Chloe Zhang
Plastic recycling is a hot topic, with global plastic production reaching 360 million tonnes in 2018. To recycle plastic, it must first be sorted. Sorting plastic to a high degree of purity means polymers can be reused and retain their value. Plastic waste collected from homes, businesses, and recycling centres is sent to Material Recovery Facilities (MRFs) and/or Plastic Recovery Facilities (PRFs). Here, plastic is sorted by type, colour, thickness, size, and usage, either manually or by machines. Sorting machines use optical scanners, near-infrared systems, and other cutting-edge sensors to distinguish between different types of plastics. After sorting, the plastic is washed, shredded, and melted to create new recycled plastic pellets, which are sold to be made into new products.
| Characteristics | Values |
|---|---|
| Importance of sorting recycled plastic | Sorting to a high degree of purity means polymers can be reused and retain their value |
| Collection | Plastic waste is collected from homes, businesses, schools, and local recycling centres |
| First sorting stage | Material Recovery Facilities (MRF) separate plastic from non-plastic |
| Second sorting stage | Plastic Recovery Facilities (PRF) sort plastic by type |
| Sorting methods | Optical sorting machines, sink/float separators, magnets, manual sorting |
| Washing | Removes adhesives, food waste, labels, and other impurities |
| Shredding | Plastic is shredded into smaller pieces |
| Further sorting | Sorting by colour, thickness, size, and plastic type |
| Re-processing | Plastic is melted and extruded into new recycled plastic pellets |
| Use of recycled plastic | Recycled plastic pellets are sold to be made into new products |
| Novel materials | May not be sorted and may end up as impurities in sorted plastic |
| Sorting challenges | Complexity of packaging design, e.g. black plastic, plastic sleeves on PET bottles, multiple layers |
| Consumer role | Consumers can help by rinsing plastic to remove food and other materials |
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What You'll Learn
Sorting plastic by colour
Sorting Methods
There are two main methods for sorting plastic by colour: manual and automated. Manual sorting involves human workers visually inspecting and sorting plastic objects by hand, which is labour-intensive and less effective. Automated sorting, on the other hand, utilises machines equipped with advanced technology to sort plastic by colour.
Technology Used
Several technologies are employed in automated plastic colour sorting:
- Optical Sorting: Optical sorting machines use photoelectric detection technology to identify and separate plastics based on colour differences. These machines rely on light reflection and detection systems to distinguish between colours accurately.
- Visible Reflectance Spectroscopy (VIS): This technique uses light reflection to identify the colour of plastics accurately. It is effective for sorting polypropylene (PP) resins and can distinguish between colours like blue, red, green, white, and yellow.
- Machine Vision and Cameras: Charged Coupled Device (CCD) cameras, in combination with techniques like quadratic discriminate analysis (QDA) and tree classifiers, can accurately detect and sort plastic by colour.
- Spectrophotometers: These instruments use geometric technology to perceive colour similarly to the human eye. They help obtain accurate colour measurements, accommodating various light sources and textures.
Sorting Process
The process of sorting plastic by colour typically involves the following steps:
- Collection and Transportation: Plastic waste is collected from homes, businesses, and recycling centres, then transported to Material Recovery Facilities (MRF) or Plastic Recovery Facilities (PRF).
- Preliminary Sorting: At the facilities, plastic is initially sorted manually or by machines to separate it from other materials and into different plastic types.
- Washing and Shredding: The plastic is washed to remove impurities like adhesives, food waste, and labels. It is then shredded into smaller pieces to facilitate further processing and remove additional impurities.
- Colour Sorting: Using the technologies mentioned above, the shredded plastic is sorted by colour. Optical sorting machines, for example, use light and colour detection to separate plastics of different colours.
- Further Processing: After colour sorting, the plastic may undergo additional sorting by thickness, size, or other characteristics. It is then melted and extruded into new recycled plastic pellets, which are sold to create new products.
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Sorting plastic by thickness
The first step in the recycling process is the collection of plastic waste from residential, commercial, and institutional sources. This is typically done by government agencies or private companies. Once the plastic waste has been collected, it is transported to sorting facilities. At these facilities, the plastic waste is unloaded and undergoes an extensive sorting process.
Sorting can be done manually or by machines. Machines used for sorting include optical sorting machines, sink/float separators, and ballistic separators. Sink/float separators use the density of the plastic to separate different thicknesses, with lighter plastics floating in a brine solution and heavier plastics sinking. Optical sorting machines use infrared technology to distinguish between different types of plastics and separate them.
After the plastic has been sorted by thickness and type, it is shredded into smaller pieces and then washed to remove impurities such as food scraps, glue, and labels. The plastic is then ready to be processed into new products.
