Research Article pubs.acs.org/journal/ascecg
Method for Recycling Tantalum from Waste Tantalum Capacitors by Chloride Metallurgy Bo Niu, Zhenyang Chen, and Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
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S Supporting Information *
ABSTRACT: The demand for tantalum (Ta) is rapidly increasing due to the manufacture of Ta capacitors (TCs) for electronic devices. With the increasing awareness of environmental protection and conservation of rare metal Ta, recycling of Ta from waste TCs (WTCs) is becoming a hot topic in current society. In this study, an efficient and environmentally friendly process for recycling Ta from WTCs by chloride metallurgy (CM) is proposed. In the CM process, the nontoxic FeCl2 is chosen as the chlorination agent. Thermodynamic analysis demonstrates that Ta can selectively react with FeCl2, and the generated TaCl5 can be easily separated and then condensed in the condensation zone. The recovery of Ta can reach 93.56% under the optimal chlorination parameters as follows: heating temperature of 500 °C, FeCl2 addition amount of 50%, holding time for 2 h, and particle size of Ta-rich powder less than 0.24 mm. Moreover, the kinetic mechanism is discussed, and the rate-controlling step in the chlorination reaction of Ta is determined by mixed control. No hazardous gas and liquid waste is produced during the whole process. Therefore, this study presents an environmentally friendly and promising method for the cyclic regeneration of rare metal Ta from WTCs. KEYWORDS: Waste tantalum capacitors, Tantalum, Chloride metallurgy, Cyclic regeneration
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INTRODUCTION Tantalum (Ta) is a rare metal which is chiefly used to manufacture tantalum capacitors (TCs) for electronic products. For example, a mobile phone (3G technology) contains about 36, the motherboard of a notebook (2 GHz), 22, a digital camcorder, about 13, of these capacitors.1 As the number of electronic products has undergone rapid growth, the global TCs consumption had been raised from 10 billion pieces in 2001 to 30 billion pieces in 2013.2 The huge market demand for TCs resulted in increasing tantalum (Ta) resource demand. Currently, the world annual production of Ta is only about 2000 t, and 42% of Ta consumption (777 t) is used for TCs.1 Due to the strong demand for Ta in capacitor manufacturing, the price of capacitor- grade Ta reached $585/kg in 2016. In addition, Ta and niobium (Nb) are almost always paired together in nature. These metals are difficult to separate because of their similar physical and chemical properties.3 Consequently, large amounts of energy and chemical will be required during the Ta separation and purification processes. Owing to the increasing environmental awareness, it is significant to balance the demand for Ta resource and minimize the impact of Ta processing on the environment. The recycling of Ta is an effective approach to balance the demand for Ta resource and increase resource efficiency in production.4 In fact, a current estimate shows that nearly 45 million tons of electronic wastes are generated globally per year,5 and large quantities of waste TCs (WTCs) will be © 2016 American Chemical Society
discarded accordingly. The WTCs contain about 45 wt % Ta, which is free of Nb.6 Therefore, WTCs could be considered as a high quality Ta resource for recycling. However, the recycling Ta from WTCs is difficult due to the coexistence of Ta with several materialsa capacitor consists of a sintered Ta anode, MnO2 or polymer cathode, a silver paste and graphite cathode layer, terminals (Fe−Ni), and mold epoxy resin (containing SiO2 powder), as shown in Figure 1. Thus far, several related research works such as those for phase separation, 4
Figure 1. Schematic illustration of a TC. Received: August 3, 2016 Revised: December 11, 2016 Published: December 21, 2016 1376
DOI: 10.1021/acssuschemeng.6b01839 ACS Sustainable Chem. Eng. 2017, 5, 1376−1381
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Main Composition of the WTCs Used in This Study composition
