Research Article pubs.acs.org/journal/ascecg
Application of Chloride Metallurgy and Corona Electrostatic Separation for Recycling Waste Multilayer Ceramic Capacitors Bo Niu and Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: Electronic-waste recycling has become a global issue owing to its potential environmental pollution and rich metal resource. Waste multilayer ceramic capacitors (MLCCs), widely existing in discarded electronic devices, are rich in valuable metals such as silver (Ag) and palladium (Pd). However, how to effectively and in an environmentally friendly manner recycle waste MLCCs is seldom considered. This study developed an efficient and integrated process for recovering valuable materials from waste MLCCs by chloride metallurgy (CM) and corona electrostatic separation (CES). In CM, NH4Cl was used as the chlorinating agent. The results demonstrated that BaTiO3, Ag, and tin (Sn) can be chlorinated. The generated gas-phase SnCl4 was collected in the condensing zone. BaCl2 and AgCl were sequentially separated by leaching with water and sodium thiosulfate solution. The recovery efficiencies of Ag, Ba, and Sn could reach 99.21, 98.76, and 99.83%, respectively. The optimal CM parameters were considered to be 400 °C, 30 min, and NH4Cl/scrap mass ratio of 4:1. Then, the residual Pd and TiO2 were performed by CES. Pd and TiO2 can be well separated under a voltage of 30 kV and a roll speed of 25 rpm. This study contributes to the efficient recycling of valuable resources from waste MLCCs. KEYWORDS: E-waste, Waste multilayer ceramic capacitors, Resource recovery, Chloride metallurgy, Corona electrostatic separation
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INTRODUCTION
The global amount of electronic waste (e-waste) is significantly increasing with the rapid update of electric products. It is estimated that 20−50 million tons of e-waste are generated around the world each year.1 The treatment of e-waste has been a global issue that needs to be urgently addressed. On one hand, the hazardous material in e-waste will cause environmental problems and jeopardize human health if it is not properly treated.2,3 On the other hand, e-waste is a rich resource because of its high content of precious metals and rare metals.4,5 Therefore, the treatment of e-waste is of great significance for environmental protection and resource regeneration. Multilayer ceramic capacitors (MLCCs) are widely used in various electronic devices to keep the electrical noise at a low level of power supply.6 Statistics show that the number of MLCCs used is estimated around 150 in a mobile phone, 200 in a personal digital assistant, and 300 in a digital television set.7 Undoubtedly, large quantities of waste MLCCs are continuously discarded within the e-waste stream. The MLCC has a structure in which many dielectric layers (mainly BaTiO3) and internal electrodes are alternately stacked and the internal electrodes are connected in parallel, as presented in Figure 1.8 The internal electrode is usually manufactured with silver− palladium alloy (Ag−Pd) or nickel (Ni). The end electrode consists of three layers of Ag, Ni, and Sn.9 The concentrations © 2017 American Chemical Society
Figure 1. Schematic illustration of MLCC.
