Incorporation of zinc in continuous jarosite precipitation

Ammonium jarosite was precipitated in a 1.5 l continuous Mixed Suspension Mixed Product Removal crystallizer in the presence of varying feed Zn²⁺, Mg²⁺, Li⁺ and NH⁺₄ concentrations. The feed Fe³⁺ concentration and SO^2-₄ concentration were kept constant at 0.25 M and 1.6 M, respectively. The experiments were carried out at 95°C and pH 1.5, using residence times varying between 0.8 and 2.0 h. The following parameters were determined: steady state conversion, the jarosite crystal growth rate, the nucleation rate, the specific surface area, the solids density in the suspension, the production rate, the supersaturation and the incorporation of zinc or magnesium in the jarosite lattice. The residence time distribution of the system was determined by adding alunite as tracer. Additional experiments were carried out at a pH of 1.2 and 1.8 respectively, in order to determine the influence of the pH on the precipitation. Based on the results obtained the incorporation of zinc in jarosite or the jarosite production rate may be predicted for a wide range of process conditions. The incorporation of zinc in ammonium jarosite, probably via isomorphous substitution, is about 2-10 g/kg depending on the process conditions. This implies that in industrial practice the loss of zinc due to the removal of iron as jarosite ranges between 0.12 and 0.6 wt%, for a zinc concentrate containing 52 wt% zinc and 8 wt% iron.