Waste Management xxx (2015) xxx–xxx
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Mineralogical analysis of dust collected from typical recycling line of waste printed circuit boards Fangfang Wang a, Yuemin Zhao a,b,⇑, Tao Zhang b, Chenlong Duan b, Lizhang Wang a a b
School of Environment Science and Spatial Informatics, China University of Mining and Technology, Xuzhou 221116, China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 3 March 2015 Revised 14 April 2015 Accepted 11 June 2015 Available online xxxx Keywords: Waste printed circuit boards Recycling Mechanical separation Dust Mineralogical analysis
a b s t r a c t As dust is one of the byproducts originating in the mechanical recycling process of waste printed circuit boards such as crushing and separating, from the viewpoints of resource reuse and environmental protection, an effective recycling method to recover valuable materials from this kind of dust is in urgent need. In this paper, detailed mineralogical analysis on the dust collected from a typical recycling line of waste printed circuit boards is investigated by coupling several analytical techniques. The results demonstrate that there are 73.1 wt.% organic matters, 4.65 wt.% Al, 4.55 wt.% Fe, 2.67 wt.% Cu and 1.06 wt.% Pb in the dust, which reveals the dust is worthy of reuse and harmful to environment. The concentration ratios of Fe, Mn and Zn can reach 12.35, 12.33 and 6.67 respectively by magnetic separation. The yield of dust in each size fraction is nonuniform, while the yield of 0.75 mm size fraction is up to 51.15 wt.%; as the particle size decreases, the content of liberated metals and magnetic materials increase, and metals are mainly in elemental forms. The F, Cl and Br elements combing to C in the dust would make thermal treatment dangerous to the environment. Based on these results, a flowsheet to recycle the dust is proposed. Ó 2015 Published by Elsevier Ltd.
1. Introduction Being a secondary resource of valuable metals, waste electrical and electronic equipment (WEEE) has drawn increasing concern by the government as well as environmental protection organization due to its huge volume and hazardous material contents. With the advanced science and technology as well as remarkable improvement of people’s living standard, it is estimated that the current global production of WEEE is expected to increase rapidly at an alarming rate of 20–25 million tons per year, China will become one of the major WEEE producers in the next ten years (Robinson, 2009), and the growth will remain due to their short lifespan (Huang et al., 2009; Hao et al., 2014). Besides, large quantities of WEEE have been exported to China for recycling by transboundary movement through clandestine operations or legal loopholes (Li et al., 2013). As an essential part of almost all the EEE and the base of the electronic industry, printed circuit boards (PCBs) account for the weight for about 3% (Zhou and Qiu, 2010). Compared to natural resource, it is pointed out that not only the ⇑ Corresponding author at: School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China. Tel./fax: +86 51683590092. E-mail address: [email protected] (Y. Zhao).
total amount but also the concentration of metals contained in PCBs should be considered in. Waste PCBs constitute a heterogeneous mixture of metals, nonmetal, and some toxic substances. By containing many electronic components, such as resistors, relays, capacitors, and integrated circuits (Duan et al., 2011), waste PCBs have a metal content of nearly 28% (copper: 10–20%, lead: 1–5%, nickel: 1–3%) (Veit et al., 2005), especially the purity of precious metals in PCBs is more than 10 times that of content-rich minerals (Li et al., 2007; Betts, 2008). From the viewpoints of environmental preservation, it is of immediate significance to find a cautious process to recover the valuable parts of waste PCBs as well as safely dispose the harmful ones (Hadi et al., 2013). Currently, the main options for the treatment of waste PCBs are involved physical separation processing, hydro-metallurgical processing, pyro-metallurgical processing and bio-metallurgical processing (Zeng et al., 2013, 2012; Zhu et al., 2014; Hadi et al., 2015; Duan et al., 2015). In most cases, several kinds of recycling techniques should be combined together to achieve high recovery rates of the main targeted metal species (Yang et al., 2014; Fogarasi et al., 2014; Zhou et al., 2013). In a typical recycling line of waste PCBs, physical processing operations such as grinding, sieving, magnetic, electrostatic, gravity separations and density-based separation are applied as pretreatments to liberate and concentrate the metallic fractions (MFs) and non-metallic fractions (NMFs)
http://dx.doi.org/10.1016/j.wasman.2015.06.021 0956-053X/Ó 2015 Published by Elsevier Ltd.
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(Al-Thyabat et al., 2013; Guo et al., 2011; Duan et al., 2009; Zhou and Qiu, 2010; Flandinet et al., 2012). Then the preparation products will be conducted to a series of metallurgical recycling process (Zhou and Qiu, 2010; Yamane et al., 2011; Fogarasi et al., 2014). In physical processes, a great deal of dust and poisonous gas are produced during the process of crushing, sieving, air float table separation, etc. In general, a well-designed recycling line must be equipped with dusting system and waste gases disposal system. According to the statistical data from several recycling lines of waste PCBs in China, Brazil, America, Pakistan and Canada, the dust collected from the dusting system is up to about 3.7% of its capacity. And the disposal or treatment of the dust becomes a difficult problem. Chemical and mineralogical characterization analysis is a very useful method to provide basic information for recycling research (Zhang et al., 2014). In this study, with the purpose of obtain basic information for recycling the dust collected from the recycling line of waste PCBs, the detailed chemical and mineralogical characterizations of the dust were undertaken by coupling several analytical techniques. Based on these analysis results, a possible flowsheet for recycling this kind of dust was proposed.
