In the production process of asphalt mixture mixer, the processes of drying, sieving, weighing and stirring will have a large amount of dust and harmful exhaust gas generated by fuel combustion, which will cause pollution of the natural environment. Modern mixing mixers use a two-stage dust removal device consisting of a cyclone dust collector and a bag filter to collect dust contaminants as much as possible to reduce pollution to the natural environment and meet the requirements of national environmental protection regulations. . Taking the 4000 type asphalt mixture mixer as an example, the bag type dust collector has more than 600 heat-resistant (>200 °C) synthetic fiber bags in the box, with a working area of 1400 m2 and a maximum exhaust capacity of 65300 Nm3/h. The mesh is small, and dust of 0.3 μm or more can be collected in the filtration. At present, the dust removal efficiency of the bag type dust collector for the asphalt mixture mixer with advanced technical performance can reach 97% to 99%, and the dust-containing gas is purified to the extent of 15 mg/Nm3. 1Select the principle of filter cloth
The filter cloth is usually made of natural fibers such as cotton or wool or synthetic fibers. Raw materials have different physical and chemical properties, suitable conditions for use or working environment (temperature, humidity, corrosion...). Therefore, the correct selection principle of the filter material of the bag mixer should be to fully understand the physical and chemical properties of the dust-containing gas discharged from the mixing machine during the production process, and carefully compare the technical properties of various fibers. , to choose the right one.
2 the correct choice of filter cloth
The method of correctly selecting the filter cloth includes: selecting according to the physical and chemical properties of the dust-containing gas; selecting according to the properties of the dust; selecting according to the cleaning method of the bag type dust collector, and the like.
2.1 Select filter cloth according to the physical and chemical properties of dust-containing gas
The physical and chemical properties of dust-containing gases include temperature, humidity, corrosivity, flammability and explosiveness.
2.1.1 Temperature of dusty gas
The temperature of the dust-containing gas discharged from the mixer is the primary factor for the correct selection of the filter cloth for the bag filter. According to the continuous use temperature, the filter cloth can be divided into three categories: normal temperature filter cloth (less than 130 ° C), medium temperature filter cloth (130 ~ 200 ° C) and high temperature filter cloth (more than 200 ° C). For working conditions with high temperature fluctuations of dusty gas, the safety factor should be chosen to be slightly larger, but the instantaneous peak temperature should not exceed the upper limit temperature of the filter cloth. For the high-temperature dust-containing gas discharged from the mixer, the high-temperature filter cloth can be directly used.
2.1.2 Humidity of dusty gas
The humidity of the dust-containing gas is another important factor in the correct selection of the filter material for the bag mixer. The humidity of the dust-containing gas indicates how much water vapor is contained in the gas, and is usually expressed by the volume fraction of water vapor Xw or relative humidity ψ in the dust-containing gas. When Xw is more than 8%, or ψ is less than 80%, it is called a wet dust-containing gas. For wet dusty gases, the following points should be noted when selecting filter cloth:
1) The wet dust-containing gas wets and bonds the dust trapped on the surface of the filter bag, especially for water-absorbing, deliquescent dust, and even causes a paste bag. For this purpose, nylon, glass fiber and other filter cloths with smooth surface and easy-to-clear fiber material should be used, and the filter cloth should be impregnated with silicone oil or fluorocarbon resin, or acrylic or PTFE on the surface of the filter cloth. Coating treatment.
2) When the flue gas discharged from the mixture mixer has high temperature and high humidity at the same time, it will affect the temperature resistance of the filter cloth, especially the materials with poor hydrolytic stability such as polyamide, polyester and imide. Therefore, the filter cloth material should be carefully selected when repairing the bag filter (replacement filter bag), and the water content (≤ 3%) of the cold aggregate should be strictly controlled during the production process of the mixture mixer.
2.1.3 Corrosiveness of dusty gases
The chemical resistance of different fibers varies and is often affected by the influence of various factors such as temperature and humidity. In comparison, PTFE fibers known as "Plastic King" have the best chemical resistance, but are more expensive. Therefore, when using the filter cloth, it is necessary to grasp the main factors according to the chemical composition of the dust-containing gas, and to select a suitable material.
2.2Select filter cloth according to the properties of dust
The traits of dust include the chemical and physical properties of dust. The emphasis is on the physical selection of dust, the correct selection of materials, structures and post-treatments.
