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Recycling printed circuit boards
Chapter
11
Recycling printed circuit boards
Abhishek Kumar Awasthi1, 2, Xianlai Zeng1,2 1
School of Environment, Tsinghua University, Beijing, China; 2Key Laboratory for Solid Waste Management and Environment Safety, (Tsinghua University), Ministry of Education of China, Beijing, China
CHAPTER OUTLINE
11.1 Introduction 311 11.2 Economic benefits of recycling of PCBs 313 11.3 Emerging technologies for recycling of waste printed circuit boards
314
11.3.1 11.3.2 11.3.3 11.3.4
Disassembling 315 Physical-mechanical recycling process of PCBs 315 Size reduction and separation 316 Human health affected owing to the physical recycling process of waste PCB 317 11.3.5 The best available technology with opportunities and challenges 318 11.3.6 Dismantling 318 11.3.7 Technology for recovery of copper and other valuable metals 319
Acknowledgments 322 References 322 Further reading 325
11.1 INTRODUCTION Printed circuit boards (PCBs) are the supplier of electronic components and electronic interconnections (Akcil et al., 2015; Awasthi et al., 2016a). The key advantage of PCBs lies in its capability to greatly reduce the errors of routing and assembly and to increase the degree of automation and fabrication efficiency. Due to decades of development, PCBs have been constantly contributing to the improvement and progress of people’s modern lives (Arshadi and Mousavi, 2014).
Waste Electrical and Electronic Equipment (WEEE) Handbook. https://doi.org/10.1016/B978-0-08-102158-3.00011-2 Copyright © 2019 Elsevier Ltd. All rights reserved.
311
312 CHAPTER 11 Recycling printed circuit boards
These PCBs are used to mechanically support and electrically connect electronic components using conductive pathways, tracks, or signal traces etched from copper sheets laminated onto a nonconductive substrate, employed in the manufacturing of business machines and computers, as well as communication, control, and home entertainment equipment. PCBs are an essential part of almost all electric and electronic equipment, and have revolutionized the electronics industry (Ghosh et al., 2015). China shares almost half of the overall PCB industries’ market and is the fastest developing country in the PCBs production. In China, the total WEEE that is domestically generated and illegally imported, over 500,000 tons of waste PCBs, needs to be treated every year, and the quantity is mounting every year owing to falling average lifetime of electronic goods. There are three major types of PCB construction: (1) single-sided, (2) double-sided, and (3) multilayered. Single-sided boards have the components on one side of the substrate. When the number of components becomes too much for a single-sided board, a double-sided board may be used. Electrical connections between the circuits on each side are made by drilling holes through the substrate in appropriate locations and plating the inside of the holes with a conducting material. The third type, a multilayered board, has a substrate made up of layers of printed circuits separated by layers of insulation. The components on the surface connect through plated holes drilled down to the appropriate circuit layer. This greatly simplifies the circuit pattern (Sanapala, 2008). Copper is the most commonly used material for traces. Simple methods involve plating the entire board with copper, and then etching away unnecessary areas through a mask (stencil) to leave the required traces. More complex methods allow traces to be added onto a bare board. Each approach has associated pros and cons (Awasthi et al., 2016c, 2017a). Some boards require the use of gold for sensitive, low-voltage applications or lead-free (RoHS) compliance. Copper traces usually demand the use of a nickel barrier layer before gold-plating. This is to prevent gold from migrating into the copper. Indiscriminate use of nickel can result in huge losses to impedance (Yu et al., 2016a,b). A typical circuit board module includes a PCB and a variety of circuit board components soldered to the printed circuit board. The PCB is generally a laminated board with circuit traces on external surfaces of the board or at interlayer levels within the board, and the electrical components are typically light-emitting diodes (LEDs), processors, memory devices, clock generators, resistors, cooling units, capacitors, and virtually any other type of electrical component (Ghosh et al., 2015). PCBs generally comprise a
11.2 Economic benefits of recycling of PCBs 313
Table 11.1 Typical PCB components and their major compositional components Electronic component
Majority composition or materials
Resistors Capacitors
Ceramic, carbon Aluminum, electrolyte, plastics, etc., copper leads Steel, copper Plastic cases, copper leads, silicon Plastic cases, copper leads, silicon Copper, plastic Aluminum, steel Aluminum
Inductors, transformers Integrated circuits Transistors, diodes Connectors Mounting brackets Heat sinks
Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, pp. 287e311.
composite of organic and inorganic materials with external and internal metal traces, permitting assembled electronic components to be mechanically supported and electrically connected. The components themselves are a mixture of often quite sophisticated construction and include the components listed in Table 11.1. Additionally, the value of precious metals in waste PCBs are more profitable than in mining ores, which makes their recycling significantly important in terms of both environmental and economic views (Table 11.3). The average content and value ratio of different metals in PCBs is as presented in Table 11.2. As clearly seen, Au, Cu, Pd, and Ag metals account for nearly all of the economic material value in waste PCBs. For that reason, waste PCB recycling mainly is concerned with extraction of these valuable metals above all else.
11.2 ECONOMIC BENEFITS OF RECYCLING OF PCBS Waste PCBs are about 3% by weight of the total amount of waste EEEs. The recycling for waste PCB outcomes in quite a large variety of materials being part of the whole assembly, with the possibility of significant environmental impacts arising from both the material resources use and the effects of disposal. Precisely, a significant percentage of the embodied materials are metals that are valuable recycling as presented in Table 11.4.
314 CHAPTER 11 Recycling printed circuit boards
Table 11.2 Metal composition of waste PCBs (%) Element
Sn
Pb
Cu
Fe
Al
Sb
Zn
Ni
Cr
Cd (g/t)
Au (g/t)
Ag (g/t)
PCB ECs Total
10.12 3.20 6.0
3.20 0.68 1.7
21.62 13.80 16.9
0.21 19.49 11.8
1.36 6.91 4.7
0.001 6.11 3.7
0.056 5.66 3.4
0.036 0.65 0.4
0.027 0.53 0.3
0.53 14.45 8.9
e 40.76 24.4
194.91 112.68 145.7
Yang, C., Li, J., Tan, Q., Liu, L., Dong, Q., 2017. Green process of metal recycling: co-processing waste printed circuit boards and spent tin stripping solution. ACS Sustainable Chemistry and Engineering 5, 35243534.
