Waste plastic refining technology

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Section 1: The Generation of Waste Plastics and Their Hazards 1.1 The Development of the Plastic Industry Since the 20th century, the material revolution has played a crucial role in the profound changes in human life. Especially in the past 50 years, advances in polymer synthesis technology have greatly promoted the development of the synthetic resin industry, and various plastics produced using synthetic resins as raw materials have emerged rapidly. Due to its wide range of applications and excellent performance, plastic is increasingly used in various fields such as agriculture, packaging, light industry, textiles, construction, automotive, electronics, as well as aerospace and defense industries. Along with steel, wood, and cement, it constitutes the four fundamental materials of modern industry, driving the continuous development of industry and agriculture as well as the emergence of contemporary high technologies. From 1980 to 1990, global plastic production increased by 67.2%, while steel production declined by 1.5%. In terms of volume, world plastic production has exceeded that of crude steel since 1991, and the ratio of plastics to steel continues to increase year by year. In 1992, the world’s production of synthetic resins exceeded 100 million tons. In 1999, the world production of synthetic resins reached 156.7 million tons. In 1997, China’s plastic consumption reached 16.3 million tons, and by 2000 the production of plastic products in China had exceeded 10 million tons; the country also continued to import plastic raw materials and semi-finished products each year. Entering the 21st century, with the continuous advancement of high-tech fields such as information technology, the production of synthetic resins will increase further, their performance will improve even more, and their applications will become more widespread. They will exert an increasingly important influence on the national economy and social development, as well as on the improvement of people’s living standards. According to the plans, by 2010 China’s demand for plastics will exceed 40 million tons. 1.2 Sources of waste plastics As chemical products, the environmental impacts that synthetic resins may have during production have been relatively well addressed through improved catalyst efficiency, process enhancements, advances in control technologies, and larger-scale manufacturing facilities. However, to everyone’s surprise, although the valuable properties of synthetic resins meet the needs of various plastic products, they have had unexpected negative effects on the environment after being used. The increasing widespread use of plastic products has brought great convenience to people’s lives, but it has also resulted in a significant amount of white pollution. Due to the tendency of plastics to age and become damaged easily, their useful life is very short; a large number of plastic products, especially packaging materials, are discarded after 6 to 12 months, and 40% of plastics turn into waste plastic after 1 to 2 years. Over the past few decades, waste plastics have been considered part of municipal solid waste (MSW). According to studies, in MSW from industrially developed regions, waste plastics account for 4%~10% (by mass) or 10%~20% (by volume), with the main sources being packaging waste, automotive waste, and processing residues. The percentage distribution of various types of waste plastics is as follows: low-density polyethylene (LDPE) at 27% ； High-density polyethylene (HDPE) 21% ； Polypropylene (PP) 18% ； Polystyrene (PS) 16% ； Polyvinyl chloride (PVC) 7%. In recent years, the proportion of waste plastics in urban household waste in our country has ranged from 0.4% to 1.5%. Plastic products come in a wide variety of types and have numerous applications. The main areas where they are used include the industrial sector, agriculture, commerce, and household use, and it is from these same areas that the waste resulting from them originates. Waste in the industrial sector mainly consists of waste generated during the synthesis of plastic materials, as well as scraps and defective products produced during the processing and manufacturing of plastic products. In the agricultural sector, plastic products are primarily used in four areas: ① agricultural plastic films and greenhouse films ； ②Textile bags, such as those used for packaging fertilizers, seeds, grains, etc ； ③Agricultural water supply fittings, including rigid and flexible drainage and water conveyance pipes ； ④Plastic ropes and nets. The aforementioned plastic products are mostly made of polyethylene resin (such as plastic mulch, pipes, ropes, and nets), followed by polypropylene resin (such as textile bags), as well as polystyrene resin (such as drainage hoses and grid membranes), among others. Our country is a major agricultural nation, and agricultural plastics account for a significant proportion of all plastic products. At present, agricultural films alone make up around 15% of annual plastic product production, and this proportion is continuing to rise year by year. Plastic waste from the commercial sector comes from