2.2.1 Dust shape and particle size distribution
Usually, the high temperature combustion process is mostly regular dust, and most of the dust particles generated by the process are irregular. The regular-shaped dust has a smooth surface and a small specific surface area, and is not easily intercepted and agglomerated when passing through the filter cloth; on the contrary, the irregular shaped dust has different shapes, a rough surface, a large specific surface area, and is easily intercepted and aggregated when passing through the filter cloth. Pay attention to the following points when using the filter cloth for the bag filter:
1) Fiber should be selected as thin, short, curly, irregular section;
2) The structure is excellent in needle felt, if the fabric is used for twill weave or the surface is subjected to brushing;
3) Needle felt with density gradient when mixing thick and fine cotton wool layer, and new technology such as surface spraying, dipping or film coating to realize surface filtration is the development direction of filter cloth for ultrafine dust;
4) Fine particle dust is difficult to capture and it is easy to form a dense dust layer after trapping, which is not conducive to cleaning. On the contrary, the coarse particle dust is easy to be collected, and the dust layer formed after the collection is loose, which is favorable for cleaning. In a sense, the dust of the combination of thickness and thickness is beneficial to both filtration and cleaning. 2.2.2 Adhesion and cohesion of dust
The dust has characteristics of adhering to each other or adhering to the surfaces of other objects, and when the suspended dusts come into contact with each other, they are adsorbed and aggregated with each other. The cohesive force of dust is related to various factors such as its type, shape, particle size distribution, water content and surface characteristics. The angle of repose can be used to characterize the cohesive force of dust. For example, an angle of repose of less than 30° is called low adhesion, and fluidity is good; when the angle of repose is greater than 45°, it is called high adhesion and poor fluidity. In addition, the viscosity (adhesion and cohesiveness) of the dust and the solid surface is also related to the roughness and cleanliness of the solid surface.
2.2.3 Moisture absorption and deliquescence of dust
The ability of dust to absorb moisture in a gas is called hygroscopicity. If hydraulic power is the main component, it is called moisture infiltration. Hygroscopicity and wettability are achieved by the action of a capillary formed between dust particles. Hygroscopicity and wettability are related to factors such as the atomic chain of the dust, the surface state, and the surface tension of the liquid. The combination can be characterized by the wetting angle—hydrophilicity of less than 60° and hydrophobicity of greater than 90°. When the humidity of the hygroscopic dust increases, the cohesive force and adhesion of the powder increase, and the fluidity and chargeability decrease, which causes the dust to adhere to the surface of the filter bag. Over time, the dust is invalidated and the dust forms a plate.
In the dust discharged from the mixer, such as CaO, CaCl2, KCL, MgCl2, etc., the chemical reaction will further occur, and its nature and morphology will change accordingly. This is deliquescence. They stick to the surface of the filter bag, which is the least desirable phenomenon for bag filters.
For hygroscopic and deliquescent dust, the precautions to be taken when selecting a filter cloth are the same as "selecting the filter cloth according to the characteristics of the dust-containing gas".
2.2.4 Dust of the dust
When the filter cloth filters, intercepts, and agglomerates dust, the destructive effect of dust (especially irregular dust) on the filter cloth is called abrasion. It is related to the nature and shape of the dust, as well as the airflow velocity and dust concentration of the dust. For example: aluminum powder, silicon powder, carbon powder, hard materials such as sintered ore, a high grinding dust pecking; particle surfaces rough, irregular sharp-edged surface area smooth dust, grinding dust pecking spherical particles many times larger to be (About about 10); the dust with a particle size of about 90μm has the highest sharpness, and when the particle size of the dust is reduced to 5~10μm, the sharpness is very weak; the dust is grinded and the airflow speed is 2 ~3.5 times the square is proportional. Therefore, in order to reduce the grinding property of the dust on the filter cloth, the speed and uniformity of the exhaust gas flow of the mixture mixer must be strictly controlled. In addition, for dust with high abrasion resistance, a filter cloth with good abrasion resistance should be used.
For the grinding of the dust, the following points can be used for reference when using the filter cloth:
1) Chemical fiber is superior to glass fiber, and expanded fiber is superior to general glass fiber;
2) Fine, short, crimped fibers are superior to coarse, long, straight fibers;
3) The felt material is superior to the fabric. In the felt material, the interlacing property between the fibers should be strengthened by needle punching. The satin fabric is optimal in the fabric, and the brushed surface of the fabric can improve the wear resistance.
4) Surface coating, calendering and other post-treatment can improve the wear resistance of the filter cloth. For the glass fiber filter cloth, silicone oil, graphite , and polytetrafluoroethylene resin can improve the wear resistance and folding resistance.
