Discarding printed circuit boards as a typical e-waste, its resource research has become a hot issue in current e-waste disposal. Pyrometallurgical metallurgy, hydrometallurgy and mechanical/physical methods are currently the main methods of recovery. Fire and hydrometallurgy aim at recovering metals. Wastewater, waste slag and toxic fumes generated during the treatment process are likely to cause serious secondary pollution. The mechanical/physical method utilizes physical processes such as crushing, screening, and sorting to enrich and separate metals and nonmetals. It has a small environmental pollution and occupies a dominant position in resource recovery. The existing recycling methods mostly focus on the recovery of metals in the circuit boards, and are less concerned with the recycling and non-hazardous use of non-metal components that account for more than 50 aggregates. In addition to being used as a filler, non-metal materials are mostly used as landfills. Not only are valuable materials such as resin and glass fiber not fully utilized, they are lost, and the harmful substances such as flame retardants and residual metals are also easily passed. Various ways to pollute the environment. Therefore, the rational recovery and disposal of these substances has become a new topic for the recycling of waste circuit boards.
Pyrolysis is the process of heating organics to a certain temperature under anoxic or anoxic conditions to decompose them into gases, liquids (oils), solids (coke) and recover them. In recent years, organic waste pyrolysis technology has been increasingly used due to its lower pollution emissions and higher energy recovery. The use of pyrolysis technology to dispose of waste circuit boards can not only recover the metals in the circuit boards, but also can realize the recycling of non-metal components such as resin and glass fibers, and has certain attractiveness. Therefore, some scholars have carried out the theoretical research and engineering practice of waste circuit board pyrolysis technology.
1 Pyrolysis process The general process flow for pyrolysis of waste circuit boards is shown. The components on the circuit board are removed first, and then the plates are crushed to a certain size and sent to the reactor for pyrolysis. Epoxy resin and other polymer materials are heated to a certain temperature under the protection of an inert gas and thermally decomposed to generate low molecular weight substances. Condensate the pyrolysis oil and gas from the reactor to obtain non-condensable gas and liquid pyrolysis oil. Metal and glass fiber and other components do not substantially change in nature and remain in the reactor as solid residue, and can be separated and recovered by simple physical methods.
1.1 Pyrolysis Products The main components of the pyrolysis gases in the circuit board are CO2, CO, HBr, lower aliphatic hydrocarbons and some aromatic hydrocarbons with low molecular weight. Pyrolysis gas has a certain calorific value, which can be used for heat recovery as a heat source for the pyrolysis process.
The pyrolysis oil has a complex composition, a large boiling point range, a high calorific value, and a similar crude oil property. Pyrolysis oil contains many valuable components. If it can be reasonably recycled, it will greatly increase the economic efficiency of the entire pyrolysis process. At present, the recycling of pyrolysis oil has the following two aspects.
(1) Use as fuel When the scale of treatment is small and the yield of pyrolysis oil is low, it is simple and feasible to use it as a fuel. Chien et al. pyrolyzed oil by atmospheric distillation to obtain light naphtha, heavy naphtha, light oil and gas, and heavy oil and gas four fractions. High-temperature fractions with higher calorific value are sold as low-grade fuels. Moderate hydrogenation, deoxygenation, and dehydration of low-temperature distillate naphtha and light oil and gas can be used as the main components of gasoline and diesel.
(2) Extraction of high value-added substances as the main components of the pyrolysis oil of chemical raw materials Phenol and isopropyl phenol are important organic chemical raw materials, and are widely used in plastics, pharmaceuticals, pesticides, dyes, coatings and other fields. If these substances can be separated and extracted, they will be more valuable than pure fuel. Wang Qing et al. explored the separation process conditions of phenol, isopropyl phenol and water in pyrolysis oil through batch distillation experiments. The isopropyl phenol content of the separated product was not less than 90, and the phenol complied with the GB3079-1997 standard. Iji used rotary kiln pyrolytic glass fiber reinforced epoxy resin waste to obtain high-purity glass fiber, and used it as an epoxy resin polymer filler and insulation material for recycling, and achieved good results.