of these metals, especially the precious metals Ag and Pd, in waste MLCCs are much higher than their respective primary resources. Therefore, waste MLCCs are considered as a high quality resource for recycling. So far, only two research studies were reported regarding the recycling of waste MLCCs. Kim et al.7 investigated the leaching behavior of Ni present in waste MLCCs using different acidic leaching reagents. They found that HNO3 was the most Received: July 2, 2017 Revised: August 11, 2017 Published: August 14, 2017 8390
DOI: 10.1021/acssuschemeng.7b02190 ACS Sustainable Chem. Eng. 2017, 5, 8390−8395
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. Automatic system for waste MLCC disassembly from waste printed circuit board. middle part of the quartz tube is the heating zone, and the two ends of the tube far from the heating zone are the condensation zone. The schematic diagram of the experimental equipment is shown in Figure S1 of the Supporting Information. A certain amount of powder scrap and NH4Cl were blended well in a quartz boat, and then put into the quartz reactor. The samples were heated at 300−500 °C for 10−60 min in an O2−N2 mixture gas flow (O2/N2 = 2/8, 100 mL/min). After the reactions, the products were collected from the condensing zone and the residues were sequentially soaked in distilled water and Na2S2O3 solution to recover BaCl2 and AgCl. The extraction of Ag from the residues can be expressed as follows:18
effective leaching reagent. The leaching rate of Ni was limited by the diffusion of HNO3 solution throughout thin channels formed between BaTiO3 layers. Prabaharan et al.10 also proposed a hydrometallurgical process for recovering total metal values from waste MLCCs. To extract the precious metals and minimize the impurities, base metals such as Ba, Ti, and Ni were first leached out in two stages using HCl. Then, the precious metals were leached out using aqua regia and nitric acid. Finally, the base and precious metals were recovered by selective precipitation. As for a pyrometallurgical process, chloride metallurgy (CM) has proved to be a quite attractive technology for the extraction of metals from ores and wastes, as it can separate the target elements in a single step according to the different vapor pressures of the chlorides.11,12 CM has the advantages of high efficiency and simple operation compared with the hydrometallurgical process.13 In our previous studies, CM has successfully separated indium from a waste liquid crystal display panel14,15 and extracted tantalum from a waste tantalum capacitor.16 However, some metal chlorides with high boiling point and unreacted components will still remain in the residues, which should be further separated. Currently, there is no correlative study on the further separation after CM. In order to achieve the maximum recovery of waste MLCCs, the residues after CM were performed on by corona electrostatic separation (CES) in this study. In the present work, an integrated process of CM and CES to recycle waste MLCCs was proposed. In CM, NH4Cl was used as the chlorinating reagent. The separation principles and operating parameters for CM and CES were investigated. The objective of this study was to develop an efficient and environmentally friendly process for maximum recycling of waste MLCCs.
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AgCl + 2Na 2S2 O3 = Na3Ag(S2O3)2 + NaCl The metal recovery efficiency was calculated by eq 2:
R = (M 0 − M )/M 0 × 100%
RPd = MPd /MCES × 100%
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RESULTS AND DISCUSSION Chloride Metallurgy (CM) for Extracting Ag, Ba, and Sn. Feasibility Analysis of CM. Prior to the experimental study, the reactions between NH4Cl and powder scrap were discussed from thermodynamic viewpoints. It is known that NH4Cl begins to decompose into HCl and NH3 above 230 °C, and the generated HCl is acting as the chlorinating agent.19 Figure 4 shows the Gibbs free energy variation of reactions between the
Table 1. Main Composition of Metals in the Sample for CM Ti 19.36
Ag 3.48
Sn 2.75
(3)
where RPd is the Pd recovery efficiency; MPd and MCES are the weights of recovered Pd and Pd in the samples conducted by CES, respectively. Chemical Analysis. The compositions of the raw material and the solid residue were quantitatively analyzed by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7500a, Agilent Corp., USA) and X-ray fluorescence spectrometry (XRF-1800, Shimadzu, Japan). The crystal structure of the raw material and products were identified by X-ray diffraction (XRD; D8 ADVANCE, BRUKER, Germany) with Cu Kα radiation. All the experiments were repeated three times, and only the mean values were given.
MATERIALS AND METHODS
Ba 48.65
(2)
where R is the metal recovery efficiency, M0 is the initial mass of metal in powder scrap, and M is the mass of metal in the residues after the experiment. Corona Electrostatic Separation (CES). To further recover Pd, CES experiments were conducted by a roll-type corona electrostatic separator. Figure 3 presents the diagram of the separator and the movement behavior of the particles. The separator consists of a grounded rotating electrode and two active electrodes (corona electrode and electrostatic electrode) connected to a dc high-voltage supply. A high intensity electric field is generated between the grounded and active electrodes. The residual mixture was dried and then fed onto the vibration feeder that ensures a monolayer of material on the surface of the rotating roll. The particles with different electrical conductivities have different movement behaviors and then are collected in several collecting tanks. The samples were performed by CES three times, and the Pd recovery efficiency was calculated by eq 3:
Materials. The waste MLCCs used in this study were obtained by an automatic disassembly system, as presented in Figure 2.17 The obtained waste MLCCs were first pulverized to a particle size less than 0.125 mm by a ball mill, and then Ni was recovered by magnetic separation. Finally, the sample after magnetic separation was used for CM. The major composition of metals in the sample is listed in Table 1. Chemical reagents used in the experiments were all analytical reagents unless otherwise mentioned. Chloride Metallurgy (CM). The CM experiments were carried out in a quartz tube furnace, which consists of a tubular electric furnace, temperature controller, gas supply system, and gas collector. The
composition content (wt %)
(1)
Pd 1.24 8391
DOI: 10.1021/acssuschemeng.7b02190 ACS Sustainable Chem. Eng. 2017, 5, 8390−8395
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Figure 3. Diagram of corona electrostatic separator and force analysis of particles on the rotating roll (red, metal; blue, semiconductor).
