Sofia Martinez
The separation of plastics by density is a crucial process in the recycling industry. Plastics are separated based on their density, which is calculated by dividing mass by volume. The simple float/sink principle of density separation in water has proven effective, with heavy plastics sinking in brine solutions and lighter plastics floating. Other methods include flotation techniques, jigging, magnetic projection and levitation, and triboelectrostatic separation. Density separation aims to produce pure fractions of plastics, improving the quality of recycled materials.
| Characteristics | Values |
|---|---|
| Method | Flotation technique |
| Principle | Selective attachment of bubbles to the hydrophobic surface of plastics |
| Plastic condition | Completely immersed in the medium |
| Density difference | 0.05 g cm-3 |
| Separation techniques | Jigging, float/sink, magnetic projection and magnetic levitation |
| Separation equipment | Jig, reflux classifier, hydraulic separator multidune, cylindrical cyclone-type media separators |
| Plastic types | High-density polyethylene (HDPE), low-density polyethylene (LDPE), PVC, PP |
| Density calculation | Mass divided by volume |
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What You'll Learn
Flotation technique
The flotation technique is a good method for separating plastics of similar density. It is based on the principle of selective attachment of bubbles to the hydrophobic surface of plastics. The efficiency of the technique depends on the dissimilarity in the wettability of the particle surface.
Froth flotation is a type of flotation technique that has been used to separate plastics. It 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 six granulated post-consumer plastics was evaluated: Polystyrene (PS), Polymethyl methacrylate (PMMA), Polyethylene Terephthalate (PET-S, PET-D) and Polyvinyl Chloride (PVC-M, PVC-D). The performance of the flotation separation of five bi-component plastic mixtures was also evaluated: PS/PMMA, PS/PET-S, PS/PET-D, PS/PVC-M and PS/PVC-D.
The results showed that floatability differed with the size and shape of plastic particles. For regular-shaped plastics (PS, PMMA and PVC-D), floatability decreased with the increase of particle size, while for lamellar-shaped particles (PET-D) floatability was slightly greater for coarser particles. Thus, plastic particles with a small size, lamellar shape and low density have greater floatability.
The three-stage sink-float method is another flotation technique that has been used to separate plastics. In this method, plastics are separated into two groups: a low-density plastic group and a high-density plastic group. Plastic with a density less than that of the medium solution floats to the surface, while plastic with a density greater than that of the medium solution sinks to the bottom. This method has been used to separate high-density polyethylene (HDPE) and polypropylene (PP) from a group of six mixed-plastic wastes.
Overall, the flotation technique, including froth flotation and the three-stage sink-float method, has shown potential for the separation of plastics, particularly those with particle sizes greater than 2.0 mm.
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Magnetic projection and levitation
The process involves the use of magnets to artificially change the apparent density of a paramagnetic medium in a vertical or horizontal direction. The magnetic force is provided by permanent magnets, and the plastic particles are collected in chambers within a separation container. The magnetic projection separator consists of a separation container, permanent magnets, and a paramagnetic medium, which is usually an aqueous MnCl2 solution.
The unidirectional magnetic projection method involves placing a single magnet next to the separation chamber with an ejection muzzle along the magnet's edge. The number of magnets used can vary from one to three, and as the number of magnets increases, the magnetic field strength also increases. However, the projection distance will not increase proportionally with the increase in magnetic field strength.
Magnetic levitation, or MagLev, is a configuration that uses two identical magnets with like poles facing each other. This method can separate mineral, polymer, and biological samples based on their densities. MagLev offers higher efficiency, lower costs, and a promising processing capacity compared to existing methods. However, the collection process is limited due to the narrow operational space between the magnets and the converging effect of the magnetic force on the desired particles.
Overall, magnetic projection and levitation technology provides a promising solution for the separation of waste plastics, offering advantages such as higher efficiency and lower costs.
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Jigging
The quality of jigging separation varies with the mixture, density differences, and the size and shape of the particles. For particles with more regular shapes, the quality of separation improves with an increase in particle size. In contrast, for lamellar particles, the influence of particle size is minimal. Jigging is most effective for separating bi-component plastic mixtures that combine a low-density plastic with a high-density one. The separation efficiency is enhanced when the less dense plastic, which overflows, has a lamellar shape, and the denser plastic, which sinks, has a regular shape.
There are three main types of jigging: common jigging, reverse jigging, and hybrid jigging. Common jigging is suitable for the separation of plastics with a specific density heavier than water, and the process is dominated by the settlement velocity of plastic particles on the bottom bed. Reverse jigging, on the other hand, is designed for plastics with a lighter density than water. In this process, plastic particles move up and down with the help of a top screen, and they are separated due to their different levitation velocities. Hybrid jigging uses a pulsating medium with air bubbles to separate light hydrophobic plastics from heavy hydrophilic materials.
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Float/sink separation
The float/sink separation method, also known as density separation, is a widely used technique for separating plastics based on their densities. This method involves placing plastic pieces into a tank of water or another fluid, where they separate into layers based on their density relative to the fluid.
During float/sink separation, plastic pieces with a density greater than that of the fluid will sink, while those with a lower density will float. This process can be adjusted by using different fluids or additives to alter the fluid's density, allowing for the separation of plastics with varying densities. This technique is particularly useful for separating co-mingled plastic streams, such as those found in PET bottle recycling, where the PET plastic needs to be separated from polypropylene (PP) and polyethylene (PE) plastic.
The float/sink method is cost-effective and simple to execute, as it relies on gravity and the inherent properties of water to forgive differences in the size, shape, and composition of the materials being separated. This method was borrowed from the mining industry in the 1970s and has since been applied to plastic recycling, with the earliest application likely being PP battery cases at MA Polymers in 1969.
In practice, the float/sink separation process can be enhanced through mechanical preparation of the waste streams. This involves using equipment such as a jig, which utilizes pulsating water or air currents to create a bed where lighter materials, typically plastics, remain on top while denser particles settle at the bottom. This layering by density allows for efficient separation and collection of the different plastic fractions.
Overall, the float/sink separation method is a valuable technique for recycling plastics, offering a cost-effective and automated way to separate mixed waste into relatively pure plastic fractions. This process has been further developed and commercialized by companies worldwide, contributing to the advancement of plastic recycling technologies.
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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 liquid 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 is different at different heights in the fluid. When a mixture of plastic particles is introduced, the particles move to regions where their mass density equals the apparent density of the fluid.
The Magneto-Archimedes method is an efficient way to separate different types of plastic particles. It has been shown to be more effective than traditional sink-float methods, which can only separate floating (light) and sinking (heavy) materials. The Magneto-Archimedes method can continuously separate different plastic types from a flow of waste materials.
The performance efficiency of the Magneto-Archimedes method depends on particle size, shape, and the pre-separation processes used to treat the plastic mixture before separation. Larger particles separate more quickly, and spherical particles separate faster than disk-shaped particles. Pre-processing the plastic mixture so that it consists of larger, roughly spherical particles can help decrease the frequency of particle collisions and increase separation efficiency.
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Frequently asked questions
The simplest method to separate plastics by density is the "float/sink" method. This method was borrowed from the mining industry in the 1970s. Heavy plastics sink in a brine (salt) solution, while lighter plastics float and can be separated.
Other methods to separate plastics by density include jigging, triboelectrostatic separation, flotation, magnetic projection and magnetic levitation, and the use of a hydraulic separator multidune.
The quality of separation depends on the mixture type, the hydrophobicity, size, density, and shape of the particles.
