The separate roles of shear rate and mixing on gibbsite precipitation

Methods are presented for quantifying the shear rate and mixing during labo ratory precipitation. These are used to investigate the separate effects of shear rate and mixing on gibbsite precipitation from caustic aluminate sol utions. Experiments were performed under conditions where the "chemistry" ( i.e. the liquor composition, temperature and seed) and the shear rates were the same but the "mixing" was different. Precipitation experiments were co nducted in both the laminar and turbulent flow regimes. The laminar flow re gime experiments were performed in a Taylor-Couette precipitator, whereas t he turbulent regime experiments were performed in a stirred tank. It was ob served that in a high-shear, turbulent, well-mixed, stirred precipitator th e effect of mixing on the product CSD is negligible compared to the effect of mean shear rate. This suggests that, unlike many precipitation systems f or which the kinetics estimates are affected by micromixing effects, it sho uld be possible to estimate the underlying gibbsite kinetics, at least in h igh-shear stirred tank experiments. The turbulent and laminar flow regime e xperiments both suggest that mixing does not significantly affect the desup ersaturation kinetics. (C) 2001 Elsevier Science Ltd. All rights reserved.