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An integrated and environmental-friendly technology for recovering valuable materials from waste tantalum capacitors
Journal of Cleaner Production 166 (2017) 512e518
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An integrated and environmental-friendly technology for recovering valuable materials from waste tantalum capacitors Bo Niu, Zhenyang Chen, Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2017 Received in revised form 13 July 2017 Accepted 5 August 2017 Available online 7 August 2017
Waste printed circuit boards (WPCBs) are valuable urban ore for recycling. Many efforts have been done to recover resources from basal boards of WPCB. In addition to basal boards, WPCBs contain many electronic components. However, the recycling technology for electronic components has been poorly developed. This study proposed an integrated and environment-friendly technology for recycling waste tantalum capacitors (WTCs) of WPCBs. Firstly, the WTCs were treated by argon pyrolysis to decompose mold resin. The pyrolysis temperature of 550 C and holding time for 30 min were considered as the optimal parameters. Then, the pyrolysis residues were performed by crush and magnetic separation to recover nickel-iron terminals. Finally, chloride metallurgy (CM) was used to recover rare metal tantalum (Ta). The Ta recovery rate could reach 92.87 ± 0.36%. The optimal parameters were determined as temperature of 493 C, adding FeCl2$4H2O of 53 wt% and holding time for 150 min based on the response surface methodology (RSM). This study exploits an efficient approach for recycling WTCs and provides a reference for future industrial application. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Waste tantalum capacitor Resource recovery Pyrolysis Chloride metallurgy
1. Introduction With the development of science and technology, the number of electronic products has dramatically increased in the last two decades. Meanwhile, technological innovation and intense marketing shorten the life cycle of electronic products, resulting in the generation of large amounts of e-waste (Ghosh et al., 2015). It is estimated that nearly 45 million tons of e-waste are discarded globally per year, and the number is growing exponentially (Prabaharan et al., 2016). Printed circuit boards (PCBs), the integral parts of any electronic products, are particularly rich in metallic materials, including base, precious and rare metals (Zhang and Xu, 2016). The concentrations of these metals in PCBs are much higher than their respective rich-content minerals, which make waste PCBs (WPCBs) economically attractive urban ore for recycling (Li et al., 2015). During the recycling process of WPCBs, electronic components (ECs), mounted on the WPCBs, were usually firstly dismantled. Then the waste printed wiring boards (WPWBs, WPCBs without ECs) were undergone the resource recovery process (Wang and Xu, 2015). Some advance technologies such as integrated mechanicalphysical methods (Li et al., 2007; Li and Xu, 2010),
* Corresponding author. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.jclepro.2017.08.043 0959-6526/© 2017 Elsevier Ltd. All rights reserved.
hydrometallurgy (Havlik et al., 2010; Kim et al., 2011) and supercritical water (Li and Xu, 2015; Xiu et al., 2013; Liu et al., 2016) have been successfully applied to recover resources from WPWBs. However, the recycling technologies for ECs were poorly developed. Actually, WPCBs contain various ECs such as chips, resistors, transistors and capacitors (ceramic, aluminum and tantalum). Among them, tantalum capacitor (TC) is present in almost all electronic products owing to its advantage of small volume, large capacity and high thermal stability. For example, the PCB of mobile phone (3G technologies), notebook (2 GHz) and digital camcorder respectively contains about 36, 22 and 13 of these special capacitors (Angerer et al., 2013). Moreover, TC contains about 45 wt % of rare metal tantalum (Ta), and other valuable metals such as Ni (nickel) and iron (Fe), so it is attractive for recycling (Mineta and Okabe, 2005). However, recovering valuable materials from WTCs is difficult because of the tightly covered mold resin, as presented in Fig. 1. The mold resin consists of silica, epoxy resin, phenolic novolac resin and flame-retardants (Katano et al., 2014; Iji, 1995). If WTCs were not disposed or recycled properly, the hazardous organics will be released into the environment, and then causing serious environmental pollution. So far, several related research such as combustion (Mineta and Okabe, 2005; Fujita et al., 2014), steam gasification with NaOH (Katano et al., 2014), chemical treatment (Mineta and Okabe, 2005; Spitczok von Brisinski1 et al., 2014) and phase
B. Niu et al. / Journal of Cleaner Production 166 (2017) 512e518
Fig. 1. Schematic illustration of a TC.
separation (Kikuchi et al., 2014), have been done to remove the organics and recover valuable materials from WTCs. Pyrolysis is a quite promising technology for recycling organics compared with combustion and solvent leaching (Chen et al., 2010; Tripathi et al., 2016). On one hand, pyrolysis is conducted without oxygen, so the organics are decomposed to oils and gases, which can be recycled as fuel or feedstock; on the other hand, pyrolysis tends to be more effective than solvent leaching. Therefore, pyrolysis can be used as a pretreatment to recycle the organics of WTCs. However, the pyrolysis characteristic of the organics in WTCs has not been investigated. Besides, the recovery of valuable metals, especially rare metal Ta, is a critical process. Since Ta is a highmelting and corrosion-resistant metal (melting point: 2995 C, insoluble in aqua regia), the recovery of Ta is usually achieved by dissolving or smelting the other components (Mineta and Okabe, 2005; Kikuchi et al., 2014). Consequently, large amounts of energy and chemicals were consumed during the recycling process. Besides, the obtained Ta had a low-grade. Chloride metallurgy has been proved to be an effective technology to extract rare metals from their ores and metal wastes (Jena and Brocchi, 1997; Kanari et al., 2009; Ma et al., 2012). In the CM process, valuable metals
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are converted to their corresponding chlorides and then separated based on their different volatility between the metal chlorides (Jena and Brocchi, 1997). This process does not produce liquid waste and is suitable for industrial production. In the previous study, we investigated the fundamental principles of chloride metallurgy (CM) for extracting Ta from WTCs. The results suggested that Ta could selectively react with FeCl2 and the generated TaCl5 can be easily separated and then condensed in the condensation zone. As a result, tantalum oxide with 99% purity could be obtained (Niu et al., 2017). However, the optimized parameters for industrial application were lacking. Based on above considerations, this study proposed an integrated process including argon pyrolysis, mechanical-physical separation and chloride metallurgy, as presented in Fig. 2. The pyrolysis characteristic of the organics from WTCs was investigated. The CM process was optimized by applying central composite design (CCD) under response surface methodology (RSM) for industrial application. In addition, Ni-Fe terminals were also recovered by mechanical-physical separation. This study aims to exploit an effective and environment-friendly process for the maximum recovery of WTCs. 2. Materials and methods 2.1. Materials The WTCs used in this study were obtained by an automatic disassembly system, as presented in Fig. 3. The major composition of the WTCs was listed in Table 1. FeCl2$4H2O (99.5%, Aladdin) was chosen as the chlorinating agent, argon (Ar, 99.99% purity) was used as the shielding gas. 2.2. Apparatus The pyrolysis and chloride metallurgy (CM) experiments were conducted in a quartz tube furnace, as shown in Fig. S1 of Supporting Information (SI). The main body consisted of a body of
Fig. 2. Flow sheet of recycling process for WTCs.
