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Abstract

The mechanism of floe formation in kaolin-polyacrylamide systems and the subsequent floe disruption in the course of continued agitation were investigated by using two stirring systems, one based on a magnetic stirrer and the other on an impeller stirrer. For similar turbulent conditions in the impeller zone, the energy input of the magnetic stirrer was 16 watt/m3 compared with 1 watt/m3 for the impeller stirrer. This difference in energy input resulted in differences in floc formation and disruption which were manifested by differences in supernatant turbidities, levels of absorption of flocculants, and the size and shape of the flocs. Thus, energy input appeared to be more important in floc disruption than the degree of turbulence. In the kaolin-polyacrylamide systems studied, the adsorption of flocculant increased with agitation time and an upward inflection in the curve was noted at higher doses of flocculant. When the flocculant was totally adsorbed, floc disruption proceeded rapidly.

Because of the nonequilibrium nature of focculant adsorption, kinetic factors related to agitation dominate the adsorption behavior. Thus, flocculation by polymer bridging cannot be considered independently of the agitation conditions imposed on the system, especially if the extent of polymer adsorption is being determined. For example, short periods of moderate agitation may produce a well-flocculated suspension, whereas a longer period of agitation (or higher energy input) may produce a suspension of relatively higher turbidity and higher flocculant adsorption. In comparative studies of flocculation, therefore, the conditions of agitation (which may be described readily for simple vessels and impellers) must be clearly specified.

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