Table 11.3 Metal content and economic value of waste PC boards (per ton) Metals
Content (%)a
Metal price ($/kg)b
Potential value ($)
Value ratio (%)
Cu Al Fe Ni Pb Sn Ag Au Pd Total
9.7 5.8 9.2 0.69 2.24 2.15 0.06 0.023 0.01 29.87
3.6 1.7 0.4 10.5 1.2 13 315 24,434 6100 e
349.2 98.6 36.8 72.5 27 279.5 189 5620 610 7282
4.8 1.35 0.51 0.99 0.37 3.84 2.6 77.17 8.38 e
a
Chris et al., 2007. London Metal Exchange, Nov., 2008. Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, 287e311. b
High worth of PCBs and possible risk to both the environment and human health drive the need for waste PCBs to be treated in an environmentally sound way (Awasthi et al., 2017a). The recycling of waste PCBs is a big challenge, due to the components, diversity and material complexity, as well as manufacturing processes (Li et al., 2015).
11.3 EMERGING TECHNOLOGIES FOR RECYCLING OF WASTE PRINTED CIRCUIT BOARDS Several technologies have been established and studied by the global scientific community, and it is very important to know the recycling technological innovations in management of PCBs. Therefore, the recent status of the international articles dealing with recycling of PCB is briefly discussed.
11.3 Emerging technologies for recycling of waste printed circuit boards 315
Table 11.4 The estimated benefits in terms of energy saving by recovery of resources from PCBs S.No.
Material
Energy savings (%)
1. 2. 3. 4. 5. 6. 7.
Aluminum Copper Iron and steel Lead Zinc Paper Plastic
95 85 74 65 60 64 >80
Data Source: Cui, J., Forssberg, E., 2004. Mechanical recycling of waste electric and electronic equipment: a review. Journal of Hazardous Materials B99, 243e263.
11.3.1 Disassembling The present recycling technologies offered for e-waste recycling must include a preliminary sorting or initial disassembly step. The dismantling/depollution of hazardous components is very important along with a very valuable step to get components, such as cables and PCBs, cell batteries and capacitors, etc., in order to achieve the successful recovery of secondary resource materials (Awasthi et al., 2017b,c). The automated disassembly of EEEs is innovative, it is mainly appropriately applied in developed countries (Duan et al., 2011). In addition, the hazardous materials are partly removed, mainly in case of small product WEEEs. This suggests that significant amounts of hazardous substances are still contained in the mechanical crushing step, which causes substantial distribution of different pollutants and perhaps decreases the amounts of valued recyclable resources (Song et al., 2015). Electronic components (ECs) have to be dismantled from PCB assembly as the best option to minimize the environmental pollution, and for resource conservation and reuse of components (Zlamparet et al., 2017).
11.3.2 Physical-mechanical recycling process of PCBs Waste PCB mechanical recycling can be generally divided into two main steps. The first step is disassembly and/or separation of different components as well as materials, commonly by mechanical or metallurgical treating to upgrade the desirable material content (Awasthi et al., 2017b). Shredding, electrostatic separation, and supercritical extraction are the
316 CHAPTER 11 Recycling printed circuit boards
main technologies employed in this step. The second step is the further separation or screening and processing of metal streams; this is possibly the most important step from environmental and economic perspectives. Several methods are available to extract metals from postprocessing waste PCBs (Yang et al., 2013, 2016; Xue et al., 2013; Wang et al., 2016). These technologies can be varied in terms of their economic feasibility, recovery efficiency, and environmental impact (Zeng et al., 2017). In the recycling of waste PCBs, selective dismantling is an essential process as subsequently: (1) The reuse of components has been become first priority; (2) Dismantling the hazardous components is very important; and (3) It is also important to recover the highly valuable components and high-grade materials such as batteries in order to simplify the subsequent recovery of materials. Most of the recycling plants operate with manual dismantling. For example, a typical dismantling process is operated at Ragn-Sells Elektronikåtervinning AB in Sweden (Cui and Forssberg, 2004). A multiple-use tool is used in the dismantling process for eliminating hazardous components in addition to recovery of valuable components and reusable materials.
11.3.3 Size reduction and separation Firstly, cutting, crushing, and grinding are procedures used to reduce waste PCBs in pieces of different particle sizes. It is recognized that cutting methods are considered better choices than crushing for the recycling processes of waste PCBs (Fig. 11.1). The purpose of crushing is to strip metals from the base plates of waste PCBs. Crushing technology is intimately related to not only energy consumption of crushing equipment but also further selective efficiency (Li and Zeng, 2012). A number of methods are available in order to separate the valuable material and nonmaterial, established principally on three different bases: physical, magnetic, and electrostatic. Out of these, the magnetic separation is applicable in a dry environment; a stable magnet iron separator then uses an eddy current system and wet environment. Certainly additional methods applied to separate both metallic and non-metallic materials must be able to separate based on electrostatic theory. Normally, these approaches depend on high electrostatic voltage, although we can use the multiroller separation devices with high voltage. With the electrostatic separation it is possible to get a powder of copper with a high content. Other mechanical procedures depend on the range of size, mass, shape, as well as density of the particles. Screening has not only been utilized to prepare a uniformly sized feed to certain mechanical processes but also to upgrade metals
11.3 Emerging technologies for recycling of waste printed circuit boards 317
Waste PCB
Unit components
Dismantling
Residues PCBs
Crushing
Screening / separation Metals such as copper
Non-metals
n FIGURE 11.1 Brief outline of mechanical processes for the recycling of waste PCBs. Redrawn from
source: Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, 287e311.
contents. Screening is necessary because the particle size and shape properties of metals are different from that of plastics and ceramics. The primary method of screening in metals recovery uses the rotating screen, or trammel, a unit that is widely used in both automobile scrap and municipal solid waste processing.
11.3.4 Human health affected owing to the physical recycling process of waste PCB As using the spraying water method and sound insulation measures are adopted, the human health damage resulting from industrial dust and noise is very little from the above analysis. So we mainly present the damage from the metal ions in wastewater. The main metal ions are Cu, Au, Cd, Pd, Pb, Sn, Ni, and Ag in wastewater (Table 11.5). However, there are
Table 11.5 The human toxicity of metal in wastewater (mg 1,4-dichlorobenzene eq) Metal ion
Cu
Cd
Pb
Sn
Ni
The equivalency factor for human toxicity Environmental impact on human toxicity Total of environmental impact on human toxicity
1.3 0.546
2.3 0.345
1.2 0.24 14.3
1.7E-2 8.5E-4
3.3E2 13.2
Reproduced from Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, 287e311.
318 CHAPTER 11 Recycling printed circuit boards
metals, viz. Cu, Cd, Pb, Sn, and Ni, that cause damage to human health or to human toxicity, which are presented in Table 11.5.
