An investigation of the mechanism of gibbsite nucleation using molecular modelling

Abstract

Two key experimental observations involving the crystallisation of gibbsite are: (1) in caustic aluminate solutions the main aluminium-containing species is the tetrahedral Al(OH)4− ion, whereas in gibbsite, crystallised from these solutions, aluminium is present with sixfold octahedral hydroxyl coordination; (2) precipitation is initiated and kinetics enhanced by dilution. In order to elucidate the mechanism(s) responsible for these observations, a semi-empirical quantum mechanical molecular modelling analysis of the heats of formation of a aluminium species has been performed. These calculations confirm the experimental finding that the Al(OH)4− monomer is the predominant (most stable) species in caustic aluminate solutions. The calculations suggest, however, that other monomeric species, for example Al(OH)3(H2O)2, Al(OH)3(H2O) and Al(OH)52− may be present in minor concentrations. On the basis of the heats of formation calculated for a series of dimers, trimers and tetramers, a mechanism for crystallisation has been proposed. The driving force for this crystallisation mechanism is provided by the freeing of bound (chelated) water molecules. The mechanism, in which water plays a catalytic role, provides a possible route for the change from fourfold hydroxyl coordination of aluminium in solution to sixfold coordination in gibbsite.