Microbiology of leaching process – iron, copper,gold, nickel, cobalt

Microbiological leaching has been use as an alternative approach to conventional hydrometallurgical methods of extraction. Future sustainable development involves measures that reduce our dependence on non-renewable raw materials and on the demand for primary resources. New resources for metals are developed with the help of novel technologies. In addition, improvement of previously existed mining techniques resulted in metal recovery by the sources that have not been of economic interest up to today. The metal winning processes based on the activity of microorganisms provides a possibility to obtain metals from mineral resources which are not accessible by conventional mining. Generally bioleaching is a process described as the

“Dissolution of metals from their insoluble mineral source by biological oxidation and complexation process carried out by certainly and naturally occurring living organisms” (Rohwerder 2003)

However, bacterial leaching is the term that is defined as the

“Extraction of metals from their ores using microorganism especially bacteria.”

Metals for which this technique is mainly employed include copper, cobalt, nickel, zinc, and uranium. These are extracted either from insoluble sulfides or—in the case of uranium—from oxides. However, for recovery of gold and silver, the activity of leaching bacteria is applied only to remove interfering metal sulfides from ores bearing the precious metals prior to cyanidation treatment. Here the term bio-oxidation is used preferentially because the bioleached metals, in most cases iron and arsenic, are not recovered. A general term covering both bioleaching and bio-oxidation techniques called “bio-mining” is used. (Bosecker 1997; Ehrlich 2002; Olson et al. 2003; Rawlings 1997, 2002; Rohwerder et al. 2002). In early days of development of this technique, mesophilic bacteria were considered to be important but nowadays (new) genera of moderately and extremely thermophilic bacteria have also become attractive (Johnson 1998; Norris et al. 2000) because industrial processes like tank leaching suffer from cost of cooling (in processes using mesophilic bacteria) but the use of thermophiles helped to make the process economical.

The biochemical fundamentals of the leaching reactions have been the subject of intensive research in recent years. “Direct mechanism” of biological metal sulfide oxidation, i.e., the direct enzymatic oxidation of the sulfur moiety of heavy metal sulfides (reviewed by Ehrlich 2002, Sand et al. 1995 and Rohwerder 2003), does not exist. The “indirect mechanism”, i.e., non-enzymatic metal sulfide oxidation by iron(III) ions combined with enzymatic (re) oxidation of the resulting iron(II) ions, remains and now comprises two sub-mechanisms: “contact” and “non-contact” mechanisms (Rawlings 2002; Sand et al. 2001).

The non-contact mechanism is carried by planktonic bacteria, which oxidize iron (II) ions in solution. The resulting iron (III) ions come in contact with a mineral surface, where they are reduced, and enter a cycle again. In a strict sense, this is in effect the previously designated indirect mechanism (Sand et al. 1995). In contact mechanism most of the cells attaches to the surface of sulfide minerals resulting in the dissolution of sulfide minerals at the interface between the bacterial cell (wall) and the mineral sulfide surface. In both contact and non-contact mechanisms, the bacteria contribute to mineral dissolution by generation of the oxidizing agent, the iron (III) ion, and by the oxidation of the sulfur compounds resulting from the dissolution.

Diversity among leaching bacteria

The predominant metal-sulfide-dissolving microorganisms are extremely acidophilic bacteria i.e. thrive at pH values below 3. They are able to oxidize either inorganic sulfur compounds and/or iron (II) ions. Due to the limited types of substrates available in mining environments, these biotop es* are extremely poor with respect to the diversity of their microbial flora. However, mining biotopes show a great microbial diversity (Johnson 1998; Hallberg and Johnson 2001) with at least 11 putative prokaryotic divisions living at AMD sites (Baker and Banfield 2003).

