Recycling and recycling of plastic waste

- Sep 18, 2019-

Plastic products are widely used due to their light weight, waterproof, durable, beautiful products and mature production technology. They have become the world's top ten producers of plastic products and the largest consumer of plastic materials. . While plastic technology brings huge material civilization, the emergence of a large number of plastic waste presents severe challenges to people, but it also gives us new opportunities. The pollution caused by plastic waste is commonly known as “white pollution”. It is attracting people's great attention. Indifferent to its existence or improper treatment will cause some serious risks, such as: (1) plastic waste in the environment is not easy to rot , stacking causes more garbage dumps and increased pollution points; (2) plastic film, such as agricultural film, enters the soil, is not easy to decompose and prevent soil permeability, which makes soil quality deteriorate, affecting crop growth; (3) plastic waste accounts for 60% of marine floating matter Above, endangering marine life and maritime safety.


Therefore, the recycling and utilization of plastic waste has reached a point where it cannot be ignored. At present, the recycling technology for plastic waste can be divided into five aspects.


1 Direct recycling and modification of waste plastics Such utilization is the most important method for the utilization of waste plastics. Its technology investment and cost are relatively low, and it has become the main method for the utilization of renewable resources in many countries. Mature craftsmanship. Compared with modified recycling, the technology investment and cost of direct recycling are relatively lower, and the selection is more common, but the modified recycling is the development direction.


1.1 Direct recycling This is the process of sorting, cleaning, crushing and granulating the recovered waste plastic products directly, and also including adding appropriate auxiliary components (such as stabilizers, anti-aging agent colorants, etc.). In combination, the addition of these additives only serves to improve processability, appearance or anti-aging, and does not improve the basic properties of the recycled product. Japan's construction company pulverized waste foam and heated it by infrared radiation. Its volume was reduced to less than 20%, and then mixed with special cement to make "rice sugar"-like building materials. As a low-cost soundproof material, it has a good The sound insulation effect is very versatile.


1.2 Modification and Recycling This is a technique of modifying the recycled material by mechanical blending or chemical grafting, such as toughening, strengthening, combination, composite, activated particle filling, blending, and crosslinking, grafting, and chlorine. Chemical modification. The properties of the modified recycled product, especially the mechanical properties, can be significantly improved or improved, and can be used as a high-grade recycled product. However, the routing is complex and sometimes requires specific machinery. Almost all thermoplastic waste plastics can be collected, sorted, cleaned, pulverized and dried, and then melted. Various plastic molding processing equipment is used to produce recycled plastic sheets, pipes, rods, parts and containers. For example, disposable expanded polystyrene (EPS) foam tableware that has been banned in the country can also be modified for recycling, which can solve the problem of the tableware being banned.


The method is as follows. Firstly, the EPS foam lunch box can be cleaned by a complete saponification method, and the washed and dried EPS foam is further pulverized and defoamed by a single screw extruder, granulated by melt extrusion, and blended with LDPE. . The tensile strength of the obtained LDPE/PS composite is higher than that of pure LDPE, and the elongation at break is relatively significant compared with the recycled material. It can be used to produce products with low strength requirements, such as cover sheets, daily miscellaneous items, etc. . The flow chart is as shown.


Foam lunch box; soaking pool coarsely crushed hot water rinse dry pulverized blended recycled products discarded EPS bubble fast food box modified regeneration process chemical degradation technology of waste plastics The basic principle is to make the original resin polymer in waste plastic products more thorough The macromolecular chain is separated into a low molecular state, and some components are monomers, and other components are basic organic materials. Chemical degradation is inseparable from degradation temperatures, catalysts, and degradation equipment. In general, degradation has an optimal degradation temperature point or temperature range, and most of the waste plastics are added to the catalyst in the cracking practice. The catalyst is mainly silicon-aluminum compounds, as shown in Table 1, and other metal oxides are also useful. But there are fewer reports. Due to the poor thermal conductivity of plastics, it will become a highly viscous melt that is difficult to transport after heating, and carbon will be deposited on the reactor wall during pyrolysis of waste plastics, which makes it difficult to discharge. Therefore, special degrading equipment is necessary. There are many research applications in tubular and fluidized bed reactors.


