For any filter aid, particle size distribution and ideal dosage are very important. Too low addition will lead to poor definition; Too much will form a thick filter cake. In general, a good filter aid should form a filter cake with high porosity (usually 0.85 to 0.9), low surface area and good particle size distribution. The filtration resistance of acceptable filter aids shall be much lower than that of the materials mixed with them. When not more than 25% (by weight) of filter aid (as part of total solids) is added, the filtration resistance shall be reduced by 67% to 75%. Adding only a small amount of filter aid (e.g. 5% of solid impurities) will actually increase the filtration resistance. When the amount of filter aids is so small that particles do not interact, they will form coherent structures, and the resistance may be adversely affected.

The filter aid is evaluated according to the filtration rate and the clarity of the filtrate. Finely dispersed filter aid can produce clear filtrate; However, they make a significant contribution to the filtration resistance of the medium. Therefore, the use of additives must be small, because the specific resistance of filter aid composed of coarse particles is much smaller. Therefore, the use of filter aid can achieve high filtration rate, but it reduces the clarity of filtrate to a certain extent.

The best filter aid should have the maximum pore size on the premise of ensuring the preset clarity of filtrate. The ideal characteristics of the best filter aid include:

1. The filter aid shall provide a thin solid layer with high porosity (0.85 to 0.90) on the outer surface of the filter medium. Ideally, suspended particles will form layered cake on the filter aid cake layer. The high porosity of the filter aid layer will ensure high filtration rate. Porosity is not just determined by pore size. Small pores may still have high porosity.

2. The filter aid should have a low specific surface area, because the friction loss caused by the liquid flowing through the particle surface will produce hydraulic resistance. The specific surface area is inversely proportional to the particle size. The quality of different kinds of filter aids depends on particle dispersion and specific surface difference. For example, most diatomite filter aids have roughly the same porosity; However, the piezoresistance of coarse-grained materials is smaller and much smaller than that of fine-grained particles.

3. The particle size distribution of filter aid should be very narrow. Fine particles increase the piezoresistance of filter aid, while coarse particles show poor separation characteristics. The ideal particle size distribution is usually prepared by air sieve, in which the finer components are removed.

4. In applications where the filter aid layer is formed on an open woven synthetic fabric or wire mesh, a wider size distribution may have to be prepared during operation. The filter aid shall have certain flexibility and can be mixed with a large number of coarse filter aids. This will provide rapid particle bridging and settling of the filter aid layer. For example, the average particle size is 80 μ The diatomite can be easily deposited to a mesh size of 175 by simply adding a small amount of filter aid μ The size of filter aid and mesh size are at the same level.

5. The filter aid is chemically inert to the suspension and will not be destroyed or decomposed in it.

In addition, using some filter aids (such as diatomite or perlite) and adding a small amount of fibrous material will produce a tighter precoated filter cake. At low suspension concentration (usually 0.01%), the filter aid acts as a medium under the condition of gradually plugging the pores. In this case, the pre coating thickness is usually 3 to 10mm. In this case, the selected filter aid shall have sufficient pore size to allow the penetration and retention of suspended particles in the precoat. Commonly used filter aids include diatomite, perlite, etc.

Generally speaking, diatomite has more complex particle shape, which provides a more tortuous path for suspended particles to be trapped. Cellulose fiber is usually used in high temperature and high alkaline environment, and the solubility of filter aid will be a problem.

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Because filtration is basically a surface capture phenomenon, people always try to use a filter aid whose median pore size (the opening of the curved channel) is slightly smaller than the average particle size of the suspended solids you want to remove.