THE SEPARATE ROLES OF SHEAR RATE AND MIXING ON GIBBSITE PRECIPITATION

摘要

The separate effects of shear rate and mixing were investigated by precipitation experiments performed under conditions where the “chemistry” (i.e. the liquor composition, temperature and seed) and the shear rates were kept the same but the “mixing” was different. Two quantitative parameters of mixing are presented. One is an axial dispersion coefficient and the other a stretching index. These precipitation experiments were conducted in both the laminar flow regime and in the turbulent flow regime. 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 the flow patterns and mixing on the product CSD is negligible compared to the effect of mean shear rate. These findings suggest that, unlike many precipitation systems for which the 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 precipitation yield.