Recycling of waste plastics

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Melting and recycling of waste plastics The melting and recycling method of plastics includes the following processes: Collection, sorting, cleaning, drying, recycling, granulating and molding of waste plastics 1. Sorting of waste plastics The sources of waste plastics are complex and often mixed with metal, rubber, fabric, mud yarn and other impurities, and different types of plastics are often mixed together. This not only causes difficulties in processing recycled waste plastics and affects the quality of the products produced, but the mixed metal impurities can also damage the processing equipment. Therefore, when using waste plastics to produce products, not only must all kinds of impurities in the waste plastics be removed, but also different types of plastics must be separated. Only in this way can high-quality recycled products be produced. The methods for sorting waste plastics include manual sorting, magnetic sorting, density sorting, electrostatic sorting, flotation, temperature difference sorting and air screening sorting. 1． Manual sorting The steps for manual sorting are as follows:: (1) Remove metal and non-metal impurities and discard plastic waste with serious weight loss. (2) First classify by products, such as films (agricultural films, natural packaging films), bottles (mineral water bottles, carbonated drink bottles), cups and boxes, soles, sandals, foam plastics, scraps, etc., and then classify different plastic varieties according to the identification method in the previous section, such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyester, etc. (3) Sort the waste plastic products classified above according to color depth and quality. The colors can be divided into the following categories:: Black, red, brown, yellow, blue, green and transparent colorless. 2． Magnetic separation The main purpose of magnetic separation is to remove impurities of steel scraps mixed in waste plastics. Because these fine steel scraps are difficult to remove by manual sorting, they must be removed by magnetic separation. 3． Density sorting Density sorting is a method that uses the different densities of different plastics for sorting. There are usually solution sorting, hydraulic sorting and centrifugal density sorting. The solution sorting method is to put mixed waste plastics into a solution with a certain density, and then sort the waste plastics according to their sinking and floating state in the solution. The advantage of the solution sorting method is that it is simple and feasible. As long as you choose to prepare one or several solutions, you can perform large-scale sorting and avoid tedious manual sorting. The disadvantage is that the densities of some types of plastics are very close, so it is difficult to obtain high-purity isolates. Hydraulic separators are commonly used in hydraulic sorting. In order to improve the sorting efficiency, it is often necessary to clean first and then sort the solution. Centrifugal density separation uses a centrifugal density separator. 4． Electrostatic sorting Electrostatic sorting is a method that uses the different electrostatic attractions of various plastics to sort. The step is to first crush the plastic into small pieces with an area of ​​about 10mm2. After drying, use high-voltage electrodes for sorting. A variety of mixed waste plastics need to be sorted multiple times. This is because only one kind of plastic can be sorted out each time through the high-voltage electrode with a pre-selected voltage. The electrostatic separation method is especially suitable for polar polyvinyl chloride, and the separation purity can reach 100%. 5. Flotation and separation Flotation is a method that uses a wetting agent to change the wettability of water on the plastic surface, so that some plastics change from hydrophobic to hydrophilic and sink, while the still hydrophobic plastic surface adheres to bubbles and floats, thereby achieving the purpose of separation. When flotation separates different types of plastics, it has nothing to do with the density, shape, size, etc. of the plastics. It uses the different wetting properties of water on the plastic surface to sort. 6． Temperature difference sorting: Temperature difference sorting is also called low temperature sorting. It is a method that uses different embrittlement temperatures of various plastics to sort. The specific method is as follows: The mixed waste plastics are crushed once and then placed in a cooler with a temperature of -50 degrees to be cooled and crushed. Plastics with a brittleness temperature below -50 degrees are crushed and separated after screening. The plastic that has not been crushed is then placed in a cooler with a temperature of -100 degrees to cool and crush, and then the brittle plastic is screened out. The coolant is liquid nitrogen, which can be cooled to -140 degrees. Solution Type Density/(g/cm3) Preparation Method Application Example Plastic floating in the solution Plastic sinking into the solution Water 1 - Polyethylene, polypropylene, polyvinyl chloride, polystyrene salt water solution (saturated) 1.19 (25 degrees) water 74ml salt 26g polystyrene, ABS polyvinyl chloride, organic glass alcohol solution (58. 