At present, the recycling and processing methods of waste PCB mainly include physical method, chemical method and biological method. Physical methods mainly include mechanical crushing, air separation and magnetic adsorption and other technologies. Chemical methods are divided into pyrometallurgy, hydrometallurgy, etc.
1. Stripping of PCB.
The surface of the PCB is generally coated with a layer of paint to protect the metal, and the paint should be removed before recycling. Paint strippers include organic paint strippers and alkaline paint strippers. Organic paint strippers are highly toxic and cause great harm to the human body and the environment. Alkaline paint strippers are relatively less toxic. We got the best formula for paint stripping through experiments: put the cut PCB pieces into 10% sodium hydroxide solution. Add 0.5% Auxiliary A, 0.5% Auxiliary B, 0.05% corrosion inhibitor mercaptobenzotriazole. Heat it in a water bath, and the paint on the surface can be completely removed within 30min, and the exposed metal can be further recovered. Alkaline paint stripping is to separate the paint and the circuit board where the paint is mainly in the original structure, which can be recycled.
2. The Physical Method of PCB Recycling
The physical method is to recycle according to the difference in physical properties such as material density, electrical conductivity, magnetic properties, and surface wettability. The main processes are dismantling, crushing, sorting, recovering precious metals, and disposing of harmful substances.
2.1 The general sequence of selective dismantling for dismantling electronic waste is selecting reusable parts. Dismantle harmful components and classify parts of various materials. Disassembly methods include manual disassembly, mechanical disassembly, and automatic disassembly. The automatic disassembly of the PCB uses methods such as bath washing or hot air heating to melt the solder, and then use a vacuum clamp or a robot to remove the PCB surface components.
2.2 Breaking. The crushing of PCB includes impact crushing, extrusion crushing and shearing crushing. At present, the most successful application is the ultra-low temperature freezing and crushing technology, which can crush tough materials to 0.074mm after embrittlement at low temperature, so that the metal and non-metal are completely dissociated. Two-stage crushing technology is often used in China, that is, coarse crushing with a shearing crusher, and then finely crushing the PCB particles to a specified particle size with a dry or wet crusher to achieve complete dissociation of metals and non-metals.
2.3 Sorting. The pulverized substances are sorted according to the differences in the density, particle size, magnetic conductivity, electrical conductivity and other characteristics of each component. Usually, there are dry and wet sorting. Dry sorting includes dry screening, magnetic separation, electrostatic, density and eddy current sorting. Wet separation includes hydrocyclone classification, flotation, hydraulic shaker, etc. Wet separation has a high recovery rate, but also a high cost. The reagents used pollute the environment, and the waste residue and liquid after separation cause secondary pollution to the environment. Dry sorting has relatively low cost and low pollution, and the main disadvantage is that the sorting rate of fine particles is low.
The movement of particles in a fluid depends not only on the particle density, but also on its size and shape. To reduce the scale effect and control the relative movement of particle density, the metals in PCB were separated and enriched, and the effect of air separation and enrichment of metals under different particle sizes was investigated.
In summary, the physical method of recycling PCB is simple, easy to scale, and produces relatively small secondary pollution, low energy consumption, low cost, high separation efficiency, and meets the requirements of environmental protection and resource utilization. However, there are disadvantages such as the overlapping of various physical properties, the inability to achieve complete separation between metals, and the large initial investment.
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