Many years of technological development now allow plastic waste to be recycled by a number of methods. They can be divided into 3 major types.(1) Mechanical recycling(2) Feedstock recycling (monomerization, blast furnace reducing agent, coke oven chemical feedstock recycling, liquefaction, etc.) (3) Thermal recycling (cement kilns, energy recovery from waste, RDF, RPF).
Recycling technology has made enormous progress and its use is becoming widespread, but recycling is not an end in itself. The purpose of recycling is to curb the consumption of finite natural resources such as oil and minimize the burden on the environment through the cyclical use of resources. This means that it is necessary to carefully consider whether the method used reduces the input of new resources or limits the burden on the environment when promoting recycling.
It is important to select the plastics recycling method that imposes the lowest social cost in addition to limiting the environmental impact. The recycling methods currently recognized by the law relating to the recycling of containers and packaging are mechanical recycling, recycling of raw materials (monomerization, liquefaction, use asa blast furnace reducing agent, coke oven chemical feedstock recycling and conversion to chemical feedstock by gasification), and thermal recycling (liquefaction and gasification).
Under the amendment in 2006, RDF and other forms of thermal recycling were added as supplementary methods, albeit with some limitations.
Methods of Plastic Recycling
● Mechanical recycling① W ash bowl ② road bollard ③ imitation wood post ④ pallet ⑤ anti-weed sheeting ⑥ heat/sound-insulating sheeting ⑦ PVC pipe ⑧ waterbutt lid ⑨colored box⑩ central reservation block ⑪ parking block ⑫ duckboard ⑬ survey and boundary markers ⑭ bricks ⑮ cross-ties for steel products ⑯video cassettes⑰weight for colored cone ⑱ plant pots◆ Used for containers, benches, building materials, textiles, sheeting…
Mechanical recycling is a way of making new products out of unmodified plastic Industrial plastic waste generated in the manufacture, processing, and distribution of plastic products is well suited for use as the raw material for mechanical recycling thanks to clear separation of different types of resins, a low level of dirt and impurities.
All kinds of recycled products are made from industrial plastic, including containers, benches and fences, children’s play equipment, construction sheeting, products for packaging, transport, construction, houses, parks, roads, railways, and other goods and installations for agriculture, forestry, and fishing.
Recycled products have several attractive characteristics: they are durable, lightweight, easy to work with, and easy to cut and assemble, just like wood. We can expect greater adoption of recycled products with these characteristics used in place of other materials, such as steel, concrete, and wood.
Waste plastics emitted from the house, such as PET bottles and expanded polystyrene, are transformed into textiles, packaging materials, bottles, stationery, daily necessities, video cassettes, and similar products.
Mechanical recycling process
Remelted to make products PET bottles from a sorted household the waste is collected, compressed and packaged by the municipalities to be transported to factories managed by recycling companies. At the recycling plant, the waste is sorted to remove impurities, and the remaining PET bottles are then shredded and cleaned, foreign bodies are non-resins are removed and the remaines turned into flakes and pellets (granules made from flakes, thermally processed by granulator) for recycling.
The recycled materials are then sent to textile and linen processing plants, where they are remelted to form textile and linen products.
Resin molding techniques(1) Extrusion molding resin is melted and continually extruded through a mold by a screw to form a molded product. Products include tubes, sheets, films, and metal sheaths. (2) Injection molding heated molten resin is injected into a mold and solidifies to form a molded product. Products made this way range from washbowls, buckets, and plastic models to larger products such as bumpers and pliers. (3) Blow molding – A preform obtained by extrusion or injection molding is fixed in a mold and inflated with air to make bottles for all kinds of uses, such as shampoo bottles. PET bottles are made by stretch blow molding to make them less prone to breakage. (4) Vacuum molding -A heat-softened sheet is sandwiched in a mold and the space between the sheet and mold is sealed and evacuated to form products such as cups and trays. (5) Inflation method – This is a type of extrusion molding where melted resin is inflated into a cylinder to form a film. This is used to make products like shopping bags.
Feedstock Recycling
Monomerization
From PET bottles to PET bottles While PET bottles can be recycled to make textiles and linens, cannot be used to make PET beverage bottles. This is because used pet bottles are unsuitable for use as raw materials for soft drinks, alcohol, or soy sauce bottles for reasons of hygiene and smell. However, converting PET bottles back to an earlier stage of processing is a more economic use of resources than making PET resin from scratch out of petroleum and naphtha.
