Waste Management xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review Ruixue Wang, Zhenming Xu ⇑ School of Environmental Science & Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 21 November 2013 Accepted 5 March 2014 Available online xxxx Keywords: Waste electrical and electronic equipment (WEEE) Non-metallic fractions (NMFs) Recycling Environmental assessment
a b s t r a c t The world’s waste electrical and electronic equipment (WEEE) consumption has increased incredibly in recent decades, which have drawn much attention from the public. However, the major economic driving force for recycling of WEEE is the value of the metallic fractions (MFs). The non-metallic fractions (NMFs), which take up a large proportion of E-wastes, were treated by incineration or landfill in the past. NMFs from WEEE contain heavy metals, brominated flame retardant (BFRs) and other toxic and hazardous substances. Combustion as well as landfill may cause serious environmental problems. Therefore, research on resource reutilization and safe disposal of the NMFs from WEEE has a great significance from the viewpoint of environmental protection. Among the enormous variety of NMFs from WEEE, some of them are quite easy to recycle while others are difficult, such as plastics, glass and NMFs from waste printed circuit boards (WPCBs). In this paper, we mainly focus on the intractable NMFs from WEEE. Methods and technologies of recycling the two types of NMFs from WEEE, plastics, glass are reviewed in this paper. For WEEE plastics, the pyrolysis technology has the lowest energy consumption and the pyrolysis oil could be obtained, but the containing of BFRs makes the pyrolysis recycling process problematic. Supercritical fluids (SCF) and gasification technology have a potentially smaller environmental impact than pyrolysis process, but the energy consumption is higher. With regard to WEEE glass, lead removing is requisite before the reutilization of the cathode ray tube (CRT) funnel glass, and the recycling of liquid crystal display (LCD) glass is economically viable for the containing of precious metals (indium and tin). However, the environmental assessment of the recycling process is essential and important before the industrialized production stage. For example, noise and dust should be evaluated during the glass cutting process. This study could contribute significantly to understanding the recycling methods of NMFs from WEEE and serve as guidance for the future technology research and development. Ó 2014 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling of plastics from WEEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Composition of plastics from WEEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Recycling plastics from WEEE by pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Recycling plastics from WEEE in SCF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Recycling plastics from WEEE by gasification recycling technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling glass from WEEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Recycling of CRT glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Recycling CRT glass by removing lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Reutilization of CRT glass as other products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Recycling of LCD glass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
00 00 00 00 00 00 00 00 00 00 00 00 00 00
⇑ Corresponding author. Tel./fax: +86 21 54747495. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.wasman.2014.03.004 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
2
R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
1. Introduction With the development of science and technology, the electronics industry has become one of the fastest-growing sectors in the world. At the same time, large amounts of waste electrical and electronic equipment (WEEE) are generated worldwide, which could bring serious environmental problems (Ni and Zeng, 2009). It is estimated that 20–50 tones of WEEE are generated per year (Robinson, 2009), and this figure is growing by about 40 million a year. However, WEEE contains not only toxic and hazardous contaminants, but also valuable and precious resources if treated in a proper way (Cui and Forssberg, 2003; Ongondo et al., 2011). In recent years, recycling of E-waste has drawn more and more attention from the public and research institutions because of the environmental protection and economic benefits. WEEE contains a variety of valuable materials, such as metals, glass, plastics and other materials (shown in Fig. 1) (Widmer et al., 2005). Much research has been done to recover and reuse the valuable materials of the E-wastes. However, most of the works are focus on recycling the MFs (Huang et al., 2009; Li and Xu, 2010; Ma et al., 2012b; Xue et al., 2012; Zhan and Xu, 2009) because of the economic benefits. The NMFs, which take up a large proportion of E-wastes, were mostly treated by incineration or landfill in the past (Zhou and Xu, 2012). However, the NMFs from WEEE contain heavy metals, brominated flame retardants (BFRs) and other toxic and hazardous substances. Combustion of the NMFs may cause the formation of polybrominated dibenzodioxins and dibenzofurans (PBDD/Fs), and landfill may lead the pollution of the groundwater which caused by brominated flame retardants (BFRs) and heavy metals (Guo et al., 2009). It has been reported that illegal and unsafe recycling operations of WEEE have been conducted in the town of Guiyu in Guangdong, China, which has become a booming recycling center for WEEE arriving from various regions of the world since 1995. Metals were recycled from WPCBs, PVC-coated wire and cables by open burning (seen in Fig. 2). Such unsafe recycling practice causes severe environmental pollution from heavy metals, fire retardants, and the secondary formation of polyhalogenated dibenzo-p-dioxins and dibenzofurans (Chan et al., 2007; Söderström and Marklund, 2002; Wang et al., 2005). In Dongli Wang’s study, the PBDEs were detected in the soil and sediment samples at levels of 0.26–824 ng/g (dry weight) (Wang et al., 2005). Moreover, children living in Guiyu had significantly higher blood lead levels (BLLs) or blood cadmium levels (BCLs) as compared with those living in Chendian (p < 0.01). 70.8% of children (109/154) had BLLs > 10 lg/dL, and 20.1% of children (31/154) had BCLs > 2 lg/L, compared with 38.7% of children (48/124) had BLLs > 10 lg/dL and 7.3% of children (9/124) had BCLs > 2 lg/L in Chendian (p < 0.01, respectively) (Zheng et al., 2008).
