The bag dust collector has a history of more than 100 years. Its biggest advantage is that the dust removal efficiency can be as high as 99.99%, the discharge concentration can reach below 10mg/m3, and its classification efficiency is also very high. It also has a good capture efficiency for fine particles below 2.5 μ m. So it is widely used. Bag filter can produce a large number of broken waste and old filter bags at the same time it is difficult to be degraded in nature because most of these filter bags are made of synthetic chemical fiber raw materials and their biodegradability is poor. Because there is no effective disposal method at present, it becomes a new source of pollution after stacking at will. Therefore, how to deal with the broken waste filter bag effectively has become an urgent problem to be solved at present.

1 .current situation of domestic filter bag market

Bag type dust collector bag for early domestic use of industrial woven ordinary, individual high-end applications by industry, 1974 Wuhan metallurgical Safety Research Institute developed 208 industrial polyester cloth, widely used, until now there are still some places in the.20 century at the beginning of 80s, Northeastern University and Fushun third woolen mill jointly developed by weaving needled nonwovens, the traditional filter to realize the filter nonwovens, the revolutionary transformation, began the domestic media process. Compared with the non weaving material, weaving machine filter short production flow, high production efficiency, high degree of automation, low prices, can be widely used in various occasions of dust.

The properties of the filter media depend on the properties of the fibers, and the types of fibers are developing rapidly while the process is changed. From the early natural fibers, polyester fibers, glass fiber filter materials to synthetic fibers such as NOMEX PPS PTFE, At present, all the fibers, such as NOMEXY PPS PTFE and so on, have been made into domestic P84 fiber, and the simultaneous dust removal for MSW incineration is also in the process of being developed.

At the same time, the production of filter media is developing rapidly. The domestic production of filter media from 1985 to 2008 is shown in Table 1.

As can be seen from Table 1, in the 20 years from 1985 to 2005, the production of filter media in China has increased by more than 23 times. Since the beginning of the 21st century, the development of filter media has been even faster, from 2005 to 2008 in only three years. The production of filter media has almost doubled. In 2008, there were 15 filter media enterprises with an annual output of more than 1.5 million meters and five foreign-funded enterprises in our country. There are 2 large enterprises with annual output of more than 5 million meters. The domestic production of filter media is expected to reach more than 90 million meters in 2010. It can be seen that the number of broken waste filter bags that have been eliminated and accumulated over the years is also astronomical.

2.Treatment method of broken and used filter bag

2.1 Furnace melting and redrawing

The broken waste filter bag is recovered, cleaned and melted back to the furnace, drawn, remade into fiber and recycled. This is the best way to solve the broken waste filter bag. However, in practice, it is difficult to achieve because of the following problems:

2.1.1 Collection, transport issues

The number of broken and used filter bags accumulated over the years is enormous, which is distributed throughout the country and is difficult to collect and deal with in a unified manner when the time of damage is uncertain; secondly, Because a large amount of dust is attached to the surface of the waste filter bag, it is difficult to ensure that the dust does not fly around in transportation, especially in the long distance transportation, and become a flowing pollution source and pollute the surrounding environment.

2.1.2 cleaning problems

The cleaning of broken waste filter bags is a very troublesome and difficult problem. Because there is a large amount of dust on the surface of the waste filter bags, and these dust components are complex (such as waste incineration, the filter bags used in industries such as waste incineration, the surface contains heavy metals, dioxin, etc., toxic, etc.). Harmful, corrosive substances, such as waste filter bags, will produce a large amount of sewage and sludge. Moreover, these sludge containing heavy metals, dioxins and other toxic, harmful, corrosive substances are dangerous chemicals. The disposal of hazardous chemicals is much more complicated. This complicates the cleaning of waste filter bags.

