Against the backdrop of rapid development in global information technology and the electronics industry, the popularity of electronic products is constantly increasing, and the cycle of product updates is shortening, leading to an explosive growth in the amount of electronic waste generated. As a core component of electronic products, circuit boards play an important role in discarded electronic devices. According to statistics, the gold content in each ton of waste circuit boards can reach several hundred grams, and the copper content is as high as over 20%, with a metal grade much higher than that of primary ore. However, waste circuit boards also contain various toxic and harmful substances such as lead, mercury, cadmium, brominated flame retardants, etc. If disposed of casually or using outdated recycling processes, it will cause serious pollution to soil, water bodies, and the atmosphere.
At present, the recycling and treatment technologies for waste circuit boards can be mainly divided into several categories, including mechanical physical methods, wet metallurgy, pyrometallurgy, and emerging bio leaching technologies.
1. Mechanical physics method is a technology that utilizes the differences in physical properties (such as density, conductivity, magnetism, etc.) of each component of a circuit board for separation and recovery. Typical processes include crushing, screening, magnetic separation, eddy current separation, and electrostatic separation.
The main advantages of this method are relatively low equipment investment, mature process, no involvement of chemical reactions, and minimal secondary pollution.
2. Wet metallurgy is a method of recovering valuable metals from a solution by dissolving metals in a circuit board using acid, alkali, or other chemical reagents, and then using steps such as displacement, precipitation, solvent extraction, and electrodeposition. This technology has a high recovery rate for precious metals such as gold, silver, and palladium, and the equipment is relatively simple and the operating temperature is low.
However, wet metallurgy also has obvious shortcomings: on the one hand, the consumption of chemical reagents is large, and the cost of waste liquid treatment is high; On the other hand, the leaching process may produce harmful gases (such as chlorine gas, nitrogen oxides, etc.), and if wastewater containing heavy metals is not disposed of properly, there is a risk of secondary pollution.
3. Pyrometallurgy is a method of melting the metal in a circuit board under high temperature conditions (usually above 1000 ℃) and separating it from other components such as glass fiber, ceramics, plastics, etc. This technology has high processing capacity and strong adaptability to various complex circuit boards. It can effectively recover base metals such as copper, tin, lead, and zinc, while precious metals such as gold and silver can be enriched in copper or lead melts for subsequent extraction.
The main problem with pyrometallurgy is high energy consumption, and the brominated flame retardants in circuit boards may produce highly toxic substances such as hydrogen bromide, dioxins, and furans at high temperatures, which require extremely high requirements for flue gas purification systems.
4. Biological leaching is the process of utilizing the metabolic activity of certain microorganisms (such as ferrous sulfide bacteria, sulfur oxidizing bacteria, etc.) to convert metals in circuit boards from solid to soluble ions, which are then recovered through subsequent methods. This technology has outstanding advantages such as mild reaction conditions, low reagent consumption, and minimal environmental pollution, and is considered one of the most promising green recycling technologies in the future.
However, the current biological leaching technology is still in the laboratory and pilot stage, with problems such as long leaching cycles, limited tolerance of bacterial strains to certain components in circuit boards (such as high concentration copper ions), and unstable leaching efficiency.
In the future, the recycling and processing technology of waste circuit boards will show the following development trends.
Efficiency: Multi technology collaboration and cascading recovery of valuable components
A single technology has its own advantages and disadvantages in selectivity, recovery rate, and economy, and the future development trend is multi technology coupling and system integration. For example, the majority of iron, aluminum, and plastic are separated using mechanical and physical methods, followed by pyrolysis treatment of organic matter. The metal rich body is then subjected to wet or pyrometallurgy, and finally, the residue is treated using biological leaching. At the same time, a cascade recycling process is established for different metals such as gold, silver, palladium, tantalum, and indium to maximize the comprehensive utilization of resources.
