Alexei Volkov
Plastic recycling is a complex process that involves multiple steps and technologies. The first stage is the collection of plastic waste, which is then taken to a Materials Recovery Facility (MRF) for sorting and processing. At the MRF, the plastic waste undergoes pre-sorting to remove non-recyclable items, followed by automatic or manual sorting of recyclables. Various technologies are employed, including trommels (screened cylinders or drums), optical scanners, ballistic separators, magnets, and eddy current separators. The sorted plastics are then processed and recycled into new products, contributing to a more sustainable economy. However, the challenge of sorting plastics lies in the diverse types of plastics with distinct characteristics, requiring vigilant sorting to prevent cross-contamination and ensure the safety of workers.
| Characteristics | Values |
|---|---|
| Sorting methods | Optical sorters, density separators, magnets, eddy current separators, trommels, ballistic separators |
| Sorting by colour | Optical scanners identify different plastics by colour |
| Sorting by weight | Glass, which is heavier than plastic and aluminium, falls through star screens and lands in bins below |
| Sorting by material | Ferrous metals are picked up by magnets; aluminium is separated by eddy current separators |
| Sorting by shape | Large star screens lift out corrugated cardboard; smaller items fall through the screens |
| Sorting by resin number | Tests can be conducted to identify the type of plastic by its reaction to fire |
| Sorting by safety | Some plastics cause dangerous chemical reactions when mixed; PVC and PET, for example, create harmful acids and toxic dioxin emissions |
| Sorting by efficiency | Artificial intelligence and machine vision technologies can be used to identify and sort plastic packaging |
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What You'll Learn
Sorting plastics by colour
Automated sorting systems, on the other hand, use advanced technologies such as optical scanners, sensors, and near-infrared (NIR) spectroscopy to identify and categorize plastic items based on their distinct attributes, including colour. NIR spectroscopy involves shining near-infrared light onto plastic items, which absorb and reflect this light differently due to their varying molecular structures. By analysing the reflected light, sensors can determine the resin type of each plastic and sort them accordingly.
One example of an automated plastic colour sorter is the machine developed by Hongshi Hi-Tech, which uses photoelectric detection technology to sort heterochromatic particles in granular materials. This machine uses a combination of vibration and light sources to identify and separate particles of different colours, directing them into different waste hoppers.
Overall, sorting plastics by colour is a complex but essential process in the recycling journey. It ensures that plastics are properly processed according to their specific properties and recycling capabilities, promoting a circular economy and minimizing the environmental impact of plastic waste.
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Sorting plastics by grade
Sorting plastic by grade is a crucial step in the recycling process. While technology has advanced to include automated sorting systems, manual intervention is still sometimes necessary for accurate sorting.
One method of sorting plastics is through the use of automated systems equipped with sensors and optical scanners. These systems use Near-Infrared (NIR) spectroscopy, which involves shining near-infrared light onto plastic items. Different types of plastic have unique molecular structures, causing them to absorb and reflect light differently. By analyzing the reflected light, sensors can identify the resin type of each plastic. This technology allows for high-speed and accurate sorting. However, irregularly shaped or unusually coloured plastics can pose challenges for these automated systems, requiring skilled workers to intervene and ensure correct sorting decisions.
Another innovative approach to sorting plastics is through the use of fluorescent markers developed by the Prism consortium. These markers provide a unique signal to sorting equipment, allowing for the identification and separation of specific polymers and multilayer packaging. This technique is particularly effective for recovering food-grade polymers, such as polypropylene (PP) and high-density polyethylene (HDPE), commonly used for milk containers. The markers also enable the identification of black plastics, which are typically invisible to NIR systems. A second pass through the detection systems can result in a purity level of up to 99.6%, meeting stringent food contact standards.
To further enhance sorting accuracy, some collection points employ machine vision technology. This approach uses computer learning to identify different components and facilitate separation into specific waste streams. Artificial intelligence (AI) plays a pivotal role in this process, enabling intelligent and precise sorting. Unilever and the Alibaba Group have successfully implemented AI in their pilot programme, aiming to expedite the return of high-grade recyclable plastics into the circular economy.
The sorting process also involves separating non-recyclable plastics, such as PVC, which is challenging to recycle due to its chemical composition. By employing a combination of advanced technologies and human expertise, recycling centres strive to accurately identify, sort, and prepare plastics for subsequent stages of recycling.
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Sorting plastics by chemical composition
Sorting plastics by their chemical composition is a complex but necessary task. Each type of plastic has a unique chemical composition, with its own mix of dyes and additives, that affects its colour, shape, structure, toughness, adaptability, and melting point. This means that different types of plastics need to be processed separately.