Fig. 3. An automatic system for WTCs disassembly from WPCBs.
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Table 1 Main composition of the WTCs used in this study.
Table 2 Experimental ranges and levels of independent variables.
Composition
Ta
Organics
SiO2
Ni
Fe
Mn
Content (wt %)
35.99
14.41
44.78
6.10
1.40
0.03
furnace (chamber dimension is F 40 mm 600 mm), a quartz tube reactor (F 35 mm 800 mm), a gas supply system, a temperature controller, and product collectors. Pyrolysis and CM products could be collected in devices (a) and (b), respectively. 2.3. Experimental procedures In a typical run, about 20 g of WTCs were taken into a quartz boat, which was placed in the middle of the quartz tube. The pyrolysis product collector was connected to the quartz tube. Ar gas passed through the reactor at a flow rate of 300 ml/min. The reactor was adjusted to setting temperature and the samples were heated. The pyrolysis lasted to 30 min to ensure complete reaction. The recycling effect of organics in WTCs was characterized by organic decomposition rate (RD), which was defined as eq. (1).
RD ¼ ðW WR Þ=WO 100%
(1)
Where, WO is the mass of organics in raw material; W and WR are the mass of raw material and residue after pyrolysis, respectively. After pyrolysis, the residues were crushed. Fe-Ni terminals were collected by magnetic separation. Subsequently, the residues were classified into fractions of 0.125e0.3 mm as the feedstock of CM stage. In CM process, FeCl2$4H2O powder were blend with the residues, and put into the quartz tube. Ar gas passed through the reactor with a flow rate of 300 ml/min. The samples were heated from room temperature to the presetting temperature. When the temperature reached about 200 C (evaporating the crystal water from FeCl2$4H2O), Ta collector unit was connected to the quartz tube. During a planned time, the samples were cooled to room temperature, and the residues were taken out to calculate the recovery rate. The Ta recovery rate was calculated by eq. (2).
R ¼ ðM0 MÞ=M0 100%
(2)
Where, M0 and M are the initial and remaining amount of Ta, respectively. 2.4. Chemical analysis
Ranges and levels Independent variables FeCl2$4H2O, wt % (XFe) Temperature, C (XT) Time, min (Xt)
a 41.591 415.91 69.5462
1 45 450 90
0 50 500 120
1 55 550 150
a 58.409 584.09 170.454
analyze the interaction of several independent factors by the Design-Expert software (version 8.0.6, Stat-Ease, Inc., Minneapolis, MN). The experiments were conducted in a standard RSM design called central composite design (CCD) for the optimization of Ta recovery rate. The FeCl2$4H2O adding amount, reaction temperature, and holding time were selected as factors on the response of Ta recovery rate with the coded values at 3 levels (1, 0 and þ1). The ranges of variables were referred to the results obtained in previous study (Niu et al., 2017). The ranges and levels of variables are given in Table 2. 3. Results and discussion 3.1. Pyrolysis To investigate the pyrolysis characteristic of mold resin from WTCs, the mold resin powder (scraped from WTCs) was measured by TGA-DTA, and the results were shown in Fig. 4. The mold resin was thermally decomposed in two major steps with maximum rate at 355 and 450 C followed by a slow weight loss in the temperature region 550e800 C. Similar results have also been observed in the studies dealing with the decompositions of phenolic resin and epoxy resin containing flame retardant, where the decompositions, are completed in at least two steps (Evangelopoulos et al., 2015; Wu et al., 2002). In addition, as shown from the TGA curve, the weight loss was about 9.75% in the first step, and in the second step, the resin underwent about 4.76% of weight loss. After the two major steps, the weight continued slow decrease, which could be attributed to the carbonization of the pyrolysis residue (Wu et al., 2002). To further obtain the optimal pyrolysis parameter for mold resin, the effect of pyrolysis temperature and holding time on the decomposition rate of organic were investigated. Fig. 5a showed the relationship between organic decomposition rate and temperature (holding time: 30 min). It showed that the organic decomposition rate sharply increased from 40.66% to 75.88% with increasing the temperature from 350 C to 450 C. After that, the
The metal content in WTCs and product were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500a, Agilent Corporation, US). The content of SiO2 and organics in WTCs were examined by X-ray Fluorescence Spectrometer (XRF1800, Shimadzu, Japan) and combustion method (Xiu et al., 2013). Thermal gravimetric (TG) and differential scanning calorimetry (DSC) were performed using a simultaneous TG-DSC instrument (Mettler Toledo, Shimadzu, Japan). The pyrolysis oils and gases were analyzed by gas chromatography-mass spectroscopy (GC-MS, TurboMass, Perkin Elmer Corporation, US). The crystal structure of product and residues were identified by X-ray diffraction (XRD, D8 ADVANCE, BRUKER, Germany) with Cu Ka radiation. All the experiments were repeated three times, and the mean values were reported. 2.5. Statistical analysis The response surface methodology (RSM) was applied to
Fig. 4. TG-DTA curves of the mold resin powder scraped from WTCs.
