Research Article pubs.acs.org/journal/ascecg
Recovery of Valuable Materials from Waste Tantalum Capacitors by Vacuum Pyrolysis Combined with Mechanical−Physical Separation 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
ACS Sustainable Chem. Eng. 2017.5:2639-2647. Downloaded from pubs.acs.org by FORDHAM UNIV on 06/29/18. For personal use only.
S Supporting Information *
ABSTRACT: Waste tantalum capacitors (WTCs), dismantled from waste printed circuit boards, are considered a high quality tantalum (Ta) resource. Ta is a rare and strategic material. Nearly half of the Ta consumption is used for manufacturing tantalum capacitors. Furthermore, large amounts of energy and chemicals are consumed during the Ta purification process. Therefore, recovering Ta from WTCs can sustainably utilize the Ta resource and reduce pollution. However, the recycling technology has been poorly developed. This study proposed vacuum pyrolysis (VP) and mechanical−physical separation (MPS) to recover Ta and other materials from WTCs. First, the WTCs were treated by VP to decompose the organics. The suitable VP parameters were considered as 400 °C, 50 Pa, and 60 min. Then, the residues were performed by MPS. Consequently, nickel−iron terminals were recovered by magnetic separation. Ta and silica were separated by corona electrostatic separation. The recovery rate and purity of Ta could reach 97.02 ± 0.85% and 71.35 ± 0.63%, respectively. The optimal parameters were determined as voltage of 28 kV, roll speed of 50 rpm, and particle size of 0.125−0.3 mm, based on the response surface methodology (RSM). This study contributes to the efficient recycling of valuable resources from WTCs. KEYWORDS: Waste tantalum capacitors, Tantalum recovery, Vacuum pyrolysis, Mechanical−physical separation
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INTRODUCTION Waste printed circuit boards (WPCBs), rich in copper, aluminum, gold, silver, and other rare metals, are attractive
Over the past decades, environmently friendly technologies such as mechanical−physical separation,5,6 vacuum metallurgy,7 and supercritical water8−10 have been applied in recycling WPWBs. However, there is little information about the recovery technology of ECs. Generally, most ECs after dismantling were collected and recycled mainly aiming at ECs reuse or precious metal recovery by improper recycling processes such as open acid washing and open incineration in China.11 Consequently, abundant organic pollutants were generated in these crude processes, which caused serious harm to the environment and human health. Also, reuse of ECs was also deprecated due to the consideration of stability of these refurbished products.11 Tantalum capacitors (TCs), with the unique properties of high capacitance to volume ratio and thermal stability, are widely used in the electronic industry.12 Statistics show that the PCB of a mobile phone, the motherboard of a notebook, and a digital camcorder, respectively, contain about 36, 22, and 12 of these special capacitors.13 Undoubtedly, a great many TCs were continuously discarded within the e-waste stream. WTCs contain about 40−50 wt % of tantalum (Ta), 10 wt % of
Figure 1. (a) Schematic illustration of TC. (b) Structure of sintered Ta electrode.
secondary resources for recycling.1,2 During the WPCBs recycling process, the electronic components (ECs), mounted on the PCBs, are usually first dismantled by heat or selective sealing-off.3,4 Then, the waste printed wiring boards (WPWBs, WPCBs without ECs) went through metal recovery procedures. © 2017 American Chemical Society
Received: December 8, 2016 Revised: February 5, 2017 Published: February 9, 2017 2639
DOI: 10.1021/acssuschemeng.6b02988 ACS Sustainable Chem. Eng. 2017, 5, 2639−2647
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Figure 2. Flowchart showing the obtained WTCs from waste printed circuit boards.
