|
|
|
|
Extraction
of
Metals - In 1947, for the first time, U.S. microbiologists discovered that copper was leached from its ores due to the activity of a bacterium, Thiobacillus ferrooxidans. These bacteria are 'acidophilic autotrophs', meaning that they are able to live solely on sulphides and thrive in an acidic environment.
In recent years, it has been shown that T. ferrooxidans and T. thioxidans both can be used for extraction of copper and uranium from insoluble minerals by either of the following two activities:
(i) by redox processes at the mineral surfaces :
(ii) biosorption of metals on bacterial cell surfaces.
|
These bacteria, as their names indicate, .live by oxidizing the sulphur that binds copper, zinc, lead and uranium into their respective sulphide minerals. This releases the valuable metal. In contrast to this biometallurgy, the conventional metallurgy involves smelting ores at high temperature, a process which is far more polluting and energy intensive approach.
In view of this, biometallurgy is an approach which is environment friendly and therefore its use should be encouraged. Us use also needs to be encouraged to extract metal from low grade ores as done in U.S.A., since conventional metallurgy requires high grade ores, which are gradually becoming exhausted.
|
Biohydrometallurgy was first used in 1970, in Canada for extraction of uranium and in South Africa for extraction of gold. In 1980s, when copper industry was in great difficulty, different firms started looking for technology, which may be more cost-effective and less polluting.
This technology involving Thiobacillus is now responsible for 30 per cent of the copper produced in U.S.A., and for 10 - 20 per cent of copper supply on world basis.
This technology also led to
(i) reduction in cost of production of copper from 60 - 90 US dollars to only 30 US dollars per pound and
(ii) elimination of the release of SO2 in the atmosphere, which is estimated at the level of two tonnes of SO2 from one tonne of copper smelted.
|
|
The bacteria like T. ferredoxins can also be used for removing sulphur from coal to reduce thc emission of SO2 from coal fired power stations. This will bring enormous environment benefits for fast developing industry in India and China.
Biohydrometallurgy of copper involves the following steps:
(i) the low grade ore and tailings, left from any earlier conventional mining, are piled up in an area, where the ground has been made permeable;
(ii) the pile is sprayed with a leaching solution that contains iron in the form of the Fe3+ ion, sulphate ions, (SO4)2- and T. ferrooxidans,' the sulphate ions in the leaching solution form sulphuric acid giving a suitable acidic solution essential for growth and activity of bacteria;
|
|
|
(iii) copper, released due to catalytic chemical reaction (facilitated by T. ferrooxidans), is drained in the form of a solution;
(iv) the metal (copper) is then removed from the solution with the help of another solvent;
(v) thc remaining leaching solution flows into an open pond, where T. ferredoxins catalyses a reaction that oxidases Fe2 + into Fe + ions and sulphide into sulphate ions, so that the leaching solution is now recharged;
(vi) the recharged leaching solution is pumped back to the top of the pile for the cycle to begin again;
|
(vii) thc copper is eventually extracted from the solution in
(viii) above as sheet’s through electrowinning process by passing electricity, so that copper is collected on negative terminals; this process is costly and non biological, but 'bioabsorption filters' such as algae will be used in future to make the process entirely biological.
Biohydrometallurgy is also used in mining. For instance, in Arizona (USA), in a mine five holes were created by detonating an explosive underground. A mixture of acidic solution and T. ferrooxidans is pumped down the central hole, where bacteria do their work.
The resulting solution, rich in valuable copper, is pumped from the other four holes, processed and recycled. However, mining by biological means is much slower and takes decades, where conventional methods take a few months/years. If both methods are compared, biological method (though cheaper) delays cash flow due to slower production and therefore hindered its adoption. Research is, therefore, in progress to increase the speed as is being attempted at School of Biotechnology, University of New South Wales (Australia).
Some progress using heterotrophic (not autotrophic like T. ferrooxidans) bacteria for extraction of manganese has been made. Genetically engineered bacteria may he used in future to increase the speed of reaction. Metals can also be separated from waterways, polluted with heavy metals, using algae and bacteria. These microbes oxidise dissolved manganese which may be separated through process of bioabsorption mentioned earlier. More progress will by made in this area in future.
| |
