Alexei Volkov
Plastic waste is a pressing environmental concern, with improper disposal leading to harmful consequences for wildlife and the planet. To address this, recycling has emerged as a crucial solution, involving the collection, sorting, and reprocessing of plastic waste into new products. However, the effectiveness of recycling hinges on accurately sorting plastics, which is challenging due to their diverse properties, colours, shapes, and packaging designs. Sorting plastic by density is a critical step in the recycling process, as different plastics must be separated to create structurally sound recycled products. This process involves using advanced technologies, such as automated systems, optical scanners, and near-infrared spectroscopy, alongside manual labour, to identify and categorise plastics based on their unique attributes, including density, colour, and behaviour under specific light wavelengths.
Why sort plastic by density for recycling?
| Characteristics | Values |
|---|---|
| To separate different types of plastics | Different plastics have different properties, and when melted together, they tend to fractionate like oil and water. |
| To ensure accurate sorting | Automated systems have improved accuracy, but manual intervention is sometimes needed for irregularly shaped or unusually colored items. |
| To identify non-recyclable plastics | For example, PVC is difficult to recycle due to its chemical composition and must be separated from recyclable plastics. |
| To separate plastic from other materials | Foreign materials such as metal and glass can be removed using gravity, air classifiers, or water streams. |
| To prepare for further processing | After sorting, plastics are shredded, washed, and melted to create new recycled plastic pellets. |
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What You'll Learn
Different densities of plastic have different properties
The varying densities of plastic are due to their unique molecular structures, which cause them to absorb and reflect light differently. This is utilised in the recycling process, where near-infrared light is shone onto the plastic items, and the reflected light is analysed to determine the resin type of each plastic. This method of using near-infrared spectroscopy allows for high-speed and accurate sorting of plastics.
Additionally, the different densities of plastic cause them to behave differently when exposed to water. High-density plastics sink, while low-density plastics float, and this property is used in the Sink-float separation method to separate plastics. This method involves filling a tank with water and allowing the high-density plastics to sink while the low-density plastics float, allowing them to be easily separated.
The different densities of plastic also affect their behaviour when exposed to specific wavelengths of light, which is another factor that can be used to distinguish between different types of plastics during the recycling process. Furthermore, the density of plastic is related to its rigidity, with light plastics like polyethylene (PE) and polypropylene (PP) films being more flexible and easily filtered off in an airstream, while heavier and rigid plastics are less flexible and can continue in a 3D stream for further processing.
Overall, the different densities of plastic have distinct properties that are utilised in the recycling process to accurately identify, sort, and separate the various types of plastics.
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Sorting by density is done by floatation
Different plastics have different properties, which is why it is crucial to sort them meticulously before recycling. Sorting by density is one of the critical steps in the recycling process. This process is known as a sink-float separator, and it is often done by floatation in many commercial firms.
The sink-float separator is a wet process where a tank is filled with water, and the plastic is placed inside. The high-density plastic sinks, while the low-density plastic floats on the water surface. This process is also known as brine solution separation, where heavy plastics sink in the brine (salt) solution, and the lighter ones float. Different brine solutions can be used to separate all the major plastics.
After the separation, the plastics are recovered for further processing. They are then transferred to a reprocessor for the next stage. The reprocessor may undertake further sorting to ensure any remaining contamination is removed. This additional sorting may include optical sorting machines and sink-float separators to separate plastic by thickness, colour, size, and type.
The use of automated systems has significantly improved the accuracy and speed of plastic sorting. These systems employ a range of sensors and optical scanners to identify and categorise plastic items based on attributes such as density, colour, and behaviour when exposed to specific light wavelengths. One of the technologies used is near-infrared spectroscopy (NIR), which involves shining near-infrared light onto the plastic items. Different plastics absorb and reflect this light uniquely due to their varying molecular structures. By analysing the reflected light, sensors can determine the resin type of each plastic.
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Automated systems sort by density, but manual intervention is sometimes needed
Plastic waste is first collected from homes, businesses, and local recycling centres and sent to Material Recovery Facilities (MRF) and/or Plastic Recovery Facilities (PRF). The former handles plastic and non-plastic waste, while the latter handles just plastic waste. Sorting equipment is then used to separate plastic from other materials and into different types of plastic.
