The adaptation of Thiobacillus ferrooxidans for the treatment of nickel–iron sulphide concentrates

Thiobacillus ferrooxidans was successfully adapted to copper–nickel–iron and pyrrhotite concentrates. Bacterial leaching in a culture medium at initial pH 2 resulted in increased solubilisation of the metals for both concentrates. Two strains, designated NatTf and ConTf, both showed similar overall % metal solubilisation in the order Ni>Fe>Cu. Extraction corresponded to the growth phase but the leaching rate for ConTf exceeded that for NatTf during the exponential phase. However, during the stationary phase the rate for ConTf was much lower than for NatTf. X-ray diffraction indicated the early production of elemental sulphur and reduced iron compounds in leach residues from ConTf limiting the rate. No evidence was found for direct attachment leaching. The order of oxidation of the minerals was pyrrhotite>sphalerite>chalcopyrite>violarite>pentlandite>pyrite because oxidation of Fe(II) to Fe(III) led to galvanic interactions where Fe(III) oxidises minerals with lower mixed-potential. Leaching models indicated a mixed kinetic regime with initial film diffusion control and subsequent shrinking-core product layer diffusion control.