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.