Fig. 1. Typical material fractions in WEEE (Widmer et al., 2005).
Fig. 2. Open burning of wires and other parts to recover metals (Guo et al., 2009).
Therefore, research on resource reutilization and safe disposal of the NMFs from WEEE has a great significance from the viewpoint of environmental protection. Among the enormous variety of NMFs from WEEE, some of them are quite easy to recycle, such as outer shells of household appliances, which could be recycled by crushing, melting, pelleting and regeneration. Others are difficult to recycle, such as NMFs from waste WPCBs, which contain complicated and hazardous ingredients; CRT glass, which contains heavy metal lead; LCD glass, which contains indium tin oxide (ITO); metal-plastic mixture, which need to be separated before the recycling process (shown in Fig. 3). However, some authors have previously written review papers on recycling of NMFs of WPCBs (Guo et al., 2009; Marques et al., 2013). In their reviews, both chemical recycling methods and physical recycling methods (e.g., reusing the NMFs as fillers of different kinds of materials) for recycling the NMFs from WPCBs were reviewed in detail. In this study, we mainly focus on two kinds of intractable NMFs from WEEE, plastics and glass. As a matter of fact, recycling of the NMFs from WEEE is a big challenge. Firstly, the composition of the NMFs from WEEE is very complicated. Laptops, for example, mainly contain plastics, glass, NMFs from WPCBs, etc. Secondly, NMFs from WEEE may contain organic and inorganic contaminants which may cause serious environmental problems (Schlummer et al., 2007b; Weber and Kuch, 2003). However, much work has been carried out to recover and reuse the NMFs from WEEE, of which material recycling and energy recovery are two key points of the recycling process. In this paper, we mainly focus on the intractable NMFs from WEEE, plastics and glass. Methods and technologies of recycling the two types of NMFs from WEEE, analysis and treatment for the hazardous substances contained in the NMFs were reviewed. Chemical recycling methods consisting of pyrolysis, SCF depolymerization and gasification were widely used in recycling plastics from WEEE. Even more important, environmental risks such as heavy metals and BFRs will be the major problems during the recycling processes. Therefore, environmental assessments of the recycling processes are essential. As for WEEE glass, the main problem for recycling the CRT glass is the funnel glass which contains a significant amount of lead. Therefore, lead removing is requisite before the reutilization. With regard to LCD glass, the recycling of precious metals (indium and tin) will be economically viable. Acid leaching is efficient recycling indium and tin, however, large quantities of waste corrosive and volatile acid might increases the environmental risk. Chloride metallurgy seems to be a promising method for recovery of indium from LCD glass without producing hazardous substances. In short, this study could contribute significantly to understanding the recycling methods of NMFs from WEEE and serve as guidance for the future technology research and development.
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
3
Percentage of Plastic (Wt%)
Fig. 3. (a) Waste outer shells of household appliances, (b) WPCBs, (c) waste CRT glass, and (d) waste LCD glass.
60 49.1
50 40 30
23.6 17.5
15.7
20
16.5
10.4
10
3.5
0
Fig. 4. The percentage of plastic in different appliances.