A “bottle-to-bottle” scheme to make recycled resin equivalent to newly made resin suitable for drinks bottles started in2003 on this basis. The method chemically decomposes the used PET bottles into their component monomers(depolymerization), and they are made into newPET bottles from this stage.
Tenjin Ltd. already uses its own proprietary decomposition method, combining ethylene glycol (EG) and methanol to decompose waste PET resin into DMT (dimethyl terephthalate) to turn it into the raw material used to make fabrics and films. This technique was improved upon to break PET bottles down further from DMT to PTA (purified terephtalic acid) to make PET resin, and Tenjin Fiber Ltd. commenced operation of a facility with the capacity to process around 62,000 tons a year.
The resin produced was deemed suitable for use in food packaging by the Japan Food Safety Commission in 2004, and bottle-to-bottle production began in April, ltd also developed a technique to produce resin by decomposing it into high purity BHET monomer ( bishydroxyethyl terephthalate) using a new depolymerization process using EG.
Creation of a new company , PET Reverse Co., Ltd in 2004, which can process about 23,000 tons per year. However, the dramatic increase in PET bottle waste exports has already retired from bottle-to-bottle production, while the PET business Reverse Co., Ltd. continued in by Toyo Seikan Kaisha, Ltd. (PET Refine Technology Co., Ltd.).
Blast Furnace Raw Material Recycling
Plastics as Reducing Agents In steelworks, iron ore, coke and auxiliary raw materials are fed into a blast furnace and the iron ore is melted into pig iron. Coke is used as a fuel to raise the temperature in the blast furnace and also acts as a reducing agent by removing oxygen from iron oxide, one of the main components of iron ore.
Since plastics are made from petroleum and natural gas, their main components are carbon and hydrogen. This means that it should be possible to use them instead of coke as a reducing agent in the blast furnace process. The process in which plastics are used as reducing agents is as follows.
Plastic waste collected from factories and homes is cleaned of non-combustibles and other contaminants such as metals, then finely pulverized and packaged to reduce its volume. Plastics that do not contain PVC are granulated and then fed to the blast furnace with coke.
Plastics containing PVC are fed into the blast furnace after hydrogen chloride has been removed at a high temperature of about 350℃ in the absence of oxygen, since the hydrogen chloride emission can damage a furnace. This dehydrochlorination process was developed by the JFE Steel Corporation started a large-scale operation in May 2004 with a processing capacity of,30,30,000,000 tons.
Recycling chemical raw materials in coke ovens
Coke is made by burning coal, and the process generates volatile compounds that produce hydrocarbon oil and coke oven gas. But also coke, hydrocarbon oil, and coke oven gas can be made from plastic waste. The plastic waste collected from households is first ground and impurities like iron are removed. The PVC is removed before the plastics are heated to 100℃ and granulated, then mixed with carbon and fed to the carbonization chamber.
The charring chamber has combustion chambers on both sides, which indirectly heat the contents. The plastic waste is not burned inside the chamber due to lack of oxygen but is thermally cracked at a high temperature to produce coke for use as a reducing agent in coke ovens, hydrocarbon oil used as a chemical feedstock coke oven gas, the used to generate electricity.
Gasification
Plastics are gasified to be used as feedstocks in the chemical industry. Plastic is mainly composed of carbon and hydrogen, and therefore typically produces carbon dioxide and water when burned.
Gasification process involves heating plastics and adding oxygen and steam. Oxygen supply is limited, resulting in most plastics turning into hydrocarbons, carbon monoxide and water. Sand heated to 600-800℃ is circulated in a low-temperature first-stage gasification furnace produce hydrogen chloride. If plastic products contain metal or glass, these are recovered as non-combustible materials. Gas from the low-temperature gasification furnace is reacted with steam at a temperature of 1,300 – 1,500 ℃ in a high-temperature second-stage gasification furnace to produce a gas mainly composed of carbon monoxide and oxygen.
At the exit of the oven, the gas is quickly cooled,200°C or less to prevent dioxin formation. The gas then passes through a scrubber and any remaining hydrogen chloride is neutralized with caustic and removed from the synthesis gas.