amount on the metal recovery efficiency. The NH4Cl dosage was varied from 1 to 5 (the mass ratio of NH4Cl/scrap) at 350 °C for 10 min. The metal recovery efficiency increased significantly with increasing the NH4Cl/scrap mass ratio from 1 to 3. When the NH4Cl/scrap mass ratio was greater than 4, the metal recovery efficiency was almost the same. The effect of temperature on the metal recovery efficiency was investigated in the range from 300 to 500 °C, with the NH4Cl/scrap mass ratio of 4:1 and holding time for 10 min. As shown in Figure 5b, the effect of temperature on the metal extraction efficiency was significant. The metal recovery efficiency could reach above 93.2% when the reaction temperature was raised to 400 °C. However, the metal recovery efficiency began to decrease with further increasing the temperature, indicating that excess temperature will not facilitate the chlorination reaction. It could be attributed to the evaporation of NH4Cl by raising the reaction temperature.21 Figure 5c shows the relationship between the metal recovery efficiency and holding time. These experiments were conducted at 400 °C with the NH4Cl/scrap mass ratio of 4:1. It shows that the metal recovery efficiency increased with the increase of holding time from 10 to 30 min. After that, the holding time had little influence on the metal recovery efficiency. Recovery efficiencies of Ag, Ba, and Sn at 30 min could reach 99.21, 98.76, and 99.83%, respectively. Based on the above experiments, the optimal CM conditions were considered as temperature of 400 °C, NH4Cl/scrap mass ratio of 4:1, and holding time for 30 min. Product Characterization. The products under optimal conditions were collected and analyzed by XRD. Figure 6 shows the XRD patterns of powder scrap and obtained products. The XRD pattern of the unreacted powder scrap was assigned as the main phase of BaTiO3 (curve a). After CM treatment, the peaks of BaTiO3 disappeared, while the peaks corresponding to BaCl2 and AgCl were observed in the residues (curve b). In addition, Sn was also detected in the condensing products (the results are shown in Figure S2). Meanwhile, the diffraction peaks of Pd were found in the residues after water filtration (curve c). Although Pd could not be recovered by CM, Pd was effectively concentrated. Corona Electrostatic Separation (CES) for Separating Pd and TiO2. Feasibility Analysis of CES. As is well-known, TiO2 is a semiconductor material, which has different electrical conductivity compared with the conductor Pd. Therefore, CES
Figure 4. Gibbs free energy variations of reactions between HCl and powder scrap at 100−800 °C.
main components in powder scrap and HCl (the thermodynamic data is calculated by HSC Chemisty 5.0). It shows that BaTiO3 can thermodynamically react with HCl to form BaCl2 and TiO2. Besides, AgCl and SnCl4 can also be generated in the presence of oxygen. Since the thermodynamic data of Ag−Pd alloy is deficient, the chlorination reaction of Ag−Pd cannot be judged by the Gibbs free energy variation. For chloride metallurgy, the metal chloride with high vapor pressure and low boiling point can be separated through distillation from the other chlorides, and then be recovered. Under standard atmospheric pressure, the boiling points of SnCl4, BaCl2, and AgCl are 115, 1560, and 1550 °C, respectively.20 This means that SnCl4 can be evaporated into the gas phase above 115 °C, while the other metal chlorides will remain in the residues (the maximum CM temperature is 500 °C). Although the metal chlorides with high boiling points cannot be separated by CM treatment, the water-soluble BaCl2 and water-insoluble AgCl can be sequentially separated by leaching with water and Na2S2O3 solution. Consequently, the mixture of TiO2 and Pd obtains (the XRD results can be seen in the section Product Characterization). Factors in CM. In order to obtain the maximum metal recovery efficiency, the effects of NH4Cl added amount, temperature, and holding time on the metal recovery efficiency were investigated. Figure 5a shows the effect of NH4Cl added 8392
DOI: 10.1021/acssuschemeng.7b02190 ACS Sustainable Chem. Eng. 2017, 5, 8390−8395
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Figure 5. Metal recovery efficiency as a function of (a) mass ratio of NH4Cl/scrap, (b) reaction temperature, and (c) holding time.
