Analysis:
Recovery and recycling of waste plastics
Recycling of waste plastics;
The recycling of waste plastics is the basis of reuse. The difficulty of recycling lies in the large quantity, wide distribution, many varieties and large volume of waste plastics. Many waste plastics are mixed with other municipal wastes, which makes recycling very difficult.
At present, foreign countries have accumulated a lot of experience in plastic recycling. They regard the recycling of waste plastics as a systematic project, involving * * *, enterprises and residents. Germany began to recycle packaging containers from 1993. In 1997, the recycled waste plastics reached 600,000 tons, which was 75% of the consumption of 800,000 tons in that year. Before the project, Germany set up more than 300 recycling and sorting outlets for packaging containers all over the country, and uniformly classified plastic products into bottles, films, cups, ps foamed products and other products, with uniform color signs. The secret of successful resin recycling in Japan lies in the establishment of recycling system. The core of recycling management system is to minimize recycling links, and manufacturers should also consider establishing recycling outlets while establishing sales outlets. Manufacturers are responsible for recycling their own waste products. When recycling the waste products produced by ourselves, it is easy to master the performance of the original standard parts and materials, which can be fully and effectively recycled and ensure the performance of the recycled products. At the same time, it can also reduce heat recovery and reduce cumbersome procedures and environmental pollution. Due to the modularization of products, the direction of technology research and development of recycling part is more clear.
For further utilization, the recycled waste plastics are often separated. The main separation technologies include density separation, dissolution separation, filtration separation, electrostatic separation and flotation separation, as shown in Figure 2. 1 The one-time separation rate of water flotation separation device of Japan Plastics Treatment Promotion Association can reach above 99.9%, and Dow Chemical Company of the United States has also developed a similar separation technology, using liquid hydrocarbon instead of water to separate mixed waste plastics, and achieved good results. Kellogg Company and Rensselaer Institute of Technology jointly developed a solvent separation and recovery technology, which can separate mixed waste plastics without manual sorting. In this method, chopped waste plastics are added into a solvent, and the solvent can be selected at different temperatures.
Effectively dissolve different polymers and separate them. Xylene is the best solvent, and the operating temperature should not be too high. Some new separation technologies, such as electromagnetic rapid heating method and reactive mixing method, have also been reported. Metal-polymer assembly can be recovered and separated by electromagnetic rapid heating method, and coated waste bumper can be recovered and separated by reactive mixing method. In addition, foreign countries have also developed a computer automatic sorting system to realize the continuous automation of the sorting process. Swiss Bueher company uses halogen lamp as strong light source. After four kinds of filters are identified, waste plastics such as PE, PP, PS, PVC and PET can be separated by computer, and the production capacity is IT/h. ..
Directly used or mixed with other polymers to make polymer alloys. These products can be used to manufacture raw plastic products, plastic fillers, filter materials, barrier materials, coatings, building materials and adhesives. This is a simple and feasible method, which can be reused and divided into two types: melting regeneration and modified regeneration.
(1) melt regeneration
This method 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 has been widely used, mainly recycling the corner wastes produced in the production process of resin factories and plastic products factories, and it can also include those disposable wastes that are easy to clean and select. This part of the waste is characterized by relatively clean and simple components. Through simple technology and equipment, recycled plastics with good properties can be obtained, and their properties are similar to those of new materials. At present, about 20% of plastic waste is recycled by this method, and most plastic recycling plants at this stage belong to this category.
The waste plastics used for composite recycling are collected from different channels, with many impurities, many kinds, miscellaneous and dirty. Due to the differences and incompatibility of physical and chemical characteristics of various plastics, their mixture is not suitable for direct processing, and different kinds of plastics must be separated before regeneration, so the recovery process is complicated. The advanced separation equipment adopted in the world can systematically separate different materials, but the one-time investment of the equipment is high. Generally speaking, composite recycled plastics are unstable and fragile, so they are often used to prepare low-grade products, such as building fillers, garbage bags, microporous sandals, raincoats and mechanical packaging materials.
At present, China's Dalian, Chengdu, Chongqing, Zhengzhou, Shenyang, Qingdao, Zhuzhou, Handan, Baoding, Zhangjiakou, Guilin, Beijing, Shanghai and other places have introduced more than 20 sets of devices for recycling waste plastics by melting, which are mainly used to produce building materials, recycled plastic products, civil materials, coatings and plastic fillers.
(2) Modification and regeneration
It refers to the modification of waste plastics by chemical or mechanical methods. The mechanical properties of modified recycled products are improved and can be made into high-grade products.