1.2 Secondary pollution prevention and debromination Due to the flame retardant requirements of electronic components, bromine flame retardants are often added to circuit board substrates and plastics. Therefore, when the circuit board is pyrolyzed or burned, it will generate more toxic fumes such as smog smoke, elemental bromine and hydrogen bromide gas, brominated phenols, and polybrominated biphenyls and dioxins/furans. These substances not only pollute the environment, corrode processing equipment, but also reduce fuel quality. Therefore, secondary pollution prevention and product debromination are issues that must be considered in the research of circuit board pyrolysis.
High-temperature treatment of flue gas can reduce emissions of two oxic substances and halogenated hydrocarbons. Iji et al. use secondary combustion methods to treat the gases generated during the pyrolysis of brominated epoxy resins. At a high temperature of 1100°C, not only organic bromide is decomposed, but also the concentration falls below the safety standard. SbBr3 generated from some inorganic flame retardants in the flue gas is also converted into Sb2O3, which facilitates subsequent dry recovery. In order to avoid environmental pollution caused by the discharge of halogenated gas products, wet scrubbing systems can be used for gas scrubbing. Studies have shown that alkaline solutions can efficiently elute the bromine and hydrogen bromide gases in the flue gas.
The excessively high concentration of brominated phenols in the pyrolysis oil limits the further use of the oil as a fuel or a chemical raw material, so debromination is an important process for oil refining. Hornung et al. studied pyrolysis oils 2,6 dibromophenol, tetrabromobisphenol A and other substances and found that the addition of polypropylene co-pyrolysis removes the bromine from the pyrolysis oil, and the main components are phenol and alkyl. The product of phenol. The debromination effect is best at 350 °C and residence time 20 min. Blazso et al. studied the co-thermal desorption and debromination of circuit board powders (mainly brominated epoxy resins) with various alkaline additives (Na2SiO3, 5A molecular sieves, 13X molecular sieves, NaOH). It was found that these alkaline additives not only react with HBr but also remove the bromine on aromatics, significantly reducing the content of brominated phenols in the pyrolysis oil. Yoshiki et al. used tetralin and decahydronaphthalene as hydrogen sources and hydrogenated the pyrolysis oil at a reaction temperature of 400-440° C. and an initial nitrogen pressure of 2 MPa. The content of bromine in the pyrolysis oil after the reaction was 2600 mg/l. The kg was reduced to 102 mg/kg and 54 mg/kg, respectively; on this basis, CaCO3, Na2CO3, K2CO3 and other alkaline substances were added, the bromine content was reduced to less than 1 mg/kg, and the oil quality was improved.
The circuit board usually contains 5 to 15 bromine, which is several hundred times higher than that of seawater and brine. Bromine is a precious resource. If we can choose a suitable recycling route to achieve the recycling of bromine, it can not only reduce the consumption of bromine resources, but also reduce the economic cost of pyrolysis. Foreign scholars have proposed the bromine recovery process shown. Using these technologies, Europe is expected to recycle 11 kt of bromine from electronic waste plastics each year.
1.3 Influencing factors Pyrolysis is a process that is influenced by heat transfer, mass transfer and chemical reactions. Factors such as temperature, heating rate, particle size, pyrolysis atmosphere, and catalyst all affect the yield and distribution of the product.
(1) Temperature pyrolysis temperature is the most important factor that affects the yield and distribution of pyrolysis products. Pyrolysis is an endothermic process. Raising the temperature can accelerate the pyrolysis reaction. Chien et al. investigated the effect of temperature on the pyrolysis products of waste circuit board resin on a fixed bed. Studies have shown that increasing the pyrolysis temperature can increase the production of gas and liquid products and reduce the solid product. However, after a certain temperature is reached, the temperature continues to rise, causing the secondary decomposition of the liquid phase product, resulting in an increase in gas production and a decrease in liquid phase production. Therefore, it is important to select the appropriate pyrolysis temperature for the target product.