Table 1. Main Composition of 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. Diagram of corona electrostatic separator and force analysis of particles on the rotating roll (red: metal; blue: nonmetal).
nickel−iron (Ni−Fe), and a certain amount of silver (Ag).14 Particularly, Ta is a rare and strategic metal. The world annual production of Ta is only about 2000 tons, and 42% of Ta consumption is used for TCs.13 Moreover, Ta and niobium are usually paired together in nature. These metals are difficult to separate due to their similar physical and chemical properties.13 Consequently, vast amounts of energy and chemicals will be consumed during the Ta purification process. Therefore, recycling WTCs is not only for resource sustainable utilization but also for environmental protection. However, recycling Ta and other materials from WTCs is difficult, due to the tightly covered mold resin, as shown in Figure 1. The mold resin is composed of silica, o-cresol novolac-type epoxy resin, phenolic novolac resin, and flame retardants.15,16 Serious environmental pollution will be caused if these organics were improperly disposed. Thus far, several researches, such as pyrometallurgy,17,18 chemical treatment,19 and steam gasification,16 have been done to remove the mold resin and recycle Ta from WTCs. Although these technologies were efficient to recover Ta from WTCs, gas pollutants or high volumes of liquid wastes will be generated in these recycling processes. Pyrolysis is a promising thermal technique that can be used to convert organics into energy or chemical feedstock.20,21 Generally, pyrolysis can be performed under inert and vacuum atmosphere. Vacuum pyrolysis (VP) has many advantages over other types of pyrolysis methods due to low decomposition
temperature and short residence time of the organic vapor in the reactor, which could reduce the occurrence and intensity of secondary reactions.22 Therefore, VP technology could be chosen as a pretreatment for recovering organics from WTCs. In addition, material separation is also a critical process for resource recovery. During the separation and recovery of metals, chemical treatments (including a series of acid leach and solvent extraction) were usually used. Compared with chemical treatment, mechanical−physical separation (MPS), with the obvious superiority of being environmentally friendly and having a high disposal rate and easy operation, was widely applied in the e-waste recycling process.23,24 Generally, MPS is based on the differences in material physical properties such as density, shape, magnetism, conductivity, and so on.25 In fact, WTCs contains magnetic material (Ni−Fe terminal), metals (Ta and Ag), and nonmetals (SiO2 and graphite). Therefore, MPS technology can be a good choice to separate valuable materials from WTCs after VP treatment. However, to our knowledge, there are no papers on VP and MPS for recycling Ta and other materials from WTCs. Accordingly, this study developed an integrated process of VP and MPS to recycle the organics and recover valuable resources from WTCs. The effects of VP parameters on organic decomposition were investigated. The VP products (gas and oil) were analyzed. After VP treatment, the valuable materials such as Ta, Ni−Fe, and SiO2 were recovered by MPS. In summary, the aim of this work is to develop a recycling process 2640
DOI: 10.1021/acssuschemeng.6b02988 ACS Sustainable Chem. Eng. 2017, 5, 2639−2647
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of time. Oil products were gathered in the condensing zones, and the gases were collected in an air bag. Then, the reactor was cooled to room temperature naturally, and the solid residue after VP was weighed. The yield of solid residue was calculated by eq 1.
without a negative impact on the environment for recycling resources from WTCs.
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EXPERIMENTAL SECTION
Materials. The WTCs used in this study were obtained by the automatic disassembly system26 shown in Figure 2. The major compositions of the WTCs are listed in Table 1.
R = MR /MO × 100%
where R is yield of solid residue, MR and MO are the weight of WTCs and residue after pyrolysis in WTCs, respectively. Mechanical−Physical Separation (MPS). After VP treatment for WTCs, the organics in the mold resin were removed. However, SiO2 and small amounts of carbon residue still covered the other parts. For further separating the valuable materials, VP residues were first crudely crushed, and the dissociated Ni−Fe terminals were collected by magnetic separation (MS). Then, the residues were further crushed using a mill crusher. The obtain powder was divided into different levels: 1# (0.0385−0.091), 2# (0.091−0.125), 3# (0.125−0.3), 4# (0.3−0.45), and 5# (0.45−0.6) mm. Finally, the different size samples were subjected to corona electrostatic separation (CES) three times. The diagram of a corona electrostatic separator and the movement behavior of the particles are shown in Figure 3. As a result, Ta, SiO2, and carbon residues were separated. The Ta recovery rate was calculated by eq 2.