Japan Baozhong Industrial Technology Research and Development Laboratory invented a method to process waste paper and waste polyethylene into synthetic wood, which can be processed like natural wood and has the same good texture as natural wood. Clayton polymer cooperative research center in Australia has developed a production process to produce wood substitutes in construction industry by using polyethylene film fragments and waste paper fibers. The processing process is carried out in a twin-screw extruder, and the processing temperature is lower than 200℃, which can avoid fiber degradation. The appearance, density and mechanical properties of newsprint/polyethylene composite material produced by this method are similar to those of hardboard, and it can be cut and formed by standard tools, and it also has good crack resistance and waterproof performance. Saitama's "Ainu Wood" technology uses dry grinding and cleaning to recycle plastic waste, and uses recycled mixed sawdust such as PE, PP, PVC, ABS and other raw materials to produce new wood boards with sawdust content exceeding 50%. The appearance of Eynde Wood technology has attracted the attention of all countries in the world, especially the developed countries, and has had a strong response.
In terms of chemical additives, Ciba-Cargill produced a mixed additive containing active and inactive additives such as antioxidants and stabilizers, which can basically restore the properties of recycled materials to the original level; Some people in the Netherlands have also developed a new chemical compatilizer, which can bond recycled plastics containing different polymers together. It is reported that in the United States, solid shear crushing (S3P) is used for machining. Without heating and melting, the resin can be sheared at the molecular level to form a * * * mixture with mutual capacity. * * * Most of the mixture is composed of HDPE and LLDPE, and its ultimate tensile strength and flexural modulus are equivalent to those of pure HDPE and LLDPE. In recent two years, solid shear extrusion, reactive mixing, multi-layer sandwich injection molding technology and reactive extrusion rule have made it possible to recycle some waste plastics that are difficult to recycle.
(3) Wood flour is filled with modified waste plastics.
Wood flour filled with modified waste plastics is a brand-new green wood-plastic material, and its processing method is also a physical modification and regeneration method. In recent years, there are more and more researches in this field at home and abroad, which have developed rapidly, and commercial products have appeared, so it has become a trend to develop plastic-wood materials and related technologies.
The development and research of wood flour and waste plastics composites not only provide opportunities for making full use of natural resources, but also reduce the pollution of waste plastics to the environment. Therefore, this wood-plastic composite is an energy-saving and environment-friendly green material. It has a wide range of applications, mainly in building materials, automobile industry, goods packaging and transportation, decorative materials and daily necessities, and has broad development prospects. This can also be seen from the patent research at home and abroad. As an organic filler of plastics, wood flour has many excellent properties that other inorganic fillers can't match: wide source, low price, low density, good insulation and little wear on processing equipment. However, as an inorganic filler, it has not been widely used, mainly for the following two reasons: poor compatibility with matrix resin; The poor dispersion effect in molten thermoplastic leads to poor fluidity and difficult extrusion molding and processing.
① Treatment of wood flour: It is best to cook wood fiber materials, such as poplar and cedar sawdust. This kind of wood fiber has regular shape and length-width ratio, so it needs to be cleaned and dried as much as possible before use, and then processed into wood powder similar to sawdust specifications. The specifications and dimensions of wood flour are specified in various patents: the best length is 1- 10 mm, the thickness is 0.3- 1.5 mm, the aspect ratio is 2.5-6.0, and the moisture absorption rate is less than 12% (by weight).
(2) Processing requirements of wood-plastic composites: When the composite particles are extruded into wood, if the unventilated extrusion process is adopted, the particles should be as dry as possible, and the moisture content should be between 0.0 1% ~ 5% (mass fraction), preferably less than 3.5%. Under the condition of ventilation, the water content below 8% is acceptable. Otherwise, the extruded material will produce cracks or other surface defects.
The cross-sectional shape of composite particles is studied, and it is considered that the cross-sectional shape with regular geometry is more favorable, including triangle, square, rectangle, hexagon, ellipse and circle. A regular cylinder with an approximately circular or elliptical cross section is preferred.
In the process of extrusion, wood fibers should be oriented along the extrusion direction, so that adjacent parallel wood fibers can overlap with polymers coated on oriented wood fibers, thus improving the physical properties of materials. Generally, the degree of orientation is 20%, preferably 30%. The material of this structure has sufficiently enhanced strength and tensile modulus, and is suitable for manufacturing doors and windows.
The mixing ratio of wood flour and waste plastic was studied. The optimum technological conditions are 45% plastic and 55% wood powder. It is also found that the mixing ratio from 40% plastic and 60% wood fiber to 60% plastic and 40% wood fiber can produce useful products. The composition of the mixture depends on the characteristics of the final product and the types of plastic and wood fibers.
