Monovalent cation concentrations determine the types of Fe(III) hydroxysulfate precipitates formed in bioleach solutions

Iron oxidation in bioleaching systems commonly produces mixtures of hydroxysulfate minerals. The purpose of this study was to characterize iron precipitates formed in liquid media inoculated with iron-oxidizing acidophiles (Acidithiobacillus ferrooxidans). Air-dried precipitates were characterized by X-ray diffraction (XRD), elemental analysis, specific surface area, color, and scanning electron microscopy. The Fe(III)-hydroxysulfates produced in this study included schwertmannite (Fe8O8(OH)6SO4) and various solid solution jarosites (K,Na,NH4,H3O)Fe3(SO4)2(OH)6). The precipitates were composed entirely of schwertmannite in the presence of insufficient concentrations of monovalent ions. The levels of NH4+, K+, and Na+ were determined for the formation of the corresponding jarosites. The levels varied depending on the jarosite type and were the lowest for K+ and highest for Na+. With natrojarosite especially, when synthesized with 500 mM Na+, the highest level tested in this study, the precipitate still contained poorly ordered schwertmannite as a major phase.