pressed method to prepare the samples. The chemical composition of magnetic production and nonmagnetic production from the dust were also analyzed in the same way. 2.3. Micro-characterization Powder structure and particle configuration of the 0.5 mm dust were analyzed by a scanning electron microscope (SEM, FEI quanta 250, America) equipped with a tungsten filament and coupled with an energy dispersive spectrometer (EDS, Bruker QUANTAX 400-10, Germany) using a silicon drift detector. 2.4. Mineral phases Mineral phase analysis of the crushed products in 0.075 mm size fractions were carried out by an X-ray powder diffractometer (XRD, Bruker D8 advance, Germany). The setting conditions for the XRD were: Cu K a radiation, 40 keV accelerating voltage, 30 mA current, 3–90° scanning range, 0.1 s/step (0.01945°/step) scan speed. 2.5. Chemical state analysis
2. Materials and methods 2.1. Sampling The 20 kg dust used in our study was collected from a typical PCBs recycling plant in Anhui Province, China. In this PCBs recycling line, the dust as shown in Fig. 1 was collected by dry dust collectors from the process of crusher, vibrating screen, and air float table. The sieving test was carried out by a standard set of screens (Retsch AS200, Germany) in dry way and 6 groups of sieved products as shown in Fig. 1 were obtained, they were +1, 1 + 0.5, 0.5 + 0.25, 0.25 + 0.1, 0.1 + 0.075 and 0.075 mm respectively. 2.2. Dust chemical composition analysis The major constituents of the dust were plenty of organic matter, a small quantity of inorganic matter and metals. For chemical characterization, the dust samples were firstly burned in plasma asher (K1050X Plasma Asher, UK) at a low temperature of 200 °C, and then the ash was frozen at 196 °C and grounded into fine powder ( 0.074 mm) by a freezing grinder (Retsch Cryomill, Germany). The chemical composition of the freezing ground product was obtained with the help of an X-ray fluorescence spectrometer (XRF, Bruker S8 Tiger, Germany) using a powder
+1 mm
-1+0.5 mm
- 0.5+0.25 mm
This experiment was carried out at room temperature in an ultra-high vacuum (UHV) system with the surface analysis system (ESCALAB 250Xi, America). The base pressure of the analysis chamber during the measurements was lower than 1.0 10 9 mbar. Al Ka radiation (hv = 1486.6 eV) from a monochromatized X-ray source was used for XPS. The take-off angle of the photoelectrons was 90° and the spot size was 900 lm. The spectra of survey scan were recorded with the pass energy of 100 eV, the energy step size was 1.00 eV. High resolution spectra were recorded with the pass energy of 20 eV, and the energy step size was 0.05 eV. The data processing (peak fitting) was performed with the Avantage (version 5.927) software provided by Thermo Fisher Scientific Corporation, using a Smart type background subtraction and Gaussian/Lorentzian peak shapes. The binding energies were corrected by setting the C1s hydrocarbon (–CH2–CH2–bonds) peak at 284.8 eV. 3. Results and discussion 3.1. Particle size distribution Fig. 1 showed that the dust mainly contains tiny particles, besides some small sheets of plastics, thin-films and fibers. After
- 0.25+0.1 mm
- 0.1+0.075 mm
-0.075mm
Fig. 1. The dust collected from dusting systems and sieved products.
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sieving, small sheets of plastics and thin-films were primarily concentrated in +1 mm size fraction, fibers were mainly enriched in 1 + 0.5, 0.5 + 0.25 and 0.25 + 0.1 mm size fractions. In the size fractions 0.1 + 0.075 and 0.075 mm, there were only tiny particles. Fig. 2 listed the yield of dust in each size fraction. The yield of +1, 1 + 0.5 and 0.1 + 0.075 mm size fractions were only 5.31 wt.%, 6.76 wt.% and 6.86 wt.% respectively, while the yield of 0.075 mm size fraction was up to 51.15 wt.%. It was indicated that the size fractions of the dust collected from PCBs recycling line were not well-distributed, the yield in the fine size fractions was extremely high, but the yield in the coarse fractions was relatively low. Since PCBs were composed of many different materials with
different mechanical crushing properties, materials with similar mechanical crushing property would have similar sizes and shapes in the process of crushing. Sieving was applied to gain materials with similar characterizations in a certain size fraction for a proper following separation. Therefore, a clear understanding of the components in different size fraction must be acquired, and SEM and EDS were applied for this experiment. 3.2. SEM & EDS analysis The samples from 0.5 + 0.25, 0.25 + 0.1, 0.1 + 0.075 and 0.075 mm size fraction were analyzed by coupling SEM and
Fig. 2. Yield of dust in each size fraction (wt.%).
A
B
C
D
Fig. 3. SEM images ((A)
0.5 + 0.25 mm, (B)
0.25 + 0.1 mm, (C)
0.1 + 0.075 mm and (D)
0.075 mm).