Table 2. Experimental Ranges and Levels of Independent Variables levels independent variables
−1
0
1
size grade of sample (Xs) voltage, kV (Xv) roll speed, rpm (Xr)
1# 25 35
3# 30 50
5# 35 65
(1)
Methods. Vacuum Pyrolysis (VP). The schematic diagram of the experiment equipment is presented in Figure S1 of the Supporting Information (SI). The main body consists of the body of a furnace (chamber dimension is Φ 40 mm × 600 mm), a quartz tube reactor (Φ 35 mm × 800 mm), a vacuum pump, a temperature controller, and product collectors. About 5 g of WTCs was taken into a quartz boat and placed in the middle of the quartz tube. After the quartz tube was sealed, the vacuum pump could take the vacuum degree of the reactor to 50 Pa. The vacuum degree could be adjusted to different vacuum degrees through regulating the vacuum fine-tuning valve. When the vacuum degree reached a set value, the samples were heated from room temperature to the preset value and then were held for a period
RTa = M Ta /MCES × 100%
(2)
where RTa is the Ta recovery rate, MTa and MCES are the weight of recovered Ta and Ta in the samples conducted by CES, respectively. Chemical Analysis. The pyrolysis products were qualitatively and quantitatively analyzed by gas chromatography−mass spectroscopy (GC-MS, TurboMass, PerkinElmer Corporation, U.S.A.). The WTCs and solid products were analyzed by inductively coupled plasma−mass spectrometry (ICP-MS, Agilent 7500a, Agilent Corporation, U.S.A.)
Figure 4. Effect of (a) pyrolysis temperature, (b) pressure, and (c) holding time on the solid residue. 2641
DOI: 10.1021/acssuschemeng.6b02988 ACS Sustainable Chem. Eng. 2017, 5, 2639−2647
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Figure 5. GC-MC spectra of (a) oil and (b) gas products from WTCs under different VP conditions.
Table 3. Main Compositions of Oil Products from WTCs Treated at 400 °C, 50 Pa, and 30 min ret. time (min)
components
area (%)
27.178 29.152 29.641 30.242 30.717 31.017 31.429 31.747 31.875 32.296 32.747 35.34 38.632 41.594 42.726
1-methyl-2-(4-methylphenoxy)-benzene 2-[(4-hydroxyphenyl)methyl]-phenol 2-[(4-hydroxyphenyl)methyl]-phenol 4,4′-methylenebis[2-methyl]-benzenamine 4,4′-methylenebis-phenol 2-bromomethylbenzoic acid, N-methyl-N′-phenyl-hydrazide 2,2′-methylenebis[4-methyl-phenol 4-[(4-amino-3-methylphenyl)amino]-phenol 2,2′-methylenebis[4-methyl-phenol 4,4′-(1-methylethylidene)bis-phenol 2,2′-methylenebis[4-methyl-phenol 2,3,4,5-tetramethyl-1-(2,3,4,5-tetramethylbenzyl)-benzene heneicosane hexatriacontane tetratriacontane
1.7 1.21 7.73 1.63 4.43 2.46 8.49 1.72 1.15 4.5 5.85 6.27 4.21 3.39 4.02
Table 4. Main Compositions of Oil Products (retention time < 27.178) from WTCs Treated at 400 °C, 2000 Pa, and 30 mina ret. time (min)
components
area (%)
12.259 14.162 14.43 17.861 20.846 22.19 22.886 23.189 23.513 24.229 25.046 25.685 26.998
2,5-dimethyl-phenol 3-(1-methylethyl)-phenol 1-ethyl-4-methoxy-benzene biphenyl dibenzofuran fluorene 4-methyl-dibenzofuran 4-methyl-dibenzofuran 9H-xanthene heptadecane 1,2-dimethyl-naphtho[2,1-b]furan 9,9-dimethyl-xanthene 1,8,10-trimethylpyrazino[1,2-a]indole hydrochloride
5.25 1.31 0.51 0.3 2.43 1.05 1.07 1.04 2.6 2 2.75 2.99 3.27
a
Main components of oil (after the retention time of 27.178 min) from WTCs treated at 2000 Pa were similar to those of the sample treated at 50 Pa.
