Alkaline decomposition-cyanidation kinetics of argentojarosite

Abstract

The cyanidation of argentojarosite in alkaline medium in the range 25–60°C and 5–30 m M NaCN, is a two-step process in series: a step of alkaline decomposition that controls the overall process, followed by a fast step of complexation of the decomposition products. The alkaline decomposition is characterized by the removal of sulfate ions from the lattice and the formation of a gel consisting of iron and silver hydroxides, which evolves to silver ferrite in the absence of cyanides. The decomposition curves show an induction period, followed by a conversion period. The induction period is independent of particle size and decreases exponentially with increasing temperature. The conversion period is characterized by the creation of a reaction halo of hydroxides around an unreacted jarosite core. The kinetic data are consistent with chemical control of the process. The rate expression obtained in NaOH media and Ca(OH) 2 media for [OH −] ≤ 1 m M is: r 0 / V[1-(1-x) 1 3 ]=59.1 exp (−42,000/RT) [OH −] 1 2 t . In Ca(OH) 2 the resulting expression for [OH −] > 1 m M is: r 0 / V[1-(1-x) 1 3 ]=2.34 exp (−42,000/RT) [OH −] 0 t .