From waste metallized film capacitors to valuable materials

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From waste metallized film capacitors to valuable materials: hexagonal flake-like micron zinc powder, copper-iron electrodes and energy resource Bo Niu, and Zhenming Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02702 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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ACS Sustainable Chemistry & Engineering

From waste metallized film capacitors to valuable materials: hexagonal flake-like micron zinc powder, copper-iron electrodes and energy resource Bo Niu1 and Zhenming Xu1, 2* 1

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China

2

Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, People’s Republic of China

Corresponding author: Zhenming Xu E-mail: [email protected] Tel: +86 21 5474495 Fax: +86 21 5474495 School of Environmental Science and Engineering Shanghai Jiao Tong University 800 Dongchuan Road, Shanghai 200240, People’s Republic of China

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Metallized film capacitors (MFCs) are widely applied in electronic appliances. The rapid replacement of electronic products leads to producing a great many waste MFCs. Waste MFCs, containing organics (plastic dielectric and brominated epoxy resin) and metals (zinc, copper, iron, etc.), are considered as hazardous waste but also a valuable resource for recycling. However, how to recycle waste MFCs effectively is seldom concerned. This work provided an integrated technology for recovering waste MFCs. Firstly, waste MFCs were treated by pyrolysis to recycle the organics. The decomposition characteristic, product and mechanism of the organics were studied. The pyrolysis temperature of 500 oC and holding time for 30 min were determined as the optimal parameters. Then, the residues were conducted by grinding and screening to recover copper-iron electrodes. Finally, hexagonal flake-like zinc powder with particle size of 15 µm was recovered by vacuum metallurgy separation (VMS). The VMS principle for recovering zinc and the growth process of hexagonal flake-shape zinc powder were analyzed. The recovery rate and purity of zinc could reach 95.66 and 99.87%, respectively, under 650 oC, 100 Pa and 2 h. In short, our study contributes to the efficient and maximum recycling waste MFCs. Key words: waste metallized film capacitors; pyrolysis; vacuum metallurgy separation; resource recovery; e-waste

2


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Introduction Metallized film capacitors (MFCs), with high specific energy and reliability characteristics (self-healing), are widely used in electronic appliances.1 As the number of electronic products is rapidly growing the global MFCs consumption has reached 62 billion pieces in 2008, and the number is growing with 15-20% each year.2 In the meantime, technological innovation and intense marketing reduce the life cycle of electronic products, which results in generating large amounts of e-waste.3, 4 It was reported

that about 40-50 million tons of e-waste are abandoned around the world

every year.5 Undoubtedly, a large number of waste MFCs were continuously discarded within the e-waste stream. The MFCs uses a plastic film as dielectric and nanometer thick metal layers (zinc, aluminum or their combination) evaporated onto the polymer film are acted as the electrodes, as shown in Fig. 1.6 The most common plastics used as dielectric are polypropylene (PP), polystyrene (PS) and polyethylene terephthalate (PET). The end electrode consists of copper and iron. The spray layers are usually silver or gold.6 Considering the metals rich in MFCs, waste MFCs are valuable resources for recycling. To

achieve

the

resource

recovery

from

waste

MFCs,

firstly,

environmental-friendly disposing the organics (plastic dielectric and brominated epoxy resin package) is significant since the organics are regarded as hazardous materials.7 On one hand, improperly dealing with the organics will cause environment pollution, such as the

toxic gases emission by open burning.7 On the other hand,

efficiently removing the organics is a precondition for recycling the valuable 3



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materials from waste MFCs.5 However, thus far, few researches have been done to study the efficient recycling of waste MFCs. Pyrolysis is proved as one of the

most

promising technologies for disposing organics, because the organics can be decomposed to low molecular weight liquid and gaseous products during the heating process without oxygen.8, 9 Furthermore, the pyrolysis product could be potentially reused as chemical feedstock or gas fuel.9, 10 The pyrolysis treatments for e-waste has been studied in waste liquid crystal displays,10, 11 waste printed circuit boards,12, 13 waste tantalum capacitor,14 and so on. Thereby, pyrolysis can be applied as a pretreatment to dispose the organics of waste MFCs.

Fig. 1. Schematic illustration of a MFC. In addition, the separation and recycling of valuable metals from waste MFCs is a critical process. Generally, hydrometallurgy and pyrometallurgy are applied to recovering valuable metals from e-waste.15, 16 Hydrometallurgy has the advantages of easy and flexible operability, but this method is not encouraged because it will need large amount of chemicals and long time. Additionally,

the waste acids discharge

and sludge produced during the process could cause potential environmental risks.17 Vacuum metallurgy separation (VMS), as a pyrometallurgy, has been widely used in the separation of metals or metal oxides.18,

19

It has the superiorities

4


of high

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efficiency, less energy consumption and environmental friendliness.19,

20

Most

importantly, this technology can prepare fine particles with the free surface and controlled chemical purity.21,

22

Zhan et al. successfully prepared spherical lead

nanoparticles from solders of waste printed circuit boards (WPCBs) and synthesize fine PbS powders from the scrap lead-acid battery through VMS.22, 23 In our previous studies, this technology has been successfully applied to separating gallium from waste solar panel,24 and recycled copper, zinc and lead from WPCBs.25, 26 Hence, VMS can be expected to effectively separate and produce ultrafine metal powders from waste MFCs.

