Omar Khalid
Plastic waste is sent to Material Recovery Facilities (MRFs) to be sorted into different types of plastic. Sorting machines use advanced technology such as NIR sensors, infrared lasers, and star screens to separate plastic by colour, size, thickness, and type. Once sorted, the plastic is shredded, washed, and melted down into pellets, which are then used to create new products.
| Characteristics | Values |
|---|---|
| Sorting techniques | Manual picking, optical sorting machines, sink/float separators, magnets, star screens, conveyor belts, lasers, NIR sensors |
| Cleaning techniques | Friction washers, water baths/tanks, chemicals, detergents, disinfectants |
| Final stage | Crushing, melting, pelletizing, compounding |
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What You'll Learn
Sorting by hand
Sorting plastic by hand is a crucial but often overlooked aspect of the recycling process. The integrity of the finished, recycled plastic product depends on the sorting process. Sorting by hand is necessary because different plastic polymers have different characteristics, and mixing them will alter these characteristics. Therefore, it is essential to sort plastics by type before they are sold to be recycled.
In the UK, for example, recycling rates have stagnated in recent years, but new government measures are expected to increase rates. One such measure is a tax on plastic packaging with less than 30% recycled content. With China's ban on importing waste, the UK must now sort and recycle more waste domestically.
Sorting plastic by hand is a meticulous task, as objects whizz along conveyor belts in recycling facilities at speeds of around three meters per second. Humans sort plastic by identifying and removing items by touching them on a screen. This hands-free sorting method corrects errors made by machines. Humans can also separate plastics by colour, an important distinction because colour impacts whether a polymer can be recycled and what it can be used for.
After sorting, the plastic pieces are cut and washed to remove traces of dirt and contaminants like paper, glue, sand, grit, and mixed plastic types. This washing process involves washers and water baths/tanks that continuously spray hot water over the plastic pieces to remove dirt and any labels stuck to their surfaces. Chemicals, detergents, disinfectants, or other products may be added to improve cleaning and separation.
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Sorting by float tanks
One of the most valuable techniques for separating plastics is the use of float tanks, also known as sink-float separators. This method is used to separate plastics by thickness, colour, size, and type, as well as to separate contaminants. Float tanks are particularly useful for recycling PET bottles, which are made of PET plastic.
Float tanks are filled with water, and depending on the materials being separated, additives may be added to alter the fluid's density. The mixed plastic is then fed into the tank, where the high-density materials sink and the low-density materials float. For example, in the case of PET bottles, the PET plastic will sink to the bottom of the tank, while the plastic wrappers and bottle caps, made from PP and PE, will float. The separated plastics are then recovered for further processing.
The float tank method is a wet process that can effectively separate co-mingled plastic streams. It is also applicable to multiple types of plastics, such as PET, PP/PE, and PS/ABS. The use of float tanks offers several advantages, including high selectivity through the basic principle of gravity separation, cost-effectiveness due to low operational and energy costs, and the ability to handle various plastic types.
The size of float tanks can vary, with standard tanks typically ranging from 4 to 6 meters in length. These tanks are equipped with rotating drums that move the plastic pieces into the water, allowing for a longer separation and soaking period. This results in a cleaner material stream. The separated materials are then conveyed for further processing or transferred to a reprocessor for the next stage of recycling.
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Sorting by magnets
Sorting plastic for recycling is a complex and costly process that typically involves several stages. One of the most important steps is sorting plastics by type, which can be done using various methods, including magnetic density sorting.
Magnetic density sorting is a technique developed by researchers from the W2Plastics European project to separate different types of plastics in a single step. This method involves passing plastic waste through a tank containing a suspension of 5-nanometre iron oxide nanoparticles placed on top of a magnet. The magnet attracts the iron oxide particles, increasing the density of the liquid at the bottom of the container, with the density gradually decreasing towards the top. As the plastic mixed with the iron oxide suspension flows through the tank, it separates into distinct layers based on the densities of the different plastic types. The flakes of plastic are then collected at various heights, resulting in separate streams of polypropylene, polyethylene, PET, and polystyrene. This process is highly precise, capable of differentiating plastics with density variations of less than 0.1%. It also helps eliminate contaminants like wood and foam, ensuring a pure form of recycled plastic that can be used for manufacturing new products.
The magnetic density sorting technique offers a cost-effective solution, with processing costs below 100 euros per tonne. It has gained attention within the recycling industry, particularly for its potential in metal recovery from shredded electronics. However, the adoption of such innovative recycling technologies may be hindered by the lack of involvement of polymer manufacturers in the recycling process and their conflicting interests.
It is worth noting that other methods, such as sink-float separation, optical sorting machines, and manual picking, are also employed in combination with magnetic separation to further refine and purify plastic streams before they undergo reprocessing into new products.
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Sorting by colour
The process of recycling plastic begins with sorting plastic from other materials at a Material Recovery Facility (MRF). The plastic is then sent to a Plastic Recovery Facility (PRF) for further sorting into different types of plastic. Sorting machines are used to identify and separate large amounts of plastic.
One of the methods to separate different types of plastic is by colour. Sorting machines can use infrared, x-ray, or other cutting-edge sensors to recognise a polymer's unique signature and, in some cases, its colour. For example, colour detection cameras for plastics are used in conjunction with infrared cameras for continuous process separation.
Infrared spectroscopy makes it possible to identify different plastics by analysing the light reflected by the surface of the material when subjected to infrared radiation. Each material emits a series of characteristic radiations depending on its molecular structure.
Another method to separate plastics is by density difference, also known as the sink/float separation method. A tank is filled with water and plastic recycling, the high-density plastic sinks, and low-density plastic floats. The small difference in density values between different plastics is sufficient to achieve good results with this system.
In addition to colour and density, plastic can also be separated by thickness, size, and type. Optical sorting machines and magnets are used to separate plastic by these characteristics.
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Sorting by size
The first step in the plastic recycling process involves sending plastic waste to Material Recovery Facilities (MRFs) that handle plastic and non-plastic materials. Here, the plastic is sorted into different types of plastic by sorting equipment, such as sink/float separators, and may be further sorted by size, colour, and thickness.
Size reduction ensures that the plastic pieces are consistent in size, which is crucial for effective sorting and processing. Larger plastic items may also be removed during the initial sorting stages to facilitate easier processing of the plastic waste. This can be done manually or through the use of specialised equipment.
Additionally, some facilities utilise star screens, a series of offset, star-shaped discs, to separate materials based on size. Smaller items fall through the screens and continue down the conveyor belt for further processing, while larger items are separated and removed. This process helps in sorting plastics by size and separating them from other materials, such as paper, glass, or metal.
Overall, sorting plastics by size is an essential step in the recycling process as it helps create a uniform feedstock for further processing. This step not only facilitates easier handling and processing of the plastic waste but also ensures that the recycled plastic is of consistent quality, contributing to a more efficient and effective recycling system.
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Frequently asked questions
The plastic is sent to a Material Recovery Facility (MRF) where it is sorted by type and baled. It is then transported to a recycling centre where it is cleaned, shredded, melted down into pellets and used to make new products.
Sorting machines are used to identify and separate large amounts of plastic by colour and type. Advanced sorting machines can use infrared, x-ray, or other cutting-edge sensors that can recognise a polymer’s unique signature.
The final stage of plastic recycling is when the plastics are crushed and melted to form pallets called nurdles. These nurdles are then transformed to take a new shape and make a new product.
It is important to rinse plastic to get off food or other materials before they become dry and stick. This ensures the recycling system operates more effectively. It is also helpful to separate paper, glass, and plastic before they are put out for collection.
