Chloe Zhang
The separation of plastics by density is a crucial process in the recycling industry. Plastics are separated based on their density, which is the mass per unit volume, through various methods such as flotation, jigging, magnetic separation, and hydraulic separation. The simple float/sink principle, for instance, involves placing plastics in water, brine, or another medium, allowing lighter plastics to float and heavier ones to sink. This method was borrowed from the mining industry in the 1970s and has been further developed with techniques like the float-sink cascade and the use of magnetic forces. Other approaches include triboelectrostatic separation and froth flotation, which takes advantage of differences in hydrophobicity and particle properties. Density separation plays a vital role in producing high-quality secondary raw materials from recycled plastics, ensuring compliance with industrial standards.
| Characteristics | Values |
|---|---|
| Method | Floatation technique |
| Principle | Selective attachment of bubbles to the hydrophobic surface of plastics |
| Plastic condition | Plastic must have adequate dissimilarity in the wettability of the particle surface |
| Plastic type | Applicable for plastic components with similar density |
| Plastic preparation | Physical modification of the plastic surface may be required |
| Separation technique | Magnetic projection and magnetic levitation |
| Jigging | Jig, reverse jig, and hybrid jig |
| Separation by density | Heavy plastics sink in brine solution, lighter ones float |
| Density difference | 0.05 g cm-3 required for acceptable separation performance |
| Separation by shape | Lamellar shape enhances flotation separation |
| Magneto-Archimedes method | Control levitation and settlement of particles by controlling the density and magnetic susceptibility of the medium and the magnetic field |
| Cyclone-type media separators | Cylindrical and cylindroconical separators have superior production capacities and can process a larger range of particle sizes |
| Modelling | Accurate modelling of hydrodynamic forces on particles is crucial to predicting their trajectories and separation efficiency |
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What You'll Learn
Flotation technique
The flotation technique is a good method for isolating plastics. It uses the principle of selective attachment of bubbles to the hydrophobic surface of plastics. The effectiveness of the flotation sorting method is influenced by the dissimilarity in the wetability of the particle surface.
Froth flotation is a technique that is based on hydrophobicity differences between particles. Most plastics are naturally hydrophobic, requiring the addition of chemicals that promote the selective wettability of one of its components. The floatability of granulated post-consumer plastics such as Polystyrene (PS), Polymethyl methacrylate (PMMA), Polyethylene Terephthalate (PET-S, PET-D) and Polyvinyl Chloride (PVC-M, PVC-D) can be evaluated in the presence of a wetting agent such as tannic acid. The performance of flotation separation of bi-component plastic mixtures such as PS/PMMA, PS/PET-S, PS/PET-D, PS/PVC-M and PS/PVC-D can also be analysed.
The flotation technique can be used to separate plastics of different densities. Plastic with a density lower than that of the medium solution floats, while plastic with a higher density sinks. For example, a three-stage sink-float method can be used to separate mixed-plastic wastes, including high-density polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET) and acrylonitrile-butadiene-styrene copolymers (ABS). The low-density plastic group (HDPE and PP) can be separated from the high-density plastic group (PET, PVC, PS and ABS) by tap water.
The flotation technique can also be used to separate post-consumer PET from other packaging plastics with similar densities. In a study, a sample composed of 85% PET, 2.5% PVC and 11.9% PS was subjected to a combination of alkaline treatment and surfactant adsorption followed by froth flotation. The result was a concentrate grade of 97.2% PET, 1.1% PVC and 1.1% PS.
The efficiency of the flotation technique is influenced by various factors such as particle size, shape, density, weight, and hydrophobicity. For regular-shaped plastics such as PS, PMMA and PVC-D, floatability decreases with an increase in particle size, while for lamellar-shaped particles such as PET-D, floatability is slightly greater for coarser particles. Therefore, plastics with a small size, lamellar shape and low density have greater floatability.
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$59.98
Magnetic projection and magnetic levitation
Magnetic projection requires a low initial velocity of the mixtures near the inlet port before separation. The magnetic projection separator consists of a separation container, permanent magnets, and a paramagnetic medium (normally an aqueous MnCl2 solution). The separation container has many chambers to collect purified plastic particles.
Magnetic levitation (MagLev) is a novel technology for the density-based analysis of diamagnetic materials submerged in a paramagnetic medium sandwiched between two magnets. MagLev can separate multiple plastic mixtures with 100% purity and high recovery rates. It can also separate materials like minerals, polymers, and biological samples based on their densities.
