Classification of filter media

- Aug 18, 2017 -

The porous material, which is used for the separation of solids, is known as the filter medium. It is the key component of the filter, which determines the separation precision and efficiency of the filtration operation, and directly affects the production strength and power consumption of the filter.

There are various kinds of filter media used in industry, which are divided into flexible medium, rigid medium and loose filter medium.

• flexible filtering media:

O metal filtration medium

O non-metallic filter medium: cotton fabrics, wool fabrics, silk fabrics, synthetic fabrics, fiberglass fabric, nonwoven fabric, nonwoven filter cloth ( felt of filter paper, filter, filter pad)

O metal, non-metallic mixing medium

• rigid filtering medium

O sintering metal mesh, metal fiber sintering felt, powder sintering material, porous ceramics, sintered porous plastic, sintered aluminum oxide, glass filter medium

• loose filtering media

O diatomite, expanded perlite powder, cellulose, sand, charcoal powder, smokeless

The mechanism of filtration is related to the mechanism of filtration. Filter medium for filter cake filtration technology characteristics must meet the special requirements of this kind of filter: the structure of the medium can guarantee began to filter particles can quickly "bridge" in the medium surface, the fine particles from erosion wear (filter);The ratio of the holding particles in the pore of the medium is low, and the media is the smallest. The filter cake can be easily removed completely; The structure of dielectric facilitates cleaning and regeneration.Commonly used filter cake filtration medium main filter cloth, filter, filter, the side type filter, etc., of deep used as filter medium, insist on the structure to meet the specified intercept precision, can stop for stop particles; The bed should have enough capacity to slow down the process of particle plugging to extend the operating cycle.

The common requirements of all kinds of filtering media are: excellent filtration characteristics (smaller than resistance, high precision of trunnion);Excellent mechanical and physical properties (high strength, creep, softness and wearability of higher), to a certain process operating conditions and environment, good chemical stability, corrosion resistance, high temperature resistant and microbe, etc.), convenient clean, renewable, cheap, reliable source.

Commonly used filtration medium and its main properties

1.3.1 filter cloth

This is the most widely used in industry and the most widely used filter medium. The filter cloth has a textile cloth and non-woven fabric. Its constituent materials are natural fibers (cotton, wool, silk, hemp) or synthetic fibers. Filtration properties of filter cloth are determined by the material, fiber weaving and post-processing.

(1) textile filter cloth

Textile filters are woven from three different types of yarns: single strand (single strand), filament (fixed-length fiber yarn) and staple fiber (poly yarns).Single filaments are generally 0 in diameter.16-0.32 (in some places even 1) single filament, by its woven filter cloth has a smooth surface, small space pure, specific resistance, clogging the smallest, easy to clean and best slag discharge performance advantages, but its capture particles diameter trap, precision filtration, unfavorable use. The separation effect was affected by the separation of suspension liquid with wide distribution range. The filament yarns are spun from two strands of raw silk, which are woven into a filter cloth. The tensile strength is good, the retention performance of the particles is better than that of the single wire, and the slag discharging performance is slightly worse. Staple is using natural cotton, wool fiber or synthetic staple fiber strand twist into the system, because of its villous fiber and rendering performance of particle entrapment, the sealing is good, but the disadvantage is that the pore by particle blocking, cleaning and slag discharge performance was poor.

There are three types of cloth: plain, twill and satin. Generally speaking, the plain pattern filter cloth is compact and small, so the grain interception is good, the filtrate is high and the service life is long and the price is cheaper. The disadvantage is that it is bigger than blocking, easy to clog, and bad discharge. The pore of satin weave fabric is the largest, smaller than the resistance, not easy to block, the slag discharging performance is good. However, the ability of particle retention is low, the filter is serious and the filter effect is poor. The various properties of the twill filter are in the center, the anti-friction ability is very strong, the filtration speed is also big, the life is the longest, so it is widely used.

The following is a brief introduction of the filter cloth usage conditions of different materials for reference.

Cotton:

The filter cloth made of ordinary cotton yarn can only be separated from the neutral filter pulp below 100, the acidic filter below 20 and the alkaline filter below 10.In alkaline media, the cotton fabric filter produces swelling. There is a contraction in the water and the aluminum sulfate solution. Therefore, the solution should be soaked in solution before using. The erosion resistance of cotton fabric filter to mold is weak, and it is necessary for pretreatment of copper ammoniac solution for the occasion to be used for the development of microblight and mildew, and the resistance will be improved.