11.3.5 The best available technology with opportunities and challenges In developing countries, mainly primitive technologies are the main obstacle to the recycling of waste PCBs. During the manual dismantling process in informal recycling sites, e-waste recyclers use chisels, hammers, and cutting torches to open solder connections and separate numerous types of metals and components. Many times PCBs are simply cooked on a coalheated plate and melted. Undoubtedly, the manual dismantling process in informal sites in China was quite common about 15 years ago, however, it has been prohibited according to Chinese environmental law. In fact, such practice is still reported with little development by using an electric heating plate system. The fact that the PCB assembly is one of the fastest growing sources of waste in many developing countries has focused attention on the need to recycle, recover, and reuse materials that have been consigned to informal dismantling sites. In developing countries such as China, India, or Nigeria, the above-mentioned methods have been widely used as well. The major common point of these disassembling technologies is the recovery of the solder remaining on the board by subjecting it to a temperature much higher than the molten point of the solder. In these processes of PCB assembly dismantling, pyrolysis under high temperature heating, during which the toxic products from resins and adhesives are decomposed, is a common occurrence (Williams et al., 2008).
11.3.6 Dismantling Dismantled PCB assemblies have a significant environmental impact because they contain different heavy metals and halogen-containing flame retardant, for example, cadmium (pins), lead (soldering tin), mercury (switches, round cell batteries), brominates, and mixed plastics that can seep into the environment if not properly managed (Song et al., 2015). Cell batteries might ignite or leak potentially hazardous organic vapors if exposed to excessive heat or fire. Explosion may result if a capacitor is subjected to high currents and heating. The round cell batteries and capacitors that are large or contain polychlorinated biphenyl should be manually removed and separately disposed by following an appropriate method. The circuit boards can then be sent to a facility for further dismantling
11.3 Emerging technologies for recycling of waste printed circuit boards 319
(for reuse or reclamation from ICs that contained precious substances or soldering tin) and copper recovery (from bare board) works (Zeng et al., 2017). Substitutes for lead in solders are currently being developed, but are not yet in production (Duan et al., 2011). While the melting of soldering tin could lead to the separation and recycling of electronic components, in addition to the melting of soldering tin, the mechanical strength of the pin that is packaged to the through hole is another key factor in separating the components.
11.3.7 Technology for recovery of copper and other valuable metals In developing new technology for waste PCBs, most researchers have focused on the technology by which the valuable metals can be separated and recovered from waste PCBs. Firstly, very simple incineration (without any control system in incineration, open burning, etc.) was used to recover valuable metals from waste PCBs, this practice lacked proper environmental and human health protection (Awasthi et al., 2016b); although this process is banned in China, it is still summarized (Song and Li 2014). Pyrometallurgy is a technology for recovery of non-ferrous metals and precious metals from waste PCBs. Pyrometallurgy involves incineration, smelting in a plasma arc furnace or blast furnace, drossing, sintering, melting, and reactions in a gas phase at high temperature. The pyrolysis process is the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents (Quan et al., 2010; Yang et al., 2013). Pyrolysis of organic materials contained in waste PCBs leads to the formation of gases, oils, and chars that can be used as chemical feedstocks or fuels (Ebin and Isik, 2016; Vehlow and Mark, 1997). There are some pilot-scale studies on the recovery of metals from waste PCBs by using pyrolysis in China (Li et al., 2010). Hydrometallurgy is one more conventional technology for the recovery of precious metals from waste PCBs. The main steps in hydrometallurgy consist of a series of acid or caustic leaches (such as cyanide leaching, halide leaching, thiourea leaching, and thiosulfate leaching, etc.) of solid materials (Akcil et al., 2015). The solutions are then subjected to separation and purification procedures, for example, precipitation of impurities, solvent extraction, adsorption, and ion exchange to isolate and concentrate the metals of interest. Consequently, the solutions are treated by electrorefining process, chemical reduction, or crystallization for metal recovery (Yang et al., 2017; Ha et al., 2014).
320 CHAPTER 11 Recycling printed circuit boards
Mechanical/physical recycling method for waste PCBs is based on the differences of materials in physical characteristics (including density, electric conductivity, magnetic susceptibilities, etc.). Owing to its better environmental properties (such as comparatively generating less wastewater), easier operability and high efficiency, additionally nonferrous metals and precious metals contents have gradually decreased in concentration in PCBs (Song et al., 2015; Zeng et al., 2017). Through mechanical recycling process for waste PCBs, the materials can be separated out into metallic and non-metallic. There are about 30% metallic materials after separation, which is hard to recover, because the fractions concentrated on metallic materials obtained from these processes are still a mixture of various metals (copper, aluminum, lead, zinc, etc.) (Wang et al., 2017; Yoo et al., 2009). The present mechanical technologies (pneumatic separation, electrostatic separation, etc.) focus on recovering the copper, but the studies on further separation of the mixed metals are relatively fewer in cases of studies in China (Wu et al., 2009). In order to further separate the concentrated fraction in metals and increase the copper content in the metallic mixture, vacuum metallurgy separation method was presented in some studies (Huang et al., 2009). Zheng et al. (2009a,b) studied a novel fluidized bed process technology for recycling glass fibers for non-metallic materials. The glass fibers are collected at high recovery rate by cyclone separators without violating the environmental regulations. Physical recycling of the non-metallic fractions is an efficient recycling method without environmental pollution, such as waste water generation, but needs capital investments in terms of equipment setup (Li and Zeng, 2012). Thus, more research should be done to develop comprehensive and industrialized usage of the non-metallic fractions recycled by physical methods. The trend in chemical recycling methods for the non-metallic fractions from waste PCBs is in order to make the best of advantages over physical recycling of the non-metallic fractions to compensate the higher cost of chemical recycling methods. The treatment and elimination of hazardous substances contained in the non-metallic fractions is an ultimate method to delete the pollution. The research in this area is just beginning and the challenges caused through technical and economic feasibility should not be underestimated (Guo et al., 2009). To achieve a clean separation among the metallic and the non-metallic fractions from waste PCBs is a way to reduce the contents of heavy metals in the non-metallic portions and therefore is an approach to minimize the potential environmental risk for the recycling of the non-metallic fractions. The removal of flame retardants and bromine recovery seem to be a way to treat hazardous substances contained
11.3 Emerging technologies for recycling of waste printed circuit boards 321
in the non-metallic fractions from e-waste and to reduce the pollution caused by the formation of PBDD/Fs. The flame retardants contained in waste PCBs are usually reactive ones, which cannot be extracted before the degradation of the thermosetting resins. Catalytic hydrogenation can be an effective way to remove most of the hazardous toxic compounds in the oil produced by chemical recycling of the non-metallic fractions from waste PCBs (Yu et al., 2016a). The bioleaching method is based on microbial capability to utilize organic and inorganic substrates, in that way removing the metals. Many investigators have studied the potential of bioleaching method for leaching metals from waste PCBs (Arshadi and Mousavi, 2014; Awasthi et al., 2016a,b,c; Karwowska et al., 2014). In this context, Willner and Fornalczyk (2013) studied the different parameters, such as the effect of pH, inoculum concentration, and the oxidationereduction potential of bioleaching of copper metal from waste PCBs applying a bacterial culture of Acidithiobacillus ferrooxidans. Generally, fungi have a promising ability to produce various types of organic acids, and different kind of metabolites can be used in the process of metal leaching from waste PCBs. These fungal-produced organic acids plays a significant role in the metal speciation step in key fungalmediated processes (fungi such as Aspergillus niger and Penicillium sp.) (Awasthi et al., 2017a; Brandl et al., 2001). In addition, a number of researchers have studied the feasibility of copper bioleaching from finemilled waste PCBs (at concentration up to 10.0 g/L), and around 90% Cu recovery was achieved by using mesophilic microbes (Bas et al., 2013; Brandl et al., 2001). Each technology has certain limitations, advantages, and disadvantages (Awasthi et al., 2016c, 2017a). Therefore, it is essential to develop a more appropriate process for the more effective and ecofriendly recycling of waste PCBs. In this context, Awasthi et al. (2016a,b,c) suggested an integrated system by means of combining physical and biological methods for better metal recovery, however, this approach needs detail research for better understanding. Therefore, this chapter highlights the significant progress with the waste PCBs recycling, in standings of technological improvement. Besides, the existence of various kinds of PCBs in electronic equipment could be a possible supply of secondary resources (e.g., raw materials) in terms of resourcefully achieving the circular economy approaches. Without any doubt, these waste PCBs show great potential value, but the number of formal recycling plants appropriate for its management, is relatively less globally.