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Fig. 5. Effect of (a) pyrolysis temperature; (b) holding time on the organic decomposition rate.
organic decomposition rate slowly increased. When the temperature was above 550 C, the decomposition rate remained the same. The relationship between the organic decomposition rate and holding time was shown in Fig. 5b (temperature: 550 C), which showed that the decomposition rate could reach up to 52.13% at 10 min. A higher decomposition rate could be obtained at 30 min and the organic decomposition rate was 87.91%. The decomposition rate could not reach to 100%, since some of the organics will be transformed into carbon residues under pyrolysis condition (oxygen-free). Therefore, the temperature of 550 C and holding time for 30 min could be considered as the optimal pyrolysis parameter for mold resin in this study. After pyrolysis process, pyrolysis gas and oil were collected and then analyzed by GC-MC. The results of GC-MC analysis were shown in Tables S1 and S2 of SI. As shown in Table S1, the oils were mainly composed of acetophenone (12.34 area %), phenol (9.76 area %), phenol congeners (16.13 area %), 2, 5-Dimethylbenzophenone (6.52 area %) and so on. Table S2 showed the composition of pyrolysis gases, which mainly contained ethylene, ethane and propene. Under high temperature combustion, these oils and gases will release energy and ultimately transform into CO2 and H2O. Thus, the pyrolysis products are suggested to be disposed by the approach of combustion as fuel. The utilization of the pyrolysis oils and gases will be investigated in a future study. 3.2. Chloride metallurgy (CM) After pyrolysis, the mold resin was decomposed, and then the solid residues were performed by mechanical-physical separation (crush, magnetic separation and screen). Ni-Fe terminals were collected by magnetic separation. Then, CM was used to extract the Ta resource. According to the previous study, Ta could selectively react with FeCl2, and the chlorination reaction between Ta and FeCl2 is as follows (Niu et al., 2017):
2Ta ðsÞ þ 5FeCl2 ðs; lÞ ¼ 2TaCl5 ðgÞ þ 5FeðsÞ ðT > 380 CÞ
(3)
The boiling points of TaCl5 and FeCl2 at standard atmospheric pressure are 234 C and 1026 C, respectively. Within the chlorination temperatures, the generated TaCl5 will be evaporated into gas phase, and then condensed in the condensation zone. Consequently, Ta was separated from the reactants (Niu et al., 2017). During the chlorination process, the extraction of Ta could be affected by some parameters such as temperature, holding time, the adding amount of chlorinating agent and so on. To obtain the maximum Ta recovery rate, the CM experiments were conducted in
a standard response surface methodology (RSM) design called central composite design (CCD) with the levels in Table 2. The response results were listed in Table S3 of SI and the analysis results were outputted by Design-Expert software. The analysis of variance (ANOVA) was shown in Table 3, demonstrating that the response surface quadratic model was significant at F value of 24.32 and pvalue < 0.0001. The statistical significance of the model was also confirmed by the R-Squared value of 0.9563, which means that 95.63% of the variations could be explained by the independent variables. The regression analysis was carried out using all the independent variables and their interactions. The coefficients for the second-order model were also calculated by the Design-Expert software. The empirical relationship between Ta recovery and the three variables can be written as following eq. (4):
Y ¼ 414:66487 þ 5:60265 XFe þ 1:40049 XT þ 0:18382 Xt ð8:0E 4Þ XFe XT ð1:83333E 4ÞXFe Xt 2 ð1:51667E 4ÞXT Xt 0:048996XFe ð1:35405E 3ÞXT2
ð3:08203E 4ÞXt2 (4) Where Y is the Ta recovery rate; XFe, XT and Xt are the actual values of FeCl2$4H2O addition amount (wt %), temperature ( C), and time (min), respectively.
Table 3 ANOVA for response surface quadratic model. Source
Sum of squares
df
Mean square
F value
p-value (prob > F)
Model
216.93
9
24.10
24.32
A- FeCl2$4H2O B-temperature C-time AB AC BC A2 B2 C2 Residual Lack of fit Pure error Cor total
26.96 4.72 7.59 0.32 6.05E-003 0.41 21.62 165.14 1.11 9.91 9.91 0.000 226.85
1 1 1 1 1 1 1 1 1 10 5 5 19
26.96 4.72 7.59 0.32 6.05E-003 0.41 21.62 165.14 1.11 0.99 1.98 0.00
27.21 4.76 7.66 0.32 6.104E-003 0.42 21.82 166.62 1.12
<0.0001 (significant) 0.0004 0.0541 0.0199 0.5824 0.9393 0.5326 0.0009 <0.0001 0.3151
R-Squared ¼ 0.9563 Adj R-Squared ¼ 0.9170.
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For better understanding the interaction of these variables, the three-dimensional response surface plots are shown in Fig. 6. As shown in Fig. 6a and b, the Ta recovery rate increased first and then descended with the increase of temperature, which demonstrated that the excessive temperature will not facilitate the chlorination reaction of Ta. Fig. 6c shows the effect of time and FeCl2$4H2O addition amount on the Ta recovery rate. It showed that the Ta recovery rate slowly increased and then kept invariability with increasing the chlorination time and FeCl2$4H2O addition amount. To optimize the CM process, the desirability function was applied to seek the maximum Ta recovery rate. The goal seeking began at a random starting point and proceeded up the steepest slope to a maximum. The temperature within range of 450e550, the FeCl2$4H2O addition amount within range 45e55 and the chlorination time within range of 90e150 were set for the maximum desirability. By investigating 39 starting points in the response surface changes, the optimal parameter was considered to be temperature of 493.16 C, FeCl2$4H2O addition amount of 52.87 wt%, chlorination time for 150 min and Ta recovery rate of
93.05%. According to the above results, 493 C, adding 53 wt% of FeCl2$4H2O and 150 min were chosen as the optimal parameters. The experiments were performed as the optimal parameters, and the results suggested that the Ta recovery rate was 92.87 ± 0.36%, which was similar to the predictions. 3.3. Environmental and economic aspects In the existing recycling processes, hydrometallurgy (including many wet steps) and pyrometallurgy (calcination and smelting) were most applied to remove the organics and recover Ta from WTCs. However, some problems exist in these processes, such as waste liquid or toxic gas generating and high energy consumption. In comparison, this study proposed an integrated and environment-friendly process for recycling WTCs (Fig. 7). At first, pyrolysis was adopted to decompose the resin into gases and oils. Then, Ni-Fe terminals were recovered by crush and magnetic separation. Since the organics were recycled, the emission of organic
Fig. 6. Response surface 3D plots for the effect of (a) temperature and FeCl2$4H2O addition amount; (b) temperature and time; (c) FeCl2$4H2O addition amount and time.