Ta
organics
SiO2
Ni
Fe
Ag
content (wt %)
35.99 ± 0.40
11.21 ± 0.15
44.78 ± 0.51
6.10 ± 0.08
1.40 ± 0.03
0.48 ± 0.03
Table 2. Content of Ta in Different Particle Sized Ta-rich Powders particle size (mm)
90% high energy consumption high efficiency
high efficiency only FeCl2 consumed Pyrolysis oil and gas were collected and could be recycled as energy resources
(3)
1 − 2R /3 − (1 − R )2/3 = Kt
(4)
R = K ln t + C
(5)
long periods of time large amount of mineral acid and oxalate organic solvent strong alkali large amount wastewater and acid
reaction residues were also generated and then formed an ash layer (mainly Fe, as shown in Figure S2 of the SI). As the reaction progresses, the reaction residues gradually increased, leading to the thickening of the ash layer. The ash layer would provide a barrier of the reactions between Ta and FeCl2. Meanwhile, the gaseous products TaCl5 penetrated through the ash layer and reactants (Ta rich particle and FeCl2), which was facilitated to the chlorination reaction. When Ta or FeCl2 was exhausted, the reaction is terminated. Therefore, the mixed control could be considered as the chlorination rate-controlling step. For the higher recovery of Ta, decreasing the particle size of reactants and increasing the contact area between reactants can be beneficial to the reaction (Figure 4d). Environmental Comparison with Other Processes. As stated in the introduction, recycling Ta from WTCs is an effective approach to balance the demand for Ta resource for social and environmental need. Therefore, some research has been done to recycle Ta from WTCs. In the existing research, WTCs are most treated via pyrometallurgy (high-temperature
ash diffusion control, and mixed control are investigated in eqs 3, 4, and 5, respectively. 1 − (1 − R )1/3 = Kt
gaseous pollutant
hydrometallurgy many wet processing steps >90%
Where R is the recovery rate of Ta; K is the reaction rate constant; t is the chlorination time; and C is a constant. The analysis results of the chlorination process are shown in Figure 6. The best correlation of the experimental data for the chlorination process of Ta was obtained by eq 5, which indicates that the kinetic model for the chlorination reaction of Ta could be described as the mixed control (chemical reaction and ash diffusion controls). The chlorination process can be explained as the follows. First, Ta will react with FeCl2 under certain temperature, and the chlorination reaction proceeded with time prolonging. During the chlorination process, the 1380
DOI: 10.1021/acssuschemeng.6b01839 ACS Sustainable Chem. Eng. 2017, 5, 1376−1381
Research Article
ACS Sustainable Chemistry & Engineering
(4) Kikuchi, R.; Yamamoto, T.; Nakamoto, M. Preliminary information of laboratorial tantalum recovery and considerations for a potential solution for conflict mineral and wildlife conservation. Environ. Nat. Resour. Res. 2013, 4 (1), 1 DOI: 10.5539/enrr.v4n1p31. (5) Ghosh, B.; Ghosh, M. K.; Parhi, P.; Mukherjee, P. S.; Mishra, B. K. Waste printed circuit boards recycling: an extensive assessment of current status. J. Cleaner Prod. 2015, 94, 5−19. (6) Mineta, K.; Okabe, T. H. Development of a recycling process for tantalum from capacitor scraps. J. Phys. Chem. Solids 2005, 66 (2−4), 318−321. (7) Fujita, T.; Ono, H.; Dodbiba, G.; Yamaguchi, K. Evaluation of a recycling process for printed circuit board by physical separation and heat treatment. Waste Manage. 2014, 34 (7), 1264−1273. (8) Katano, S.; Wajima, T.; Nakagome, H. Recovery of tantalum sintered compact from used tantalum condenser using steam gasification with sodium hydroxide. APCBEE Proc. 2014, 10, 182−186. (9) Spitczok von Brisinski, L.; Goldmann, D.; Endres, F. Recovery of metals from tantalum capacitors with ionic liquids. Chem. Ing. Tech. 2014, 86 (1−2), 196−199. (10) Jena, P.; Brocchi, E. Metal extraction through chlorine metallurgy. Miner. Process. Extr. Metall. Rev. 1997, 16 (4), 211−237. (11) Kanari, N.; Allain, E.; Joussemet, R.; Mochón, J.; Ruiz-Bustinza, I.; Gaballah, I. An overview study of chlorination reactions applied to the primary extraction and recycling of metals and to the synthesis of new reagents. Thermochim. Acta 2009, 495 (1), 42−50. (12) Ma, E.; Lu, R.; Xu, Z. An efficient rough vacuum-chlorinated separation method for the recovery of indium from waste liquid crystal display panels. Green Chem. 2012, 14 (12), 3395−3401. (13) Hua, Z.; Wang, J.; Wang, L.; Zhao, Z.; Li, X.; Xiao, Y.; Yang, Y. Selective extraction of rare earth elements from NdFeB scrap by molten chlorides. ACS Sustainable Chem. Eng. 2014, 2 (11), 2536− 2543. (14) Uda, T. Recovery of rare earths from magnet sludge by FeCl2. Mater. Trans. 