falun Mine in central Sweden since 1687 (7). Commercial copper leaching from partially roasted ore at the Rio Tinto mine in Spain was first recorded in 1752 (6), but to what extent bacteria were involved in this process remains unclear. The dissolved copper in all these cases was precipitated from pregnant solution on contact with metallic iron, a process first described by the Chinese (6).

2. PRESENT

First reports demonstrating the involvement of bacteria in copper leaching date back to the 1950's, following reports on the role of bacteria in the formation of acid mine drainage from pyrite inclusions in bituminous coal deposits (8-10). L.C. Bryner and J.V. Beck and their students at Brigham Young University in Provo Utah found the same bacteria, Thiobacillus ferrooxidans and Thiobacillus thiooxidans, in copper mine drainage from Kennecott's openpit mine in Bingham Canyon, Utah, that had been found in acid coal mine drainage. They showed in laboratory experiments that T. ferrooxidans was capable of leaching various copper sulfide minerals as well as molybdenite, but the latter only in the presence of pyrite (11-13). Demonstration of bioleaching of some other metal sulfides like ZnS, NiS, and PbS soon followed (14-16). The chief process in bioleaching of sulfidic ores is the mobilization of metal constituent(s). This is accomplished through microbially promoted oxidation of the metal sulfide(s). Silverman and Ehrlich (17) distinguished between two modes of bacterial attack. In one of these modes (indirect attack), the chief function of T. ferrooxidans, which was the only organism capable of promoting leaching that was recognized at the time, was to regenerate the oxidant, ferric ion, in the bulk phase from ferrous ion, 2Fe 2+ + 02 + 21-1+ ==> 2Fe 3+ + H20

(1)

The ferrous ion resulted from the chemical oxidation of the metal sulfide in the ore by ferric ion, 2Fe 3+ + MS ==> 2Fe 2+ + M 2+ + S~

(2)

MS in equation (2) represents a metal sulfide, and M z+ the divalent metal ion formed in the oxidation of MS. In addition to oxidizing Fe 2+, T. ferrooxidans and/or T. thiooxidans, which is also detected in bioleach processes, were visualized as oxidizing the S~ formed in the chemical oxidation in reaction (2), to H2SO4 (17,18), S~ + 1.502 + HzO ==> H2804

(3)

In the other mode of bacterial attack of metal sulfide (direct attack), the bacteria, according to Silverman and Ehrlich (17) attacked a metal sulfide by attaching to the mineral surface and oxidizing it enzymatically by conveying electrons from the reduced moiety of the mineral, usually the sulfide but in the case of CuzS also from the cuprous copper, to 02, MS + 02 +2H + ==> M 2+ + H20

(4)

Clear evidence of the ability of T. ferrooxidans to attach readily to the surface of metal sulfides was developed subsequently (19-28). In direct attack, electron transfer from sulfide-S, or from cuprous copper in the case of CuzS, involves Fe(III) bound in the cell envelope and exopolymer (29-31). This bound Fe acts as an electron shuttle between the electron donor and the electron transport system of the cell, which conveys a major portion of the electrons to 02 and the rest to CO2 [see (32) for more detail]. Thus, the Fe(III) bound in the cell envelope and exopolymer is thought to undergo reversible reduction and oxidation in this electron transfer. The sites on a metal sulfide particle for bacterial attachment and attack seem to be limited. Thus, once maximum attachment has been achieved, further multiplication of attached cells, if it occurs, should result in the displacement into the bulk phase of one of the two daughter cells of each dividing bacterium. This model of direct attack is in contrast to the model for indirect attack of metal sulfides in which Fe 3+ in the bulk phase, produced by unattached bacteria from dissolved Fe 2+, is the electron acceptor. If these two models for biooxidation of metal sulfides describe the process correctly, the iron requirement for an optimal rate of metal sulfide oxidation by the direct mode of attack should be significantly smaller than for the indirect mode. Differences in reaction kinetics between exclusively direct and indirect modes of attack are also likely. Sand et al. have recently suggested that because Fe(III) oxidizes metal sulfide in both the direct and indirect mechanisms, there is no difference between the two mechanisms (33). Their model emphasizes a similarity in the chemistry of attack of the sulfide moiety by iron, and makes no distiction between ferric iron in the bulk phase and ferric iron bound in the cell envelope. Although initial studies of bioleaching suggested that T. ferrooxidans was the only actor in bioleaching of metal sulfides, subsequent studies showed that other, phylogenetically unrelated organisms could also be active. These include not only autotrophs but also heterotrophs (34) and not only mesophiles but also thermophiles, all of them acidophilic and all of them Fe(II) oxidizers (34,35). Indeed, in many cases Leptospirillum ferrooxidans, which cannot oxidize reduced forms of sulfur, seems to dominate the metal-sulfide oxidizing microbial flora (36,37). Further study of the microbes in pregnant solution from bioleaching operations showed that the acidophilic iron oxidizers were accompanied by many other kinds of organisms, including heterotrophic bacteria, fungi, and protozoa (38-40). Indeed, heap, dump, and in-situ leaching by native microbial flora in the field is probably the result of a consortium of acidophilic microorganisms including autotrophic and heterotrophic bacteria, fungi, and even protozoa. The autotrophs can be assumed to be the chief promoters of the actual metal leaching process, whereas the most important role of the heterotrophs can be assumed to be to limit the concentration of organics that might otherwise inhibit the autotrophs (41-45). Some of the heterotrophs can also promote formation of floe, as in the case of L. Ferrooxidans (37). Protozoans, in addition to aiding in the removal of dissolved organics, may control the size of the microbial population by preying on it (38). Both autotrophs and heterotrophs contribute to the weathering of the host rock (gangue) to expose ore mineral that is encapsulated in the gangue (43). Indeed, sufficient A1 could be mobilized from aluminosilicates to make its separation desirable (43). The weathering action is due in part to

