Keywords: waste plastics, white pollution, recovery, regeneration, pyrolysis, technical progress
Waste plastics are usually disposed of by landfill or incineration. Incineration will produce a large number of toxic gases, causing secondary pollution. Landfill will take up a lot of space; It takes more than one hundred years for plastics to degrade naturally; The precipitated additives pollute the soil and groundwater. Therefore, the development trend of waste plastics treatment technology is recycling, but the recycling rate of waste plastics is low at present. There are problems in management, policy and recycling, but more importantly, the recycling technology is not perfect enough.
There are various technologies for recycling waste plastics, including technologies for recycling various plastics and technologies for recycling single resin. In recent years, plastic recycling technology has made many gratifying progress. This paper mainly summarizes the common technologies.
1 separation and separation technology
One of the key links in plastic recycling is the collection and pretreatment of waste plastics. Especially in China, the important reason for the low recovery rate is the low degree of garbage classification and collection. Because the melting point and softening point of different resins are quite different, in order to recycle waste plastics better, it is best to classify a single variety of resins, so separation and screening is an important link in the utilization of plastic recycling. For small batches of waste plastics, manual sorting can be used, but the efficiency of manual sorting is low, which will increase the recycling cost. A variety of separation and separation methods have been developed abroad.
1. 1 instrument identification and separation technology
The Italian company Govoni used X-ray detector and automatic classification system to separate PVC from mixed plastics for the first time [1]. The American Plastic Recycling Technology Center has developed an X-ray fluorescence spectrometer, which can automatically separate PVC containers from hard containers. Germany's Refrakt Company used heat source identification technology to separate molten PVC from mixed plastics at a lower temperature by heating [1].
Near infrared has the function of identifying organic matter. The optical filter [1] using near infrared technology can identify plastics at a speed of more than 2000 times per second, and ordinary plastics (PE, PP, PS, PVC, PET) can be clearly distinguished. When the mixed plastics pass through the near infrared spectrum analyzer, the device can automatically sort out five common plastics at the speed of 20 ~ 30 pieces per minute.
1.2 hydraulic spinning technology
According to the principle of cyclone separation and the density difference of plastics, Japan Plastics Processing Promotion Association developed a hydraulic cyclone separator. The mixed plastics are put into a storage tank after pretreatment such as crushing and cleaning, and then quantitatively transported to a stirrer, and the formed slurry is sent to a cyclone separator through a centrifugal pump, and plastics with different densities are discharged respectively. Dow Chemical Company of the United States also developed a similar technology, using liquid hydrocarbon instead of water for separation, and achieved good results [2].
1.3 selective dissolution method
Kellogg Company of the United States and Rensselaer Institute of Technology jointly developed the technology of solvent selective dissolution for recycling waste plastics. Adding mixed plastics into xylene solvent can selectively dissolve and separate different plastics at different temperatures, in which xylene can be recycled with little loss [1, 3].
Vinyloop technology was developed by Solvay SA company in Belgium, which used methyl ethyl ketone as solvent to separate and recover PVC. The density of recycled PVC is almost the same as that of new raw materials, but the color is slightly gray. German also has Delphi technology for solvent recovery, and Bivini nylon technology for ester and ketone solvents is much less.
1.4 flotation separation method
A material research institute in Japan successfully separated PVC, PC (polycarbonate), POM (polyoxymethylene) and PPE (polyphenylene ether) by using common wetting agents, such as sodium lignosulfonate, tannic acid, aerosol OT and saponin [4].
1.5 Electroseparation Technology [5]
SePAration of mixed plastics (such as PAN, PE, PVC and pa) by triboelectricity. The principle is that when two different non-conductive materials rub, they get opposite charges through the gain and loss of electrons, in which the material with high dielectric constant is positively charged and the material with low dielectric constant is negatively charged. Plastic recycling mixture often contacts in a rotating tank to generate charge, and then it is sent to another tank with a charged surface for separation.
Incineration to recover energy.
The combustion heat of polyethylene and polystyrene is as high as 46000 kJ/kg, which exceeds the average value of fuel oil of 44000 kJ/kg, and the calorific value of PVC is as high as18,800 kJ/kg. Waste plastics have fast combustion speed and low ash content, and are used to replace coal or oil in blast furnace injection or cement rotary kiln abroad. Because the combustion of PVC will produce hydrogen chloride, which will corrode boilers and pipelines, and the waste gas contains furan, dioxin and so on. The United States has developed RDF technology (garbage solid fuel), which mixes waste plastics with waste paper, sawdust and fruit shells. It not only dilutes the chlorine-containing components, but also facilitates storage and transportation. For those waste plastics that are technically unrecoverable (such as various composite materials or alloy mixed products) and difficult to regenerate, incineration can be used to recover heat energy. Its advantages are large processing capacity, low cost and high efficiency. Disadvantages are that harmful gases are generated, special incinerators are needed, and equipment investment, loss, maintenance and operation costs are high.
