Abandoned circuit boards (PCBs containing electronic components) are classified as hazardous waste (HW49 “other waste” in China’s National Hazardous Waste List, code 900-045-49). They contain heavy metals such as lead, mercury, cadmium, and chromium, as well as toxic substances such as brominated flame retardants (PBDEs). Improper disposal can cause serious soil, water, and air pollution. The processing should follow the principles of “reduction, resource utilization, and harmlessness”, combined with pre-treatment, resource recycling, and environmental protection disposal technologies. The specific process is as follows:
Table of Contents
Ⅰ.Pre treatment: Reduce waste volume, separate recyclable components
Preprocessing is the foundation of subsequent processing, with the aim of removing impurities, separating different materials, and improving resource recovery efficiency.
1. Disassembly
Manually or mechanically disassemble electronic components (such as capacitors, resistors, chips, connectors, etc.) on circuit boards, and separate them into individually recyclable components (such as chips containing precious metals and connectors containing copper). For small or complex circuit boards, disassembly efficiency can be improved through automated equipment such as laser cutting and robotic arm sorting.
2. Crushing and Grinding
Crush the remaining circuit board substrate (mainly composed of resin, glass fiber, and metal) after disassembly, and use crushers (such as shear and impact crushers) to break it into particles of 1-5mm. Then, refine it into finer powder (usually<1mm) through a grinder to fully separate metal and non-metal (resin, glass fiber).
3. Sorting
Using physical methods to separate metal and non-metal components:
-Magnetic separation: separating ferromagnetic metals such as iron and nickel;
-Eddy current sorting: By using a high-frequency magnetic field to generate eddy currents in non-magnetic metals such as copper and aluminum, separation from non metals is achieved;
-Wind selection/reselection: Utilizing the density and air resistance differences between metals and non metals, further separating metals (such as gold, silver, and other precious metal particles) from fine particles.
Ⅱ.Resource recycling: Extracting valuable resources and achieving circular utilization
Abandoned circuit boards contain abundant recyclable resources (such as copper accounting for 8-20%, gold 0.02-0.05%, silver 0.1-0.3%, as well as tin, palladium, etc.), and resource recycling is the core link, which can reduce waste volume and create economic value.
1. Metal recycling technology
-Wet metallurgy (mainstream technology):
Dissolve the broken metal particles with acid (such as nitric acid, sulfuric acid) or cyanide reagents, and separate and purify copper, gold, silver, etc. through precipitation, extraction, electrolysis and other steps. For example, dissolving copper in dilute sulfuric acid and electrolytically depositing pure copper; Leaching gold with thiourea solution, and then recovering it through activated carbon adsorption. This method has a high metal recovery rate (copper can reach over 95%, gold and silver can reach over 90%), but it requires strict treatment of waste liquids containing heavy metals to avoid pollution.
-Pyrometallurgy:
Mix the circuit board with fuel and burn it at high temperature (1000-1500 ℃), where the metal melts and separates (such as copper and tin forming alloys), and non-metallic materials (resin, glass fiber) burn to ashes. But it will produce dioxins (high-temperature decomposition products of brominated flame retardants) and heavy metal smoke, causing serious pollution. Currently, it is only used in a few factories with strict exhaust gas treatment and is gradually being phased out.
-Biometallurgy:
Using organic acids or enzymes secreted by microorganisms (such as ferrous sulfide bacteria) to dissolve metals (such as copper and gold) in circuit boards, and then recovering them through solid-liquid separation. This method is environmentally friendly (low pollution, low energy consumption), but has a long cycle (several days to several weeks), suitable for low-grade metal recovery, and is currently in the experimental application stage.
2. Non metallic recycling and utilization
After sorting, non-metallic components (resin, glass fiber, accounting for about 70%) in the circuit board can be utilized in the following ways:
-Producing recycled composite materials: mixing with plastics, rubber, etc., processing into building templates, trays, decorative materials, etc. (residual heavy metals and brominated flame retardants need to be removed first to avoid toxicity);
-Pyrolysis energy conversion: High temperature pyrolysis (400-800 ℃) in an anaerobic environment produces combustible gases (such as methane and hydrogen) and carbon black, which can be used as fuel. Carbon black is used as a rubber filler (dioxin emissions need to be controlled).
Ⅲ.Harmless disposal: Treat residual residue to avoid secondary pollution
After resource recycling, a small amount of unrecoverable residue will still be generated (such as non-metallic ash containing heavy metals and sludge from waste liquid treatment), which needs to be disposed of harmlessly:
1. Curing/stabilization
Add solidifying agents (such as cement, asphalt, chelating agents) to the residue, fix heavy metals in the solidified body through chemical reactions, and reduce their leaching toxicity (such as chelating agents forming stable chelates with heavy metals). The solidified body must meet the “Pollution Control Standards for Hazardous Waste Landfills” (GB 18598) before being sent to a hazardous waste safe landfill for disposal.
2. High temperature incineration (strict control conditions)
For a small amount of organic toxic residue (such as resin containing brominated flame retardants), it is incinerated at high temperature (≥ 1200 ℃) in a dedicated rotary kiln or fluidized bed, and equipped with a flue gas purification system (such as activated carbon adsorption, bag dust removal, deacidification tower) to remove dioxins, heavy metal smoke and acidic gases (such as HCl, HBr), ensuring that the exhaust emissions meet the standards.
Ⅳ.Environmental and compliance requirements for processing
1. Qualification management: Processing enterprises must obtain a “Hazardous Waste Operation License”, and it is strictly prohibited for unqualified units to dismantle and burn without authorization (illegal processing is the main cause of pollution, such as the release of toxic gases from open-air burning, and the direct discharge of acid washing waste liquid polluting soil and water bodies).
2. Pollution prevention and control: Throughout the entire process, measures must be taken to prevent leakage (such as impermeable floors and waste liquid storage tanks) and scattering (such as enclosed workshops and dust removal equipment). Wastewater, exhaust gas, and solid waste residues must be discharged in compliance with standards or safely disposed of.
3. Policy support: China encourages the research and development of “resource utilization+harmless” technologies (such as low toxicity wet extraction and efficient sorting equipment), and promotes the standardized recycling of electronic waste through the extended producer responsibility system (EPR) (such as enterprises setting up recycling points and assuming responsibility for disposal).
Summarize
The core of disposing of discarded circuit boards is to first recycle resources and then dispose of them harmlessly. Through pre-treatment, metals and non metals are separated, and valuable metals such as copper and gold are recovered using wet metallurgy and other technologies. Non metallic components are used to produce recycled materials, and the remaining residue is safely buried after solidification or high-temperature treatment. The key is to strictly follow the regulations on hazardous waste management, avoid secondary pollution, and improve resource utilization efficiency, achieving a win-win situation between “turning waste into treasure” and environmental protection.
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