Effect of impeller type and position in a batch cooling crystallizer on the growth of borax decahydrate crystals

The effects of impeller type and position in a batch cooling crystallizer on the crystal growth kinetic as well as the shape, agglomeration and crystal size distribution of disodium tetraborate decahydrate (borax) were investigated in detail. Three types of impellers applied in this work generated completely different fluid flow patterns in the crystallizer allowing investigation of the influence of the liquid flow on the kinetic parameters as well. The ratio of impeller off-bottom clearance, C, to the liquid height, H, was varied from 0.1 to 0.5 for all impeller types used. Results indicate that the impeller type and off-bottom clearance influence the overall crystal growth rate and the crystal size distribution of the final product. It was also found that agglomeration of crystals produced was less pronounced as the radial fluid flow in the crystallizer became more dominant. In that system borax crystals tend to be smaller, but more regularly shaped.

► The effects of impeller type and impeller position on the crystal growth of borax were investigated.
► Impeller type and off-bottom clearance influence the overall crystal growth rate as well as crystal size distribution.
► Growth rates of borax were controlled by combination of diffusion and surface-integration mechanism at all impeller positions examined.
► The agglomeration is less pronounced as radial flow in the crystallizer becomes more dominant.