3 melting regeneration technology
Melting regeneration is to re-plasticize waste plastics after heating and melting. According to the properties of raw materials, it can be divided into simple regeneration and compound regeneration. Simple recycling mainly recycles leftover waste from resin factories and plastic products factories, as well as disposable consumer goods such as polyester beverage bottles and food packaging bags, which are easy to select and clean. The performance after recycling is similar to that of new materials.
The raw material of compound regeneration is waste plastics collected from different channels, which has the characteristics of many impurities, complex varieties, diverse forms and dirt, so the regeneration treatment procedure is complicated and the separation technology and screening workload are heavy. Generally speaking, composite recycled plastics are unstable and fragile, and are usually used to prepare lower-grade products. Such as building fillers, garbage bags, microporous sandals, raincoats, equipment packaging materials, etc.
4 cracking to recover fuel and chemical raw materials
4. 1 thermal cracking and catalytic cracking technology
Due to the in-depth study of cracking reaction theory [6- 1 1], the development of cracking technology at home and abroad has made many progress. Cracking technology can be divided into two types due to different final products: one is to recover chemical raw materials (such as ethylene, propylene, styrene, etc.). ) [12], and the other is to obtain fuel (gasoline, diesel, tar, etc.). ). Although all waste plastics are converted into low molecular substances, the process routes are different. The preparation of chemical raw materials is to heat waste plastics in a reaction tower and reach the decomposition temperature (600 ~ 900℃) in a fluidized bed, which generally does not produce secondary pollution, but it has high technical requirements and high cost. Cracking oil technology usually includes thermal cracking and catalytic cracking.
Japan's Fuji Cycle Company converts waste plastics into gasoline, kerosene and diesel oil, and uses ZSM-5 catalyst to crack plastics into fuel through conversion reactions in two reactors. Each kilogram of plastic can produce 0.5L gasoline, 0.5L kerosene and diesel oil. Amoco has developed a new technology to convert waste plastics into basic chemicals in refineries. Pretreated waste plastics are dissolved in hot refined oil and decomposed into light products under the action of high temperature catalytic cracking catalyst. Recovering liquefied petroleum gas and aliphatic fuel from polyethylene; Aliphatic fuel can be recovered from polypropylene and aromatic fuel can be obtained from polystyrene. Yoshio Uemichi et al. [13] developed a composite catalytic system for polyethylene degradation. The catalysts are silica/alumina and HZSM-5 zeolite. The experimental results show that the catalyst can effectively and selectively prepare high-quality gasoline with a gasoline yield of 58.8% and an octane number of 94.
Domestic Li Mei et al. [14] reported that gasoline of MON73 and diesel oil of SP- 10 can be obtained from waste plastics after reacting at 350 ~ 420℃ for 2 ~ 4 s, which can be continuously produced. Li et al. [3] studied the catalysts in the degradation of waste plastics. In the process of catalytic cracking with polyethylene, polystyrene and polypropylene as raw materials, the ideal catalyst is a molecular sieve catalyst with acidic surface, operating temperature of 360℃, liquid yield of over 90% and gasoline octane number of over 80. Liu [15] developed a pilot plant for catalytic cracking of waste plastics to produce steam and diesel oil once, with a daily output of 2 tons, which realized the continuous operation of separating steam and diesel oil and discharging slag. The cracking reactor has the characteristics of good heat transfer effect and large production capacity. When the amount of catalyst is 1 ~ 3% and the reaction temperature is 350 ~ 380℃, the total yield of gasoline and diesel oil can reach 70%. The octane number of gasoline made of waste polyethylene, polypropylene and polystyrene is 72, 77 and 86 respectively, and the freezing point of diesel oil is 3,-1 1, -22℃. This process Yuan [16] studied the catalytic cracking technology of waste plastics in fluidized moving bed reactor, and solved the problems of slag cleaning at the bottom of the reactor and pipeline cementation. It lays a foundation for realizing safe, stable and long-term continuous production, reducing energy consumption and cost, and improving output and product quality.
