Ans:
Recycling and Recovery of Waste Plastics
Recycling of Waste Plastics:
Recycling of waste plastics is the basis for reuse. The difficulty of recycling lies in the fact that waste plastics are large in quantity, wide in distribution, many varieties and large in volume, and many waste plastics are mixed with other municipal garbage, which makes it very difficult to be recycled.
At present, foreign countries have accumulated a lot of experience in the recycling of waste plastics, they take the recycling of waste plastics as a systematic project, ***, enterprises, residents **** with the participation. Germany in 1993 began the implementation of packaging containers recycling, recycling waste plastics in 1997 reached 600,000 tons, was 800,000 tons of consumption of 75%. At present, Germany has set up more than 300 recycling and sorting outlets throughout the country, and each outlet is unified to divide plastic products into bottles, films, cups, PS foam products and other products, and has a unified color marking. The secret of Japan's success in resin recycling lies in the establishment of a recycling system. The core of the recycling management system is to minimize the recycling process, and each manufacturer must consider establishing a recycling network at the same time as it establishes a sales network. Manufacturers take responsibility for recycling their own used products, and when they do so, the performance of the original standard parts and their materials can be easily grasped, so they can be recycled efficiently and effectively, and the performance of the recycled products can be ensured. At the same time, heat recovery can be minimized, reducing cumbersome procedures and environmental pollution. The modularization of the product makes the direction of technological research and development in the recycling section clearer.
For further utilization, the recycled waste plastics are often separated, the main separation techniques used are density separation, dissolution separation, filtration separation, electrostatic separation and flotation separation, etc., see Figure 2.1. Japan Plastics Processing Promotion Association of the water flotation separation device can reach a separation rate of 99.9% or more, the U.S. DOW Chemical Company has developed a similar separation technology to liquid hydrocarbons instead of water to separate mixed waste plastics. DOW Chemical Company in the United States has also developed a similar separation technology, using liquid hydrocarbons instead of water to separate mixed waste plastics, and has achieved better results. Kellogg and Rensselaer Polytechnic Institute jointly developed a solvent-based separation and recycling technology, which can separate mixed waste plastics without manual sorting. The method is to add chopped waste plastics to a solvent, at different temperatures the solvent can selectively
dissolve different polymers and separate them. The best solvent to use is xylene, and the operating temperature is not too high. Some new separation techniques such as electromagnetic fast heating method, reactive **** mixing method has also been reported. Electromagnetic rapid heating method can recover and separate metal-polymer components, and reactive *** mixing method can realize the recovery and separation of waste bumpers with coating layer. In addition, foreign countries have developed computerized automatic sorting systems, realizing the continuous automation of the sorting process. Switzerland's Bueher company with halogen lamps as a strong light source irradiation, after four kinds of filter identification, by the computer can be separated from PE, PP, PS, PVC and PET waste plastics, production capacity of It / h.
Directly used or blended with other polymers to make polymer alloys. These products can be used in the manufacture of raw plastic products, plastic fillers, filtration materials, barrier materials, coatings, building materials and adhesives. This is a simple and viable method of reuse and can be divided into melt recycling and modified recycling.
(1) Melt regeneration
The method is to melt the waste plastics after heating and re-molding. According to the nature of the raw material, can be divided into simple regeneration and composite regeneration two kinds.
Simple regeneration has been widely used, the main recovery of resin production plants and plastic products factory production process generated by the edge of the waste, but also can include those easy to clean, select the one-time use of waste products. This part of the waste material is characterized by cleaner, more single composition, using a simple process and equipment can be good nature of the recycled plastics, its performance is similar to the new material. Now about 20% of the plastic waste products using this recycling method, most of the plastic recycling plant at this stage belongs to this category.
Composite regeneration of waste plastics used from different channels collected, more impurities, with diverse, mixed, dirty and other characteristics. Due to the differences in physical and chemical properties of various plastics and incompatibility, their mixture is not suitable for direct processing, before regeneration must be separated from different types, so the recycling process is more complicated, the international advanced separation equipment can be used to systematically sort out different materials, but the one-time investment in equipment is higher. Generally speaking, the nature of composite recycled plastics is not stable, easy to become brittle, so it is often used to prepare lower-grade products, such as construction filler, garbage bags, micro-perforated sandals, raincoats and packaging materials for machinery, etc.