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EDS. SEM images were demonstrated in Fig. 3. In 0.5 + 0.25 mm size fraction (Fig. 3A), there were mainly irregular shaped pieces of PCBs substrates and regularly arranged claviform fibers. And with the size decreasing, the components of the size fractions of 0.25 + 0.1 (Fig. 3B) and 0.1 + 0.075 mm (Fig. 3C) changed little.
However, when the size was less than 0.075 mm (Fig. 3D), the shapes of the particles changed a lot, more individual claviform fibers and debris of PCBs substrates were enriched in this fraction. With the help of EDS, it is easy to get the element composition of each particle. It was found that the fibers were constituted of Al,
Fig. 4. Chemical composition of the dust samples.
Table 1 Magnetic substance content of the fine PCB powders in each size fraction (wt.%). Size fraction (mm)
+1
Yield (wt.%)
0
1 + 0.5 0
0.5 + 0.25 0.59
0.25 + 0.1 2.84
0.1 + 0.075 6.88
0.075 23.03
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F. Wang et al. / Waste Management xxx (2015) xxx–xxx
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100 lm), and some did not existed in metallic forms but compounds forms such as Cu–Cl and Ti–Mg–S. In this case, it was impossible to achieve the goal of recovery by mechanical separation, chemical leaching may be a preferable choice. 3.3. Dust chemical composition Chemical composition of the dust was given in Fig. 4A. Apart from 73.1 wt.% organic matter, the dust was mainly composed of Al, Si, Ca, Fe, Cu, Br and Pb, and also some trace elements, including Na, Mg, P, S, Cl, K, Ti, Cr, Mn, Ni, Zn, Sr, Ag, Sn, Ba and Bi. From the viewpoint of valuable metals recovery, 46.5 kg aluminum, 45.5 kg iron, 26.7 kg copper, 10.6 kg lead, 8.6 kg manganese and 7.6 kg zinc could be recovered per ton of this dust. But the level of lead in the dust was particularly high from the viewpoint of pollution control. The primary task is to remove the metals from the organic matter as much as possible if we want to recycle this dust. After separated by a magnet, in the nonmagnetic production (Fig. 4B), apart from 78.37 wt.% organic matter, the major constituents were Al, Si, Ca, Cu, Sn and Pb. Valuable metals including Al, Cu and Pb only accounted for 8.91 wt.%. So, magnetic separation could make the metal element removal process easier. However, in magnetic production (Fig. 4C), valuable metals such as Fe, Mn and Zn were concentrated with the total content up to 71.87 wt.%, their the concentration ratios were 12.35, 12.33 and 6.67 respectively. The content of Fe exceeded 55%. Though the content of Ni was very low, the concentration ratio was 12.14. Consequently, magnetic separation is necessary in the recycling process, and the separated magnetic products could be used as feed directly for the steel works. Table 1 showed the uneven distribution of magnetic substance in all size range. Though there was none magnetic substance exit in +0.5 mm size fractions, as the particle size decreased, the content of magnetic substance increased obviously, it accounted for 23.03 wt.% in 0.075 mm size fraction. And the outcome was in conformity to the analysis result of EDS. 3.4. Mineral phase analysis Fig. 5. X-ray diffraction patterns of the dust.
Si and Ca. Besides the PCBs substrates which were mainly composed of C and O, there were some liberated metals such as Al, Cu and Fe. And the content of metals increased as the particle size decreased. Meanwhile, it is necessary to point out that almost of the metal elements existed in very tiny particles (less than
Since the component of the dust was complicated, and its high content of organic matter and iron would aggrandize the background of diffraction profile as well as strengthened the fluorescent effect, it is hard to analysis the XRD pattern of the dust directly. So nonmagnetic and magnetic parts of the dust were measured by XRD respectively, the results were shown in Fig. 5. The XRD pattern of the nonmagnetic part suggested that there were
Fig. 6. Survey scan XPS spectrum of the fine crushed products.
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SiO2, metallic stannum, metallic copper, metallic aluminum and metallic lead. The metals were mainly in simple substance form. Nevertheless, it is difficult to analyze the XRD pattern of magnetic part. Peaks marked with F were not assigned to a single substance, but to a series of substances with a similar crystal form. Peaks marked with F were assigned to the form of M3O4 such as Zn2TiO4, Ca0.15Fe2.85O4 and Fe3O4. Through the above analysis, it can be sure Fe3O4 was the main form. Peaks marked with G were assigned to metallic iron.