2. Recycling of plastics from WEEE Typical E-wastes contain about 10–30 wt% of plastics (Taurino et al., 2010), which often contain brominated flame retardants (BFRs), including polybrominated diphenyl ethers (PBDE) and tetrabromobisphenol A (TBBA) (Luda and Balabanovich, 2011; Menad et al., 1998; Schlummer et al., 2007a). WEEE plastics may include more than 15 different polymers which make the recycling process difficult (Schlummer and Maeurer, 2006; Schlummer et al., 2006). However, the main problem of the plastics recycling process is the containing of BFRs, which might trigger serious environmental pollutions. Therefore, environmental assessments of the recycling processes are of the essence. 2.1. Composition of plastics from WEEE A direct analysis study to characterize the plastic constituents of WEEE was developed (Martinho et al., 2012). 3400 items, including cooling appliances, small WEEE, central processing units, copying equipment, printers, CRT monitors and CRT televisions were
characterized. The percentage of plastic in different appliances is summarized in Fig. 4, which showed that most WEEE contains 10–30% of plastics. For small WEEE, this figure was particularly up to 49.1%, while CPUs had the lowest ratio of plastics. The types of plastics found in the analyzed appliances according to Graca Martionho et al. are shown in Fig. 5. The results showed that polystyrene (PS), acrylonitrile–butadiene–styrene (ABS), polycarbonate blends (PC/ABS), high-impact polystyrene and polypropylene (HIPS) were the dominant polymers. With regard to the variety of polymers, large cooling appliances, CRT monitors and CRT televisions had the lowest numbers of different polymers, with up to about 10 different types, while printers, copying equipments and CPUs have exceed 13 different types of polymers. In the small WEEE case, the number reached 21 different types. Furthermore, BFRs, metals and color additives were also found in the WEEE plastics. 2.2. Recycling plastics from WEEE by pyrolysis Incineration has been widely applied in the plastic waste recycling process during which plenty of energy could be obtained and used for electricity generation or some other processes (Astrup et al., 2009). However, incineration could be dangerous to the environment because of the emission of toxic components including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dioxins (PCDs) which can be found in the fly ash and the residues (Ban et al., 1997). Pyrolysis is a process during which materials are heated to high temperatures in the absence of oxygen and decompose into smaller molecules. Compared with combustion, pyrolysis has been proved to be a more appropriate technology (Alston and Arnold, 2011; Alston et al., 2011; Chien et al., 2009) for several reasons. Firstly, the generation of gas would reduce to 5–10% compared with combustion. Secondly, the contaminants would be generated in the residue while large amount of toxic gas would leak to the environment during the combustion process. Thirdly, the contaminants would not leak out since the whole pyrolysis system is sealed (Scheirs, 1998). The pyrolysis products of the waste plastics can potentially be used as
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
4
R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
100% 80%
80%
Large cooling appliances
76%
60%
69%
CRT monitors
60% 40% 20%
40%
20%
20%
8% 1% 1% 4% 0% 0% 4% 0%
6%
4%
1%
0%
0%
2%
2%
2%
0%
0%
40%
60%
CRT televisons
43%
23%
40%
20%
20%
19%
15% 4%
8%
2% 1%
0% 1% 2%
0%
5% 0% 0% 0% 0% 0%
3%
0%
60%
Copying equipments 40%
21%
14%
10%
7% 2%
Printers
31% 30%
60% 44%
38% 32%
CPUs
40% 22%
20%
14%
20% 8%
6% 0% 1% 1% 0% 0% 0% 1% 1%
0%
5%
1%
1% 3%
8% 0%
5%
7% 1% 1%
1% 1% 2%
0%
40%
Small WEEE
28%
30%
20% 20% 10%
14% 10%
8% 3%
1% 0% 0% 0%
3%
6% 1% 1% 0%
2% 0% 1% 0% 0% 0% 0% 0% 0% 0%
0%
Fig. 5. Plastic compositions for WEEE appliances (Martinho et al., 2012).
