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. Plants, in particular legumes, evolved a way to use them millions of years ago. They have nodules on their roots that are filled with bacteria capable of converting nitrides into soluble nitrates.
These are then absorbed by the roots of the plant and used to make proteins and other organic molecules. Another place in which bacteria, that can release energy by breaking up metal compounds, support higher organisms is at hydrothermal vents on the sea floor. These entire ecosystems take their energy, indirectly, from the bacteria breaking down metal sulphides, in the plumes of superheated water, to metal sulphates. These bacteria are preyed upon by other, larger micro-organisms and these in turn, by invertebrates, and fish.
It is the only ecosystem on earth that does not take sunlight as its primary source of energy. The earliest record of the practice of leaching is from Spain in AD 50. Water was allowed to percolate through the rocks and the resulting CuSO4 solution was collected in amphorae and evaporated. Because the process of electrolysis had not been invented the copper was retrieved from this solution by cementation (precipitation) (6) The process of heap leaching was started in the 1700s with layers of crushed ore and burning wood alternated on an impervious slope.
Water was sprayed on top and, because of the heat, the copper and iron sulphides oxidised and became soluble sulphates, which were leached out by the water. Obviously, this process evolved huge clouds of SO2 gas and it was banned for this reason in 1888. But in one single mine, the Rio Tinto mine in Spain, the process appeared to work just as well without the roasting step and continued right up to the depletion of the ore in 1970. 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 of Bioleaching has many advantages over more traditional ore refining techniques. It uses no components that aren’t naturally occurring (ore, air, water and micro-organisms) so danger from spillages is minimal. The equipment (stirred tanks and ore grinders) is easy to set up and maintain. It also takes place at low pressure and temperature so is not dangerous. The amount of hazard to the people operating the equipment is lower than other mining processes because Bioleaching is low dust, SO2 and noise emitting and finally the simple fact that Bioleaching can easily handle ores, which have a very low concentration of metal in them.
Bioleaching does have disadvantages. Firstly, even using the newest thermophillic bacteria, the process is very slow compared to smelting, for example. Also, depending on the exact variant of the process used it may be possible for H+ ions to leach into the surface water and turn this acidic, in turn acidifying the water table slightly. This leaching can also happen with some of the heavy metals being extracted causing “Yellow-Boy” pollution. (2) Such as this in Colorado: Work is being done to minimise the disadvantages of Bioleaching.
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.