The classical leaching bacteria belong to the genus Acidithiobacillus (formerly Thiobacillus, Kelly and Wood 2000) which include extremely acidophilic sulfur- and/or iron (II)-oxidizing bacteria, the mesophilic At. Thio-oxidans and At. Ferro-oxidans. Together with the moderately thermophilic At. caldus, these leaching bacteria belong to the Gram-negative γ-proteobacteria. Other leaching proteobacteria are species of the genus Acidiphilium such as Ac. acidophilum (Hiraishi et al. 1998), whereas members of the genus Leptospirillum belong to a new bacterial division (Hippe 2000; Coram and Rawlings 2002). Gram-positive leaching bacteria are moderately thermophilic members of the genera Acidimicrobium, Ferromicrobium, and Sulfobacillus (Clark and Norris 1996; Johnson and Roberto 1997; Norris et al.1996). Leaching archaebacteria are also known and they all belong to the Sulfolobales, a group of extremely thermophilic, sulfur- and iron(II) ion-oxidizers including genera such as Sulfolobus, Acidianus, Metallosphaera, and Sulfurisphaera (Fuchs et al. 1995, 1996; Kurosawa et al. 1998; Norris et al. 2000). Mesophilic and acidophilic iron (II)-oxidizing archaebacteria have also been discovered recently that belongs to the Thermoplasmales, and two species, Ferroplasma acidiphilum (Golyshina et al. 2000) and F. acidarmanus (Edwards et al. 2000). Among these bacteria are species with an extremely limited substrate spectrum. In particular, L. ferrooxidans and L. ferriphilum can grow only by aerobically oxidizing iron (II) ions. In contrast, At. ferrooxidans is endowed with a remarkably broad metabolic capacity. This species lives on the oxidation of reduced sulfur compounds and is able to oxidize molecular hydrogen, formic acid, iron (II) ions and other metal ions.

Anaerobic growth is possible by oxidation of sulfur compounds or hydrogen coupled with iron (III) ion reduction (Pronk et al. 1992; Das et al. 1992, Ohmura et al. 2002). Like Acidianus spp., At. Ferro-oxidans reduces elemental sulfur in the course of anaerobic hydrogen oxidation (Ohmura et al. 2002). The application of electron acceptors other than oxygen is reflected by the presence of various electron transport components. For example, at least 11 different cytochromes of the c type have been identified in the genome of At. Ferrooxidans.

Many leaching bacteria control the complete aerobic and anaerobic components of the sulfur and iron cycles could be of especially great importance for AMD treatment. If natural bioleaching in waste heaps and tailings is stopped by flooding or with organic covers—both common AMD countermeasures that create an anoxic environment —leaching bacteria could remain active due to their anaerobic capacities.

However, the above mentioned anaerobic physiology of leaching bacteria and their presence in anoxic biotopes support the hypothesis of an anaerobic leaching process. For example, the existence of Acidithiobacillus-like species has been demonstrated in an anoxic reactor designed to clean contaminated groundwater and lignite. Further metabolic diversity among leaching bacteria has been found with respect to their carbon assimilation pathways. Acidithiobacillus spp. and Leptospirillum spp. can grow only chemolithoautotrophically. In contrast, Acidiphilium acidophilum and Acidimicrobium ferrooxidans are able to grow autotrophically with sulfur and iron (II) compounds, heterotrophically with glucose or yeast extract, and mixotrophically with all of these substrates. In addition, several Acidiphilium spp. and Acidisphaera rubrifaciens possess pigments that may confer the ability for some photosynthetic activity (Hiraishi and Shimada 2001; Hiraishi et al. 1998, 2000). An obligate mixotrophic iron (II)-oxidizing bacterium is Ferromicrobium acidophilus (Johnson and Roberto 1997). In addition, some Sulfobacillus spp. shows poor chemolithotrophic growth, as do many thermophilic Sulfolobales (Johnson 1998).

Metal sulfides are leached via two different pathways.

Two different reaction mechanisms control the dissolution of metal sulfides:
• The thiosulfate pathway
• The polysulfide pathways.

In general, dissolution is achieved by a combination of proton attack and oxidation processes. The reaction pathway is determined by the mineral species (Sand et al. 2001). However, the crystal structure (e.g., monosulfide or disulfide structure) does not control the pathway of dissolution. The reactivity of metal sulfide with protons (acid solubility) is the relevant criterion.