③ Improvement of compatibility: Because the main component of wood flour is cellulose, which contains a large number of hydroxyl groups, these hydroxyl groups form intermolecular hydrogen bonds or intramolecular hydrogen bonds, which makes wood flour absorbent, and the moisture absorption rate can reach 8%- 12%, and it is very polar, while thermoplastics are mostly nonpolar and hydrophobic, so the compatibility between them is very poor and the interfacial adhesion is very small. Surface modification of polymer and wood flour with appropriate additives can improve the interfacial affinity between wood flour and resin. The performance of modified wood flour filler is enhanced, which can transfer the stress between filler and resin well, thus improving the strength of composite materials. Therefore, the first problem to be solved is compatibility in order to obtain plastic-wood composites with excellent properties and meet the requirements.
The compatibility problem is mainly solved by adding various additives.
Coupling agent method: Coupling agent can improve the compatibility between inorganic filler and inorganic fiber and matrix resin, and also improve the interface between wood flour and polymer. Silane coupling agent and titanate coupling agent are the two most widely used coupling agents. Experiments show that both of them can improve the compatibility between filler and resin.
Compatibilizer method: Adding compatilizer method is the simplest and most effective method. It is reported that suitable compatibilizers include maleic anhydride grafted plant fibers or maleic anhydride modified polyolefin resins, acrylate polymers and ethylene acrylic polymers. Most of these compatibilizers contain hydroxyl groups or anhydride groups, which can react with hydroxyl groups in wood flour to reduce the polarity and hygroscopicity of wood flour, so they have good compatibility with resin.
(4) Influence of additive dosage on the properties of composites: The dosage of coupling agent is not proportional to the activation effect of filler. When the additive content is 1%, the tensile strength and tensile modulus of the material are the best, but with the increase of additive content, the properties of the material decrease. Therefore, the dosage of additives should not be too much, otherwise it will not only affect the performance, but also cause unnecessary waste.
⑤ Improvement of fluidity: For extrusion processing, the processed materials are required to have certain fluidity. In most cases, filled plastics need to be melted, stressed and deformed, and then cooled and molded to make various products. Therefore, it is necessary to study the effect of wood flour filler on the rheological properties of melt. Among them, the most important is the influence on melt viscosity.
With the increase of wood flour content, the viscosity of polymer melt increases, which is related to the dispersion of wood flour in matrix resin. Wood flour particles exist in the matrix in a certain aggregate state, and the effect of aggregate wood flour on the fluidity of the filling system is unfavorable. Appropriate amount of stearic acid can be added to reduce the aggregation number of wood flour particles, improve the agglomeration phenomenon and make them fully dispersed in the matrix resin. In addition, wood-plastic composites belong to pseudoplastic fluid in molten state, and the apparent viscosity decreases with the increase of shear rate. Therefore, in order to make the filling system have good processing fluidity, higher shear stress should be adopted as much as possible to reduce the shear viscosity of the filling system and make it suitable for extrusion molding.
⑥ Improvement of processing conditions: Extrusion molding, hot pressing molding and injection molding are the main molding methods for processing plastic-wood composites. Due to the short processing cycle, high efficiency and simple molding process, extrusion molding is a better choice.
Single screw extruder can complete the plasticizing and conveying tasks of materials. Because the filling of wood powder increases the viscosity of polymer melt and the difficulty of extrusion, the single screw extruder used for wood powder filling modification must adopt a specially designed screw with strong mixing and plasticizing ability.
Because of the fluffy structure of wood flour, it is not easy to feed into the extruder screw, so the materials should be mixed and granulated before extrusion. Because wood flour has water absorption, it should be dried before granulation, and the drying temperature is about 150℃, and the appropriate time is 3 hours. If the drying is insufficient, bubbles will be generated in the product, thus reducing the mechanical strength of the material. It is also very important to control the processing temperature. If the temperature is too high, the wood flour will carbonize due to heating, which will affect the apparent color of the material. Therefore, the processing temperature should be properly controlled during the processing.
Chemical methods:
It refers to the conversion of waste plastics into low molecular compounds or oligomers through chemical reactions. These technologies can be used to produce fuel oil, natural gas, polymer monomers and petrochemical and chemical raw materials from waste plastics.
