Recovery of Tantalum from Waste Tantalum Capacitors by

Research Article pubs.acs.org/journal/ascecg

Recovery of Tantalum from Waste Tantalum Capacitors by Supercritical Water Treatment 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 S Supporting Information *

ABSTRACT: Recycling e-waste has been a significant topic for environmental protection and also for resource recovery. Waste tantalum capacitors (WTCs), widely existing in discarded small appliances, are rich in high quality tantalum (Ta) resources. However, recovering Ta from WTCs is difficult due to the tightly covered mold resin. This study proposes an efficient and environmentally friendly process using supercritical water (SCW) to decompose the organics and recover Ta from WTCs. Two methods of SCW were studied: SCW depolymerization (SCWD) and SCW oxidation (SCWO). The results indicated that the mold resin (packing) of WTCs were effectively decomposed by SCW treatments. The organic decomposition efficiency in SCWD was lower than that in SCWO. The optimal parameters for SCWD and SCWO were, respectively, considered to be 425 °C, 25 MPa, and 120 min and 400 °C, 25 MPa, and 90 min, adding a 10% volume ratio of H2O2. After SCWO treatment, Ta electrodes were directly recovered, and the purity of Ta could reach 93.18%. For the sample treated by SCWD, Ta powder could be further recovered by mechanical separation. This study presents an efficient and environmentally friendly process to recover Ta from e-waste, which is significant for resource regeneration and environmental conservation. KEYWORDS: E-waste, Waste tantalum capacitors, Tantalum recovery, Supercritical water, Environmental conservation

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INTRODUCTION A large quantity of e-waste is being generated with the rapid replacement of electric products. It is estimated that the global production of e-waste has reached 45 million tons per year, and this figure is growing exponentially.1 E-waste has been an exigent environmental problem that has to be solved because of its fast growing and its containing toxic materials.2 However, e-waste is also a valuable and precious resource if treated in a proper way. Actually, e-waste contains a variety of valuable metals, such as base metal (copper), precious metal (gold, silver, and platinum), rare metal (tantalum, germanium), and so on.3 Moreover, the concentrations of these metals in e-waste are much higher than their respective primary resources, so e-waste is attractive for recycling.4,5 Waste tantalum capacitors (WTCs) widely exist in small household appliances. For instance, a discarded mobile phone, the motherboard of a notebook, and a digital camcorder, respectively, contain about of 36, 22, and 12 of these capacitors.6 WTCs, containing about 45 wt % of Ta, are considered as a high quality Ta resource.7 In fact, Ta is a rare metal. Currently, the world annual production of Ta is only about 2000 tons, and 42% of Ta is consumed in manufacturing tantalum capacitors.6 Moreover, Ta and niobium are usually paired together in nature. The metals are difficult to separate because of their similar physical and chemical properties.8 As a result, large amounts of energy and chemicals will be consumed during the © 2017 American Chemical Society

Ta purification process. Therefore, the recovery of Ta from WTCs is significant in achieving sustainable utilization of Ta resources and minimizing the impact of Ta processing on the environment. However, recovering Ta from WTCs is difficult owing to the tightly covered mold resin, as shown in Figure 1. The mold resin mainly contains silica, o-cresol novolac-type epoxy resin, phenolic novolac resin, and flame-retardants.9,10 To achieve the recovery of Ta resources, first of all, environmentally friendly

Figure 1. Schematic illustration of a TC. Received: February 16, 2017 Revised: March 16, 2017 Published: April 5, 2017 4421

DOI: 10.1021/acssuschemeng.7b00496 ACS Sustainable Chem. Eng. 2017, 5, 4421−4428

Research Article

ACS Sustainable Chemistry & Engineering

Figure 2. Flowchart briefing the obtained WTCs from e-waste.

