Summarization of Recycling and Recycling Technologies of Used Plastic Products (I)

With the increasing use of plastics, plastic products have become an important part of people's lives. However, due to the fact that waste plastics are difficult to degrade naturally and are not affiliated with the natural environment, the environmental pollution caused by it is also becoming increasingly serious. According to reports, the world has dumped a large amount of plastic waste into the oceans and rivers each year, which has destroyed the living environment of marine life and caused the death of fish and other marine life. In addition, a large amount of plastic waste is dispersed in the soil and affects the permeability of the soil. It is conducive to crop growth; especially disposable packaging materials, including waste boxes, garbage bags, food packaging bags and other wastes, are called "white pollution." In recent years, serious plastic pollution incidents in many cities have caused people's strong reaction, even provoked the "anti-plastic" sentiment. Therefore, it is extremely urgent to seek a practical comprehensive prevention and control strategy for waste plastics. The key to solving this problem is how to handle and recycle it, and develop new environmentally friendly materials.

1 Sources and Classification of Waste Plastics

Waste plastics mainly come from two aspects: First, municipal solid waste; second, industrial solid waste. In terms of total amount, with the continuous deepening and promotion of cleaner production strategies, the quantity of waste plastics brought by industrial production has been on a downward trend. However, the proportion of waste plastics in urban solid waste has been rising rapidly. According to relevant information, at present, waste plastics account for about 7% of urban solid waste [1]. The waste plastics are of complex composition, mainly including polyethylene (PE), polypropylene (PP), polystyrene (PS), foamed polystyrene (PSF) and polyvinyl chloride (PVC), and others include polyethylene terephthalate. Diol (PET), Polyurethane (PU) and ABS plastics [2]. Except for a small number of waste plastics such as plastic products, the transition material and scrap are in the form of a single plastic, which can be directly recycled. Most of the waste plastics are present in municipal solid waste in a variety of plastic hybrid forms. Since most plastic types are incompatible, products made of mixed plastics have poor mechanical properties. Therefore, plastics (chemical structures) should be classified before recycling plastics.

Classification can be based on the use and nature of different plastics. For example, the commonly used polyvinyl chloride, polystyrene, polypropylene and other plastics can be classified by simple methods such as visual inspection, feel, specific gravity, and burning. For another example, depending on the density difference between different plastics, different kinds of plastics can be placed in a specific solution (such as water, saturated salt solution, alcohol solution, calcium chloride solution, etc.) according to the plastic's ups and downs in the solution. Sex classification and identification. For another example, by using the difference in the solubility of different plastics in the solvent, it is possible to adopt the dissolution and precipitation method for separation. The method is to add waste plastic pieces to a specific solution, control different temperatures, and selectively dissolve and sort various plastics. . In addition, when there is a large amount of waste and there is a lot of debris, wind-screening technology can also be used. This method is to put the crushed waste plastic from above into the gravity screening room, spray air from the horizontal direction, use the weight of the plastic and the resistance to the air. Different screening [3].

2 Waste Plastics Processing Technology

2.1 Health Care

Because waste plastics have a large molecular structure, they are not easy to decompose and rot after being discarded for a long time, and they are light in weight and large in size. Exposure to the space can fly with the wind or float in the water. Therefore, people often use hilly depressions or pits with natural depressions to build landfills for sanitary landfills. Sanitary landfilling method has the advantages of low construction investment, low operating cost, and recovery of biogas, etc. It has become the final disposal method for waste plastics widely used in various countries. In the process of landfill, if reasonable dispatching and operation mechanization can greatly reduce the processing cost. In general, all landfills are provided with impervious layers and compacted by mechanical pressure, covered with soil layers, greened, grassed, built in parks or natural landscapes for people to rest and play. However, landfill disposal also has serious drawbacks: (1) Due to the small density and large volume of plastic waste, plastic waste has a large space area, which increases the pressure on land resources; (2) Plastic waste is hard to degrade and will be buried after landfilling. Become permanent waste, seriously impede water penetration and groundwater circulation; (3) Additives in plastics such as plasticizers or pigments will also cause secondary pollution. At the same time, the Act fills in a large amount of waste plastic that can be used, which runs counter to the sustainable use. Therefore, it is advisable to first crush the waste plastic and its packaging when landfilling, and to landfill the residues that have been comprehensively utilized and comprehensively treated.

2.2 Incineration

Incineration of recovered heat energy is another major method of waste plastics processing. The method of incinerating waste plastics has the advantages of large quantity, low cost, and high efficiency. There are three main methods [4]: ​​(1) The use of a dedicated incinerator to incinerate waste plastics to recover heat energy, and the incinerator used has a flow Bed-type burners, plank-type burners, converter-type burners, etc.; (2) The use of waste plastics as a supplemental fuel in combination with other fuels for the production of steam. This is a viable and relatively advanced energy recovery technology, such as Thermal power plants can use waste plastics as supplementary fuel; (3) By hydrogenation or anaerobic decomposition, the waste plastics are converted into flammable gases or other forms of combustibles, and the heat energy is recovered through their combustion [2]. At present, there are nearly 2,000 incinerators in Japan, and the heat energy recovered from the use of incineration waste plastic accounts for about 38% of the total amount of plastics recovered. There are more than 40 waste plastics incineration plants in Germany. They use the recovered thermal energy for thermal power generation and generate about 6% of the total power generated by thermal power generation. The main products of waste plastics incineration are carbon dioxide and water. However, with the changes in plastic types and incineration conditions, harmful substances such as polycyclic aromatic hydrocarbon compounds and carbon monoxide are also produced. For example, PVC generates HCl, polyacrylonitrile produces HCN, and polyurethane. Will produce cyanide and so on. In addition, waste plastics also contain heavy metal compounds such as cadmium, lead, etc. These heavy metal compounds will be emitted together with smoke, dust and incineration residues during the incineration process, polluting the environment [5]. Therefore, gas emission treatment facilities must be arranged to prevent pollution. Otherwise, if these substances directly enter the atmosphere, the result will be the destruction of the ozone layer, the formation of a greenhouse effect, acid rain, and endanger human health.

(to be continued)