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Prokaryotes and Man - Bioleaching

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Introduction

Prokaryotes and Man Bioleaching G.J.E.Dickens Introduction When extracting metals from deposits, physical methods of mining can only go so far before the yield of pure metal relative to the amount of ore extracted renders the process uneconomical. Under such circumstances, a method called leaching is employed. This involves pumping hot water or acid through a vat containing pulverised low-grade ore and allowing the soluble metal compounds to dissolve in it, before recovering the metal from this solution by electrolysis. However, not all metal compounds are soluble, when this is the case then a slightly different process called Bioleaching is employed. This process makes use of some bacteria's ability to convert insoluble metal compounds (like sulphides) directly into ions in solution. A culture of bacteria in hot water is passed into the vat. The metal is then recovered from this solution exactly like the other leaching methods. (7) Using bacteria to make valuable compounds soluble is nothing new. ...read more.

Middle

Many other mines tried to mirror the non-roasting process without any success and numerous theories were put forward as to why the leaching only worked at Rio Tinto. Today we know that it was because of the presence of a bacterium, Thiobacillus ferro-oxidans, in the water used by the miners. However, the existence of these bacteria and their chemical abilities was discovered in 1941 and the process of Bioleaching as we know it was started soon after in 1946. The only significant change in the procedure since then has been the discovery of micro-organisms that live in hot springs that can operate at higher temperatures and, therefore, faster. (3) The process is relatively simple compared to some chemical processes used to extract metals from their ores. Here is a list of the steps: 1. Low-grade ore extracted physically from the mine or quarry. 2. Ore pulverised and placed into a large thermally insulated vat. 3. Hot (50-70 degrees Celsius) water containing culture of bacteria added. Most commonly, the bacteria used are T. ferro-oxidans or T. thio-oxidans. ...read more.

Conclusion

Firstly, the rate of the procedure can be increased if it can be carried out at higher temperatures because of the increased thermal kinetics. To do this new species of bacteria are being engineered to withstand higher and higher temperatures. The geneticists working on these took genes from bacteria, which were found to be living in hot springs, to incorporate into the genetic loop of the Thiobacillus species. This will create a type of bacteria that can live at high temperatures and still make the insoluble compounds soluble. Secondly the introduction of a second species to break apart some of the other chemicals in the ore (intentionally the ones that hold the rock together) means that it doesn't have to be so finely pulverised. This is more economical. (1) Finally, there is research being undertaken to try and isolate and replicate the chemical process that takes place at the membrane of the bacteria. If this can be done the temperature of the reaction (and therefore the rate) can be increased since there will be no bacteria that need to be kept alive. ...read more.

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