Table 1. Main Composition of the WTCs Used in This Study composition

Ta

organics

SiO2

Ni

Fe

Ag

content (wt %)

36.49 ± 0.42

14.32 ± 0.16

41.06 ± 0.48

6.12 ± 0.08

1.38 ± 0.03

0.46 ± 0.02

Figure 3. Effect of SCW temperature (a), pressure (b), holding time (c), and H2O2 adding amount on the yield of residue.

halides, and aluminum trichloride to dissolve the mold resin. In our previous studies, chloride metallurgy was adopted to recover Ta resources.13 Also, an integrated vacuum pyrolysis and mechanical−physical separation process was proposed to decompose the mold resin and recycle Ta from WTCs.14 Although these technologies were efficient at removing the mold resin and recovering Ta from WTCs, some problems still exist in the processes. For instance, a relatively high temperature is needed in pyrolysis, and some liquid wastes are produced during the chemical treatment. Recently, supercritical water (SCW) technology has been widely used to decompose toxic organic wastes.15,16 The physical or chemical characteristics of SCW (T ≥ 374 °C, P ≥ 22.1 MPa) are different from those of water at room temperature and

removal of the mold resin is significant. On one hand, improperly disposing of organics will bring environmental pollution, such as the emission of toxic and harmful gases by open incineration. On the other hand, effectively removing the mold resin from WTCs is a prerequisite for recycling valuable Ta resources. Thus far, several studies have been done to deal with the mold resin and recycle Ta from WTCs. Mineta et al.7 removed the mold resin by combustion and then recovered Ta by chemical treatment. Similarly, Fujita et al.11 recovered Ta powders by heat treatment at 723 and 823 K in the air. Katano et al.10 developed a steam gasification process with sodium hydroxide to destroy the mold resin. The halogen gas generated from the decomposition of mold resin could be trapped in sodium hydroxide. Endres et al.12 applied ionic liquids, mixed dialkyimidazolium 4422


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ACS Sustainable Chemistry & Engineering atmospheric pressure. For example, the dielectric constant is much lower; the number and persistence of hydrogen bonds are both diminished. Consequently, SCW can behave like many organic solvents.17 Moreover, SCW can provide abundant H+ and OH−, which can be an efficient acid or base catalyst for organic reactions.18 Currently, there are two main SCW processes used for the treatment of organic wastes. One is the SCW depolymerization (SCWD) process under a reducing atmosphere, and the other is the SCW oxidation (SCWO) process in the presence of oxygen. Generally, SCWD is applied to the recovery of polymer materials because SCW is an excellent hydrolysis reagent.19 On the other hand, SCWO is used to degrade the organic matter into small molecules (even CO2 and H2O).17 To our knowledge, however, there are no papers on SCWD or SCWO for disposing of the mold resin and recovering the Ta resources from WTCs. Accordingly, the aim of this work was to explore the feasibility of SCWD and SCWO technologies to remove the organics and recycle Ta from WTCs. The effects of SCWD and SCWO on the organic decomposition and Ta recovery from WTCs were investigated. The decomposition products of organics in SCWD and SCWO were analyzed, and the reaction mechanisms were also discussed. In one word, this study proposed an efficient and environmentally friendly process for recycling Ta from e-waste.

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MATERIALS AND METHODS

The WTCs used in this work were obtained by the flowchart, as presented in Figure 2. The detailed automatic disassembling system can be found in our previous study.20 The main compositions of the WTCs are presented in Table 1. Chemical reagents used in the experiments were all analytical reagents unless otherwise mentioned. SCW Treatment. The SCW treatments were carried out using a 100 mL high pressure semibatch-type reactor made of Hastelloy alloy (Nantong Huaxing Oil Equipment Co., Ltd., China). The schematic diagram of the semibath SCW reactor was presented in Figure S1 of the Supporting Information (SI). About 5 g of WTCs was taken into the reactor tube, and then the constant-flux pump was turned on at a flow rate of 10 mL/min (SCWO: mixing a certain amount of H2O2, 30 wt %). The heating unit was turned on with a heating rate of 20 °C/min after the pressure reached the set value. When the temperature reached the set value, the reaction was maintained for a certain time. The liquid products were gathered by the condensing system, and the gas was collected by the air bag. After that, the reactor was cooled to room temperature, and the solid residue was removed from the reactor. The inner wall of the reactor and pipes were washed by deionized water. All of the liquid products were filtrated to obtain solid residue. Two parts of the residue were dried together to obtain the total mass of solid residue. The yield of solid residue could be used to present the decomposition efficiency of organics in WTCs. The yield of residue was calculated by eq 1.

RY = W /W0 × 100%

Figure 4. Spectra of (a) oil and (b) gas products after SCWD and SCWO treatments.

