Studies on the mechanism and kinetics of bioleaching

The use of off-gas analysis and redox potential measurement has shown that bioleaching involves at least three important sub-processes. The primary at tack of the sulphide mineral is a chemical ferric leach. The role of the ba cteria is to convert the iron from the ferrous to the ferric form, thereby maintaining a high redox potential. The kinetics of bacterial ferrous iron oxidation by Thiobacillus ferrooxida ns and a Leptospirillum-like bacterium, and the chemical ferric leach kinet ics of pyrite have both been described as functions of the ferric/ferrous-i ron ratio. Thus, the chemical ferric leach of the mineral and the bacterial oxidation of the ferrous iron are linked by the redox potential, and are i n equilibrium when the rate of iron turnover between the mineral and the ba cteria is balanced. These rate equations have been used to predict the steady state redox poten tial and sulfide mineral conversion in a continuous bioleach reactor. The m odel successfully predicts laboratory data and is being tested against data from pilot-plant and full-scale bioleach systems. Furthermore, the model p redicts which bacterial species will predominate and which mineral will be preferentially leached under specific operating conditions. Enzyme restrict ion analysis has shown that in pyrite-arsenopyrite bioleach reactors the do minant iron oxidizer is L. ferrooxidans, which is in agreement with the pre dictions of the model. (C) 1999 Published by Elsevier Science Ltd. All righ ts reserved.