1. Currently, widely used technologies include wind shaker technology, flotation separation technology, cyclone separation technology, float-sink separation, and eddy current separation technology.
2. **Supercritical Technology Processing Method**
Supercritical fluid extraction technology refers to a purification method that utilizes the influence of pressure and temperature on the solubility of supercritical fluids for extraction and separation without altering the chemical composition. Compared to traditional extraction methods, supercritical CO2 extraction offers advantages such as environmental friendliness, convenient separation, low toxicity, minimal residue, and the ability to operate at room temperature.
The primary research directions regarding the use of supercritical fluids to treat waste PCBs focus on two aspects: First, supercritical CO2 fluid can effectively extract resin and brominated flame retardant components from printed circuit boards. When the resin bonding material is removed using supercritical CO2, the copper foil layer and glass fiber layer can be easily separated, enabling efficient material recycling from PCBs. Second, supercritical fluid can directly extract metals from waste PCBs. Wai et al. reported the extraction of Cd²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Pd²⁺, As³⁺, Au³⁺, and Ga³⁺ from simulated cellulose filter paper or sand using lithium fluorinated diethyldithiocarbamate (LiFDDC) as a complexing agent. Research on Sb³⁺ showed extraction efficiencies exceeding 90%.
However, supercritical processing technology also has significant drawbacks: the high selectivity of extraction necessitates the addition of entrainers, which can be harmful to the environment; the relatively high extraction pressures require expensive equipment; and the high temperatures used in the extraction process lead to increased energy consumption.
3. **Chemical Method**
Chemical treatment technology involves using the differing chemical stabilities of various PCB components for extraction.
3.1 **Heat Treatment Method**
The heat treatment method primarily separates organic matter and metals using high temperatures. It includes incineration, vacuum cracking, microwave methods, and others.
3.1.1 **Incineration Method**
The incineration method involves crushing electronic waste to a specific particle size and sending it to a primary incinerator for combustion, decomposing organic components and separating gas from solids. The residue after incineration consists of bare metals or their oxides and glass fiber, which can be recovered through physical and chemical methods after further crushing. The gases containing organic components then enter a secondary incinerator for combustion treatment before being discharged. A significant disadvantage of this method is the substantial production of waste gases and toxic substances.
3.1.2 **Cracking Method**
Pyrolysis, also known as dry distillation, involves heating electronic waste in a container under air isolation, controlling temperature and pressure to decompose the organic matter into oil and gas, which can be recovered after condensation and collection. Unlike incineration, the vacuum pyrolysis process occurs in an oxygen-free environment, significantly reducing the production of dioxins and furans and minimizing environmental pollution.
3.1.3 **Microwave Processing Technology**
The microwave recovery method first crushes electronic waste, followed by microwave heating to decompose organic matter. Heating to approximately 1400 degrees Celsius melts glass fiber and metals into a vitrified substance. After cooling, gold, silver, and other metals are separated in bead form, while the remaining glass can be recycled as building materials. This method significantly differs from traditional heating methods, offering advantages such as high efficiency, rapid processing, increased resource recovery, and low energy consumption.
3.2 **Hydrometallurgy**
Hydrometallurgy technology primarily utilizes the solubility of metals in acidic liquids like nitric acid, sulfuric acid, and aqua regia to extract metals from electronic waste and recover them from the liquid phase. This method is currently the most widely used for processing electronic waste. Compared to pyrometallurgy, hydrometallurgy has advantages such as reduced exhaust gas emissions, easier disposal of residues post-metal extraction, significant economic benefits, and a simpler process flow.
4. **Biotechnology**
Biotechnology leverages the adsorption of microorganisms on mineral surfaces and microbial oxidation to recover metals. Microbial adsorption can be categorized into two types: using microbial metabolites to immobilize metal ions and directly immobilizing metal ions with microbes. The former involves utilizing hydrogen sulfide produced by bacteria to fix metal ions, leading to floc formation and sedimentation when saturation is reached. The latter employs the oxidizing properties of ferric ions to oxidize other metals in precious metal alloys, such as gold, making them soluble for easier recovery. Although the extraction of precious metals like gold through biotechnology offers a simple process, low cost, and ease of operation, it suffers from longer leaching times and lower rates, limiting its practical application.
**Concluding Remarks**
E-waste is a valuable resource. Strengthening research and application of metal recycling technologies for e-waste is crucial from both economic and environmental perspectives. Due to the complex and diverse nature of e-waste, no single technology can effectively recover metals. The future development trend of e-waste processing technology should focus on industrialized processing forms, maximizing resource recycling, and implementing scientific processing techniques. In summary, studying the recycling of discarded PCBs not only protects the environment and prevents pollution but also facilitates resource recovery, conserves energy, and promotes sustainable economic and social development.
—
Feel free to adjust any specific terminology or phrasing to better fit your audience!