the sulfiwic acid generated by the autotrophs in attacking pyrite and chalcopyrite minerals, which causes rupture of Si-O and AI-O bonds in aluminosilicates. Wheathering may also be promoted by some of the less acidophilic heterotrophs that generate organic acids and/or ligands that sequester Ca and Mg from the crystal lattice of aluminosilicates in the early stages of a leaching operation. With the demonstration of bacterial involvement in bioleaching, four distinct approaches have been taken in its commercial exploitation. These are heap, dump, in-situ, and reactor leaching. S.R. Zimmerley, D.G. Wilson and J.D. Prater were issued the first patent on heap bioleaching on 24 October 1955 (43). They assigned it to Kennecott Copper Corporation. This patent described a cyclic process of heap leaching of copper-, zinc-, copper-molybdenum, chromite- and titanium-ores. The last three ores were meant to be upgraded (beneficiated) by the process, i.e., the ore was enriched in metal value instead of the metal value being extracted. Cu recovery from pregnant solution described in this patent was by cementation with metallic iron. The process of heap bioleaching has gone through various improvements over the years, as will be discussed by Brierley and Brierley in a subsequent chapter (44). Change in the design of heaps to prevent slumping and optimization of aeration has been a major factor in this improvement. Much effort has been expended to design a commercially viable process for bioleaching of ore concentrate in reactors. Progress has been gradual, with the chief stumbling block having been slow leaching rates. But as Brierley and Brierley will discuss (44), a breakthrough has now been achieved, making ore-concentrate bioleaching commercially feasible in certain instances. Advances in reactor design and, in at least one instance, the use of a moderately thermophilic acidophile as an agent of leaching (45) have been at the heart of this breakthrough. A rationale for turning to moderate thermophiles is a more limited cooling requirement for reactors. Ore concentrate leaching with hyperthermophiles in reactor leaching has been tried because of observations that leaching rates with such strains were higher than with mesophiles at ambient temperatures (35). However, more recent studies have shown that acidophilic hyperthermophiles tested in reactors have much more limited tolerance for high pulp density than moderate thermophiles or mesophiles (46). The observed accelerating effect at elevated temperature was probably mostly on indirect leaching. Although commercial leaching was initially restricted to copper ores, reactor- based processes have recently been developed for the extraction of other metals such as Co, Ni and Zn (47-50). Ehrlich reported in 1964 that T. ferrooxidans was capable of oxidizing arsenopyrite (51). In his study, he measured mob~ation of Fe and As. He did not follow sulfide. The mobilized iron appeared as Fe(II) and Fe(III). The mobilized arsenic appeared as arsenite [As(III)] and as arsenate [As(V)]. Some of the arsenite and arsenate were precipitated by iron. The iron arsenate compound was later shown to be scorodite [FeAsO4.2H20] (52). Although the possibility suggested itself that the arsenate resulted from oxidation of arsenite by T. ferrooxidans, this could not be confirmed by direct testing. However, the thermophilic archeon, Sulfolobus acidocaldarius strain BC, is capable of such oxidation (53). Current evidence indicates that the arsenate formed in the presence of T. ferrooxidans is the result of

chemical oxidation of arsenite by the bacterially generated Fe(III) (54). Transient formation of S~ was also observed in this recent study (54). The ability of T. ferrooxidans to oxidize arsenopyrite led the late Eric Livsey-Goldblatt in 1983 to propose its use in biobeneficiating pyritic gold ores in a bioleaching process that he estimated to be significantly more economical than pyrometallurgical treatment (55), and this has turned out to be the case. In pyritic gold ores, pyrite and arsenopyrite encapsulate the gold, making it inaccessible to lixiviants such as cyanide or thiourea. Partial oxidation of the pyrite and arsenopyrite uncover the gold sufficiently for extraction, and at the same time lessen the non-specific, irreversible consumption of cyanide during subsequent extraction of the ore. 3. FUTURE