The process of sorting plastics by chemical composition typically begins with a conveyor belt moving the plastic waste through an array of sensors and optical scanners that analyse factors like density, colour, and behaviour when exposed to specific wavelengths of light. For example, heavy plastics sink in a brine (salt) solution, while lighter ones float and can be separated. Different brine solutions can be used to separate all the major plastics.
Optical detection systems also allow colours to be separated. For instance, Veolia has developed a hands-free sorting system where operators can remove items by touching them on a screen. The plastics are usually washed to remove impurities like glue and ink, then ground into flakes, which may be further sorted by colour.
However, the variety of packaging designs can make the sorting task more challenging. Black plastic, plastic sleeves on PET bottles, and products with multiple layers mean that valuable materials are often thrown away. Multi-layered products are difficult and expensive to separate and often contain polyamides or ethylene vinyl alcohol, which can change the colour, chemical, physical, and mechanical properties of the recycled material.
There are also safety reasons for sorting plastics by chemical composition. Some types of plastics can cause dangerous chemical reactions when mixed together. For example, adding one PVC container to 900 lbs of PET bottles can contaminate the entire load by producing harmful acids or toxic dioxin emissions.
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Sorting plastics by size
Another technique for sorting plastics by size is through the use of ballistic separators. These machines use air to separate materials based on their weight and size. Light plastics, such as polyethylene (PE) and polypropylene (PP) films, are carried away in an airstream, while heavier and more rigid plastics continue on a different path. This method ensures that plastics of different sizes are effectively separated.
In addition to trommels and ballistic separators, recycling centers also employ wet processes such as sink-float separators. In this method, a tank is filled with water, and the plastic waste is placed inside. The high-density plastics sink, while the low-density plastics float, allowing for an effective separation based on density and size.
Furthermore, recycling centers may utilize optical sorting machines that can sort plastics by thickness, color, size, and type. These machines, equipped with sensors and optical scanners, analyze factors such as density, color, and the behavior of plastics when exposed to certain conditions. This technology enables the accurate sorting of plastics by size, ensuring that the recycled material is pure and ready for the next stages of processing.
While automation plays a significant role in sorting plastics by size, human involvement is still crucial. Workers are often positioned along the line to manually remove smaller contaminants and items that might jam the machinery, such as plastic bags and coat hangers. This collaborative effort between technology and human intervention ensures the accuracy and efficiency of the plastic sorting process.
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Sorting plastics by weight
Sorting plastic by weight is one of the methods used in recycling centres. It is a complex process that involves several technologies and equipment. Objects to be sorted move at high speeds of around three metres per second on conveyor belts in recycling facilities. Light plastics like polyethylene (PE) and polypropylene (PP) films are separated from the heavier and rigid plastics using an airstream.
The heavier plastics are then sorted using near-infrared (750–2500 nm) systems to distinguish between different polymer types. This technology relies on the varying reflectivity of polymers and their individual wavelength signatures. Optical detection systems further along in the process allow colours to be separated.
To ensure purity in the sorted plastics, which is crucial for their reuse, recycling centres also employ hands-free sorting methods. Operators can remove incorrectly sorted items by touching them on a screen. Sorting by weight is just one of the methods used, and the entire process involves multiple technologies and steps to ensure the purity and quality of the recycled plastics.
The recycling process can be made more efficient and effective through proper waste separation at the point of collection. Initiatives such as the Waste-Free World, which uses automatic scanning and sorting, and digital watermarks, which can be read by high-resolution cameras or scanners, are also being developed to improve sorting accuracy and increase recycling rates.
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Frequently asked questions
The first step in sorting plastics at a recycling center is to separate the different types of plastics. This can be done manually by workers or automatically by machines using various technologies such as optical scanners, density separators, and magnets.
Sorting plastics before recycling them is important to increase the quality and quantity of recycled products. It also helps to prevent cross-contamination, as some types of plastics can cause dangerous chemical reactions when mixed together.
Recycling centers use a variety of technologies to sort plastics, including machine learning, artificial intelligence, and optical sorters that use infrared light to identify the type of plastic.
One challenge of sorting plastics for recycling is that some plastics are not easily sorted or available in sufficient quantities to make mechanical recycling economically and environmentally viable. Another challenge is cross-contamination, where different types of plastics are mixed together, which can compromise the quality of the recycled product.
Individuals can help improve the sorting process by separating their waste correctly at the point of collection. This includes separating different types of plastics and ensuring that only recyclable materials are placed in the recycling bin.
