| With the rapid technology innovation, the upgrade and replacement of electrical and electronic equipments (EEE) have been immensely accelerated in the last two decades, resulting in the ever-increasing generation of waste electrical and electronic equipments (WEEE) or so-called e-waste. WEEE has become the fastest growing solid waste. As a big country of the production and consumption of electrical and electronic products, meanwhile with the illegal flow of electronic waste from Europe and America, China is facing an unprecedented wave of electronic waste. As a kind of hazardous waste, WEEE is also very valuable because it is rich in a variety of metals and precious metals. Increasingly stringent policies and the growing awareness of environmental protection and resource recycling have greatly promoted the launching of e-waste recycling. Thus, the efficient and environment-friendly disposal and recycling technology is of great significance not only as regards environmental protection but also for the recovery of valuable materials. However, present research and applications are not deep enough. More work is also urgently needed.As one of the most important branches of the WEEE stream, waste printed circuit boards (WPCB) are generally considered to be representative of WEEE, and have received increasing attention from the public and researchers worldwide because the reasonable disposal and effective recycling of WPCB is a very complicated issue. Due to complex material properties and structural features of WPCB, the disposal and recycling process are limited by some unfavorable factors, such as:WPCB can not be crushed easily due to its tough substrate laminated structure; the recovery rate of rare metals is low because they are often wrapped in non-metallic materials; environmental problems will be caused in the crushing process because high temperatures cause the decomposition of organic polymer; nonmetallic materials are often underutilized; the efficiency of conventional pyrolysis is not high enough due to the large material size, porosity, low thermal conductivity, and other factors. Currently, the research and development on the recovery technology of WPCB is still immature. In this paper, WPCB is targeted as research object and its efficient recycling technology is widely researched.Microwave heating is characterized by selective, holistic, real-time and efficient. On basis of these characteristics, microwave heating has been widely used in the field of biomass, medical waste, living garbage and so on. When microwave heating is applied to WPCB pyrolysis, the difficult crushing problem of WPCB can be effectively solved. Moreover, a comprehensive recovery of valuable components can be achieved; rare metals can be easily dissociated from the non-metallic materials; the value of the pyrolysis products can be increased, among others. Thus, microwave pyrolysis technologies of WPCB have received increasing attentions. However, due to the metal-rich material property of WPCB and the common avoidance of the application of microwave heating to metal materials, research on microwave prolysis of WPCB is very rare. The existing researches are solely based on the rapid heating characteristics of microwave absorbers which can be instantly heated to a high temperature. Through the heat conduction of the preheated absorbers, WPCB can be pyrolyzed. From the perspective of heating mode, it is still external heating, or hybrid heating of external heating and microwave heating. Thus, the advantages of microwave overall heating have not been fully explored. While research on the direct exertion of microwaves to WPCB to induce its pyrolysis is still nearly blank.Based on the research status of microwave pyrolysis technology applied in the field of the solid waste recycling and energy conversion, combined with the material composition and structural characteristics of WPCB, in particular, its metal-rich material property and the microwave heating mechanism, especially discharge phenomenon can be triggered by the exposure of metal tips to microwaves, the research on microwave induced pyrolysis of WPCB is put forward in this paper. This research is based on the principle of microwave heating and corona discharge, aiming to fully utilize the microwave heating effect of WPCB itself or added absorbers and the heating effect of corona discharge caused by microwave-metal interaction. Through the coupling of these two kinds of heating effect, direct pyrolysis of WPCB characterized by internal heating style can be achieved. On the basis of this thinking, the following studies have been carried out:Systematic research on the dynamics of temperature rising and weight-loss during the microwave-induced pyrolysis of two kinds of WPCB (waste circuit board substrates without any electronic components, denoted as WPCB-A; waste computer motherboard with the removal of resistors and capacitors, denoted as WPCB-B) have been carried out in the industrial microwave pyrolysis apparatus. Moreover, the kinetic study of the thermal decomposition of WPCB under both conventional and microwave heating schemes have been studied by using thermogravimetric analyzer (TGA) and above-mentioned industrial microwave pyrolysis apparatus respectively. The following conclusions can be obtained. Microwave power is an important factor to affect the characteristics of temperature rising and weight-loss. When microwave power is low, only partial pyrolysis of the material can be achieved; the total weight-loss rate is very low; and the highest temperature that the material can reach is also relatively low. When the microwave power exceeds a certain limit, material can be pyrolyzed rapidly and completely and the pyrolysis reactions can be described by a one-phase reaction. At the same microwave power, the pyrolysis process of WPCB-B is accompanied by frequent spark phenomena, resulting in accelerated heating rate and weight-loss rate when compared to WPCB-A, who is pyrolyed with rare spark phenomenon. These results indicate that the metal discharge can promote the microwave heating process and pyrolysis reaction. As regards WPCB-A, the pyrolysis process can be significantly accelerated by adding a suitable microwave absorber, especially pyrolytic char and activated carbon. While, for WPCB-B, the addition of microwave absorbers should be cautious because it may weaken the heating effect caused by microwave-metal discharge, lead to the competition between dielectric heating and microwave-metal discharge which may be inferior to WPCB-B pyrolysis. The kenetics of microwave-induced pyrolysis of WPCB-B is different from its conventional pyrolysis kinetics. Under the same heating rate, the activation energy in microwave-induced pyrolysis is significantly smaller than that in conventional heating scheme, indicating that there may be some kind of special effects (such as non-thermal effects).The effective separation and recovery as well as analytical characterization of the pyrolysis products is another important part of this paper. Through the effective separation and recovery of the pyrolyzed products, the distribution rules of WPCB under microwave heating conditions can be obtained. Based on the advanced analytical instruments and scientific analytical methods, the elemental composition, molecular structure, chemical composition, and physical properties of the pyrolysis products are