two sources: ① The distribution sector, where most of the plastic products used are single-use packaging materials such as packaging bags, tying ropes, shock-absorbing foam plastics, packing boxes, and partition boards. There are many types of such plastic products, but they generally cause little pollution, and can be recycled for reuse; ② The consumer sector, where the discarded plastic products include things like food containers, plastic bottles, packaging bags, plates, dishes, and various other plastic items. These products have usually been used and contain pollutants, and in addition to being sorted for recycling, they also require further treatment. Plastic products used in daily life account for a large proportion of all plastic products, and the share of plastics used in daily life is increasing continuously. These plastic products can be divided into three categories: the first category is packaging materials, such as packaging bags, boxes, PS foam padding used in household appliances, and packing ropes ； The second type are disposable plastic products, such as beverage bottles, milk bags, cans, cups, basins, containers, etc ； The third category consists of non-disposable items, such as various utensils, plastic shoes, lighting fixtures, stationery, cooking equipment, toilet fixtures, makeup supplies, and other miscellaneous items. There are many types of resins used in everyday plastic products; in addition to the four most common resins, there are also resins such as polyethylene terephthalate (PET), acrylonitrile-butadiene-styrene (ABS), and nylon. In addition, there are also waste materials resulting from the use of plastic products in areas such as transportation, home appliances, and environmental materials. 1.3 Hazards of waste plastics: When waste plastics are discarded in the environment after use, they mainly cause two types of hazards: one is pollution of the landscape environment ； Another category is the harm to ecosystems. The pollution of the landscape environment by waste plastics refers to the damage caused by such plastics to the landscape; this is manifested in the disposal of used plastic products in cities, tourist areas, water bodies, along roads and railways, which creates an unpleasant visual impact on people. It affects the overall aesthetic appeal of the landscape. Among them, discarded light-colored plastic films, plastic bags, plastic packaging materials, etc. are known as \"white pollution\". The harm of waste plastics to ecosystems mainly lies in their impact on animals, aquatic systems, and land systems. As early as the mid-1960s, it was discovered that residual vinyl chloride monomers in polyvinyl chloride plastic could cause a strange disease known as \"acral osteolysis,\" which involves the dissolution of the metacarpal bones. Workers involved in the production of polyvinyl chloride resin often suffer from symptoms such as numbness and tingling in their fingers, a condition known as Raynaud’s syndrome. When people are exposed to vinyl chloride monomer, skin hardening occurs, characterized by swelling of the fingers, wrists, and face, as well as thickening and stiffening of the skin that results in a loss of elasticity and an inability to grip objects firmly. Additionally, conditions such as an enlarged spleen, varices in the stomach and esophagus, liver damage, and elevated portal vein pressure may arise. After the 1970s, a very rare type of liver cancer – hepatic angiosarcoma – was also found in some PVC manufacturing plants. Thereafter, although manufacturers tried their best to control the monomer content in polyvinyl chloride resin, the problem was not completely resolved; therefore, in 1975, the United States was the first to propose a ban on using polyvinyl chloride plastics for packaging food and beverages. Since plastic products cannot be digested or broken down in animals, ingesting them can lead to stomach discomfort, abnormal behavior, reduced fertility, and even death. In zoos in our country, there have been unfortunate incidents where animals died after ingesting plastic food bags discarded by visitors. Studies show that waste plastic has had a significant impact on marine ecosystems, with some large marine animals dying as a result of consuming such waste plastic. Between 1970 and 1987, 543 large seabirds such as petrels were studied in the Pacific Ocean; since they could not distinguish between plastic and seaweed, plastic items were found in the stomachs of 458 of them, and plastic was also detected in the stomachs of sea turtles. The pollution of the oceans by discarded plastics has become an international issue; foam polystyrene accounts for 22% of marine debris, while other plastics account for 23%. These waste plastics not only wrap around the ship’s propellers but also damage the ships and machinery, causing shutdowns and accidents and resulting in huge losses for the shipping industry. And it costs 10 times more to remove 1 ton of marine waste than to remove the same amount of waste on land. Waste agricultural films, plastic bags, and the like in farmland can also cause livestock to ingest them accidentally, leading to loss of appetite and death. Furthermore, when they remain in agricultural fields for a long time, they affect soil aeration, hinder water flow and the growth of crop roots, and can also entangle agricultural machinery, interfering with field operations. Over time, this has an impact on the deeper layers of soil and leads to a deterioration of the soil environment. Thermosetting plastics also cause severe environmental pollution. For example, small and medium-sized ship hulls made of fiber-reinforced plastic (FRP) are difficult to dispose of once they become obsolete. Every year, a large number of such abandoned ships are dumped on coasts, riversides, and lakes around the world, causing severe environmental pollution and has become a major environmental problem. 