Abstract
Homogeneous nucleation of Al(OH)3 crystals from synthetic, optically clear, caustic aluminate solutions and the influence of alkali metal ion (Na+ versus K+) have been investigated under isothermal, batch crystallization conditions. The nucleation kinetics showed a seventh-order dependence upon Al(III) relative supersaturation and a strong temperature effect. Activation energy of 160 kJ mol−1 and interfacial energy of 33 mJ m−2 were estimated and found to be independent of alkali ion, as was the Al(OH)3 equilibrium 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 Al(III)-containing, supramolecular clusters which grow more rapidly into Al(OH)3 crystallites than do K+ ions. The development of the Al–OH octahedral structure of Al(OH)3 nuclei is an alkali metal ion-mediated, chemical reaction-controlled condensation process, displaying specific gibbsite (γ-Al(OH)3)–bayerite (α-Al(OH)3) dimorphism. Furthermore, significant differences in the level of alkali ion incorporation, reflecting in the purity and morphology of the crystalline product, were observed.
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