Fig. 6. Schematic diagram of the fixed bed reactor (Hall and Williams, 2007).
fuels, raw materials for the petrochemical industry (Mastellone and Arena, 2004). However, pyrolysis is not so perfect since the containing of BFRs make the recycling process problematic. Some research has been done and developed to recycle WEEE plastics using the pyrolysis method. Three types of plastic (respectively from cathode ray tubes, refrigeration equipment, and mixed WEEE) fractions were collected and investigated to analysis the pyrolysis products. (Hall and Williams, 2007) The plastic wastes were pyrolyzed in the fixed bed reactor which was heated to 600 °C at a heating rate of 10 °C min 1. The schematic diagram of the fixed bed reactor is shown in Fig. 6. The pyrolysis products consist of oil, char and gas, among which oil make up the overwhelming majority (seen in Fig. 7). GC–MS was used to characterize the pyrolysis products, which showed that the oils mainly contained styrene, phenol, and other aromatic compounds. Meanwhile, few nitrogenated and oxygenated compounds as well as very few organic halogens were found in the pyrolysis oils. The GC–MS results are shown in Table 1, which indicated that the oil can be used as fuels. Furthermore,
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
5
R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
several works have been developed by Hall et al. to dispose the WEEE plastics using the pyrolysis process (Hall et al., 2008, 2007; Hall and Williams, 2006a,b, 2008). Two types of plastics contained BFRs have been pyrolyzed in the presence of two zeolite catalysts. (Hall and Williams, 2008). The flame retarded plastics were, acrylonitrile–butadiene–styrene (ABS) that was flame retarded with tetrabromobisphenol A (TBBA) and high impact polystyrene (HIPS) that was flame retard with
Oils Char Gas
80
Weight (%)
60
40
20
0
CRT
Refrigeration
Mxied WEEE
Fig. 7. Composition of the pyrolysis products (Hall and Williams, 2007).
decabromodiphenyl ether. They investigated the pyrolysis in the presence of two catalysts, zeolite ZSM-5 and zeolite Y-Zeolite, in a fixed bed reactor at a final temperature of 440 °C. It had been showed that the presence of the catalysts led to a reduction of the amount of the oil because of the formation on the surface of the catalysts. The Y-Zeolite was found to be very effective at removing the organobromine compounds. However, both catalysts were not so effective at removing the inorganobromine from the volatile pyrolysis products. The environmental impact of pyrolysis of mixed WEEE plastics was studied (Alston and Arnold, 2011; Alston et al., 2011). Waste WEEE plastics were pyrolyzed in the designed equipment based on a pilotplant. The schematic of experimental equipment is shown in Fig. 8. Data were collected from the pyrolysis experiments, the results showed that around 70 wt% of the mixed plastic can be converted to potential fuel. A life cycle assessment (LCA) was studied to assess the different disposal processes, that is, landfill, incineration and pyrolysis. The comparison of disposal processes for main impact categories is shown in Table 2. It turned out that none of them was the ‘‘best’’ option. Landfilling has the least short-term impact on climate change, but requires large amounts of space. Incineration was the most effective method of reducing carbon deposits, but produced the most greenhouse gases. Compared with the former two processes, pyrolysis seems to be a strong compromise candidate, since it saved most resources without emissions of a large amount of greenhouse gas or requirement of much space.
Table 1 Composition of the oil resulting from the pyrolysis process (Hall and Williams, 2007). CRT plastic
Refrigeration plastic
Mixed WEEE plastic
Name
Area (%)
Name
Area (%)
Name
Area (%)
Toluene Ethylbenzene Styrene Cumene Propylbenzene a-Methylstyrene 1-Ethenyl-2-methylbenzene 2,5-Dimethylphenol (E)-2-Phenyl-2-butene Benzyl nitrile 2-Methylindene a-Methyl-benzeneacetonitrile 2-Phenylcyclopropanecarbonitrile Benzenebutanenitrile 2-Phenylcyclopropanecarbonitrile (1-Methyl-3-butenyl)benzene 2-Phenylcyclopropanecarbonitrile Bibenzyl 1,2-Diphenylpropane 1,3-Diphenylpropane 1-Naphthaleneacetonitrile 1,2-Diphenylcyclopropane 1,3-Diphenyl-1-butene (3-Phenylbutyl)benzene (E)-Stilbene 1,2-Diphenylcyclopropane 1,4-Diphenylbutane 1,3-Diphenyl-1-butene 1,3-Diphenyl-1-butene Hexadecanenitrile 2-Phenylnaphthalene Unknown m-Terphenyl Unknown 1,3,5-Triphenylbenzene
3.2 8.3 19.3 1.7 0.1 6.8 0.1 0.9
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review Ruixue Wang, Zhenming Xu ⇑ School of Environmental Science & Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 21 November 2013 Accepted 5 March 2014 Available online xxxx Keywords: Waste electrical and electronic equipment (WEEE) Non-metallic fractions (NMFs) Recycling Environmental assessment
