The mechanism of `direct and `indirect bacterial oxidation of sulphide minerals

The chemical oxidation rate of pyrite with ferric iron increases with increasing ferric-to-ferrous iron concentration ratios in the bulk-phase of the solution. In batch experiments, these high ratios were due to the presence of Leptospirillum ferrooxidans bacteria. It was proposed that the observed maximum chemical pyrite oxidation rate (mole pyrite oxidised/mole pyrite s), is determined by the mass transfer rate of chemically produced ferrous iron from the pyrite surface to the bulk-phase. The overall kinetics of chemical pyrite oxidation at pseudo-steadystates in these batch experiments are appropriately described by rate equations for the three independent sub-processes involved: (i) a linear equation that describes the chemical pyrite oxidation kinetics at the pyrite surface (i.e. production rate of ferrous iron) in terms of the ferric iron concentration in the bulk-phase, and the surface concentration of ferrous iron, (ii) mass transfer of ferrous iron from the pyrite surface to the bulk of the solution, where the solid¯liquid mass transfer coefficient of ferrous iron, kL(m/s), was estimated using a Sherwood correlation, and (iii) bacterial oxidation of ferrous iron by L. ferrooxidans.