An experimental study of the long-term bioleaching of large sphalerite ore particles in a circulating fluid fixed-bed reactor

Abstract

Although the bioleaching of sulphide ores is well studied, the relatively coarse particle size typical of heap and dump leach operations, presents unique challenges. Leaching from such large particles, especially over long-term process, is poorly understood. In this study, three large particle size classes (+23/−25, +14/−16, +5.25/−6.75mm) were prepared from a sphalerite ore from the Northern Cape, South Africa, by two different methods of comminution (HPGR and cone crusher) and packed into leach reactors, which were operated continuously and well mixed through internal circulation of leach solution. Each reactor was inoculated by a mixed culture of mesophilic acidophilic chemolithotrophs (1.86×1010cell/kg ore) in which L. ferriphilum was predominant. This culture had been adapted to the sphalerite ore sample previously. Media containing 1g/L ferrous iron were used as continuous feed to the reactors. The pH, redox potential, Fe+3 and Fe+2 concentration as well as total Fe, Zn, Mg, Al and planktonic cell concentrations in the effluent solution were measured regularly. The reactors were stopped from time to time to investigate the progress of leaching and also the colonisation of the surface of the ore particles by sessile cells was investigated. Planktonic and sessile microbial populations were monitored by quantitative real time polymerase chain reaction (qRT PCR). A comparison of the effect of the different comminution devices on metal extraction indicated that HPGR crushed ore leached more rapidly in all particle size classes and showed 10 to 15% additional zinc leach extraction. On average, larger particles showed a greater difference in the cell population densities (HPGR crushed ore has more cell population) than the finer particles, and L. ferriphilum continued to dominate the microbial population over the course of leaching.