From a technical point of view, chemical methods mainly include pyrolysis, catalytic cracking, hydrocracking, supercritical fluid method, solvolysis and so on. Thermal cracking produces hydrocarbons with a wide boiling point range and low recovery value. Because of the existence of catalyst, the reaction temperature can be reduced by several tens of degrees, and the product distribution is relatively easy to control, so that gasoline with high crystallinity can be obtained. Supercritical fluid method has become a research hotspot because of its environmental protection, economy, fast decomposition speed and high conversion rate. It is not only suitable for oiling waste plastics, but also suitable for solvent decomposition of polycondensates. Solvolysis is mainly used for depolymerization of polycondensation waste plastics.
Receiving monomer.
As far as use is concerned, chemical methods can be divided into two types according to the difference of final products. First, prepare fuel (gasoline, kerosene, diesel oil, liquefied gas, etc.). ), and the other is to prepare basic chemical raw materials and monomers.
(1) Oilization technology for manufacturing fuel (oil and gas)
Foreign countries began to develop refueling technology as early as the stone sleeve crisis in the 1970s.
Cracking, 1kg waste plastics can produce up to iL. This technology does not use stirring device, and is only suitable for polyolefin, not for halogen-containing plastics.
APME (European Plastics Manufacturers Association) believes that the recycling process must be able to accept a wide range of mixed plastics in order to have vitality. At present, the industry has carried out laboratory project research and preliminary pilot test on waste plastics rich in PVC (up to 60%), but the best technological conditions for building demonstration devices have not yet been provided.
In April 2000, Japan fully implemented the "packaging container recycling law" for waste plastics. In order to solve the problem of oiling mixed plastics, the Japan Waste Plastics Recycling Promotion Association and the Waste Research Consortium, with the support of * * * *, successfully developed a universal oiling technology for mixed waste plastics. Its technological process includes pretreatment, dechlorination and thermal decomposition. In order to improve the quality of oil products, catalysts were added for upgrading.
Japanese companies such as Mitsubishi Heavy Industries, Toshiba and Nippon Steel have successively carried out pilot or industrial tests, which can produce oil crystals such as gasoline, diesel oil and heavy oil. The technology has passed, but the economy has not. Therefore, the relevant companies are greatly reducing the cost by improving the process, especially the test results of Northeast Electric Power and Mitsubishi Heavy Industries using supercritical water to refuel waste plastics. After the reaction time was greatly shortened from 2 hours in the past to 2 minutes, the oil recovery rate remained at a high level of over 80%, which was beneficial to reducing the cost. Considering that the rising oil price will be beneficial to improving economic benefits, the 0.5t/h industrial test currently under way is expected to be put into practical use soon after its success.
(2) Basic chemical raw material preparation and monomer recovery technology:
Mixed waste plastics are pyrolyzed to produce liquid hydrocarbons, and ultra-high temperature gasification to produce water gas, which can be used as chemical raw materials. In recent years, Germany's Hust Company, Rules Company, BASF Company, Kansai Electric Power Company, Mitsubishi Heavy Industries, etc. have developed the technology of producing synthesis gas by ultra-high temperature gasification of waste plastics, and then producing methanol and other chemical raw materials, and have been industrialized.
In recent years, more and more attention has been paid to the recycling technology of waste plastic monomers, which has gradually become the mainstream direction, and its industrial application is also under study. 1998 at the 14 international academic conference on analysis and applied pyrolysis held in Munich, Germany in May, a new trend of polymer waste recycling and development appeared. From the papers published in this conference, it can be seen that the problem of "white pollution" of polymer materials has been basically solved internationally, and after the research and industrialization of preparing fuel by pyrolysis of polymer wastes, it has tended to transform polymer wastes into new raw materials for polymer synthesis through effective catalytic-pyrolysis methods.
Stage. At present, the research level has reached the monomer recovery rate of 90% for polyolefin, 97% for polyacrylate, 92% for fluoroplastics, 75% for polystyrene and 80% for nylon and synthetic rubber. The industrial application of these achievements is also under study, which will bring great benefits to the environment and resource utilization.
Bater Memorial Institute (U.S. patent. No.5136117) has successfully developed the technology of recovering ethylene monomer from mixed waste plastics such as LDPE, HDPE, PS, PVC, etc. The recovery rate is 58% (mass fraction), the cost is 3.3 cents/kg, and the target is to realize industrialization within two years. Mitsubishi Corporation, the general agent of Japan, introduced this technology and commercialized it, and built a continuous reaction device with a flow rate of 20L/h. ..
Solvolysis (including hydrolysis and alcoholysis) is mainly used for depolymerization of polycondensation polymer materials to recover monomers, which is suitable for a single variety of waste plastics after strict pretreatment. At present, it is mainly used to treat polar waste plastics such as polyurethane, thermoplastic polyester and polyamide. Such as polyurethane foam hydrolysis to prepare polyester and diamine, polyurethane soft and hard products alcoholysis to prepare polyol, waste PET depolymerization to prepare crude terephthalic acid and ethylene glycol.