Metal sulfides with valence bands that are derived only from orbitals of the metal atoms cannot be attacked by protons (acid-non-soluble metal sulfides). In contrast, metal sulfides with valence bands derived from both the metal and sulfide orbitals, are more or less soluble in acid (acid-soluble metal sulfides).

Acid-non-soluble metal sulfides: thiosulfate pathway

Metal sulfides such as pyrite, molybdenite, and tungstenite (FeS2, MoS2, and WS2, respectively) are exclusively oxidized via electron extraction by iron (III) ions, i.e., the transfer of valence band electrons to iron (III) ions. In this group of metal sulfides, the chemical bonds between sulfur and metal moiety do not break until a total of six successive one-electron oxidation steps have been conducted and thiosulfate is liberated (Figs. 1A, 3B). This mechanism is called thiosulfate pathway (Schippers et al. 1996). Mainly, free thiosulfate is oxidized via tetrathionate and other poly-thionates to sulfate (Fig. 1A), but significant amounts of elemental sulfur (10–20%) may also produce in the absence of sulfur-oxidizing bacteria (Schippers et al. 1999). From the requirement of an electron extraction reaction it can be concluded that only iron (II)-oxidizing bacteria are able to leach acid-non-soluble metal sulfides under acidic conditions. Only these bacteria can regenerate the iron (III) ions consumed in the initial oxidation processes because, at around pH 2, iron (II) ions are not oxidized abiotically at significant rates (Singer and Stumm 1970).

Acid-soluble metal sulfides: Polysulfide pathway

Metal sulfides such as sphalerite (ZnS), galena (PbS), arsenopyrite (FeAsS), chalcopyrite (CuFeS2), and hauerite (MnS2) are dissolved by the combined action of electron extraction by iron (III) ions and proton attack, i.e., the binding of protons by the sulfide moiety via valence band electrons. In this group of metal sulfides, the chemical bonds between metal and sulfur moiety can be broken by proton attack and, after binding two protons, hydrogen sulfide (H2S) is liberated. However, in the presence of iron (III) ions the sulfur moiety is oxidized in a one-electron step concomitantly with the proton attack. Therefore, the first free sulfur compound is most likely a sulfide cation (H2S+, i.e., H2S minus one electron), which can spontaneously dimerize to free disulfide (H2S2) and is further oxidized via higher polysulfides and polysulfide radicals to elemental sulfur (Fig. 1B). This mechanism is named as polysulfide pathway. (Schippers and Sand 1999).

In the course of polysulfide oxidation, more than 90% of the sulfide is transformed to elemental sulfur in the absence of sulfur oxidizing bacteria (Schippers and Sand 1999). Minor products formed are thiosulfate, poly-thionates, and sulfate (Schippers and Sand 1999). As the oxidizing action of iron (III) ions is not an absolute prerequisite in the polysulfide pathway (because here chemical bonds between metal and sulfide can be broken by proton attack), acid-soluble metal sulfides may also be dissolved by the activity of sulfur-oxidizing bacteria. In the absence of iron (III) ions, these bacteria oxidize free sulfide (H2S) resulting from the proton attack on the metal sulfide via elemental sulfur to sulfuric acid and, thus, regenerate the protons previously consumed by the metal sulfide dissolution.