semiconductor releases partial negative charge. Meanwhile, the particles are polarized by the electrostatic electrode during the transit time. The charge can uniformly distribute on the surface of the conductor. However, the semiconductor induces less charge due to the different conductivity.23 The difference in the charging property further affects the forces characteristic of the conductor and semiconductor. During the CES, the particles are subject to gravity force (Fg), electrostatic force (Fe), electric image force (Fi), air friction (Fa), and centrifugal force (Fc), as shown in Figure 3. The particles will detach from the rotating roll speed when the radial forces achieve equilibrium: Fg sin θ = Fe + Fc − Fi
(4)
Generally, Fi is related to the charge. Fe is related to the charge and electric field strength. Fc is proportional to the roll speed.23 Therefore, the separation of TiO2 and Pd can be achieved by regulating the input parameters including voltage and roll speed. Factors in CES. The effect of voltage on the Pd separation was investigated in the range from 15 to 35 kV with a roll speed of 35 rpm. As shown from Figure 7a,b, the voltage had little influence on the Pd recovery efficiency in the range from 15 to 30 kV. However, with increasing the voltage, the mass of products falling into the metal collecting box decreased, while the mass of products falling into the nonmetal collecting box increased. Accordingly, the purity of Pd increased. In fact, high voltage could enhance the electric image force acting on the nonmetal particles. Then, the nonmetal particles are more strongly pinned to the surface of the rotating electrode.24 Therefore, TiO2 particles were more effectively separated at
Figure 6. XRD pattern of powder scrap and products: (a) initial powder scrap, (b) residues after CM, (c) residues after water filtration, and (d) residues after leaching with 5 wt % Na2S2O3 solution.
can be a good choice for separating TiO2 and Pd. The CES principle is based on the different movement behavior of the particles because of their difference in electrical conductivity.22 The charging mechanism of conductor and semiconductor in CES is shown in Figure S3. The mixture particles are fed onto the surface of the roll electrode and pass through the electric field. All the particles are negatively charged by ion bombardment with the same polarity as the corona electrode. The conductor discharges rapidly to the roll electrode, while the 8393
DOI: 10.1021/acssuschemeng.7b02190 ACS Sustainable Chem. Eng. 2017, 5, 8390−8395
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Figure 7. CES for Pd: mass of products as a function of (a) high voltage level and (c) roll speed; recovery efficiency and purity of Pd as functions of (b) high voltage level and (d) roll speed.
Figure 8. Proposed flow sheet for recycling of waste MLCCs.
coupled plasma atomic electron spectroscopy (ICP-AES) result showed that the recovered Pd contained 17.18% Ti. Though Pd and TiO2 were not completely separated by CES, Pd was effectively enriched, which could benefit the subsequent Pd purification.