(2) Heating rate The heating rate increases, and the characteristic temperature (initiation temperature, reaction final temperature, etc.) of the pyrolysis increases accordingly, and the main reaction interval increases. When the same temperature is reached, the reaction time of the sample at a low heating rate is prolonged, the conversion of the reactants is increased, and the reaction proceeds more completely.
Li Aimin and others investigated the effects of fast and slow heating methods on pyrolysis tar yield of waste circuit boards. It was found that below 550°C pyrolysis final temperature, slow heating is more efficient than rapid heating. However, with the increase of temperature, due to the rapid heating energy molecules in the material can be obtained in a very short period of time to accelerate the decomposition, tar yield will increase slightly.
(3) Particle size The particle size not only means the difference in the degree of pulverization during the pretreatment of the circuit board, but also influences the heat and mass transfer of the particles during the pyrolysis process and the escape speed of the product, resulting in different product distribution. Sun Lushi et al. compared the distribution of large particles (15mm×15mm), small particles (8mm×8mm), and powder (0.2mm) at the same pyrolysis final temperature (600°C). The results show that the radial temperature of the powder particles is uniform, the pyrolysis is carried out thoroughly, almost all the volatile components are precipitated, and the gas yield is high. With the increase of particle size, pyrolysis tends to produce longer molecular chain compounds, and large particle pyrolysis results in higher oil production. Therefore, for the pyrolysis of liquid oil as the target product, an appropriate increase in the particle size facilitates the production of liquid oil. However, the increase in particle size will result in uneven temperature distribution and coking. Therefore, the particle size should be controlled within a suitable range.
(4) The presence of other oxygen has little effect on the degree of thermal decomposition, but Chen et al. found that the presence of oxygen affects the activation energy of the pyrolysis reaction. The catalyst can reduce the activation energy of pyrolysis and increase the yield and quality of the target product. A US plant uses catalytic pyrolysis to process electronic waste such as printed circuit boards, chassis enclosures, and electrical wires, which not only shortens the reaction time, but also lowers the pyrolysis temperature much lower than conventional pyrolysis temperatures, resulting in fewer side reactions and damage to the by-product glass fibers. Also small. Vacuum pyrolysis not only greatly reduces the reaction temperature, reduces the formation of two types of dioxin-like substances, but also shortens the residence time of the product in the high-temperature pyrolysis zone, reduces the secondary reaction, and helps to increase the liquid product yield. In addition, the system is closed and there is a certain negative pressure, which prevents the spread of toxic substances in the system. The introduction of vacuum pyrolysis provides an effective solution to the secondary pollution caused by pyrolysis of electronic waste.
2 Pyrolysis Kinetics and Mechanism Studies Pyrolysis of waste circuit boards is a complex reaction that involves innumerable elementary reactions. Most scholars use thermogravimetric analysis to study the pyrolysis kinetics of waste circuit boards and their major components, and combine the pyrolysis products with the corresponding pyrolysis mechanism.
Based on the thermogravimetric analysis, Chen et al. discussed the thermal decomposition characteristics of epoxy resin, the main component of the waste circuit board. The results show that the pyrolysis reaction order of epoxy resin in nitrogen atmosphere is 0.4, and the average activation energy is 172.5kJ/mol. Barontini et al. believe that the pyrolysis of waste circuit board in the range of 260～300°C is simplified under nitrogen atmosphere. In the first-order reaction kinetics model, the activation energy at a heating rate of 10°C/min was calculated to be 146.3 kJ/mol. Sun Lushi et al. studied the thermogravimetric curves of waste circuit board powders in a nitrogen atmosphere, and considered the total reaction of the pyrolysis reaction. The rate is controlled by the speed of the chemical reaction. Sun Lushi also studied the pyrolysis behavior of samples in an aerobic atmosphere. The apparent activation energy Ea in the initial stage is small. It is considered that the reaction at this stage is controlled by physical processes such as heat and mass transfer, and the apparent activation energy in the second stage is significantly higher than the first stage. It is considered that the reaction at this stage is controlled by the chemical reaction. .
