Influence of dissolved oxygen on the bioleaching efficiency of a mesophile to moderate thermophile consortium under oxygen enriched atmosphere

摘要

The use of oxygen is a well-known practice in high-temperature bioleaching reactors whereas air is usually preferred in medium and low-temperature operations due to economic constraints. Under high-sulfide loading conditions, as it is the case with high-grade metal sulfide concentrates, the microbial and chemical demand for oxygen is significantly increased during the bioleaching process, which requires the injection of large amounts of air and thus increases the energy costs of the process. Sparging with oxygen enriched gas instead of air may offer an interesting alternative process option to improve gas transfer in the bioleaching reactor and to provide an adequate oxygen supply in order to satisfy the oxygen demand. However the use of such conditions can lead to much higher dissolved oxygen (DO) concentrations than those encountered with air sparging. Very few papers have been devoted to the study of the optimal range of DO concentrations for bioleaching processes. However most of them reported an inhibitory effect of DO concentrations above 5 ppm. The purpose of this study was to investigate the influence of DO on the bioleaching efficiency under oxygen-enriched atmosphere in 21L stirred tank reactor at 42°C. Bioleaching experiments were performed in continuous mode on a sulfide-rich tailings wastes composed mainly of pyrite (51%) and quartz using the "BRGM-KCC" bacterial consortia. The solid load was closed to 20% (w/w). The DO concentration in the reactor was varied between 4 and 17 ppm. For a DO ranging from 4 to 13 ppm the results obtained show a good bacterial oxidizing activity, no inhibitory effect of the DO was observed and the sulfide dissolution increased with the DO concentration. It is assumed that this improvement of the bioleaching efficiency was linked to an increase of the oxygen transfer rate from the gas phase to the liquid phase rather than a direct effect of the DO level. When the DO concentration reached 17 ppm a significant decrease of the microbial activity and consequently of the oxygen consumption was noticed. This results show that there is a critical value above which the DO concentration is detrimental to the activity of the bioleach microorganisms present in the "BRGM-KCC" consortia but this value is much higher than the one usually mentioned in the literature.