Gasification with electricity generation of melting furnace waste first converts the waste into high-temperature gas and then uses the emitted pyrolysis gas and coal as fuel to spin a steam turbine and generate power. This method turns the burnt ash into a solid. Gasification with reformer furnace power generation subjects the waste to pyrolysis, then adds oxygen to the resulting gas, carbonized solids, tar and other substances.
The gas rich in carbon monoxide and vapor is recovered and used as fuel for power generation or as a chemical feedstock. Any waste gasification method can be used with shaft furnaces, fluidized bed furnaces, or rotary kilns. Also, power can be generated not only via steam turbines, but also with high efficiency gasengines, gas turbines and fuel cells.
Thermal Recycling
RDF
Conversion of waste plastics into waste molten fuel oil (RDF) by catalytic pyrolysis in which it is mechanically separated from solid waste and processed this splitting method is not foolproof, also another fragmentary waste is separated with the plastic.
Then, the differentiated waste is sent through a conveyor belt equipped with an optical separation device for the separation of 100% of plastic waste at the source. These various plastic wastes turn into useful fuel, e.g. liquid RDF oil through catalytic pyrolysis.All of the feed material is converted to liquid RDF, gas and sludge.
No effluent is generated in the process and the remaining gases from the reactor are released through the vents. Plasma Pyrolysis is a recent advancement in the world of technology that couples the thermo-chemical features of plasma along with the pyrolysis technique.
pyrolysis is the thermal disintegration of any carbonaceous material in an oxygen-free environment. While optimization is carried out, the most likely compounds produced are methane, carbon monoxide, hydrogen, carbon dioxide and water vapor. The method requirements are maintained such that it completely reduces any likelihood of formation of toxic dioxins and furans (in case of chlorinated waste). The transformation of organic waste into non-toxic gases exceeds 99%.
The severe conditions of plasma kill stable bacteria and the pyrolysis method aids in reducing carbon dioxide emissions and landfills.
Thermo Gasification
This is another way to reduce landfill space and the cost of incinerating plastic waste. The main advantage of the gasification process is that it uses inert air containing nitrogen instead of oxygen, which makes it a much simpler process and reduces costs. This is avertical fixed bed gasification system, where a thermo-chemical process turns carbon-based material into gases such as carbon dioxide, carbon monoxide, hydrogen and methane, which can be used for heat or power generation.
The gasification process using air as a gasifying agent is considered an environmentally friendly way to convert biomass and plastic waste into combustible gases. Immediate gasification has the advantages of a simple method and an economical process, but the inevitable presence of Plastic waste includes polyethylene, polypropylene, and polystyrene, which are softened by heating up to 100-150 degrees.
Innovations in plastic recycling incorporate increasingly stable chemical sensors and refined software that together improve the accuracy and productivity of automatic sorting. The recycling method uses a minimal amount of water to recover the plastic, where water is used only as a coolant. The waterless approach makes recycled plastic pellets more useful than others, which can be useful in road construction.
The incineration of plastics releases toxic gases, which pollute the atmosphere. And it is generally costly to recycle plastics. Save for these methods, biodegradation by microbes has been determined to be an environment-friendly means for the degradation of plastic waste. Although recycling is still the most favored approach, biodegradation is fit for plastics with particular applications, such as agricultural mulch films.
Following are the various steps of plastic degradation by microorganisms:
Bio-deterioration is the action of microbial constructs and additional decomposed bodies which are responsible for the physical and chemical degeneration that transforms the mechanical, physical and chemical characteristics of plastic.
Bio-fragmentation points to the catalytic actions that divide polymeric plastics into oligomers, dimers, or monomers by ectoenzyme or free-radicals discharged from microorganisms.
Assimilation distinguishes to the combination of molecules carried in the cytoplasm the microbial metabolism.
Mineralisation attributes to the whole degeneration of molecules that resulted in the excretion of completely oxidized metabolites (CO2, N2, CH4, H2O).
Erzsebet Frey (Eli Frey) is an ecologist and online entrepreneur with a Master of Science in Ecology from the University of Belgrade. Originally from Serbia, she has lived in Sri Lanka since 2017. Eli has worked internationally in countries like Oman, Brazil, Germany, and Sri Lanka. In 2018, she expanded into SEO and blogging, completing courses from UC Davis and Edinburgh. Eli has founded multiple websites focused on biology, ecology, environmental science, sustainable and simple living, and outdoor activities. She enjoys creating nature and simple living videos on YouTube and participates in speleology, diving, and hiking.