Bacterial Leaching Process

As early as the days of the Romans, the action of water-entrained oxygen on metallic sulphides has been recognized and used, particularly on copper-iron minerals. As it turns out, a microscopic organism known as thiobacillus ferroxidans is responsible for this action. This one-celled bacterium breaks down the sulphide minerals and generates weak sulphuric and suiphurous acids, which take the metal into solution as sulphates and sulphites, often leaving a precipitate of iron.

This bacterial leaching process is now used deliberately, mainly to recover copper and uranium from mine drainage waters and from surface waters percolating through old mine umps and waste heaps.

The phenomenon of bacterial leaching has given rise to an active field of research and is producing economic benefits in the heap leaching of low-grade copper ores which are cheaply mined in open pits, particularly in the southwestern U.S.

In the low-grade, underground uranium mines, blasted ore was left in the stopes and leached with bacteria-rich solutions in situ. This reduced production costs considerably since only the uranium-bearing solution was pumped to surface, instead of tonnes of ore.

Another area where bacterial leaching is being evaluated is in the treatment of refractory gold ores, in which the gold occurs as sub-microscopic particles inside the suiphide minerals. Bacterial oxidation destroys the suiphide grains, freeing the gold for subsequent cyanidation.