a b s t r a c t The world’s waste electrical and electronic equipment (WEEE) consumption has increased incredibly in recent decades, which have drawn much attention from the public. However, the major economic driving force for recycling of WEEE is the value of the metallic fractions (MFs). The non-metallic fractions (NMFs), which take up a large proportion of E-wastes, were treated by incineration or landfill in the past. NMFs from WEEE contain heavy metals, brominated flame retardant (BFRs) and other toxic and hazardous substances. Combustion as well as landfill may cause serious environmental problems. Therefore, research on resource reutilization and safe disposal of the NMFs from WEEE has a great significance from the viewpoint of environmental protection. Among the enormous variety of NMFs from WEEE, some of them are quite easy to recycle while others are difficult, such as plastics, glass and NMFs from waste printed circuit boards (WPCBs). In this paper, we mainly focus on the intractable NMFs from WEEE. Methods and technologies of recycling the two types of NMFs from WEEE, plastics, glass are reviewed in this paper. For WEEE plastics, the pyrolysis technology has the lowest energy consumption and the pyrolysis oil could be obtained, but the containing of BFRs makes the pyrolysis recycling process problematic. Supercritical fluids (SCF) and gasification technology have a potentially smaller environmental impact than pyrolysis process, but the energy consumption is higher. With regard to WEEE glass, lead removing is requisite before the reutilization of the cathode ray tube (CRT) funnel glass, and the recycling of liquid crystal display (LCD) glass is economically viable for the containing of precious metals (indium and tin). However, the environmental assessment of the recycling process is essential and important before the industrialized production stage. For example, noise and dust should be evaluated during the glass cutting process. This study could contribute significantly to understanding the recycling methods of NMFs from WEEE and serve as guidance for the future technology research and development. Ó 2014 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling of plastics from WEEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Composition of plastics from WEEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Recycling plastics from WEEE by pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Recycling plastics from WEEE in SCF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Recycling plastics from WEEE by gasification recycling technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recycling glass from WEEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Recycling of CRT glass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Recycling CRT glass by removing lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Reutilization of CRT glass as other products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Recycling of LCD glass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
00 00 00 00 00 00 00 00 00 00 00 00 00 00
⇑ Corresponding author. Tel./fax: +86 21 54747495. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.wasman.2014.03.004 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
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R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
1. Introduction With the development of science and technology, the electronics industry has become one of the fastest-growing sectors in the world. At the same time, large amounts of waste electrical and electronic equipment (WEEE) are generated worldwide, which could bring serious environmental problems (Ni and Zeng, 2009). It is estimated that 20–50 tones of WEEE are generated per year (Robinson, 2009), and this figure is growing by about 40 million a year. However, WEEE contains not only toxic and hazardous contaminants, but also valuable and precious resources if treated in a proper way (Cui and Forssberg, 2003; Ongondo et al., 2011). In recent years, recycling of E-waste has drawn more and more attention from the public and research institutions because of the environmental protection and economic benefits. WEEE contains a variety of valuable materials, such as metals, glass, plastics and other materials (shown in Fig. 1) (Widmer et al., 2005). Much research has been done to recover and reuse the valuable materials of the E-wastes. However, most of the works are focus on recycling the MFs (Huang et al., 2009; Li and Xu, 2010; Ma et al., 2012b; Xue et al., 2012; Zhan and Xu, 2009) because of the economic benefits. The NMFs, which take up a large proportion of E-wastes, were mostly treated by incineration or landfill in the past (Zhou and Xu, 2012). However, the NMFs from WEEE contain heavy metals, brominated flame retardants (BFRs) and other toxic and hazardous substances. Combustion of the NMFs may cause the formation of polybrominated dibenzodioxins and dibenzofurans (PBDD/Fs), and landfill may lead the pollution of the groundwater which caused by brominated flame retardants (BFRs) and heavy metals (Guo et al., 2009). It has been reported that illegal and unsafe recycling operations of WEEE have been conducted in the town of Guiyu in Guangdong, China, which has become a booming recycling center for WEEE arriving from various regions of the world since 1995. Metals were recycled from WPCBs, PVC-coated wire and cables by open burning (seen in Fig. 2). Such unsafe recycling practice causes severe environmental pollution from heavy metals, fire retardants, and the secondary formation of polyhalogenated dibenzo-p-dioxins and dibenzofurans (Chan et al., 2007; Söderström and Marklund, 2002; Wang et al., 2005). In Dongli Wang’s study, the PBDEs were detected in the soil and sediment samples at levels of 0.26–824 ng/g (dry weight) (Wang et al., 2005). Moreover, children living in Guiyu had significantly higher blood lead levels (BLLs) or blood cadmium levels (BCLs) as compared with those living in Chendian (p < 0.01). 70.8% of children (109/154) had BLLs > 10 lg/dL, and 20.1% of children (31/154) had BCLs > 2 lg/L, compared with 38.7% of children (48/124) had BLLs > 10 lg/dL and 7.3% of children (9/124) had BCLs > 2 lg/L in Chendian (p < 0.01, respectively) (Zheng et al., 2008).
