The impact of plastics on the environment As a new type of material, plastics are widely used in various fields, and their annual growth rate reaches 6%, which is higher than other materials. At present, plastics widely replace a large number of metals, wood, paper, glass, and leather in various sectors of industry and agriculture and people's lives. Plastics play an important role in reducing environmental pressure in our modern life, mainly in the following areas:
(1) Packaging: Plastics are widely used in packaging due to their light weight, high strength, good impact resistance, transparency, moisture resistance and corrosion resistance, which can reduce the weight of packages and reduce the damage of packaged articles. 1/4 of the amount;
(2) Automobiles: Automobiles use plastic parts to reduce the emission of fuel exhaust gas;
(3) Construction: Good thermal insulation properties of plastic building materials reduce indoor heating and energy consumption, and can be maintained or painted within 50 years;
(4) Agricultural plastic film: Since the 1980s, agricultural plastic film and greenhouse film have been used and applied more and more widely, which has greatly increased the output of crops. The use of agricultural mulch is of great significance to a large agricultural country like ours.
However, plastic has a resulting weakness, that is, its natural degradation time is longer than 100 years. The degradability of plastics can lead to the long-term existence of wastes. In particular, many of the packaging materials, agricultural plastic film, beverage bottles, etc., belong to the short-lived category of plastics, and are often discarded after they are consumed. Therefore, plastic waste Has become an increasingly prominent environmental issue, the formation of the so-called "white pollution." Therefore, it is necessary to discuss the methods and ways of recycling plastic waste.
Sources of plastic waste Sources of plastic waste mainly include:
1) Production waste refers to wastes in the production process such as waste products, scraps, etc.;
2) Packaging includes various plastic bags, plastic buckets, beverage bottles, foam plastics, etc.;
3) Industrial waste including plastic parts after the scrap of electrical appliances, automobiles, etc.;
4) Agricultural and construction waste includes agricultural plastic film, greenhouse film and various architectural plastic products.
Plastic waste disposal landfill Landfill is the easiest way to dispose of plastic waste. However, the disadvantages of landfilling are obvious: (1) Landfilling means promoting all available resources; (2) Because of the small specific gravity and large volume of plastic waste, the space occupied by landfills is large; (3) Plastic waste is hard to degrade, and it will hinder the reuse of landfills after landfilling (if various plants cannot grow); (4) The additives in plastics precipitate on behalf of contaminated soil and water resources. In summary, it is not advisable to land a large amount of plastic waste. At present, a number of research institutes abroad have been working on research and development of microorganisms or enzymes that can decompose plastics. It has been reported that the Institute of Life Sciences Industry of Japan’s Ministry of International Trade and Industry discovered two new strains of bacteria that can decompose plastics, enabling % of the polyvinyl alcohol was completely degraded in four days and the polyurea resin degraded by 30% in two weeks. Therefore, if a certain kind of microorganism can be introduced into a landfill to degrade plastic, it will be an effective way to solve the plastic waste.
The recycling of recycled plastics is divided into two steps. The first is to separate different types of plastics from mixed waste plastics and then to regenerate them.
Pre-classification and material identification In order to facilitate the reclassification of plastics, many countries in the world have established a plastic identification code, usually divided into seven categories: 01 for PET, 02 for HDPE, 03 for PVC, 04 for LDPE; 05 stands for PP; 06 stands for PS; 07 stands for other plastics. Plastic products should have the above marks to facilitate pre-classification of recycled plastics.