Sulfur chemistry—implications for bioleaching kinetics

In both pathways, the main role of leaching bacteria consists of regeneration of iron (III) ions—the most important oxidants in acidic biotopes (Fig. 1). Thus, the acidophilic iron (II) ion oxidizers control the redox potential in their environment, which is determined mainly by the iron (III)/iron (II) ratio in leaching solutions. Besides this, acidophilic sulfur oxidizers contribute to the transformation of the intermediary sulfur compounds to sulfuric acid (Schippers and Sand 1999; Schippers et al. 1999). In the case of elemental sulfur, oxidation is exclusively carried out by bacteria because this sulfur species is inert to abiotic oxidation in acidic environments (Fig. 1B). As a result elemental sulfur may accumulate in the course of metal sulfide dissolution if sulfur-oxidizing bacteria are absent or inhibited. In general, the production of sulfuric acid from reduced sulfur compounds is needed to regenerate protons consumed by the initial leaching processes via the polysulfide pathway (Fig. 1B). In addition, sulfur oxidizers can influence leaching kinetics in a particular manner. Elemental sulfur may occur suspended as free aggregates and crystals, or can form a layer on the metal sulfide surface (Fowler and Crundwell 1999). In the latter case, the electrochemical properties of the metal sulfide surface might change and/or a barrier is formed that reduces diffusion rates for ions and oxygen. Both phenomena negatively influence leaching kinetics. Leaching rate decreases the sulfur layers on acid-soluble sphalerite at low redox potentials in the absence of sulfur oxidizers (Fowler and Crundwell 1999). Similar problems occur for chalcopyrite. In contrast, at high redox potentials (about 750 mV vs standard hydrogen electrode (SHE)) no inhibiting sulfur layers were observed, either with acid-soluble or with acid-non-soluble metal sulfides (Fowler and Crundwell 1998; Fowler et al. 1999). Although elemental sulfur was also formed in the latter two cases, it probably occurred only as free aggregate, which does not decrease leaching rates under pH-controlled conditions. The factors that influence the properties of surface sulfur layers forming during metal sulfide oxidation are not yet fully understood. Therefore, the details of this process have to be elucidated in order to prevent the formation of inhibiting layers in leaching plant operations.
Surface science—extracellular polymeric substances, attachment, and contact mechanism

Most leaching bacteria grow by attaching to surface of mineral sulfidesout of which some cells always remain in the planktonic state. The reason for which is unknown. On the other hand, attachment process is predominantly mediated by the extracellular polymeric substances (EPS) surrounding the cells and that attachment also stimulates EPS production. In the case of At. ferrooxidans R1 and pyrite, EPS consisted of the sugars glucose, rhamnose, fucose, xylose, mannose, C12–C20 saturated fatty acids, glucuronic acid, and iron(III) ions. Attachment occurs due to a mainly electrostatic interaction of the positively charged cells (2 mol negatively charged glucuronic acid residues complex 1 mol positively charged iron (III) ions resulting in a net positive charge) with negatively charged pyrite (at pH 2 in sulfuric acid solution). In contrast, hydrophobic interactions do not contribute significantly to attachment to metal sulfide surfaces. Cells grown on elemental sulfur do not attach to pyrite due to a considerably modified EPS composition. These EPS contain considerably less sugars and uronic acids but much more fatty acids than pyrite grown EPS. The most important difference is the total lack of complexed iron (III) ions or other positively charged groups. As a result hydrophobic interactions are relevant in attachment of At. ferrooxidans to sulfur. This means that the bacteria are able to adapt the composition and amount of their EPS according to the growth substrate (planktonic cells grown with soluble substrates, e.g., iron (II) sulfate, produce almost no EPS). The site of attachment and the detection/sensing of this site are not random. For example, atomic force microscopy (AFM) images demonstrate that cells of At. ferrooxidans preferentially (>80%) attach to sites with visible surface imperfections (scratches). Furthermore, attachment to areas with low crystallization is favored and the sessile cells seem to orient along crystallographic axes, in whose direction oxidation fronts propagate. Adhesion to scratches could be by mere contact area enhancement. Areas with low crystallization and crystallographic axis are often not related to changes in surface topography. Therefore, attachment to specific sites on the mineral surface is principally related to different attractant forces, most likely caused by oxidation processes. Both At. ferrooxidans and L. ferrooxidans have clearly been shown to possess a chemosensory system—chemotaxis—reacting positively to gradients of iron(II)/(III) ions, thiosulfate, etc. (Acuña et al. 1992). These compounds occur compulsorily in the course of metal sulfide dissolution (Fig. 1). Dissolution occurs (in an electrochemical sense) at local anodes (Fig. 3B) [bringing iron (II) ions and thiosulfate in solution in the case of pyrite]. It may be speculated that these local anodes are the sites towards which the cells are chemotactically attracted.

