PVC is widely used in the construction industry. If the waste can be recycled, the pollution of PVC to the environment can be greatly reduced.
Recycling plastics to build the Rubik's cube exterior wall of the World Expo
In fact, PVC waste is often mixed with other plastic materials. Generally, plastics should be classified first, and then comprehensively utilized by physical or chemical means.
Classification of PVC
Sorting is a difficult point in the process of plastic waste treatment, and it is also one of the key factors that determine the treatment effect.
Common plastics are polyvinyl chloride, polystyrene, polypropylene, low (high) density polyethylene, polyamine, polyurethane and so on. Manual sorting has many disadvantages: low efficiency; Workers are in a harmful environment; Large material classification error and so on.
Automatic sorting system has been mostly used in industrial production, and there are many researches in developed countries. A chemical science and technology association in Germany once invented the technology of screening PVC from waste plastics by infrared rays, but the object was limited to transparent bottles, which had the problems of low screening accuracy and high cost.
Recently, Japan has successfully developed the technology of screening polyvinyl chloride resin (PVC) from waste plastics with high precision. This technology uses the difference of heating characteristics (within 50°C) and charging characteristics between PVC and other plastics to detect, and the detected PVC is marked with red light, which is easy for operators to identify. This method is more accurate than near infrared technology, and its cost is only 1/2.
The American Rubber Recycling Center (CPRR) invented a method to separate PVC bottles from other discarded containers. The key to this process is to use the X-ray fluorescence detector developed by Asoma Instruments. Researchers at CPRR have developed a computer system. It uses X-rays from radioisotopes to excite all electrons in the compound, and chlorine-containing molecules will emit X-ray reflection images after being irradiated, which are easy to be displayed by X-ray fluorescence analyzer. The separation work is completed by a set of mechanical devices. It can not only remove all bottles containing chlorine atoms through the conveyor belt, but also remove bottles without chlorine atoms.
Recovery technology of PVC
At present, there are three main technologies for recycling PVC: one is physical recycling; The second is the comprehensive utilization of chemical recovery method; The third is to recycle energy by burning PVC materials. Among them, the comprehensive utilization of physical methods and chemical methods is commendable. They can recycle waste products again and give full play to the properties of materials.
Physical recovery method
Physical recycling refers to the recycling process of PVC waste only through mechanical treatment, mainly through chopping, screening, grinding and other procedures, and finally get film, powder, particles or other forms of recycling. These recycled materials can be used to produce heavy-duty packaging bags, agricultural water pipes and soles.
PVC plastic products are divided into soft products and hard products. Soft products mainly include film, cable sheath, plastic shoes and leather products; Hard products mainly include bottles, pipes, profiles and plates.
In soft products, agricultural films are used in large quantities. The agricultural film is made of PVC, PE, EVA and other materials. When recycling, we should first distinguish PVC membrane from other materials. Agricultural film often contains soil, sand, grass roots and nails. These impurities should be removed and cleaned, then dried and chopped into pieces or particles, and then made into agricultural product packaging films, agricultural reclaimed water pipes, soles and so on. Due to repeated heating and mechanical action, the recycled waste agricultural PVC film will inevitably be partially degraded and its strength will be reduced. In order to ensure the product quality, some new PVC resins and corresponding additives need to be added. If recycled materials are made into secondary plastic films through internal mixing, two-roll winding and four-roll winding, and different pigments are added in the internal mixing process, the produced calendered films will also show different colors. In addition, about 30% of the residual plasticizer (mainly DOP) in the waste agricultural PVC film can be recycled after refining.
PVC bottles in hard products are also a kind of products with large usage, and their wastes can be processed into profiles, pipes, folding plates and so on. In the plastic processing plant, firstly, the PVC fragments are cleaned, the caps and labels are removed, then dried and crushed into small pieces less than 0.5mm. BFGoodrich, one of the major PVC manufacturers in the United States, made an experiment to extrude new PVC and recycled PVC. The results show that the recycled PVC retains the excellent properties of new materials and can be used in occasions with low performance requirements.
For the physical recycling method, the quality of PVC recycled materials has a great relationship with the pollution degree and the composition of the selected materials. The quality of recycled materials determines the extent to which recycled materials can replace the original materials: "high-quality" recycled materials can be reused in the same type of PVC applications, while "low-quality" recycled materials from mixed wastes can only be "recycled at a low level" into products usually made of other materials.
Of course, the quality of the final product is also inseparable from the recycling equipment itself. At present, the equipment and technology for recycling PVC waste in Germany have reached a very high level. Wherein the pollutant content is less than 1%, and the obtained secondary crushed PVC material can be processed into various products by various methods. The PVC Association of the American Plastics Industry Association also strongly encourages the recycling of PVC, and plans to promote the recycling of other PVC products from the recycling of PVC bottles.