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Sorting plastic by type
At the recycling plant, objects to be sorted move along conveyor belts at speeds of around three metres per second. Light plastics like polyethylene (PE) and polypropylene (PP) films are filtered off in an airstream, while heavier and rigid plastics continue in a 3D stream. Near-infrared (750–2500 nm) systems are used to sort the different polymer types by distinguishing between their individual wavelength signatures. Optical detection systems separate colours, and magnets are used to remove metals.
To ensure effective recycling, it is important to correctly sort plastic waste before collection. Recycling labels on plastic packaging are designed to help people recycle their waste correctly. However, these symbols can often be confusing. The Resin Identification Code, indicated by a triangle with a number from 1 to 7, tells you the type of plastic used. For example, a water bottle with a "3" or a "5" cannot be recycled in most jurisdictions in the US. A "3" indicates that the bottle is made from polyvinyl chloride, while a "5" means it is made from polypropylene.
Once collected, the plastic waste is sent to Material Recovery Facilities (MRF) or Plastic Recovery Facilities (PRF) to be sorted into plastic types. The plastic is then shredded into smaller pieces and washed to remove impurities such as adhesives, food waste, and labels. After washing, the plastic is further sorted by colour and then melted and extruded into new recycled plastic pellets, which are sold to be used in new products.
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Sorting plastic by usage
At the commercial level, recycling companies rely on their providers to appropriately sort the plastic before sending it for recycling. This means that the onus is on the providers to ensure that the plastic they send for recycling is correctly sorted by usage.
When sorting plastic by usage, it is essential to separate items based on their intended purpose or the type of product they were used for. For example, plastic packaging, such as bottles, containers, and wrappers, would be sorted separately from plastic items such as toys, furniture, or electronic devices.
Additionally, the type of plastic resin used in the product also plays a crucial role in sorting by usage. Different types of plastic resins have distinct properties and are used for specific applications. For instance, PET (polyethylene terephthalate) is commonly used for beverage bottles, food packaging, and other consumer goods, while HDPE (high-density polyethylene) is used for items such as milk jugs, detergent bottles, and plastic pipes. Sorting by resin type ensures that the recycled plastic can be processed and used for specific applications.
It is worth noting that some plastics may be challenging to sort by usage due to their complex composition. For example, black plastic, plastic sleeves on PET bottles, and products with multiple layers can be difficult to recycle and may require specialized technology for detection and sorting.
Overall, sorting plastic by usage is a critical step in the recycling process, ensuring that recycled plastic can be processed effectively and safely while also retaining its value for reuse in new products.
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Sorting plastic by chemical composition
One of the groundbreaking technologies used to sort plastic by chemical composition is Near-Infrared (NIR) spectroscopy. This technique involves shining near-infrared light onto plastic items. Different types of plastic have distinct molecular structures, which cause them to absorb and reflect light in unique ways. By analysing the reflected light, sensors can determine the resin type of each plastic. For example, Polyethylene Terephthalate (PET) and High-Density Polyethylene (HDPE) emit different NIR spectra, allowing them to be differentiated.
Another method used in sorting plastic by chemical composition is optical sorting. Optical detection systems can separate colours, and optical sensors can identify and sort plastic items based on their visual characteristics, such as colour and shape. Additionally, some technologies use digital watermarks or codes to identify the packaging material. These codes are invisible to the naked eye but can be read by cameras on the sorting line.
The sorting process typically begins with collection, where plastic waste is gathered from various sources, including households, businesses, and recycling centres. The plastic waste then moves along conveyor belts in recycling facilities, where it is sorted using various technologies. After sorting, the plastics are directed to their appropriate recycling or disposal pathways. Mechanical recycling involves cleaning, shredding, and melting the plastics to produce new products, while plastics unsuitable for mechanical recycling may undergo chemical recycling or energy recovery processes.
Overall, sorting plastic by chemical composition is essential to ensure the effectiveness of the recycling process. It helps to identify and separate non-recyclable plastics, such as PVC, and ensures that each type of plastic is processed appropriately, maximising the value of recycled materials and reducing negative environmental impacts.
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Frequently asked questions
Plastic waste collected from homes, businesses, and recycling centres is sent to Material Recovery Facilities (MRF) and/or Plastic Recovery Facilities (PRF). Here, the plastic is sorted into plastic types, shredded, washed, and sorted further.
Sorting can be done manually or by machines. Sorting machines use optical scanners, sink/float separators, magnets, and infrared systems to separate plastic by colour, thickness, size, and plastic type.
Sorting to a high degree of purity means polymers can be reused and retain their value. It also helps eliminate cross-contamination, which would render a batch unsuitable for reprocessing.
The sorted plastic is then melted and extruded into new recycled plastic pellets, which are sold to be made into new products.
It is important to correctly sort your plastic waste and rinse plastic containers to remove food waste and other materials before they dry and stick.