and X-ray diffraction (XRD, D8 ADVANCE, BRUKER, Germany) with Cu Kα radiation. The contents of SiO2 and organics in WTCs were examined by an X-ray fluorescence spectrometer (XRF-1800, Shimadzu, Japan) and a combustion method.27 All the experiments were repeated three times, and only the mean values were reported. Statistical Analysis. The response surface methodology (RSM) was applied to 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 the Box-Behnken Design (BBD) for the optimization of the Ta
recovery rate and purity of Ta. The size grade of the sample, CES voltage, and roll speed were selected as factors for the response of the recovery rate and purity of Ta with the coded values at 3 levels (−1, 0, and +1). The ranges and levels of variables are given in Table 2. Actually, the preliminary experiment had been done to determine the experimental scope. It was found that when the voltage was higher than 35 kV, the spark discharge occurred, which is adverse to metal separation. In addition, when the roll speed was above 65 rpm, the SiO2 particles fall into the metal collecting box. So, 35 kV and 65 rpm 2642
DOI: 10.1021/acssuschemeng.6b02988 ACS Sustainable Chem. Eng. 2017, 5, 2639−2647
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%), heneicosane (4.21 area%), and so on. When the pressure reached 2000 Pa, some phenol homologues, dibenzofuran homologues, and xanthene were detected. For pyrolysis gas products from WTCs, as shown in Figure 5b, the components of the gas products had an obvious change at 300 and 400 °C. Then, the components were similar to increasing temperature and pressure. The main compositions of the gas products from WTCs (treated at 400 °C, 50 Pa, 30 min) are listed in Table 5, which demonstrated that most gaseous products were hydrocarbon, consisting of carbon monoxide, ethylene, ethane, ethanol, and so on. The pyrolysis oils can be recycled as chemical feedstock, and the pyrolysis gases can be also used as gaseous fuel. On the basis of the above study of VP parameters and analysis of products, we can draw the conclusion that temperature and pressure had a great influence on the pyrolysis process of organics in WTCs. Higher temperature and pressure could facilitate the pyrolysis of organics. However, more energy is consumed by increasing temperature, and some toxic gas is released when increasing pressure near to normal atmosphere.29 It could be seen from the GC-MC results that the components of the pyrolysis oil from WTCs treated at 2000 Pa became more complex, which is not beneficial to the recycling of oil. In addition, the pyrolysis of organics was not completed at 500 Pa, 400 °C (Figure 4b). Therefore, the optimal VP parameters are suggested to be 400 °C, 50 Pa, and 60 min in this study. MPS Process. After VP treatment for WTCs, the organics in mold resin were decomposed, and the emission of hazardous organics was eliminated. Then, the residues obtained by VP were crushed, and Ni−Fe terminals were collected by magnetic separation. Finally, the residues were classified into different size fractions for corona−electrostatic separation (CES). Consequently, Ta and SiO2 could also be separated. The CES separation principle is based on the different movement behavior of nonconductive and conductive particles because of their extreme difference in electrical conductivity.25 The force analysis of particles on the rotating roll and the movement behavior of the samples are shown in Figure 3. The mixture particles are fed onto the surface of the roll electrode and pass through the electric field. Nonconductive particles (mainly SiO2) and conductive particles (mainly Ta) are charged by ion bombardment with the same polarity as the corona electrode. Since nonconductive particles are not affected by the electrostatic induction, they are pinned to the surface of the roll electrode as a result of the electric image force (Fi).25 When the gravity force (Fg), air drag force (Fr), centrifugal force (Fc), and Fi achieve equilibrium, the nonconductive particles are detached from the rotating roll electrode (or removed by a brush). For conductive particles, they are charged by electrostatic induction and attracted to the electrostatic electrode. Under the function of the field force (Fe), Fr, and