The role of iron-oxidizing bacteria in stimulation or inhibition of chalcopyrite bioleaching

A series of bacterial and chemical leaching experiments were conducted to clarify contradictory reports in the literature regarding the role of bacteria in the bioleaching of chalcopyrite. Tests containing a high bacterial concentration showed inhibited leaching, even lower than non-inoculated controls. However, when bacterial cells were washed before inoculation, it was apparent that it was not the bacterial cells but rather the chemical species introduced with them that influenced the leaching rate. In addition, the results of comparative tests with 0.1 M ferrous sulphate or ferric sulphate showed that copper was leached from the ore 2.7 times faster in leach solutions containing ferrous ion, suggesting that ferric ions inhibit chalcopyrite dissolution. The results indicated that the chalcopyrite dissolution rate is strongly dependent on the reduction potential ( E h) in solution, and that this parameter is far more influential than the number or activity of bacterial cells. These results imply that the role of bacteria may only be stimulatory when the prevailing electrochemical conditions are also favourable.