2002, 43 (1), 55−62. (15) Zheng, H.; Okabe, T. H. Recovery of titanium metal scrap by utilizing chloride wastes. J. Alloys Compd. 2008, 461 (1), 459−466. (16) Havlik, T.; Orac, D.; Petranikova, M.; Miskufova, A.; Kukurugya, F.; Takacova, Z. Leaching of copper and tin from used printed circuit boards after thermal treatment. J. Hazard. Mater. 2010, 183 (1−3), 866−73. (17) Dai, Y.; Yang, B. The vacuum metallurgy of nonferrous metals; Metallurgical Industry Press: Beijing, 2009. (18) Cable, M.; Frade, J. R. Diffusion-controlled growth of multicomponent gas bubbles. J. Mater. Sci. 1987, 22 (3), 919−924. (19) Lu, C. H.; Lee, J. T. Kinetic analysis of the serial reactions of lead magnesium tungstate ceramics using a multiple core-shell model. J. Mater. Sci. 1998, 33 (8), 2121−2127. (20) Frade, J. R.; Cable, M. Theoretical solutions for mixed control of solid state reactions. J. Mater. Sci. 1997, 32 (10), 2727−2733.
combustion) and hydrometallurgy including many wet steps to extract Ta. However, gas pollutants and liquid waste will be inevitably generated during the combustion and chemical treatment. An overview of the comparison between the proposed route and other processes for WTCs recycling is given in Table 3. Therefore, the proposed CM technology in this study can be regarded as an environment-friendly and promising process for recycling Ta from WTCs due to its excellent environmental and economic benefits.
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CONCLUSIONS An efficient and environmentally friendly chloride metallurgy (CM) process has been proposed for the recovery of tantalum (Ta) from waste tantalum capacitors (WTCs). Compared to other hazardous and corrosive chlorination agents (Cl2, HCl, and CCl4), the nontoxic FeCl2 was chosen as the chlorination agent in this study. Thermodynamic analysis demonstrates that Ta can selectively react with FeCl2, and the generated TaCl5 can be easily separated and then condensed in the condensation zone. The recovery of Ta could reach to 93.56% under the optimal chlorination parameters as follows: heating temperature of 500 °C, FeCl2 addition amount of 50%, holding time for 2 h, and particle size of Ta-rich powder less than 0.24 mm. In addition, the kinetic mechanism is discussed, and the ratecontrolling step in the chlorination reaction of Ta is determined by the mixed control (chemical reaction and ash layer diffusion controls). No hazardous gas and liquid waste are produced during the whole process. Therefore, this study presents an environmentally friendly and promising technology for the cyclic regeneration of the rare metal Ta from WTCs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01839. Schematic illustration of the pyrolysis equipment; XRD patterns for the residue after CM; saturated vapor pressure calculation of TaCl5 and FeCl2 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +86 21 5474495. Fax: +86 21 5474495. ORCID
Zhenming Xu: 0000-0002-4605-9409 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51534005). REFERENCES
(1) Angerer, T.; Luidold, S.; Antrekowitsch, H. Recycling potentials of the two refractory metals tantalum and niobium. European Metallurgical Conference, 2013; pp 1069−1084. (2) The market distribution and outlook of Tantalum capacitors, http://www.chyxx.com/industry/201407/269121.html (accessed July 29, 2014). (3) Gunn, G. Tantalum and niobium; John Wiley & Sons, 2013. 1381
DOI: 10.1021/acssuschemeng.6b01839 ACS Sustainable Chem. Eng. 2017, 5, 1376−1381
