Mechanistic study of the pyrite-solution interface during the oxidative bacterial dissolution of pyrite (FeS2) by using electrochemical techniques

The beneficiation of mineral substances through biotechnology forms the obj ective and basis of the present study of the pyrite bacterial leaching (bio leaching) process. The use of ferrous and sulfide oxidizing bacteria in bio leaching processes is a recent technique that has found industrial applicat ions in copper, uranium, cobalt and gold extraction. To pursue its applicat ion and extend it to the upgrading of other ores, its technical and economi c viability must be continually demonstrated and optimized through credible technological innovations in terms of scale-up and better control of bioch emical activity. This is what this work aims to achieve by improving our kn owledge of the intimate mechanisms governing the action of microorganisms d uring pyrite bacterial oxidative dissolution. The substrate used is formed of finely ground pyrite. The cultures (classic al nutritive medium without ferrous iron) are batch-prepared and kept at a temperature of 35 degreesC. agitated and aerated. The bacterial population used comprises three species: Thiobacillus ferrooxidans. Thiobacillus thioo xidans and Leptospirillum ferrooxidans. Specific equipment was developed an d adapted for the study, including the design of special pyrite electrodes and the use of electrochemical methods for corrosion and interface investig ations. These tools served to identify and monitor the electrochemical reac tions occurring during bioleaching, both in solution and on the surface of the pyrite electrodes. The work consisted in relating the observations of the changes in certain k ey electrochemical parameters to the presence of Fe2+ and Fe3+ ions in solu tion. The electrochemical behavior of pyrite during bioleaching was studied by continuous measurement of certain electrochemical parameters in differe nt situations, both natural and induced. The overall chemical process of pyrite bioleaching was determined and subdi vided into distinct elementary stages. The key factors of each elementary s tage and their respective roles were identified, This made it possible, for each stage, to differentiate the electrochemical reactions occurring in so lution and at the interfaces. which, when combined, lead to overall reactio ns that advance the bioleaching of pyrite. The results completely contradict the theory of direct dissolution of the p yrite by the bacteria and indicate that (i) the ferric ion hence remains th e only powerful oxidant of pyrite during its bacterial leaching, (ii) the o nly role of the bacteria is (re)generation of ferric ions in solution, and (iii) the process of bacterial adhesion or contact or attack no longer has the same meaning as was hitherto attributed. (C) 2001 Elsevier Science B.V. All rights reserved.