1 Environmental impact of waste plastics
1. 1 "white pollution" caused by waste plastic packaging materials
Plastics are widely used as packaging materials for various products because of their low price and convenient molding. With the development of modern plastics industry and the improvement of consumption level, most of them are disposable, causing environmental pollution. For example, white foamed polystyrene plastics are used as packaging materials for household appliances, industrial instruments, fast food boxes, beverage cups, etc. It is characterized by its large volume, light weight, non-decay, non-decomposition, and discarded after use, resulting in white foam everywhere along the railway, rivers, cities and scenic spots, which seriously affects the environment and urban sanitation, and is called "white pollution".
According to literature reports, the total output of plastic products in China was nearly 7 million tons in 1995, of which about1700,000 tons were used as packaging materials. About 500 thousand tons of these packaging materials enter the municipal solid waste treatment system as solid waste, and the other part is thrown away by users, forming plastic waste. These plastic wastes are scattered on the roadside, green space or street trees in busy cities and tourist attractions, which destroys the urban landscape and affects the appearance of the city. In windy cities, littered plastic bags dance with the wind, which poses a great threat to the urban power supply system. 1996 there have been more than a dozen short-circuit accidents caused by plastic bags winding on overhead power supply lines in Tianjin. Plastic packaging of food and beverage is a hotbed for mosquitoes, flies and bacteria to survive and reproduce, which can easily cause the spread of germs and affect environmental sanitation and urban public health.
1.2 Impact of waste plastic landfill on environment
At present, sending waste plastics as garbage to municipal solid waste landfill is the main method to treat waste plastics. With the rapid development of plastics industry, the output of waste plastics is directly proportional to it, and the environmental problems caused by it are increasingly prominent. First of all, landfill encroaches on limited cultivated land and wastes land and resources seriously. Second, plastic waste will not rot for a long time after landfill, which will endanger the future. Corrosion resistance and bacteria resistance are one of the advantages of plastic products, but they have become a headache for scientists after becoming garbage. In the absence of air and light, it is difficult for microorganisms to decompose organic matter, and plastic waste can also be decomposed after 200 years of landfill. Thirdly, the plastic waste is washed away by rain for a long time after landfill, which brings a lot of harmful substances into the living environment of human beings and causes harm to future generations. To sum up, it is a long-term effect to treat the environmental pollution of plastic waste by landfill, and this method should be avoided in actual disposal.
The use of agricultural plastic film has brought great development to agricultural production, but the damaged and littered plastic film has also caused great harm to the soil. Film fragments form a barrier layer on the soil, which makes the cultivated land worse, hinders the development of plant roots and the absorption of water and nutrients, and poisons the soil.
1.3 Impact of waste plastics incineration on the environment
In order to solve the problems of large area, high cost and long-term damage to the environment, many countries are actively developing waste plastic incineration equipment, and using the heat generated by incineration to generate electricity to realize resource reuse. However, this seemingly simple method hides great harm.
In the process of thermal decomposition of plastics, the polymer is cracked and a large number of harmful gases are released. For example, the composition of polystyrene plastic can remain unchanged below 80℃, and when it exceeds 280℃, the molecular weight begins to decrease, resulting in volatile gas, which contains 44% styrene monomer, 22% dimer and trimer, and a small amount of environmentally harmful substances such as toluene and ethylbenzene. Burning PVC plastics not only produces chlorine gas, hydrogen chloride and dioxin gas, but also produces harmful gases such as C0, NOx, formaldehyde, vinyl chloride and styrene, which has a great impact on the ecological environment.
In the process of plastic incineration, inorganic metals such as plastic filling and dyeing also volatilize into the atmosphere, such as harmful substances such as lead and arsenic, causing air pollution.
As can be seen from the above, with the continuous development of plastic industry, plastic waste has an increasing impact on the environment, which has caused certain harm to people's production and life. Therefore, how to prevent the harm of plastic waste has become the focus of attention of plastic workers and environmental protection workers in various countries. At present, in addition to landfill and incineration, there are also chemical methods and physical methods developed according to the principles of reduction, harmlessness and recycling, which make plastic waste resources, turn waste into treasure and benefit mankind.
2 Waste and resource treatment methods and technologies
The recycling of waste plastics is an important means to protect the environment and an important way to make full use of existing human resources. It is reported that the average annual growth rate of recycled plastics in the United States reaches 16%, and 1988 reaches 2 15000 tons. China's plastics industry started late, and the recycling of waste plastics has just started. In major cities, some self-employed people treat waste plastics with cheap equipment and cheap labor. These stalls often bring secondary pollution in the treatment and cause more serious damage to the environment. For example, Chengdu Business Daily reported in June that more than 80 primary school students in Qinjiamiao Primary School in Pixian County were collectively poisoned, 1999, because toxic gas leaked when an operator used cheap equipment to treat waste plastics. It is also reported that an individual processing stall in Shuangliu uses hospital waste plastic needles to make folk packaged food bags, and the harm caused by its inflow into society can be imagined.
