The effect of impeller type on floc size and structure during shear-induced flocculation

The effect of impeller type and shear rate on the evolution of floc size and structure during shear induced flocculation oi polystyrene particles with aluminum sulfate is investigated by image analysis, One radial Bow (six-blade Rushton turbine) and two axial flow (three-blade fluid foil, four-blade 45 degrees pitch) impeller configurations are examined, The steady state average floc size is shown to depend on the frequency of recirculation, io the impeller zone and its characteristic velocity gradient. The concepts of fractal geometry are used to characterize the floc structure, For all impellers, the two-dimensional floc fractal dimension, D-pf, increases during floc growth, indicating formation of more open structures. Later on, D-pf levels off at a steady slate value as breakage becomes significant and the floc size distribution approaches steady state, The sheer rate does not affect the steady state D-pf of the flocs within experimental uncertainty. (C) 1996 Academic Press, Inc.