The separate roles of shear rate and mixing on gibbsite precipitation

Abstract

Methods are presented for quantifying the shear rate and mixing during laboratory precipitation. These are used to investigate the separate effects of shear rate and mixing on gibbsite precipitation from caustic aluminate solutions. 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 conducted in both the laminar and turbulent flow regimes. The laminar flow regime experiments were performed in a Taylor–Couette precipitator, whereas the turbulent regime experiments were performed in a stirred tank. It was observed that in a high-shear, turbulent, well-mixed, stirred precipitator the effect of mixing on the product CSD is negligible compared to the effect of mean shear rate. This suggests that, unlike many precipitation systems for which the kinetics estimates are affected by micromixing effects, it should be possible to estimate the underlying gibbsite kinetics, at least in high-shear stirred tank experiments. The turbulent and laminar flow regime experiments both suggest that mixing does not significantly affect the desupersaturation kinetics.