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

Abstract

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.