In my opinion, some future fundamental research effort in biohydrometallurgy should be directed toward leaching and/or beneficiation of oxide, silicate and carbonate ores. Sporadic research in this area has been done in the past, but it has not led to industrial applications so far (56-61). Although the metal in some of these ores can be mobilized by sulfuric acid generated from sulfur by the autotroph T. Thiooxidans (56), organic acids and ligands such as 2ketogluconate generated by some heterotrophic bacteria (57), and oxalate and citrate generated by fungi can also be useful (57,60). In the case of metal oxide ores, anaerobic bacterial processes in which bacteria reduce the metal oxide and thereby solubilize it may be the most promising for industrial exploitation (62). In such processes, the bacteria use the metal oxide as terminal electron acceptor. The electron donor may be organic carbon, formate, or HE, depending on the organism (for a review, see (63)). An example of a reaction in which MnO2 is bacterially reduced to Mn 2+ with acetate is the following, 4MnO2 + CH3COO- + 7H + ==> 4Mn 2+ + 2HCO3- + 41-120

(5)

For other examples, see Ehrlich (64). Since ores are not sterile and cannot be sterilized on a commercial scale, heterotrophic leaching presents some process design challenges that autotrophic leaching does not. Aerobic heterotrophic leaching based on the action of microbially produced acidulants and/or ligands, would best be operated in a two-reactor system. In such a system, the first reactor would be the generator in which desired microbes would produce the acidulant/ligand in pure culture trader optimal growth conditions, preferably in a continuous mode. The spent culture solution from this reactor would be bled into a second reactor containing the ore to be leached. Growth of microbes that might destroy the acidulant/ligand in the second reactor could be controlled by ensuring a very low level of residual nitrogen source in the spent culture medium and by temperature manipulation. In anaerobic heterotrophic leaching of metal oxides by a reductive process, the maintenance of selective growth conditions is extremely important, just as it is in aerobic autotrophic leaching of metal sulfides. In bioleaching of metal sulfides, the selective growth conditions are high acidity and the absence of a major organic carbon and energy source, which are readily

established and maintained. In heterotrophic leaching of metal oxides, circumneutral to moderately acid pH and an adequate supply of a carbon/energy source is essential. However, by themselves, these conditions are not sufficiently selective, but when combined with anaerobiosis and use of a very specialized carbon/energy source, the resultant conditions can be highly selective. In the ideal case, anaerobiosis excludes potentially interfering obligate aerobes, and the specialized carbon/energy source utilizable only by the leaching organism(s) prevents overgrowth by anaerobic heterotrophs incapable of leaching a mineral oxide. Phenol is an example of a specialized carbon/energy source that is toxic to many microorganisms but can be used as carbon and energy source by some iron oxide- and MnOE-reducers (65). Acetate is another specialized carbon/energy source. It is non-fermentable except by acetoclastic methanogens, and it is inadequate as a sole source of carbon for most anaerobes. But it can be used as a carbon/energy source by some reducers of iron oxide, MnO2, and UO22+ (65,66). Thus in designing a heterotrophic leaching process, important considerations are selective conditions in a one-reactor system, or axenic conditions in the first reactor of a two-reactor system. In choosing a carbon/energy source for commercial heterotrophic leaching, cost becomes another important consideration. If it has to be sugar, industrial molasses, whether a byproduct of cane-sugar, beet sugar, or corn-starch hydrolyzate processing, would be a prime candidate, but it is not very selective. If it is to be an aromatic electron donor, industrial phenolic waste streams from chemical industry might be worth considering. If it is to be acetate, production in a reactor with an acetogen like Clostridium thermoaceticum growing on sugar (glucose or fructose, e.g., invert sugar, or corn starch hydrolyzate, but not sucrose) or some other feed stocks could take place at the site of the bioleaching plant (67).

4. CONCLUSIONS From the foregoing brief survey it is apparent that scientifically based biohydrometallurgy using acidophilic autotrophs has made significant strides in its developent as a commercially viable technology for the processing of sulfidic ores. The technology for heterotrophic leaching of metal oxides, carbonates, and silicates on an industrial scale awaits development. In view of its potential as an environmentally more benign process than pyrometallurgy, biohydrometallurgy promises to replace many if not all pyrometallurgical ore extraction methods in the not too distant future. REFERENCES

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