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Fig. 7. Schematic illustration of recycling process for WTCs (the XRD pattern for Ta product was shown in Fig. S2).
Table 4 Comparison of different methods for WTCs recycling. Method
Proposed technology
Pyrometallurgy (Fujita et al., 2014; Kikuchi et al., 2014)
Hydrometallurgy (Mineta and Okabe, 2005; Katano et al., 2014; Spitczok von Brisinski1 et al., 2014)
Processing steps
calcination or smelting
many wet processing steps
Material recovery Energy consumption Chemical use
integrated process containing pyrolysis, mechanical separation and chloride metallurgy tantalum oxide, Ni-Fe lower energy consumption than pyrometallurgy only FeCl2$4H2O consumed
mixture of metals high energy consumption e
Emissions
gas and oil were collected
gaseous pollutant
Ta, Ni-Fe, e large amount of strong acid, alkali and organic solvent liquid waste
pollutants was eliminated in the mechanical-separation process. In the CM process, compared to the hazardous Cl2 or HCl gas, we choose the non-toxic FeCl2$4H2O as the chlorinating agent. Although the excess chlorination agent and the generated Fe will remain in the residues (Fig. S3 of SI), these materials could be easily recycled by filtration and magnetic separation. An overview of the comparison between the proposed route and other recycling technologies is given in Table 4. Therefore, the proposed technology in this study could be considered as an environment-friendly process for recycling WTCs. Besides the environmental impact, the economic benefit of this recycling process was also estimated. Assuming that 10 kg of WTCs were treated by this system, and the equipment used in this study is fabricated at laboratory scale. The depreciation cost of equipment and electric charge as well as cost of FeCl2$4H2O consumption were considered. Detailed calculations can be found in the SI. By this process, about 3.65 kg of tantalum oxide and 0.75 kg of Ni-Fe could be obtained (SiO2 and the generated Fe were not considered in this evaluation), and the profit was evaluated about 2151.86 dollars. Nerveless, our study remained at the laboratory stage and more detailed assessment should be done before an industrial application. Besides, the revenues have not been taken into account the cost of raw materials or labor, but the Chinese government would provide some subsidies for recycling e-waste. Based on above environmental and economic assessment, this integrated recycling process for WTCs is environment-friendly and economically viable,
which provides a good prospect for industrial application. 4. Conclusion This study developed an integrated process to recycle valuable materials from WTCs. The technological process contained pyrolysis, mechanical-physical separation and chloride metallurgy (CM). The pyrolysis temperature of 550 C and holding time for 30 min were considered as the optimal parameters for decomposing the organics of WTCs. After pyrolysis, Ni-Fe terminals were recovered by crush and magnetic separation. In the CM process, the Ta recovery rate could reach 92.87 ± 0.36%, with the temperature of 493 C, adding FeCl2$4H2O of 53 wt% and holding time for 150 min. Through the integrated process, the organics of WTCs were removed, tantalum oxide and Ni-Fe were recovered. Environmental and economic assessment suggested that the recycling process is environment-friendly and economically viable. In short, this study proposed an efficient and promising process for recycling WTCs, which is significant for e-waste treatment and sustainable utilization of resource. Acknowledgments This work was supported by the National Natural Science Foundation of China (51534005).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jclepro.2017.08.043. References Angerer, T., Luidold, S., Antrekowitsch, H., 2013. Recycling potentials of the two refractory metals tantalum and niobium. In: European Metallurgical Conference. Chen, T.C., Shen, Y.H., Lee, W.J., Lin, C.C., Wan, M.W., 2010. The study of ultrasoundassisted oxidative desulfurization process applied to the utilization of pyrolysis oil from waste tires. J. Clean. Prod. 18, 1850e1858. Evangelopoulos, P., Kantarelis, E., Yang, W., 2015. Investigation of the thermal decomposition of printed circuit boards (PCBs) via thermogravimetric analysis (TGA) and analytical pyrolysis (PyeGC/MS). J. Anal. Appl. Pyrol. 115, 337e343. Fujita, T., Ono, H., Dodbiba, G., Yamaguchi, K., 2014. Evaluation of a recycling process for printed circuit board by physical separation and heat treatment. Waste manag. 34, 1264e1273. Ghosh, B., Ghosh, M.K., Parhi, P., Mukherjee, P.S., Mishra, B.K., 2015. Waste printed circuit boards recycling: an extensive assessment of current status. J. Clean. Prod. 94, 5e19. Havlik, T., Orac, D., Petranikova, M., Miskufova, A., Kukurugya, F., Takacova, Z., 2010. Leaching of copper and tin from used printed circuit boards after thermal treatment. J. Hazard. Mater 183, 866e873. Iji, M., 1995. Recycling of IC molding resin wastes from IC package production processes. In: Ieee/ucs/semi International Symposium on Semiconductor Manufacturing, pp. 197e200. Jena, P.K., Brocchi, E.A., 1997. Metal extraction through chlorine metallurgy. Min. Proc. Ext. Met. Rev. 16, 211e237. n, J., Ruiz-Bustinza, I., Gaballah, I., 2009. Kanari, N., Allain, E., Joussemet, R., Mocho An overview study of chlorination reactions applied to the primary extraction and recycling of metals and to the synthesis of new reagents. Thermochim. Acta 495, 42e50. Katano, S., Wajima, T., Nakagome, H., Katano, S., Wajima, T., Nakagome, H., 2014. Recovery of tantalum sintered compact from used tantalum condenser using steam gasification with sodium hydroxide. APCBEE Proc. 10, 182e186. Kikuchi, R., Yamamoto, T., Nakamoto, M., 2014. Preliminary information of laboratorial tantalum recovery and considerations for a potential solution for conflict mineral and wildlife conservation. Environ. Nat. Resour. Res. 4. Kim, E.Y., Kim, M.S., Lee, J.C., Pandey, B.D., 2011. Selective recovery of gold from waste mobile phone PCBs by hydrometallurgical process. J. Hazard. Mater 198,