At present, the recycling process is very complicated.
At present, China's Dalian, Chengdu, Chongqing, Zhengzhou, Shenyang, Qingdao, Zhuzhou, Handan, Baoding, Zhangjiakou, Guilin, as well as Beijing, Shanghai and other places were imported from Japan, Germany, more than 20 sets of (Taiwan) melt regeneration processing and utilization of waste plastics device, mainly used for the production of building materials, recycled plastics, civil engineering materials, coatings, plastic fillers and so on.
(2) modified regeneration
refers to the modification of waste plastics through chemical or mechanical methods. The mechanical properties of the modified recycled products have been improved, and can be made into higher-grade products.
Japan's Takarazuka City Industrial Technology Research and Development Laboratory invented a way to waste paper and waste polyethylene processing into synthetic wood, this synthetic wood can be processed as natural wood, texture and natural wood as good. The Cooperative Polymer Research Centre in Clayton, Australia, has developed a process for producing wood substitutes for the construction industry from polyethylene film trimmings and waste paper fibers in a twin-screw extruder at temperatures below 200°C, which avoids degradation of the fibers. The newsprint/polyethylene composites produced by this method are similar in appearance, density and mechanical properties to hard fiberboard, can be cut and shaped with standard tools, have good crack resistance when nailed, and are more water resistant than hard fiberboard. Sadao Nishikori's "Einwood" technology uses dry milling and cleaning to recycle plastic waste, and uses recycled raw materials such as PE, PP, PVC, and ABS mixed with waste wood chips to produce a new type of board with a wood chip content of more than 50%. Einwood technology has aroused the attention of countries around the world, especially developed countries and has produced a strong reaction.
In terms of chemical additives, Ciba - Cargill produced a mixture of additives containing antioxidants, *** stabilizers and other active and inactive additives can make the performance of recycled materials basically restored to the original level; the Netherlands has also developed a new type of chemical capacitance enhancers that can contain different polymers recycled plastics bonded together. The United States reported the use of solid shear pulverization process (Solid State Shear Pulverization, S3P) for mechanical processing, without heating and melting can be carried out on the molecular level of the resin shear, the formation of inter-compatible **** mixture, **** mixture of most of the composition of HDPE and LLDPE, the ultimate tensile strength and flexural modulus can be comparable to HDPE and LLDPE pure materials, and the ultimate tensile strength and flexural modulus can be comparable to HDPE and LLDPE pure materials. The ultimate tensile strength and flexural modulus are comparable to those of pure HDPE and LLDPE. In the past two years, solid phase shear extrusion, reactive **** blending, multi-layer sandwich injection molding technology and reactive extrusion method makes the recycling of some difficult-to-recycle waste plastics possible.
(3) wood powder filled modified waste plastic
Wood powder filled modified waste plastic is a new green wood materials, the processing method is also a physical modification of the regeneration method. Due to more research and rapid development in this area in recent years at home and abroad, and there have been commercialized products, the development of plastic wood materials and its related technology has become a trend
The development and research of composite materials of wood flour and waste plastics not only can provide the opportunity to make full use of the natural resources, but also can alleviate the environmental pollution caused by waste plastics, so this wood-plastic composite material is a kind of energy-saving, protective and environmental protection material, which can be used to protect the environment. Therefore, this kind of wood-plastic composite material is a kind of green material that saves energy and protects the environment. It has a wide range of applications, mainly used in building materials, automobile industry, packaging and transportation of goods, decorative materials and daily life utensils, etc., and has a broad development prospect. This can also be seen from the domestic and foreign patent research. Wood flour as an organic filler for plastics, with many other inorganic fillers can not be compared with the excellent performance: wide range of sources, low price, low density, good insulation, processing equipment wear and tear. However, it is not as widely used as inorganic fillers, mainly for the following two reasons, poor compatibility with the base resin; poor dispersion in the molten thermoplastics, resulting in poor fluidity and extrusion molding, processing difficulties.
