At present, the recycling methods of waste PCB mainly include physical method, chemical method and biological method. Physical methods mainly include mechanical crushing, air separation and magnetic adsorption; chemical methods include pyrometallurgy and hydro-metallurgy.
1) Paint removal of PCB.
PCB surface is generally coated with a layer of paint to protect metal, and the paint should be removed before recycling. The organic paint removers are organic paint remover and alkaline paint remover. The organic paint remover has high toxicity and great harm to human body and environment. We get the best paint removal formula by experiment: put the cut PCB chip into 10% sodium hydroxide solution. Adding 0.5% additive A, 0.5% additive B and 0.05% mercaptobenzotriazole, the paint on the surface can be completely removed within 30min, and the exposed metal can be further recovered. Alkaline depainting is to separate the paint from the contact place of PCB, and the paint is mainly in the original structure, which can be recycled.
2) Physical method of PCB recycling
Physical method is based on the material density, conductivity, magnetism, surface wettability and other physical characteristics of recovery. The main processes include dismantling, crushing, sorting, recovering precious metals and treating harmful substances.
<1> The general order of selective disassembly of e-waste is: select reusable parts. Disassemble harmful components and classify parts of various materials. In the automatic disassembly of PCB, the soldering tin is dissolved by bath or hot air heating, and then the surface components of PCB are removed by vacuum clamp or robot.
<2> Crushing. PCB crushing includes impact crushing, extrusion crushing and shearing crushing. At present, the technology of ultra-low temperature freeze crushing is more successful. The tough material can be crushed to 0.074 mm after embrittlement at low temperature, so that the metal and nonmetal can be completely dissociated. In China, two-stage crushing technology is often used, that is, shearing crusher is used for coarse crushing, and then dry or wet crusher is used to finely crush PCB particles to the specified size, so as to realize the complete dissociation of metal and nonmetal.
<3> Separation the crushed materials are sorted according to the density, particle size, magnetic conductivity, conductivity and other characteristics of each component, usually dry separation and wet separation. Dry separation includes dry screening, magnetic separation, electrostatic separation, density and eddy current separation. Wet separation includes hydro-cyclone classification, flotation, hydraulic shaking table, etc. The recovery rate of wet separation and the cost both are high. The reagent used pollutes the environment, and the waste residue and waste liquid after separation cause secondary pollution to the environment. Dry separation has the advantages of low cost and low pollution. The main disadvantage is the low separation rate of fine particles.
-Density separation: the movement of a particle in a fluid depends not only on its density, but also on its size and shape. In order to reduce the scale effect and control the relative movement of particle density. The separation and enrichment of metals in PCB were carried out, and the effect of gas separation and enrichment of metals under different particle sizes was investigated.
To sum up, the physical method of PCB recovery process is simple, easy to scale, and produces relatively small secondary pollution, low energy consumption, low cost, high separation efficiency, in line with the requirements of environmental protection and recycling. However, there are some disadvantages, such as overlapping of various physical properties, unable to realize the complete separation between metals and large investment in the early stage.