Fig. 1. Typical material fractions in WEEE (Widmer et al., 2005).
Fig. 2. Open burning of wires and other parts to recover metals (Guo et al., 2009).
Therefore, research on resource reutilization and safe disposal of the NMFs from WEEE has a great significance from the viewpoint of environmental protection. Among the enormous variety of NMFs from WEEE, some of them are quite easy to recycle, such as outer shells of household appliances, which could be recycled by crushing, melting, pelleting and regeneration. Others are difficult to recycle, such as NMFs from waste WPCBs, which contain complicated and hazardous ingredients; CRT glass, which contains heavy metal lead; LCD glass, which contains indium tin oxide (ITO); metal-plastic mixture, which need to be separated before the recycling process (shown in Fig. 3). However, some authors have previously written review papers on recycling of NMFs of WPCBs (Guo et al., 2009; Marques et al., 2013). In their reviews, both chemical recycling methods and physical recycling methods (e.g., reusing the NMFs as fillers of different kinds of materials) for recycling the NMFs from WPCBs were reviewed in detail. In this study, we mainly focus on two kinds of intractable NMFs from WEEE, plastics and glass. As a matter of fact, recycling of the NMFs from WEEE is a big challenge. Firstly, the composition of the NMFs from WEEE is very complicated. Laptops, for example, mainly contain plastics, glass, NMFs from WPCBs, etc. Secondly, NMFs from WEEE may contain organic and inorganic contaminants which may cause serious environmental problems (Schlummer et al., 2007b; Weber and Kuch, 2003). However, much work has been carried out to recover and reuse the NMFs from WEEE, of which material recycling and energy recovery are two key points of the recycling process. In this paper, we mainly focus on the intractable NMFs from WEEE, plastics and glass. Methods and technologies of recycling the two types of NMFs from WEEE, analysis and treatment for the hazardous substances contained in the NMFs were reviewed. Chemical recycling methods consisting of pyrolysis, SCF depolymerization and gasification were widely used in recycling plastics from WEEE. Even more important, environmental risks such as heavy metals and BFRs will be the major problems during the recycling processes. Therefore, environmental assessments of the recycling processes are essential. As for WEEE glass, the main problem for recycling the CRT glass is the funnel glass which contains a significant amount of lead. Therefore, lead removing is requisite before the reutilization. With regard to LCD glass, the recycling of precious metals (indium and tin) will be economically viable. Acid leaching is efficient recycling indium and tin, however, large quantities of waste corrosive and volatile acid might increases the environmental risk. Chloride metallurgy seems to be a promising method for recovery of indium from LCD glass without producing hazardous substances. In short, this study could contribute significantly to understanding the recycling methods of NMFs from WEEE and serve as guidance for the future technology research and development.
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
3
Percentage of Plastic (Wt%)
Fig. 3. (a) Waste outer shells of household appliances, (b) WPCBs, (c) waste CRT glass, and (d) waste LCD glass.
60 49.1
50 40 30
23.6 17.5
15.7
20
16.5
10.4
10
3.5
0
Fig. 4. The percentage of plastic in different appliances.
