The methods of Waste Plastics sorting
The separation technology of waste plastics mixture can be divided into dry and wet methods. It is generally believed that wet method is easier to obtain higher separation accuracy than dry method. Optical separation, electric separation, wind separation, density separation and flotation have been applied to the separation of waste plastics in varying degrees.
1 dry separation
(1) Optical sorting
Different plastics have different infrared spectra, for example, the infrared spectra of PVC are different from those of PET, PP, etc.]; X-ray can detect chlorine in PVC. When infrared spectroscopy or X-ray irradiation is applied to the bulk plastic mixture on the belt conveyor, if the detector obtains the information of PVC, the nozzle ejects air to blow it out. By this method, a variety of plastics can be sorted. Optical sorting method is suitable for the separation of some plastics in bulk plastics mixture. For broken fine plastics, the separation process is difficult to complete due to the displacement of some bands in the spectrum. In addition, it is difficult to separate black plastics by spectral method.
(2) Electrostatic sorting
Electrostatic sorting is divided into electrostatic separation and frictional electrification separation, that is, using corona discharge or frictional electrification to electrify the object of study, and sorting plastic particles with different electrification and electric quantity in turn. The order of plastics from negatively charged to positively charged is: PVC, PET, PP, PE, PS, ABS, PC. Because the difference of charged plastics is not very obvious, especially for the actual plastic waste, the charged properties are different from that of pure plastics, and the electric separation is greatly affected by the attached moisture and humidity, so there are many limitations in the separation of waste plastics by electric separation technology.
(3) Wind separation
Wind separation sorting the plastic particles in the air flow due to differences in particle size, shape, density and other separation, suitable for the separation of materials with large density difference. The equipment can be divided into wind separator and wind shaker. Besides wind factors, the difference of particle friction coefficient is also used. Generally speaking, wind separation is more suitable for the separation of metal and plastics. The efficiency of separating waste plastics is not high, and the treatment capacity of wind shaker is not high.
2 wet separation
(1) Density sorting
If a suitable density medium can be chosen to make one of the two plastics float and the other sink, the separation of the two plastics can be achieved. Because waste plastics are often treated with flame retardants and reinforcing agents, the density of plastics with the same name is often different.
The commonly used sorting media are water, saturated sodium chloride solution (density 1200 kg/m3), saturated CaCl2 solution (density 1500kg/m3), and mixture of acetone and tetrabromoethane.
(2) Hydrocyclone sorting
Japan Plastic Processing Promotion Association has successfully separated plastics by using special hydrocyclone to separate plastics according to density difference. It has successfully separated plastics with density less than water and more than water. If multi-stage cyclone or repeated separation of the same cyclone is used, plastics with more similar density can be separated.
(3) Flotation
Flotation is the most effective way to obtain high quality concentrate in mineral processing. Its mechanism is based on the selective fixation of particles to bubbles. Under natural conditions, most plastics are hydrophobic and floatable, but selective wetting of each component of plastics to be separated can be achieved by controlling liquid-gas interfacial tension, plasma treatment and surfactant adsorption. Flotation is capable of separating waste plastics with similar density and charge properties, and can achieve high separation accuracy.
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