Green Packaging: PVC Environmental Issues (6)

4.4 Other recycling technologies including co-incineration

A newly developed dissolution-precipitation process is based on the physical process of the polymer chain that does not destroy the feed mixture. This process is mainly applied to composites containing PVC and other ingredients. The PVC in the mixture can be separated by a selective solvent, and then the entire mixture is recovered by a precipitation process. This way, PVC and other ingredients can be reused.

Now, an experimental factory has already begun operation and plans to run the first formal factory in 2001. This technical plant is a closed loop system where solutions can be recycled. 

This process is mainly used to selectively process the collected PVC products. The quality of these products is the same as mechanical recycling, which means that the cost of making these materials manageable can be compared. Advocates of this process hope that such technology can be used to deal with a mixture of complex molecular forms, such as: tarpaulins, cables, medical blister packs, floor coatings, automotive mudguards, and hopefully Other recycling methods compete economically. 

Mixed plastic waste is used by German ironworks as a decrement agent for blast furnace ironmaking. Mixed plastic waste can also be used as a substitute for coal, oil, and gas in cement kilns to generate heat. 

There is some controversy when assessing the environmental impact of using mixed plastic waste in blast furnaces and cement kilns. According to some biosphere analyses, blast furnaces and cement kilns are more favorable than energy incineration of municipal solid waste for energy recovery and prevention of global warming. Considering the potential contribution of PVC to the release of dioxins, it is even more difficult to draw accurate conclusions and therefore further research is needed. 

Blast furnaces and cement kilns can handle mixed plastic waste at lower investment. Therefore, the use of blast furnaces and cement kiln for mixed plastic waste disposal is very competitive with other treatment facilities. On the other hand, blast furnaces and cement kilns have severe restrictions on the content of chlorine in mixed plastics. This is because chlorine can have negative effects on the produced cement and iron, and the formation of HCI may cause corrosion to the equipment. In theory, burning mixed plastic waste in a cement kiln is still a very important process in the future. 

4.5 Incineration

If used for incineration, PVC waste is generally disposed of in a municipal incinerator. Since PVC products are also used in hospitals, PVC waste can also be seen in hospital waste incinerators. About 60,000 tons of PVC are burned every year in a community. PVC accounts for 10% of the total incinerated plastics and accounts for 0.7% of the total incineration waste. In the entire incineration waste, PVC accounts for 38% to 66% of the total chlorine. Other major sources of chlorine are perishable materials (about 17%) and paper (about 10%). On average, 50% of chlorine in the incinerator is due to PVC. 

During the incineration process, PVC waste generates hydrochloric acid (hydrogen chloride) and is released with the flue gas. If there is no technology for reusing hydrochloric acid (hydrogen chloride), the hydrochloric acid (hydrogen chloride) needs to be neutralized. Now in Germany, only five factories have used this special recycling technology and another three are under construction. All acid gases (except hydrogen chloride gas and various oxides of sulfur) from the burning of municipal solid waste must be neutralized before being released into the air. Community legislation has already set the minimum value of hydrogen chloride gas release. This minimum value can be modified in more stringent legislation. 

In order to achieve the minimum release of this hydrogen chloride gas, it is necessary to add neutralizing substances to the incinerator. The most common application is limestone, which can neutralize the acidic components of the incineration gas. The four main processes of neutralization are dry neutralization, semi-dry neutralization, semi-wet neutralization, and wet neutralization.

The evaluation of flue gas scrub residues obtained from the incineration of PVC waste leads to the conclusion that the average per kilogram of PVC can be generated after limestone dry neutralization, semi-dry neutralization and semi-wet neutralization of flue gas. 1 to 1.4 kg of residue; in the process of semi-dry neutralization, using sodium bicarbonate as a neutralizing substance, 1 kg of PVC can produce approximately 0.88 kg of residue; likewise, the wet process can produce 0.4 to 0.9 kg of residual liquid objects. During the incineration of rigid PVC and flexible PVC, there is a clear difference in the amount of neutralizing material required and the amount of residue generated. In contrast, the demand for neutralizing substances and the residue generated from flexible PVC are higher (1 kg of flexible PVC produces 0 to 5 to 0.78 kg of residue).

Flue gas scrubbing residue is defined as hazardous waste. These residues are mixed with fly ash and can also be produced separately in semi-wet and wet systems. These residues include salts produced by neutralization, excessive amounts of neutralizing substances, and also some contaminants such as heavy metals and dioxins that have not been decomposed. With the exception of a few exceptions, the only option for Member States is to landfill these residues. 

A method has been devised to recover calcium chloride and sodium chloride in dry and wet processes, but these methods have not been used for commercial purposes. Except for some special cases, in most cases, it is impossible to determine whether these technological processes can be used to recover useful substances in a large number of residues. These techniques can be thought of as "end-of-the-range" solutions. In contrast, a preventive measure aimed at reducing the source of residue is more welcome. 

Compared with incineration of municipal solid waste that does not contain PVC, the current level of PVC in urban solid waste has the following impact on flue gas scrub residues:

• PVC incineration increased the number of flue gas scrubbing residues (37% for dry, 34% for semi-dry, and 42% for semi-wet). 

• PVC incineration increases the amount of leachable salts in the residue, which are mainly calcium chloride, sodium chloride, and potassium chloride. 

• PVC incineration increased the amount of leachate in the residue sent to the landfill (19% for dry, 18% for semi-dry, 15% for semi-wet, and 4% for wet). The leaking fluid must be treated before it is discharged. 

· There is a theoretical possibility of the increase in complex chlorides due to the incineration of PVC, which leads to increased leakage of material. However, this possibility also needs to be verified through data. 

· In the current temperature range of municipal solid waste incineration, higher chlorine content will not affect the transfer of heavy metals and some trace elements from bottom ash to gas processing residues. 

Since PVC is the largest supplier of chlorine in incinerators, the potential impact of the incineration of PVC waste on the release of dioxin has become a central issue of scientific debate. Between 1993 and 1995, communities accounted for 40% of the contribution of PVC to the emission of dioxins. 

Although the true mechanism has not yet been clarified, it can be estimated that reducing the amount of chlorine in the waste can lead to the formation of dioxins. Even if the actual formation mechanism is not yet clear. It is hoped that this effect of reducing the chlorine content on the formation of dioxins can be the second or third element. Of course, the most important factor affecting the formation of dioxin is incineration parameters such as temperature, oxygen concentration, and so on. 

According to the level of chlorine in the current municipal waste, it seems that there is no direct relationship between the chlorine content and the formation of dioxins. There is a possibility that when the chlorine content in the waste exceeds a certain threshold, It will lead to the formation of dioxins in the incinerator. It has been suggested that this threshold is determined to contain 1% chlorine, but this threshold cannot yet be determined. Further experiments are needed to verify the above threshold. This threshold will increase as the amount of chlorine-containing waste increases. 

At present, not all incinerators in the community have been operating in accordance with the latest air emissions standards for dioxins. Guidance on waste incineration indicates a 01 ng/m3 emission limit. This will reduce the emission of dioxins in the incinerator.  (to be continued)