Earth Filtration and Filter Aid

Diatomaceous earth filtration, also known as precoat filtration or diatomite filtration, relies on a layer of diatomaceous earth about 0.3-cm (1/8-in.) thick placed on a septum or filter element. The septums may be placed in pressure vessels or operated under vacuum in open vessels. A schematic diagram of a typical pressure system is shown in Fig. 2. 

Any substances that can enhance the filtration efficiency are termed a filter aid. Diatomaceous earth is the most common filter aid for the precoat filtration system. An efficient, economical filter aid must (a) have rigid, intricately shaped, porous, individual particles (b) form a highly permeable, stable, incompressible filter cake (c) remove even the finest solids at high rates of flow and (d) he as chemically inert and essentially insoluble in the Uquid being filtered. Commercial diatomaceous earth, such as Celite diatomite, meets these requirements due to the wide variety of intricately shaped particles and inert composition that makes it practically insoluble in all hut a few liquids. 

The manufacturer of filter aids produces filter aid grades in a wide range of particle sizes to meet practically any industrial filtration requirement. The relative flow rates of these grades are determined by a standard filtration test and are shown in Fig. 4. Typical properties of some commercial filter aids are shown in Tables 1-3. Filter Cel, the finest grade, shown in Fig. 4, giving the highest clarity and lowest flow rate, is a natural diatomite, which has been selectively quarried, dried, milled, and air-classified. To make coarser, faster flow rate filter aids, Filter-Cel is calcined and air-classified. These straight 

It is axiomatic in the use of filter aids that the ability of the filter aid to remove small particles of suspended matter decreases as the particle size, and thus the flow rate, increases. Conversely, as filter aid particle size, and therefore the flow rate, decreases, the ability of the filter aid to remove small particles of suspended matter increases. The extent to which this takes place will depend very much on the type and particle size distribution of the undissolved solids being removed. 

For a given liquid, clarity of filtrate is governed principally by (a) grade and amount of filter aid for body feed (b) grade and amount of filter aid for precoat (c) length of cycle and (d) filtration rate. 

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