① wood powder processing: wood fiber materials preferred for cooking wood materials, such as aspen, cedar sawdust, etc., this wood fiber has a regular shape and aspect ratio, need to be processed and dried before use, as dry as possible, and then processed into a similar sawdust specifications of the wood powder. Patents on the specifications of wood flour, size are made accordingly: length preferably 1-10mm, thickness 0.3-1.5mm, aspect ratio 2.5-6.0, moisture absorption rate of less than 12% (by weight).
② processing requirements for plastic wood composite: composite particles extruded into material, if the extrusion process without ventilation equipment, particles should be as dry as possible, the moisture content should be between 0.01% to 5% (mass fraction), preferably less than 3.5%. With ventilation, a moisture content of less than 8% is acceptable. Otherwise, the extruded material will produce cracks or other surface defects.
The shape of the cross-section of the complex particles has been studied and it is considered more favorable to have a cross-section with a regular geometrical shape, including triangular, square, rectangular, hexagonal, elliptical, circular, etc.', preferably a regular cylinder with an approximately circular or elliptical cross-section.
In the extrusion process the wood fibers are more preferably oriented in the extrusion direction, and this orientation enables adjacent parallel wood fibers to overlap with the polymer encapsulated on the oriented wood fibers, which in turn can improve the physical properties of the material. Typically the degree of orientation is 20%, preferably 30%. This structure of the material has sufficiently enhanced strength, tensile modulus, suitable for making windows and doors.
Studied the mixing ratio of wood flour and waste plastic, the preferred conditions for the plastic 45% (mass fraction, the same later), wood flour 55%, also found that from 40% of the plastic, 60% of the wood fiber to 60% of the plastic, 40% of the wood fiber mixing ratio can be used to produce a suitable product. The selection of mixture components depending on the characteristics of the end product, plastic and wood fiber type.
③ compatibility improvement: as the main component of wood flour is cellulose, cellulose contains a large number of hydroxyl, these hydroxyl form intermolecular hydrogen bonding or intramolecular hydrogen bonding, so that the wood flour has a water-absorbent, hygroscopicity up to 8% to 12%, and strong polarity, and most of the thermoplastic non-polar, with hydrophobicity, so compatibility between the two is poor, the interface of the bonding force is very small. The use of appropriate additives to modify the surface of the polymer and wood flour can improve the interfacial affinity between the wood flour and resin, modified wood flour filler has the nature of enhancement, can be very good to transfer the stress between the filler and the resin, so as to achieve the role of enhancing the strength of the composite material. Therefore, in order to obtain excellent performance, eligible plastic wood composite materials, the first problem to be solved is the problem of compatibility.
The compatibility problem mainly relies on the addition of various additives to solve.
Coupling agent method: coupling agent can improve the compatibility between inorganic filler and inorganic fiber and matrix resin, but also can improve the interface between wood powder and polymer. Silane coupling agent and titanate coupling agent are the two most widely used types of coupling agent, and experiments have shown that both coupling agents can improve the compatibility between filler and resin.
Compatibilizer method: Adding compatibilizer method is the simplest and very effective method. Suitable compatibilizers have been reported to be grafted plant fibers such as maleic anhydride or maleic anhydride-modified polyolefin resins, acrylates*** polymers, and vinyl acrylate*** polymers. Most of these compatibilizers contain hydroxyl or anhydride groups, which can esterify with the hydroxyl group in the wood powder, reducing the polarity and hygroscopicity of the wood powder, so it has good compatibility with the resin.
④ The dosage of additives on the performance of composites: the dosage of coupling agent and the activation effect of filler is not proportional to the relationship, when the additive content of 1%, the material's tensile strength and tensile modulus is the best, with the increase in the dosage of additives, the performance of the material instead of decreasing. Therefore, the amount of additives can not be too much, otherwise, both affect the performance, but also cause unnecessary waste.