1.4 Introduction to Treatment and Disposal Methods for Waste Plastics 1.4.1 Landfill disposal Waste plastics are usually mixed with municipal solid waste and landfilled. Landfilling requires no pretreatment, has low costs, and the technology involved is relatively simple; therefore, landfilling is currently the most common method for dealing with waste plastics. However, during landfilling, plastics remain in the soil without decomposing over time, putting the soil in an unstable state. It is also possible for harmful substances in the plastics, such as plasticizers and additives, to leach out, causing secondary pollution. Moreover, as the amount of solid waste increases, the available land for landfilling keeps decreasing, and landfilling waste plastics wastes resources that could be recycled. It can be seen from this that landfilling is the worst option for dealing with waste plastics. 1.4.2 Incineration Treatment – Incinerating waste plastics is also a common treatment method; by controlling the burning temperature, it is possible to make full use of the heat generated during the combustion of waste plastics. The method of incinerating waste plastics to convert them into thermal energy has the following advantages: ① Waste plastics do not require preprocessing nor do they need to be separated from municipal waste, especially when it comes to waste plastics that are difficult to decompose and are mixed together ； ②The calorific value of burned waste plastic is comparable to that of similar fuel oils: ③ Incineration can reduce the mass of waste plastic by 80% and its volume by over 90%; the residue left after combustion has a high density, making it easy to dispose of through landfilling. In Japan, waste plastics whose heat energy is recovered through incineration account for about 38% of the total amount of recycled waste plastics. Japan has nearly 2,000 incinerators, while Germany has over 40 plants for incinerating waste plastics; the heat energy recovered in these processes is used for power generation, accounting for around 6% of the total electricity generated from fossil fuels. There are mainly three ways to recover thermal energy by burning waste plastics: ① Using specialized incinerators to burn waste plastics in order to recover thermal energy; the incinerators used include fluidized bed incinerators, floating bed incinerators, rotary kiln incinerators, etc ； ②Combusting waste plastics as a supplementary fuel alongside other fuels that produce steam is a viable and relatively advanced energy recovery technique; for example, thermal power plants can use waste plastics as a supplementary fuel ； ③Through hydrogenation or anaerobic decomposition, waste plastics are converted into combustible gases or other forms of combustible materials, and thermal energy is recovered through their combustion. However, studies have shown that the burning of waste plastics generates large amounts of harmful gases that pollute the environment. The main products of burning waste plastic are carbon dioxide and water; however, depending on the type of plastic and the burning conditions, harmful compounds such as polycyclic aromatic hydrocarbons, acidic compounds, carbon monoxide, and heavy metal compounds are also produced. If these substances enter the atmosphere directly, they can pollute the environment and pose risks to human health. Therefore, it can be said that burning waste plastics is not the best way to dispose of them. 1.4.3 Recycling and Reuse Recycling is divided into simple recycling and modified recycling. Simple recycling refers to the direct processing and molding of recycled plastic waste after sorting, cleaning, crushing, and granulation ； Products obtained through simple recycling have poor performance, and generally can only be used to manufacture products of lower quality. Since the 1970s, mechanical recycling technology for waste plastics has emerged in the towns and villages of Jiangxi and Zhejiang provinces in China. Plastic is crushed and then mixed with some new materials; it is melted, fed into molds, and used to produce plastic containers, kitchen items, slippers, and other products. However, the quality of these products is not guaranteed; plastic products mixed with recycled plastic cannot compete with those made from pure new material in terms of strength, elasticity, toughness, and durability, and such technologies also cause serious secondary pollution. Modified recycling refers to the reprocessing and shaping of recycled materials after modifying them through