Abstract: Under isothermal batch crystallization condition, unseeded nucleation of Al(OH)₃crystals from supersaturated caustic aluminate solution was studied by in situ conductivity and absorbance measurements. The nucleation kinetics of Al(OH)₃crystals were obtained and the influence of H₂O and Na⁺on the unseeded nucleation were investigated. The experimental results indicate that, when the molar ratio of Na₂O to Al₂O₃is fixed in Na₂O-Al₂O₃-H₂O system. The crystallization of Gibbsite display two kinds of nucleation mechanism with variation of H₂O content. When Na₂O in the crystallization system is substituted by K₂O, the induction period of the crystallization is prolonged, but the active energy and the reaction order are all found to be independent of the alkali ion. It can be concluded that, the nucleation of Al(OH)₃crystals in supersaturated alminate solution is a chemical reaction controlled process, H₂O and Na⁺take part in the reconstruction of alminate ions though they are not present in the overall-reaction of Al(OH)₃crystallization. H₂Ois related to the key procedure of the decomposition of supersaturated sodium alminate solution. Yet, Na⁺has little effects on the controlling-step.

Key words: Sodium aluminate solutions, Al(OH)₃, Unseeded nucleation, Crystallization kinetic