This also seems to be fully applicable to bioleaching of metal sulfides. To summarize, cells are attracted to dissolution sites by their chemotactic sensory system and determine the anodes and cathodes on the metal sulfide surface to become permanent.

The dissolution process occurs in the EPS layer (Fig. 3). The EPS fills the void between the outer membrane (of the cells) and the surface layer (of the metal sulfide). The pioneering work of Tributsch and coworkers (Rodriguez- Leiva and Tributsch 1988) demonstrated that this distance is 10–100 nm wide. More precise measurements do not exist. In the case of metal sulfides such as pyrite, which need an oxidizing attack by iron(III) ions for dissolution, the EPS-complexed iron(III) ions must fulfill this function (Fig. 3B). However, this very process is not at all clear. Currently, the most likely explanation is based on two plausible assumptions. In order that these ions are reduced the first assumption considers the electron tunneling effect.

It is known that electrons can bridge distances of up to2 nm by tunneling from one electron hole to another. Consequently the iron(III) ions have to be within this distance from the pyrite surface (to be reducible by tunneling electrons). Considering the distance between the cell membrane and substrate surface, this hypothesis seems to be reasonably sound and would explain the reduction of the iron(III) ions. The second assumption is that iron(II) ion-glucuroni acid complexes are less stable than the corresponding iron (III) ion complexes. This has been demonstrated for various iron-carbonic acid complexes. Consequently, iron(II) ions produced by the cathodic electron transfer are released from their EPS chelators. Th remaining uronic acid complex at the pyrite surface will recruit a new iron(III) ion out of solution as it stands in equilibrium with the dissolved, as well as other complexed, iron(III) ions. If mobile iron(II) ions diffuse towards the outer membrane, they will be (re)oxidized by the enzymatic system of the cells. These two assumptions currently underlie the most likely explanation of the electrochemical mechanism of (bio)leaching of metal sulfides. The chemical reactions occur outside the cells, in fact outside the outer membrane, but still within the EPS-generated microenvironment (Fig. 3B).
Conditions for growth of microbes in commercial bioleaching operations

Microorganisms that biooxidize sulfide minerals at low pH are resistant to acidic conditions and most heavy metals in process solutions. Typically, microbial cultures are pre-grown or adapted to a particular ore feed in the laboratory or pilot plant. In this manner, potentially inhibitory agents in the feed select for organisms able to oxidize iron and sulfur in the presence of the agents. For example, the tolerance to arsenic has been increased under selective pressure in CSTRs. Mercuric ions select for strains that volatilize mercury by mercuric reductase (Olson et al. 1982). However, their unique physiology makes acidophiles sensitive to inhibition by organic acids and certain anions. Acidophiles maintain an intracellular pH near neutrality. As a consequence, the proton gradient between the environment and the cytoplasm may be 4 or 5 orders of magnitude. This gradient is maintained, at least in part, by an inside-positive membrane potential. While this potential helps to exclude protons, it renders the cell sensitive to lipophilic anions, such as thiocyanate and, to a lesser extent, nitrate. These ions cross the cytoplasmic membrane and accumulate in response to the membrane potential. This accumulation inhibits cells in two ways: (1) increased proton uptake and resultant acidification of the cytoplasm, and (2) anions may react with cell constituents, such as binding to active sites of enzymes. The latter appears to be the case with thiocyanate and bisulfite ions. Many organic acids are protonated at low pH and hence are membrane permeable. Inside the neutral pH cytoplasm, the molecule dissociates, releasing protons and acidifying the cell. This mechanism applies also to fluoride, which occurs predominantly as HF below pH 3.2. These sensitivities require attention to chemistry of process solutions. For example, thiocyanate is produced by reaction of cyanide with reduced sulfur species during cyanidation of biooxidized gold ore concentrates. Process solutions containing thiocyanate that became mixed with bio-oxidation solutions have caused problems at bio-oxidation plants. Second, the supply of water for bio-oxidation processes at a mine must be evaluated for potentially inhibitory agents. For example, water high in chloride (more than a few grams per liter) inhibits metal sulfide bio-oxidation. Consequently, a saline water supply or the presence of soluble Cl-containing minerals in ore may restrict bio-oxidation options. Temperatures are controlled in CSTR bio-oxidation plants. The high rates of exothermic metal sulfide oxidation require cooling of commercial reactors. Cooling requirements are lessened by operating in the moderate thermophile temperature range, as at the Youanmi plant. Exothermic oxidation of sulfides also causes heating in Bio-heaps. For example, Newmont Mining Company’s commercial bio-oxidation operation in Nevada has measured temperatures up to 810C in heaps containing 1.4– 1.8% sulfide-sulfur. Temperatures as high as 660C were noted in a 960,000 t test heap of low grade, run of mine chalcopyrite ore containing 4% pyrite at Kennecott Utah Copper . Heaps heat as mesophilic microorganisms grow and oxidize sulfides. As temperatures increase to >400C, mesophiles are displaced by moderately thermophilic iron- and sulfur-oxidizers. Extreme thermophiles may displace moderate thermophiles where temperatures increase to above 600C. Although moderate thermophiles are cultured from hot heaps, extremely thermophilic archaea are much less commonly detected. However, large populations of extremely thermophilic archaea can be maintained in heaps inoculated with these organisms.

