Using a model suspension of rice starch granules coagulated with alum, the paper describes an experimental investigation into the factors which control strength by the sweep floc mechanism. Electron micrographs, backed by electrophoresis measurements, indicated that the precipitate exists both as a positively charged adhesive coating and also in a mobile state, possibly similar to ‘free’ precipitate without significant charge. A comparison of strength measurements on flocs derived from rice starch with those based on low charged latex particles, led to the conclusion that coulombic attraction between the primary charges and shared coating in the form of an electrostatic bridge was the dominant bonding mechanism when the primary charge was significant. Rheological measurement on a suspension of the ‘free’ precipitate at a concentration likely to be encountered within the floc showed that it behaved as a Bingham plastic i.e with an identifiable yield stress at zero shear and contributes to cohesion. It is suggested that floc strength depends on an interplay between these two binding mechanisms, overdosing reducing electrostatic bridging and tending towards the weaker cohesion of the internal gel.

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