Continuous bioleaching of a pyrite concentrate in stirred reactors: Population dynamics and exopolysaccharide production vs. bioleaching performance

A continuous bioleaching operation was carried out in a laboratory-scale unit using a cobaltiferous pyrite concentrate. The objective was to investigate mechanisms of microbial activity and mineral oxidation for a better understanding and optimisation of biomining operations, particularly the ones using stirred tank technology. A combination of scientific and technical approaches (molecular ecology and biochemistry) was used and various key operating parameters were tested (temperature, nitrogen source, CO 2 availability). The bacterial strains that dominate the culture were present in the concentrate and originated from the deposit on the mining site. Whatever the operating conditions tested, the culture composition was very stable. The iron-oxidiser Leptospirillum ferriphilum BRGM1 was the dominant organism in standard (not limiting) conditions, and was always very well represented during the first 3–4 days of residence time. Sulfobacillus sp. BRGM2 also played a significant role in the process. The role and the presence of Acidithiobacillus caldus BRGM3 seemed of lesser importance. An increase in operating temperature from 35 °C to 45 °C had no major impact on bioleaching efficiency. When the nitrogen source was limiting, there was a negative impact on both bacterial growth and bioleaching efficiency. This was the result of a combination of factors such as less precipitate formation and decreased bacterial attachment to the pyrite surface. CO 2 limitation had a very significant negative effect on bacterial productivity and consequently on bioleaching efficiency. However, when CO 2 was a limiting factor, the population composition remained unchanged but a significant decrease in exopolysaccharide production was observed. This study gives new insights for the application of stirred tank technology to the processing of sulphide concentrates, and more specifically on the impact of key operating parameters on bioleaching performances, population dynamics and attachment of bacteria to the solid surfaces.