Transformation of waste plastic treatment to green recycling
I. People pay more and more attention to the treatment of waste plastics
Plastics are considered one of the greatest inventions of the 20th century, which greatly facilitates people's production and life. However, a large amount of waste plastics are piled up in landfills or discarded into the environment, posing a serious threat to the natural ecosystem. Traditional treatment methods cannot change the status quo of resource waste and environmental pollution. Therefore, exploring environmentally friendly and economical ways to recycle plastic waste and realizing the transformation of plastic consumption from a disposable economy to a circular carbon economy have become hot spots and problems in the field of plastic recycling.
Developed countries have developed early in the direction of waste plastic recycling technology and have relatively mature application systems. The relevant experience is worth summarizing and learning from. Japan has issued special production requirements for plastic companies. For example, for PET bottles, it is stipulated that handles should not be used, coloring is prohibited, and physically removable labels and plastic bottle caps should be used; companies need to put waste plastics into production to form a closed loop of resources. The United States has more than 1,700 waste plastic recycling companies. The American Plastics Industry Association has proposed a method of labeling and classifying plastic types for recycling waste plastics; NASA has conducted research on using satellite remote sensing to identify waste plastics in the ocean; energy companies located in coastal areas are required to provide plastic product training to employees working on the outer continental shelf, and volunteers are encouraged to remove waste plastics in coastal areas. Some EU countries have established a relatively comprehensive recycling method for household waste plastic packaging, and have issued policies that require packaging manufacturers to pay a certain fee to recyclers or that packaging manufacturers are responsible for recycling and processing.
Thanks to the rapid development of materials and manufacturing industries, the disposal and utilization of waste plastics are no longer limited to traditional disposal modes such as landfill and incineration. With the continuous enrichment of waste plastic types and the improvement of processing technologies, it is helpful to comprehensively sort out the corresponding disposal and utilization technology system and clarify the development path based on the actual national conditions to grasp the development focus of the field. After a technical review, this paper forms a classification of waste plastic disposal and utilization technologies from four aspects: mechanical disposal, energy and resource conversion, recycling and reuse, and new technologies; compares the characteristics, usage conditions, and current development status of various technologies, grasps the industry status and development challenges of waste plastic disposal and utilization, and provides a direct reference for the research on clean and efficient recycling, disposal, and utilization of waste plastics.
II. Classification of waste plastic disposal and utilization technologies
1. Mechanical disposal technology
Mechanical disposal technology for waste plastics is mainly aimed at convenient reduction, but there are generally problems of subsequent ecological and environmental impacts. For example, landfill and ocean dumping will spread the impact range of microplastics. Considering the continuous ecological impact, landfill and ocean dumping are not ideal waste plastic disposal technologies, nor do they conform to the principle of green sustainable development. It is necessary to carry out ecological and environmental impact assessment in advance and improve the implementation plan for the landfill disposal of waste plastics. Although the building material filler method is in the research and development stage, it has certain development prospects in the recycling and utilization of waste plastics.
2. Energy and resource conversion of waste plastics
From the 1960s to the end of the 20th century, the problem of energy and resource shortages received widespread attention. With the introduction of concepts such as sustainable development and circular economy, the plastic industry has developed rapidly, and the output of waste plastics has increased. Researchers have turned their attention to the energy and resource utilization of waste plastics. The recycling and disposal of waste plastics are mainly aimed at achieving harmlessness, reduction, and energy or resource utilization. Chemical recycling technology for the purpose of environmental protection is a representative energy and resource recycling technology, which decomposes high-molecular waste plastic polymers into small-molecule compounds for secondary recycling. Energy and resource conversion mainly includes thermochemical technology, hydrolysis, alcoholysis, and biodegradation.
3. Recycling of waste plastics
Plastics can be divided into thermoplastics and thermosetting plastics according to their physical properties: the former can be melted into liquid at high temperatures, and then made into objects of different shapes according to requirements, and can be recycled and repeatedly molded; the latter cannot be melted or reshaped, and can only be processed into hard plastic objects and used once, and heating will increase their hardness. Thermoplastics are mainly used for resource and energy recycling, while thermosetting plastics are only used for energy recycling to avoid waste.
(1) Simple recycling method
Simple recycling refers to the technology of sorting, cleaning, crushing, melting and remaking recycled waste plastics without modification, and directly using them for plastic molding processing. It has low cost and low investment, but it has requirements for the types of plastics that can be processed. The simple recycling method is applicable to almost all thermoplastic waste plastics and a small amount of waste plastics mixed with thermosetting plastics. It is mainly divided into three types of disposal: ① The scraps from the production process of plastic processing plants and resin production plants are usually clean and have a single component. They can be directly crushed and plasticized without sorting; ② Waste plastics from recycled plastics, such as various packaging materials, films, etc. The materials need to be sorted, cleaned, crushed and plasticized; ③ Waste plastics from special-purpose plastics, such as cable sheaths, need special pretreatment before reuse, and can be reused or blended with other polymers after dissolution, precipitation and drying; waste plastics are added to react in the process of making polyester from chemicals, such as terephthalic acid (TPA) and EG. Waste plastics can be added in the process of making polyester.