Table 1 Catalyst catalyst for thermal cracking of polyolefins. Alumina for silica gel, silica gel for chromatographic separation, noble metal oxide-Y zeolite, silica-alumina, H-Y zeolite, basic oxide, 0. 2.1 Depolymerization is the degradation of plastic polymers into monomers or other chemical materials, which can be reused in synthetic polymers. This technology requires relatively clean waste plastics, removal of additives, and purification of monomers. It is the key to technology. Depolymerization can be further divided into Hydrolysis and Alcoholysis.


2.2 Pyrolysis refers to the cracking of plastics at high temperatures (greater than 500 ° C) under anaerobic conditions. Taking the 10t/d test equipment of the German heavy oil company as an example, the mixed waste plastic from the garbage recycling center can be decomposed to 35%~58% diesel and 23% by heating at 699~800°C and 1000°C for 30min~ 40% gas. Research by China University of Petroleum pointed out that the use of accelerators alone to pyrolyze waste polyethylene can result in oil and qualified compensation. The wax yield is 50% to 90%, and the economic benefits of making wax are higher than that of oil. Anglo-American has similar patents.


2.3 Hydrogenation is carried out under the conditions of low hydrogen pressure (about 30 MPa hydrogen pressure) and low temperature (less than 500C). The purity of the cracked product is lower than that of the thermal cracking product, which can be directly refined in the refinery. The hydrocracking requires strict separation and pulverization in advance, which requires expensive equipment investment. The 2X 104t/a waste plastics processing unit of V.O.AG of Germany can decompose mixed waste plastic into 80% liquid fuel and other products at 460~490C and 20MPa with hydrogen base as catalyst. 2.4 Gasification refers to the degradation of waste plastics at very high temperatures (up to 1500 C). The products are CO and hydrogen, which can be used to prepare methanol and related products.


Germany has been working on polymer material cracking for many years, and Deutsch-ReifenNelHunststoffe Thermal Decomposition and Salzgitter Thermal Decomposition have been established.


Table 2 Overview of biodegradable plastics that have been industrially produced abroad. National company main component commodity name production capacity (t/a) price (dollar/kg) US starch US UK Italy polyvinyl alcohol-starch alloy China has "light/microbial degradation plastics" The mulch film has been included in the national key scientific and technological research project, and has made great progress. After covering a few months, the bare part can be decomposed into pieces of 4cm×4cm or less, compared with soil water movement, soil volume, gravity, porosity and agglomeration. The results of studies with heavy metal components indicate that they do not cause pollution to the soil and have no effect on crop growth. Han Changtai and other researches on non-starch type controllable light and biodegradable mulch film have achieved preliminary results. Agricultural films made from fully biodegradable plastic PLLA have also been used.


At present, degradable plastics are not as good in quality as ordinary plastics, and their prices are higher than ordinary plastics. At the same time, the nature of their natural degradation remains to be studied. In addition, degradable plastics are not conducive to the recycling of plastics due to the addition of other substances. .


5 Landfill is the best way to deal with waste plastics, but it has the lowest cost. Buried is deep buried plastic waste, at least not affecting the growth of surface plants. Landfill as the final treatment method of garbage has certain characteristics: (1) low processing cost; (2) the treatment technology is relatively simple, which is conducive to popularization; (3) landfill can use non-cultivated land as the site, such as: beach land, valley, depression, ditch, etc. (4) There is no need to pre-treat the garbage. In the landfill process, the landfill unit needs to be treated with anti-seepage treatment, and the non-toxic and harmless covering material is used to cover the surface of the garbage according to the specified technical requirements, and the collected percolation is collected. Water, etc. will be treated. On the basis of landfill, the waste plastic processing facilities such as incineration plants will be gradually demonstrated and constructed.


It can be seen from the above methods that in addition to direct recycling, the more economical method of comprehensive utilization of waste plastics is to use modified recycling, and for waste plastics with serious pollution and poor sorting or thermal decomposition (recovery of oil products) It is also possible to use a resin material such as a monomer intermediate or to use incineration (utilization of energy) to finally use a landfill method.


Plastic waste has an extremely wide range of applications, especially with the increasing attention to environmental pollution issues and the implementation of sustainable development strategies, the recycling and utilization of waste plastics will be further expanded, and huge economic benefits and Social benefits, promote the in-depth research and development of related technologies in this field.