322 CHAPTER 11 Recycling printed circuit boards
ACKNOWLEDGMENTS This work is supported by “National Key Technology R&D Program” of China (2017YFF0211604), and Key Laboratory for Solid Waste Management and Environment Safety (Tsinghua University), Ministry of Education of China (No. SWMES 2017-12). We are also thankful to Dr. Jinhui Li, Professor, School of Environment, Tsinghua University, Beijing China, for guidance, suggestions and kind support.
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Ebin, B., Isik, M.I., 2016. WEEE recycling. CHAPTER 5- Pyrometallurgical Processes for the Recovery of Metals from WEEE. https://doi.org/10.1016/B978-0-12-8033630.00005-5. Ghosh, B., Ghosh, M.K., Parhi, P., Mukherjee, P.S., Mishra, B.K., 2015. Waste printed circuit boards recycling: an extensive assessment of current status. Journal of Cleaner Production 94, 5e19. Guo, J.Y., Guo, J., Xu, Z.M., 2009. Recycling on non-metallic fractions from waste printed circuit boards: a review. Journal of Hazardous Materials 168, 567e590. Ha, V.H., Lee, J.C., Huynh, T.H., Jeong, J., Pandey, B.D., 2014. Optimizing the thiosulfate leaching of gold from printed circuit boards of discarded mobile phone. Hydrometallurgy 149, 118e126. Huang, K., Guo, J., Xu, Z., 2009. Recycling of waste printed circuit boards: a review of current technologies and treatment status in China. Journal of Hazardous Materials 164, 399e406. Jadhav, U., Su, C., Hocheng, H., 2016. Leaching of metals from printed circuit board powder by an Aspergillus niger culture supernatant and hydrogen peroxide. RSC Advances 6 (49), 43442e43452. Karwowska, E., Andrzejewska-Morzuch, D., Lebkowska, M., Tabernacka, A., Wojtkowska, M., Telepko, A., Konarzewska, A., 2014. Bioleaching of metals from printed circuit boards supported with surfactant producing bacteria. Journal of Hazardous Materials 264, 203e210. Li, J., Zeng, X., Chen, M., Ogunseitan, O.A., Stevels, A., 2015. “Control-Alt-Delete”: rebooting solutions for the E-waste problem. Environmental Science and Technology 49 (12), 7095e7108. Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, pp. 287e311. Li, J., Duan, H., Yu, K., et al., 2010. Characteristic of low-temperature pyrolysis of printed circuit boards subjected to various atmosphere. Resources, Conservation and Recycling 54, 810e815. Quan, C., Li, A., Gao, N., et al., 2010. Characterization of products recycling from PCB waste pyrolysis. Journal of Analytical and Applied Pyrolysis 89, 102e106. Sanapala, R., 2008. Characterization of FR-4 Printed Circuit Board Laminates Before and After Exposure to Lead-Free Soldering Conditions. University of Maryland [Thesis]. https://drum.lib.umd.edu/bitstream/handle/1903/8362/umi-umd-5671.pdf?sequence¼ 1&isAllowed¼y. Song, Q., Zeng, X., Li, J., Duan, H., Yuan, W., 2015. Environmental risk assessment of CRT and PCB workshops in a mobile e-waste recycling plant. Environmental Science and Pollution Research 22, 1e8. Song, Q., Li, J., 2014. Environmental effects of heavy metals derived from the e-waste recycling activities in China: a systematic review. Waste Management 34 (12), 2587e2594. Vehlow, J., Mark, F., 1997. Electrical and Electronic Plastics Waste Co-Combustion with Municipal Solid Waste for Energy Recovery. Available from: www.APME.Com. Wang, J., Guo, J., Xu, Z., 2016. An environmentally friendly technology of disassembling electronic components from waste printed circuit boards. Waste Management 53, 218e224.