Chemical recovery method
Chemical recovery refers to using the chemical properties of polymers to convert them into small molecular compounds or simple compounds. Chemical decomposition method has high resource utilization rate, but the process is complex and the cost is high, which affects the recycling of plastic products.
For PVC, not only the polymer chain is crushed, but also the chlorine atoms attached to the chain are released in the form of HCI. Hydrogen chloride must be purified and reused, or it must be neutralized to form different products. Some products can be reused and some products must be treated, which are determined by the technology used in the treatment process. All countries in the world have strengthened their research in this field.
Plastic recycling utilization technology developed in China can recycle waste PVC mulch film and shed film, and then produce oil, paraffin and building materials. Under the condition of continuous production, the equipment using this technology has strong daily treatment capacity of waste agricultural film, the oil yield can reach 40%~80%, and the conversion rate of gasoline and diesel oil is high, which meets the vehicle fuel and environmental emission standards. In addition, because the PVC film contains 30%~37% dioctyl phthalate, DOP can be extracted with solvent at room temperature, and the solvent can be recovered below 800℃. DOP is a widely used plasticizer. At present, the market price is between 7000 yuan and 8000 yuan/ton. Recycling waste PVC film 1 ton can make a profit of more than 2000 yuan and eliminate environmental pollution.
There are also two advanced PVC solvent recovery processes in Europe. Solvi Belgium invested 6.5438+0.06 million euros to build the world's first set of soft PVC resin recycling device in Italy. The device adopts Vinylon batch process, and can regenerate 8,500 tons of PVC from 6,543,800 tons of waste plastics (mostly composed of PVC) every year. In this technology, methyl ethyl ketone mixture is used as solvent to selectively dissolve PVC and additives under the pressure of100 c ~140 c, and the solution is precipitated to obtain soft PVC containing additives. The cost of recycling and producing PVC by this process is equivalent to that of producing new PVC, and the obtained PVC contains additives, so the resin price is higher. Vinylon process can reduce the environmental impact of PVC, especially soft PVC. The second ethylene plant will be put into operation in France in 2004, aiming at recycling PVC coated varnished cloth and fabrics.
Another new process for recovering PVC by solvent method in Europe is to recover PVC from the whole automobile circuit board. This process was jointly developed by automobile manufacturers in Wuppertal and Delphi, Germany, and Wuppertal University. Three years ago, WiefekGmbH, an auto parts recycling company in Feilden-Fei Sen, Germany, industrialized it. The cost of recycling PVC by this process is nearly 20% lower than that of new PVC. Esters and ketones are used as solvents in this process, but the amount of solvents is much less than that of ethylene ring process. Solvent does not completely dissolve waste PVC, but softens it, making it easy to separate from copper wire, and the obtained copper wire can be used to pay the cost of recycling PVC. Wiefek company invented the process of centrifugal separation of plastics and solvents, and obtained the patent right. The PVC recycling production line established by the company can recycle 225 tons of reusable PVC every year.
Heat recovery method
At present, obtaining energy by burning PVC products is also one of the ways to reuse PVC. However, this method tends to be eliminated.
PVC waste will produce hydrochloric acid (hydrogen chloride) during incineration, and these gases need to be neutralized with the release of flue gas. The method is to add neutralizing substances, most commonly limestone, into the incinerator to neutralize the acidic components in the incineration gas. Because chlorine gas released by PVC combustion has strong corrosive destructive power and is the chief culprit of dioxins, environmental groups in various countries strongly oppose the use of incineration to recover heat energy.
The recycling of PVC plastics generally means that waste PVC is made into granular PVC by crushing, cleaning, spin-drying, heating and plasticizing, stretching, cooling, granulating and processing, which can be used to manufacture PVC-related products, especially as raw materials. Because PVC is widely used in pipes and utensils, and can only be used once for sanitary reasons, the equipment for recycling PVC has always been concerned by people. Generally, PE and pp are separated from PVC, and can also be recycled and granulated centrally (DY recycling granulator is used for PP, PE, ABS, etc. ). The recycling of all kinds of waste plastics has attracted wide attention. Generally speaking, it is common in plastic crusher, and other equipment is the same as PVC recycling equipment. The quality of recycled plastics is different with different equipment configurations, and the key is the quality of cleaning and granulation.
The general plastic recycling process is:
Pvc plastic →pvc plastic → automatic cleaning → plasticizing and molding → wire drawing and granulation → quality inspection and packaging.
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