The filter cloth made of nitrification is hard surface, easy to discharge slag, concentrated sulfuric acid, nitric acid and mixed acid and hydrochloric acid solution, the zinc chloride solution has stability, but not alkali. In reductive material and organic solvent, easy to be corroded. Nitrocellulose cloth has a dangerous flammability, which limits its use.

Wool fabric:

It is generally made of sheep wool, and the corrosion resistance of the acid filter is better than that of cotton cloth, and the life of the fabric is close to the service life of cotton cloth in neutral materials in the treatment of 5% to 6% concentrated inorganic acid material. The effect of high temperature and alkali and the destruction of sodium hypochlorite. Cloth made of camel hair has the stability of inorganic acid. The wool fabric is a little less than cotton in strength and the retention performance of grain, and the price is much higher than that of cotton cloth.

PVC:

The function of acid, salt and mineral oil is stable. But the oxidant, the concentration of alkaloids can be destroyed. There is a smooth surface, which is good for slag discharge, but can only be used under 60.

Polypropylene cloth:

Strong acid resistance (organic and inorganic), alkaline corrosion and elasticity. The temperature can be up to 110 and the price is low, but the strength of the cloth is weak, which is used for fiber filter and coarse crystal filtration. The polypropylene cloth has a smooth surface, soft texture and good slag discharge. The plot density is low, which can resist acid, alkali and strong oxidant corrosion and microbe action. But soluble in xylene, tetrachloroethane solvent, hygroscopic low, at 0.Below 3 %, safe use temperature is below 100 below C, commonly used in pigment, dye filter.

Polyamide (nylon 66) cloth:

In wet condition, it has high strength, long life, excellent wear resistance and good thermal stability. It can be used in the temperature of 120-150.The chemical stability of alkaline filtration pulp is better, but it can be used to 100 DCS, but in the acidic medium, only the dilute acid material can be stable under normal temperature. 

Polyester cloth:

High strength, flexible, good flax resistance, long life, superior particle retention. Chemical stability can resist acid corrosion. It is slightly less alkaline, but superior to acrylonitrile fiber. Polyester fabric has a high resistance to microbial action. Polyester filter cloth varieties are widely used. The two water - acid plants in China are separated from phosphoric acid slurry and mostly used polyester 734 #

The nylon cloth:

It is composed of 85 % acrylonitrile polyacrylate and vinyl pyridine. With high thermal stability and wear resistance, the inorganic acid has good corrosion resistance and is dissolved in the concentration of sulfuric acid with over 80% concentration. The strong base of weak base and room temperature also has certain corrosion resistance, it will not be used by common solvent, grease, neutral salt and most acidic salt.

Grey cloth:

It is made of polyacrylonitrile fiber and polyester fiber. It has a very smooth surface and high mechanical strength. It is not blocked by pollutants and is easy to discharge slag. The chemical properties of two kinds of fibers. It is especially suitable for the filtration of acid gypsum slurry in sewage treatment and phosphoric acid plant. According to foreign reports, its service life is also long, is a very promising new filter cloth.

(2) non-woven fabric

Using the heating method, the various fibers together with certain resin adhesives and solvents are made of 0.2-2-2 thick sheet of paper filter cloth. The fiber tissue of this kind of cloth is not the rule but the random and tight arrangement, so its intensity is worse than the textile filter, stronger than the filter paper, and the porosity is thicker than the filter paper, so it is smaller than the resistance ratio filter paper. Non-woven filter cloth has been used in filter press and horizontal disc filter, mainly used to clarify filtration and purify the solid particles in low-concentration suspended liquid. Such as beverage, milk, varnish, lubricants and the filtration of plant gum. The filtration mechanism of non-woven filter cloth is roughly graded filtration, i.e. the surface portion of the filter cloth is larger than 100, and its inner layer intercepts highly dispersible particles. 

(3) post-processing of filter cloth

In order to improve the filtration properties and physical and mechanical properties of the filter cloth, the woven fabric can be knitted and processed. Its processing methods are:

Calendering: the filter cloth is pressed under the hot roller, so that the surface of the fabric is smooth and the porosity of the yarn can be reduced, so as to improve the ability of the filter cloth to capture and discharge the particles. The filter cloth strength after calendering is somewhat reduced.

Raising: on one side of the filter cloth fabric with micro gear steel comb brush make the yarn bundle fiber filter cloth surface produces fine soft hairs, improve the ability of filter cloth intercept tiny particles, improve the separation effect, improve the clarity of the filtrate.

