Studies on recrystallised aluminium hydroxide precipitates

Abstract

Aluminium hydroxide gels were recrystallised in high pH solution for 2–1000 h to give a series of Gibbsite powdersP11/2,P11/8,P1/48,P11/360 andP11/X, consisting of hexagonal platelet crystals; their average platelet lengths (l 0) were 0.03, 0.06, 0.13, 0.26 and 0.45 microns. The dissolution of dilute suspensions of these powders in well-stirred sodium hydroxide solutions were studied at 20–65°C Reaction solid and solution were analysed after different times by chemical and physical methods. Reaction occurred by two-directional dissolution of the platelet crystals. The reactions of GibbsiteP11/X (prepared by prolonged 1000 h crystallisation) were first order w.r.t. powder weight (and second order w.r.t. powder effective surface area); the reactions of the other Gibbsites were first order for the first twenty percent rapid dissolution and then slower. The initial rate constantsk w1 (h−1) and half-lifest 0.5 (h) for reactions ofP11/2,P11/8,P11/48.P11/360 andP11/X with sodium hydroxide solution of unit mean ionic activity at 20°C were 1.70, 0.9; 0.60, 2.2; 0.22, 5.5; 0.07, 14 and 0.02, 38 respectively. Rate constants increased linearly with the mean ionic activity of the hydroxide solution and increased exponentially with reciprocal absolute temperature, four to five times for 15°C temperature rise. Half-lifes decreased in a similar manner. Energies of activation varied from 76–83 kJ/mol. The rates of dissolution are determined by the rate of the chemical reaction between adsorbed hydroxyl ions and adjacent reactive aluminium hydroxide sites on the powder crystal surface.