high voltage. However, it should be noted that spark discharge occurred when the voltage increased to 35 kV in this study. The spark discharge will tangle the electrical field and decrease the metal recovery efficiency.25 Figure 7c,d shows the effect of roll speed on the Pd separation. The roll speed was changed from 15 to 55 rpm with voltage of 30 kV. With the increase of roll speed, both the Pd recovery efficiency and the mass of products falling into the metal and middling collecting boxes increased, while the purity of Pd decreased. This was attributed to the fact that high roll speed improved the centrifugal force acting on the particles. As a result, some TiO2 particles fell into the metal and middling collecting boxes. To obtain a higher recovery efficiency and purity of Pd, 30 kV and 25 rpm were considered as the optimal parameters in this study. The recovery efficiency and purity of Pd could reach 92.36 and 70.27%, respectively. The inductively
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CONCLUSION This study proposed an efficient and integrated process for recycling of waste MLCCs, as shown in Figure 8. NH4Cl and nontoxic Na2S2O3 were respectively chosen as the chlorinating agent and Ag extractant. After CM and extraction process, the generated gas-phase SnCl4 was collected in the condensing zone. BaCl2 and AgCl were sequentially separated by leaching with water and Na2S2O3 solution. The recovery efficiencies of Ag, Ba, and Sn could reach 99.21, 98.76, and 99.83%, respectively. Moreover, the condensing NH4Cl can be reused 8394
DOI: 10.1021/acssuschemeng.7b02190 ACS Sustainable Chem. Eng. 2017, 5, 8390−8395
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(12) 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. (13) Terakado, O.; Saeki, T.; Irizato, R.; Hirasawa, M. Pyrometallurgical Recovery of Indium from Dental Metal Recycling Sludge by Chlorination Treatment with Ammonium Chloride. Mater. Trans. 2010, 51 (6), 1136−1140. (14) 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. (15) Ma, E.; Xu, Z. Technological process and optimum design of organic materials vacuum pyrolysis and indium chlorinated separation from waste liquid crystal display panels. J. Hazard. Mater. 2013, 263, 610−617. (16) Niu, B.; Chen, Z.; Xu, Z. Method for Recycling Tantalum from Waste Tantalum Capacitors by Chloride Metallurgy. ACS Sustainable Chem. Eng. 2017, 5 (2), 1376−1381. (17) Wang, J.; Guo, J.; Xu, Z. An environmentally friendly technology of disassembling electronic components from waste printed circuit boards. Waste Manage. 2016, 53, 218−224. (18) Li, X.; Wang, L. Extraction and Refining of Precious Metals; Central South University Press: Changsha, China, 2000. (19) Andreev, A. A.; D’yachenko, A. N.; Kraidenko, R. I. Processing of oxidized nickel ores with ammonium chloride. Theor. Found. Chem. Eng. 2011, 45 (4), 521−525. (20) Dai, Y.; Yang, B. The Vacuum Metallurgy of Nonferrous Metals; Metallurgical Industry Press: Beijing, 2009. (21) Itoh, M.; Miura, K.; Machida, K. Novel rare earth recovery process on Nd-Fe-B magnet scrap by selective chlorination using NH4Cl. J. Alloys Compd. 2009, 477 (1−2), 484−487. (22) Cui, J.; Forssberg, E. Mechanical recycling of waste electric and electronic equipment: a review. J. Hazard. Mater. 2003, 99 (3), 243− 263. (23) Xue, M.; Yan, G.; Li, J.; Xu, Z. Electrostatic Separation for Recycling Conductors, Semiconductors, and Nonconductors from Electronic Waste. Environ. Sci. Technol. 2012, 46 (19), 10556−10563. (24) Wu, J.; Li, J.; Xu, Z. Electrostatic separation for recovering metals and nonmetals from waste printed circuit board: problems and improvements. Environ. Sci. Technol. 2008, 42 (14), 5272−5276. (25) Hou, S.; Wu, J.; Qin, Y.; Xu, Z. Electrostatic separation for recycling waste printed circuit board: a study on external factor and a robust design for optimization. Environ. Sci. Technol. 2010, 44 (13), 5177−5181.
and the generated NH3 gas can be absorbed by water to produce ammonia−water. For the separation of Pd and TiO2, CES was applied. The recovery efficiency and purity of Pd could, respectively, reach 92.36% and 70.27% under optimal conditions. No hazardous gas and liquid waste were produced during the CES process. Therefore, the proposed technology in this study can be regarded as a promising process for recycling of waste MLCCs.
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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.7b02190. Schematic diagram of the CM equipment; image of the condensed product in one end of the quartz tube and the XRD pattern of the yellow deposits; charge model of conductor and semiconductor (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
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