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 studie d at various shear rates (63-129 s(-1)) and aluminum sulfate, Al-2(SO4 )(3) 16H(2)O, flocculant concentrations. The competition between coagu lation and fragmentation during shear-induced flocculation determined the equilibrium or steady state particle (floc) structure and size dis tribution. The evolution of the hoc structure with time was monitored by image analysis of digitized flee images. The average hoc structure became less open or irregular as the flee size distribution attained s teady state as a result of shear-induced breakage/restructuring. At hi gh alum (flocculant) concentrations, the steady state flee size distri bution appeared to be self-preserving with respect to shear rate. In c ontrast, at lower flocculant concentrations, the steady state hoc size distribution narrowed with increasing shear rate as the large tail of the distribution was pushed to smaller particle sizes by shear-induce d fragmentation. Copyright (C) 1996 Elsevier Science Ltd.