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Separation technology of waste plastics

Plastics have good chemical stability, electrical insulation, thermal insulation and mechanical properties, and low density, easy processing, so they are widely used in agriculture, machinery industry, construction, transportation, national defense, medical and many other fields.

However, with the increasing use of plastic products, more and more plastic waste is produced, which has a serious impact on the ecological environment. Therefore, how to recycle and reuse waste plastics is an important issue for the sustainable development of plastics industry, and also the key to protect the ecological environment.

The recycling methods of waste plastics mainly include energy recovery, physical recovery and chemical recovery,

Energy recovery is to obtain the heat energy of waste plastics by burning; Physical recovery is to melt, granulate or process waste plastics into new products after sorting, cleaning and crushing; Chemical recovery is to convert the organic components in waste plastics into petrochemical raw materials by thermal decomposition or chemical decomposition.

For the separation technology of waste plastics and related equipment, the main recycling methods of waste plastics are physical recycling. Physical recycling has the advantages of simple processing technology, low equipment cost and less investment. It has a considerable production scale in China’s waste plastics recycling industry, and so far, It is also one of the main ways to recycle waste plastics in China.

For a large number of waste plastics, we need to carry out pretreatment. If the mixed waste plastics are directly machined, the product value is low, the performance is poor and unstable. Therefore, in order to increase the recycling of waste plastics and improve the value of recycled products, the separation of mixed plastics is particularly important.

The separation of mixed waste plastics is generally based on the differences of chemical composition, light transmittance, density and other characteristics of plastics. The mixed waste plastics are purposefully separated into a single component to obtain a unified category of waste plastics. After separation, the purity of specific materials can meet the relevant requirements, which is conducive to making better recycled products with higher value, Maximize economic value.

For waste plastics separation technology, it can be divided into traditional separation method and modern separation method.

Traditional separation methods include manual separation, density separation and wind separation; Modern separation methods include color separation, near infrared spectroscopy (NIR) separation, etc. Different methods are effective for separation of mixed waste plastics, but these methods have their own advantages and disadvantages in the scope of use, separation efficiency, accuracy and industrial application.

Manual separation

Manual sorting is to use the identification experience of operators or with the help of simple instruments to classify some large plastic products, such as various household appliance shells, plastic furniture, large children’s toys, etc. This method requires less equipment, but requires higher separation experience of operators. The effect of manual separation is difficult to replace by machine, but it has the disadvantages of low efficiency and high labor intensity.

The picture shows the operator sorting manually

Screening and sorting

Screening and sorting is one of the technical ways to effectively separate the materials by rolling and vibrating the broken waste plastics through a specific mesh sieve. The biggest defect of this method is that it can not identify the types of waste plastics. In practical application, it is often combined with other separation technologies to obtain the best separation efficiency of waste plastics.

The picture shows the screening and sorting equipment

Density separation

Density separation is a technology to separate waste plastics by using the density difference between different kinds of plastics. In industry, the broken mixed waste plastics are successively passed through the separation tank containing different density liquids (commonly used liquids are tap water, saturated salt water, calcium chloride solution, etc.), and then the plastics are classified according to the floating and sinking situation of the plastics in the liquid. This method is easy to operate and suitable for separating plastic particles with large density difference. The disadvantage is that it will produce a large amount of waste water that needs further treatment, increase the economic cost, and it is not easy to identify the plastics with similar density.

The figure shows the density sorting diagram

Wind Separation

Wind separation is the use of plastic particles in the air flow due to particle size, shape, density and other differences for effective separation, suitable for the separation of materials with large density difference. In the wind separation, the broken plastics are sprayed in the separation device, and the wind blows in from the transverse or reverse direction. The resultant force difference between the resistance of different plastics to the air flow and their own gravity is used for separation. This method is the most widely used separation method of mixed waste plastics. It is often used to separate plastic particles with uniform shape and size but different density. It can also be used to remove sand, metal and other impurities in plastics.

The picture shows the wind separation equipment

Electromagnetic separation and eddy current separation

Electromagnetic separation is a method that uses magnetic field produced by separation equipment to separate magnetic materials from the mixed waste plastics. This method is mainly used to remove the ferromagnetic metal impurities mixed in the waste plastics. On the other hand, eddy current separation is a separation technology which uses different conductivity of materials, which is suitable for separation between high conductivity and low density materials and between conductor and non conductor. Eddy current separation can effectively recover the non-ferrous metals (such as copper, lead, zinc, etc.) in the waste plastics. Electromagnetic separation and eddy current separation can be used together to form a high efficiency process, and the purpose of removing ferromagnetic and non ferromagnetic metal impurities in waste plastics can be achieved at the same time.

The figure shows the eddy current sorting equipment

Color sorting

Color separation is a technology of separation according to the color difference of plastics. The working principle is to make waste plastics reflect different brightness after passing through several color filters, and convert them into current values through computer system and identify them. When separating devices, different color plastics can be effectively classified. The method is suitable for separating plastic products with different colors, but the disadvantage is that it can not be used to identify the material types of waste plastics.

The picture shows the color sorting equipment

Optical selection

Different kinds of plastics have different spectral properties. By scanning the polymers with different light media, the materials of waste plastics can be identified quickly and accurately. At present, the optical separation methods used in the identification of waste plastics mainly include X-ray sorting (xrt/xrf), NIR, Mir, LIBS, etc. Near infrared spectrum separation is the most widely used and large-scale optical separation method in the field of waste plastics separation because of its advantages of easy acquisition of spectral analysis signal and abundant information, and the overlapping of spectral bands and low absorption intensity of the spectrum area. This method is suitable for most plastics identification, and it is fast, reliable and sensitive, but the disadvantage is that it is impossible to recognize dark plastics. Therefore, it is often used in the separation process of waste plastics, combined with color separation, to improve the sorting effect.

The photo shows intelligent optical selection equipment

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