Filter media precoat filtration

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Filter aids may be applied in one of two ways. The first method involves the use of a precoat filter aid, which can be applied as a thin layer over the filter before the suspension is pumped to the apparatus. A precoat prevents fine suspension particles from becoming so entangled in the filter medium that its resistance becomes exces-sive. In addition it facilitates the removal of filter cake at the end of the filtration cycle. The second application method involves incorporation of a certain amount of the material with the suspension before introducing it to the filter. The addition of filter aids increases the porosity of the sludge, decreases its compressibility, and reduces the resistance of the cake. In some cases the filter aid displays an adsorption action, which results in particle separation of sizes down to 0.1 /i. The adsorption ability of certain filter aids, such as bleached earth and activated charcoals, is manifest by a decoloring of the suspension s liquid phase. This practice is widely used for treating fats and oils. The properties of these additives are determined by the characteristics... 

The ability of an admix to be retained on the filter medium depends on both the suspension s concentration and the filtration rate during this initial precoat stage. The same relationships for porosity and the specific resistance of the cake as functions of suspension concentration and filtration rate apply equally to filter aid applications. 

In precoating, the prime objective is to prevent the filter medium from fouling. The volume of initial precoat normally applied should be 25 to 50 times greater than that necessary to fill the filter and connecting lines. This amounts to about 5-10 lb/100 fF of filter area, which typically results in a 1/16-in. to 1/8-in. precoat layer over the outer surface of the filter medium. An exception to this rule is in the precoating of continuous rotary drum filters where a 2-in. to 4-in. cake is deposited before filtration. The recommended application method is to mix the precoat material with clear liquor (which may consist of a portion of the filtrate). This mixture should be recycled until all the precoat has been deposited onto the filter medium. The... 

Precipitates or salts are used when corrosive liquor must be filtered, and where there is no available medium of sufficient fineness that is corrosion-resistant and will not contaminate the cake. In these cases, precipitates or salts are used on porous supports. In the filtration of caustic liquors, ordinary salt (sodium chloride) is used as the filter medium in the form of a precoat over metallic cloth. This procedure has the advantage that the salt medium will not be detrimental to either the cake or the filtrate if inadvertently mixed with it. 

The most commonly used filter medium is woven cloth, but a great variety of other media is also used. The main types are listed in Table 10.2. A comprehensive discussion of the factors to be considered when selecting filter media is given by Purchas (1971) and Mais (1971) see also Purchas and Sutherland (2001). Filter aids are often used to increase the rate of filtration of difficult slurries. They are either applied as a precoat... 

The filters used for gas cleaning separate the solid particles by a combination of impingement and filtration the pore sizes in the filter media used are too large simply to filter out the particles. The separating action relies on the precoating of the filter medium by the first particles separated which are separated by impingement on the filter medium fibres. Woven or felted cloths of cotton and various synthetic fibres are commonly used as the filter media. Glass-fibre mats and paper filter elements are also used. 

After the precoat is established the solids to be removed from the filter feed are trapped on the surface of the precoated bed. This thin layer of slime is removed by a knife which is caused to advance slowly towards the drum. The knife also removes a thin layer of the precoated bed so that a new surface of the filter medium is exposed. This procedure allows steady filtration rates to be achieved. 

Diatomaceous earth filtration (or precoat filtration), accomplishes particle removal by physically straining the solids out of the water. The thickness of the initial layer of diatomaceous earth (DE) filter medium is normally about 1/8-in. (3 mm), and the water... 

Precoating the filter medium prevents blinding of the medium with the product and will increase clarity. Filter aid must be an inert material, however, there are only a few cases where it cannot be used. For example, waste cells removed with filter aid cannot be reused as animal feed. Filter aid can be a significant cost, and therefore, optimization of the filtration process is necessary to minimize the addition of filter aid or precoat. Another possible detriment is that filter aid may also specifically absorb enzymes. 

As previously mentioned, it is the bed of deposited solids that constitutes the actual filter medium. Therefore, until sufficient solids are deposited to provide a filter cake the filtrate may be turbid and if so, it should be recirculated to the treatment tank. Should the turbidity continue after the bed begins to form, arrangements should be made in future runs to precoat the filter with... 

DIATOMACEOUS EARTH FILTRATION - Is a process in which a filter cake or precoat of diatomaceous earth is used as a filter medium. 

Cake filtration this is undoubtedly the most widely encountered mechanism in industrial filtration and involves the accnmulation of particles that bridge together in a porous structure on the surface of the fabric. It follows from this that, once formed, the cake effectively becomes the filter medium, with the fabric thereafter acting simply as a support. In cases where it is difficult for the particles to form a naturally porous cake, the use of a special precoat or body feed may be employed to assist in this task. 

SEM micrographs of Diatomaceous Earth (DE) are shown in Figure 1 (Angela F, 2012 and Wen-Tien Tsai, 2006). The porous structure of DE can be clearly seen in Figure 1(b). The filtering medium used in precoat filtration is composed of fossil-like skeletons of microscopic... 

Addition of another powdered solid, termed a filter aid, to the system can produce a significant improvemaat to a filtration operation. The filter aid may be used in two ways, either separately or in conjunction. The first of these methods is to precoat the filter medium with a layer of fiher ail cake. Precoat aids must filter quickfy without bleeding or penetrating throu the cloth and must give a uniform thickness with a reproducible filtering surfiice. The susp ntion is then fihered onto the precoat by surface and depth filtration mechanisms. Thus surface properties are inq>ortant in the choice of aid. 

The resistivity of the cake, however, very often increases with time. Under the pressure of filtration or the friction of the fluid that continues to pass through the cake, the filtered particles continue to compact and reduce the area available for flow. Section 7.S.4.2 also describes the empirical methods that are used to characterize the compressibility of the cake. It is significant that the particles produced by precipitation from the brine are often highly compressible. At the same time, they have a tendency to become lodged in the pores of the filter medium. Filtration of these particles can become very difficult. The usual solution to this problem is the use of a filter aid. The filter aid is applied to the surface of the filter medium before introducing the brine. This is the precoating operation. Filter aids are selected for their desirable characteristics, and the precoat material protects the pores from penetration by filtered solids while at the same time offering little resistance to the flow. Compression of the precipitates still occurs, and so filter aid ( admix ) is also added in small quantities to the brine to improve the characteristics of the cake. 

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