Aggregate size distributions in flocculation

Abstract

The evolution of aggregate size distributions resulting from flocculation under a variety of conditions has been investigated using a light scattering technique. By measuring floc size distributions it is possible to distinguish clearly between initial floc formation and growth after the flocs have been formed. Coagulation of dispersed suspensions, under conditions of vigorous agitation, by charge reduction or electrical double layer compression generally produces unimodal size distributions which shift progressively to coarser sizes. Low molecular weight polymers produce size distributions similar to those obtained with salts but with somewhat coarser limiting floc sizes. Flocculation of stable dispersions by high molecular weight polymers gives rise to bimodal size distributions comprising a fine mode consisting of the primary particles and a coarse mode made up of large flocs. Polymer addition leads initially to the formation of some flocs. Flocculation then proceeds by a continuous reduction in the quantity of residual primary particles accompanied by an increase in both the amount and size of the flocs. Complete elimination of the fine mode, corresponding to incorporation of all primary particles into flocs, generally requires high polymer dosage (about 10 mg l−1 or more for the system studied here). In contrast, the addition of high molecular weight polymers to unstable (pre-coagulated) suspensions causes rapid floc growth at low polymer dosage. (less than 2 mg l−1), with essentially complete incorporation of the primary particles into the flocs.