Fg, the particles are detached from the roll electrode at a certain angle and then fall into the metal collector.30 Generally, the Fe is proportional to the electric field strength. The Fc is proportional to the roll speed. The Fr is related to the particle size of sample. Therefore, the separation process could be controlled by regulating the input parameters including voltage, roll speed, and particle size of the sample.31 To obtain the maximum recovery rate and purity of Ta, the CES experiments were conducted in a standard response surface methodology (RSM) design called the Box-Behnken Design (BBD). The CES voltage, roll speed, and size grade of
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RESULTS AND DISCUSSION Factors on VP Process. Since the organics are decomposed to form gas, oil, and char during the VP process, the solid Table 5. Main Compositions of Gas Products from WTCs Treated at 400 °C, 50 Pa, and 30 min ret. time (min)
components
area (%)
2.258 4.377 5.25 15.576 17.969 18.078 18.336 18.754 19.032 20.566 22.172
carbon monoxide ethylene ethane acetaldehyde 1-propene 1-butene 1,3-butadiene butane 2-butene ethanol propanal
11.51 27.14 9.6 5.18 5.32 5.02 0.47 1.65 0.44 20.65 0.54
residue could represent the organics decomposition efficiency. Figure 4a shows the solid residue under different temperatures (pressure: 50 Pa; holding time: 30 min). The solid residue decreased from 95.61 ± 0.11% at 300 °C to 94.02 ± 0.12% at 400 °C. However, when further increasing the temperature, the solid residue began to slowly increase, which could be attributed to the carbonization of organics at higher temperature.28 The effect of pressure on the solid residue was investigated at 400 °C and 30 min. The results are presented in Figure 4b, showing that the solid residue was rapidly decreased when the pressure is higher than 2000 Pa, which could be due to the reaction between oxygen and residue. As is well known, some toxic gases will be released during the incineration of polymers. In order to control the emission of gas pollutants, the VP pressure was not recommended too high. The pyrolysis products under different pressure is discussed in the following section. Figure 4c shows the effect of holding time on the solid residue. These experiments were conducted at 400 °C and 50 Pa. It showed that the solid residue decreased with increasing the holding time from 10 to 60 min. Subsequently, the solid residue remained about the same. The solid residue could reach 93.56 ± 0.11% at 60 min. The weight loss of the sample and the organics content in WTCs were, respectively, measured at 6.44 and 14.32%, meaning that a given mass of organics was transformed into carbon residues during the VP process. Pyrolysis Product Analysis. After the VP process, pyrolysis oil and gas were collected and then analyzed by GC-MC. The spectra of oil products under different VP conditions are shown in Figure 5a. Within the temperatures (300−500 °C) and pressures (50−500 Pa), the components of the oil products were similar. However, when the pressure increased to 2000 Pa, the intensity of the peaks (15−25 min) increased, and some new characteristic peaks (10−15 min) appeared. The main compositions of the oil from WTCs treated at 50 and 2000 Pa are, respectively, listed in Tables 3 and 4. The oil from WTCs treated at 50 Pa was mainly composed of phenol homologues (35.08 area %), 2,3,4,5tetramethyl-1-(2,3,4,5-tetramethylbenzyl)-benzene (6.27 area 2643
DOI: 10.1021/acssuschemeng.6b02988 ACS Sustainable Chem. Eng. 2017, 5, 2639−2647
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Figure 6. Response surface 3D plots for the effect of (a) size grade and voltage, (b) size grade and roll speed, (c) voltage and roll speed on Ta recovery rate and (d) size grade and voltage, (e) size grade and roll speed, and (f) voltage and roll speed on the purity of Ta.
the sample were selected as factors of the responses, as shown in Table 2. The response results are presented in Table S1 of the SI, and the analytical results were outputted by DesignExpert software. The analysis of variance (ANOVA) is shown in Tables S2 and S3 of the SI. It demonstrated that both response surface quadratic models were significant at an F value of 5985.06 and p-value of
