1 Traditional treatment and rehabilitation techniques
The primary stage of plastic industry development is the low stage of waste plastic treatment and recycling. The traditional treatment and recovery technology is relatively simple, mainly including direct recovery, landfill or open burning.
1. 1 In the initial stage of direct recycled plastics production, the cost is relatively high and the consumption is low, so the output is not large, and people's requirements for plastic characteristics are not high. Therefore, simple cleaning, separation and crushing of waste plastics can be used as raw materials for new plastics, which can be added to new plastics in a certain proportion for recycling or mixed processing into composite plastics.
1.2 Landfill or open-air incineration For some plastics that are difficult to treat, it is an extremely quick method to use large-area landfill or open-air incineration, but it will cause secondary pollution of waste plastics, and it will be difficult to completely degrade them in a short time after landfill. Waste plastics are polluted by heavy metals and oil pollution, which is not only a kind of waste.
2 New treatment and recovery technologies
In recent years, environmental protection has attracted great attention from governments and people all over the world, which has promoted the rapid development of waste plastic treatment and recycling technology. On the basis of traditional technologies, some new treatment and recovery technologies have emerged, mainly including quantification of combustion heat, pyrolysis of monomers and modification.
2. 1 improvement of traditional treatment and recycling technology direct recycling can not only alleviate the harm of waste plastics to the environment to a certain extent, but also make up for the shortage of raw materials in plastic production, so it has development potential and needs further improvement. With the development of plastic industry, the increase of functional plastics and varieties challenges the separation process in direct recovery technology, and the research and discussion of separation technology is one of the development directions of this method.
At present, most domestic recycling enterprises mainly rely on manual separation, with poor purity, low efficiency and small scale.
2.2 Quantification of combustion heat This method is a relatively mature method abroad, and it is still in its infancy in China. 1987, Shenzhen built the first industrial waste incineration power plant in China, which can burn waste plastics and other wastes, killing two birds with one stone. Its technology and equipment were imported from America.
In Japan, it was put into production in 1988, and its process flow includes receiving, incineration, strong ventilation, waste gas purification, dust removal and ash treatment 10 systems. Since then, Sichuan, Shenyang and other places have built such power plants. Recycled waste plastics can also bring huge economic benefits as fuel for blast furnace injection. Using waste plastics as fuel to burn cement is an efficient and safe heat reuse method. The main problem to be solved by combustion method is how to eliminate the harmful gas released by combustion furnace and make it pollution-free, which restricts the popularization of combustion method.
2.3 Monomer pyrolysis of waste plastics can be used to prepare chemical raw materials (ethylene, styrene, tar, etc. ) and liquid fuels (gasoline, diesel, liquefied gas). This technology has been studied and popularized in China for more than 10 years. Usually divided into thermal cracking and catalytic cracking.
2.3. The separation of pyrolysis waste plastics is complicated. If they are classified and then pyrolyzed, it needs certain equipment investment, energy and time, and the recovery cost is high. Generally, pyrolytic waste plastics that cannot be classified and polluted are heated to their decomposition temperature (600 ~ 900℃) in a reactor, and decomposed, absorbed and purified to obtain usable decomposition products, mainly by using the pyrolysis temperature characteristics of waste plastics.
2.3.2 The thermal cracking reaction temperature of catalytic cracking is high and difficult to control. In order to reduce temperature, save cost and improve yield, catalyst [26] is often used in catalytic cracking. The technology of producing fuel oil by catalytic cracking of waste plastics has a successful precedent in the world. Some small-scale waste plastic oil chemical plants have also been established in China, such as Beijing, Xi and Guangzhou. The selection of cracking catalyst for waste plastics is the key to this technology. There are many patented technologies in this field in China.
2.4. 1 Physical modification Physical modification, that is, modification is mainly carried out by physical methods, and certain solvents and fillers are added to waste plastics to improve the mechanical properties of some general waste plastics and make them recycled. For example, waste polyvinyl chloride (PV C) and polystyrene (PS) are modified into recycled particles; Grinding waste polyester fiber into powder, surface treatment and filling it into photovoltaic products can obviously improve the tensile strength of photovoltaic products. There are many researches on the physical modification of waste polystyrene (PS) plastics in China, such as mixing waste polystyrene with solvent, adding filler to make mold forming agent, and using fiber as filler to make thermal insulation material.
2.4.2 Chemical Modification Chemical modification is a hot research field at present. For example, a widely used 1730 polyester insulating paint with excellent performance was synthesized by alcoholysis polycondensation of waste polyester plastic and polyol; Qin Mei and others esterified and regenerated the alcoholysis products of polyester waste plastics with maleic anhydride and styrene, and obtained U P resin with excellent performance, which provided a new way for the treatment and resource utilization of polyester waste plastics. Zhang Chunsheng et al. grafted waste polystyrene with acrylic acid and active monomer to prepare coatings with good flexibility, adhesion, mechanical properties and weatherability. Waste polystyrene plastic (PS), solvent oil and polyvinyl alcohol were mixed and modified to make building sealant, which was used to seal the gap between doors and windows. Waste plastic modified oil absorbent is used to deal with oil spill accidents at sea.