Research Article pubs.acs.org/journal/ascecg
Pyrolysis-Based Technology for Recovering Copper from Transistors on Waste Printed Circuit Boards Mengkun Hu, Jianbo Wang, and Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: Waste electronic components (ECs) recycling is a crucial part of waste printed circuit boards (WPCBs) recycling system. Waste transistors (WTs) are one of the largest obsolete ECs whose recycling technology has been poorly developed. This study presented a pyrolysis-based technology for recovering copper from WTs disassembled from WPCBs, including coarse crushing, pyrolysis, pulverizing, and sieving. First, the WTs were coarsely crushed to reduce the particle size of the epoxy molding compound (EMC). Second, the coarsely crushed WTs were pyrolyzed to damage the cross-linking network structure of EMC and reduce the hardness of EMC. Third, all of the pyrolysis residues were crushed and sieved to separate metals and nonmetals. Finally, an integrated recycling process was proposed based on above studies. In addition, the mechanism of the pyrolysis was analyzed based on bond theory and product analysis to deeply understand the recycling process. Under optimal conditions, 99.16% of copper in WTs were recovered with a purity of 92.75%. This paper provides a method for waste transistors recycling in an efficient and environmentally friendly way. KEYWORDS: Waste transistor, Electronic component, Copper recovery, Pyrolysis, Crushing
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and central process units,12 with the main target of noble metals. However, the “not so precious” ECs also ought to be drawn more attention to, for the purpose of completing the WPCBs recycling system, preventing environmental pollution, and promoting the sustainable utilization of resources Transistors, as fundamental electronic components mounted on PCBs, have been widely used in the electronics industry due to their advantages of low cost, flexibility, and strong reliability. As one of the largest consumable electronic components, transistors account for 16.38% of the total ECs weight.13 Not surprisingly, large quantities of waste transistors (WTs) are discarded continuously with e-waste stream. Transistors contain tons of recoverable materials, such as copper (∼50 wt %) and silver (∼0.035 wt %), whose contents are much higher than their respective primary resources. Meanwhile, WTs are classified as hazardous waste by Directory of National Hazardous Wastes (2016 Revision) in China due to the constituents of heavy metals (such as lead) and toxic organics. Hence, WTs should be disposed and recycled properly concerning both resources sustainable utilization and environmental protection. However, there has been no research aiming at WTs recycling up to now.
INTRODUCTION With the development of the electronics industry and shorter lifespans of electronic products, the output of waste electrical and electronic equipment (WEEEs) increased rapidly in recent years.1,2 It is estimated that the global generation of e-waste was to be approximately 49 million metric tons in 2012 and the number would reach 65.4 million by 2017.3 Printed circuit boards (PCBs) are integral parts of electronic products.4 Generally, waste PCBs (WPCBs) make up 3−4% by weight of the total WEEEs.5 Meanwhile, WPCBs are resource-rich but hazardous, containing value metals, heavy metals, and toxic organic substances. Therefore, safe disposal of WPCBs is necessarily demanded to protect the environment and utilize resources sustainably. A lot of efforts have been made to solve the problem of depth and refinement recycling of waste PCBs (WPCBs).6−8 At present, the WPCBs recycling system consists of (1) dismantling electronic components (ECs) from WPCBs, (2) disposing and recycling of base boards, and (3) classification and recycling of ECs. As a matter of fact, the classification and recycling of ECs is the weak link among the holistic recycling chain of WPCBs due to the difficult and complex recycling process caused by their complicated natures.5 This issue of waste ECs recycling has not been well solved by now. In recent years, there are several studies focusing on waste ECs recycling, which mostly emphasized the recycling of highly valuable ECs, such as tantalum capacitors,9,10 monolithic ceramic capacitors11 © 2017 American Chemical Society
Received: July 15, 2017 Revised: September 16, 2017 Published: October 25, 2017 11354
DOI: 10.1021/acssuschemeng.7b02375 ACS Sustainable Chem. Eng. 2017, 5, 11354−11361
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. WTs environmentally friendly disassembled from WPCBs by electronic components automatic disassembling machine (ECs-ADM) and manual sorting.
Virtually, the metallic components are tightly covered by the nonmetallic encapsulation materials (epoxy molding compound, EMC). Hence, the prerequisite of the metals recovery is the liberation of metals and nonmetals. However, on the basis of our preliminary tests, good liberation of metals and nonmetals cannot be obtained via simple crushing due to high hardness and tenacity of EMC which comprises of the epoxy resin and organic hardener with a cross-linking network structure.14,15 Hence, the cross-linking network structure should be first damaged to lower the hardness of EMC and facilitate the crushing. Pyrolysis, which has been widely used in disposal of WEEEs, is proved to be an efficient and promising technology for removing and recovering organic materials compared to other methods, such as combustion and solvent leaching.16−19 Nonetheless, the good liberation performance also cannot be obtained via simple pyrolysis, due to high thickness and dense structure of EMC, which resulted in a big temperature gradient between surface and inside of EMC and impeded the pyrolysis process of organic components. All above analyses of the preliminary tests indicated that the liberation of metals and nonmetals is the core of WTs recycling and a suitable technical route is required to recycle WTs efficiently. The objective of this research is to propose an efficient technology for recovering value materials (copper) from WTs. The pyrolysis of WTs was primarily focused on, including thermal characteristics analysis of EMC, optimum design for pyrolysis treatment, and identification of pyrolysis products. Then, the subsequent recycling processes were investigated. Finally, the pyrolysis mechanisms were analyzed to get a better understanding of the recycling process.
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Figure 2. Structure of discrete packaging DPAK transistors.
Table 1. Mass Fraction of the Main Metals of Raw Materials element
Cu
Al
Pb
Sn
Ag
content (wt %)
54.570
1.753
0.649
0.474
0.035
Table 2. Composition of EMC in WTs21−23 component
EXPERIMENTAL SECTION
content (wt %)
typical material
epoxy resin hardener
10−20 5−15
filler flame retardant
60−90 1−5
coupling agent accelerator (catalyst) others
