1 Physical Law
The physical method utilizes mechanical means and differences in PCB physical properties to achieve recycling.
1.1 Crushing
The objective of crushing is to maximize the separation of metals from organic materials in the waste circuit board, thereby enhancing separation efficiency. Studies indicate that crushing to 0.6 mm allows for nearly 100% metal dissociation; however, the choice of crushing method and the number of stages are contingent upon the subsequent processing steps.
1.2 Sorting
Separation employs variations in material density, particle size, conductivity, magnetic permeability, and surface characteristics to facilitate the process. Currently, widely adopted technologies include wind shaker technology, flotation separation, cyclone separation, float-sink separation, and eddy current separation.
**2 Supercritical Technology Treatment Method**
Supercritical fluid extraction technology is a purification method that utilizes the effects of pressure and temperature on the solubility of supercritical fluids to achieve extraction and separation without altering the chemical composition. Compared to traditional extraction methods, the supercritical CO2 extraction process offers several advantages, including environmental friendliness, ease of separation, low toxicity, minimal residue, and the ability to operate at room temperature.
The primary research directions for using supercritical fluids to treat waste circuit boards focus on two main areas: First, supercritical CO2 has the capability to extract resin and brominated flame retardant components from printed circuit boards. By using supercritical CO2 to remove the resin bonding material, the copper foil layer and glass fiber layer in the circuit board can be easily separated, thus enabling efficient material recycling. Second, supercritical fluid can be employed directly to extract metals from waste circuit boards. Wai et al. reported successfully extracting Cd2+, Cu2+, Zn2+, Pb2+, Pd2+, As3+, Au3+, Ga3+, and Ga3+ from simulated cellulose filter paper or sand using lithium fluorinated diethyldithiocarbamate (LiFDDC) as a complexing agent. The extraction efficiency for Sb3+ reached above 90%.
However, supercritical processing technology has significant drawbacks, including high extraction selectivity that necessitates the use of entrainers, which can harm the environment; the need for high extraction pressures requiring specialized equipment; and elevated temperatures during extraction, leading to high energy consumption.
**3 Chemical Method**
Chemical treatment technology involves a process that exploits the differences in chemical stability among various components in PCBs for extraction.
**3.1 Heat Treatment Method**
The heat treatment method primarily focuses on separating organic matter from metals using high temperatures. This includes incineration, vacuum cracking, and microwave methods.
**3.1.1 Incineration**
The incineration method involves crushing electronic waste to a specific particle size and feeding it into a primary incinerator. Here, organic components decompose, separating gas from solid residues. The remaining ash consists of bare metals or their oxides along with glass fiber, which can be recovered through physical and chemical methods after further crushing. The gas containing organic compounds proceeds to a secondary incinerator for combustion before being discharged. A significant downside to this method is the production of considerable waste gas and toxic substances.
**3.1.2 Cracking Method**
Pyrolysis, also referred to as dry distillation, entails heating electronic waste in a container while isolating it from air. By controlling temperature and pressure, organic materials are decomposed into oil and gas, which can be condensed and collected. Unlike the incineration of electronic waste, the vacuum pyrolysis process occurs under oxygen-free conditions, minimizing the formation of dioxins and furans, thus reducing waste gas generation and environmental impact.
**3.1.3 Microwave Processing Technology**
The microwave recovery method begins with crushing electronic waste, followed by microwave heating to decompose organic materials. Heating to approximately 1400 degrees Celsius melts glass fiber and metal, forming a vitrified substance. Once cooled, precious metals such as gold and silver can be separated into beads, while the residual glass can be recycled as construction materials. This method stands out from traditional heating techniques due to its high efficiency, rapid processing, excellent resource recovery, and low energy consumption.
**3.2 Hydrometallurgy**
Hydrometallurgy technology primarily leverages the solubility characteristics of metals in acidic solutions such as nitric acid, sulfuric acid, and aqua regia to extract and recover metals from electronic waste. This is currently the most prevalent method for processing e-waste. In comparison to pyrometallurgy, hydrometallurgy offers benefits like reduced exhaust gas emissions, easier disposal of post-extraction residues, significant economic advantages, and a straightforward process flow.
**4 Biotechnology**
Biotechnology employs the adsorption capabilities of microorganisms on mineral surfaces and their oxidation processes to address metal recovery challenges. Microbial adsorption can be categorized into two types: one involves using microbial metabolites to immobilize metal ions, while the other involves direct immobilization by microbes. The former utilizes hydrogen sulfide produced by bacteria to bind metal ions. When bacterial surfaces absorb ions to saturation, flocs can form and settle. The latter approach uses the oxidizing properties of ferric ions to oxidize metals in precious metal alloys, such as gold, making them soluble and facilitating recovery. Although the biotechnology approach to extracting precious metals like gold has advantages such as a simple process, low cost, and ease of operation, it currently suffers from longer leaching times and lower leaching rates, preventing widespread practical application.
**5 Concluding Remarks on Disposal of Waste Circuit Boards**
E-waste represents a valuable resource. Enhancing research and application of metal recycling technologies for e-waste holds great significance from both economic and environmental perspectives. Given the complex and varied nature of e-waste, recovering its metals through any single technology proves challenging. The future development of e-waste processing technology should emphasize industrial-scale processing, maximum resource recycling, and scientific advancements in processing methods. In conclusion, investigating the recycling of discarded PCBs not only helps protect the environment and prevent pollution but also facilitates resource recovery, conserves significant energy, and promotes sustainable economic and social development.