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RESULTS AND DISCUSSION Effect of SCWD and SCWO on Yield of Residue. Organic polymers in the mold resin could be removed during the SCW treatment. In this study, the solid residue was proved to be metals, SiO2, and char. So, the yield of the solid residue could represent the organic decomposition rate. The lower yield of the residue represents the higher organic decomposition rate. To obtain the maximum decomposition rate, the effects of reaction temperature, pressure, H2O2 adding amount, and holding time on the solid residue were investigated. Figure 3a presents the effect of temperature on the solid residue after SCW treatments (SCWD, 25 MPa, 120 min; SCWO, 25 MPa, 120 min, adding 10% volume ratio of H2O2). It shows that both the solid residues of samples treated by SCWD and SCWO decreased with increasing the temperature, suggesting that a higher temperature could facilitate the decomposition of organics in the SCW process. It was reported that free radical reaction was the main polymer decomposition mechanism in SCW. When the reaction temperature was high enough, the reaction system could provide enough energy to break the bond of the polymer that caused the formation of free radicals.22,23 Furthermore, it is worth noting that the weight loss of the sample treated by SCWO was much higher than that of the sample treated by SCWD at the same reaction temperature. It suggests that the organic decomposition rate during the SCWO process was higher than that during the SCWD process.

(1)

where RY is the yield of residue, W0 is the weight of WTCs, and W is the weight of solid residue after SCWD/SCWO treatment. Chemical Analysis. The organic products were analyzed by gas chromatography−mass spectroscopy (GC-MS, TurboMass, PerkinElmer Corporation, US). The metal content in the raw material as well as liquid and solid products was examined by inductively coupled plasmamass spectrometry (ICP-MS, Agilent 7500a, Agilent Corporation, US). The contents of SiO2 and organics in WTCs were measured by X-ray fluorescence spectrometry (XRF-1800, Shimadzu, Japan) and the combustion method.21 The crystalline phases of solid products were characterized 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 reported. 4423


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Figure 5. Proposed decomposition pathways of organics in mold resin by SCW treatments.

molecular weight organic compounds and dissolved in the SCW.23 In addition, it also can be found in Figure 3b that the solid residue showed little change when further increasing the system pressure. It indicated that the pressure had a small effect on the decomposition of organics when the reaction system had reached the critical condition. Figure 3c shows the effect of holding time on the solid residue after SCW treatments (SCWD, 425 °C, 25 MPa; SCWO, 400 °C, 25 MPa, adding 10% volume ratio of H2O2). The results suggested that the solid residue decreased with the increase in holding time. The minimum solid residue of the samples treated by SCWD and SCWO could respectively reach about 89.63 wt % at 120 min and 85.38 wt % at 90 min. Since the organics in WTCs were measured at about 14.32 wt %, the weight loss of organics treated by SCWD and SCWO could nearly reach 72.42 and 100%, respectively. The weight loss of the organics in SCWD could not reach 100%, since some of the organics will be carbonated during the treatment.26 In addition, the weight loss (14.62 wt %) was more than that of the organics in WTCs (14.32 wt %), which could be due to the oxidization and dissolution of metals during the SCWO process.19,27 Figure 3d shows the effect of H2O2 adding amount on solid residue after SCWO treatment (400 °C, 25 MPa, 90 min). It shows that the solid residue could dramatically decrease from 89.98 to 86.32% when only adding a 5% volume ratio of H2O2 (VH2O2/VH2O = 1/20). Actually, in the SCWO system,

Generally, the enhancement of polymer decomposition in SCW could be attributed to the enhanced dissolving capacity of polymer in SCW and diffusion of water into the molten polymer phase.24 In the SCWO reaction system, the decomposition of organics could also be improved under the oxidation atmosphere (H2O2 = H2O + 1/2O2), owing to the combinative effect of hydrolysis and oxidation.22 During the SCWO process, the organics were very effectively oxidized into small molecules, even CO and H2O.25 Compared with SCWO, hydrolysis was supposed to be the main reaction during the SCWD process.19 Polymers with high molecular weight were gradually depolymerized into low molecular weight compounds in the SCWD process. As a result, the organic decomposition rate in SCWD was lower than that in the SCWO process. The effect of pressure on the solid residue after SCW treatments is shown in Figure 3b (SCWD, 425 °C, 120 min; SCWO, 400 °C, 120 min, adding 10% volume ratio of H2O2), which demonstrates that the system pressure also plays an important role in the solid residue. The residue yield gradually decreased within the pressure of 20 MPa. When the pressure reached 25 MPa, the solid residue rapidly decreased. It was reported that the nature of the SCW changed significantly when the pressure reached the critical point (22.1 MPa). The density and viscosity of the SCW rapidly decreased while the diffusivity and mass transport coefficient increased. Consequently, the polymers could be quickly decomposed into low 4424


