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 microsco py were successfully employed to probe aging, supersaturated sodium alumina te solutions and follow the nucleation and growth of aluminium hydroxide (A l(OH)(3)) crystallites. Unseeded, batch crystallization experiments were pe rformed at 65 degrees C and solution ratios of NaOH/Al=1.37 with NaOH conce ntration, [NaOH], in the range of 4.0-6.0 M. Colloidal size polyaluminate s tructures were observed in cryo-TEM micrographs of vitrified, aging solutio ns. The latter appeared optically-clear. The evolution of these microstruct ures, followed as a function of time, indicated that they went through syst ematic 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 nucl eation behavior of unseeded, supersaturated sodium aluminate solutions, whi ch initially contain Al(OH)(4)(-) species. (C) 1999 Elsevier Science B.V. A ll rights reserved.