Effect of shear schedule on particle size, density, and structure during flocculation in stirred tanks

The effect of shear history on the evolution of the polystyrene-alum floc size, density, and structure is investigated by small-angle light scattering during cycled-shear and tapered-shear flocculation in a stirred tank using a Rushton impeller. First, various sampling schemes are experimentally evaluated. The floc structure is characterized by the mass fractal dimension, D-f, and the relative floc density. During turbulent shear flocculation, small floc structures are shown to be more open (D-f=2.1) than larger floc structures (D-f=2.5) as a result of shear-induced restructuring during steady state attainment. Flocs produced by cycled-shear flocculation are grown at shear rate G=50 s(-1) for 30 min, are fragmented at G(b)=100, 300, or 500 s(-1) for one minute, and then are regrown at G=50 s(-1). This shear schedule decreases the floc size but compacts the floc structure. When flocs are produced by gradual reduction of the shear rate from G=300 to 50 s(-1) (tapered-shear flocculation), smaller though equally dense flocs are produced compared with cycled-shear flocculation. The cycled-shear flocculation method produces the largest flocs with the highest potential for sedimentation when the fragmentation shear rate is G(b)=300 s(-1). (C) 1998 Elsevier Science S.A. All rights reserved.