physical methods (such as toughening, strengthening, blending, compounding, filling, etc.) or chemical methods (such as cross-linking, grafting, chlorination, etc.). This modification process for waste plastics is relatively complex and requires specific mechanical equipment; the properties of the modified recycled plastic products, especially their mechanical properties, are improved, allowing them to be used in the production of higher-quality plastic products. For example, Cytec Industries produces a mixed additive containing antioxidants, co-stabilizers, and other active and inactive additives, which can restore the properties of recycled materials to nearly their original levels ； Dutch researchers have developed a new type of chemical compatibilizer that can bond plastics containing different polymers together ； The United States uses solid state shear pulverization (3SP) for mechanical processing; it enables the molecular-level shearing of resins without the need for heating or melting, thereby creating compatible blends. The blend is primarily composed of HDPE and LLDPE, and its ultimate tensile strength and flexural modulus are comparable to those of pure HDPE and LLDPE. Quality issues with plastic products made from modified recycled materials, as well as problems related to secondary pollution during the production process, still exist. Reresource technology, also known as recycling technology, can be divided into energy recovery technology and material recovery technology. Energy recovery is also known as heat recovery. Due to the high heat of combustion of plastics – 46.63 GJ/kg for polyethylene, 43.95 GJ/kg for polypropylene, 18.08 GJ/kg for polyvinyl chloride, and 35.26 GJ/kg for ABS – there is great potential for recovering their thermal energy. Thermal energy recovery technologies are receiving increasing attention both domestically and internationally. Currently, the United States has over 500 incinerators. Thermal energy recovery from waste plastics does not require complex preprocessing, nor is it necessary to separate them from household waste. After incineration, the mass and volume of waste plastics can be reduced by over 80% and 90% respectively. The residue resulting from incineration has a high density, making it convenient to dispose of it through landfilling. Therefore, this industrial technology that combines environmental protection with power generation is turning waste plastics into a resource, and it has become a new focus for investment on the international stage. In 1993, Japan’s Ministry of International Trade and Industry released the \"21st Century Waste Plastic Management Plan and Implementation Measures.\" By 1995, waste plastics recycled through incineration to recover thermal energy accounted for about 28% of the total amount recycled, and this figure rose to 30% in 1997 – far higher than the 11% accounted for by recycled materials. However, to recover the heat of combustion, specialized incinerators are required, resulting in high initial investment, and the burning of some plastic wastes can also produce secondary pollutants such as HCI. Additionally, abroad, waste plastics are also used for blast furnace injection as a substitute for coal, oil, and coke, with good results. After conducting experiments of type 1A, the German steel company Bremen Steel obtained approval in February 1995 to build a facility for injecting 7×10^4 tons per year of waste plastic pellets into blast furnaces; this can replace 7×10^4 tons of heavy oil each year, and the investment can be recouped in about 2 years. This technology has begun to be adopted by large steel companies such as Mannesmann and Thyssen. Japanese company NKK also conducted a pilot test in 1995 on using recycled plastic pellets instead of coal powder in blast furnaces, and the test was successful; the Japanese Iron and Steel Association has included this approach in its energy-saving plan for 2010. Based on the use of waste tires in blast furnaces, the Tokuyama Cement Plant in Japan conducted experiments in 1996 on using waste plastics in rotary kilns. The waste plastics were crushed into particles smaller than 25 mm and injected through openings in the upper part of the coal burner. To prevent the impact of chlorine on the clinker, PVC-based materials were not considered for use at that time. Furthermore, fuel can also be produced from recycled plastics; Japan has successfully developed RDF, a solid fuel made primarily of plastics mixed with various combustible waste materials such as waste paper, wood chips, fruit peels, and sewage sludge, with a calorific value of 20,920 kJ/kg (5,000 kcal/kg) and uniform particle size. This fuel not only dilutes chlorine to improve heat generation efficiency, but it is also easy to store, transport, and use as a substitute for coal in other boilers and industrial furnaces. With the support of Japan’s Ministry of International Trade and Industry, Resource Recycling Co., a joint venture between Ito Trading and Kawasaki Steel, has begun mass-producing solid fuel from waste. Denryoku Kankaku Co. is conducting process tests on the use of this solid fuel as fuel in fluidized-bed boilers for power generation, with a target efficiency of 