Permeability cake 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. 

There are some solids, however, which form a less permeable cake, even in veiy thin layers. With these sohds, the resistance of the deposited cake will be veiy high when compared to that of the precoat bed, and the slope of the filtrate cui ve will be -t-0.5 for all v ues of form time. 

Filtrates that are acceptable with a low quantity of fines that pass trough the filter cloth in the first few seconds of cake formation. Broadly, and depending on particle size and cloth permeability, the filtrate may contain 1,000 to 5,000 ppm insolubles. 

Wells, S. A. and Dick, R. I. (1993) "Permeability, Solid and Liquid Velocity, and Effective Stress Variations in Compressible Cake Filtration," Proceedings, American Filtration Society Conference on System Approach to Separation and Filtration Process Equipment, Chicago, Illinois, May 3-6, pp. 9-12... 

When a slurry flows through a filter, the solid particles become entrapped by the filter medium which is permeable only to the liquid. Either of two mechanisms are used cake filtration or depth filtration. 

In cake filtration, the medium must oppose excessive penetration and promote the formation of a junction with the cake, to high permeability. The medium should also give free discharge of cake after washing and dewatering. 

Cake filtration characteristics such as porosity, permeability, specific resistance, and cake compactibility parameters n, p, 8, or in Equation (22.29)... 

In Figure 3 a typical result of the transient evolution of size specific collection efficiency for the cordierite sample No. 2 is presented. The decrease of the filter permeability due to the coating is beneficial for the collection efficiency of the coated filters and makes them approach the cake filtration regime more quickly. 

Both the permeability and filtration characteristics of nonwovens are dependent on the feh porosaty and fibre diameter. A medium which as been heavily calendered on both sides will possess the lowest poro. Sur ce treatments and/or use of laminations of different porosities, are aimed at inqtroving cake filtration performance and cake release. Generally speaking, the filtration efficiency at a particular particle size is inversely proportional to the fibre diameter, other Actors bdng the same. 

Figure 2 illustrates the operation of a precoat filter with cake in place. Point A represents the entrance of the cake into the unfiltered suspension. Point B Fepresents the point of emergence and C is the point at which the cake, together with its accumulated suspension particles, reaches minimum permeability. Maximum filtration... 

Low filtration rate Low permeability cake or cloth blinding during cycle a. Low air rate b. Wet cake c. Long deliquoring period... 

The two words are often used interchangeably because both processes lead to increases of the effective particle size with the accompanying benefits of higher settling or flotation rates, higher permeability of filtration cakes, or... 

Cake filtration is based on passing a suspension through a permeable, relatively thin medium (chapter 8). The sohds are deposited in the form of a cake on the upstream side of the medium. As soon as the first layer of cake is formed, the subsequent filtration takes place on top of this cake and the medium provides only a supporting function. These so-called surface filters are best used for filtration of suspensions of solids concentrations in excess of 1% by volume in order to minimize medium blinding which occurs in the filtration of dilute suspensions. If cake filtration is to be applied to clarification of liquids, which imphes low feed concentrations of sohds, addition of filter aids is usually necessary—see section 2.2. 

Cake filtration occurs after surface filtration where a cake acts as the filter medium. In some instances the cake formed has a lower permeability than the filter medium and thus becomes the limiting factor in the filtration cycle. 

Surface filters are usually used for suspensions with higher concentrations of solids, say above 1 % by volume, because of the blinding of the medium (or of the precoat) that occurs in the filtration of dilute suspensions. This can, however, sometimes be avoided by an artificial increase of the input concentration, in particular by adding a filter aid as a body feed as filter aids are very porous their presence in the cake improves permeability and often makes cake filtration of dilute and generally difficult slurries possible. 

Because gravity is too weak to be used for removal of cakes in a gravity side filter (2), continuously operated gravity side filters are not practicable but an intermittent flow system is feasible in this arrangement the cake is first formed in a conventional way and the feed is then stopped to allow gravity removal of the cake. A system of pressure filtration of particles from 2.5 to 5 p.m in size, in neutralized acid mine drainage water, has been described (21). The filtration was in vertical permeable hoses, and a pressure shock associated with relaxing the hose pressure was used to aid the cake removal. 

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