⑤ Improvement of fluidity performance: For extrusion molding process, the requirements of the processed material has a certain degree of fluidity. Most of the cases filled with plastic need to melt, force, deformation, after cooling and shaping into a variety of products, so the addition of wood powder filler on the melt rheological properties of the impact is necessary to be studied. One of the most important is the effect of the melt viscosity.
As the content of wood flour increases, the polymer melt viscosity increases, which is related to the dispersion of wood flour in the matrix resin. Wood flour particles in the matrix is in the form of a certain aggregation state, the aggregated state of the wood flour is unfavorable to the filler system flow performance, can be added to the appropriate amount of stearic acid to reduce the number of aggregation of wood flour particles to improve the phenomenon of clustering, so that it is fully dispersed in the matrix resin. In addition, wood-plastic composites are pseudoplastic fluids in the molten state, and the apparent viscosity decreases with the increase of shear rate. So in order to make the filling system has good processing flow performance, should be used as high as possible shear stress, in order to reduce the shear viscosity of the filling system, make it suitable for extrusion molding processing.
⑥ Improvement of processing conditions: extrusion molding, thermo compression molding, injection molding is the main molding method for processing plastic-wood composite materials. Due to the short cycle of extrusion molding, high efficiency, molding process is simple, so the extrusion molding method is a better choice.
The single-screw extruder can complete the task of plasticizing and conveying materials. Due to the filling of wood flour to increase the viscosity of the polymer melt, increasing the difficulty of extrusion, so the single-screw extruder for the modification of wood flour filling must be a special design of the screw, the screw should have a strong mixing and plasticizing ability.
Due to the fluffy structure of wood flour, it is not easy to feed the extruder screw, in the extrusion of the material should be mixed before granulation. As wood flour has water absorption, granulation should be dried before the wood flour, drying temperature of about 150 ℃, the time to 3h is appropriate, if the drying is not sufficient, the products will have bubbles, resulting in a decline in the mechanical strength of the material. Processing temperature control is also very important, the temperature is too high, the wood powder due to thermal effects will occur charring phenomenon, thus affecting the apparent color of the material. Therefore, the processing temperature should be properly controlled in the process.
Chemical methods:
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, gas, polymer monomers, and petrochemical and chemical feedstocks from waste plastics.
From a technical point of view, the main chemical methods are pyrolysis, catalytic cracking, hydrocracking, supercritical fluid methods, and solvent decomposition. Thermal cracking generates hydrocarbons with a wide range of boiling points and low recycling value. Catalytic cracking, due to the presence of catalysts, the reaction temperature can be lowered by tens of degrees, the product distribution is relatively easy to control, and gasoline with a high crystal level can be obtained. Supercritical fluid method is becoming the current research hotspot because of its environmental protection, economy, fast decomposition speed, high conversion rate, etc. It is suitable for waste plastics olefinization as well as solvent decomposition of polycondensates. Solvent solubilization is mainly used for the depolymerization of polycondensate waste plastics to recover
monomers.
In terms of use, the chemical method can be divided into two kinds of different end products, one is the production of fuel (gasoline, kerosene, diesel, liquefied petroleum gas, etc.), the other is the production of basic chemical raw materials, monomers.
(1) the production of fuel (oil, gas) olefinization technology
Overseas as early as the 1970s stone sleeve crisis has begun to develop the olefinization technology,
cracking, lkg of waste plastics oil production can reach up to iL. This technology does not use stirring devices, only suitable for polyolefins, can not be used for halogenated plastics.
APME (Association of Plastics Producers of Europe) believes that, in order to be viable, the recycling process must be able to accept a wide range of mixed plastics. Currently industry has conducted laboratory engineering studies and initial pilot tests on PVC-rich (up to 60%) waste plastics, but has yet to provide optimal process conditions for the construction of demonstration units.
Japan in April 2000 on the full implementation of waste plastics "packaging containers regeneration law", in order to solve the problem of mixed plastics olefinization, Japan Scrap Plastics Recycling Promotion Association and the Waste Research Consortium in the *** funding, the development of a successful general mixed waste plastics olefinization technology. The process includes pre-treatment, dechlorination and thermal decomposition. In order to improve the quality of the oil, a catalyst is added to modify the oil.
