Cyanidation kinetics of argentian jarosite in alkaline media

Abstract

The cyanidation of argentian potassium jarosite in alkaline media is characterized by an induction period, followed by a conversion period, as was observed in other synthetic jarosites. The induction period shows an apparent order of 0.7 (NaOH) and 0.2 [Ca(OH) 2] with respect to the [OH −], and an apparent activation energy of 86 kJ mol −1 (NaOH) and 36 kJ mol −1 [Ca(OH) 2]. The conversion period is characterized by the presence of a reaction front with an unreacted jarosite core. The decomposition products are amorphous. The process is chemically controlled in both media. In NaOH–CN −, the order of reaction is 0.6 with respect to the [OH −] and the activation energy is 43 kJ mol −1. In Ca(OH) 2–CN −, the order of reaction with respect to [OH −] is 0.5 and the activation energy is 80 kJ mol −1. The order of reaction with respect to the [CN −] concentration is zero in both media. The process is faster than that observed in natural arsenical potassium jarosite from gossan ores (Rio Tinto, Spain). The addition of potassium chloride to the reaction system increases the reaction rate; the order of reaction with respect to the [Cl −] was 0.13.