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Figure 6. Images of (a) WTCs treated by SCWD, (b) WTCs treated by SCWO, (c) obtained Ta electrode, and (d) Ni−Fe terminals after SCWO and washing with water.

H2O2 could turn to HO• and HO2•, which were of high oxidizability, promoting the decomposition of the organic compounds.28,29 After that, the solid residue slowly decreased, and a lower residue yield reached 85.38% by adding a 10% volume ratio of H2O2. However, the solid residue of the samples began to increase when further adding H2O2, which could be attributed to the fact that the oxidation of metals led to the increase in solid residue.19,30 On the basis of the above results, we could draw the conclusion that the organic decomposition efficiency of the samples treated by SCWO was much higher than those treated by SCWD. The optimal parameters for SCWD and SCWO were, respectively, considered to be 425 °C, 25 MPa, and 120 min and 400 °C, 25 MPa, and 90 min, adding a 10% volume ratio of H2O2. Effect of SCWD and SCWO on Products. After SCW treatments, the oil and gas products under optimal conditions were collected and analyzed by GC-MC. Figure 4 shows the spectra for oil and gas products after SCWD and SCWO treatments. As shown in Figure 4a, the spectrum of the liquid product from samples treated by SCWD and SCWO were obviously different. The oil after SCWD treatment was mainly phenol and phenol homologues. In comparison, the liquid products after SCWO treatment mainly contained long-chain hydrocarbons. The results indicated that the addition of H2O2 could open the ring of phenols by oxidative reaction, resulting in the formation of hydrocarbons.28 Actually, the color of the liquid product after SCWO treatment became lighter than that of the sample treated by SCWD (seeing the inset image from Figure 4a). For the gas products, as shown in Figure 4b, similar compositions were detected in the products after SCWD and SCWO treatments. The products mainly consisted of ethylene, 1-butene, 2-butene, methyl alcohol, and so on. The gas and oil products were collected, and thus the environmental pollution

Figure 7. XRD pattern for the separated Ta electrode.

could be avoided. The utilization of these products will be investigated in a future study. Mold Resin Decomposition Mechanism. The organics in the mold resin could be effectively decomposed by SCW treatments. On the basis of the identified products, the probable decomposition pathways of organics in mold resin were investigated, as presented in Figure 5. Generally, the decomposition of polymer in SCW belongs to a radical mechanism.22,23 According to the bond energy theory, when the reaction system provides energy greater than the bond dissociation energy of the resins, the bonds are broken and then the free radicals are formed. These free radicals could generate new compounds by conjugation and catalytic reactions. Consequently, the polymers are decomposed into small molecules.17,22 Table S1 (SI) presents the bond length and bond energy of some common covalent bonds in polymers. Though the bond energy is also related to the adjacent structures and substituents, Table S1 has great 4425


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Figure 8. Images of (a) Ni−Fe terminals, (b) nonmetal particles, (c) Ta-rich particles after mechanical−physical separation, and (d) the XRD pattern for the Ta-rich particles (Ta, JCPDF# 25-1280; Ta2O5, JCPDF# 25-0922; Ag, JCPDF# 04-0783).

