1、 Basic types of filter cloth
Filter cloth is divided into several basic types such as woven fabric, non-woven fabric, and composite fabric.
1. Woven fabric
Due to the various arrangements of warp and weft yarns in woven fabrics, different weaving patterns can be obtained. In addition, different types of yarns and processing techniques can be used, so the actual weaving style of woven fabrics is unlimited. Woven fabric is woven by interlacing warp and weft yarns in a certain pattern, and the pattern of interlacing is called the structure of the woven fabric. The commonly used weave patterns for woven fabrics are plain weave, twill weave, and satin weave.
Plain weave refers to the weaving method in which each warp yarn and each weft yarn are interwoven vertically. Plain weave fabric is the densest, therefore it has the best filtering effect and stiffness.
The twill weaving method refers to a weaving technique in which a weft yarn passes over and under two or more warp yarns simultaneously, and moves horizontally in a regular pattern from one row to the next. This weaving method can produce diagonal diagonal lines, usually moving upwards from left to right along the surface of the fabric. The advantage of twill weaving is that it can fill more weft yarns per unit length of fabric, resulting in a larger volume of woven fabric. Compared to plain weave fabric, twill fabric is more flexible and easier to install on filter machines.
Satin weaving method further expands the concept of diagonal weaving by using wider interlacing point spacing. This weaving method can produce fabric with a smooth surface, while satin fabric with diagonal lines is the softest. This type of cloth not only makes it easy to remove the filter cake, but also reduces the possibility of particles being captured inside the cloth. However, the disadvantage of satin weave fabric is that it is not resistant to friction.
1.1 Precision processing of woven fabrics
The precision processing of woven fabrics has three main purposes: ensuring the stability of the fabric, improving the surface characteristics of the fabric, and adjusting the permeability of the fabric.
(1) Heat setting. Throughout the entire production process, tension acts on the filter cloth, so the filter cloth needs to have stability. Otherwise, after removing the tension, the size of the fabric will change, which may cause the feeding hole on the filter press to not align with the hole on the filter cloth. To avoid such problems, the fabric should be treated with hot water or dry heat setting treatment, and the temperature and duration of the treatment depend on the polymer material of the fabric. Another reason why filter cloth needs stability is that tension is applied to the filter cloth in advance during use, such as on belt filters and vertical filter presses. In this case, it is necessary to carefully control the temperature to stretch the filter cloth. Pre stretching can not only reduce the further elongation of the filter cloth during use, but also ensure that the filter cloth has a good trajectory during operation, reducing deviation.
(2) Press the light. Polishing is the most commonly used method to improve the surface properties of fabrics. When operating, let the filter cloth pass between the hot pressure rollers, and select the temperature, pressure, and speed of passing through the rollers according to different fabrics. The pressed filter cloth has a smooth surface, which is conducive to removing the filter cake, and adjusts the permeability, improving the filtration efficiency.
(3) Burn hair. Burnt hair is a surface treatment method that slightly burns the fuzz on the surface of fabrics woven from short fiber yarns. The short fiber fuzz on the fabric surface hinders the removal of the filter cake. An air frame or a very hot metal strip can be used to quickly contact the fabric surface and burn off the fuzz.
(4) Fleece. Plucking is the process of using a fine steel comb to create soft fibers on one or both sides of a fabric. Plucking not only improves the ability of filter cloth to capture fine particles, but also enhances the fabric's ability to absorb dirt. The velvet method is commonly used to treat dust removal filter cloth.
(5) Surface coating. Surface coating is a special surface treatment method for filter cloth. Both woven and non-woven fabrics can be coated with microporous polymers, which not only improves filtration accuracy but also improves the conditions for removing filter cake from the fabric. For example, tetrafluoroethylene trimer can form a very strong coating layer on the surface, with numerous pores on the layer, with a pore size of 5-8 μ m.
2. Non woven fabric
The preparation of non-woven fabrics requires six steps: fiber preparation, web formation, adhesive reinforcement, drying, post finishing, and rolling. Among these 6 steps, forming a net and bonding reinforcement are the most important.
2.1 Networking
Mesh forming can be divided into dry mesh forming, wet mesh forming, and polymer extrusion mesh forming.
(1) Dry process to form a network. Dry mesh forming is divided into mechanical mesh forming and air flow mesh forming. After chemical, mechanical, solvent, or thermal bonding treatment, the fiber web formed by dry process becomes a size stable non-woven fabric.
(2) Wet networking. Wet networking is the process of using water as a medium to evenly suspend short fibers in water, and then using the action of water flow to deposit the fibers on a permeable curtain or porous drum to form a wet fiber web.
(3) Polymer is extruded into a mesh. Polymer extrusion into a web utilizes the principles and equipment of polymer extrusion. Representative spinning methods include melt spinning, dry spinning, and wet spinning for forming webs.
2.2 Reinforcement
Reinforcement can be divided into three categories: mechanical reinforcement, chemical bonding, and thermal bonding. They can be used alone or in combination.
2.2.1 Mechanical reinforcement
Mechanical reinforcement does not use chemicals such as adhesives or solvents, nor does it use hot melt bonding. Instead, mechanical methods are used to entangle the fibers in the fiber web together, or coil shaped fiber bundles or yarns are used to reinforce the fiber web.
