Heavy Metal Mining Using Microbes

The use of microorganisms to extract metals from ore involves the harnessing of a natural process in an industrial context. Biomining (a general term referring to microbially enhanced leaching or oxidation of metals) has some distinct advantages over traditional methods, in that it is more environmentally friendly. It does not have the high energy requirements of smelting and does not produce harmful gaseous emissions. The wastes produced from biomining operations are less biologically and chemically active than traditional mine tailings because it has already been bioleached. Microbial leaching of ore bodies in situ, in stockpiles, or in mine tailings also makes metal recovery feasible from low-grade ores in which the metal is not economically recoverable by smelting. For example, biooxidation of refractory gold-bearing ores allows this low-grade ore with as little as 1 g of gold (Au) per metric ton, which would otherwise be considered as waste, to be processed at a cost in range of U.S. $4-6 per metric ton (Rawlings, 2002). Biomining is commonly used for recovery of Au and uranium (U), but copper (Cu) is recovered in by far the greatest quantities. Ferric sulfides usually accompany the Cu sulfides in ore and hence Fe transformations are components of the reaction chain. Ferrous iron is formed in the ore body by reaction of ferric iron with CuS:

The mobilized Cu is recovered from the leachate by sedimentation, solvent action, or electrolysis. The Fe2+ remaining after Cu removal is reoxidized (see the previous equation). Also, sulfur-oxidizing bacteria convert S0 to sulfuric acid:

This acid fortifies the leaching solution, which is recirculated following the metal harvest.

The organisms involved in biomining are acidolithic, chemolithoautotrophic, able to use ferrous iron or reduced S or both as electron donors, and able to fix CO2. Among important species in ore leaching are the following: Acidithiobacillus ferrooxidans, T. thiooxidans, Leptospirillum ferrooxidans, Sulfobacillus thermo-sulfidooxidans, and Acidianus brierleyi. A properly aerated ore body or suspension of iron- and S-containing mineral in water is an ideal environment for these organisms: air provides the C source (CO2) and preferred electron acceptor (O2), the mineral ores supply the electron donors, and water is the medium for growth. Inoculation of the ore body with microorganisms is usually unnecessary. Normally occurring Fe and S oxidizers and acidophilic heterotrophs establish themselves unaided because the acidic environment is not suitable for the growth of other organisms.

Several commercial-scale biomining processors are operating around the world (Rawlings, 2002). The two main types are irrigation-type and stirred-tank processes. Irrigation processes involve the percolation of leaching solutions through crushed ore that has been stacked in heaps or dumps or irrigation of an ore body in situ without bringing the ore to the surface. Stirred-tank-type processors are large (1000-2000 m3) tanks arranged in series and operated in continuous flow mode, in which the feed being added to the first tank overflows from tank to tank until the process is sufficiently complete. The stirred-tank operations provide a step up in rate and efficiency of biooxidation, but are expensive to construct and operate. Their use is therefore restricted to the pretreatment of high-value gold-bearing arsenopyrite concentrate. Most current biomining processes are operated at ambient temperatures or in tanks at 40-50°C. Warmer operations are currently being developed, which will accelerate the process and make it more economical, but will also require a better understanding of the thermophilic organisms involved.

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