Controlling the temperature in heaps is difficult. However, this is being explored, especially in connection with Bio-heap leaching of chalcopyrite, which is more successful with thermophiles. Heap height is an important factor in the temperature rise, which increases with the square of heap height. Other factors important in heap heating include sulfide oxidation rate, aeration and irrigation rates, and the local climate.

High volumes of air must be supplied in CSTR sulfide bio-oxidation processes. Efficient delivery of oxygen is the single largest operating cost in refractory gold bio-oxidation plants. The volume of air that must be supplied is based on the sulfide oxidation required. For example, at Wiluna in Western Australia, about 8 t air was supplied per tonne of concentrate, at typical oxygen utilization efficiencies of 25%. Aeration of Bio-heaps can accelerate bio-oxidation reactions, reducing leach cycle time. Air may be delivered via a network of pipes installed in a gravel layer at the base of heaps. Air distribution networks typically include 500 mm headers and 50 mm diameter laterals at 2 m spacing. Holes are drilled in the bottom of the 50 mm diameter air distribution pipes. The density of holes is dependent on the amount of sulfide-sulfur to be oxidized and the oxidation rate. Air is injected into the heap using a set of low-pressure high volume fans or blowers.

Microbial processes for metals leaching and mineral bio-oxidation

The processes by which microorganisms facilitate mineral bio-oxidation and bioleaching are termed “contact” and “non-contact” mechanisms. Rohwerder et al. (2003) describe these mechanisms in detail. Iron plays a key role in metal sulfide bio-oxidation processes. The ferrous-ferric ratio has a dominating effect on the solution redox potential in bio-oxidation systems. It is the iron-oxidizing microorganisms, mesophilic or thermophilic, that increase the redox potential resulting in metal sulfide bio-oxidation. The presence of ferric ions alone is not sufficient for bio-oxidation of a metal sulfide if the redox potential is not sufficiently oxidizing.

Attached microorganisms may facilitate the process by localization of ferric ions in the exopolymeric material at the mineral surface (Sand and Gehrke 1999). Sulfide minerals vary in susceptibility to bio-oxidation. The electrochemical potential of sulfide minerals is the fundamental basis of dissolution and is related to differences in metal-sulfur bond energies in lattice structures. In a system containing a mixture of metal sulfides, the mineral with the lowest potential tends to oxidize first, sometimes to an appreciable extent before minerals with higher potentials. Sulfides like sphalerite (ZnS) or pyrrhotite (FeS) have low electrode potentials of approximately 100 mV to 300 mV versus standard hydrogen electrode (Riekkola-Vanhanen and Heimala 1993) and are readily biooxidized. In contrast, molybdenite (MoS2) has a rest potential of about 700 mV and is difficult to bioleach (Romano et al. 2001). Similarly, enargite (Cu3AsS4) resists bioleaching (Olson and Clark 2001).