Mitsubishi Heavy Industries, Toshiba, Nippon Steel and other Japanese companies have successively carried out pilot or industrialization tests, can produce gasoline, diesel, heavy oil and other oil crystals, the technology has been passed, but not yet economically passed. To this end, the relevant companies are through the improvement of technology to significantly reduce costs, prominent for the Northeast Electricity in conjunction with Mitsubishi Heavy Industries to use supercritical water for waste plastics oleophilized test results, the reaction time from the past 2h significantly shortened to 2min after the recovery rate of the oil is still maintained at a high level of more than 80%, which is conducive to the reduction of the cost. Taking into account the rise in oil prices will be conducive to improving economic efficiency, is currently underway 0.5t / h of industrialization tests, is expected to be successful will be practical sooner.
(2) the production of basic chemical raw materials, monomer recycling technology:
Mixed waste plastics thermal decomposition of liquid hydrocarbons, ultra-high-temperature gasification of water gas, can be used as chemical raw materials. Germany Hoechst, Rule, BASF, Japan Kansai Electric Power, Mitsubishi Heavy Industries in recent years have developed the use of waste plastics ultra-high-temperature gasification of syngas, and then the system of methanol and other chemical raw materials, technology, and has been industrialized production.
In recent years, the waste plastic monomer recycling technology has received increasing attention, and gradually become the mainstream direction, and its industrial applications are also in the study. 1998 May in Munich, Germany, held the 14th International Analysis and Application of Cracking Conference, appeared on the development of polymer waste recycling of new tendencies. From the papers published in this conference, for polymer materials, "white pollution" problem, the international in the basic solution to the polymer waste by cracking the preparation of fuel research and industrialization, has tended to polymer waste through effective catalytic - cracking method into polymer synthetic raw materials. New
stage. The current level of research has reached the monomer recovery rate of polyolefins for 90%, polyacrylates for 97%, 92% of fluoroplastics, polystyrene for 75%, nylon, synthetic rubber for 80% and so on. These results of industrial applications are also in the study, it will have a huge benefit on the environment and resource utilization.
The U.S. BattelleMemorial Institute (U.S. Patent US5136117) has successfully developed from LDPE, HDPE, PS, PVC and other mixed waste plastics in the recovery of vinyl monomer technology, the recovery rate of 58% (mass fraction), the cost of 3.3 U.S. cents / kg, the goal is to achieve two years after industrialization. Japan's general agent - Mitsubishi Corporation has introduced the technology and commercialization of development, has built a flow rate of 20L / h of continuous reaction device.
Solvent solubilization (including hydrolysis and alcohol solubilization) is mainly used for polymerization of polymer materials depolymerization of recycled monomers, applicable to a single species and strict pretreatment of waste plastics. Currently mainly used to deal with polyurethane, thermoplastic polyester and polyamide and other polar waste plastics. For example, the use of polyurethane foam hydrolysis method of polyester and diamine, polyurethane soft and hard products alcoholysis method of polyol, waste PET depolymerization system crude terephthalic acid and glycol.
In addition, in recent years, supercritical fluid method is also increasingly used in the depolymerization of polymerization polymer materials, the recovery of its monomer, the effect is far better than the usual solvent solution. Japan T. Sako et al. used supercritical fluid decomposition to recover waste polyester (PET), glass fiber reinforced plastic (FRP) and polyamide / polyethylene composite film. The advantage of their use of supercritical methanol for PET recovery is that PET decomposition is fast and does not require a catalyst, allowing for almost 100% monomer recovery. They also use subcritical water recycling treatment PA6 / PE composite membrane, so that PA6 hydrolyzed into monomer '- caprolactam, recovery rate is greater than 70% a 80%.
