Ethan Clarke
Plastic is one of the most versatile and widely used materials in the world, with 360 million tonnes produced globally in 2018. Recycling plastic is a complex process that involves several stages, from collection to sorting and reprocessing. Once plastic waste has been collected from homes, businesses, and recycling centres, it is sent to Material Recovery Facilities (MRFs) and Plastic Recovery Facilities (PRFs) to be sorted. Sorting is a critical step in the recycling process, as different plastics have distinct properties that require specific recycling methods. Advanced automated sorting machines use techniques such as infrared, x-ray, and other sensors to identify and separate various types of plastics. After sorting, the plastics are shredded, washed, and further processed to remove contaminants and separate mixed plastic types. The final stage typically involves melting and extruding the plastic into pellets, which can then be moulded into new products.
| Characteristics | Values |
|---|---|
| Plastic Sorting Techniques | Manual picking, Sink-float separator, NIR sensor, Pneumatic systems, Digital watermarks, Fluorescent markers |
| Plastic Recycling Facilities | Material Recovery Facilities (MRF), Plastic Recovery Facilities (PRF) |
| Plastic Shredding | Shredders and granulators with industrial blades |
| Plastic Washing | Washers and water baths/tanks |
| Plastic Separation | Wet separation, Float tanks, Air jets |
| Plastic Reprocessing | Heating, Extruding, Compounding, Pelletising, Moulding |
| Plastic Distribution | Pellet form |
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What You'll Learn
Sorting by hand
Sorting plastic by hand, or manual picking, is an important step in the recycling process. It involves humans sorting through plastic items to remove large items, non-recyclables, and obvious contaminants. This step is crucial as it helps to prevent damage to equipment, injuries to workers, and the rejection of entire shipments due to contamination.
During manual picking, items that are not meant to be recycled, such as Christmas lights, machetes, hoses, cords, wires, metal hangers, and "tanglers" like plastic bags, are removed. These items are either sent for further processing or disposed of. While machines and technology play a significant role in sorting, humans are still smarter than machines and play a critical role in ensuring the accuracy of the sorting process.
After the initial manual picking, the remaining materials are then fed into trommels, which are cylindrical drums with holes that separate finer materials from heavier ones. Paper and cardboard, for example, can be removed with a "star screen," a machine that uses spinning discs to kick up flat, lightweight materials while allowing heavier materials to pass through.
The sorting process also involves identifying and separating non-recyclable plastics, such as PVC, which is challenging to recycle due to its chemical composition. Automated systems are designed to identify and divert such materials from the recycled plastics stream. However, manual intervention is often necessary to ensure accuracy and address any errors made by machines.
Overall, sorting by hand is an essential aspect of plastic recycling, ensuring the safety of workers, the efficiency of the process, and the purity of the recycled materials. It is a complex task that requires both human intelligence and machine support to achieve successful recycling outcomes.
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Sorting by machine
Sorting plastic waste by machine is a critical stage in the recycling process. Sorting machines are used to identify and separate large amounts of plastic. There are several methods and technologies used to sort plastics by machine.
One method is through the use of optical sorters, which can distinguish between different types of plastics. Optical sorting machines can separate plastic by thickness, colour, size, and plastic type. Another method is the use of a wet process known as a sink-float separator, where a tank is filled with water and plastic recycling, with high-density plastic sinking and low-density plastic floating.
Advanced sorting machines can also sort through the use of infrared, x-ray, or other cutting-edge sensors that can recognize a polymer's unique signature and, in some cases, colour. These machines can identify and separate massive amounts of input, ensuring the purity and quality of the final products.
Digitalization and artificial intelligence also play a role in sorting plastics by machine. For example, Tomra employs deep learning, a branch of AI where the algorithm learns by looking at thousands of images of the objects to be sorted. This technology has been used to separate polyethylene (PE)-silicon cartridges from those used for two-component adhesives, ensuring the quality of the desired PE stream.
Another technology used in plastic sorting is chemical or digital markers. A European project called HolyGrail addresses the concept of digital watermarks, where codes are stamped all over the surface of an item and are invisible to the naked eye but can be seen by cameras on the sorting line. These codes can be embossed into plastics or integrated into printed labels, helping to identify the packaging material.
Overall, sorting plastics by machine is an important step in the recycling process, utilizing various methods and technologies to identify, separate, and purify large amounts of plastic efficiently.