206e215. Li, J., Xu, Z., 2010. Environmental friendly automatic line for recovering metal from waste printed circuit boards. Environ. Sci. Technol. 44, 1418e1423. Li, K., Xu, Z., 2015. Application of supercritical water to decompose brominated epoxy resin and environmental friendly recovery of metals from waste memory module. Environ. Sci. Technol. 49, 1761e1767. Li, J., Lu, H., Guo, J., Xu, Z., Zhou, Y., 2007. Recycle technology for recovering resources and products from waste printed circuit boards. Environ. Sci. Technol. 41, 1995e2000. Li, J., Zeng, X., Chen, M., Ogunseitan, O.A., Stevels, A., 2015. "Control-Alt-Delete": rebooting solutions for the E-waste problem. Environ. Sci. Technol. 49, 7095e7108. Liu, K., Zhang, Z., Zhang, F.S., 2016. Direct extraction of palladium and silver from waste printed circuit boards powder by supercritical fluids oxidation-extraction process. J. Hazard. Mater 318, 216e223. Ma, E., Lu, R., Xu, Z., 2012. An efficient rough vacuum-chlorinated separation method for the recovery of indium from waste liquid crystal display panels. Green Chem. 14, 3395e3401. Mineta, K., Okabe, T.H., 2005. Development of a recycling process for tantalum from capacitor scraps. J. Phys. Chem. Solids 66, 318e321. Niu, B., Chen, Z., Xu, Z., 2017. Method for recycling tantalum from waste tantalum capacitors by chloride metallurgy. ACS Sustain. Chem. Eng. 5, 1376e1381. Prabaharan, G., Barik, S.P., Kumar, B., 2016. A hydrometallurgical process for recovering total metal values from waste monolithic ceramic capacitors. Waste Manag. 52, 302e308. Spitczok Von Brisinski, L., Goldmann, D.I.D., Endres, P.F., 2014. Recovery of metals from tantalum capacitors with ionic liquids. Chem. Ing. Tech. 86, 196e199. Tripathi, M., Sahu, J.N., Ganesan, P., 2016. Effect of process parameters on production of biochar from biomass waste through pyrolysis: a review. Renew. Sust. Energy Rev. 55, 467e481. Wang, J., Xu, Z., 2015. Disposing and recycling waste printed circuit boards: disconnecting, resource recovery, and pollution control. Environ. Sci. Technol. 49, 721e733. Wu, C.S., Liu, Y.L., Chiu, Y.C., Chiu, Y.S., 2002. Thermal stability of epoxy resins containing flame retardant components: an evaluation with thermogravimetric analysis. Polym. Degrad. Stabil. 78, 41e48. Xiu, F.R., Qi, Y., Zhang, F.S., 2013. Recovery of metals from waste printed circuit boards by supercritical water pre-treatment combined with acid leaching process. Waste Manag. 33, 1251e1257. Zhang, L., Xu, Z., 2016. A review of current progress of recycling technologies for metals from waste electrical and electronic equipment. J. Clean. Prod. 127, 19e36.
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
An integrated and environmental-friendly technology for recovering valuable materials from waste tantalum capacitors Bo Niu, Zhenyang Chen, Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 February 2017 Received in revised form 13 July 2017 Accepted 5 August 2017 Available online 7 August 2017
Waste printed circuit boards (WPCBs) are valuable urban ore for recycling. Many efforts have been done to recover resources from basal boards of WPCB. In addition to basal boards, WPCBs contain many electronic components. However, the recycling technology for electronic components has been poorly developed. This study proposed an integrated and environment-friendly technology for recycling waste tantalum capacitors (WTCs) of WPCBs. Firstly, the WTCs were treated by argon pyrolysis to decompose mold resin. The pyrolysis temperature of 550 C and holding time for 30 min were considered as the optimal parameters. Then, the pyrolysis residues were performed by crush and magnetic separation to recover nickel-iron terminals. Finally, chloride metallurgy (CM) was used to recover rare metal tantalum (Ta). The Ta recovery rate could reach 92.87 ± 0.36%. The optimal parameters were determined as temperature of 493 C, adding FeCl2$4H2O of 53 wt% and holding time for 150 min based on the response surface methodology (RSM). This study exploits an efficient approach for recycling WTCs and provides a reference for future industrial application. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Waste tantalum capacitor Resource recovery Pyrolysis Chloride metallurgy
1. Introduction With the development of science and technology, the number of electronic products has dramatically increased in the last two decades. Meanwhile, technological innovation and intense marketing shorten the life cycle of electronic products, resulting in the generation of large amounts of e-waste (Ghosh et al., 2015). It is estimated that nearly 45 million tons of e-waste are discarded globally per year, and the number is growing exponentially (Prabaharan et al., 2016). Printed circuit boards (PCBs), the integral parts of any electronic products, are particularly rich in metallic materials, including base, precious and rare metals (Zhang and Xu, 2016). The concentrations of these metals in PCBs are much higher than their respective rich-content minerals, which make waste PCBs (WPCBs) economically attractive urban ore for recycling (Li et al., 2015). During the recycling process of WPCBs, electronic components (ECs), mounted on the WPCBs, were usually firstly dismantled. Then the waste printed wiring boards (WPWBs, WPCBs without ECs) were undergone the resource recovery process (Wang and Xu, 2015). Some advance technologies such as integrated mechanicalphysical methods (Li et al., 2007; Li and Xu, 2010),
* Corresponding author. E-mail address: [email protected] (Z. Xu). http://dx.doi.org/10.1016/j.jclepro.2017.08.043 0959-6526/© 2017 Elsevier Ltd. All rights reserved.