2.3 Select the filter cloth according to the cleaning method of the bag filter
The cleaning method of the bag filter is another important factor that must be considered when selecting the filter cloth correctly. That is to say, the bag type dust collectors of different cleaning methods should adopt different structures due to different cleaning energy and deformation characteristics of the filter bag. Variety of filter cloth.
2.3.1 Mechanical vibrating cleaning method
The bag filter of mechanical vibration cleaning method uses mechanical devices (including manual, electromagnetic vibration and pneumatic) to vibrate the filter bag, and the vibration frequency ranges from several times to several hundred times per second. In addition to small-mix mixers, these bag-type dust collectors are mostly in the form of inner-Filter Bags. The utility model is characterized in that the kinetic energy applied to the dust layer is less and the number of actions is more, and the filter cloth is required to be thin and smooth, and the texture is soft, which is favorable for transmitting vibration waves so as to form sufficient vibration force on all the filtering surfaces. Therefore, the bag mixer of the mixture mixer usually uses a satin or twill fabric woven from chemical fiber short fibers, the thickness is 0.3~0.7mm, the mass per unit area is 300~350g/m2, and the recommended filtering speed is 0.6~ 1.2m/min (small mixture mixer can be appropriately increased to 1.5~2.0m/min).
2.3.2 Sub-room back-blowing cleaning method
The baghouse type dust-removing bag type dust collector adopts a compartment structure and a valve to switch by chamber to form a reverse air flow, forcing the filter bag to contract or bulge to clear the ash. This kind of cleaning method is also a low-kinetic type cleaning, and the working pressure of the bag type dust collector is used as the cleaning power, and the back-blowing airflow power is additionally provided in special occasions. The bag type dust collector of the chamber back-flush type cleaning method requires a thin filter cloth with soft texture, easy deformation and dimensional stability. Since the bag type dust collector has internal filter and external filter, the filter cloth selection is slightly different. . Generally speaking, the inner filter type is usually a round bag with no frame, the bag diameter is 120~300mm, and the bag length to bag diameter ratio is 15~40. It is preferred to use satin (or twill) woven filter cloth, thickness 1.0~1.5. Mm, the mass per unit area is 300~400g/m2; the outer filter type commonly used flat bag, diamond bag and honeycomb bag, with support frame, prefer to use thin needle felt fabric filter cloth with good wear resistance and permeability, unit area It is 350~400g/m2.
2.3.3 Vibration backflushing and cleaning method
The bag type dust collector with vibration back-flushing and cleaning method refers to a bag type dust collector which has both the vibration and the reverse air flow double-cleaning action, and the vibration causes the dust cake to loosen and the reverse air flow to dissociate the dust. The two methods work together to improve the cleaning effect, especially for the filtration of fine particle viscous dust. The filter cloth of such bag type dust removal is selected, and is substantially the same as the bag type dust collector of the compartment back-flush type.
2.3.4 nozzle back-blowing cleaning method
The bag type dust-removing bag type dust collector uses a high-pressure fan or a blower as the back-blowing cleaning power, and sequentially sprays the filter bag through the moving nozzle to form a strong reverse airflow, so that the filter bag is sharply deformed and cleaned. It is a medium energy cleaning type. According to the nozzle form and its movement track, it can be divided into three types: rotary back-blow type, reciprocating back-blow type and gas ring sliding back-blow type.
The bag-type dust remover of the rotary back-flushing type and the reciprocating back-flushing type is a compact outer bag with a frame. Such bag type dust collectors are required to use a filter cloth which is relatively soft, structurally stable and has good wear resistance. It is preferred to use a medium thickness needle felt filter material with a mass per unit area of 350 to 500 g/m2. China has more choices of tubular satin weave filter cloth.
The bag type dust-removing bag type dust collector adopts an inner filter bag, and the nozzle is a ring-shaped slit. The sleeve is moved up and down on the outside of the bag, and the filter is required to be thick, wear-resistant, rigid, and lint-free. Cloth, wool compression felt is preferred, synthetic fiber needle felt can also be used, and the mass per unit area is 600~800g/m2.
2.3.5 Pulse blowing type cleaning method
The bag-type dust-removing bag type dust collector is powered by compressed air, and uses a pulse blowing mechanism to release the compressed air stream instantaneously, inducing several times of secondary air to be injected into the filter bag at high speed, so that the filter bag is rapidly expanded. * Shock vibration and reverse gas and clear ash, which is a type of high kinetic energy cleaning. An outer filter bag or a flat bag with a frame is usually used. Such bag type dust collectors are required to use thick filter cloth with strong wear resistance and strong tensile strength. The chemical fiber needle felt or compression felt filter cloth is preferred, and the mass per unit area is 500~650g/m2.