(1) Acupuncture reinforcement method. In dry nonwoven fabrics, needle punched nonwoven fabrics are the most common and have become increasingly widely used. They have been applied in geotextiles, carpets, automotive interior materials, papermaking blankets, filter materials, synthetic leather base fabrics, and high-temperature resistant synthetic materials.
The non-woven fabric made by needle punching method has multiple and uniform internal pores, which are irregular. Therefore, good breathability and good particle retention are also major advantages. However, after particles enter the interior of non-woven fabric, they will block the pores, and the plush on the surface of the fabric will also make it difficult to peel off the filter cake. The solution is to burn the cloth, press it, and apply resin on the surface. The needle punched non-woven fabric treated in this way has both good particle retention and good cake stripping properties. If a base cloth is laid between the upper and lower fiber webs, the strength of the non-woven fabric produced is no less than that of woven fabric.
(2) Water jet reinforcement method. Compared with the needle punching method, water jet has a smaller force on the fibers, so the density of water jet non-woven fabric is also lower and softer.
(3) Sewing method. The appearance and characteristics of the fabric are similar to traditional woven or knitted fabrics.
2.2.2 Chemical bonding reinforcement
The use of chemical methods to reinforce non-woven fabrics is the longest and most widely used method for reinforcing fiber webs. There are two types of chemical bonding methods: adhesive bonding and solvent bonding.
2.2.3 Thermal bonding reinforcement
2.3 Post organization
(1) Heat setting finishing. The heat shrink finishing method can be used to increase the tightness, strength, and dimensional stability of non-woven fabrics.
(2) Polishing and finishing. Uniform thickness calendering finishing aims to make the thickness of non-woven fabrics uniform and meet certain thickness requirements.
(3) Burn and tidy. It is to burn off the protruding floating fibers on the surface of non-woven fabric with gas in order to obtain a smooth non-woven fabric.
(4) Vacuum cleaning and tidying up. The purpose of vacuum cleaning is to increase the adsorption force of non-woven fabrics on dust. The key of vacuum finishing is to impregnate nonwovens with antistatic agent, hygroscopic agent, paraffin lotion, etc. The fibers of non-woven filter materials that have been vacuum cleaned can adsorb dust during the filtration process.
(5) Coating finishing. In order to improve the original function of non-woven fabrics or give them new functions, a layer of chemical material can be coated on their surface. By using appropriate coating polymer formulations, its strength and other properties can be improved.
(6) Stacking and organizing. The principle of layering is very simple, which is to use adhesive methods to stack two or more layers of materials together to form a composite material.
3. Composite fabric
(1) Coating. Coating a layer of chemical material on the surface of woven or non-woven fabrics can improve particle retention, enhance filtrate clarity, improve cake stripping, enhance corrosion resistance, and prolong service life.
(2) Stacking. Deep filtration media, with the upper coarse layer serving as a pre filter to trap larger particles and the lower fine layer to trap smaller particles, maximizing the media's ability to absorb pollutants. Surface filtration media utilizes the fine layer or membrane layer on the surface of the medium to collect particles, and the filtration efficiency depends entirely on the pores on the surface of the medium.
(3) Multiple layers. The upper layer of the multi-layer medium is the thinnest and used for filtration, while the lower layers are thicker and very sturdy, providing mechanical stability for the entire medium.
2、 Characteristics of filter cloth
1. Permeability of filter cloth
(1) Permeability is an important parameter that determines the initial flow velocity of a fluid through a medium, and the initial flow velocity affects the filter cake structure near the medium.
(2) Permeability is a parameter that directly affects the pressure loss during filtration. When permeability is high, the pressure loss is small and the filtration speed is fast.
(3) Permeability is the key factor affecting the success or failure of filter cake filtration.
(4) The permeability determines the amount of particles captured by the thin filter slurry inside the medium, which of course makes it difficult to backwash and regenerate the medium.
(5) Permeability is an important parameter that determines the size or power of air filtration station fans.
2. Particle bridging on filter cloth holes
The pores of the filter cloth are larger than those of the filter cake on top of it. As the larger the pores of the filter cloth, the less likely it is to clog and the easier it is to wash, it is recommended to choose a filter cloth with the largest pores possible. The difficulty of clogging the filter cloth is related to the bridging on the filter cloth holes.
Firstly, large particles form an arched bridge, followed by smaller particles forming a filter cake on the arch bridge. The ability to filter out smaller particles on the larger pores of the filter cloth is attributed to the bridging phenomenon.
3. Flow resistance of filter cloth
In most cases, the medium is only discarded after multiple uses due to wear and pore blockage. Although the medium needs to be cleaned after each use, the ability of the filter cloth to pass through the fluid inevitably decreases to some extent, and the degree of decrease is related to the flow resistance of the medium.
The initial filtration pressure and concentration of solid particles significantly affect the initial resistance of the medium. If the filtration pressure used exceeds a certain critical value, the desired conditions for cake filtration will not occur. This is because under such filtering pressure, stable bridging cannot be formed, and particles that fail to bridge will block the pores of the medium. Similarly, if the concentration of the slurry is below a certain value, no bridging will be formed.