analyzed comprehensively. And then the resource utilization of different products has been discussed from multiple perspectives. In addition, with respect to the drawbacks that pyrolysis products generally contain toxic bromides that are unsuitable for reuse, the transference rules of bromine in microwave pyrolysis process of WPCB was studied to explore effective removal pathway. Meanwhile, a complete set of analytical method for the elemental bromine content in the WPCB and pyrolysis products was established. The following conclusion can be obtained. The heating rate and the final pyrolysis temperature are two important factors to affect products distribution; high heating rate and final pyrolysis temperature is beneficial to shorten the residence time of the volatiles and reduce the secondary decomposition reaction, resulting in an increase in the liquid product, also contributing to the enhancement of the H2content in the gaseous product at the same time. With the increased addition of pyrolytic char or activated carbon, the pyrolysis process can be promoted with reduced residence time of the volatiles, leading to the increase of pyrolysis product. However, when the addition of microwave absorber was increased to a certain amount, the reaction temperature can be greatly improved. Thus, the volatile was decomposed secondly when they escape from the material bed, leading to a decrease in the liquid products and an increase of pyrolysis gas. Therefore, the addition of microwave absorber should consider both the pyrolysis efficiency and the expected products. In the microwave-induced pyrolysis of WPCB,～30%Br was transferred into the liquid and～50%Br was transferred into gaseous products while only about one fifth was left in the solid residues. Under the high temperature and rapid pyrolysis process induced by microwave, more than50wt.% bromine in WPCBs can be transformed into HBr and captured by CaCO3. As a result, the pyrolysis oils can achieve a debromination of over95%, improving its quality greatly as well as reducing corrosion to connecting pipes.The coupling mechanism between microwave-metal discharge and microwave dielectric heating is the core mechanism of microwave-induced pyrolysis of WPCB. Only the relationship between microwave-metal discharge and the dielectric heating is revealed, the performance mechanism of every part of the main ingredients in the WPCB under microwave irradiation can be clear. In this paper, the heating effect of microwave-metal discharge and dielectric heating was studied respectively and then together. Firstly, metal strips were inserted in microwave-transparent and insulated materials-quartz sand, and then irradiated by the microwaves. The heating effect was obtained through an indirect calorimetric method. Secondly, different amounts of microwave absorbers were placed in the quartz sand to obtain their heating effect as the same way. Finally, the mixture of metal strips and microwave absorbers were mixed together and placed in the quartz sand to study their combined heating effect. Based on the above experimental results and analysis, the coupling mechanism of microwave absorption and microwave-metal discharge generation was described. The following conclusion can be obtained. To a certain amount of metals strips, the stimulation and intensity of the discharge are very sensitive to the amount of absorber, with the variation of which, the heating mechanism can be collaborative as well as competitive. When the amount of microwave absorber is small relative to a certain amount of metals strips, the microwave-metal discharge phenomenon is largely unaffected by the absorber, and the heating mechanism between the two effects is collaborative, and the total heat generated could be essentially the sum of the heat produced in each type of interaction. With the increase in the amount of microwave absorber material, microwave-metal discharges become harder to trigger. Thus, the importance of microwave-metal discharges is reduced, and the total heating in the sample is largely dependent on the wave absorption in the absorber. When the amount of microwave absorber is large enough to prevent microwave-metal discharges completely, the total heat generation in the sample is mainly attributable to the wave absorption of absorber. Furthermore, the dispersed metal strips may disturb the wave propagation due to the "skin effect" and reflection of waves, leading to a reduced total heating effect when compared with that of microwave absorber alone.Through the simulation of microwave heating process, the coupling mechanism between microwave absorber and metal under microwave can be viewed theoretically. The experimental test can characterize their coupling rules from a macroscopic view, while calculation can be specific to the point and detail to obtain their coupling mechanism theoretically. Firstly, due to complete information about the variation of the physical parameters as a function of temperature-change, water was selected as microwave absorber to simulate the electromagnetic field and temperature field during its microwave heating process. The simulation results were tested by the temperature measurement experiments after a certain period of time to validate the reliability of the model. Then, the electromagnetic field and temperature field of microwave absorber (activated carbon) during microwave heating process were simulated. The influence of dielectric parameters on the energy conversion ratio was studied. By the mutual inspection and confirmation between the theoretical calculations and experimental results, the mechanism of microwave dielectric heating can be described more completely. Finally, the electromagnetic field when metal strips and dielectric medium were co-existed under microwave irradiation was simulated to complete the systematic and theoretical description of the coupling mechanism. Dielectric medium was classified into two kinds:one is microwave transparent medium, the other is strong microwave absorber. The calculation results and the experimental results can be mutually verified.The economic assessment of the recovery process by microwave-induced pyrolysis was carried out by crushing and separating the pyrolysis solid products to recover metals and assess their value and assessing the calorific value of the gas and liquid products. The economic assessment reveals that the combined treatment is amazingly profitable and very promising to tackle the challenges posed by the electronic scraps. Taking into account the demand for scale-up and industrial promotion, a continuous recycling process was designed for WEEE recycling and reasonable proposal has been put forward for the improvement of pyrolysis efficiency and economic benefits.Through this study, a more comprehensive and in-depth understanding of the microwave-induced pyrolysis process of WPCB can be obtained, enriching the understanding of the principles and mechanisms of microwave pyrolysis in different occasions. It will provide theoretical reference for the process design and system optimization for the microwave-induced pyrolysis of electronic waste and even other similar solid waste. Finally, a summary of the content and conclusions of the full text were presented, the inadequate points of my research were pointed out, and the further research work was indicated. |
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