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 si ze and structure during shear induced flocculation oi polystyrene part icles with aluminum sulfate is investigated by image analysis, One rad ial Bow (six-blade Rushton turbine) and two axial flow (three-blade fl uid foil, four-blade 45 degrees pitch) impeller configurations are exa mined, The steady state average floc size is shown to depend on the fr equency of recirculation, io the impeller zone and its characteristic velocity gradient. The concepts of fractal geometry are used to charac terize the floc structure, For all impellers, the two-dimensional floc fractal dimension, D-pf, increases during floc growth, indicating for mation of more open structures. Later on, D-pf levels off at a steady slate value as breakage becomes significant and the floc size distribu tion approaches steady state, The sheer rate does not affect the stead y state D-pf of the flocs within experimental uncertainty. (C) 1996 Ac ademic Press, Inc.