Studies on recrystallised aluminium mono-hydroxide precipitates. Kinetics of dissolution by sodium hydroxide solutions

Abstract

Boehmite powders were prepared by controlled hydrolysis of aluminium isopropoxide solution and recrystallisation in hot water; these were cylinders (fibres) of lengths (I 0) = 0.06 to 0.10μm. The dissolution of dilute suspensions of these powders in well-stirred sodium hydroxide solutions was studied at 35 ° to 65 °C. Reaction solid and solution were analysed after different times by chemical and physical methods. Reaction occurred by three-directional dissolution of the cylinders: the reactions of the first (non-aggregated) thirty percent material were four-third order w. r. t. powder weight (and second order w. r. t. powder surface area). The initial rate constantsk wi (g−″ hr−1) for reactions with sodium hydroxide of unit mean ionic activity at 20 °C varied from 0.012 to 0.040. Rate constants increased linearly with the mean ionic activity of the hydroxide solution and exponentially with reciprocal temperature, five-six times for 15 °C temperature rise. Energies of activation varied from 115–125 kJ mole−1.