Permeability, flocculated suspensions

Polymer flocculants are now used in a wide range of applications (1). Early interest in these materials was largely based on their ability to improve the dewatering rates (i.e. to increase the permeability) of suspensions. 

The structure of the suspension and the compression rheological properties determine much of the consolidation behaviour. Colloidally stable, dilute suspensions of monodisperse spherical particles are well described by the relationships described above. The effect of the shape of the particles and the particle concentration can be accounted for by multiplying the expression given in equation (9.22) by suitable factors. For flocculated suspensions, the situation is much more complex. The attractive interparticle forces can produce a cohesive network of particles, which will resist consolidation depending on its strength. Because flocculation generally affects the suspension microstructure, the permeability will change. 

Filter aids as well as flocculants are employed to improve the filtration characteristics of hard-to-filter suspensions. A filter aid is a finely divided solid material, consisting of hard, strong particles that are, en masse, incompressible. The most common filter aids are applied as an admix to the suspension. These include diatomaceous earth, expanded perlite, Solkafloc, fly ash, or carbon. Filter aids build up a porous, permeable, and rigid lattice structure that retains solid particles and allows the liquid to pass through. These materials are applied in small quantities in clarification or in cases where compressible solids have the potential to foul the filter medium. 

Traditional permeability tests are time-consuming and subject to some uncertainties (4). In the present paper, we describe an automated technique for determining the filtrability of fairly dilute suspensions, which can give useful information, on the behaviour of polymeric flocculants. 

The method described here provides a convenient means of determining the specific filtration resistance of fairly dilute suspensions. Results for clay suspensions flocculated by cationic polymers show that the specific resistance gives a sensitive indication of flocculation and is a useful guide in the selection of optimum flocculant concentrations. In a series of trials not reported here, it has been shown that the specific resistance results are very well matched by re-filtration rate data, as expected. The results also agree well with other, unrelated techniques. For more concentrated suspensions, some discrepancies have been found between permeability methods and other measures of flocculation (4). 

The permeability decreases as the applied pressure increases, due to Ui increase in the packing density [26]. Flocculation of the ceramic suspension before filtration gives much higher permeabilities, often by a factor of 10. To accoimt for fiocculation during filtration, a two-layer... 

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