hydrometallurgy (Havlik et al., 2010; Kim et al., 2011) and supercritical water (Li and Xu, 2015; Xiu et al., 2013; Liu et al., 2016) have been successfully applied to recover resources from WPWBs. However, the recycling technologies for ECs were poorly developed. Actually, WPCBs contain various ECs such as chips, resistors, transistors and capacitors (ceramic, aluminum and tantalum). Among them, tantalum capacitor (TC) is present in almost all electronic products owing to its advantage of small volume, large capacity and high thermal stability. For example, the PCB of mobile phone (3G technologies), notebook (2 GHz) and digital camcorder respectively contains about 36, 22 and 13 of these special capacitors (Angerer et al., 2013). Moreover, TC contains about 45 wt % of rare metal tantalum (Ta), and other valuable metals such as Ni (nickel) and iron (Fe), so it is attractive for recycling (Mineta and Okabe, 2005). However, recovering valuable materials from WTCs is difficult because of the tightly covered mold resin, as presented in Fig. 1. The mold resin consists of silica, epoxy resin, phenolic novolac resin and flame-retardants (Katano et al., 2014; Iji, 1995). If WTCs were not disposed or recycled properly, the hazardous organics will be released into the environment, and then causing serious environmental pollution. So far, several related research such as combustion (Mineta and Okabe, 2005; Fujita et al., 2014), steam gasification with NaOH (Katano et al., 2014), chemical treatment (Mineta and Okabe, 2005; Spitczok von Brisinski1 et al., 2014) and phase
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Fig. 1. Schematic illustration of a TC.
separation (Kikuchi et al., 2014), have been done to remove the organics and recover valuable materials from WTCs. Pyrolysis is a quite promising technology for recycling organics compared with combustion and solvent leaching (Chen et al., 2010; Tripathi et al., 2016). On one hand, pyrolysis is conducted without oxygen, so the organics are decomposed to oils and gases, which can be recycled as fuel or feedstock; on the other hand, pyrolysis tends to be more effective than solvent leaching. Therefore, pyrolysis can be used as a pretreatment to recycle the organics of WTCs. However, the pyrolysis characteristic of the organics in WTCs has not been investigated. Besides, the recovery of valuable metals, especially rare metal Ta, is a critical process. Since Ta is a highmelting and corrosion-resistant metal (melting point: 2995 C, insoluble in aqua regia), the recovery of Ta is usually achieved by dissolving or smelting the other components (Mineta and Okabe, 2005; Kikuchi et al., 2014). Consequently, large amounts of energy and chemicals were consumed during the recycling process. Besides, the obtained Ta had a low-grade. Chloride metallurgy has been proved to be an effective technology to extract rare metals from their ores and metal wastes (Jena and Brocchi, 1997; Kanari et al., 2009; Ma et al., 2012). In the CM process, valuable metals
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are converted to their corresponding chlorides and then separated based on their different volatility between the metal chlorides (Jena and Brocchi, 1997). This process does not produce liquid waste and is suitable for industrial production. In the previous study, we investigated the fundamental principles of chloride metallurgy (CM) for extracting Ta from WTCs. The results suggested that Ta could selectively react with FeCl2 and the generated TaCl5 can be easily separated and then condensed in the condensation zone. As a result, tantalum oxide with 99% purity could be obtained (Niu et al., 2017). However, the optimized parameters for industrial application were lacking. Based on above considerations, this study proposed an integrated process including argon pyrolysis, mechanical-physical separation and chloride metallurgy, as presented in Fig. 2. The pyrolysis characteristic of the organics from WTCs was investigated. The CM process was optimized by applying central composite design (CCD) under response surface methodology (RSM) for industrial application. In addition, Ni-Fe terminals were also recovered by mechanical-physical separation. This study aims to exploit an effective and environment-friendly process for the maximum recovery of WTCs. 2. Materials and methods 2.1. Materials The WTCs used in this study were obtained by an automatic disassembly system, as presented in Fig. 3. The major composition of the WTCs was listed in Table 1. FeCl2$4H2O (99.5%, Aladdin) was chosen as the chlorinating agent, argon (Ar, 99.99% purity) was used as the shielding gas. 2.2. Apparatus The pyrolysis and chloride metallurgy (CM) experiments were conducted in a quartz tube furnace, as shown in Fig. S1 of Supporting Information (SI). The main body consisted of a body of
Fig. 2. Flow sheet of recycling process for WTCs.
Fig. 3. An automatic system for WTCs disassembly from WPCBs.
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Table 1 Main composition of the WTCs used in this study.
Table 2 Experimental ranges and levels of independent variables.
Composition
Ta
Organics
SiO2
Ni
Fe
Mn
Content (wt %)
35.99
14.41
44.78
6.10
1.40
0.03
furnace (chamber dimension is F 40 mm 600 mm), a quartz tube reactor (F 35 mm 800 mm), a gas supply system, a temperature controller, and product collectors. Pyrolysis and CM products could be collected in devices (a) and (b), respectively. 2.3. Experimental procedures In a typical run, about 20 g of WTCs were taken into a quartz boat, which was placed in the middle of the quartz tube. The pyrolysis product collector was connected to the quartz tube. Ar gas passed through the reactor at a flow rate of 300 ml/min. The reactor was adjusted to setting temperature and the samples were heated. The pyrolysis lasted to 30 min to ensure complete reaction. The recycling effect of organics in WTCs was characterized by organic decomposition rate (RD), which was defined as eq. (1).
RD ¼ ðW WR Þ=WO 100%
(1)
Where, WO is the mass of organics in raw material; W and WR are the mass of raw material and residue after pyrolysis, respectively. After pyrolysis, the residues were crushed. Fe-Ni terminals were collected by magnetic separation. Subsequently, the residues were classified into fractions of 0.125e0.3 mm as the feedstock of CM stage. In CM process, FeCl2$4H2O powder were blend with the residues, and put into the quartz tube. Ar gas passed through the reactor with a flow rate of 300 ml/min. The samples were heated from room temperature to the presetting temperature. When the temperature reached about 200 C (evaporating the crystal water from FeCl2$4H2O), Ta collector unit was connected to the quartz tube. During a planned time, the samples were cooled to room temperature, and the residues were taken out to calculate the recovery rate. The Ta recovery rate was calculated by eq. (2).
