Bacterial Leaching

The action of water-entrained oxygen on metallic sulphides has been recognized and used, particularly on copper-iron minerals dating back as early as the days of the Roman Empire,. 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 sulphurous acids, which take the metal into solution as sulphites and sulphates, often leaving a precipitate of iron.

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

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 southwestem United States.

In the low-grade, underground uranium mines of Elliot Lake, Ontario, blasted ore was left in the stopes and leached with rich solutions of bacteria 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 sulphide minerals. The sulphide grains are destroyed by bacterial oxidation, liberating the gold for subsequent cyanidation.