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Washing and contaminant removal
Once plastics are sorted, they are washed and contaminants are removed. This is an important step in the recycling process as contaminants can cause a variety of problems, such as damaging machinery, creating an unsafe working environment for sorters, and reducing the value of recyclables. Contaminants can include non-recyclable materials, such as plastic bags, as well as hazardous waste, bio-hazardous waste, and frozen and refrigerated food boxes.
To remove contaminants, manual picking is often used to sort through items and remove large items, non-recyclables, and obvious contamination. After this, the waste is fed into trommels, which are cylindrical drums that allow finer materials to fall through while larger items continue on for further processing or disposal.
Washing is also a critical step in removing contaminants and preparing plastics for recycling. Different washing processes are used depending on the type of plastic being recycled. For example, water and caustic soda are effective for washing poly(ethylene terephthalate) bottles, while adding a detergent or organic solvent can increase the efficiency of washing polyethylene and polypropylene bottles. The optimal washing process should be tailored to specific packaging types to improve deodorization and meet recycling targets.
In addition to manual and chemical washing processes, mechanical processes are also used to separate and clean plastics. One such process is the sink-float separator, where a tank is filled with water and plastic recycling. The high-density plastic sinks, while the low-density plastic floats, allowing for easy separation and further processing.
Overall, the washing and contaminant removal stage of plastic recycling involves a combination of manual, chemical, and mechanical processes to ensure that plastics are clean and free of impurities before they are processed into new products.
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Shredding and resizing
The shredding process can vary depending on the type of plastic and the recycling facility's capabilities. Some facilities can only process specific types of plastic products, while others, like the Berry Circular Polymers facility in Cambridgeshire, are equipped to handle a wide range of plastic types, including single-polymer, mixed, or contaminated materials.
During the shredding process, it is essential to maintain the purity and quality of the final products. Metal detectors are often employed at each shredding machine to identify and remove any non-plastic waste that might have been missed during the initial sorting stages. This ensures that the shredded plastic is free from contamination, which is crucial for the successful reprocessing of the material.
After shredding, the plastic pieces undergo further separation to isolate different types of plastic. This can be achieved through various methods, including float tanks that separate plastics according to their density, with high-density plastics sinking and low-density plastics floating. Another method is the use of washers and water baths to remove contaminants and any remaining labels or adhesives from the plastic surfaces.
The resized plastic particles then move on to the next stages of the recycling process, which may include washing, melting, and extruding the plastic into new forms, such as pellets, to be moulded into new products. These steps contribute to the overall goal of giving recycled plastics a new lease of life, reducing waste, and promoting sustainability.
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Melting and compounding
Once plastics are sorted, they are ready for the next stage of the recycling process. This involves shredding the plastic into smaller pieces, flakes, or particles, and then melting and extruding it into new pellets. This process, known as compounding, is done using machines called compounding lines or extruders. These machines melt the plastic regrinds into pellets, which are small, rounded pieces of plastic.
The incorporation of various additives during this process can transform the plastic into a high-quality, reusable material. However, the plastic is typically distributed and remanufactured in the pellet form. This stage of the recycling process is crucial in preparing the plastic for its new life as different products.
The melting and compounding stage is an important step in the recycling journey of plastics. It ensures that the plastic waste is efficiently utilized and given a new purpose. This stage also helps to reduce the need for creating new plastic from scratch, as the recycled plastic can be used to create new products.
The compounding process can vary depending on the type of plastic being recycled and the specific requirements of the recycling facility. Some facilities may incorporate additional steps or techniques to enhance the quality and purity of the recycled plastic. Overall, the melting and compounding stage is a complex and intricate part of the plastic recycling process, requiring advanced technology and expertise.
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Frequently asked questions
The steps to recycle plastic are collecting, sorting, and reprocessing. Collection involves users depositing plastic into a recycling container. Sorting is done at Material Recovery Facilities (MRF) and Plastic Recovery Facilities (PRF), which handle just plastic. The plastic is then shredded, melted, and extruded into new pellets, which are then moulded into new products.
Sorting is mainly done automatically, with a manual sort to ensure all contaminants have been removed. Sorting machines use infrared, x-ray, or other cutting-edge sensors to recognise a polymer's unique signature. Some plastic is also sorted manually by hand, especially irregularly shaped or unusually coloured plastic.
Different types of plastic have different properties, so it is vital to sort them thoroughly before recycling. Packaging design makes the sorting task complex, and products with multiple layers are difficult and expensive to separate. Some plastics, like PVC, are also notoriously difficult to recycle due to their chemical composition.
Separate plastics collection schemes will help improve sorting output. Less variety in plastic types and simpler packaging design might also help. Digital watermarks and chemical markers can also help identify and separate different plastics.
