Kinetic study on the precipitation of alunite from manganese sulphate solutions

Abstract

The kinetics of precipitating alunite (KAl 3 (SO 4) 2(OH) 6) from manganese sulphate solutions containing 0.2 g/l potassium was evaluated at different temperatures (80–95°C), controlled solution pH's (2.5–3.5) and aluminium/potassium (Al/K) molar ratios ( 3 1 6 1 and 9 1 ). The experimental data fit a second-order kinetic model which shows two regions of precipitation with rate constant dependent on the precipitation parameters. At the stoichiometric Al/K ratio of 3, the variation of rate constants with temperatures from 80–95°C yields activation energies of 58.5 and 43.2 kj/mol for the initial induction stage and the subsequent crystal growth respectively. At Al/K molar ratios of 6 and 9, the activation energies are 110 and 132 kJ/mol for the two stages of precipitation. Seeding the solutions with alunite crystals before the precipitation (at 5 and 10 g/l) decreases the induction period and improves the kinetics significantly. An increase of sulphate concentration also improves the crystal growth rate.