The formation of Al(OH)3 crystals from supersaturated sodium aluminate solutions revealed by cryovitrification-transmission electron microscopy

The techniques of cryovitrification and cryo-transmission electron microscopy were successfully employed to probe aging, supersaturated sodium aluminate solutions and follow the nucleation and growth of aluminium hydroxide (Al(OH)(3)) crystallites. Unseeded, batch crystallization experiments were performed at 65 degrees C and solution ratios of NaOH/Al=1.37 with NaOH concentration, [NaOH], in the range of 4.0-6.0 M. Colloidal size polyaluminate structures were observed in cryo-TEM micrographs of vitrified, aging solutions. The latter appeared optically-clear. The evolution of these microstructures, followed as a function of time, indicated that they went through systematic increases in number, density and size prior to the appearance of Al(OH)(3) crystallites. The observation of the sequential formation of low and high density Alcontaining clusters which subsequently grow into colloidal size Al(OH)(3) crystallites, provides the first visual evidence of the nucleation behavior of unseeded, supersaturated sodium aluminate solutions, which initially contain Al(OH)(4)(-) species. (C) 1999 Elsevier Science B.V. All rights reserved.