2.1.3 complex fiber components in waste filter bags

The early filter bags were mainly made of natural fibers, such as cotton, hemp and animal fibers, with simple structure. At present, the filter bag is composed of two or more kinds of synthetic fibers. (some surfaces are coated with film. For example, gradient filter material, its structure from outside to inside is as follows: surface layer: ultrafine fiber; Base: fine fiber; skeleton: the coarse fiber layer after the base cloth. P84 + PPS gradient fiber composite filter material belongs to this kind of structure. Because of the low price of the glass fiber filter material, the gradient structure is made from PTFEPPS fiber + glass fiber fiber (base cloth). It can effectively reduce the price of filter media, improve the filtration performance and ash removal performance. PTFE and PPS fine fibers can effectively trap dust particles as the upwind layer of filter media, and have a better capture effect on tiny particles, and the ash removal effect is better. The substrate is mainly used to support and resist the friction corrosion of the skeleton. Therefore, the trapping ability of gradient filter media to fine particles is obviously higher than that of ordinary filter media.

At present, in order to reduce the cost of production, most of the filter materials are made from two or more kinds of synthetic fibers, except for a few manufacturers. In order to improve the properties of the filter materials and the requirements of the processing technology, For example, the key to the processing process of needle-felt filter material is fiber evenness, and the key to evenness lies in the opening and combing of the two processes. Because of the short production process, the uniformity is very difficult to control. Sometimes other fibers are mixed into the fibers for easy processing to ensure their evenness. For example, PTFE fibers, because of their very large dielectric constant, open loose, carding produce a lot of static electricity, resulting in processing difficult to open loose, Carding. After adding 5% polyester, its carding performance is greatly improved, processing has become easier.

To sum up, the modern media is a variety of hybrid fiber spinning. Therefore, when washing the waste bag again melted down, and face the difficult to solve the problem of drawing: 1) how to determine the melting temperature, melting temperature for various fiber is different; 2) to pull out of the silk material is not sure; 3) the new wire to pull out the physical and chemical properties to guarantee.

2.1.4 Chemical change of fiber composition in waste filter bag

The smoke composition of bag dust collector is complex and the use environment is bad. The chemical composition of filter bag material has also changed after a long period of use. The main reasons for the chemical change of filter bag material are as follows:

（1）Oxidation

Oxidation is one of the main factors in the damage of filter bags . The oxidation of fibers is the process of loss ( or dissociation ) of electrons in the fiber . This reaction can increase the oxygen atoms of the elements in the fiber . The most common fibers are polymeric compounds such as polypropylene , polyphenylene sulfide , etc . The principle of oxidation reaction is as follows :

It is seen from the above that the large molecular chains of the oxidized fibers are broken into small molecular structures. The molecular structure has changed, and the properties of the fibers have changed.

In the case of a polyphenylene sulfide (PPS) fiber, its molecular formula is:

The molecular main chain is formed by connecting benzene ring with sulfur atom on the opposite site to form a large molecular primary bond. At least 85% of the sulfur bond in its structure is directly bound to two aromatic rings. At 150 ℃ at high temperature, this is the "-S-" bond in the attacking molecule of oxygen molecule. And combine it to form SOx. so that the macromolecular chain is broken into a small molecular structure. Because benzene ring provides rigidity of PPS fiber and sulfide provides compliance of PPS fiber. When oxygen or oxidant in flue gas binds with -S- in PPS fiber to form so _ x, causing PPS fiber to discolor, harden, brittle, reduce strength and break down, and when serious, the fiber mesh will break away from the base cloth (see figure 1)

（2）Hydrolysis

Hydrolysis is the inverse reaction of condensation. Fiber hydrolysis is a reaction in which polymers are decomposed into two due to the involvement of water molecules in the fibers. One part of the parent molecule acquires a hydrogen ion from the water molecule, while the other group aggregates the hydroxyl group from the water molecule, causing its molecular chain to break to produce new small molecules. The tensile strength of the fibers is weakened and damaged. Therefore, synthetic fibers produced by Polycondensation polymers are not hydrolytic. For example, common polyester, polypropylene, Filtration media such as Knomex are easily hydrolyzed. Polyester and Nomex fibres are hydrolyzed as follows:

Hydrolysis will destroy the main structure of the polymer and make the fiber molecules smaller. The higher the content and temperature of water molecules in the flue gas, The more serious the hydrolysis of the filter bag, the more different the hydrolysis temperature of the different filter material is. P84 is one of the poor hydrolytic resistance of the filter bag used at present. After hydrolysis, the strength of the filter bag decreases seriously, and the bag is easily damaged, and after the sewing thread is hydrolyzed, The filter bag cracks from the sewing thread so that the bag is no longer cylindrical and becomes a piece of cloth (see figure 2).