Separation of mixed waste plastics For a large number of mixed waste plastics that cannot be separated manually, other separation technologies are needed, mainly in the following ways:
1) Relative density method Because different plastics have different densities, the crushed plastic is put into the liquid, the relatively dense plastic sinks, and the relative density rises slightly, and the purpose of separation can be achieved;
2) The air separation method utilizes flowing air to separate plastics of different densities, with the smallest density flowing farthest and the largest flowing close. The factors affecting air separation mainly include the size, density and shape of solids;
3) Water Separation A hydrocyclone that uses water instead of air can be used to break down plastics of different densities. The pulverized and washed plastic mixture forms a slurry in the storage tank, and is quantitatively fed into a hydrocyclone by a centrifugal pump, and a smaller density plastic is obtained above the cyclone, and a larger plastic is located below. Selecting the appropriate operating conditions, the plastic will be the best separation, is a successful method;
4) The flotation method uses the difference in the surface tension of plastics for separation. This method uses wetting agents and buoyant liquids, and has nothing to do with the relative density, size and shape of the material, and its separation efficiency is quite high;
5) Electrostatic Separation Method Since plastics have different polarity, there are different states of static electricity generation and electrification, so plastics can be separated. The electrodynamic separation technique is to separate plastic or plastic from paper using a corona of up to 20-40 kv generated by a rotating drum fixed electrode;
6) The X-ray fluorescence separation method uses the X-ray fluorescence emission intensity of the plastic characteristic groups to detect the plastic, and then separates. This technology has been used to separate PVC, but the sensitivity needs to be further improved;
7) The hot melt method separates molten plastics at different temperatures, but this technique has practical problems. On the one hand, there are melt-coincidence zones for different polymers. On the other hand, melt polymers may Destruction of molecular structure;
8) Solvent separation uses solvents to dissolve different polymers at different temperatures to achieve separation;
9) The infrared separation method uses infrared rays to distinguish different plastics. For example, PET and PVC bottles can be separated by infrared technology because infrared light can identify chlorine atoms and the identified PVC can be ejected from the plastic stream using air nozzles.
Recycling plastic waste There are two ways to recycle plastic waste, one is mechanical recycling, and the other is chemical recycling. The mechanical regeneration method is to separate the separated plastic waste of a single type and then melt it out after being extruded in a plastic extruder. At the same time, the pigment and the necessary additives such as ultraviolet stabilizer are added to meet the needs of users. The final small particle. The chemical regeneration method converts a polymer into a monomer or polymer short chain through pyrolysis, hydrogenation, or chemical reagents, and converts it into oil, monomers, and chemical raw materials. In addition, it is also possible to use waste plastics in the blast furnace to produce carbides as components required for steelmaking.
Marked and remanufactured products of recycled products should be marked strictly because the properties of plastics are reduced after regeneration, and the content of impurities is also increased. Moreover, recycled plastic products cannot be used for packaging that is in direct contact with food.
Energy recovery Waste plastic is called “white oil†in Europe and refers to the high energy content of plastics. It is usually the use of plastic calories to replace the fuel used in factories, such as in cement kilns or power plants. However, direct combustion of plastics produces some toxic gases, so advanced combustion furnaces are required, and industrial combustion furnaces are relatively expensive. (author Fu Zhihong, Department of Industrial Equipment and Control, South China University of Technology)
(1) Packaging: Plastics are widely used in packaging due to their light weight, high strength, good impact resistance, transparency, moisture resistance and corrosion resistance, which can reduce the weight of packages and reduce the damage of packaged articles. 1/4 of the amount;
(2) Automobiles: Automobiles use plastic parts to reduce the emission of fuel exhaust gas;
(3) Construction: Good thermal insulation properties of plastic building materials reduce indoor heating and energy consumption, and can be maintained or painted within 50 years;
(4) Agricultural plastic film: Since the 1980s, agricultural plastic film and greenhouse film have been used and applied more and more widely, which has greatly increased the output of crops. The use of agricultural mulch is of great significance to a large agricultural country like ours.
However, plastic has a resulting weakness, that is, its natural degradation time is longer than 100 years. The degradability of plastics can lead to the long-term existence of wastes. In particular, many of the packaging materials, agricultural plastic film, beverage bottles, etc., belong to the short-lived category of plastics, and are often discarded after they are consumed. Therefore, plastic waste Has become an increasingly prominent environmental issue, the formation of the so-called "white pollution." Therefore, it is necessary to discuss the methods and ways of recycling plastic waste.
Sources of plastic waste Sources of plastic waste mainly include:
1) Production waste refers to wastes in the production process such as waste products, scraps, etc.;
2) Packaging includes various plastic bags, plastic buckets, beverage bottles, foam plastics, etc.;
3) Industrial waste including plastic parts after the scrap of electrical appliances, automobiles, etc.;
4) Agricultural and construction waste includes agricultural plastic film, greenhouse film and various architectural plastic products.