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Wang, F., Zhao, Y., Zhang, T., Zhang, G., Yang, X., He, Y., Wang, L., Duan, C., 2017. Metals recovery from dust derived from recycling line of waste printed circuit boards. Journal of Cleaner Production 165, 452e457. Williams, E., Kahhat, R., Allenby, B., 2008. Environmental, social and economic implications of global reuse and recycling of personal computers. Environmental Science and Technology 42, 6446e6454. Willner, J., Fornalczyk, A., 2013. Extraction of metals from electronic waste by bacterial leaching. Environment Protection Engineering 39, 197e208. Wu, J., Li, J., Xu, Z., 2009. A new two-roll electrostatic separator for recycling of metals and nonmetals from waste printed circuit board. Journal of Hazardous Materials 161, 257e262. Xue, M., Xu, Z., 2013. Computer simulation of the pneumatic separator in the pneumaticelectrostatic separation system for recycling waste printed circuit boards with electronic components. Environmental Science and Technology 47 (9), 4598e4604. Yang, C., Li, J., Tan, Q., Liu, L., Dong, Q., 2017. Green process of metal recycling: coprocessing waste printed circuit boards and spent tin stripping solution. ACS Sustainable Chemistry and Engineering 5, 3524e3534. Yang, J., Lei, J., Peng, S., Lv, Y., Shi, W., 2016. A new membrane electro-deposition based process for tin recovery from waste printed circuit boards. Journal of Hazardous Materials 304, 409e416. Yang, X., Sun, L., Xiang, J., Hu, S., Su, S., 2013. Pyrolysis and dehalogenation of plastics from waste electrical and electronic equipment (WEEE): a review. Waste Management 33, 462e473. Yoo, J., Jeong, J., Yoo, K., et al., 2009. Enrichment of the metallic components from waste printed circuit boards by a mechanical separation process using a stamp mill. Waste Management 29, 1132e1137. Yu, G., Bu, Q., Cao, Z., Du, X., Xia, J., Wu, M., Huang, J., 2016a. Brominated flame retardants (BFRs): a review on environmental contamination in China. Chemosphere 150, 479e490. Yu, M., Zeng, X., Song, Q., Liu, L., Li, J., 2016b. Examining regeneration technologies for etching solutions: a critical analysis of the characteristics and potentials. Journal of Cleaner Production 113, 973e980. Zheng, Y., Shen, Z., Cai, C., et al., 2009a. The reuse of nonmetals recycled from waste printed circuit boards as reinforcing fillers in the polypropylene composites. Journal of Hazardous Materials 163, 600e606. Zheng, Y., Shen, Z., Ma, S., et al., 2009b. A novel approach to recycling of glass fibers from non-metal materials of waste printed circuit boards. Journal of Hazardous Materials 170, 978e982. Zeng, X., Duan, H., Wang, F., Li, J., 2017. Examining environmental management of e-waste: China’s experience and lessons. Renewable & Sustainable Energy Reviews 72, 1076e1082. Zlamparet, G.I., Ijomah, W., Miao, Y., Awasthi, A.K., Zeng, X., Li, J., 2017. Remanufacturing strategies: a solution for WEEE problem. Journal of Cleaner Production 149, 126e136.
Further reading 325
FURTHER READING Awasthi, A.K., Cucchiella, F., D’Adamo, I., Li, J., Rosa, P., Terzi, S., Wei, G., Zeng, X., 2018. Modelling the correlations of e-waste quantity with economic increase. The Science of the Total Environment 613e614, 46e53. Brandl, H., Faramarzi, M.A., 2006. Microbe-metal-interactions for the biotechnological treatment of metal-containing solid waste. China Particuology 4, 93e97. Coombs, C., 2001. Printed Circuit Handbook, fifth ed. McGraw Hill. Estrada-Ruiz, R.H., Flores-Campos, R., Gamez-Altamirano, H.A., Velarde-Sanchez, E.J., 2016. Separation of the metallic and non-metallic fraction from printed circuit boards employing green technology. Journal of Hazardous Materials 311, 91e99. Gu, W., Bai, J., Dong, B., Zhuang, X., Zhao, J., Zhang, C., Wang, J., Shih, K., 2017. Enhanced bioleaching efficiency of copper from waste printed circuit board driven by nitrogen-doped carbon nanotubes modified electrode. Chemical Engineering Journal 324, 122e129. Hadi, P., Xu, M., Lin, C.S.K., Hui, C.W., McKay, G., 2015. Waste printed circuit board recycling techniques and product utilization. Journal of Hazardous Materials 283, 234e243. Kemmlein, S., Hahn, O., Jann, O., 2003. Emissions of organophosphate and brominated flame retardants from selected consumer products and building materials. Atmospheric Environment 37, 5485e5493. Li, J., Xu, Z., Zhou, Y., 2007. Application of corona discharge and electrostatic force to separate metals and non-metals from crushed particles of waste printed circuit boards. Journal of Electrostatics 65, 233e238. Mdlovu, N.V., Chiang, C.L., Lin, K.S., Jeng, R.C., 2018. Recycling copper nanoparticles from printed circuit board waste etchants via a microemulsion process. Journal of Cleaner Production 185, 781e796. Marques, A.C., Cabrera, J.M., Malfatti, C.F., 2013. Printed circuit boards: a review on the perspective of sustainability. Journal of Environmental Management 131 (15), 298e306. Rocchetti, L., Amato, A., Beolchini, F., 2018. Printed circuit board recycling: a patent review. Journal of Cleaner Productioń 178, 814e832. ́ Sheldon, R.A., 2016. Green chemistry and resource efficiency: towards a green economy. Green Chemistry 18 (11), 3180e3183. Xiang, D., Mou, P., Wang, J., Duan, G., Zhang, H.C., 2007. Printed circuit board recycling process and its environmental impact assessment. International Journal of Advanced Manufacturing Technology 34, 1030e1036. Zhan, L., Xiang, X., Xie, B., Sun, J., 2016. A novel method of preparing highly dispersed spherical lead nanoparticles from solders of waste printed circuit boards. Chemical Engineering Journal 303, 261e267. Zeng, X.L., Gong, R.Y., Chen, W.Q., Li, J.H., 2016. Uncovering the recycling potential of “new” WEEE in China. Environmental Science and Technology 50 (3), 1347e1358.
11
Recycling printed circuit boards
Abhishek Kumar Awasthi1, 2, Xianlai Zeng1,2 1
School of Environment, Tsinghua University, Beijing, China; 2Key Laboratory for Solid Waste Management and Environment Safety, (Tsinghua University), Ministry of Education of China, Beijing, China
CHAPTER OUTLINE
11.1 Introduction 311 11.2 Economic benefits of recycling of PCBs 313 11.3 Emerging technologies for recycling of waste printed circuit boards
314
11.3.1 11.3.2 11.3.3 11.3.4
Disassembling 315 Physical-mechanical recycling process of PCBs 315 Size reduction and separation 316 Human health affected owing to the physical recycling process of waste PCB 317 11.3.5 The best available technology with opportunities and challenges 318 11.3.6 Dismantling 318 11.3.7 Technology for recovery of copper and other valuable metals 319
Acknowledgments 322 References 322 Further reading 325
11.1 INTRODUCTION Printed circuit boards (PCBs) are the supplier of electronic components and electronic interconnections (Akcil et al., 2015; Awasthi et al., 2016a). The key advantage of PCBs lies in its capability to greatly reduce the errors of routing and assembly and to increase the degree of automation and fabrication efficiency. Due to decades of development, PCBs have been constantly contributing to the improvement and progress of people’s modern lives (Arshadi and Mousavi, 2014).
Waste Electrical and Electronic Equipment (WEEE) Handbook. https://doi.org/10.1016/B978-0-08-102158-3.00011-2 Copyright © 2019 Elsevier Ltd. All rights reserved.