Table 2 Binding energy, full width at half maximum and atomic percentage from XPS spectra. Element
Chemical state
Peak BE
FWHM eV
Atomic %
Pb4f7
Lead oxide Tin lead alloy C–Br Aluminum oxide Si–Al–Ca–O SiO2 C–N PVC Tin oxide Tin lead alloy – ETFE Calcium oxide – –
139.00 137.00 70.69 74.46 102.00 103.40 399.76 200.36 487.19 485.51 1304.36 689.57 347.64 284.86 532.89
1.32 0.75 1.09 1.26 1.15 1.19 1.77 1.37 1.59 0.84 1.3 2.34 0.7 1.37 2.12
0.10 0.03 0.61 1.08 2.63 2.05 1.25 0.36 0.23 0.02 0.20 0.29 0.55 67.78 22.80
Br3d5 Al2p3 Si2p3 N1s Cl2p3 Sn3d5 Mg1s F1s Ca2p C1s O1s
Consequently, magnetic property was a significant difference among the substances in this dust. Fortunately, these conductive metals were mixed in non-conductive organic matters. Considering the particle size was less than 1 mm, triboelectric separation may be a good separation method for the materials (Wang et al., 2014a,b). 3.5. Chemical state analysis Survey scan (Fig. 6) XPS indicated that on the very surface of the dust, the elements were Pb, Br, Al, Si, N, Cl, Sn, Mg, F, Ca, C and O. High-resolution XPS was adopted to explore the chemical states of elements detected from surface in detail. Binding energy (BE), full width at half maximum (FWHM) and atomic percentage of each element were listed in Table 2 (Chastain and King, 1995). Obviously, the total content of C and O was over 90 at.%. The chemical states of N, Cl and F were very simple; each had one peak combined with C. The peak of N1s at the position of 399.76 eV was assigned to C–N, which was common in organic matter. The peak of Cl2p3 at 200.36 eV and F1s at 689.57 were respectively attributed to polyvinyl chloride (PVC) and Ethyl Tetra Fluoro Ethylene (ETFE) existed in PCBs. Fig. 7 indicated the XPS spectra of Pb, Sn, Si and Br were much more complicated. The Pb4f7 XPS spectrum showed there were two peaks, the peak at 137 eV was related to lead oxide and the other at 139 eV was assigned to tin lead alloy. Similarly, the Sn3d5 XPS spectrum was also made of two peaks, the peak at
Fig. 7. Survey scan XPS spectrum of the fine crushed products.
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Fig. 8. Proposed flowsheet for dust recycling.
485.51 eV was attributed to tin lead alloy and the other at 487.19 eV was related to tin oxide. Peaks of 2p1 and 2p3 were mixed together in the same area to analyze Si2p. After overlapping peak resolving, two chemical states were found, the peak at 102 eV was assigned to Si–Al–Ca–O and 103.4 eV to SiO2. The peak of Al2p was very close to Br3d, therefore, in Br3d XPS spectrum, Al2p peak was also got at 74.46 eV assigned to aluminum oxide. And the peak of Br3d5 at 70.69 eV was assigned to C–Br which came from brominated flame retardants applied in PCBs. Although organic matters was the main component, considering there were a lot of low-melting-point metals such as lead and tin, and some halogen elements such as F, Cl and Br in PCBs, it was unadvisable to choose thermal treatment because many severe pollutants such as dioxin and lead dust may be generated. 3.6. Proposed flowsheet for dust recycling Base on the mineralogical analysis of the dust collected from the recycling line of PCBs, a sound flowsheet is proposed as shown in Fig. 8. As mentioned before, the +1 mm size fraction is enriched with thin-films and sheets, but large particles were not completely liberated. While the 1 mm size fraction has a higher yield and contains liberated metals and organic matters, including most magnetic materials. So sieving should be carried out as the first step. For the +1 mm size fraction, thin films and sheets could be separated from large particles of dust by air separation with ease due to their significant differences in shape and density. And the large particles should be returned back to the fine crusher of the PCBs recycling line. For the 1 mm size fraction, triboelectric separation should be applied first to remove the large amount of organic matters for obtaining the metal concentrate. Then magnetic separation should be carried out due to the magnetic products enriched over 55 wt.% iron, 10 wt.% manganese and 5 wt.% zinc. The nonmagnetic products should be treated with leaching operation for they are all very
fine particles which could not be separated by mechanical separation anymore. And finer particle is a good condition for leaching as the high efficiency would be higher. At last, purer organic products could be recovered, and finer particle size is a good condition for its reuse. 4. Conclusion Detailed mineralogical analysis of dust collected from typical recycling line of waste printed circuit boards coupling several analytical techniques was carried out for the first time in order to get basic information as much as possible for improving and developing the separation treatment of this kind of waste. Distinctly, the dust contains both valuable metals and poisonous substances, which caused more hardships in recycling process. After screening, fraction size greater than 1 mm mainly was thin-films and plastic sheets while organic matters and metals were mostly enriched in fraction size less than 1 mm. SEM and EDS analysis proved that the liberation of metals as well as enrichment of magnetic materials were better as the particle size decreased. XRD and XPS analysis revealed that the metals were mainly in elemental form, few were in the form of compound such as oxide and chloride in the surface. Disparities in conductivity and magnetism among the materials provided a good condition for mechanical separation. Triboelectric separation might be an alternative way for the recycling of fine granularity dust. Considering the dust contained 1.06 wt.% Pb, 0.15 wt.% Cr and a few brominated flame retardants, it is necessary to discuss its danger to ecological environment in the future. Acknowledgements This project was Project supported by Natural Science Foundation of Jiangsu Province of China (No. BK2012136), National Natural Science Foundation of China (No. 51304196). The authors would like to thank Advanced Analysis and Computation Center of China University of Mining and Technology for their technical support.
