CO2 mass transfer in bioleaching reactors: CO2 enrichment applied to a complex copper concentrate

Abstract : In bioleaching processes using autotrophic bacteria, carbon dioxide (CO2) is the carbon source for the growth of the microorganisms and its availability is dependent on gas mass transfer. The objective of this study was to investigate the demand in CO2 in complex sulfidic copper (Cu) concentrate bioleaching operations and to optimise CO2 supply. Batch tests in 2 l-stirred reactors at 10%w/v solid concentration were performed to study the need for CO2-enrichment and to determine the adequate CO2 partial pressure in the gas inlet. The results show that ferrous iron (Fe(II)) oxidation, and thus microbial activity, is delayed when air is injected without CO2-enrichment; the carbonates present in the solid are not sufficient to provide the CO2 required for the activity of Fe oxidising bacteria. CO2-enrichment improves leaching kinetics since the copper dissolution rate increases from 84 mg L−1 h−1 with air solely to 120 mg L−1 h−1 when CO2 is added to air. A CO2 enrichment influences both the composition of the bacterial community and the abundance of the bioleaching species. This study proposes also a methodology to determine gas/liquid transfer components and to assess CO2 limitations in the system. It shows that the microorganisms are not only sensitive to the transfer rate of CO2 from the gas to the liquid phase, but also to the availability of CO2 in solution.