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Bacterial Mining

Bacterial mining, or bio-mining, represents the use of microorganisms to leach metals from ores or mine tailings (wastes), followed by the subsequent recovery of metals of interest from the leaching solution. The term has also been applied to the recovery of oil from reservoirs where the reservoir energy has been depleted over time. This could well be a process of the future for recovering energy from alternate sources that have been difficult-toimpossible to reach or recover and develop.

The use of microbes to extract metals from ores is simply the harnessing of a natural process for commercial purposes. Microbes have participated in the deposition and solubilization of heavy metals in the earth’s crust since geologically ancient times. Most of this activity is linked to the iron and sulfur cycles. Anaerobic sulfate reducing bacteria generate sulfides that can react with a variety of metals to form insoluble metal sulfides. There are two main types of processes for commercial-scale microbially assisted metal recovery. These are irrigation-type and stirred tank-type processes. Bio-mining involves a chemical process called leaching which is actually oxidation reaction and maybe called bio-oxidation. Bio-leaching processes can be carried out at a range of temperatures, and as would be expected, the iron- and sulfur-oxidizing microbes present differ depending on the temperature ranges. In mineral bio-oxidation processes that operate at 40°C or less, the most important microorganisms are believed to be a consortium of gram-negative bacteria such as Acidithiobacillus ferrooxidans. Microorganisms that dominate bio-leaching at 50°C (122°F) include Acidithiobacillus caldus and some Leptospirillum spp. At temperatures greater than 65°C (149°F), bio-mining microbial consortia are dominated by archaea rather than bacteria with species of Sulfolobus and Metallophores being most prominent.

Thus, biomining has developed into a successful and expanding area of biotechnology and the process employs microbial consortia that are dominated by acidophilic, autotrophic iron- or sulfur-oxidizing prokaryotes. Mineral bio-oxidation takes place in highly aerated, continuous-flow, stirred-tank reactors or in irrigated dump or heap reactors, both of which provide an open, non-sterile environment. Continuous-flow, stirred tanks are characterized by homogeneous and constant growth conditions where the selection is for rapid growth, and consequently, tank consortia tend to be dominated by two or three species of microorganisms. In contrast, heap reactors provide highly heterogeneous growth environments that change with the age of the heap, and these tend to be colonized by a much greater variety of microorganisms. Heap microorganisms grow as biofilms that are not subject to washout, and the major challenge is to provide sufficient biodiversity for optimum performance throughout the life of a heap.

Currently, a variety of biotechnological processes are being given serious consideration as options to the more conventional recovery methods for energy production. During the energy crisis that commenced in the 1970s, a bioleaching process was applied to oil shale in the United States in order to produce shale oil. Sulfur is actually introduced to the fractured oil shale blocks, and Thiobailli thiooxidans is used to generate a large amount of 0.1 N sulfuric acid to remove carbonate minerals. With Green River oil shale, 43% of the carbonates can be removed so that a more porous oil shale rock will remain. More oil can be produced from this treatment method due to the improved heat transfer efficiency upon retorting.