The formation of refractory surface layers may slow metal sulfide bio-oxidation. Although sphalerite is easily leached, a product layer of elemental sulfur, forming under low redox conditions, needs to be removed by microbial sulfur oxidation before sphalerite can be effectively leached (Fowler and Crundwell 1999). Chalcopyrite often exhibits parabolic bioleaching kinetics due to formation of a refractory surface layer. There is disagreement as to the nature of this passivating layer.Extreme fine grinding and use of thermophiles can overcome this problem.

Pyrite plays a key role in many bio-oxidation operations. Its oxidation produces acidity, heat and dissolved iron. Pyrites vary greatly in their chemical and biological reactivity. It appears that imperfections in crystal structure contribute to ease of oxidation. Highly crystalline pyrite resists oxidation whereas framboidal pyrite oxidizes more easily. Non-sulfide minerals occurring with the sulfide ore significantly affect growth and activity of metal sulfide oxidizing microorganisms. For example, if the carbonate content of an ore exceeds its acid producing potential, the organisms may not be able to maintain the acidic conditions required for their growth. This is less a problem in stirred tank reactor bio-oxidation processes in which ores go through a preliminary concentration step to increase the grade of metal sulfides and remove gangue minerals. Also problematic in Bio-heap leaching is the presence of excessive fines, especially clay minerals. These may migrate with leach solution to the bottom of a heap and restrict water flow and access of oxygen.

Bioleaching of base metals

There are 11 full-scale, stirred-tank bioleaching plants employing three different technologies to process sulfidic- refractory precious metal concentrates and concentrates of pyrite, cobalt and chalcopyrite (Brierley and Briggs 2002). Bio-heap leaching for recovery of copper is a common practice in the industry. Copper is leached from chalcocite by acid (Eq. 1), forming covellite (CuS), or by ferric iron formed from microbial oxidation of ferrous iron (Eq. 2).

2Cu2S + 2H2SO4 + O2 → 2CuS + 2CuSO4 + 2H2O (1)
Cu2S + Fe2(SO4)3 → 2CuSO4 + 2FeSO4 + S (2)

The mineral covellite is subsequently also leached by ferric iron (Eq. 3).

CuS + Fe2(SO4)3 → CuSO4 + 2FeSO4 + S (3)

Chalcocite leaching is generally considered a two-step process (Eqs. 1, 3). Copper in the form of the mineral chalcopyrite (CuFeS¬) leached poorly in conditions designed for recovery from chalcocite. New technologies are developed in which thermophilic microorganisms are used for bioleaching of copper from chalcopyrite, using either heap or stirred-tank reactor systems. Bio-heap leaching of chalcocite is performed on ore crushed to particle sizes that may range between about 1 cm and 4 cm, depending on results of a cost benefit analysis that takes into account copper bioleaching rate, recovery, crushing costs, pad area and other economic factors. Crushed ore is agglomerated and stacked to a height of 6–10 m in heaps that may be hundreds of meters in length and width. Leach solution is applied generally by drip irrigation. Pregnant leach solution draining from heaps is enriched in copper sulfate and is sent to a solvent extraction-electrowinning circuit for copper recovery. Copper production ranges from about 10,000 t Cu/year in smaller operations to over 100,000 t/year in larger operations at a cash cost ranging from US $0.40 to US $0.65 per pound of Cu (Brierely and Brierley 2000).
To date, there are no commercial scale bioleach operations for recovery of zinc or nickel. Bench scale studies, and in some cases pilot plant tests, have indicated potential for extension of bioleach technology to other metals.

Bio-oxidation pretreatment of precious metal ores

Stirred-tank bioleach plant is used to pretreat a sulfidic gold concentrate to enhance gold recovery (Brierley and Briggs 2002). The largest of these operations is Sansu, where nearly 1,000 t concentrate are processed daily in reactors up to 900 m3 in size.

