A model of flocculation

Abstract

The phenomenon of flocculation in liquid suspensions has a variety of applications, including mineral processing, treatment of industrial effluents, and municipal sewage sludge purification. Obtaining metals from ores would not be possible without the slurry processing route. Flocculants play here a double role: limiting environmental contamination and also slowing down depletion of raw materials — including potable, industrial and agricultural water. Development of better flocculants requires improved understanding of the mechanism of their action. We propose a model of flocculation based on the assumption that effective flocculants pervade large volumes of liquids in the suspensions. Since many flocculants are polymers, good flocculants according to the model should have large radii of gyration R G. We therefore assume a connection between R G and settling velocities y of particles out of suspension. Four different types of aqueous suspensions are studied, containing in turn silica, coal, Mn ore and Fe ore. A unique relationship is demonstrated between R G and y for several polymeric flocculants in each type of slurry. For each suspension type the corresponding equation has the form y = aR G b , where the two parameters characterize the suspended particles and the liquid medium. While polysaccharides from natural sources are used as flocculants, we show how their cationization enhances the flocculation efficiency in all media we have studied. Large effects are achieved for the flocculating agent concentrations in the range of 6–9 ppm.