In addition, in recent years, supercritical fluid method has been more and more applied to depolymerization and polycondensation of polymer materials, and its monomer is recovered, and the effect is far better than that of ordinary solvent decomposition. T. Sako and others in Japan used supercritical fluid decomposition to recover waste polyester (PET), glass fiber reinforced plastic (FRP) and polyamide/polyethylene composite film. The advantage of their recovery of PET with supercritical methanol is that the decomposition speed of PET is fast, no catalyst is needed, and almost 100% monomer can be recovered. They also treated the PA6/PE composite membrane with subcritical water recovery to hydrolyze PA6 into monomer caprolactam, and the recovery rate was above 70%-80%.
Thermal energy regeneration:
The burning of plastics will release a lot of heat. The calorific value of polyethylene and polystyrene is as high as 46 000 kJ/kg, which exceeds the average calorific value of fuel oil of 44 000 kJ/kg. The combustion test shows that waste plastics have the basic properties of being used as fuel. See Table 2.2 for the comparison test of pulverized coal and heavy oil combustion. As can be seen from Table 2.2, the calorific value of waste plastics is equivalent to that of coal and petroleum, and it does not contain sulfur. In addition, due to the low ash content, the combustion speed is fast.
Therefore, foreign countries use waste plastics instead of coal, oil and coke for blast furnace injection, cement rotary kiln instead of coal for burning cement, and refuse solid fuel (RDF) for power generation, all of which have achieved good results.
(1) refueling: solid waste fuel RDF
Japan actively promotes the production of solid waste fuel from waste plastics. RDF technology was originally developed in the United States. In recent years, in Japan, due to the shortage of landfills, the corrosion of boilers by hydrogen chloride and the environmental pollution caused by tail gas from chlorine-containing waste plastics, RDF with a calorific value of 20933 kJ/kg and uniform particle size was prepared by mixing various combustible wastes with the characteristics of high calorific value of waste plastics, which not only diluted chlorine, but also facilitated storage, transportation and combustion, and was used in other boilers and industrial kilns to replace coal. Garbage solid fuel power generation was first applied in the United States, with 37 RDF power stations, accounting for 2 1.6% of garbage power stations. In combination with overhaul, Japan changed some small-scale waste incineration stations into RDF production stations to facilitate continuous and efficient large-scale power generation after centralized, so that the steam parameters of waste power stations were reduced from
Electricity is better. At present, all cement plants in Japan are actively promoting.
(2) Blast furnace injection and cement rotary kiln injection
The technology of injecting waste plastics into blast furnace is a new method to treat waste plastics by using the high calorific value of waste plastics to make them into blast furnaces with suitable particle size instead of coke or pulverized coal. The application of injecting waste plastics into blast furnaces abroad shows that the utilization rate of waste plastics reaches 80%, and the discharge amount is 0. 1% ~ 1.0% of the incineration amount, which only produces less harmful gases and has lower treatment cost. The technology of injecting waste plastics into blast furnace opens up a new way for the comprehensive utilization of waste plastics and the treatment of "white pollution", and also provides a new means for metallurgical enterprises to save energy and increase efficiency.
1995, Bremen Iron and Steel Company of Germany first injected waste plastics into its No.2 blast furnace (with a volume of 2688m3), and established a 70kt/a injection equipment. Then Krupp/Hirsch Steel Company of the United States also established a 90kt/a injection equipment, and other German iron and steel companies are also preparing to adopt this technology. 1996, NNK Company of Japan injected waste plastics into its Jingbin Plant 1 No.1 blast furnace (volume 4093m3), and planned to treat waste plastics at 30kt/a.
It also intends to transfer this technology to other factories in Japan. Japanese environmental protection circles and public opinion circles have high hopes for this, and the Japan Iron and Steel Union has included this in the 20 10 energy-saving plan, requiring an annual injection of10 million tons, equivalent to 2% of the energy consumption of the steel industry, with bright prospects.
In addition, the experiment of injecting waste plastics into Japanese cement rotary kiln is also successful. On the basis of burning waste tires for a long time, the cement plant of Deshan Company, with the cooperation of 1996 Waste Plastic Treatment Promotion Association, successfully carried out the experiment of injecting waste plastics into rotary kiln.
fermentation method
It is reported that waste polyethylene can be transformed into microbial protein by oxidative fermentation and pyrolysis fermentation. This method is a non-mainstream method and is not commonly used at present.
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