Filtration Conditions

Two significant operating parameters influence the process of filtration: the pressure differential across the filtering plate, and the temperature of the suspension. Most cakes may be considered compressible and, in general, their rate of compressibility increases with decreasing particle size. The temperature of the suspension influences the liquid-phase viscosity, which subsequently affects the ability of the filtrate to flow through the pores of the cake and the filter medium. In addition, the filtration process can be affected by particle inhomogeneity and the ability of the particles to undergo deformation when subjected to pressure and settling characteristics due to the influence of gravity. Particle size inhomogeneity influences the geometry of the cake structure not only at the moment of its formation, but also during the filtration process. During filtration, small particles retained on the outer layers of the cake are often entrained by the liquid flow and transported to layers closer to the filter medium, or even into the pores themselves. This results in an increase in the resistances across the filter medium and the cake that is formed. Particles that undergo deformation when subjected to transient or high pressures are usually responsible for the phenomenon known as pore clogging. The addition of coagulating and peptizing agents can greatly improve filterability. These are additives which can drastically alter the cake properties and, subsequently lower flow resistance and ultimately increase the filtration rate and the efficiency of separation. Filter aids may be used to prevent the penetration of fine particles into the pores of a filter plate when processing low concentration suspensions. Filter aids build up a porous, permeable, rigid lattice structure that retains solid particles on the filter medium surface, while permitting liquid to pass through. They are often employed as precoats with the primary aim of protecting the filter medium. They may also be mixed with a suspension of diatomaceous silica type earth (>90% silica content). Cellulose and asbestos fiber pulps were typically employed for many years as well.

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