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Mineralogical analysis of dust collected from typical recycling line of waste printed circuit boards Fangfang Wang a, Yuemin Zhao a,b,⇑, Tao Zhang b, Chenlong Duan b, Lizhang Wang a a b
School of Environment Science and Spatial Informatics, China University of Mining and Technology, Xuzhou 221116, China School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 3 March 2015 Revised 14 April 2015 Accepted 11 June 2015 Available online xxxx Keywords: Waste printed circuit boards Recycling Mechanical separation Dust Mineralogical analysis
a b s t r a c t As dust is one of the byproducts originating in the mechanical recycling process of waste printed circuit boards such as crushing and separating, from the viewpoints of resource reuse and environmental protection, an effective recycling method to recover valuable materials from this kind of dust is in urgent need. In this paper, detailed mineralogical analysis on the dust collected from a typical recycling line of waste printed circuit boards is investigated by coupling several analytical techniques. The results demonstrate that there are 73.1 wt.% organic matters, 4.65 wt.% Al, 4.55 wt.% Fe, 2.67 wt.% Cu and 1.06 wt.% Pb in the dust, which reveals the dust is worthy of reuse and harmful to environment. The concentration ratios of Fe, Mn and Zn can reach 12.35, 12.33 and 6.67 respectively by magnetic separation. The yield of dust in each size fraction is nonuniform, while the yield of 0.75 mm size fraction is up to 51.15 wt.%; as the particle size decreases, the content of liberated metals and magnetic materials increase, and metals are mainly in elemental forms. The F, Cl and Br elements combing to C in the dust would make thermal treatment dangerous to the environment. Based on these results, a flowsheet to recycle the dust is proposed. Ó 2015 Published by Elsevier Ltd.
1. Introduction Being a secondary resource of valuable metals, waste electrical and electronic equipment (WEEE) has drawn increasing concern by the government as well as environmental protection organization due to its huge volume and hazardous material contents. With the advanced science and technology as well as remarkable improvement of people’s living standard, it is estimated that the current global production of WEEE is expected to increase rapidly at an alarming rate of 20–25 million tons per year, China will become one of the major WEEE producers in the next ten years (Robinson, 2009), and the growth will remain due to their short lifespan (Huang et al., 2009; Hao et al., 2014). Besides, large quantities of WEEE have been exported to China for recycling by transboundary movement through clandestine operations or legal loopholes (Li et al., 2013). As an essential part of almost all the EEE and the base of the electronic industry, printed circuit boards (PCBs) account for the weight for about 3% (Zhou and Qiu, 2010). Compared to natural resource, it is pointed out that not only the ⇑ Corresponding author at: School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China. Tel./fax: +86 51683590092. E-mail address: [email protected] (Y. Zhao).
total amount but also the concentration of metals contained in PCBs should be considered in. Waste PCBs constitute a heterogeneous mixture of metals, nonmetal, and some toxic substances. By containing many electronic components, such as resistors, relays, capacitors, and integrated circuits (Duan et al., 2011), waste PCBs have a metal content of nearly 28% (copper: 10–20%, lead: 1–5%, nickel: 1–3%) (Veit et al., 2005), especially the purity of precious metals in PCBs is more than 10 times that of content-rich minerals (Li et al., 2007; Betts, 2008). From the viewpoints of environmental preservation, it is of immediate significance to find a cautious process to recover the valuable parts of waste PCBs as well as safely dispose the harmful ones (Hadi et al., 2013). Currently, the main options for the treatment of waste PCBs are involved physical separation processing, hydro-metallurgical processing, pyro-metallurgical processing and bio-metallurgical processing (Zeng et al., 2013, 2012; Zhu et al., 2014; Hadi et al., 2015; Duan et al., 2015). In most cases, several kinds of recycling techniques should be combined together to achieve high recovery rates of the main targeted metal species (Yang et al., 2014; Fogarasi et al., 2014; Zhou et al., 2013). In a typical recycling line of waste PCBs, physical processing operations such as grinding, sieving, magnetic, electrostatic, gravity separations and density-based separation are applied as pretreatments to liberate and concentrate the metallic fractions (MFs) and non-metallic fractions (NMFs)
http://dx.doi.org/10.1016/j.wasman.2015.06.021 0956-053X/Ó 2015 Published by Elsevier Ltd.
Please cite this article in press as: Wang, F., et al. Mineralogical analysis of dust collected from typical recycling line of waste printed circuit boards. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.021
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(Al-Thyabat et al., 2013; Guo et al., 2011; Duan et al., 2009; Zhou and Qiu, 2010; Flandinet et al., 2012). Then the preparation products will be conducted to a series of metallurgical recycling process (Zhou and Qiu, 2010; Yamane et al., 2011; Fogarasi et al., 2014). In physical processes, a great deal of dust and poisonous gas are produced during the process of crushing, sieving, air float table separation, etc. In general, a well-designed recycling line must be equipped with dusting system and waste gases disposal system. According to the statistical data from several recycling lines of waste PCBs in China, Brazil, America, Pakistan and Canada, the dust collected from the dusting system is up to about 3.7% of its capacity. And the disposal or treatment of the dust becomes a difficult problem. Chemical and mineralogical characterization analysis is a very useful method to provide basic information for recycling research (Zhang et al., 2014). In this study, with the purpose of obtain basic information for recycling the dust collected from the recycling line of waste PCBs, the detailed chemical and mineralogical characterizations of the dust were undertaken by coupling several analytical techniques. Based on these analysis results, a possible flowsheet for recycling this kind of dust was proposed.
