With the acceleration of electronic product renewal, the amount of discarded printed circuit board(PCB), the main component of electronic waste, is also increasing. The environmental pollution caused by waste PCB has also attracted the attention of many countries.In the waste PCB, there are heavy metals such as lead, mercury, hexavalent chromium, and other toxic chemicals such as polybrominated biphenyl(PBB) and polybrominated diphenyl ether(PBDE) as flame retardants. These substances will cause great pollution to groundwater and soil in the natural environment, and bring great harm to people's life and physical and mental health. On the waste PCB, there are nearly 20 kinds of nonferrous and rare metals, which have high recycling value and economic value. It is a real mineral deposit waiting to be mined.
The physical method is to use the mechanical means and PCB physical properties of different methods to achieve recovery.
The purpose of crushing is to make the metal in the waste circuit board as far as possible and organic matter dissociation, in order to improve the separation efficiency.It is found that when the metal is broken at 0.6mm, the dissociation of the metal can reach 100% basically, but the choice of crushing mode and series depends on the subsequent process.
Separation is to use the material density, particle size, electrical conductivity, magnetic conductivity and surface properties and other physical properties of the difference to achieve separation. At present, there are many widely used technologies, such as wind table technology, flotation separation technology, cyclone separation technology, floatation separation technology and eddy current separation technology.
2. Supercritical technology treatment
Supercritical fluid extraction (SFE) technology is a purification method that uses pressure and temperature to influence the solubility of sfe without changing the chemical composition. Compared with traditional extraction methods, the supercritical CO2 extraction process has advantages of environmental friendliness, convenient separation, low toxicity, little or no residue, and can be operated at room temperature.
The main research directions of using supercritical fluid to treat waste PCB mainly focus on two aspects: first, because supercritical CO2 fluid has the extraction ability of resin and brominated flame retardant components in printed circuit board. When the resin adhesive material in the printed circuit board is removed by supercritical CO2 fluid, the copper foil layer and glass fiber layer in the printed circuit board can be easily separated, thus providing a possibility for the efficient recovery of materials in the printed circuit board. Second, directly utilize supercritical fluid to extract metals from waste PCB. Wai etd. reported the research results of extracting Cd2+, Cu2+, Zn2+, Pb2+, Pd2+, As3+, Au3+, Ga3+ and Sb3+ from simulated cellulose filter paper or sand with lithium fluorinated diethyl dithiocarbamate (LiFDDC) as complexant, and the extraction efficiency was over 90%.
Supercritical treatment technology also has a lot of defects, such as: high selectivity of extraction need to add entrainment agent, harm to the environment; High extraction pressure requires high equipment requirements; High temperature is used in the extraction process, so energy consumption is high.