Figure 9. Proposed SCW technology for recycling WTCs.

of the benzene oxygen free radical. This reaction involved the rupture of the O−O bond, removal of the HO2• radical, migration of free radicals, and opening of the benzene ring.28 Finally, the generating acyl-group free radicals undergo further fragmentation reactions, generating some hydrocarbon. In addition, the paths of benzene ring opening by HO• could referred to in the study of Liu et al.28 Effect of SCWD and SCWO on Metal Recovery. After SCW treatments, the organics in mold resin were removed, and the mold resin lost its original tight and solid property. Therefore, the emissions of hazardous organics were effectively eliminated, and the metals in WTCs could be recovered in the subsequent processing. Figure 6 shows the images of residues

significance in reference to their relative strength comparison. The possible bond breaking positions were marked. Then, the break of weak bonds produced free radicals which could induce further free radical reactions. As a result, phenol, phenol homologues, and some hydrocarbon were the main components of products from resins by SCWD treatment. For the sample treated by SCWO, HO2• was provided by H2O2 (H2O2 → H2O + O2, H• + O2 → HO2•). The highly oxidative free radical HO2• could further break the benzene rings.28 The paths of the benzene ring opening could be explained as follows. First, phenol may form a benzene oxygen free radical under SCWO conditions.31 Then, HO2• reacted with the cyclohexadienone free radical, which is a resonance structure 4426



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and obtained materials via SCWD and SCWO treatments. As shown in Figure 6a, some cracks appeared in the mold resin, and the color of the resin changed to black after SCWD treatment, indicating that the mold resin was damaged and a considerable part of the organics were carbonated in the SCWD process. Although the organics in the mold resin were decomposed by SCWD, the carbon residue and SiO2 package covered the other materials. Thus, the WTCs treated by SCWD still required further treatment. Compared to the samples treated by SCWD, the mold resin after SCWO treatment was completely destroyed, as shown in Figure 6b. More interestingly, the SiO2 package became powders. The Ta electrode and Ni−Fe terminals were separated. Such results were expected given the superior organic decomposition efficiency in the SCWO system (Figure 3). Moreover, the separated Ta electrode, Ni−Fe terminals, and SiO2 powder could be easily recovered. The separated materials were washed with water, and then the obtained Ta and Ni−Fe terminals were shown in Figure 6c and d. In addition, the recovered Ta electrode was ground into powder and further examined by XRD and ICP-AES. The XRD analytical result, as shown in Figure 7, showed that only the Ta phase was observed. The ICP-AES result suggested that the purity of Ta could reach 93.18%. Furthermore, it was reported that some metals may be converted from insoluble to soluble species during the SCWO process due to the oxidizing atmosphere.19,27 For investigating the metal migration characteristics in solid-aqueous phase products during the SCWO process, the metal content in the liquid product was also analyzed by ICP-AES. The content of Ta, Ag, Ni, and Fe in the liquid (about 400 mL) was 0.0055, 0.0087, 4.641, and 0.6927 ppm, respectively. It suggested that Ta and Ag were barely dissolved into the liquid during the SCWO process. For the samples treated by SCWD, since the SiO2 package and residual carbon still covered the other materials, the mechanical separation methods (crush, screen, magnetic separation, and corona electrostatic separation) were applied in the resource recovery process.14 Detailed experiments are presented in the Supporting Information (SI). The images of the recovered materials are shown in Figure 8. Therefore, Ta and Ni−Fe terminals were also recovered. In addition, the recovery rate and purity of Ta powder could reach 96.26 and 72.37%, respectively.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00496. The bond length and bond energy of some common covalent bonds in polymers, schematic diagram of the semibath SCW reactor, mechanical-physical separation for WTCs after SCWD treatment, and schematic representation of a roll-type corona electrostatic separator (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 5474495. Fax: +86 21 5474495. E-mail: [email protected]. 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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CONCLUSION This study proposed efficient and environmentally friendly processes using SCWD and SCWO technologies to decompose mold resin and recover Ta from WTCs, as presented in Figure 9. The results suggested that SCWD and SCWO could effectively decompose the mold resin. The organic decomposition efficiency in SCWD was lower than that in SCWO. The optimal parameters for SCWD and SCWO were, respectively, considered to be 425 °C, 25 MPa, and 120 min and 400 °C, 25 MPa, and 90 min, adding a 10% volume ratio of H2O2. After SCWO treatment, the Ta electrode could be directly recovered, and the purity of Ta could reach 93.18%. For the sample treated by SCWD, Ta powder could be recovered by mechanical separation methods. Therefore, this study presents efficient and promising technologies for the cyclic regeneration of Ta from e-waste. 4427


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Recovery of Tantalum from Waste Tantalum Capacitors by

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