The Objectives of Filtration

The objectives for performing filtration usually fall into one of the following categories  

separation aimed at facilitating or improving other plant operations. 

In filtration for solids recovery, the concentration of solids suspension must be high enough to allow the formation of a sufficiently thick cake for discharge in the form of a solid mass before the rate of flow is materially reduced. However, solids concentration alone is not the only criterion for adequate cake formation. For example, an 0.5% suspension of paper pulp may be readily cake-forming whereas a 10% concentration of certain chemicals may require thickening to produce a dischargeable cake. 

Filtration for both solids and liquid recovery differs from filtration for solids recovery alone in the cake building, washing and drying stages. If the filtrate is a valuable liquor, maximum washing is necessary to prevent its loss but if it is valueless, excess wash liquor can be applied without regard to quality. 

Filtration may also serve as the preparatory step for the operation following it. The latter stages may be drying or incineration of solids, concentration or direct use of the filtrate. Filtration equipment must be selected on the basis of their ability to deliver the best feed material to the next step. Dry, thin, porous, flaky cakes are best suited for drying where grinding operations are not employed. In such cases, the cake will not ball up, and quick drying can be achieved. A clear, concentrated filtrate often aids downstream treatment, whereby the filter can be operated to increase the efficiency of the downstream equipment without affecting its own efficiency. 

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