（3）Acid, alkaline corrosion

Corrosion is one of the most common causes of filter bag damage.

The main reason for the corrosion of the filter bag is that the flue gas contains acid and alkaline components, which change the dew point with the change of the concentration of these chemicals, such as when the dust collector is turned on or stopped below the dew point, When SO2 in waste gas meets water, H _ 2SO _ 3 is formed, which results in carbonization of filter bag fiber, damage of original molecular chain structure and formation of small molecular compound.

Most of the traces of the corrosion damage of the filter bag are radial, and a large area of discoloration is formed on the surface of the filter bag, which results in the filter bag becoming hard, brittle and a few irregular holes appear. Figure 3 shows a corroded filter bag in the chlorination section of a magnesium oxide plant in Henan Province. Due to the very high HCl content in the flue gas of the plant, the filter bag was corroded with holes in less than 3 months on average. Therefore, If pulse cleaning is carried out under this condition, the damage of filter bag will be accelerated.

（4）High temperature degradation of fiber

Because most of the filter bags work at high temperature, the high temperature will result in the glass state of the filter bag fiber, so that the filter bag fiber will degrade and change, resulting in the fiber macromolecular chain breaking. On the outside, the high temperature causes the filter bag to shrink and deform and harden. The filter bag is so tightly strapped to the skeleton that it cannot even be extracted from the filter bag, thus forming deep traces on the inside of the filter bag (as shown in figure 4), and making the fiber vitrification of the filter bag extremely fragile. Figure 5 is the filter bag of a garbage incinerator in Guangzhou. The filter bag is made of PPS fiber. The high temperature makes the PPS fiber extremely fragile.

2.1.5 economy

To sum up, due to the problems of collection, transportation, cleaning, secondary pollution and high cost of disposal of the waste filter bags, there are also the difficulties of heating, melting, redrawing, and the difficulty of determining the properties and prices of the newly drawn fibers. Therefore, the recovery of waste filter bags is time-consuming and difficult to achieve economically.

2.2 incineration

The waste incineration is an effective means of bag reduction, harmless, is currently one of the methods to solve the damage, the waste bag most effective. Although in the incineration process there are two possible contamination, but after incineration can realize the reduction of volume reduction and stabilization of waste incineration. The filter bag after organic synthetic fiber a CO2 and H2O gas, and the glass fiber filter bag by high temperature incineration fiber will become SiO2, so as to realize harmless.

2.3 Landfill

Landfill is the most simple and practical method to solve the waste filter bag.

2.4 other uses

There are no reports at home and abroad that the waste filter bags have been used in other fields, but it is also reported that some people have used the waste filter bags in the enclosure after cleaning, and in captivity for livestock. This may be associated with the use of the waste filter bags as a base cloth for planting lawns. It is also a good use to replace the undercloth used in the current lawn planting, but to prevent the pollution of groundwater.

3 Summary

(1) the biodegradability of the filter bag is poor because of the use of synthetic raw material of chemical fiber. The waste filter bag is difficult to biodegrade in nature and will become a new source of pollution.

(2)  incineration is a better way to treat waste filter bags at present;

(3) Collection, transportation, cleaning, drying, melting and other processes are difficult to recover the key point of waste filter bags;

(4)  economy is one of the main problems that waste filter bag is difficult to recover.

(5)  the most ideal disposal method of waste filter bag is to melt wire drawing and reuse after collection, but it is difficult to realize under the present technical condition;

(6) waste filter bags should be used to prevent groundwater pollution when planting lawns;

(7) Landfill is the most simple and practical method to dispose waste filter bags.