4%) 0.91 (25 degrees) water 100ml 95% alcohol 160ml water polypropylene polyethylene alcohol solution (55. 4%) 0.925 (25 degrees) 100ml 95% alcohol 140ml low density polyethylene high density polyethylene calcium chloride aqueous solution 1.27 water 150ml calcium chloride 100g (industrial use) polystyrene, plexiglass, polyethylene polyvinyl chloride 7. Air sieve sorting Air sieve sorting is a method in which the crushed plastics are placed in a sorting device and sprayed, and the wind is blown in from the transverse or reverse direction, and the resultant difference between the resistance of different plastics to the air flow and their own weight is used for sorting. Since the particle size after crushing will affect the sorting effect, this method requires that the particle size after crushing is uniform. This method can also be used to sort stones and sand mixed in plastics. 2. Cleaning and drying of waste plastics Waste plastics are usually contaminated with oil, garbage, sediment, etc. to varying degrees. These impurities will seriously affect the quality of recycled plastic products. Therefore, waste plastics must be cleaned. Cleaning methods include manual cleaning and mechanical cleaning. (1) Manual cleaning and drying Manual cleaning should determine the specific cleaning method according to the product variety and degree of contamination. The cleaning process of general agricultural film and packaging film is as follows: Clean with warm alkaline water (remove oil stains) - brushing - rinse with cold water - dry in the sun. The cleaning process for films and containers used to package toxic drugs is as follows:: Lime water cleaning (neutralization and detoxification) - brushing - cold water rinsing - sun drying (2) Mechanical cleaning and drying Mechanical cleaning has two types: intermittent and continuous. The intermittent method is to first soak the waste plastics in a solvent filled with hot alkali aqueous solution for a certain period of time, and then use mechanical stirring to cause the films to rub and collide with each other to achieve the purpose of removing contaminated dirt, and then take out the cleaned film. Continuous type is an improvement method of intermittent type. The shredded waste plastic is continuously fed, and the cleaned film is continuously discharged. 3. Regeneration and granulation: Cleaned and dried waste is generally granulated before molding and processing. Waste plastics generally age to varying degrees after use, and the additives they contain also suffer varying degrees of loss. Therefore, it is often necessary to add certain additives appropriately before granulation, especially for soft polyethylene plastics, where plasticizers, stabilizers, etc. are often added. Since recycled plastics are often mixed with waste materials of many different colors, dark colorants are generally added during regeneration. It should be noted that when adding additives, lower-priced varieties should be considered, because the prices of recycled products are generally lower. 4. Thermal cracking recovery of waste plastics There are many methods for thermal cracking recovery of oil and gas, such as melting tank method, screw thermal decomposition method, reaction tube evaporator method, fluidized bed reactor method, catalytic cracking method, etc. Different methods can be used for thermal cracking recovery of different types of plastics. They are briefly introduced as follows. 1． Melting tank method The melting tank method uses a molten salt as the heating medium to heat and decompose waste plastics. The decomposed hot steam passes through the electric dust collector and condenses into decomposition products in the condenser. This method can be used for polyethylene, random polypropylene, polypropylene, and polystyrene. 2． Screw thermal decomposition method The screw thermal decomposition method can be used for the recycling of polyethylene, polypropylene, polystyrene, and polymethylmethacrylate. The key part of this type of device is the screw thermal decomposition reactor. Generally, external electric heating is used, with the temperature reaching 500 to 550 degrees. The materials are preheated with a microwave heater before entering the screw thermal decomposition reactor. The reaction products are recovered as light oil and heavy oil respectively. 3． Reaction tube evaporator method The reaction tube evaporator method is used to recycle waste plastics with uniform raw materials and easy to become liquid monomers. Applicable waste plastic types include random polypropylene (APP), polystyrene, etc. 4． Fluidized bed reactor method Fluidized bed reactor method There are many devices used for oily recovery of waste plastics. This method can be applied to: Polypropylene, cross-linked polyethylene, polypropylene, polyvinyl chloride and other plastics. 