311
312 CHAPTER 11 Recycling printed circuit boards
These PCBs are used to mechanically support and electrically connect electronic components using conductive pathways, tracks, or signal traces etched from copper sheets laminated onto a nonconductive substrate, employed in the manufacturing of business machines and computers, as well as communication, control, and home entertainment equipment. PCBs are an essential part of almost all electric and electronic equipment, and have revolutionized the electronics industry (Ghosh et al., 2015). China shares almost half of the overall PCB industries’ market and is the fastest developing country in the PCBs production. In China, the total WEEE that is domestically generated and illegally imported, over 500,000 tons of waste PCBs, needs to be treated every year, and the quantity is mounting every year owing to falling average lifetime of electronic goods. There are three major types of PCB construction: (1) single-sided, (2) double-sided, and (3) multilayered. Single-sided boards have the components on one side of the substrate. When the number of components becomes too much for a single-sided board, a double-sided board may be used. Electrical connections between the circuits on each side are made by drilling holes through the substrate in appropriate locations and plating the inside of the holes with a conducting material. The third type, a multilayered board, has a substrate made up of layers of printed circuits separated by layers of insulation. The components on the surface connect through plated holes drilled down to the appropriate circuit layer. This greatly simplifies the circuit pattern (Sanapala, 2008). Copper is the most commonly used material for traces. Simple methods involve plating the entire board with copper, and then etching away unnecessary areas through a mask (stencil) to leave the required traces. More complex methods allow traces to be added onto a bare board. Each approach has associated pros and cons (Awasthi et al., 2016c, 2017a). Some boards require the use of gold for sensitive, low-voltage applications or lead-free (RoHS) compliance. Copper traces usually demand the use of a nickel barrier layer before gold-plating. This is to prevent gold from migrating into the copper. Indiscriminate use of nickel can result in huge losses to impedance (Yu et al., 2016a,b). A typical circuit board module includes a PCB and a variety of circuit board components soldered to the printed circuit board. The PCB is generally a laminated board with circuit traces on external surfaces of the board or at interlayer levels within the board, and the electrical components are typically light-emitting diodes (LEDs), processors, memory devices, clock generators, resistors, cooling units, capacitors, and virtually any other type of electrical component (Ghosh et al., 2015). PCBs generally comprise a
11.2 Economic benefits of recycling of PCBs 313
Table 11.1 Typical PCB components and their major compositional components Electronic component
Majority composition or materials
Resistors Capacitors
Ceramic, carbon Aluminum, electrolyte, plastics, etc., copper leads Steel, copper Plastic cases, copper leads, silicon Plastic cases, copper leads, silicon Copper, plastic Aluminum, steel Aluminum
Inductors, transformers Integrated circuits Transistors, diodes Connectors Mounting brackets Heat sinks
Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, pp. 287e311.
composite of organic and inorganic materials with external and internal metal traces, permitting assembled electronic components to be mechanically supported and electrically connected. The components themselves are a mixture of often quite sophisticated construction and include the components listed in Table 11.1. Additionally, the value of precious metals in waste PCBs are more profitable than in mining ores, which makes their recycling significantly important in terms of both environmental and economic views (Table 11.3). The average content and value ratio of different metals in PCBs is as presented in Table 11.2. As clearly seen, Au, Cu, Pd, and Ag metals account for nearly all of the economic material value in waste PCBs. For that reason, waste PCB recycling mainly is concerned with extraction of these valuable metals above all else.
11.2 ECONOMIC BENEFITS OF RECYCLING OF PCBS Waste PCBs are about 3% by weight of the total amount of waste EEEs. The recycling for waste PCB outcomes in quite a large variety of materials being part of the whole assembly, with the possibility of significant environmental impacts arising from both the material resources use and the effects of disposal. Precisely, a significant percentage of the embodied materials are metals that are valuable recycling as presented in Table 11.4.
314 CHAPTER 11 Recycling printed circuit boards
Table 11.2 Metal composition of waste PCBs (%) Element
Sn
Pb
Cu
Fe
Al
Sb
Zn
Ni
Cr
Cd (g/t)
Au (g/t)
Ag (g/t)
PCB ECs Total
10.12 3.20 6.0
3.20 0.68 1.7
21.62 13.80 16.9
0.21 19.49 11.8
1.36 6.91 4.7
0.001 6.11 3.7
0.056 5.66 3.4
0.036 0.65 0.4
0.027 0.53 0.3
0.53 14.45 8.9
e 40.76 24.4
194.91 112.68 145.7
Yang, C., Li, J., Tan, Q., Liu, L., Dong, Q., 2017. Green process of metal recycling: co-processing waste printed circuit boards and spent tin stripping solution. ACS Sustainable Chemistry and Engineering 5, 35243534.
Table 11.3 Metal content and economic value of waste PC boards (per ton) Metals
Content (%)a
Metal price ($/kg)b
Potential value ($)
Value ratio (%)
Cu Al Fe Ni Pb Sn Ag Au Pd Total
9.7 5.8 9.2 0.69 2.24 2.15 0.06 0.023 0.01 29.87
3.6 1.7 0.4 10.5 1.2 13 315 24,434 6100 e
349.2 98.6 36.8 72.5 27 279.5 189 5620 610 7282
4.8 1.35 0.51 0.99 0.37 3.84 2.6 77.17 8.38 e
a
Chris et al., 2007. London Metal Exchange, Nov., 2008. Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, 287e311. b
High worth of PCBs and possible risk to both the environment and human health drive the need for waste PCBs to be treated in an environmentally sound way (Awasthi et al., 2017a). The recycling of waste PCBs is a big challenge, due to the components, diversity and material complexity, as well as manufacturing processes (Li et al., 2015).
11.3 EMERGING TECHNOLOGIES FOR RECYCLING OF WASTE PRINTED CIRCUIT BOARDS Several technologies have been established and studied by the global scientific community, and it is very important to know the recycling technological innovations in management of PCBs. Therefore, the recent status of the international articles dealing with recycling of PCB is briefly discussed.
11.3 Emerging technologies for recycling of waste printed circuit boards 315
Table 11.4 The estimated benefits in terms of energy saving by recovery of resources from PCBs S.No.
Material
Energy savings (%)
1. 2. 3. 4. 5. 6. 7.
Aluminum Copper Iron and steel Lead Zinc Paper Plastic
95 85 74 65 60 64 >80
Data Source: Cui, J., Forssberg, E., 2004. Mechanical recycling of waste electric and electronic equipment: a review. Journal of Hazardous Materials B99, 243e263.
11.3.1 Disassembling The present recycling technologies offered for e-waste recycling must include a preliminary sorting or initial disassembly step. The dismantling/depollution of hazardous components is very important along with a very valuable step to get components, such as cables and PCBs, cell batteries and capacitors, etc., in order to achieve the successful recovery of secondary resource materials (Awasthi et al., 2017b,c). The automated disassembly of EEEs is innovative, it is mainly appropriately applied in developed countries (Duan et al., 2011). In addition, the hazardous materials are partly removed, mainly in case of small product WEEEs. This suggests that significant amounts of hazardous substances are still contained in the mechanical crushing step, which causes substantial distribution of different pollutants and perhaps decreases the amounts of valued recyclable resources (Song et al., 2015). Electronic components (ECs) have to be dismantled from PCB assembly as the best option to minimize the environmental pollution, and for resource conservation and reuse of components (Zlamparet et al., 2017).
