Shear-induced flocculation: The evolution of floc structure and the shape of the size distribution at steady state

The flocculation of polystyrene particles in a stirred tank was studied at various shear rates (63–129 s−1) and aluminum sulfate, Al2(SO4)3 16H2O, flocculant concentrations. The competition between coagulation and fragmentation during shear-induced flocculation determined the equilibrium or steady state particle (floc) structure and size distribution. The evolution of the floc structure with time was monitored by image analysis of digitized floc images. The average floc structure became less open or irregular as the floc size distribution attained steady state as a result of shear-induced breakage/restructuring. At high alum (flocculant) concentrations, the steady state floc size distribution appeared to be self-preserving with respect to shear rate. In contrast, at lower flocculant concentrations, the steady state floc size distribution narrowed with increasing shear rate as the large tail of the distribution was pushed to smaller particle sizes by shear-induced fragmentation.