(2) Modification and Regeneration Method
Compared with the simple recycling method, the modification and recycling method is more complicated. After the waste plastics are modified through chemical or mechanical processes, new plastics that meet specific needs are produced by mixing different materials or adding additives. It can improve the basic mechanical properties of recycled materials and meet the production needs of high-quality special products. Waste plastic modification includes blending modification, reinforcement modification, filling modification, toughening modification, grafting modification, etc., which can be roughly divided according to physical modification and chemical modification. The modification and recycling method is mostly adopted by small and medium-sized enterprises, mainly consuming waste plastics generated by industrial and mining enterprises and agriculture (such as plastic parts, packaging products, pesticide bottles, food bags, daily necessities). This type of waste plastic contains a small amount of fillers and plasticizers, and the molecular weight can be increased after a little processing to facilitate reuse. For example, chain extenders are used to extend the bond between the carboxyl group in ABS and the terminal hydroxyl group of polybutadiene in situ to achieve polymer modification and utilization; PS is debrominated and then granulated using a modified blending process to improve its physical properties and upgrade waste plastic recycling technology; bisphenol A in industrially produced polycarbonate is removed by blending to achieve polymer film surface modification and meet the normal use requirements of the product.
The demand for plastic products in society is increasing year by year, which not only consumes a lot of energy, but also causes environmental pollution and harms biological health. In this context, the recycling of waste plastics through modified regeneration methods is also a development method that adapts to the low-carbon economy. It is necessary to control the environmental safety of processing enterprises in a targeted manner, strive to reduce environmental pollution problems, and pursue comprehensive and coordinated sustainable development. Modified regeneration technology can cause performance changes in various waste plastics. For example, PE, PP, PVC, PS, ABS, and PA will all undergo color changes, viscosity, and elongation decreases during the regeneration process, while the viscosity of high-density PE will increase. Therefore, waste plastic regeneration technology has both advantages and disadvantages for the performance changes of recycled plastics. Additives can be added or technical adjustments can be made to ensure product quality.
4. New technologies for the disposal and utilization of waste plastics
(1) Supercritical fluid disposal and utilization technology
Waste plastics react in supercritical fluids, which has the advantages of short reaction time, no need for catalysts, and high recovery rate. Waste plastics have easily decomposable chemical bonds such as ether bonds, acyl bonds, and ester bonds, which can be decomposed into monomers in supercritical fluids and then reorganized to produce new plastic products. PET can be decomposed into monomers in supercritical fluids, and PU can also be recycled using supercritical fluid decomposition technology. Supercritical fluid decomposition technology has good development prospects, but there are problems with high high-pressure sealing requirements when it is used.
(2) Blast furnace injection energy recovery technology
Energy recovery using blast furnace injection is generally considered to be an effective method for recycling waste plastics. From the perspective of energy utilization, blast furnace injection technology has a high utilization rate of the energy contained in waste plastics (about 80%), mainly reducing iron ore in the form of chemical energy. From the perspective of environmental protection, the toxic gas content produced by blast furnace injection technology is low, which is convenient for large-scale application. Blast furnace injection technology has a strong processing capacity and is conducive to the development of a low-carbon circular economy. However, recycled waste plastics are usually not fully classified and do not meet the requirements of blast furnace injection for reactive materials, resulting in the fact that blast furnace injection technology has not been widely used at present.
(3) Photoprocessing technology
Photoprocessing technology uses light energy as an energy source to treat waste plastics. It has low pollution and economic advantages and has attracted much attention in recent years. Compared with thermochemical technology, the photoreaction conditions are mild and the energy consumption is low. It can accurately break specific chemical bonds to achieve high selectivity of target products. Photoprocessing technology is mainly divided into two categories: photodegradation and photocatalysis.
Although the application prospects of photoprocessing technology are good, there are still problems to be solved, such as exploring methods for accurately identifying and controlling reaction paths, developing low-cost, high-performance photocatalysts, and developing economical and environmentally friendly pretreatment methods according to different types of waste plastics.
(4) Electrocatalytic technology
There are still few applied studies on the conversion of waste plastics into valuable products by electrocatalysis. Although electrocatalytic technology has the advantages of controllable energy potential, recyclable electrolyte, and selective conversion, it faces many application challenges: it is difficult to convert experimental production equipment to industrial application, and the process of separating organic acids and redox substances from electrolytes is energy-intensive, requiring more efficient, environmentally friendly, and economical catalysts.
III. Suggestions on the development of waste plastic disposal and utilization technology
First, reduce the amount from the source and encourage the use of recycled plastics. Take a policy-driven approach to encourage commodity packaging to choose alternatives such as biodegradable plastics, reduce the consumption of disposable non-degradable plastics, and strive to reduce the output of waste plastics from the source. Formulate a standard system for recycled plastics, standardize the recycling and reuse of waste plastics, and improve the public recognition of recycled plastics and the recycling rate of waste plastics.
Second, strengthen the classification and recycling of waste plastics. The disposal and utilization of waste plastics in my country developed relatively late, and the relevant recycling and management mechanisms are still not sound. The public's environmental awareness has not yet been transformed into behavioral habits, and the understanding of the importance of garbage classification is relatively weak. It is necessary to strengthen the implementation and publicity of relevant garbage classification and recycling policies, and effectively promote the industrialization of waste plastic classification and recycling.
The third is to promote technological innovation and transformation of achievements. New technologies for waste plastic treatment are not yet mature, and have prominent shortcomings compared to mature technologies. Necessary support should be given to the layout of research on waste plastic disposal and utilization to promote breakthroughs in key technical problems. Public funding should be used to drive the enthusiasm of enterprises to participate in technological research, focus on basic research driven by application needs, and promote the smooth transformation of innovative technological achievements in waste plastic treatment and utilization.