Recovery of zinc, nickel, cobalt and other metals by bioleaching

VIERA, M., POGLIANI, C., DONATI, E.

Microbial processing of metal sulphides

Springer

Año: 2007; p. 103 - 119

Bioleaching is currently an economical alternative for treating specific sulphidic ores. Commercial-size bioleaching processes are in operation for the recovery of copper from sulphide minerals especially in developing countries. Those operations treat copper ores by using heap-leaching technology; in addition, this technology has been used for the recovery of uranium. Bioleaching is also successfully applied in the pre-treatment (biooxidation) of refractory gold minerals. Since 1986, large-scale biooxidation plants have been operating in several places of the world. These plants are using tank-leaching technologies although there are some commercial applications which treat refractory gold ores in heaps.  Bioleaching may be used as an alternative process for the extraction and recovery of other metals like nickel, zinc, cobalt, and molybdenum among others. Such metals can occur in significant quantities in sulphidic ores or can be associated with them. Although, bioleaching could have the same advantages over other technologies for the recovery of those metals, as it occurs in the case of copper and gold, practically there are no commercial applications. Exceptions are a plant where cobalt is recovered from sulphidic tailings and some reports about the technical feasibility of the technology to be applied to zinc, nickel and cobalt concentrates (Brierley and Brierley, 2001; Acevedo, 2002; Harvey et al., 2002; Rawlings et al., 2003; Olson et al., 2003; Da Silva, 2004a; Clark et al., 2005; Morin et al., 2005). The extraction of base metals from sulphide ores is based mainly on the activity of acidophilic microorganisms. The most important are the iron- and/or sulphur-oxidising bacteria and archaea which grow autotrophically fixing CO2 from the atmosphere. A complete description of these microorganisms can be found in chapter 1. The susceptibility of sulphide minerals to be bioleached depends on different characteristics: crystal structure, solubility, electrochemical potential, etc. According to the widely accepted bioleaching mechanisms (Schippers and Sand 1997; Sand et al., 1999; Rohwerder et al., 2003; chapter 2 in this book), sulphides can be classified in two different groups. Acid-insoluble sulphides, like pyrite (FeS2), molybdenite (MoS2) or tungstenite (WS2) are attacked via the thiosulphate mechanism. Acid-soluble sulphides, like ZnS, CuFeS2, NiS, CoS or PbS are degraded via the polysulphide mechanism. Deeper information on the bioleaching mechanisms is presented in chapter 2. Conditions for leaching by using autotrophic microorganisms are really restrictive and extremely favorable for the dissolution of metal (especially the low pH) but the presence of high amounts of oxides and carbonates limits the use of those microorganisms. In addition, the extraction of metals from nonsulphidic minerals (like lateritic ores) can be catalyzed by fungus, yeasts or heterotrophic bacteria (Ehrlich, 2001) which can often tolerate high pH values. These microorganisms produce large amounts of organic acids such as citric, lactic, tartaric among others which can complex and mobilize metals from nonsulphidic solids. Among bacteria, members of the genus Bacillus are the most effective, while Aspergillus and Penicillium are the most important species among fungi (Viera and Donati, 2005). Several studies on the use of heterotrophic microorganisms on the leaching of different materials including metallic oxides, metallic ores, quartz sands and silicates, as well as solid residues have been published (Willscher and Bosecker, 2003). An excellent review on studies involved bioleaching and biobeneficiation of ores to recover metals like cobalt, nickel, zinc, molybdenum, manganese, silver, gallium, platinum group metals and uranium was written by Ehrlich (1997). It also includes some discussion about leaching with heterotrophic bacteria. In this chapter, we present an up-date of that review on the use of autotrophic and heterotrophic microorganisms for the bioleaching of base metals other than iron and copper. The bioleaching of zinc will be discussed in more detail and it will include our own results about bioleaching of zinc ores by using pure and mixed cultures of iron-oxidising and sulphur-oxidising microorganisms