Biohydrometallurgical iron oxidation and precipitation: Part II — Jarosite precipitate characterisation and acid recovery by conversion to hematite

• Iron precipitated in two-stage CSTR system predominantly as jarosite
• K-jarosite was preferentially formed over NH4+, Na+ and H3O+ jarosites.
• Cu and Ni contents in precipitates were below 0.2% and 0.02%, respectively.
• Precipitates showed good filterability and settling characteristics.
• Jarosite was readily converted to hematite and acid by autoclaving at 225 °C.

This study reports the characterisation of precipitates formed in a two-stage continuous stirred tank bioreactor (CSTR) system operated for continuous iron oxidation and removal from high strength iron liquor. The system was operated at ambient temperature without biomass carrier or neutralising agent addition. The elemental composition of the precipitates, for each of the influent pH solutions (pH 1.1–2.2), contained varying amounts (%w/w) of Fe (29.2–34.0%), S (11.8–13.2%), K (0.92–5.67%), N (0.10–0.77%), Na (0.055–0.74%), Cu (0.044–0.15%) and Ni (0.002–0.016%). Importantly, the process resulted in efficient precipitation of ferric iron and sulphate with only minor co-precipitation losses of Cu and Ni, making it suitable for use in base metal process flow sheets. The precipitates were predominantly (75–99%) jarosite, with potassium jarosite formation occurring preferentially over that of hydronium, ammonium and sodium jarosite. The jarosite precipitate showed good filterability and settling characteristics which are important for larger scale industrial processes. The jarosite precipitates could be readily converted to hematite and acid by autoclaving at 225 °C. The physical and chemical characterisation of jarosite, determined in this study, is relevant in the design and deployment of iron oxidation, precipitation and residue treatment processes in hydrometallurgical flow sheets.