SIMULTANEOUS DISSOLUTION OF ZINC FERRITE AND PRECIPITATION OF AMMONIUM JAROSITE

Zinc ferrite (ZnO.Fe2O3) was converted, in a single-step seeded batch process, into ammonium jarosite (NH4Fe3(SO4)2(OH)6) at 95-degrees-C and at a pH which was kept constant at either 1.7 or 1.95. The conversion consists of the dissolution of zinc ferrite, which could be described by a surface reaction controlled shrinking core model, and the simultaneous precipitation of ammonium jarosite, which is also a surface-reaction-controlled process. The supersaturation during conversion for the precipitation of jarosite was calculated from the solution composition. At pH 1.7 virtually 100% conversion of zinc ferrite is achieved. This leads to a residual zinc content of the solids produced of below 0.1 wi%. X-ray diffraction analyses of the product showed correspondingly small amounts of zinc ferrite. This indicates that the zinc incorporation in the jarosite lattice is practically negligible. The advantages of such a controlled conversion process for industrial practice are discussed.