PCB Tech - How to turn waste PCB circuit boards into treasures

With the acceleration of the update of electronic products, the number of discarded printed circuit boards (PCBs), the main component of electronic waste, is also increasing. The pollution caused by waste circuit boards to the environment has also aroused the attention of various countries. In waste circuit boards, heavy metals such as lead, mercury, and hexavalent chromium, as well as toxic chemicals such as polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE), which are used as flame retardants, are contained in the natural environment. It causes huge pollution to groundwater and soil, and brings great harm to people's lives and physical and mental health. On the waste circuit boards, there are nearly 20 kinds of non-ferrous metals and rare metals, which have high recycling value and economic value. It is a mineral deposit that is really waiting to be mined.

1 Physical Law

Physical method is a method that uses mechanical means and the difference in PCB physical properties to realize recycling.

1.1 Broken

The purpose of crushing is to dissociate the metal in the waste circuit board from the organic matter as much as possible to improve the separation efficiency. The study found that when the metal is broken 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 the use of material density, particle size, conductivity, magnetic permeability and surface characteristics and other physical properties of the difference to achieve separation. Currently widely used are wind shaker technology, flotation separation technology, cyclone separation technology, float-sink separation and eddy current separation technology.

2 Supercritical technology treatment method

Supercritical fluid extraction technology refers to a purification method that uses the influence of pressure and temperature on the solubility of supercritical fluids to perform extraction and separation without changing the chemical composition. Compared with traditional extraction methods, the supercritical CO2 extraction process has the advantages of environmental friendliness, convenient separation, low toxicity, little or no residue, and can be operated at room temperature.

The main research directions on the use of supercritical fluids to treat waste circuit boards are concentrated in two aspects: First, because 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 circuit board is removed by the supercritical CO2 fluid, the copper foil layer and the glass fiber layer in the printed circuit board can be easily separated, thereby providing the possibility of efficient recycling of materials in the printed circuit board . 2. Directly use supercritical fluid to extract metals from waste circuit boards. Wai et al. reported the extraction of 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. According to the results of Sb3+ research, the extraction efficiency is above 90%.

Supercritical processing technology also has big shortcomings such as: the high selectivity of extraction requires the addition of entrainer, which is harmful to the environment; the relatively high extraction pressure requires high equipment; the high temperature is used in the extraction process and the energy consumption is high.

3 Chemical method

Chemical treatment technology is a process that uses the difference in chemical stability of various components in PCB for extraction.

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 so on.

3.1.1 Incineration

The incineration method is to crush electronic waste to a certain particle size and send it to a primary incinerator for incineration, decompose the organic components in it, and separate the gas from the solid. The residue after incineration is the bare metal or its oxide and glass fiber, which can be recovered by physical and chemical methods after being crushed. The gas containing organic components enters the secondary incinerator for combustion treatment and is discharged. The disadvantage of this method is that it produces a lot of waste gas and toxic substances.

3.1.2 Cracking method

pyrolysis is also called dry distillation in the industry. It is to heat electronic waste in a container under the condition of isolating air, control the temperature and pressure, so that the organic matter in it is decomposed and converted into oil and gas, which can be recovered after condensation and collection. Different from the incineration of electronic waste, the vacuum pyrolysis process is carried out under oxygen-free conditions, so the production of dioxins and furans can be suppressed, the amount of waste gas generated is small, and the environmental pollution is small.

3.1.3 Microwave processing technology

The microwave recovery method is to crush the electronic waste first, and then use microwave heating to decompose the organic matter. Heating to about 1400 degree Celsius melts glass fiber and metal to form a vitrified substance. After this substance is cooled, gold, silver and other metals are separated in the form of beads, and the remaining glass substance can be recycled for use as building materials. This method is significantly different from traditional heating methods, and has significant advantages such as high efficiency, rapidity, high resource recovery and utilization, and low energy consumption.

3.2 Hydrometallurgy

Hydrometallurgy technology mainly uses the characteristics of metals that can be dissolved in acidic liquids such as nitric acid, sulfuric acid and aqua regia to remove metals from electronic waste and recover them from the liquid phase. It is currently the most widely used method for processing electronic waste. Compared with pyrometallurgy, hydrometallurgy has the advantages of less exhaust gas emissions, easy disposal of residues after metal extraction, significant economic benefits, and simple process flow.

4 Biotechnology

Biotechnology uses the adsorption of microorganisms on the surface of minerals and the oxidation of microorganisms to solve the problem of metal recovery. Microbial adsorption can be divided into two types: the use of microbial metabolites to immobilize metal ions and the use of microbes to directly immobilize metal ions. The former is to use the hydrogen sulfide produced by bacteria to fix it. When the surface of the bacteria absorbs ions to reach saturation, it can form flocs and settle down; the latter uses the oxidizing property of ferric ions to oxidize other metals in precious metal alloys such as gold. It becomes soluble and enters the solution, exposing the precious metal to facilitate recovery. The extraction of precious metals such as gold by biotechnology has the advantages of simple process, low cost, and convenient operation, but the leaching time is longer and the leaching rate is low, so it has not been actually put into use at present.

5 Concluding remarks on disposal of waste circuit boards

E-waste is a precious resource. Strengthening the research and application of metal recycling technology for e-waste is of great significance both from an economic and environmental point of view. Due to the complex and diverse characteristics of e-waste, it is difficult to recover the metals in it with any technology alone. The future development trend of e-waste processing technology should be: industrialization of processing forms, maximum recycling of resources, and scientific processing technology. In summary, studying the recycling of discarded PCBs 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.