2. 1 plastic recycling treatment method
The recycling of plastics is different from the recycling of metals, paper and glass. Due to the differences in physical and chemical characteristics of various plastics and the incompatibility of various plastics, their mixtures are not suitable for processing. Therefore, each plastic must be collected separately and then treated separately. In the process of collection and classification, it is necessary to prevent dirt and foreign impurities from mixing, otherwise it will affect the reuse of recycled materials.
Different types of plastics have different service lives. For example, low-density polyethylene is mainly a film product with a service life of 1 ~ 2 years; The service life of PVC pipes is longer, reaching more than 10 years. Therefore, when processing and utilizing waste plastics, appropriate recycling methods should be selected according to the sources, types, pollution levels and mixing conditions of plastics, so as to transform waste plastics into materials suitable for reprocessing. Table 1 shows the recycling methods of different kinds of plastics.
Table 1 Treatment Methods of Different Kinds of Plastics
raw material
Treatment method product category
kind
colour
A PE, PP, PS, bedding, plastic boxes, plastic parts (industrial waste, 100% plastic), monochromatic classification, specific gravity screening, air separation, coarse crushing, washing, drying and granulating. Commercially available recycled particles (PE, PP, PS) B PE, PP, PS, ABS are used as wire sheaths and molded plastic parts.
colour
black
Winnowing, sorting, coarse crushing, crushing, washing with water, drying, melting and forming line rollers, piles, U-shaped grooves, rods, plates, racks, raw materials, benches, containers, artificial reefs, C APP, PE, polymers, recycled waste products, containers, industrial products (plastics discharged from cities, excluding PVC), removing mottled PVC, coarse crushing, etc. Buried fuel (gasoline, calcium carbide), heat energy and chemical products (ethylene, propylene and styrene monomer) PVC A and B are the same as above. Class C is buried after melting and solidification or buried in blocks, mainly by incineration.
The primary problem encountered in plastic recycling is the collection of waste plastics. In order to make the collection work smoothly, it is necessary to raise citizens' awareness of environmental protection, change passive collection into active payment, and need the close cooperation between the enterprise departments that produce waste plastics and the treatment departments. If the production department wants to collect and transport the waste plastics separately, it will provide a strong guarantee for the recycling of the processing department.
2.2 plastic recycling technology
2.2. 1 Melting treatment of waste plastics
This technology is the most economical and convenient way to recycle waste plastics at present, because it can make full use of all substances and energy. Its basic principle is that the waste plastics are crushed and sent to a melting device, and the waste plastics are melted at its melting temperature, and the secondary master batch is obtained after extrusion granulation and cooling granulation. The main problems encountered by this technology are high energy consumption and the influence of fillers in waste plastics.
2.2.2 Hydrolysis recovery technology of waste plastics
Plastic resins produced by polycondensation, such as polyurethane, polyamide, polyester and polycarbonate, can be hydrolyzed to restore these polymers to original monomers or intermediates. The structure of plastics is stable in processing and use, and the decomposition of polymers requires certain external conditions. Figure 1 is the decomposition schematic diagram of polyurethane flexible foam, and the hydrolyzed product can be used as the initial raw material for foam production.
2.2.3 Oil recovery technology of waste plastics
Plastics are polymer compounds synthesized from petroleum. When energy is applied to cut off the atomic chain, substances with similar oil molecular structure can be obtained. Using this principle, petroleum fuels such as gasoline and diesel oil can be obtained through thermal decomposition and distillation. This process is called plastic oiling. Fig. 2 is a flow chart of the plastic oiling process.
The oil yield of oil injection process can reach above 75% ~ 80%, and the generated oil can be used as oil for oil-fired boilers. This technology is more difficult and more expensive than other waste plastic treatment technologies, but it is a suitable treatment technology from the perspective of environmental protection.
2.2.4 High temperature pyrolysis treatment of waste plastics
Pyrolysis at high temperature refers to the thermal degradation of polymer materials at high temperature and the release of a large number of gases. The treatment process is as follows: there is a layer of sand in the combustion zone, and gas is introduced to make the sand flow like liquid to form a fluidized bed. The waste to be treated is placed on the fluidized bed. The reactor is a completely closed system, and the temperature can reach 600 ~ 900℃. When treating waste plastics, 44% of fuel gas, 26% of aromatics, a mixture of light gasoline and tar and 30% of solid residue can be obtained.
This method is suitable for treating plastic products with metal foil or metal coating. The mixture of gas and oil can be used as fuel, but the remaining 30% residue must be reprocessed.
2.2.5 Disposal of waste plastics by combustion method
For those polluted waste plastics and recycled waste plastic products, incineration is their ultimate destination, so that the heat energy of waste plastics can be obtained. The calorific value of waste plastics is equivalent to that of similar fuel oil (see Table 2), and the heat energy obtained from combustion can be used for power generation.