Brief description of the processing method of waste circuit board

As electronic products update faster, the number of printed circuit boards (PCBs) that are a major component of e-waste is also growing. The environmental pollution caused by waste PCBs has also attracted the attention of various countries. In waste PCBs, heavy metals such as lead, mercury, hexavalent chromium, and toxic chemicals such as polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE), which are flame retardant components, will be in the natural environment. Groundwater and soil cause huge pollution, which brings great harm to people's life and physical and mental health. On the waste PCB, containing nearly 20 kinds of non-ferrous metals and rare metals, it has high recovery value and economic value, and it is a real mine waiting for mining.

1 Physical method

The physical method is a method of recycling using mechanical means and physical properties of the PCB.

1.1 broken

The purpose of the crushing is to dissociate the metal in the waste circuit board as much as possible from the organic matter to improve the sorting efficiency. It has been found that when the fracture is at 0.6 mm, the metal can basically reach 100% dissociation, but the choice of the crushing method and the number of stages depends on the subsequent process.

1.2 Sorting

Separation is achieved by utilizing differences in physical properties such as density, particle size, electrical conductivity, magnetic permeability, and surface properties of the material. At present, there are widely used wind shaker technology, flotation separation technology, cyclone separation technology, floating sedimentation separation and vortex sorting technology.

2. Supercritical technology processing

Supercritical fluid extraction technology refers to a purification method for extracting and separating using the influence of pressure and temperature on the solubility of supercritical fluid without changing the chemical composition. Compared with the traditional extraction method, the supercritical CO2 extraction process has the advantages of environmental friendliness, convenient separation, low toxicity, little or no residue, and operation at room temperature.

The main research directions in the treatment of waste PCBs with supercritical fluids are concentrated in two aspects: First, the supercritical CO2 fluid has the ability to extract the resin and brominated flame retardant components in the printed circuit board. When the resin bonding material in the printed wiring board is removed by the supercritical CO2 fluid, the copper foil layer and the glass fiber layer in the printed wiring board can be easily separated, thereby providing efficient recycling of materials in the printed wiring board. . Second, the use of supercritical fluid directly extracts the metal in the waste PCB. Wai et al. reported the extraction of Cd2+, Cu2+, Zn2+, Pb2+, Pd2+, As3+, Au3+, Ga3+ from simulated sample cellulose filter paper or sand using lithium fluoride dithiocarbamate (LiFDDC) as a complexing agent. According to the results of Sb3+, the extraction efficiency is above 90%.

Supercritical treatment technology also has great defects such as: high selectivity of extraction requires the addition of entrainer, which is harmful to the environment; high extraction pressure requires high equipment; high temperature is used in the extraction process, so energy consumption is high.

3 Chemical methods

Chemical processing techniques are processes that utilize different chemical stability of various components in the PCB.

3.1 Heat treatment method

The heat treatment method is mainly a method of separating organic matter and metal by means of high temperature. It mainly includes incineration method, vacuum cracking method, microwave method and the like.

3.1.1 Incineration

The incineration method breaks the electronic waste to a certain particle size, feeds it into an incinerator for incineration, and decomposes the organic components to separate the gas from the solid. The residue after incineration is a bare metal or its oxide and glass fiber, which can be recovered by physical and chemical methods after being pulverized. The gas containing organic components is discharged into the secondary incinerator after combustion treatment. The disadvantage of this method is that it produces a large amount of exhaust gas and toxic substances.

3.1.2 Lysis method

Cracking is also called retorting in the industry. It is to place the electronic waste in a container and heat it under the condition of insulating air. The temperature and pressure are controlled, and the organic matter is decomposed and converted into oil and gas. After being collected by condensation, it can be recovered. Unlike the incineration of electronic waste, the vacuum pyrolysis process is carried out under anaerobic conditions, so it can be suppressed. The production of English and furan has less exhaust gas and less environmental pollution.

3.1.3 Microwave processing technology

The microwave recovery method first breaks the electronic waste and then uses microwave heating to decompose the organic matter. Heating to about 1400 °C, the glass fiber and metal are melted to form a vitrified material. After the material is cooled, gold, silver and other metals are separated as beads, and the remaining glass material can be recycled and used as building materials. This method is significantly different from the traditional heating method, and has significant advantages such as high efficiency, rapidity, high resource recycling efficiency, and low energy consumption.

3.2 Hydrometallurgy

Hydrometallurgical technology mainly utilizes the characteristics that metals can be dissolved in acid liquids such as nitric acid, sulfuric acid and aqua regia, and the metal is removed from the electronic waste and recovered from the liquid phase. It is currently the most widely used method for handling electronic waste. Compared with pyrometallurgy, hydrometallurgy has the advantages of less exhaust gas emission, easy disposal of residues after metal extraction, significant economic benefits, and simple process flow.

4 Biotechnology

Biotechnology solves the problem of metal recovery by utilizing the adsorption of microorganisms on the mineral surface and the oxidation of microorganisms. Microbial adsorption can be divided into two types: the use of microbial metabolites to immobilize metal ions and the use of microorganisms to directly immobilize metal ions. The former is fixed by hydrogen sulfide generated by bacteria. When the surface of the cell is adsorbed, the ions can form a floc, and the latter is formed by using the oxidizing property of ferric ions to oxidize other metals in the precious metal alloy such as gold. It becomes soluble and enters the solution, so that the precious metal is exposed to facilitate recovery. The extraction of gold and other precious metals by biotechnology has the advantages of simple process, low cost and convenient operation, but the leaching time is longer and the leaching rate is lower, which is not actually put into use at present.


E-waste is a valuable resource. The research and application of metal recycling technology to enhance e-waste is of great significance both economically and environmentally. Due to the complex and diverse characteristics of e-waste, it is difficult to recover the metal by any technology. The future development trend of e-waste technology should be: industrialization of processing forms, maximization of resource recovery, and scientific processing technology. In summary, researching the recycling of waste PCB can not only protect the environment, prevent pollution, but also facilitate the recycling of resources, save a lot of energy, and promote the sustainable development of the economy and society.

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