The magnetic levitation configuration involves placing the samples in a paramagnetic medium between two identical magnets with like poles facing each other. The number of magnets affects the magnetic field strength and projection distance.
Overall, magnetic projection and magnetic levitation offer efficient, cost-effective, and promising solutions for separating waste plastics and other materials.
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Jigging
The common jig is suitable for the separation of plastics with a specific density heavier than water. The separation is dominated by the settlement velocity of plastic particles on the bottom bed. The quality of separation varies with the mixture, the density differences, and the size and shape of the particles. For particles with more regular shapes, the quality of separation improves with the increase of particle size. For lamellar particles, the influence of particle size is minimal.
Reverse jigging is designed for plastics with a lighter density than water. In this method, plastic particles move up and down with the help of a top screen, and the particles are separated due to their different levitation velocities. Hybrid jigging, on the other hand, uses a pulsating medium with air bubbles to separate light hydrophobic plastics from heavy hydrophilic materials.
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Float/sink principle
The float/sink principle, also known as density separation, is a simple and cost-effective method for separating plastics from other materials, such as wood, metal, and contaminants. This technique was borrowed from the mining industry in the 1970s and has been successfully applied to plastic recycling.
The basic concept of the float/sink principle is that materials with different densities will either float or sink when submerged in a liquid, typically water. By placing a mixture of plastics and other materials in a separation tank, the materials will naturally separate based on their densities. The lighter materials, such as plastics with lower densities, will float to the top, while heavier materials, like metals and denser plastics, will sink to the bottom.
This method is advantageous because gravity is a free force, and water accommodates variations in the size, shape, and composition of the materials being separated. Additionally, the construction of separation tanks is relatively simple and inexpensive. The float/sink principle can be applied to separate plastics of different densities, as well as to separate plastics from other materials.
To enhance the effectiveness of the float/sink method, certain modifications can be made. For instance, brine (salt) solutions can be used instead of plain water. Different brine solutions have varying densities, allowing for the separation of plastics with specific density ranges. This technique has been successfully employed by commercial firms to separate major plastics.
Another variation of the float/sink principle is the use of flotation techniques, which take advantage of the hydrophobic nature of plastics. By selectively attaching bubbles to the hydrophobic surfaces of plastics, they can be made to float and separate from other materials. However, this method is more effective when plastics have distinct hydrophobicity, and physical modifications may be required for aged plastics that have lost their inherent hydrophobic properties.
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Magneto-Archimedes method
The Magneto-Archimedes method is a promising high-resolution mass density-based separation technique that uses magnetically responsive liquids and magnets to separate particles by means of magneto-Archimedes levitation. This method is based on Archimedes' principle, which states that the upward buoyant force exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces.
In the Magneto-Archimedes method, a fluid is magnetized by magnets located at the top and bottom of a flow channel. The magnets change the hydrostatic pressure in the fluid and create a gradient of "apparent mass density" in the fluid. This means that the apparent density of the fluid differs at various heights. When the plastic particle mixture is introduced, the particles move to regions where their mass density matches the apparent density of the fluid.
The Magneto-Archimedes method is particularly effective for separating different types of plastics. It is important to note that the performance efficiency of this method depends on particle size, shape, and the pre-separation processes used to treat the plastic mixture before entering the system. For instance, larger particles separate more quickly, and spherical particles separate faster than disk-shaped particles. Therefore, it is beneficial to preprocess the plastic mixture to consist of larger, roughly spherical particles, which also helps decrease the frequency of particle collisions.
The Magneto-Archimedes method offers a continuous one-step separation of multiple types of polymers, which cannot be achieved by conventional sink-float techniques. By utilizing magnetically responsive liquids and magnets, this method provides a robust and efficient framework for investigating particle-laden flows and enhancing the separation of plastics by density.
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Frequently asked questions
The simplest method is the "float/sink" principle of density separation in water. Heavy plastics sink in a brine (salt) solution, while lighter plastics float and can be separated.
Flotation techniques can be used to separate plastics with similar densities. This technique uses the principle of selective attachment of bubbles to the hydrophobic surface of plastics.
Density is calculated by dividing mass by volume. The formula is: Density = Mass/Volume.
Advanced techniques include jigging, triboelectrostatic separation, magnetic projection and magnetic levitation, and the use of specific hydraulic separators such as the Multidune separator.
