Kinetic model for the cyanidation of silver ammonium jarosite in NaOH medium

A synthesis of silver ammonium jarosite has been carried out obtaining a single-phase product with the formula: [(NH 4) 0.71(H 3O) 0.25Ag 0.040]Fe 2.85(SO 4) 2(OH) 5.50. The product consists on compact spherical aggregates of rhombohedral crystals. The nature and kinetics of alkaline decomposition and also of cyanidation have been determined. In both processes an induction period followed by a conversion period have been observed. During decomposition, the inverse of the induction period is proportional to [OH −] 0.75 and an apparent activation energy of 80 kJ mol − 1 was obtained; during the conversion period, the process is of 0.6 order (OH − concentration) and an activation energy of 60 kJ mol − 1 was obtained. During cyanidation, the inverse of the induction period is proportional to [CN −] 0.5 and an apparent activation energy of 54 kJ mol − 1 was obtained; during the conversion period the process is of 0 order (CN − concentration) and an activation energy of 52 kJ mol − 1 was obtained. Results obtained are consistent with the spherical particle model with decreasing core and chemical control, in the experimental conditions employed. For both processes and in the basis of the behaviour described, two mathematical models, including the induction and conversion periods, were established, that fits well with the experimental results obtained. Cyanidation rate of different jarosite materials in NaOH media have also been established: this reaction rate at 50 °C is very high for potassium jarosite, high and similar for argentojarosite and ammonium jarosite, lower for industrial ammonium jarosite and negligible for natural arsenical potassium jarosite and beudantite. These results confirm that the reaction rate of cyanidation decreases when the substitution level in the jarosite lattice increases.