11.3.2 Physical-mechanical recycling process of PCBs Waste PCB mechanical recycling can be generally divided into two main steps. The first step is disassembly and/or separation of different components as well as materials, commonly by mechanical or metallurgical treating to upgrade the desirable material content (Awasthi et al., 2017b). Shredding, electrostatic separation, and supercritical extraction are the
316 CHAPTER 11 Recycling printed circuit boards
main technologies employed in this step. The second step is the further separation or screening and processing of metal streams; this is possibly the most important step from environmental and economic perspectives. Several methods are available to extract metals from postprocessing waste PCBs (Yang et al., 2013, 2016; Xue et al., 2013; Wang et al., 2016). These technologies can be varied in terms of their economic feasibility, recovery efficiency, and environmental impact (Zeng et al., 2017). In the recycling of waste PCBs, selective dismantling is an essential process as subsequently: (1) The reuse of components has been become first priority; (2) Dismantling the hazardous components is very important; and (3) It is also important to recover the highly valuable components and high-grade materials such as batteries in order to simplify the subsequent recovery of materials. Most of the recycling plants operate with manual dismantling. For example, a typical dismantling process is operated at Ragn-Sells Elektronikåtervinning AB in Sweden (Cui and Forssberg, 2004). A multiple-use tool is used in the dismantling process for eliminating hazardous components in addition to recovery of valuable components and reusable materials.
11.3.3 Size reduction and separation Firstly, cutting, crushing, and grinding are procedures used to reduce waste PCBs in pieces of different particle sizes. It is recognized that cutting methods are considered better choices than crushing for the recycling processes of waste PCBs (Fig. 11.1). The purpose of crushing is to strip metals from the base plates of waste PCBs. Crushing technology is intimately related to not only energy consumption of crushing equipment but also further selective efficiency (Li and Zeng, 2012). A number of methods are available in order to separate the valuable material and nonmaterial, established principally on three different bases: physical, magnetic, and electrostatic. Out of these, the magnetic separation is applicable in a dry environment; a stable magnet iron separator then uses an eddy current system and wet environment. Certainly additional methods applied to separate both metallic and non-metallic materials must be able to separate based on electrostatic theory. Normally, these approaches depend on high electrostatic voltage, although we can use the multiroller separation devices with high voltage. With the electrostatic separation it is possible to get a powder of copper with a high content. Other mechanical procedures depend on the range of size, mass, shape, as well as density of the particles. Screening has not only been utilized to prepare a uniformly sized feed to certain mechanical processes but also to upgrade metals
11.3 Emerging technologies for recycling of waste printed circuit boards 317
Waste PCB
Unit components
Dismantling
Residues PCBs
Crushing
Screening / separation Metals such as copper
Non-metals
n FIGURE 11.1 Brief outline of mechanical processes for the recycling of waste PCBs. Redrawn from
source: Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, 287e311.
contents. Screening is necessary because the particle size and shape properties of metals are different from that of plastics and ceramics. The primary method of screening in metals recovery uses the rotating screen, or trammel, a unit that is widely used in both automobile scrap and municipal solid waste processing.
11.3.4 Human health affected owing to the physical recycling process of waste PCB As using the spraying water method and sound insulation measures are adopted, the human health damage resulting from industrial dust and noise is very little from the above analysis. So we mainly present the damage from the metal ions in wastewater. The main metal ions are Cu, Au, Cd, Pd, Pb, Sn, Ni, and Ag in wastewater (Table 11.5). However, there are
Table 11.5 The human toxicity of metal in wastewater (mg 1,4-dichlorobenzene eq) Metal ion
Cu
Cd
Pb
Sn
Ni
The equivalency factor for human toxicity Environmental impact on human toxicity Total of environmental impact on human toxicity
1.3 0.546
2.3 0.345
1.2 0.24 14.3
1.7E-2 8.5E-4
3.3E2 13.2
Reproduced from Li, J., Zeng, X., 2012. Chapter 13 e Recycling Printed Circuit Boards. Waste Electrical and Electronic Equipment (WEEE) Handbook. A Volume in Woodhead Publishing Series in Electronic and Optical Materials, 287e311.
318 CHAPTER 11 Recycling printed circuit boards
metals, viz. Cu, Cd, Pb, Sn, and Ni, that cause damage to human health or to human toxicity, which are presented in Table 11.5.
11.3.5 The best available technology with opportunities and challenges In developing countries, mainly primitive technologies are the main obstacle to the recycling of waste PCBs. During the manual dismantling process in informal recycling sites, e-waste recyclers use chisels, hammers, and cutting torches to open solder connections and separate numerous types of metals and components. Many times PCBs are simply cooked on a coalheated plate and melted. Undoubtedly, the manual dismantling process in informal sites in China was quite common about 15 years ago, however, it has been prohibited according to Chinese environmental law. In fact, such practice is still reported with little development by using an electric heating plate system. The fact that the PCB assembly is one of the fastest growing sources of waste in many developing countries has focused attention on the need to recycle, recover, and reuse materials that have been consigned to informal dismantling sites. In developing countries such as China, India, or Nigeria, the above-mentioned methods have been widely used as well. The major common point of these disassembling technologies is the recovery of the solder remaining on the board by subjecting it to a temperature much higher than the molten point of the solder. In these processes of PCB assembly dismantling, pyrolysis under high temperature heating, during which the toxic products from resins and adhesives are decomposed, is a common occurrence (Williams et al., 2008).
11.3.6 Dismantling Dismantled PCB assemblies have a significant environmental impact because they contain different heavy metals and halogen-containing flame retardant, for example, cadmium (pins), lead (soldering tin), mercury (switches, round cell batteries), brominates, and mixed plastics that can seep into the environment if not properly managed (Song et al., 2015). Cell batteries might ignite or leak potentially hazardous organic vapors if exposed to excessive heat or fire. Explosion may result if a capacitor is subjected to high currents and heating. The round cell batteries and capacitors that are large or contain polychlorinated biphenyl should be manually removed and separately disposed by following an appropriate method. The circuit boards can then be sent to a facility for further dismantling
11.3 Emerging technologies for recycling of waste printed circuit boards 319
(for reuse or reclamation from ICs that contained precious substances or soldering tin) and copper recovery (from bare board) works (Zeng et al., 2017). Substitutes for lead in solders are currently being developed, but are not yet in production (Duan et al., 2011). While the melting of soldering tin could lead to the separation and recycling of electronic components, in addition to the melting of soldering tin, the mechanical strength of the pin that is packaged to the through hole is another key factor in separating the components.
