Precoat filters are used to treat solutions having traces of insoluble contaminations. They embody a rigid, semi-flexible or flexible screen on which a filter is deposited medium. During the filtration process the filter medium and the filtrated solids form a filter bed, which works as an additional strainer element to collect much finer contaminants. The “bed” medium may filter by adsorption and by mechanical means. The filter vessel is filled with the suspension under pressure and it passes through the filter bed, leaving the solids in the filter bed. The purified water is then collected after the filter element.
The name “precoat filter” is sometimes given to other filter elements, which comprise a coating material permanently bonded to a mechanical screen.
Filter media are to a large extent chemically inert materials of fine fibrous or granular structure, which develop filter layers of high permeability. The filter bed consists mostly of diatomaceous earth (diatomite or kieselguhr), which is manufactured from fossil remains of diatoms, a type of algae having a silica skeleton. A number of other media may be employed for precoated filters. Chief of these is perlite, which is processed from a vitreous rock of organic origin. Other precoat media include powered organic rock, activated carbon, asbestos and cellulose.
Before start-up a primary filter medium layer has to be deposited on the basic screen. The correct amount of filter medium is mixed with clean liquid and then pumped into the filter body, where it is deposited uniformly over the filter elements. The precoat layer is formed so evenly because, while it is formed, a slight pressure differential is produced and the flow of liquid always takes the path of least resistance. To obtain extremely fine filtration, the process may be repeated with a further dose of finer medium to superimpose a second layer of this medium on the first. If suspensions with predominantly colloidal, compressible or very fine solid particles are to be filtered, then the filter achievement will decrease fast by the firm filter bed. This problem can be avoided, if the suspension filter mediums are added, which produce a loose and more permeable filter bed.
Principle of a Precoatfilter
The precoat filtration can be used in vertical leaf filters, horizontal pressure leaf filters, candle filters, filter presses, layer filters and rotating drum filters. A special type of precoat filters is the vacuum rotary drum filter. Before beginning of the filtration a 50... 70 mm thick basic filter medium layer is applied. The scraper on a precoat filter moves slowly towards the drum and shaves-off the blinding layer of the contaminants together with a thin layer of the precoating material. This movement exposes continuously a fresh layer of the precoat surface so that when the drum submerges into the tank, it is ready to polish the solution. The blade movement mechanism is equipped with a precision drive having an adjustable advance rate of 1…10 mm/hr.
After the precoating the filtration process starts. If there is no constantly removal of the filter bed, as described by the rotary drum filters, filter regeneration becomes necessary. For cleaning purposes this will be done through removal or regeneration of the filter medium. The removal process dislodges the filter bed together with contaminants, which have collected. That can be done by backwashing or centrifugation. The bed has then to be reformed. When using the regeneration process the filter will be “re-coated” by running the bed medium through in a suitable carrier fluid. This fluid passes through the strainer element, leaving the coating material arrested and deposited on the surface to form a new bed.