Bag filter has a history of more than 100 years. The biggest advantage is that the dust removal efficiency can reach 99.99% or more, the emission concentration can reach 10mg/m3 or less, and the classification efficiency is also high. It also has fine particles below 2.5μm. Because of its good collection efficiency, it has been widely used. Bag filter will produce a large number of damaged waste and old filter bags while it is in operation. However, because most of these filter bags are made of synthetic chemical fiber materials, they have poor biodegradability and are difficult to be degraded in nature; The method of disposal was randomly stacked and became a new source of pollution. Therefore, how to effectively dispose of damaged waste filter bags has become an urgent problem to be solved.
1. Status of Domestic Filter Bag Market
The filter bags used in the domestic early bag filter were ordinary industrial woven fabrics, and the individual high-end applications used the industry. In 1974, the Wuhan Institute of Metallurgy and Safety developed 208 industrial polyester fabrics, which have been widely used. There are some occasions to use. In the early 1980s, Northeastern University and Fushun No. 3 Woolen Textile Factory jointly developed a non-woven filter material for needle felts, from the traditional weaving filter material to the non-woven filter material, and realized a revolutionary change of the filter material. Localization process. Compared with woven filter material, non-woven filter material has short production flow, high production efficiency, high automation degree and low product price, and can be widely applied to various dust removal occasions.
The properties of the filter media depend on the characteristics of the fibers, and at the same time the process changes, the types of fibers are also rapidly developing. From the early natural fiber, polyester fiber, glass fiber filter, to NOMEX, PPS, PTFE and other synthetic fibers, NOMEX, PPS, PTFE and other fibers have been realized in China. P84 fiber and waste incineration use synchronous dust removal to remove the two British filters. Materials are also under development.
2 Damaged Waste Filter Bag Disposal Method
2.1 Remelting and re-drawing
The damaged waste filter bag is recovered, cleaned, returned to the furnace and melted and drawn; the fibers are re-formed and recycled. This is the best way to solve broken and worn filter bags. However, it is difficult to achieve in practice because of the following problems:
2.1.1 Collection and Transportation Issues
The number of damaged and worn out filter bags accumulated over the years is huge. These filter bags are distributed throughout the country. In the case of indeterminate damage times, it is difficult to achieve a unified collection of all of them. Second, due to the waste filter A lot of dust is attached to the surface of the bag, so it is difficult to ensure that the dust does not run around during transportation, especially during long-distance transportation, and becomes a source of mobile pollution and pollutes the surrounding environment.
2.1.2 Cleaning Issues
The cleaning of broken and worn filter bags is a very troublesome and thorny issue. Due to the large amount of dust on the surface of waste filter bags, and the dust components are complex (such as filter bags used in industries such as waste incineration, the surface contains heavy metals, dioxins and other toxic, harmful, corrosive substances), when the waste filter bags are cleaned Produce a lot of sewage and sludge. Moreover, these sludges containing heavy metals, dioxins and other toxic, harmful and corrosive substances are dangerous chemicals, and the disposal of hazardous chemicals is much more complicated. This makes the washing of waste filter bags complicated.
2.1.3 Waste filter bag with complex fiber composition
The early filter bags were mainly made of natural fibers, such as cotton, hemp, and animal fibers. The structure was simple and the composition was simple. At present, the filter bags used are basically two or more types of synthetic fibers mixed and spun (some surfaces are also coated). Such as the gradient filter, the structure from the outside to the inside: the surface layer: microfiber; base layer: fine fiber; skeleton: the back surface layer of the base fabric coarse fiber layer. P84+PPS gradient fiber composite filter belongs to this kind of structure. Due to the low price of glass fiber filter media, gradient structures are made of PTFE, PPS fiber and glass fiber (base cloth), which can effectively reduce the price of filter media and improve filtration performance and cleaning performance; PTFE and PPS fine fibers As a filter, the windward surface layer can effectively trap dust particles, and also has good trapping effect on fine particles; the effect of cleaning is better; and the underlying fabric is mainly used to support and resist frame friction corrosion. Therefore, the gradient filter’s ability to trap tiny particles is significantly higher than that of ordinary filters.
At present, in addition to individual manufacturers in order to reduce production costs, shoddy, most of the filter material is basically made of two or more synthetic fibers mixed spinning. In order to improve the performance of the filter material and the processing process requirements, for example, the key to the processing technology of the needled felt filter material is the uniformity of the fiber, and the key to the uniformity is the opening and the combing of two processes. Due to the short production process flow of needle felt, the uniformity is difficult to control. Therefore, in order to facilitate processing, other fibers are sometimes mixed into the fiber to ensure its uniformity. Such as PTFE fiber, because its fiber dielectric constant is very large, when opening, combing, there is a lot of static electricity, which makes it difficult to open and comb when processing. With the addition of 5% polyester, the opening and carding properties have been greatly improved and processing has also become easier.
In summary, modern filters are made by blending multiple fibers. Therefore, when the cleaned and used filter bags are returned to the furnace for melting and drawing again, they are faced with the following difficult problems: 1) How to determine the melting temperature because the melting temperatures of the various fibers are different; 2) The material of the redrawn yarn It is uncertain; 3) How to guarantee the physical and chemical properties of the re-extracted new silk.
2.1.4 Chemical changes in the fiber components of the original waste filter bag
Bag filter dust components are complex, the use of harsh environments, chemical composition of the filter bag material after long-term use have also undergone chemical changes. The main reasons for chemical changes in the filter bag material are:
(1) Oxidation
Oxidation is one of the main factors of filter bag damage. Fiber oxidation is the process by which molecules in a fiber lose (or dissociate) electrons. This reaction increases the oxygen atoms of the elements in the fiber. Among the commonly used fibers, oxidized mainly are polymeric compounds such as polypropylene, polyphenylene sulfide, and the like. The oxidation reaction principle is as follows:
From the above formula, it can be seen that after oxidation, the macromolecular chains of the fibers are broken up and become small molecule structures. The molecular structure has changed and the fiber properties have also changed.