11.3.7 Technology for recovery of copper and other valuable metals In developing new technology for waste PCBs, most researchers have focused on the technology by which the valuable metals can be separated and recovered from waste PCBs. Firstly, very simple incineration (without any control system in incineration, open burning, etc.) was used to recover valuable metals from waste PCBs, this practice lacked proper environmental and human health protection (Awasthi et al., 2016b); although this process is banned in China, it is still summarized (Song and Li 2014). Pyrometallurgy is a technology for recovery of non-ferrous metals and precious metals from waste PCBs. Pyrometallurgy involves incineration, smelting in a plasma arc furnace or blast furnace, drossing, sintering, melting, and reactions in a gas phase at high temperature. The pyrolysis process is the chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents (Quan et al., 2010; Yang et al., 2013). Pyrolysis of organic materials contained in waste PCBs leads to the formation of gases, oils, and chars that can be used as chemical feedstocks or fuels (Ebin and Isik, 2016; Vehlow and Mark, 1997). There are some pilot-scale studies on the recovery of metals from waste PCBs by using pyrolysis in China (Li et al., 2010). Hydrometallurgy is one more conventional technology for the recovery of precious metals from waste PCBs. The main steps in hydrometallurgy consist of a series of acid or caustic leaches (such as cyanide leaching, halide leaching, thiourea leaching, and thiosulfate leaching, etc.) of solid materials (Akcil et al., 2015). The solutions are then subjected to separation and purification procedures, for example, precipitation of impurities, solvent extraction, adsorption, and ion exchange to isolate and concentrate the metals of interest. Consequently, the solutions are treated by electrorefining process, chemical reduction, or crystallization for metal recovery (Yang et al., 2017; Ha et al., 2014).
320 CHAPTER 11 Recycling printed circuit boards
Mechanical/physical recycling method for waste PCBs is based on the differences of materials in physical characteristics (including density, electric conductivity, magnetic susceptibilities, etc.). Owing to its better environmental properties (such as comparatively generating less wastewater), easier operability and high efficiency, additionally nonferrous metals and precious metals contents have gradually decreased in concentration in PCBs (Song et al., 2015; Zeng et al., 2017). Through mechanical recycling process for waste PCBs, the materials can be separated out into metallic and non-metallic. There are about 30% metallic materials after separation, which is hard to recover, because the fractions concentrated on metallic materials obtained from these processes are still a mixture of various metals (copper, aluminum, lead, zinc, etc.) (Wang et al., 2017; Yoo et al., 2009). The present mechanical technologies (pneumatic separation, electrostatic separation, etc.) focus on recovering the copper, but the studies on further separation of the mixed metals are relatively fewer in cases of studies in China (Wu et al., 2009). In order to further separate the concentrated fraction in metals and increase the copper content in the metallic mixture, vacuum metallurgy separation method was presented in some studies (Huang et al., 2009). Zheng et al. (2009a,b) studied a novel fluidized bed process technology for recycling glass fibers for non-metallic materials. The glass fibers are collected at high recovery rate by cyclone separators without violating the environmental regulations. Physical recycling of the non-metallic fractions is an efficient recycling method without environmental pollution, such as waste water generation, but needs capital investments in terms of equipment setup (Li and Zeng, 2012). Thus, more research should be done to develop comprehensive and industrialized usage of the non-metallic fractions recycled by physical methods. The trend in chemical recycling methods for the non-metallic fractions from waste PCBs is in order to make the best of advantages over physical recycling of the non-metallic fractions to compensate the higher cost of chemical recycling methods. The treatment and elimination of hazardous substances contained in the non-metallic fractions is an ultimate method to delete the pollution. The research in this area is just beginning and the challenges caused through technical and economic feasibility should not be underestimated (Guo et al., 2009). To achieve a clean separation among the metallic and the non-metallic fractions from waste PCBs is a way to reduce the contents of heavy metals in the non-metallic portions and therefore is an approach to minimize the potential environmental risk for the recycling of the non-metallic fractions. The removal of flame retardants and bromine recovery seem to be a way to treat hazardous substances contained
11.3 Emerging technologies for recycling of waste printed circuit boards 321
in the non-metallic fractions from e-waste and to reduce the pollution caused by the formation of PBDD/Fs. The flame retardants contained in waste PCBs are usually reactive ones, which cannot be extracted before the degradation of the thermosetting resins. Catalytic hydrogenation can be an effective way to remove most of the hazardous toxic compounds in the oil produced by chemical recycling of the non-metallic fractions from waste PCBs (Yu et al., 2016a). The bioleaching method is based on microbial capability to utilize organic and inorganic substrates, in that way removing the metals. Many investigators have studied the potential of bioleaching method for leaching metals from waste PCBs (Arshadi and Mousavi, 2014; Awasthi et al., 2016a,b,c; Karwowska et al., 2014). In this context, Willner and Fornalczyk (2013) studied the different parameters, such as the effect of pH, inoculum concentration, and the oxidationereduction potential of bioleaching of copper metal from waste PCBs applying a bacterial culture of Acidithiobacillus ferrooxidans. Generally, fungi have a promising ability to produce various types of organic acids, and different kind of metabolites can be used in the process of metal leaching from waste PCBs. These fungal-produced organic acids plays a significant role in the metal speciation step in key fungalmediated processes (fungi such as Aspergillus niger and Penicillium sp.) (Awasthi et al., 2017a; Brandl et al., 2001). In addition, a number of researchers have studied the feasibility of copper bioleaching from finemilled waste PCBs (at concentration up to 10.0 g/L), and around 90% Cu recovery was achieved by using mesophilic microbes (Bas et al., 2013; Brandl et al., 2001). Each technology has certain limitations, advantages, and disadvantages (Awasthi et al., 2016c, 2017a). Therefore, it is essential to develop a more appropriate process for the more effective and ecofriendly recycling of waste PCBs. In this context, Awasthi et al. (2016a,b,c) suggested an integrated system by means of combining physical and biological methods for better metal recovery, however, this approach needs detail research for better understanding. Therefore, this chapter highlights the significant progress with the waste PCBs recycling, in standings of technological improvement. Besides, the existence of various kinds of PCBs in electronic equipment could be a possible supply of secondary resources (e.g., raw materials) in terms of resourcefully achieving the circular economy approaches. Without any doubt, these waste PCBs show great potential value, but the number of formal recycling plants appropriate for its management, is relatively less globally.
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ACKNOWLEDGMENTS This work is supported by “National Key Technology R&D Program” of China (2017YFF0211604), and Key Laboratory for Solid Waste Management and Environment Safety (Tsinghua University), Ministry of Education of China (No. SWMES 2017-12). We are also thankful to Dr. Jinhui Li, Professor, School of Environment, Tsinghua University, Beijing China, for guidance, suggestions and kind support.
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