Alkaline decomposition–cyanidation kinetics of argentian natrojarosite

The cyanidation of argentian natrojarosite in alkaline medium shows an induction period, followed by a conversion period. The induction period decreases exponentially with increasing temperature and [OH −]. The conversion period is characterized by the unreacted core model, the reaction halo is formed by amorphous hydroxides and the core by jarosite. The kinetic data are consistent with chemical control of the process. In the experimental conditions employed, the order of reaction is 0.4 with respect to [OH −] with an activation energy of 96 kJ mol −1 in NaOH medium. In Ca(OH) 2 medium the order of reaction with respect to [OH −] is 0.5 and the activation energy is 39.6 kJ mol −1. The alkaline decomposition–cyanidation of argentian natrojarosite, as for argentojarosite and plumbojarosite, is a two-step process in series: a slow step of alkaline decomposition, followed by a fast step of complexation of the silver from the decomposition solids. The addition of soluble salts such as NaCl or Na 2SO 4 in the reaction system increases the reaction rate by a factor higher than 2 in the experimental conditions employed.