pressed method to prepare the samples. The chemical composition of magnetic production and nonmagnetic production from the dust were also analyzed in the same way. 2.3. Micro-characterization Powder structure and particle configuration of the 0.5 mm dust were analyzed by a scanning electron microscope (SEM, FEI quanta 250, America) equipped with a tungsten filament and coupled with an energy dispersive spectrometer (EDS, Bruker QUANTAX 400-10, Germany) using a silicon drift detector. 2.4. Mineral phases Mineral phase analysis of the crushed products in 0.075 mm size fractions were carried out by an X-ray powder diffractometer (XRD, Bruker D8 advance, Germany). The setting conditions for the XRD were: Cu K a radiation, 40 keV accelerating voltage, 30 mA current, 3–90° scanning range, 0.1 s/step (0.01945°/step) scan speed. 2.5. Chemical state analysis
2. Materials and methods 2.1. Sampling The 20 kg dust used in our study was collected from a typical PCBs recycling plant in Anhui Province, China. In this PCBs recycling line, the dust as shown in Fig. 1 was collected by dry dust collectors from the process of crusher, vibrating screen, and air float table. The sieving test was carried out by a standard set of screens (Retsch AS200, Germany) in dry way and 6 groups of sieved products as shown in Fig. 1 were obtained, they were +1, 1 + 0.5, 0.5 + 0.25, 0.25 + 0.1, 0.1 + 0.075 and 0.075 mm respectively. 2.2. Dust chemical composition analysis The major constituents of the dust were plenty of organic matter, a small quantity of inorganic matter and metals. For chemical characterization, the dust samples were firstly burned in plasma asher (K1050X Plasma Asher, UK) at a low temperature of 200 °C, and then the ash was frozen at 196 °C and grounded into fine powder ( 0.074 mm) by a freezing grinder (Retsch Cryomill, Germany). The chemical composition of the freezing ground product was obtained with the help of an X-ray fluorescence spectrometer (XRF, Bruker S8 Tiger, Germany) using a powder
+1 mm
-1+0.5 mm
- 0.5+0.25 mm
This experiment was carried out at room temperature in an ultra-high vacuum (UHV) system with the surface analysis system (ESCALAB 250Xi, America). The base pressure of the analysis chamber during the measurements was lower than 1.0 10 9 mbar. Al Ka radiation (hv = 1486.6 eV) from a monochromatized X-ray source was used for XPS. The take-off angle of the photoelectrons was 90° and the spot size was 900 lm. The spectra of survey scan were recorded with the pass energy of 100 eV, the energy step size was 1.00 eV. High resolution spectra were recorded with the pass energy of 20 eV, and the energy step size was 0.05 eV. The data processing (peak fitting) was performed with the Avantage (version 5.927) software provided by Thermo Fisher Scientific Corporation, using a Smart type background subtraction and Gaussian/Lorentzian peak shapes. The binding energies were corrected by setting the C1s hydrocarbon (–CH2–CH2–bonds) peak at 284.8 eV. 3. Results and discussion 3.1. Particle size distribution Fig. 1 showed that the dust mainly contains tiny particles, besides some small sheets of plastics, thin-films and fibers. After
- 0.25+0.1 mm
- 0.1+0.075 mm
-0.075mm
Fig. 1. The dust collected from dusting systems and sieved products.
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sieving, small sheets of plastics and thin-films were primarily concentrated in +1 mm size fraction, fibers were mainly enriched in 1 + 0.5, 0.5 + 0.25 and 0.25 + 0.1 mm size fractions. In the size fractions 0.1 + 0.075 and 0.075 mm, there were only tiny particles. Fig. 2 listed the yield of dust in each size fraction. The yield of +1, 1 + 0.5 and 0.1 + 0.075 mm size fractions were only 5.31 wt.%, 6.76 wt.% and 6.86 wt.% respectively, while the yield of 0.075 mm size fraction was up to 51.15 wt.%. It was indicated that the size fractions of the dust collected from PCBs recycling line were not well-distributed, the yield in the fine size fractions was extremely high, but the yield in the coarse fractions was relatively low. Since PCBs were composed of many different materials with
different mechanical crushing properties, materials with similar mechanical crushing property would have similar sizes and shapes in the process of crushing. Sieving was applied to gain materials with similar characterizations in a certain size fraction for a proper following separation. Therefore, a clear understanding of the components in different size fraction must be acquired, and SEM and EDS were applied for this experiment. 3.2. SEM & EDS analysis The samples from 0.5 + 0.25, 0.25 + 0.1, 0.1 + 0.075 and 0.075 mm size fraction were analyzed by coupling SEM and
Fig. 2. Yield of dust in each size fraction (wt.%).
A
B
C
D
Fig. 3. SEM images ((A)
0.5 + 0.25 mm, (B)
0.25 + 0.1 mm, (C)
0.1 + 0.075 mm and (D)
0.075 mm).
