What to know about extremophiles used in the bioleaching of pyrite in stirred bioreactors?

Only one industrial installation in the world used bacterial leaching in stirred bioreactors for the extraction of another metal than gold. Pure example of “Green mining”, this plant “digested” one million tonnes of cobaltiferrous pyrite (FeS₂) stockpiled near the city of Kasese in the eastern part of Uganda. Cobalt that constituted the valuable metal to recover was solubilized as the result of the controlled microbial degradation of the pyrite. From the bioleach pregnant solution a relatively complex series of chemical operations could then lead to the production of pure cobalt in a directly marketable form.

Bioleaching of pyrite was easy and robust, operated by local personnel with no special qualification in biotechnological sciences. The main constraint was to keep a stable efficiency of the mixing and aeration system in 1,300 m³ bioreactors.

At the beginning of the story, in the early 1990’s, only two microbial species were isolated from the consortium used to induce a catalytic effect on the oxidation of the sulfide matrix trapping the cobalt. Further studies showed a much larger diversity and a relative versatility of the biological population growing in the bioreactors according to the operating conditions ensuring an optimal return of the commercial operation.

The communication aims at showing the progress in the knowledge of the micro-organisms used for the pyrite bioleaching in such an installation, what was useful from that knowledge to conduct an industrial operation and what remains to be acquired for improving the process.