Understanding gibbsite particles interactions and agglomeration behaviour for improved bayer process crystallization

Abstract

Aluminium trihydroxide (-AlOH)3) or gibbsite crystallization in alumina (Al2O3) refineries is a pivotal step in the Bayer process for commercial production of alumina and aluminium metal from bauxite ores. Since the crystal growth kinetics in supersaturated Bayer liquors at elevated temperatures (60-90 oC) are notoriously slow, conditions conducive to rapid agglomeration of fine gibbsite crystals are sought for the production of coarse particles of commercial interest. This work focuses on studies performed to elucidate the reluctance of colloidal size gibbsite crystals to undergo rapid aggregation and agglomeration during crystallization. Seeded, isothermal batch crystallization of gibbsite from synthetic liquors were carried out in which the role of alkali metal ion (Na+ versus K+) and the incidental particle interactions were probed at 65 oC. Interactions between gibbsite particles dispersed in supersaturated sodium and potassium aluminate liquors were quantified in terms of temporal interparticle forces (by Colloid probe AFM) and dispersion rheology. The results show that both particle aggregation and agglomeration processes were faster in sodium than potassium aluminate liquors. Furthermore, strong repulsive forces which are not due to electrical double layer but structural or electro-steric interactions, initially exist between gibbsite surfaces, delaying the on-set of aggregation and agglomeration. With time, the interparticle repulsion attenuated and completely disappeared, followed by development of adhesive particle interactions. The particle interaction forces and the rates of dispersion thixotropic structure, shear yield stress and visco-elastic moduli development were faster in sodium than in potassium liquors, consistent with the agglomeration rates. The findings underscore the important role the non- crystallizing alkali metal ((Na+ versus K+) ion plays in the interfacial phenomena underpinning the Bayer process gibbsite agglomeration mechanism.