2. Recycling of plastics from WEEE Typical E-wastes contain about 10–30 wt% of plastics (Taurino et al., 2010), which often contain brominated flame retardants (BFRs), including polybrominated diphenyl ethers (PBDE) and tetrabromobisphenol A (TBBA) (Luda and Balabanovich, 2011; Menad et al., 1998; Schlummer et al., 2007a). WEEE plastics may include more than 15 different polymers which make the recycling process difficult (Schlummer and Maeurer, 2006; Schlummer et al., 2006). However, the main problem of the plastics recycling process is the containing of BFRs, which might trigger serious environmental pollutions. Therefore, environmental assessments of the recycling processes are of the essence. 2.1. Composition of plastics from WEEE A direct analysis study to characterize the plastic constituents of WEEE was developed (Martinho et al., 2012). 3400 items, including cooling appliances, small WEEE, central processing units, copying equipment, printers, CRT monitors and CRT televisions were
characterized. The percentage of plastic in different appliances is summarized in Fig. 4, which showed that most WEEE contains 10–30% of plastics. For small WEEE, this figure was particularly up to 49.1%, while CPUs had the lowest ratio of plastics. The types of plastics found in the analyzed appliances according to Graca Martionho et al. are shown in Fig. 5. The results showed that polystyrene (PS), acrylonitrile–butadiene–styrene (ABS), polycarbonate blends (PC/ABS), high-impact polystyrene and polypropylene (HIPS) were the dominant polymers. With regard to the variety of polymers, large cooling appliances, CRT monitors and CRT televisions had the lowest numbers of different polymers, with up to about 10 different types, while printers, copying equipments and CPUs have exceed 13 different types of polymers. In the small WEEE case, the number reached 21 different types. Furthermore, BFRs, metals and color additives were also found in the WEEE plastics. 2.2. Recycling plastics from WEEE by pyrolysis Incineration has been widely applied in the plastic waste recycling process during which plenty of energy could be obtained and used for electricity generation or some other processes (Astrup et al., 2009). However, incineration could be dangerous to the environment because of the emission of toxic components including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dioxins (PCDs) which can be found in the fly ash and the residues (Ban et al., 1997). Pyrolysis is a process during which materials are heated to high temperatures in the absence of oxygen and decompose into smaller molecules. Compared with combustion, pyrolysis has been proved to be a more appropriate technology (Alston and Arnold, 2011; Alston et al., 2011; Chien et al., 2009) for several reasons. Firstly, the generation of gas would reduce to 5–10% compared with combustion. Secondly, the contaminants would be generated in the residue while large amount of toxic gas would leak to the environment during the combustion process. Thirdly, the contaminants would not leak out since the whole pyrolysis system is sealed (Scheirs, 1998). The pyrolysis products of the waste plastics can potentially be used as
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
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R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
100% 80%
80%
Large cooling appliances
76%
60%
69%
CRT monitors
60% 40% 20%
40%
20%
20%
8% 1% 1% 4% 0% 0% 4% 0%
6%
4%
1%
0%
0%
2%
2%
2%
0%
0%
40%
60%
CRT televisons
43%
23%
40%
20%
20%
19%
15% 4%
8%
2% 1%
0% 1% 2%
0%
5% 0% 0% 0% 0% 0%
3%
0%
60%
Copying equipments 40%
21%
14%
10%
7% 2%
Printers
31% 30%
60% 44%
38% 32%
CPUs
40% 22%
20%
14%
20% 8%
6% 0% 1% 1% 0% 0% 0% 1% 1%
0%
5%
1%
1% 3%
8% 0%
5%
7% 1% 1%
1% 1% 2%
0%
40%
Small WEEE
28%
30%
20% 20% 10%
14% 10%
8% 3%
1% 0% 0% 0%
3%
6% 1% 1% 0%
2% 0% 1% 0% 0% 0% 0% 0% 0% 0%
0%
Fig. 5. Plastic compositions for WEEE appliances (Martinho et al., 2012).