Hot rolling processing: this is limited to synthetic fiber filter cloth. The purpose is to improve the thermal stability or mechanical strength of the filter cloth so as to make it more suitable for operation at higher temperature. It is a hot rolling process for single filaments.

1.3.2 mesh

Mesh is woven filter medium, and its main materials are metals, those who can be brushed metal and alloys, such as stainless steel, brass, monel nickel copper alloy, nickel, silver and so on all can be used as the material of wire mesh. There have also been a filter made of plastic. Is because the material is the metal material, so the mesh surface is smooth, wear-resistant, resistant to corrosion and high temperature resistant, basically do not produce contraction, stretch, not easy be particle plugging, easy to clean, renewable, than fiber filter cloth for long service life. Only one-time investment is more expensive, but the operating cost is lower than the filter cloth.

There are also plain and twill weave.

Strainer can be installed directly on the leaf filter or drum filter machine for filtering. It can also be used as cloth and filter paper support. At this point, the filter must be rolled with roller to make its surface smooth, and then install with filter media such as filter cloth, so as not to damage the supporting filter medium. We now have copper, tin, bronze, stainless steel, carbon steel and other materials.

1.3.3 filter felt

Synthetic fiber (such as polyacrylonitrile fiber, nylon, polypropylene, etc.) or wool fiber dense array without rules, can add or not add resin adhesive (hot glue), after compaction resistance, made of different thickness of the blanket. Install on filter press or drum vacuum filter. When a pleated metal mesh is used as a filter element. The filtering mechanism of the filter is deeply filtration. Synthetic fiber filter mat has the advantages of anti-corrosion, mildew and particle retention.

1.3.4 filter liner

Filtering liner is copying model made of asbestos and cellulose filter medium, the thickness of 2 ~ 5 mm, fiber diameter of 200 ~ 250, the other with a diameter of 20 ~ 30 of cellulose fibre to support body, asbestos fibers in which constitute a space, is used to filter machine, such as diatomite particles, in which the filling can improve the porosity of the mat. Because the porosity of the filter liner is very small, it is mainly used for precision filtration, which can be used to deal with suspended fluid containing low solids and small particles.

1.3.5 filtration membrane (porous plastic film)

With cellulose ester, polyvinyl chloride, polyethylene or polytetrafluoroethylene, the thickness is 0 through certain process.2 ~ 150um, aperture 0.3 ~ 10um thin film installed on special equipment. The filtration membrane of this porous membrane can reach 0.1 um. It is mainly used in the treatment of medical industry and biochemistrum sewage treatment.

1.3.6 hard (rigid) porous media

The rigid porous filter medium includes porous ceramics, metal ceramics, porous sintered plastic, metal fiber sintering felt, multi-layer metal sintering network, etc.

They are made of ceramic, metal and plastic powder that is sintered at high temperature. By means of fine powder particle size, sintering temperature, pressure, time and binder variety, the control of the formula can be used to make the product of porous medium, permeable, porosity and different shapes. The thickness is generally above 20mm (the thickness of the sintered metal is usually 1.6 mm).Cylinder (including tubular) is suitable for pressure filtration, and the plate is used for vacuum filtering gravity filtration. The filtration mechanism of porous media is generally suitable for deep filtration, but it can also be used for surface filtration. Because of their unflexibility, the pressure fluctuation in the filtration has little influence on the filter cake, and its pores can range from several thousand to one, so most of it is used for precision filtration.

Porous ceramics including ordinary porous ceramics and corundum ceramic two kinds, the former is the main raw material, refractory clay, the latter is the alumina powder, it will respectively with thermosetting resin and solvent mixture into the required shape, calcined at 1400 DHS C stove. With high temperature and corrosion resistance, the base corundum is stable to concentrated inorganic acid, but it is unstable to the aqueous solution of weak alkaline and alkaline salt. Ordinary ceramics can be resistant to thin, thick inorganic acids and their salt water solutions. However, the function of alkaline liquid is not stable. Porous ceramics are widely used in gas purifying filtration (such as air filtration in the production of nitrate) and also for the clarification of inorganic acids and acidic saline solutions.

Metal fiber bonded felt:

It is a new type of filter material, and the filtration precision of the product is 3-100 micron. It is widely used in petroleum, chemical, chemical fiber, aviation and aerospace, and is an irreplaceable high-tech material


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