Another example is polyphenylene sulfide (PPS) fiber, its molecular formula is:
The main chain of the molecule consists of a phenyl ring at the para position to connect a sulfur atom to form a macromolecular main bond. In its structure, at least 85% of the thioether bond (-S-) is directly bonded to two aromatic rings. At high temperature (150°C) conditions, the oxygen molecule attacks the “-S-” bond in the molecule and combines with it to generate SOx. The macromolecular chains are broken up and become small molecular structures. Since the phenyl ring provides rigidity to the PPS fiber, the thioether provides flexibility to the PPS fiber. Therefore, when the oxygen or oxidant in the flue gas is combined with the -S- in the PPS fiber to generate SOx, the PPS fiber discolors, becomes hard, becomes brittle, and the strength is reduced and broken. In severe cases, the web breaks off from the base cloth.
(2) Hydrolysis
Hydrolysis is the reverse reaction of condensation. Fiber hydrolysis is a reaction in which molecules of water are decomposed into two by the introduction of water molecules into the fiber. One part of the parent molecule takes a hydrogen ion (H+) from the water molecule, and the other group gathers from the water molecule. Hydroxy (OH-), the process of breaking its molecular chains to form new small molecules. As the molecular weight becomes smaller, the tensile strength of fibers is weakened and damaged. Therefore, the synthetic fibers produced by polycondensation polymers are not resistant to hydrolysis. Such as commonly used polyesters, polypropylene, NOMEX and other media are prone to hydrolysis.
Hydrolysis will destroy the main structure of the polymer and make the fiber molecules smaller. The higher the water molecule content and temperature in the flue gas, the more severe the hydrolysis of the filter bag. Different filters have different hydrolysis temperatures. Among them, P84 is a kind of poor hydrolysis resistance in the filter bag currently used. After hydrolysis, the strength of the filter bag is seriously reduced and easily damaged. After the hydrolysis of the sewing thread, the filter bag is cracked from the sewing thread so that the filter bag is no longer a tube. It becomes a cloth.
(3) Acid and alkaline corrosion
Corrosion is one of the most common causes of filter bag damage. Flue gas contains a variety of corrosive substances, which have a greater effect of corrosion under high temperature conditions, which can cause damage to the filter bag.
The main reason for filter bag corrosion is that the flue gas contains acid and alkaline components. The dew point changes due to changes in the concentration of these chemicals. For example, when the dust collector is turned on or stopped below the dew point, the SO2 in the exhaust gas is affected by water. H2SO3 will form, which will cause carbonization of the filter bag fiber and damage to the original molecular chain structure, resulting in the formation of small molecule compounds.
The traces of corrosion damage of the filter bag are mostly radial and form a large area discoloration on the surface of the filter bag, causing the filter bag to become hard, brittle, and a few irregular holes, which can be discerned with the naked eye.
(4) High-temperature degradation of fibers
Since most filter bags are operated at high temperatures, high temperatures can cause the filter bag fibers to become vitrified, causing degradation of filter bag fibers, resulting in fiber macromolecular chain breaks. From an external point of view, the high temperature causes the filter bag to shrink and harden, the filter bag is tightly clamped on the frame, and even the frame in the filter bag cannot be pulled out, thus forming deep marks on the inner surface of the filter bag (as shown in Figure 4). ), and make the filter bag fiber vitrification becomes extremely fragile, reduced strength.
2.1.5 Economicality
In summary, due to the problems of collection, transportation, cleaning, secondary pollution, and high disposal costs of waste filter bags, there are also difficulties in reheating, remelting, and drawing, as well as difficult-to-determine problems such as the performance and price of newly drawn fiber yarns. Therefore, the recycling of used filter bags is economically time-consuming and difficult to implement. There is no economic benefit.
2.2 Incineration
Incineration is an effective means to realize the reduction and harmlessness of waste filter bags, and it is also one of the most effective methods to solve the damage and waste filter bags. Although secondary pollution may also occur during incineration, it can be reduced, reduced, and stabilized after incineration. After the waste filter bag is incinerated, the organic synthetic fiber will become CO2 and H2O and other gases, and the glass fiber filter bag will become SiO2 after high-temperature incineration, thus achieving harmlessness.
2.3 Landfill
Landfill is the simplest and most practical method to solve waste filter bags, and it is the most widely used method at present.
2.4 Other uses
At home and abroad, there are currently no reports that waste filter bags have been used in other areas. However, it is reported that some people have also used waste filter bags to clean the livestock after being washed and used in the fences. Therefore, it can be thought that if the used filter bag is used as a base cloth for planting lawns, it can replace the current base cloth used in lawn planting and is also a good use, but it should prevent its contamination of groundwater.
3. Summary
(1) The biodegradability of the filter bag is poor due to the use of chemical fiber synthetic raw materials. Waste filter bags are difficult to be biodegraded in nature and will become new sources of pollution.
(2) Incineration is currently a better way to dispose of used filter bags;
(3) Difficulties in the collection, transportation, cleaning, drying, melting and other processes are the key to the recovery of waste filter bags;
(4) Economy is also one of the major problems in the recycling of waste filter bags;
(5) The most ideal disposal method for waste filter bags is after collection, melting, drawing and reuse, but it is difficult to achieve under the current technical conditions;
(6) Waste filter bags are used to prevent contamination of groundwater when turfs are planted;
(7) Landfilling is the simplest and most practical way to dispose of used filter bags, and is the most widely used method.