At present, the main recycling methods of waste PCB multilayer circuit boards include physical, chemical and biological methods. Physical method mainly includes mechanical crushing, air separation and magnetic adsorption and other technologies; Chemical method is divided into pyrometallurgy, hydrometallurgy and so on.
1. Paint removal of PCB multilayer circuit board.
PCB multilayer surface is generally coated with a layer of paint to protect the metal, and the paint should be removed before recycling. Paint remover has organic paint remover and alkaline paint remover, organic paint remover is toxic, harmful to human body and environment, alkaline paint remover toxicity is relatively small. We obtained the best formula for paint removal through experiments: put the cut PCB multilayer board small pieces into 10% sodium hydroxide solution. Add 0.5% agent A, 0.5% agent B, 0.05% corrosion inhibitor mercaptobenzotriazole, and heat the water bath. Within 30 minutes, the paint on the surface can be completely removed, and the exposed metal can be further recovered. Alkaline paint removal is to separate the place where the paint contacts with the circuit board. The paint mainly exists in the original structure, which can be recycled.
2. Physical method of recycling PCB multilayer board
Physical method is based on the material density, electrical conductivity, magnetism, surface wettability and other physical characteristics of the difference in recovery. The main processes are dismantling, crushing, sorting, recovery of precious metals, disposal of harmful substances.
(1) The general sequence of selective disassembly of electronic waste is: select reusable parts. Disassemble harmful components and categorize parts of various materials. Disassembly methods include manual disassembly, mechanical disassembly and automatic disassembly. Automatic disassembly of PCB multilayer board adopts bath or hot air heating method to dissolve solder, and then uses vacuum clamp or robot to remove PCB multilayer board surface components.
(2) broken. The crushing of PCB multilayer board includes impact crushing, extrusion crushing and shear crushing. At present, the most successful application is ultra-low temperature frozen crushing technology, which can pulverize tough materials to 0.074mm after embrittlement at low temperature, so that metal and non-metal can completely dissociate. Two-stage crushing technology is often used in China, that is, the shear crusher is coarse crushed, and then the multilayer board particles are finely crushed to the specified particle size with dry or wet crusher to achieve complete dissociation of metal and non-metal.
(3) Separation The pulverized material is sorted according to the differences in the density, particle size, magnetic permeability, electrical conductivity and other characteristics of each component, usually with dry and wet separation. Dry separation includes dry screening, magnetic separation, electrostatic separation, density and eddy current separation. Wet separation includes hydrocyclone classification, flotation, hydraulic shaker and so on. Wet separation rate is high, but the cost is also high, the reagent used pollute the environment, the waste residue and liquid after separation cause secondary pollution to the environment. Dry separation is relatively low cost and low pollution, the main disadvantage is the low separation rate of fine particles.
Density sorting: The movement of particles in a fluid depends not only on particle density, but also on its size and shape. In order to reduce the scale effect and control the relative movement of particle density. Metal separation and enrichment in PCB multilayer board were studied. The effect of gas separation and enrichment in different particle sizes was investigated.
To sum up, the physical recovery process of PCB board is simple, easy to scale, and the secondary pollution is relatively small, low energy consumption, low cost, high separation efficiency, in line with the requirements of environmental protection and resource recovery. However, due to the overlapping of various physical properties, it is impossible to realize the complete separation of metals, and the initial investment is large.