Typically, refractory gold plants operate with a 15– 20% slurry density. The slurry is fed continuously to a primary reactor. Most of the microbial growth occurs in the primary reactor, which has a typical slurry residence time of 2–2.5 days. The primary reactor overflows to a series of smaller secondary reactors connected in series. This design increases the efficiency of sulfide oxidation by reducing short-circuiting of sulfide particles. Total residence time in the circuit is about 4–6 days (Brierley and Briggs 2002).

In December 1999, the first bio-oxidation heap facility for pretreatment of refractory gold ore was commissioned. Bio-oxidation pretreatment of P80 19 mm ore stacked with haul trucks in pads each 305 m ×147 m ×12.8 m was conducted over a period of 150 days to oxidize 40–45% of the sulfide-S content of the ore. The sulfides occur as pyrite and arsenianpyrite occluding the gold. Following bio-oxidation, the oxidized ore is removed from the heap and processed by milling and carbon-in-leach for cyanide recovery of the gold.

Gold recovery was 30–39% before bio-oxidation, increasing to 49–61% after bio-oxidation. Gold recovery was less than the targeted 71% due to construction and process changes necessitated by declining gold prices. Original design criteria were for smaller ore particle size (P80 10 mm), longer bio-oxidation time (270 days), and ore stacking with radial stackers that would lessen heap compaction from haul trucks. The plant uses a mix of microorganisms including mesophilic A. ferrooxidans, L. ferrooxidans, moderately thermophilic Sulfobacillus species and thermophilic archaea Acidianus and Metallospheara species. A mix is required as pyrite oxidation increases areas within the heap to temperatures as high as 81oC, necessitating the use of thermophilic archaea as well as the mesophilic bacteria (Brierley 2001). The microbe mix is inoculated on the ore prior to stacking in the heap in order to provide uniform distribution of an active mix of the culture throughout the heap (Brierley 1994).

Future developments

“Much of the future of biomining is likely to be hot” (Rawlings 2002). Use of moderately thermophilic bacteria and thermophilic archaea is gaining significant attention for commercial applications. Billiton Process Research report benefits of moderate thermophiles for bioleaching of nickel concentrates. Pilot scale test work shows extreme thermophiles achieve efficient bioleaching of primary copper sulfide and nickel sulfide concentrates, giving higher recoveries than achieved by bioleaching with either mesophilic or moderate thermophilic cultures. BHP-Billiton is actively developing proprietary technologies for stirred-tank bioleach of chalcopyrite concentrates. BacTech/Mintek with Industrias Penoles of Mexico recently operated a stirred-tank bioleach pilot plant (170 m3 capacity) in Monterrey, Mexico using moderately thermophilic microorganisms. The project focused on a polymetallic (chalcopyrite, sphalerite, and galena) concentrate containing precious metals. Recoveries of 96–97% Cu, 99% Zn, 98–99% Au and 40% Ag were achieved at a feed rate of 2.7 t/day. Stable closed-circuit operation was maintained and 0.5 t/day of high purity copper cathode was produced. Bioleaching of “dirty” concentrates, with their high smelter penalty costs, represents some of the most attractive new applications for stirred reactor bioleaching. In addition to stirred reactors, bioleaching of copper from chalcopyrite ore in heaps using thermophiles will likely become a reality within the next few years.

The GEOCOAT process, developed by Geobiotics, Lakewood, Colorado, is a unique heap leach system for bio-oxidation pretreatment of refractory precious metal concentrates and bioleaching copper, zinc or nickel sulfide concentrates. Concentrates or finely ground ore are agglomerated onto coarse ore particles or inert substrates. The coated particles then bioleach in a heap configuration. A committee of the United States National Research Council (2002) states “…the application of biotechnology to the extraction and recovery of metals, is becoming an increasingly important hydrometallurgical processing tool.” The mining industries also recognize that biotechnology offers another tool for economic recovery of metal values. Future developments will expand the role of biotechnology in the extraction and recovery of many metal values.

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