Thermal regeneration:
Plastic combustion can release a large amount of heat, polyethylene and polystyrene calorific value of up to 46000kJ/kg, more than the average fuel oil 44000kJ/kg calorific value. Combustion tests show that waste plastics are fully equipped with the basic properties of fuel. It and coal dust, heavy oil combustion comparison test is shown in Table 2.2. From Table 2.2, it can be seen that the waste plastic heating capacity and coal and oil equivalent, and does not contain sulfur. In addition, because of the ash content is less, the combustion speed is fast.
Therefore, foreign waste plastic used in blast furnace blowing instead of coal, oil and coke, used in cement rotary kiln instead of coal to burn cement, as well as made of refuse solid fuel (RDF) used for power generation, received a very good result.
(1) fuel: solid waste fuel RDF
Japan actively promote the use of waste plastics solid waste fuel (RDF). RDF technology was originally developed by the United States, Japan in recent years in view of the lack of landfills, incinerators to deal with chlorine-containing waste plastics, resulting in the corrosion of the boiler of the HCI and the tailpipe gas produced by the two D8-engine pollution of the environment, the use of waste plastics with high heating value characteristics mixed with a variety of combustible garbage made of plastic, plastic and other waste materials, and the use of waste plastics to make the waste solid fuel (RDF), the use of waste plastics to make the waste solid fuel (RDF). Characteristics of waste plastic with high calorific value mixed with a variety of combustible garbage made of calorific value of 20933kJ / kg and uniform particle size of RDF, both chlorine dilution, but also for easy storage, transportation and for other boilers, industrial kilns, burning instead of coal. Garbage solid fuel power generation was first applied in the United States, and there have been 37 RDF power station, accounting for 21.6% of the garbage power station. Japan combined with the overhaul of some of the small waste incineration stations into RDF production station, in order to focus on continuous high-efficiency scale power generation, so that the steam parameters of the waste power station from & lt;30012 to 45012 or so, power generation efficiency from the original 15% to 20% to 25%. Chichibu Onoda Cement Company has been in the rotary kiln test firing RDF success, not only instead of coal, and ash has become a useful component of cement, the effect is better than for the generation of
electricity. At present, Japan's cement plants are actively promoted.
(2) blast furnace blowing, cement rotary kiln blowing
Blast furnace blowing waste plastics technology is the use of waste plastics, high calorific value of waste plastics as raw materials made of suitable granularity of spraying people in the blast furnace to replace the coke or pulverized coal in a new method of handling waste plastics. Foreign blast furnace blowing waste plastics application shows that the utilization rate of waste plastics up to 80%. Emissions for the incineration amount of 0.1% ~ 1.0%, only produces less harmful gases, treatment costs are low. Blast furnace blowing waste plastic technology for the comprehensive utilization of waste plastics and governance of "white pollution" opened up a new way, but also for metallurgical enterprises to provide a new means of energy efficiency.
Germany's Bremen Iron and Steel Company in 1995, the first in its No. 2 blast furnace (volume 2688m3) on the blowing of waste plastics, and the establishment of a set of 70kt / a blowing equipment, followed by Krupp / Hirsch Iron and Steel Company has also established a set of 90kt / a blowing equipment, Germany's other iron and steel companies are also ready to use this technology. Japan's NNK company in 1996 in its Keihin plant No. 1 blast furnace (volume 4093m3) on the blowing of waste plastics, plans to deal with waste plastics 30kt / a, it
also intends to transfer the technology to other Japanese factories. Japan's environmental community and public opinion has high hopes, the Japanese Iron and Steel Alliance has been included in the 2010 energy saving plan, the requirements of the annual blowing more than 1 million tons, equivalent to the iron and steel industry, 2% of energy consumption, the future is promising.
In addition, Japan's cement rotary kiln blowing waste plastic test success. Tokuyama company cement plant in the long-term burning of waste tires on the basis of in 1996 in the waste plastics processing to promote the cooperation of the Association successfully carried out the rotary kiln blowing waste plastics test.
Fermentation method
It has been reported that waste polyethylene can be converted into microbial protein by both oxidative fermentation and pyrolytic fermentation. This method is a non-mainstream method and is not commonly used at present.
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