Until recently, two types of bioreactors were used for bioleaching: the stirred tank reactor (STR), borrowed from chemical engineering, and the so-called Pachuca tank airlift reactor (ALR), borrowed from hydrometallurgy. These reactors perform very satisfactorily in the fields for which they were developed, but they have a number of major shortcomings when used for bioleaching. These drawbacks stem from the stressful conditions that the mechanics and fluid dynamics of STRs andALRs create for the microorganisms. A rotating barrel batch bioreactor, designed, built and tested in the authors' laboratory in the 1990s, was considered to be the most attractive option for incorporating into a continuous machine to overcome these drawbacks. This paper describes a novel type of continuous bioreactor that was tailored to suit certain physiological requirements of bioleaching microorganisms, established by recent microbiological research. The original batch reactor was modified to enable integration with the continuous machine. With the continuous bioreactor, 4 g/L of net volume per hour of a 40% solids mineral suspension, consisting of a sulfide flotation concentrate that was predominantly pyrite, was completely leached in 24 hours. The final leach liquor contained concentrations of as much as 53 g/L of ferric ion, complexed with fragments of extracellular polymeric substance (EPS). Just three units arranged in series are sufficient to ensure complete leaching of the feed. The improved performance of this innovative bioreactor vis-a-vis STRs andALRs, is interpreted on the grounds of experimental evidence and of the most recent advances in bacterial physiology.