Luda et al. proposed a three-step pyrolysis mechanism for brominated epoxy resins. The first step is the pyrolysis of the brominated part of the resin to form bromoalkanes and bromophenols, dibromophenols; the second step is the non-brominated part of the resin pyrolysis to generate alkyl phenols, bisphenol A and other substances; the third step is The unsaturated substances formed during the first two steps form coke after reactions such as cyclization and polymerization. Balabanovich et al. discussed in detail the role of amine curing agents in the pyrolysis of brominated epoxy resins. It is believed that the amine curing agent causes the pyrolysis temperature of the brominated epoxy resin to be lower by about 100°C than the non-brominated epoxy resin. By using this property, the mixed resin can be pyrolyzed step by step, so that the bromine atoms in the brominated epoxy resin are removed in the form of HBr at the initial stage of heating, and the remaining bromine-free resin is further decomposed at high temperatures to generate fuel oil and gas. Some EU research institutes have jointly developed a pyrolysis process called "Haloclean" to recycle electronic wastes based on two-stage pyrolysis. The process was used to process waste circuit boards and the raw materials were pyrolyzed in two stages (the pyrolysis temperatures were 350°C and 450°C, respectively) to obtain 80% cracked oil containing phenolic compounds. This process is currently used by a German factory.
3 Research and Prospects Currently, the pyrolysis technology of waste printed circuit boards is still at the stage of experimental research in foreign countries. There is not much research in China, and there is still much work to be done to truly realize industrialization.
3.1 Pyrolysis reaction mechanism The reaction mechanism study helps to understand the pyrolysis process and provides theoretical guidance for pyrolysis technology. Most of the existing research results are based on thermogravimetric analysis, and the actual pyrolysis process is far more complicated than that of thermal and heavy conditions.
Only on the basis of understanding the chemical reaction, fully take into account the effects of heat transfer and mass transfer factors, the parameters obtained can fully reflect the complex changes in the process, in order to provide the basis for optimizing reactor design and reaction operations. Therefore, the in-depth study of the pyrolysis reaction mechanism will be the focus of future research work.
3.2 The catalyst and reactor pyrolysis reaction rate is slow, after adding the catalyst can not only improve the reaction rate, shorten the reaction time, but also the catalytic decomposition of pyrolysis products, improve the quality of pyrolysis products. Based on the research on catalytic pyrolysis of waste plastics, the search for catalysts and catalysts for pyrolysis of waste circuit boards will be one of the future research directions.
Organic matter is prone to coke coking at high temperatures, the temperature of the equipment requires a higher material, the type of reactor and heat and mass transfer methods will directly affect the yield of pyrolysis products. Based on an in-depth understanding of the laws of pyrolysis, a pyrolysis device with excellent performance and high processing efficiency was developed, which provided the basis for the industrial application of pyrolysis technology.
3.3 Economic and secondary pollution prevention Explore various technological conditions, achieve optimal distribution of products and reasonable recycling, and strive to improve the economics of circuit board pyrolysis process.
In addition, the conversion mechanism of pollution components during the pyrolysis process and the research of harmless control technology are indispensable content.
The use of pyrolysis technology to recycle and dispose of waste printed circuit boards not only eliminates pollution, but also recovers resources from pyrolysis products. It is a resource technology with good development prospects. With the further development of the basic theoretical research of pyrolysis technology and the development of pyrolysis equipment, this method will become an important recycling method for the recycling of electronic waste in the future.