Study of Bacterial Attachment During the Bioleaching of Pyrite, Chalcopyrite, and Sphalerite

In this study the degree of bacterial attachment of the main microorganisms involved in the bioleaching of metal sulfides and their influence on the dissolution rate and final metal extractions were determined. Three different mineral sulfides were bioleached: chalcopyrite (CuFeS 2 ), sphalerite (ZnS) and pyrite (FeS 2 ). A mixed culture of mesophilic bacteria ( Acidithiobacillus and Leptospirillum spp.) was tested at 35°C and thermophilic bacteria ( Sulfolobus spp.) were tested at 68°C. The results confirmed that a relationship exists between attachment and mineral dissolution rates. The bioleaching process can be divided into three stages. An initial stage with extensive bacterial attachment to the pyritic phase, which is of major importance in order to obtain high dissolution rates (since attached cells liberate Fe 2+ by contact bioleaching and oxidize Fe 2+ to Fe 3+ avoiding iron accumulation at the surface). In a second stage the bacterial attachment diminishes due to the saturation of the surface by the attached cells. This limitation, together with the presence of Fe 2+ in solution, produces an increase in the concentration of free cells, which use Fe 2+ as an energy source. Finally, in a third stage, a balance between free and attached cells is reached, giving rise to a cooperative mechanism. In this case, the attached cells attack the pyrite phase of the mineral generating Fe 2+ . This in turn is oxidized by the free cells in solution, regenerating the oxidizing agent (Fe 3+ ) for the indirect bioleaching of the mineral.