R ¼ ðM0 MÞ=M0 100%
(2)
Where, M0 and M are the initial and remaining amount of Ta, respectively. 2.4. Chemical analysis
Ranges and levels Independent variables FeCl2$4H2O, wt % (XFe) Temperature, C (XT) Time, min (Xt)
a 41.591 415.91 69.5462
1 45 450 90
0 50 500 120
1 55 550 150
a 58.409 584.09 170.454
analyze the interaction of several independent factors by the Design-Expert software (version 8.0.6, Stat-Ease, Inc., Minneapolis, MN). The experiments were conducted in a standard RSM design called central composite design (CCD) for the optimization of Ta recovery rate. The FeCl2$4H2O adding amount, reaction temperature, and holding time were selected as factors on the response of Ta recovery rate with the coded values at 3 levels (1, 0 and þ1). The ranges of variables were referred to the results obtained in previous study (Niu et al., 2017). The ranges and levels of variables are given in Table 2. 3. Results and discussion 3.1. Pyrolysis To investigate the pyrolysis characteristic of mold resin from WTCs, the mold resin powder (scraped from WTCs) was measured by TGA-DTA, and the results were shown in Fig. 4. The mold resin was thermally decomposed in two major steps with maximum rate at 355 and 450 C followed by a slow weight loss in the temperature region 550e800 C. Similar results have also been observed in the studies dealing with the decompositions of phenolic resin and epoxy resin containing flame retardant, where the decompositions, are completed in at least two steps (Evangelopoulos et al., 2015; Wu et al., 2002). In addition, as shown from the TGA curve, the weight loss was about 9.75% in the first step, and in the second step, the resin underwent about 4.76% of weight loss. After the two major steps, the weight continued slow decrease, which could be attributed to the carbonization of the pyrolysis residue (Wu et al., 2002). To further obtain the optimal pyrolysis parameter for mold resin, the effect of pyrolysis temperature and holding time on the decomposition rate of organic were investigated. Fig. 5a showed the relationship between organic decomposition rate and temperature (holding time: 30 min). It showed that the organic decomposition rate sharply increased from 40.66% to 75.88% with increasing the temperature from 350 C to 450 C. After that, the
The metal content in WTCs and product were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500a, Agilent Corporation, US). The content of SiO2 and organics in WTCs were examined by X-ray Fluorescence Spectrometer (XRF1800, Shimadzu, Japan) and combustion method (Xiu et al., 2013). Thermal gravimetric (TG) and differential scanning calorimetry (DSC) were performed using a simultaneous TG-DSC instrument (Mettler Toledo, Shimadzu, Japan). The pyrolysis oils and gases were analyzed by gas chromatography-mass spectroscopy (GC-MS, TurboMass, Perkin Elmer Corporation, US). The crystal structure of product and residues were identified by X-ray diffraction (XRD, D8 ADVANCE, BRUKER, Germany) with Cu Ka radiation. All the experiments were repeated three times, and the mean values were reported. 2.5. Statistical analysis The response surface methodology (RSM) was applied to
Fig. 4. TG-DTA curves of the mold resin powder scraped from WTCs.
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Fig. 5. Effect of (a) pyrolysis temperature; (b) holding time on the organic decomposition rate.
organic decomposition rate slowly increased. When the temperature was above 550 C, the decomposition rate remained the same. The relationship between the organic decomposition rate and holding time was shown in Fig. 5b (temperature: 550 C), which showed that the decomposition rate could reach up to 52.13% at 10 min. A higher decomposition rate could be obtained at 30 min and the organic decomposition rate was 87.91%. The decomposition rate could not reach to 100%, since some of the organics will be transformed into carbon residues under pyrolysis condition (oxygen-free). Therefore, the temperature of 550 C and holding time for 30 min could be considered as the optimal pyrolysis parameter for mold resin in this study. After pyrolysis process, pyrolysis gas and oil were collected and then analyzed by GC-MC. The results of GC-MC analysis were shown in Tables S1 and S2 of SI. As shown in Table S1, the oils were mainly composed of acetophenone (12.34 area %), phenol (9.76 area %), phenol congeners (16.13 area %), 2, 5-Dimethylbenzophenone (6.52 area %) and so on. Table S2 showed the composition of pyrolysis gases, which mainly contained ethylene, ethane and propene. Under high temperature combustion, these oils and gases will release energy and ultimately transform into CO2 and H2O. Thus, the pyrolysis products are suggested to be disposed by the approach of combustion as fuel. The utilization of the pyrolysis oils and gases will be investigated in a future study. 3.2. Chloride metallurgy (CM) After pyrolysis, the mold resin was decomposed, and then the solid residues were performed by mechanical-physical separation (crush, magnetic separation and screen). Ni-Fe terminals were collected by magnetic separation. Then, CM was used to extract the Ta resource. According to the previous study, Ta could selectively react with FeCl2, and the chlorination reaction between Ta and FeCl2 is as follows (Niu et al., 2017):
2Ta ðsÞ þ 5FeCl2 ðs; lÞ ¼ 2TaCl5 ðgÞ þ 5FeðsÞ ðT > 380 CÞ
(3)
The boiling points of TaCl5 and FeCl2 at standard atmospheric pressure are 234 C and 1026 C, respectively. Within the chlorination temperatures, the generated TaCl5 will be evaporated into gas phase, and then condensed in the condensation zone. Consequently, Ta was separated from the reactants (Niu et al., 2017). During the chlorination process, the extraction of Ta could be affected by some parameters such as temperature, holding time, the adding amount of chlorinating agent and so on. To obtain the maximum Ta recovery rate, the CM experiments were conducted in
a standard response surface methodology (RSM) design called central composite design (CCD) with the levels in Table 2. The response results were listed in Table S3 of SI and the analysis results were outputted by Design-Expert software. The analysis of variance (ANOVA) was shown in Table 3, demonstrating that the response surface quadratic model was significant at F value of 24.32 and pvalue < 0.0001. The statistical significance of the model was also confirmed by the R-Squared value of 0.9563, which means that 95.63% of the variations could be explained by the independent variables. The regression analysis was carried out using all the independent variables and their interactions. The coefficients for the second-order model were also calculated by the Design-Expert software. The empirical relationship between Ta recovery and the three variables can be written as following eq. (4):
Y ¼ 414:66487 þ 5:60265 XFe þ 1:40049 XT þ 0:18382 Xt ð8:0E 4Þ XFe XT ð1:83333E 4ÞXFe Xt 2 ð1:51667E 4ÞXT Xt 0:048996XFe ð1:35405E 3ÞXT2
ð3:08203E 4ÞXt2 (4) Where Y is the Ta recovery rate; XFe, XT and Xt are the actual values of FeCl2$4H2O addition amount (wt %), temperature ( C), and time (min), respectively.
