Bioleaching
Bioleaching is the leaching of sulphide mineral ores or concentrates using microorganism to increase the leaching mechanism through to catalyze the oxidation of sulphide minerals
Bioleaching is the extraction of a metal from sulfide ores or concentrates using materials found native to the environment; namely, water, air and microorganisms. In other words, bioleaching is the commercialization of the ability of certain bacteria, found in nature, to catalyze the oxidation of sulfide minerals. It is the leaching of sulfide minerals that distinguishes bioleaching from conventional acid leaching wherein only oxidized minerals are leached.
Bioleaching is used today in commercial operations to process ores of copper, nickel, cobalt, zinc and uranium, whereas, biooxidation is used in gold processing and coal desulfurization.
The original bacteria is used in bioleaching were those that were native to the site where the leaching will be accrued However, once bioleaching gained acceptance as a potentially fixed method for the recovery of metals from low-grade ores, flotation tailings and waste materials.
The development is occurred in this area.There are two dominant views on the mechanisms involved in bioleaching of copper sulphide minerals:
Indirect mechanism
The first mechanism is that the overall leaching process occurs by the microbial oxidation of ferrous to ferric ions followed by the chemical oxidation of the sulphide mineral by the ferric ion.
4Fe2++02+4H+=4Fe3++2H2O
2S+3O2+2H2O=2H2SO4
Direct mechanism
The second mechanism is that the microbial catalysis is involved of the overall dissolution of the mineral. It has been proposed that the microorganisms interact with the mineral directly, enhancing the rate of oxidation, over and above that achieved by chemical oxidation.
The complete reaction of chalcopyrite are:
4CuFeS 2 + 11O 2 + 6H 2O Thiobacillus ferrooxin = 4CuSO 4 + 4Fe(OH) 3 + 4S
2S + 3O 2 + 2H 2O Thiobacillus thiooxan = 2H 2SO4
Most sulphide mine waters contain the active autotrophic bacteria. Use of these mine waters for the Leach solution automatically provides the initial cultures of bacteria for leach systems .For optimum bacterial action the research is necessary to find the best condition such as pH, temperature and etc.