35%. As mentioned above, incineration for recycling is an important method for the reuse of waste plastics; the heat generated is used for power generation or heating. However, in China, the technology for incinerating urban waste is still in its early stages and operates on a small scale, so the heat produced is not sufficient for power generation. Moreover, large and medium-sized cities do not have the infrastructure needed for centralized heating. Therefore, the recovered thermal energy has no suitable way to be utilized, and dealing with the residues in the incinerator also increases costs. At the same time, the burning process generates some toxic and harmful gases and substances that pose risks to human health and the environment; therefore, it is not appropriate to promote this practice at the current stage in our country. Material recovery technology involves the thermal or catalytic cracking of waste plastics to recover fuel oil and chemical raw materials. Under the action of heat, the polymer chains in waste plastic products break apart, resulting in compounds with lower relative molecular masses. The thermal cracking of plastics is divided into three types: monomeric cracking, random cracking, and intermediate cracking. The thermal pyrolysis of polyolefin plastics is a typical random-type pyrolysis. After cleavage, it produces low-molecular-weight compounds with no fixed pattern regarding chain length or structure ； Under appropriate temperature, pressure, and catalyst conditions, the chain length and structure of the resulting low-molecular-weight compounds can be kept within a certain range. By utilizing this property, high-quality gasoline and diesel can be produced. The main advantages of waste plastic pyrolysis technology are: ① The pyrolyzed products have high utility value ； ②The number of times that waste plastics can be processed is theoretically unlimited: the waste plastics are first cracked down into monomers, which are then polymerized into polymers; once these products are discarded, they can be cracked again, and this process can be repeated indefinitely. However, due to the gradual decline in mechanical properties, the number of times that recycling can be carried out is generally limited ； ③Pyrolysis technology can be used to process mixed recyclables (such as mixtures of polypropylene and polyethylene products), but it is necessary to classify them into chlorinated and non-chlorinated products. The main disadvantages of pyrolysis technology are high investment costs and strict technical requirements. In addition to thermal pyrolysis, chemical decomposition is also used to treat waste plastics; this technique is suitable for waste plastics of a single type that have undergone strict pretreatment. Although various plastics can be decomposed chemically, they are currently mainly used to treat polar waste plastics such as polyurethanes, thermoplastic polyesters, and polyamides. For example, polyester and diamines can be produced by the hydrolysis of polycarbonate foam plastics; polyols can be obtained through the alcoholysis of soft and hard polyurethane products; and crude terephthalic acid and ethylene glycol can be synthesized by the depolymerization of waste PET. In material recycling technologies, in addition to the pyrolysis of waste plastics to produce liquid fuels, there are many other methods such as the thermal pyrolysis of waste plastics to recover monomers, as well as the gasification and carbonization liquefaction of waste materials. 1.4.4 Replacing existing plastics with biodegradable plastics: Biodegradable plastics refer to a type of polymer material that, during a certain period of use, possesses the same functional properties as conventional plastics; however, after fulfilling their purpose, their chemical structure undergoes significant changes, allowing them to degrade rapidly and integrate into the environment. Biodegradable plastics are divided into photodegradable plastics and biodegradable plastics. In the long term, fully biodegradable plastics represent the best way to completely eliminate \"white pollution,\" but many unresolved issues still exist at present. With current technology, both photodegradation and biodegradation can only achieve partial degradation; however, the changes in their physicochemical properties after degradation cause their volume to shrink rapidly, **which facilitates subsequent processing. However, biodegradable plastics still cannot be widely adopted at present, mainly due to their performance and cost. In traditional plastic production, it is always desired that the strength be as high as possible and the lifespan as long as possible. In biodegradable plastics, the addition of biodegradable components inevitably affects certain properties of the plastic itself. How to maintain the original excellent properties of plastics while enabling them to degrade when necessary is, in itself, a contradiction ； To achieve unity between the two, there are many technical challenges that have not been fully resolved. Another important reason why users find it difficult to accept is that the production cost of biodegradable plastics is currently significantly higher than that of traditional plastics.