Dissolution Kinetics of Crystalline and Thermally Decomposed Gibbsite, Boehmite and Bauxite in Sodium Hydroxide Solution

Results of a study comparing the dissolution of crystalline and thermally decomposed (i.e. amorphous) gibbsite, boehmite and bauxite in sodium hydroxide solution at 50, 70 and 90 C are presented. Analysis of reaction solids and solutions at various time intervals has shown the dissolution rate of each amorphous and crystalline material to vary with solution temperature, particle size and geometric surface area. In all instances, dissolution was sigmoidal in nature, being rapid initially, and slowing thereafter. For amorphous materials, this behaviour varied in accordance with calcination temperature of the crystalline precursor. Reactivity diminished with increasing time and temperature of calcination, yet under optimal calcination conditions, alumina was extracted into solution at concentrations above the equilibrium solubility limit of both boehmite and gibbsite. Results are attributed to structure and surface reactivity differences between crystalline gibbsite and boehmite, and the thermally decomposed forms of alumina obtained therefrom.