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EDS. SEM images were demonstrated in Fig. 3. In 0.5 + 0.25 mm size fraction (Fig. 3A), there were mainly irregular shaped pieces of PCBs substrates and regularly arranged claviform fibers. And with the size decreasing, the components of the size fractions of 0.25 + 0.1 (Fig. 3B) and 0.1 + 0.075 mm (Fig. 3C) changed little.
However, when the size was less than 0.075 mm (Fig. 3D), the shapes of the particles changed a lot, more individual claviform fibers and debris of PCBs substrates were enriched in this fraction. With the help of EDS, it is easy to get the element composition of each particle. It was found that the fibers were constituted of Al,
Fig. 4. Chemical composition of the dust samples.
Table 1 Magnetic substance content of the fine PCB powders in each size fraction (wt.%). Size fraction (mm)
+1
Yield (wt.%)
0
1 + 0.5 0
0.5 + 0.25 0.59
0.25 + 0.1 2.84
0.1 + 0.075 6.88
0.075 23.03
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F. Wang et al. / Waste Management xxx (2015) xxx–xxx
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100 lm), and some did not existed in metallic forms but compounds forms such as Cu–Cl and Ti–Mg–S. In this case, it was impossible to achieve the goal of recovery by mechanical separation, chemical leaching may be a preferable choice. 3.3. Dust chemical composition Chemical composition of the dust was given in Fig. 4A. Apart from 73.1 wt.% organic matter, the dust was mainly composed of Al, Si, Ca, Fe, Cu, Br and Pb, and also some trace elements, including Na, Mg, P, S, Cl, K, Ti, Cr, Mn, Ni, Zn, Sr, Ag, Sn, Ba and Bi. From the viewpoint of valuable metals recovery, 46.5 kg aluminum, 45.5 kg iron, 26.7 kg copper, 10.6 kg lead, 8.6 kg manganese and 7.6 kg zinc could be recovered per ton of this dust. But the level of lead in the dust was particularly high from the viewpoint of pollution control. The primary task is to remove the metals from the organic matter as much as possible if we want to recycle this dust. After separated by a magnet, in the nonmagnetic production (Fig. 4B), apart from 78.37 wt.% organic matter, the major constituents were Al, Si, Ca, Cu, Sn and Pb. Valuable metals including Al, Cu and Pb only accounted for 8.91 wt.%. So, magnetic separation could make the metal element removal process easier. However, in magnetic production (Fig. 4C), valuable metals such as Fe, Mn and Zn were concentrated with the total content up to 71.87 wt.%, their the concentration ratios were 12.35, 12.33 and 6.67 respectively. The content of Fe exceeded 55%. Though the content of Ni was very low, the concentration ratio was 12.14. Consequently, magnetic separation is necessary in the recycling process, and the separated magnetic products could be used as feed directly for the steel works. Table 1 showed the uneven distribution of magnetic substance in all size range. Though there was none magnetic substance exit in +0.5 mm size fractions, as the particle size decreased, the content of magnetic substance increased obviously, it accounted for 23.03 wt.% in 0.075 mm size fraction. And the outcome was in conformity to the analysis result of EDS. 3.4. Mineral phase analysis Fig. 5. X-ray diffraction patterns of the dust.
Si and Ca. Besides the PCBs substrates which were mainly composed of C and O, there were some liberated metals such as Al, Cu and Fe. And the content of metals increased as the particle size decreased. Meanwhile, it is necessary to point out that almost of the metal elements existed in very tiny particles (less than
Since the component of the dust was complicated, and its high content of organic matter and iron would aggrandize the background of diffraction profile as well as strengthened the fluorescent effect, it is hard to analysis the XRD pattern of the dust directly. So nonmagnetic and magnetic parts of the dust were measured by XRD respectively, the results were shown in Fig. 5. The XRD pattern of the nonmagnetic part suggested that there were
Fig. 6. Survey scan XPS spectrum of the fine crushed products.
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SiO2, metallic stannum, metallic copper, metallic aluminum and metallic lead. The metals were mainly in simple substance form. Nevertheless, it is difficult to analyze the XRD pattern of magnetic part. Peaks marked with F were not assigned to a single substance, but to a series of substances with a similar crystal form. Peaks marked with F were assigned to the form of M3O4 such as Zn2TiO4, Ca0.15Fe2.85O4 and Fe3O4. Through the above analysis, it can be sure Fe3O4 was the main form. Peaks marked with G were assigned to metallic iron.
