On the kinetics and mechanism of the dissolution of pyrite in the presenceof Thiobacillus ferrooxidans

In this paper, the results of a study of the leaching of pyrite in the pres ence and absence of bacteria at the same solution conditions are summarised , These results indicate that the leaching of pyrite occurs at a higher rat e in the presence of bacteria than in the absence of bacteria. Analysis of the rate of reaction as a function of concentration indicates that the orde r of reaction with respect to ferric ions is the same in the presence and t he absence of bacteria. However, the order of reaction with respect to H+ i s -0.5 in the absence of bacteria, and -0.39 in the presence of bacteria. T he results of a study of the mixed potential of pyrite are also presented. These results indicate that the mixed potential of pyrite decreases with ti me in the presence of bacteria, while it is constant in the absence of bact eria. A detailed mechanism of the leaching of pyrite in ferric sulphate sol utions is presented, and this theory is used to interpret the results. It i s argued that the effect of the bacteria on the rate of leaching and on the mixed potential is not consistent with the direct contact mechanism of bac teria leaching. Instead, it is shown that the results can be explained by a n increase in the pH at the mineral surface as a result of bacterial activi ty. This means that the increase in the leaching rate is a result of the in direct contact mechanism. (C) 2001 Published by Elsevier Science B.V.