Mechanistic study of the pyrite-solution interface during the oxidative bacterial dissolution of pyrite (FeS2) by using electrochemical techniques.
Résumé
The beneficiation of mineral substances through biotechnology forms the objective and basis of the present study of the pyrite bacterial leaching (bioleaching) process. The use of ferrous and sulfide oxidizing bacteria in bioleaching processes is a recent technique that has found industrial applications in copper, uranium, cobalt and gold extraction. To pursue its application and extend it to the upgrading of other ores, its technical and economic viability must be continually demonstrated and optimized through credible technological innovations in terms of scale-up and better control of biochemical activity. This is what this work aims to achieve by improving our knowledge of the intimate mechanisms governing the action of microorganisms during pyrite bacterial oxidative dissolution. The substrate used is formed of finely ground pyrite. The cultures (classical nutritive medium without ferrous iron) are batch-prepared and kept at a temperature of 35°C, agitated and aerated. The bacterial population used comprises three species: Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans. Specific equipment was developed and adapted for the study, including the design of special pyrite electrodes and the use of electrochemical methods for corrosion and interface investigations. These tools served to identify and monitor the electrochemical reactions 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 key electrochemical parameters to the presence of Fe2+ and Fe3+ ions in solution. The electrochemical behavior of pyrite during bioleaching was studied by continuous easurement of certain electrochemical parameters in different situations, both natural and induced. The overall chemical process of pyrite bioleaching was determined and subdivided into distinct elementary stages. The key factors of each elementary stage and their respective roles were identified. This made it possible, for each stage, to differentiate the electrochemical reactions occurring in solution and at the interfaces, which, when combined, lead to overall reactions that advance the bioleaching of pyrite. The results completely contradict the theory of direct dissolution of the pyrite by the bacteria and indicate that (i) the ferric ion hence remains the only powerful oxidant of pyrite during its bacterial leaching, (ii) the only 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.