Table 2 Binding energy, full width at half maximum and atomic percentage from XPS spectra. Element
Chemical state
Peak BE
FWHM eV
Atomic %
Pb4f7
Lead oxide Tin lead alloy C–Br Aluminum oxide Si–Al–Ca–O SiO2 C–N PVC Tin oxide Tin lead alloy – ETFE Calcium oxide – –
139.00 137.00 70.69 74.46 102.00 103.40 399.76 200.36 487.19 485.51 1304.36 689.57 347.64 284.86 532.89
1.32 0.75 1.09 1.26 1.15 1.19 1.77 1.37 1.59 0.84 1.3 2.34 0.7 1.37 2.12
0.10 0.03 0.61 1.08 2.63 2.05 1.25 0.36 0.23 0.02 0.20 0.29 0.55 67.78 22.80
Br3d5 Al2p3 Si2p3 N1s Cl2p3 Sn3d5 Mg1s F1s Ca2p C1s O1s
Consequently, magnetic property was a significant difference among the substances in this dust. Fortunately, these conductive metals were mixed in non-conductive organic matters. Considering the particle size was less than 1 mm, triboelectric separation may be a good separation method for the materials (Wang et al., 2014a,b). 3.5. Chemical state analysis Survey scan (Fig. 6) XPS indicated that on the very surface of the dust, the elements were Pb, Br, Al, Si, N, Cl, Sn, Mg, F, Ca, C and O. High-resolution XPS was adopted to explore the chemical states of elements detected from surface in detail. Binding energy (BE), full width at half maximum (FWHM) and atomic percentage of each element were listed in Table 2 (Chastain and King, 1995). Obviously, the total content of C and O was over 90 at.%. The chemical states of N, Cl and F were very simple; each had one peak combined with C. The peak of N1s at the position of 399.76 eV was assigned to C–N, which was common in organic matter. The peak of Cl2p3 at 200.36 eV and F1s at 689.57 were respectively attributed to polyvinyl chloride (PVC) and Ethyl Tetra Fluoro Ethylene (ETFE) existed in PCBs. Fig. 7 indicated the XPS spectra of Pb, Sn, Si and Br were much more complicated. The Pb4f7 XPS spectrum showed there were two peaks, the peak at 137 eV was related to lead oxide and the other at 139 eV was assigned to tin lead alloy. Similarly, the Sn3d5 XPS spectrum was also made of two peaks, the peak at
Fig. 7. Survey scan XPS spectrum of the fine crushed products.
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Fig. 8. Proposed flowsheet for dust recycling.
485.51 eV was attributed to tin lead alloy and the other at 487.19 eV was related to tin oxide. Peaks of 2p1 and 2p3 were mixed together in the same area to analyze Si2p. After overlapping peak resolving, two chemical states were found, the peak at 102 eV was assigned to Si–Al–Ca–O and 103.4 eV to SiO2. The peak of Al2p was very close to Br3d, therefore, in Br3d XPS spectrum, Al2p peak was also got at 74.46 eV assigned to aluminum oxide. And the peak of Br3d5 at 70.69 eV was assigned to C–Br which came from brominated flame retardants applied in PCBs. Although organic matters was the main component, considering there were a lot of low-melting-point metals such as lead and tin, and some halogen elements such as F, Cl and Br in PCBs, it was unadvisable to choose thermal treatment because many severe pollutants such as dioxin and lead dust may be generated. 3.6. Proposed flowsheet for dust recycling Base on the mineralogical analysis of the dust collected from the recycling line of PCBs, a sound flowsheet is proposed as shown in Fig. 8. As mentioned before, the +1 mm size fraction is enriched with thin-films and sheets, but large particles were not completely liberated. While the 1 mm size fraction has a higher yield and contains liberated metals and organic matters, including most magnetic materials. So sieving should be carried out as the first step. For the +1 mm size fraction, thin films and sheets could be separated from large particles of dust by air separation with ease due to their significant differences in shape and density. And the large particles should be returned back to the fine crusher of the PCBs recycling line. For the 1 mm size fraction, triboelectric separation should be applied first to remove the large amount of organic matters for obtaining the metal concentrate. Then magnetic separation should be carried out due to the magnetic products enriched over 55 wt.% iron, 10 wt.% manganese and 5 wt.% zinc. The nonmagnetic products should be treated with leaching operation for they are all very
fine particles which could not be separated by mechanical separation anymore. And finer particle is a good condition for leaching as the high efficiency would be higher. At last, purer organic products could be recovered, and finer particle size is a good condition for its reuse. 4. Conclusion Detailed mineralogical analysis of dust collected from typical recycling line of waste printed circuit boards coupling several analytical techniques was carried out for the first time in order to get basic information as much as possible for improving and developing the separation treatment of this kind of waste. Distinctly, the dust contains both valuable metals and poisonous substances, which caused more hardships in recycling process. After screening, fraction size greater than 1 mm mainly was thin-films and plastic sheets while organic matters and metals were mostly enriched in fraction size less than 1 mm. SEM and EDS analysis proved that the liberation of metals as well as enrichment of magnetic materials were better as the particle size decreased. XRD and XPS analysis revealed that the metals were mainly in elemental form, few were in the form of compound such as oxide and chloride in the surface. Disparities in conductivity and magnetism among the materials provided a good condition for mechanical separation. Triboelectric separation might be an alternative way for the recycling of fine granularity dust. Considering the dust contained 1.06 wt.% Pb, 0.15 wt.% Cr and a few brominated flame retardants, it is necessary to discuss its danger to ecological environment in the future. Acknowledgements This project was Project supported by Natural Science Foundation of Jiangsu Province of China (No. BK2012136), National Natural Science Foundation of China (No. 51304196). The authors would like to thank Advanced Analysis and Computation Center of China University of Mining and Technology for their technical support.
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