Plastic waste disposal landfill Landfill is the easiest way to dispose of plastic waste. However, the disadvantages of landfilling are obvious: (1) Landfilling means promoting all available resources; (2) Because of the small specific gravity and large volume of plastic waste, the space occupied by landfills is large; (3) Plastic waste is hard to degrade, and it will hinder the reuse of landfills after landfilling (if various plants cannot grow); (4) The additives in plastics precipitate on behalf of contaminated soil and water resources. In summary, it is not advisable to land a large amount of plastic waste. At present, a number of research institutes abroad have been working on research and development of microorganisms or enzymes that can decompose plastics. It has been reported that the Institute of Life Sciences Industry of Japan’s Ministry of International Trade and Industry discovered two new strains of bacteria that can decompose plastics, enabling % of the polyvinyl alcohol was completely degraded in four days and the polyurea resin degraded by 30% in two weeks. Therefore, if a certain kind of microorganism can be introduced into a landfill to degrade plastic, it will be an effective way to solve the plastic waste.
The recycling of recycled plastics is divided into two steps. The first is to separate different types of plastics from mixed waste plastics and then to regenerate them.
Pre-classification and material identification In order to facilitate the reclassification of plastics, many countries in the world have established a plastic identification code, usually divided into seven categories: 01 for PET, 02 for HDPE, 03 for PVC, 04 for LDPE; 05 stands for PP; 06 stands for PS; 07 stands for other plastics. Plastic products should have the above marks to facilitate pre-classification of recycled plastics.
Separation of mixed waste plastics For a large number of mixed waste plastics that cannot be separated manually, other separation technologies are needed, mainly in the following ways:
1) Relative density method Because different plastics have different densities, the crushed plastic is put into the liquid, the relatively dense plastic sinks, and the relative density rises slightly, and the purpose of separation can be achieved;
2) The air separation method utilizes flowing air to separate plastics of different densities, with the smallest density flowing farthest and the largest flowing close. The factors affecting air separation mainly include the size, density and shape of solids;
3) Water Separation A hydrocyclone that uses water instead of air can be used to break down plastics of different densities. The pulverized and washed plastic mixture forms a slurry in the storage tank, and is quantitatively fed into a hydrocyclone by a centrifugal pump, and a smaller density plastic is obtained above the cyclone, and a larger plastic is located below. Selecting the appropriate operating conditions, the plastic will be the best separation, is a successful method;
4) The flotation method uses the difference in the surface tension of plastics for separation. This method uses wetting agents and buoyant liquids, and has nothing to do with the relative density, size and shape of the material, and its separation efficiency is quite high;
5) Electrostatic Separation Method Since plastics have different polarity, there are different states of static electricity generation and electrification, so plastics can be separated. The electrodynamic separation technique is to separate plastic or plastic from paper using a corona of up to 20-40 kv generated by a rotating drum fixed electrode;
6) The X-ray fluorescence separation method uses the X-ray fluorescence emission intensity of the plastic characteristic groups to detect the plastic, and then separates. This technology has been used to separate PVC, but the sensitivity needs to be further improved;
7) The hot melt method separates molten plastics at different temperatures, but this technique has practical problems. On the one hand, there are melt-coincidence zones for different polymers. On the other hand, melt polymers may Destruction of molecular structure;
8) Solvent separation uses solvents to dissolve different polymers at different temperatures to achieve separation;
9) The infrared separation method uses infrared rays to distinguish different plastics. For example, PET and PVC bottles can be separated by infrared technology because infrared light can identify chlorine atoms and the identified PVC can be ejected from the plastic stream using air nozzles.
Recycling plastic waste There are two ways to recycle plastic waste, one is mechanical recycling, and the other is chemical recycling. The mechanical regeneration method is to separate the separated plastic waste of a single type and then melt it out after being extruded in a plastic extruder. At the same time, the pigment and the necessary additives such as ultraviolet stabilizer are added to meet the needs of users. The final small particle. The chemical regeneration method converts a polymer into a monomer or polymer short chain through pyrolysis, hydrogenation, or chemical reagents, and converts it into oil, monomers, and chemical raw materials. In addition, it is also possible to use waste plastics in the blast furnace to produce carbides as components required for steelmaking.
Marked and remanufactured products of recycled products should be marked strictly because the properties of plastics are reduced after regeneration, and the content of impurities is also increased. Moreover, recycled plastic products cannot be used for packaging that is in direct contact with food.
Energy recovery Waste plastic is called “white oil†in Europe and refers to the high energy content of plastics. It is usually the use of plastic calories to replace the fuel used in factories, such as in cement kilns or power plants. However, direct combustion of plastics produces some toxic gases, so advanced combustion furnaces are required, and industrial combustion furnaces are relatively expensive. (author Fu Zhihong, Department of Industrial Equipment and Control, South China University of Technology)
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