5. Catalytic cracking method Catalytic cracking method is a method that uses a suitable catalyst to oily waste plastics at low temperature. It is generally used to oily a single type of plastic. Applicable plastics include polyethylene, polypropylene, polyvinyl chloride, etc. Because low-temperature oiling is used, the oil yield of this method is higher. In addition, the use of catalysts makes the oil obtained more uniform, and the additional price of the product is higher. 5. Energy recovery of waste plastics The energy recovery of waste plastics is a method to achieve the purpose of recycling by effectively utilizing the heat energy released when it is burned in the incinerator. Combustion heat is usually utilized by using a heat exchanger to convert thermal energy into warm water, or by converting it into steam through a boiler to generate electricity and heat. Waste plastics - crushing - combustion - heat recovery - smoke exhaust treatment. The key issues in energy recovery of waste plastics are the incineration technology and the treatment of combustion exhaust gas. The former has certain requirements for the design of the incinerator due to the higher calorific value of plastic and the different types of waste plastics. ； Due to the requirements of environmental protection, the latter requires the exhaust gas to be non-pollution-free, so it must be treated. 1. The waste plastic incinerator has the following design requirements for the incineration of waste plastics: 1. It can be mechanically operated and has stable incineration. 2. It can be effectively incinerated even if it is a mixture of various plastics or mixed with other municipal solid waste. 3. It can prevent the release of harmful gases to the maximum extent. 4. Large incineration capacity and few failures. 5. Complete combustion and no smoke particles. 6. Wastewater emissions must comply with environmental protection requirements. When thermoplastic plastics are put into high-temperature incinerators, part of the plastic melts quickly and rapidly decomposes or gasifies. As a result, gas phase combustion occurs, and the carbonaceous residue produced burns slowly, and part of the melt covers its surface, causing lack of oxygen and insufficient combustion. Thermosetting plastics do not melt when heated, but undergo decomposition and surface combustion. The ignition temperature is high and the burning speed is slow. When the above-mentioned thermoplastic plastics and thermosetting plastics are put into incineration at the same time, slow-burning carbonaceous residues and thermosetting plastic residues often cause combustion stagnation. For this reason, there must be appropriate stirring and good ventilation in the incinerator to promote the burning of solid matter. The combustion chamber structure used in waste plastic incinerators is as follows:: (1) Fire grid incinerator (2) Bed type incinerator (3) Downfire grid incinerator (4) Fluidized bed incinerator (5) Cyclone incinerator (6) Micro-pulverized coal incinerator 2. Energy recovery of waste plastics The heat energy from the incineration of waste plastics is used through a heat exchanger to turn water into warm water or steam. 6. Recycling of chemical raw materials The method of recycling chemical raw materials can be used to recycle many kinds of waste plastics. This method is actually a chemical decomposition method. Compared with the thermal cracking method, it can obtain more uniform decomposition products, but this method requires a single type of clean waste. The chemical decomposition method of recycling chemical raw materials mainly includes hydrolysis and alcoholysis methods. The introduction is as follows. (1). Hydrolysis method Hydrolysis method is a method of recovering raw materials by adding water to decompose polymers containing easily hydrolyzable groups. This method is suitable for plastics such as polyester, polyester, polycarbonate and polyamide. These plastics are stable under normal conditions, so hydrolysis must be carried out under special conditions. Industrially, hydrolysis technology is used to decompose polyurethane foam and recover polyester polyol devices. (2)． Alcoholysis Alcoholysis method is a method that uses the hydroxyl group of alcohols to alcoholyze certain polymers to recover raw materials. Alcoholysis method can be used for polyester, polyester and other plastics. An alcoholysis process for polyester foam is as follows: Ethylene glycol - polyurethane foam - crushing - reactor - distillation - polyester polyol N2 alcoholysis uses ethylene glycol. The reactor is equipped with a stirring device and the temperature is controlled at 185 to 210 degrees. The reaction sometimes uses organic metal compounds as catalysts. Polyester (polyethylene terephthalate) alcoholysis recovery can obtain ethylene terephthalate and ethylene glycol, which can be used to produce new raw materials of high quality polyester.