Characterization and alkaline decomposition-cyanidation kinetics of industrial ammonium jarosite in NaOH media

A complete characterization was carried out on a jarositic residue from the zinc industry. This residue consists of ammonium jarosite, with some conte nts of H3O+, Ag+, Pb2+, Na+ and K+ in the alkaline "sites" and, Cu2+ and Zn 2+ as a partial substitution of iron. The formula is: [Ag0.001Na0.07K0.02Pb 0.007(NH4)(0.59)(H3O)(0.31)]Fe-3(SO4)(2)(OH)(6). Some contents of franklini te (ZnO . Fe2O3), gunninguite (ZnSO4.H2O) and quartz were also detected. Th e jarosite is interconnected rhombohedral crystals of 1-2 mum, with a size distribution of particles of 2-100 mum. which could be described by the Ros in-Rammler model. The alkaline decomposition curves exhibit an induction period followed by a progressive conversion period; the experimental data are consistent with t he spherical particle with shrinking core model for chemical control. The a lkaline decomposition of the ammonium jarosite can be shown by the followin g stoichiometric formula: NH4Fe3(SO4)(2)(OH)(6(s)) + 3OH((aq))(-) --> (NH)(4(aq))(+) + 3Fe(OH)(3(s)) + 2SO(4(aq)r)(2-) The decomposition (NaOH) presents an order of reaction of 1.1 with respect to the [OH-] and an activation energy of 77 kJ mol(-1). In NaOH/CN- media, the process is of 0.8 order with respect to the OH- and 0.15 with respect t o the CN-. The activation energy was 46 kJ mol(-1). products obtained are a morphous. Franklinite was not affected during the decomposition process. Th e presence of this phase is indicative that the franklinite acted like a nu cleus during the ammonium jarosite precipitation. (C) 2001 Elsevier Science B.V. All rights reserved.