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

Abstract

The development is described of a constant supersaturation crystalliser (CSC) designed primarily to investigate the size dependency of the agglomeration 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°C. The results supported findings reported in the literature that the gibbsite agglomeration rate kernel is size independent at shear rates above 480 s −1 and has a strong inverse dependency on the mean shear rate. The results indicate that under these conditions the crystal collision processes are not rate determining for gibbsite agglomeration in turbulent stirred tanks. The agglomeration kernel was shown to be independent of the crystalliser's solids content. Other CSC results showed that gibbsite linear growth rates are: (1) independent of the average shear rates, which is consistent with the surface reaction controlled growth mechanism proposed in the literature, and (2) independent of crystal size (i.e. McCabe Δ L law).