EFFECT OF SHEAR ON THE STRENGTH OF POLYMER-INDUCED FLOCS

Micromechanics was used to show that the rupture strength of polymer-i nduced flocs varied with the hydrodynamic conditions at which the floc s were formed, A maximum flee strength at an intermediate shear rate w as observed, The overall performance of the polymeric flocculants, as determined by two independent methods, showed that the conditions for optimal flocculation did not coincide with those for maximum flee stre ngth. The amount of flocculation was explained in terms of the competi ng effects of the particle collision frequency and the destructive hyd rodynamic forces, The contributing forces to the flee strength are, ho wever, more likely to be densification of the flee by shear and the we akening of reattachment strengths, The evidence presented here may be useful for explaining flocculation data that depart from the constant yield stress theory. Thus, the notion of the flee strength varying wit h shear rate may offer an alternative to multilevel flee structure mod els in the description of flocculation kinetics. (C) 1997 Academic Pre ss.