The influence of alkali metal ions on homogeneous nucleation of Al(OH)(3) crystals from supersaturated caustic aluminate solutions

Homogeneous nucleation of Al(OH)(3) crystals from synthetic, optically clea r, caustic aluminate solutions and the influence of alkali metal ion (Nai v ersus Kt) have been investigated under isothermal, batch crystallization co nditions. The nucleation kinetics showed a seventh-order dependence upon AI (III) relative supersaturation and a strong temperature effect. Activation energy of 160 kJ mol(-1) and interfacial energy of 33 mJ m(-2) were estimat ed and found to be independent of alkali ion, as was the AI(OH)3 equilibriu m solubility. The nucleation rate, however, was faster in aging sodium than in potassium aluminate solutions. It appears that Na+ ions provide greater stability for the formation and densification of AI(III)containing, supram olecular clusters which grow more rapidly into Al(OH)(3) crystallites than do K+ ions. The development of the AI-OH octahedral structure of Al(OH)(3) nuclei is an alkali metal ion-mediated, chemical reaction-controlled conden sation process, displaying specific gibbsite (gamma-Al(OH)(3))-bayerite (al pha-Al(OH)(3)) dimorphism. Furthermore, significant differences in the leve l of alkali ion incorporation, reflecting in the purity and morphology of t he crystalline product, were observed. (C) 2000 Academic Press.