Views: 33 Author: Site Editor Publish Time: 2019-09-06 Origin: Site
Plastic recycling ways | Five ways to completely separate plastics and metals!
In daily life, there are many kinds of plastic products, but there are few pure plastic products. Plastic products often contain metals and even large amounts of rare metals because of performance requirements. How to completely separate metal from plastic is an important part of plastic recycling.
According to whether the plastic and metal have a clear dividing line to divide the plastic products, the plastic products can be divided into the connected plastic products and the phase-fused plastic products. The former has a clear boundary between plastic and metal in the structure, such as plastic products molded by in-mold injection molding technology, while the latter has no obvious boundary line between plastic and metal, such as circuit board.
Whether it is a junction-type plastic product or a phase-blown plastic product, there is a corresponding method in the industry for recovering metal from these two types of plastic products. High temperature separation, chemical immersion and freeze crushing can be used to recover metals from phased plastic products, while freeze-crushing, pyrometallurgy and wet metallurgy can be used to recover from phase-fused plastic products. metal.
(1) High temperature separation method
The so-called high-temperature separation method is to heat the plastic above the glass transition temperature or above the melting point. When the plastic is above the glass transition temperature and the texture is soft, the plastic can be separated from the metal by hand or device, and when the plastic is heated to the melting point. Above the temperature, the plastic will automatically separate from the metal due to its melting state. Commonly used high temperature separation methods include incineration and heat transfer oil heating.
(2) Chemical immersion method
The chemical immersion method firstly immerses the plastic product in a mixture of cyclohexanone and butyl ester. After the plastic product is softened by the corrosion of the chemical syrup, the plastic and the metal insert are separated by hand. However, cyclohexanone and butyl ester are both toxic substances, cyclohexanone can irritate the skin and have anesthetic and carcinogenic effects on the human body. But butyl ester is easy to induce a variety of neuritis, and can also cause allergic reactions.
Chemical immersion is the first recycling method used by most plastics companies. The recycling efficiency of this recycling method is relatively low. Ordinary plastic products must be immersed in the mixture for more than 1 hour before they can be separated from the metal by hand. The heat transfer oil heating method can soften or melt the plastic in just tens of seconds. The recycling efficiency of the two is not comparable. In addition, chemical immersion has been gradually replaced by other recovery methods due to the toxicity of cyclohexanone and butyl ester.
(3) Freezing and breaking method
The working principle of the cryogenic crushing method is to first use the liquid nitrogen to freeze and embrittle the plastic in the plastic product, and then pulverize the plastic by mechanical force, and the metal in the plastic product is retained due to its high strength.
The freeze-crushing method using liquid nitrogen as a freezing medium has the following characteristics: 1 The boiling point of liquid nitrogen is -196 ° C, which can make the plastic brittle quickly; 2 The liquid nitrogen is inert and does not cause harmful gas to the plastic, which is environmentally friendly and The human body is harmless; 3 liquid nitrogen is cheap, but the storage cost is high. At present, the commonly used crushing methods include grinding and crushing methods, impact crushing methods, shear crushing methods, and extrusion crushing methods.
The PCB is one of the phase-fused plastic products. In the industrial sector, pyrometallurgy and wet metallurgy are the most commonly used methods for recovering precious metals from waste PCBs. Before recycling the PCB by pyrometallurgy or wet metallurgy, the PCB must be pre-treated to separate the metal and non-metallic components in the PCB for subsequent processing.
The material used to form the main body of the PCB is epoxy resin. The epoxy resin has a crisp texture and is poor in heat resistance, easily broken and melted. Therefore, most companies currently use mechanical separation after mechanical crushing to separate metal and non-metal from PCB boards.
(4) Fire metallurgy
The use of pyrometallurgy to recover the metal in the PCB is to separate the non-metallic substances in the PCB by the high temperature of the metallurgical furnace, and dissolve the metal in the plastic product in other metals or molten salts, and then The desired metal species are separated by subsequent processing. In the usual recycling process, the non-metallic substances in the PCB are mainly the substrate, and the substrate floats above the solution due to the high temperature and is then removed. The metal material must be separated from the precious metal by refining or electrolysis.
The fire metallurgy method has the following characteristics: 1 simple operation and high recovery efficiency; 2 applicable electronic waste types; 3 easy to produce environmental pollution; 4 equipment is relatively expensive and energy-consuming.
(5) Wet metallurgy
The basic working principle of the wet metallurgy method is to pretreat the metal particles, and then place the metal particles in an acidic or alkaline solution for chemical treatment or organic solvent extraction, separation of impurities, extraction of metals and their compounds.
The wet metallurgy method has the following process characteristics: 1 It can better recover precious metals such as Au and Ag, and non-ferrous metals such as Cu and Zn, and the recovery cost is low; 2 only the metal recovery effect on the surface of the electronic waste is better, and the opposite The internal metal recovery effect is relatively poor; 3 the leachate in the recovery process is not only toxic, but also harmful to the environment.