Automated sorting systems, equipped with sensors and optical scanners, identify and categorise plastic items based on their attributes, including density, colour, and behaviour when exposed to specific wavelengths of light. For instance, near-infrared (NIR) light is shone onto plastic items, and sensors analyse how the different types of plastic reflect and absorb the light uniquely due to their varying molecular structures. This technology allows for high-speed and accurate sorting.
However, there are limitations to automated systems, and manual intervention is sometimes required. For instance, plastic items with irregular shapes or unusual colours may confuse the sensors, requiring skilled workers to step in and ensure correct sorting decisions are made. Additionally, some plastics, such as PVC, are challenging to recycle due to their chemical composition and may require manual separation from the stream of recycled plastics.
Other methods of sorting plastics include the use of sink-float separators, where high-density plastics sink in a tank of water while low-density plastics float, and the removal of foreign materials such as metal and glass using gravity, air classifiers, or magnetic properties.
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Density is mass per unit volume
Density is defined as a substance's mass per unit volume. The formula for density is d = M/V, where d is density, M is mass, and V is volume. The unit for density is often expressed in grams per cubic centimetre (g/cm3). For instance, the density of water is 1 gram per cubic centimetre. However, density can also be expressed in kilograms per cubic metre (in metre-kilogram-second or SI units). An example of this unit expression is the density of air, which is 1.2 kilograms per cubic metre.
Density is an essential factor in the recycling process, especially when sorting plastic. Different plastics have different properties, and therefore, different densities. When various plastics are melted together without being sorted, they tend to fractionate, resulting in structurally unsound products. Thus, it is crucial to sort plastics by density before recycling.
The process of sorting plastic for recycling can be done manually or through modern automated systems. In manual sorting, skilled workers sort plastic items by hand, while automated systems employ advanced technologies such as sensors, optical scanners, and near-infrared spectroscopy to identify and categorise plastic items based on their distinct attributes, including density, colour, and behaviour when exposed to specific wavelengths of light.
One common method of sorting plastics by density is through flotation. In this method, heavy plastics are submerged in a brine (salt) solution, causing them to sink, while lighter plastics float and can be separated. Different brine solutions can be used to separate all the major types of plastics. This process is similar to how eggs float on highly salted water, as the density of the water increases with the addition of salt, allowing objects with lower density to float.
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Sorting by density is one of many steps in the recycling process
Sorting plastic by density is one of the many steps in the recycling process. The recycling process involves collecting, sorting, and reprocessing plastic waste into new products. Sorting plastic by density is crucial because different plastics have different properties, and when melted together, they tend to fractionate like oil and water. This results in structurally unsound products made from recycled plastics.
The process of sorting plastic by density can be done through a wet process known as a sink-float separator. In this method, a tank is filled with water, and plastic is placed inside. The high-density plastic sinks, while the low-density plastic floats, allowing for easy separation. Different brine solutions can also be used to separate the major plastics.
Additionally, automated sorting systems equipped with sensors and optical scanners play a significant role in the recycling process. These systems identify and categorize plastic items based on attributes such as density, color, and behaviour when exposed to specific wavelengths of light. One of the technologies used is near-infrared spectroscopy (NIR), which involves shining near-infrared light onto plastic items. The light is absorbed and reflected differently due to the varying molecular structures of the plastics, allowing sensors to determine the resin type.
However, manual intervention is still necessary in some cases, especially with irregularly shaped or unusually coloured plastic items. Skilled workers ensure correct sorting decisions and identify non-recyclable plastics, such as PVC, which is challenging to recycle due to its chemical composition. Sorting plastic by density is, therefore, a critical step in the recycling process, ensuring the production of high-quality recycled materials.
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Frequently asked questions
Sorting plastic by density is important because different plastics have different properties. When different plastics are melted together, they tend to fractionate like oil and water, so the resulting products are structurally unsound unless the plastics are carefully sorted.
Plastic is sorted by density using a wet process known as a sink-float separator. A tank is filled with water and plastic recycling, the high-density plastic sinks, and low-density plastic floats. The separated plastics are then recovered for further processing.
Automated sorting systems use a range of sensors and optical scanners to identify and categorize plastic items based on attributes such as density, colour, and behaviour when exposed to specific wavelengths of light. One of the technologies used in the sorting process is NIR (near-infrared spectroscopy), which involves shining near-infrared light onto the plastic items. Different types of plastic absorb and reflect this light uniquely due to their varying molecular structures, allowing sensors to determine the resin type of each plastic.
