Development and application of a constant supersaturation, semi-batch crystalliser for investigating gibbsite agglomeration

The development is described of a constant supersaturation crystalliser (CS C) designed primarily to investigate the size dependency of the agglomerati on kernel for the commercially important, but difficult to study, gibbsite- caustic aluminate system. The capabilities of the CSC were illustrated by a series of experiments conducted at supersaturation ratios of 1.67 and 1.86 , at 80 degreesC. The results supported findings reported in the literature that the gibbsite agglomeration rate kernel is size independent at shear r ates above 480 s (1) and has a strong inverse dependency on the mean shear rate. The results indicate that under these conditions the crystal collisio n processes are not rate determining for gibbsite agglomeration in turbulen t stirred tanks. The agglomeration kernel was shown to be independent of th e crystalliser's solids content. Other CSC results showed that gibbsite lin ear growth rates are: (1) independent of the average shear rates, which is consistent with the surface reaction controlled growth mechanism proposed i n the literature, and (2) independent of crystal size (i.e. McCabe DeltaL l aw). (C) 2001 Elsevier Science B.V. All rights reserved.