It is an important way to recycle resources and avoid secondary pollution to make chemical raw materials and fuels by cracking wastes. Germany, the United States, Japan and other countries have large factories, and China has also built small waste plastic oiling plants in Beijing, Xi, Guangzhou and other places, but there are still many problems to be solved. Due to the poor thermal conductivity of waste plastics, plastics produce high viscosity melt when heated, which is not conducive to transportation; PVC in waste plastics will produce HCl, which will corrode equipment and reduce catalyst activity. Carbon residue adheres to the reactor wall, which is difficult to remove and affects continuous operation; The catalyst has low service life and activity and high production cost; At present, there is no better treatment method for oil residue produced in production, and so on. There are still many reports on pyrolysis to produce oil in China [43-54], but how to absorb the existing achievements and overcome the technical difficulties is an urgent task before us.
4.2 Supercritical refueling method
The critical temperature of water is 374.3℃, and the critical pressure is 22.05Mpa. Critical water has the properties of normal organic solution, which can dissolve organic matter but not inorganic matter, and is completely miscible with air, oxygen, nitrogen and carbon dioxide. Japanese patent reports waste plastics (PE, PP, PS, etc. ) can be recycled with supercritical water. The reaction temperature is 400 ~ 600℃, the reaction pressure is 25Mpa, the reaction time is below 65438±00min, and the oil yield can reach above 90%. The advantages of using supercritical water to degrade waste plastics are obvious: the cost of using water as medium is low; Carbonization can be avoided during pyrolysis; The reaction is carried out in a closed system, which will not bring new pollution to the environment; Fast reaction speed, high production efficiency, etc. Qiu Ting et al [17] summarized the progress of supercritical technology in plastic recycling.
4.3 gasification technology
The advantage of gasification method is that it can treat municipal solid waste in a mixed way without separating plastics, but the operation needs to be higher than that of thermal decomposition method (generally around 900℃). Schwaize Pumpe Refinery in Espag, Germany can process 1700 tons of waste plastics into city gas every year. RWE plans to gasify 220,000 tons of lignite, more than 654.38 million tons of plastic waste and petroleum sludge produced by town petroleum processing plants every year. Hoechst Company of Germany gasifies the mixed plastics by high-temperature Winkler process, and then converts it into water gas as the raw material for alcohol synthesis.
4.4 Hydrocracking Technology
Vebaeol Company of Germany has set up a hydrocracking unit to hydrogenolysis waste plastic particles at 15 ~ 30 MPa and 470℃ to produce a synthetic oil, which contains 60% paraffin, 30% cycloalkane and 1% aromatic hydrocarbon. The effective energy utilization rate of this processing method is 88%, and the effective substance conversion rate is 80%.
5 Other utilization technologies
Waste plastics also have a wide range of uses. Texas State University uses yellow sand, stones, liquid PET and curing agent as raw materials to make concrete, while Bitlgosz [18] uses waste plastics as cement raw materials. Xie Liping [19] prepared mesoporous activated carbon from waste plastics, wood and paper, Lei et al. [20] reported that waste polystyrene was used as coating, [2 1] reported that plastics could be turned into wood. Song Wenxiang [22] introduced that foreign countries used HDPE as raw material, and through special methods, glass fibers with different lengths were made to flow in the same direction along the axial direction of the material in the mold, thus producing high-strength plastic sleepers. Pu et al. [23] used waste polyethylene to make high value-added polyethylene wax. Li Chunsheng et al. [24] reported that compared with other thermoplastics, polystyrene has the characteristics of low melt viscosity and high fluidity, which can well infiltrate the contact surface after melting and play a good bonding role. Zhang Zhengqi et al. [25] modified asphalt with waste plastics, and dissolved one or several plastics evenly in asphalt according to a certain proportion, which improved the road performance of asphalt, thus improving the quality of asphalt pavement and prolonging the service life of pavement.
Concluding remarks
Controlling white pollution is a huge systematic project, which requires the joint efforts of all departments and industries, the participation and support of the whole society in thought and action, and the improvement of the scientific and technological awareness and environmental awareness of the whole people. While formulating laws and regulations to strengthen management, government departments can take the development of environmental protection technologies and industries as an important channel to stimulate the economy and expand employment, so as to industrialize the collection, treatment and recycling of waste plastics. At present, China's recycling enterprises are scattered and small in scale, and many new technologies and equipment for plastic recycling at home and abroad cannot be popularized and implemented, and the quality of recycled products is low. Therefore, we should standardize the management of plastic recycling enterprises to improve their scientific and technological content and economic benefits. At the same time of recycling, we should develop environmentally friendly plastics and seek practical substitutes.