Alunite is a potential resource for production of alumina and potassium sulfate.Hydrometallurgy is the conventional process employed for this purpose and direct leaching in KOH is suitable as one of the process steps because it does not require prior calcination. In this paper dissolution kinetics of pure natural alunite in KOH is described. Kinetics of dissolution of alunite in potassium hydroxide follows the shrinking core model. The rate of reaction is controlled by surface chemical reaction step and the order of reaction with respect to KOH is 1.327. The activation energy of this reaction was estimated as 94.18kJ/mol and the following kinetic model for alunite dissolution in potassium hydroxide was obtained:

In this study, dehydroxylation of alunite was studied. On the basis of thermogravimetric results and sulfur loss, dehydroxylation of alunite occurs from 508 to 577 °C. In this temperature range, temperature and time, as well as their interaction, have the greatest effect on alunite conversion (dehydroxylation) with 99.85% contribution, while the contribution of particle size was only 0.11%. Dehydroxylation increased with time up to a maximum and reached a plateau beyond that time. The time required (t max) for maximum dehydroxylation (i.e., maximum conversion) for various temperatures was different and showed decreasing trend with increasing temperature. Kinetic studies below t max showed that dehydroxylation of alunite follows Avrami–Erofeev models with different reaction indexes. In the temperature range of 508–534 °C, dehydroxylation occurs according to the model α = e((−1.77×1011e–23314/T )(t–0.9r 1/3))1.5 , while in the 534–575 °C temperature range, the α = e((−11065.64e–9924/T )(t–0.213r 1/3) is applicable.