Table 3 ANOVA for response surface quadratic model. Source
Sum of squares
df
Mean square
F value
p-value (prob > F)
Model
216.93
9
24.10
24.32
A- FeCl2$4H2O B-temperature C-time AB AC BC A2 B2 C2 Residual Lack of fit Pure error Cor total
26.96 4.72 7.59 0.32 6.05E-003 0.41 21.62 165.14 1.11 9.91 9.91 0.000 226.85
1 1 1 1 1 1 1 1 1 10 5 5 19
26.96 4.72 7.59 0.32 6.05E-003 0.41 21.62 165.14 1.11 0.99 1.98 0.00
27.21 4.76 7.66 0.32 6.104E-003 0.42 21.82 166.62 1.12
<0.0001 (significant) 0.0004 0.0541 0.0199 0.5824 0.9393 0.5326 0.0009 <0.0001 0.3151
R-Squared ¼ 0.9563 Adj R-Squared ¼ 0.9170.
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For better understanding the interaction of these variables, the three-dimensional response surface plots are shown in Fig. 6. As shown in Fig. 6a and b, the Ta recovery rate increased first and then descended with the increase of temperature, which demonstrated that the excessive temperature will not facilitate the chlorination reaction of Ta. Fig. 6c shows the effect of time and FeCl2$4H2O addition amount on the Ta recovery rate. It showed that the Ta recovery rate slowly increased and then kept invariability with increasing the chlorination time and FeCl2$4H2O addition amount. To optimize the CM process, the desirability function was applied to seek the maximum Ta recovery rate. The goal seeking began at a random starting point and proceeded up the steepest slope to a maximum. The temperature within range of 450e550, the FeCl2$4H2O addition amount within range 45e55 and the chlorination time within range of 90e150 were set for the maximum desirability. By investigating 39 starting points in the response surface changes, the optimal parameter was considered to be temperature of 493.16 C, FeCl2$4H2O addition amount of 52.87 wt%, chlorination time for 150 min and Ta recovery rate of
93.05%. According to the above results, 493 C, adding 53 wt% of FeCl2$4H2O and 150 min were chosen as the optimal parameters. The experiments were performed as the optimal parameters, and the results suggested that the Ta recovery rate was 92.87 ± 0.36%, which was similar to the predictions. 3.3. Environmental and economic aspects In the existing recycling processes, hydrometallurgy (including many wet steps) and pyrometallurgy (calcination and smelting) were most applied to remove the organics and recover Ta from WTCs. However, some problems exist in these processes, such as waste liquid or toxic gas generating and high energy consumption. In comparison, this study proposed an integrated and environment-friendly process for recycling WTCs (Fig. 7). At first, pyrolysis was adopted to decompose the resin into gases and oils. Then, Ni-Fe terminals were recovered by crush and magnetic separation. Since the organics were recycled, the emission of organic
Fig. 6. Response surface 3D plots for the effect of (a) temperature and FeCl2$4H2O addition amount; (b) temperature and time; (c) FeCl2$4H2O addition amount and time.
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Fig. 7. Schematic illustration of recycling process for WTCs (the XRD pattern for Ta product was shown in Fig. S2).
Table 4 Comparison of different methods for WTCs recycling. Method
Proposed technology
Pyrometallurgy (Fujita et al., 2014; Kikuchi et al., 2014)
Hydrometallurgy (Mineta and Okabe, 2005; Katano et al., 2014; Spitczok von Brisinski1 et al., 2014)
Processing steps
calcination or smelting
many wet processing steps
Material recovery Energy consumption Chemical use
integrated process containing pyrolysis, mechanical separation and chloride metallurgy tantalum oxide, Ni-Fe lower energy consumption than pyrometallurgy only FeCl2$4H2O consumed
mixture of metals high energy consumption e
Emissions
gas and oil were collected
gaseous pollutant
Ta, Ni-Fe, e large amount of strong acid, alkali and organic solvent liquid waste
pollutants was eliminated in the mechanical-separation process. In the CM process, compared to the hazardous Cl2 or HCl gas, we choose the non-toxic FeCl2$4H2O as the chlorinating agent. Although the excess chlorination agent and the generated Fe will remain in the residues (Fig. S3 of SI), these materials could be easily recycled by filtration and magnetic separation. An overview of the comparison between the proposed route and other recycling technologies is given in Table 4. Therefore, the proposed technology in this study could be considered as an environment-friendly process for recycling WTCs. Besides the environmental impact, the economic benefit of this recycling process was also estimated. Assuming that 10 kg of WTCs were treated by this system, and the equipment used in this study is fabricated at laboratory scale. The depreciation cost of equipment and electric charge as well as cost of FeCl2$4H2O consumption were considered. Detailed calculations can be found in the SI. By this process, about 3.65 kg of tantalum oxide and 0.75 kg of Ni-Fe could be obtained (SiO2 and the generated Fe were not considered in this evaluation), and the profit was evaluated about 2151.86 dollars. Nerveless, our study remained at the laboratory stage and more detailed assessment should be done before an industrial application. Besides, the revenues have not been taken into account the cost of raw materials or labor, but the Chinese government would provide some subsidies for recycling e-waste. Based on above environmental and economic assessment, this integrated recycling process for WTCs is environment-friendly and economically viable,
which provides a good prospect for industrial application. 4. Conclusion This study developed an integrated process to recycle valuable materials from WTCs. The technological process contained pyrolysis, mechanical-physical separation and chloride metallurgy (CM). The pyrolysis temperature of 550 C and holding time for 30 min were considered as the optimal parameters for decomposing the organics of WTCs. After pyrolysis, Ni-Fe terminals were recovered by crush and magnetic separation. In the CM process, the Ta recovery rate could reach 92.87 ± 0.36%, with the temperature of 493 C, adding FeCl2$4H2O of 53 wt% and holding time for 150 min. Through the integrated process, the organics of WTCs were removed, tantalum oxide and Ni-Fe were recovered. Environmental and economic assessment suggested that the recycling process is environment-friendly and economically viable. In short, this study proposed an efficient and promising process for recycling WTCs, which is significant for e-waste treatment and sustainable utilization of resource. Acknowledgments This work was supported by the National Natural Science Foundation of China (51534005).
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