THE MUTUAL EFFECT OF MIXED THIOBACILLI AND LEPTOSPIRILLI POPULATIONS ON PYRITE BIOLEACHING

Although current bio-oxidation processes with mesophilic bacteria resu lt from the occurrence of mixed populations, the mutual effect of the various species has not been studied very extensively to date. Mixed c ultures made up of pure Thiobacillus ferrooxidans, Thiobacillus thioox idans and Leptospirillum ferrooxidans strains of the DSM collection we re batch tested for their ability to oxidize a cobaltiferous pyrite or e. The most efficient population for pyrite oxidation was composed of the three bacterial species. The influence of the relative abundance o f the different strains in the inoculum was studied. The cobalt solubi lization rate obtained with T. ferrooxidans increased when L. ferrooxi dans was present but was not affected by the initial concentration of L. ferrooxidans. The bioleaching with T. ferrooxidans was only improve d by adding T. thiooxidans when the initial concentration of T. thioox idans was higher than the initial concentration of T. ferrooxidans. Du ring continuous bioleaching of the cobaltiferous pyrite at 20% solids with a natural mesophilic mixed population, rod-shaped and Leptospiril lum-like bacteria were enumerated in the liquid phase. The 100 l biole aching unit is made up of 3 or 4 reactors arranged in cascade. The con centration of Leptospirillum-like organisms rose exponentially versus dissolved ferric iron, whereas the concentration of rod-shaped. bacter ia did not change from the value obtained in the first reactor, provid ing the solution contained less than 60 g.l(-1) ferric iron. At higher Fe3+ concentrations, the rod-shaped population in the liquid phase de creased. These results suggest that the rod-shaped bacteria performed the earlier steps of pyrite oxidation, whereas Leptospirillum-like org anisms participated in the later phase of bioleaching. The effluent fr om the last reactor was treated with CaCO3 in order to precipitate iro n. Recycling of partially neutralized bioleach solution to the feed se emed to increase the concentration of Leptospirillum-like bacteria in all reactors. Two hypotheses are proposed to explain this phenomenon: some bacteria may have been brought into the first tank with the recyc led liquid, or the cobalt concentration may have affected the distribu tion of Leptospirillum-like bacteria between liquid and solid phases. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.