The mutual effect of mixed Thiobacilli and Leptospirilli populations on pyrite bioleaching

Although current bio-oxidation processes with mesophilic bacteria result from the occurrence of mixed populations, the mutual effect of the various species has not been studied very extensively to date. Mixed cultures made up of pure #Thiobacillus ferrooxidans, #Thiobacillus thiooxidans and #Leptospirillum ferrooxidans strains of the DSM collection were batch tested for their ability to oxidize a cobaltiferous pyrite ore. The most efficient population for pyrite oxidation was composed of the three bacterial species. The influence of the relative abundance of the different strains in the inoculum was studied. The cobalt solubilization rate obtained with #T. ferrooxidans increased when #L. ferrooxidans was present but was not affected by the initial concentration of #L. ferrooxidans. The bioleaching with #T. ferrooxidans was only improved by adding #T. thiooxidans when the initial concentration of #T. thiooxidans was higher than the initial concentration of #T. ferrooxidans. During continuous bioleaching of the cobaltiferous pyrite at 20% solids with a natural mesophilic mixed population, rod-shaped and #Leptospirillum-like bacteria were enumerated in the liquid phase. The 100 l bioleaching unit is made up of 3 or 4 reactors arranged in cascade. The concentration of #Leptospirillum-like organisms rose exponentially versus dissolved ferric iron, whereas the concentration of rod-shaped bacteria did not change from the value obtained in the first reactor, providing the solution contained less than 60 g/l ferric iron. At higher Fe3+ concentrations, the rod-shaped bacteria performed the earlier steps of pyrite oxidation, whereas #Leptospirillum$-like organisms participated in the later phase of bioleaching. The effluent from the last reactor was treated with CaCO3 in order to precipitate iron... (D'après résumé d'auteur