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Recovery of Metals - Microbial ore leaching consists of the processes involving bacteria, mainly T. ferrooxidans that lead to leaching of metal sulphates from low grade ores containing insoluble metal sulphides. The metal solubilizing action of the bacteria is both direct as well as indirect. In direct conversion, the metal sulphide is itself converted into metal sulphate as follows.

In contrast, indirect conversion is based on the reduction of ferric sulphate [Fe2 S04h] to ferrous sulphate (FeS04); in this reaction, metal sulphate (CUS04) and elemental sulphur (S) are produced as follows.

T. ferrooxidans also oxidises sulphur to H2S04 and converts FeS04 to Fe2 (S04)3, both reactions providing energy used by the bacterium.
The process involving T. ferrooxidans is being used since 1960s in Canada for uranium recovery and since 1970s in South Africa for recovery of gold. In USA and some other countries it is being used mainly for the recovery of copper; about 10-20% of the world copper supply is derived by this process.

The use of Thiobacillus for copper recovery has resulted in about 2-3-fold reduction in copper prices, and has eliminated the release of SO2 into the atmosphere; this process is briefly outlined below.

1. The ore is collected in a huge pile, preferably on a surface that is impermeable to the resulting metal sulphate solution.
2. The pile is periodically sprayed with a leaching solution having ferric ions [as Fe2(S04)3] and H2SO4. H2SO4 maintains a low pH conducive for leaching. This solution slowly percolates through the ore pile.
3. Copper is released into the leaching solution as CUS04 by the chemical reaction (indirect conversion of Cu S). Bacteria present in the ore will also convert Cu S directly into CUS04.
4. The leaching solution ultimately reaches the bottom of pile and is drained to a copper recovery plant; the leachate may contain about 1-3 g Cull of leachate. The leachate is passed over scrap iron; this produces iron sulphate and elemental copper; the latter forms a precipitate and is easily recovered.

5. The solution remaining after copper recovery is pumped into a regeneration tank/pond. The bacterium T. ferrooxidans oxidises FeS04 into Fe2(S04h; aeration may be provided to facilitate this conversion. Additional H2SO4 may be added to maintain low pH. The leaching solution thus becomes recharged and ready for reuse.

Alternatively, holes may be blasted into the mine and the leaching solution is pumped into these holes. After a period of time, the solution is pumped out and copper is extracted from the leachate; the remaining solution is recharged and reused. This is called in situ ore leaching.
 

 

 
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