Fig. 6. Schematic diagram of the fixed bed reactor (Hall and Williams, 2007).
fuels, raw materials for the petrochemical industry (Mastellone and Arena, 2004). However, pyrolysis is not so perfect since the containing of BFRs make the recycling process problematic. Some research has been done and developed to recycle WEEE plastics using the pyrolysis method. Three types of plastic (respectively from cathode ray tubes, refrigeration equipment, and mixed WEEE) fractions were collected and investigated to analysis the pyrolysis products. (Hall and Williams, 2007) The plastic wastes were pyrolyzed in the fixed bed reactor which was heated to 600 °C at a heating rate of 10 °C min 1. The schematic diagram of the fixed bed reactor is shown in Fig. 6. The pyrolysis products consist of oil, char and gas, among which oil make up the overwhelming majority (seen in Fig. 7). GC–MS was used to characterize the pyrolysis products, which showed that the oils mainly contained styrene, phenol, and other aromatic compounds. Meanwhile, few nitrogenated and oxygenated compounds as well as very few organic halogens were found in the pyrolysis oils. The GC–MS results are shown in Table 1, which indicated that the oil can be used as fuels. Furthermore,
Please cite this article in press as: Wang, R., Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): A review. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.03.004
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R. Wang, Z. Xu / Waste Management xxx (2014) xxx–xxx
several works have been developed by Hall et al. to dispose the WEEE plastics using the pyrolysis process (Hall et al., 2008, 2007; Hall and Williams, 2006a,b, 2008). Two types of plastics contained BFRs have been pyrolyzed in the presence of two zeolite catalysts. (Hall and Williams, 2008). The flame retarded plastics were, acrylonitrile–butadiene–styrene (ABS) that was flame retarded with tetrabromobisphenol A (TBBA) and high impact polystyrene (HIPS) that was flame retard with
Oils Char Gas
80
Weight (%)
60
40
20
0
CRT
Refrigeration
Mxied WEEE
Fig. 7. Composition of the pyrolysis products (Hall and Williams, 2007).
decabromodiphenyl ether. They investigated the pyrolysis in the presence of two catalysts, zeolite ZSM-5 and zeolite Y-Zeolite, in a fixed bed reactor at a final temperature of 440 °C. It had been showed that the presence of the catalysts led to a reduction of the amount of the oil because of the formation on the surface of the catalysts. The Y-Zeolite was found to be very effective at removing the organobromine compounds. However, both catalysts were not so effective at removing the inorganobromine from the volatile pyrolysis products. The environmental impact of pyrolysis of mixed WEEE plastics was studied (Alston and Arnold, 2011; Alston et al., 2011). Waste WEEE plastics were pyrolyzed in the designed equipment based on a pilotplant. The schematic of experimental equipment is shown in Fig. 8. Data were collected from the pyrolysis experiments, the results showed that around 70 wt% of the mixed plastic can be converted to potential fuel. A life cycle assessment (LCA) was studied to assess the different disposal processes, that is, landfill, incineration and pyrolysis. The comparison of disposal processes for main impact categories is shown in Table 2. It turned out that none of them was the ‘‘best’’ option. Landfilling has the least short-term impact on climate change, but requires large amounts of space. Incineration was the most effective method of reducing carbon deposits, but produced the most greenhouse gases. Compared with the former two processes, pyrolysis seems to be a strong compromise candidate, since it saved most resources without emissions of a large amount of greenhouse gas or requirement of much space.
Table 1 Composition of the oil resulting from the pyrolysis process (Hall and Williams, 2007). CRT plastic
Refrigeration plastic
Mixed WEEE plastic
Name
Area (%)
Name
Area (%)
Name
Area (%)
Toluene Ethylbenzene Styrene Cumene Propylbenzene a-Methylstyrene 1-Ethenyl-2-methylbenzene 2,5-Dimethylphenol (E)-2-Phenyl-2-butene Benzyl nitrile 2-Methylindene a-Methyl-benzeneacetonitrile 2-Phenylcyclopropanecarbonitrile Benzenebutanenitrile 2-Phenylcyclopropanecarbonitrile (1-Methyl-3-butenyl)benzene 2-Phenylcyclopropanecarbonitrile Bibenzyl 1,2-Diphenylpropane 1,3-Diphenylpropane 1-Naphthaleneacetonitrile 1,2-Diphenylcyclopropane 1,3-Diphenyl-1-butene (3-Phenylbutyl)benzene (E)-Stilbene 1,2-Diphenylcyclopropane 1,4-Diphenylbutane 1,3-Diphenyl-1-butene 1,3-Diphenyl-1-butene Hexadecanenitrile 2-Phenylnaphthalene Unknown m-Terphenyl Unknown 1,3,5-Triphenylbenzene
3.2 8.3 19.3 1.7 0.1 6.8 0.1 0.9