Fig. 2. Flow sheet of the recycling process for waste MFCs. Accordingly, this work proposed an integrated process for recycling waste MFCs, including pyrolysis, grinding and screening, and vacuum metallurgy separation (VMS), as presented in Fig. 2. The decomposition characteristic, product and mechanism of the organics were studied. The principle of VMS for recovering zinc and the growth process of hexagonal flake-shape micron zinc powder were analyzed. The pyrolysis and VMS parameters were optimized. In addition, the copper-iron (Cu-Fe) electrodes were also separated by grinding and screening. Our study aims to develop an efficient and integrated process for the maximum recycling of waste MFCs. 5



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Experimental Materials The waste MFCs (model: MKS-metallized polystyrene capacitors) used in this work were obtained by Yangzhou Ningda Noble Metal Co., Ltd. (China) and Shanghai Xin Jinqiao Environmental Protection Co., Ltd. (China). The materials obtained by manual opening of a waste capacitor are shown in Fig. 3. The mass of each material was weighed and the results demonstrated that the capacitor contained 53.98 wt.% resin package, 35.3 wt.% metalized polymer and 10.72 wt.% electrode pins (Cu-Fe alloy). For further determined the composition, the metal and the organic contents in a waste capacitor were examined by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500a, Agilent Corporation, US) and combustion method, respectively. The results are listed in Table 1.Argon (Ar, 99.99%) was used as the shielding gas. Table 1.The main composition of the waste MFCs used in this study Composition

organics

Zn

Cu

Fe

Al

Ag

Content

61.02 ±

27.60±

8.07 ±

2.62 ±

0.68±

0.01±

(wt. %)

1.05

0.54

0.13

0.04

0.02

0.001

6


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Fig. 3 The comprising materials dismantled from a waste capacitor . Apparatus The pyrolysis and VMS experiments were performed by a quartz tube furnace, as depicted in Fig. 4. The main body composed of a furnace body (chamber dimension is Φ 40 mm × 600 mm), a quartz tube reactor (Φ 35 mm × 800 mm), an Ar gas supply system, a temperature controller, a vacuum pump and product collectors. The middle part of the quartz tube is the heating zone (200 mm) and the right end of the tube far away from the heating zone was the condensation zone. For VMS experiments, the vacuum pump instead of the gas supply system was connected with the furnace.

7



Fig. 4. Schematic illustration of the quartz tube furnace Experimental procedures 60 of waste MFCs (about 10 g, without any treatment) were taken into a quartz boat (25 mm × 15 mm× 150 mm), and placed in the heating zone (middle of the quartz tube). The pyrolysis product collector was connected to the quartz tube. Ar gas with a flow rate of 100 ml/min passed through the reactor. The pyrolysis was heated from room temperature to the setting values (350 - 550 oC) with a heating rate of 20 o

C/min. The pyrolysis process was hold for 30 min. During the pyrolysis process, the

organics decomposed into oil, gas and char. The pyrolysis oil was collected in the condensing device, and the pyrolysis gas was gathered

by a commercial gas bag, as

shown in Fig. 4. After the pyrolysis treatment, the samples were removed from the furnace when the temperature dropped to room temperature. The decomposition efficiency of organics in WMFC was determined as eq.1. R ⁄ 100%

(1)

Where, WO is the organic mass of raw material; W and WR are the mass of raw 8


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sample and residue after pyrolysis, respectively. After pyrolysis, the resin package was destroyed, and the pyrolysis residue was easily ground. The pyrolysis residue (about 4.87 g) was ground by a mortar and all the Cu-Fe electrodes were collected by screening using a sieve (pore size: 0.6 mm). Subsequently, the residues were performed by VMS. In a typical experiment, about 5 g of sample was put into a quartz boat, and then placed in the heating zone (the middle of quartz tube). The vacuum pump was opened to maintain the vacuum atmosphere in the furnace body, and then sample was heated from room temperature to certain temperature (500 - 700 oC) with a heating rate of 20 oC/min under vacuum (100 - 10000 Pa) for 1 - 4 h. During the VMS process, metal Zn will be evaporated into gas phase, and then mainly condensed and collected in one side of the quartz tube (the orientation of vacuum pumping). Since the quartz tube was long enough to capture the Zn vapor, Zn condensing product was collected by scraping.. After VMS process, the residues were taken out to calculate the recovery rate. The Zn recovery rate was calculated by the following eq.

R ⁄ 100%

(2)

Where, M0 and M are the initial and residual amount of Zn, respectively. Chemical Analysis The metal content in waste MFCs and product were examined by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7500a, Agilent Corporation, US). The samples were subjected to complete digestion with laboratory reagent grade nitric acid and perchloric acid with a volume ratio of 4:1, followed by dilution using 1% 9



aqua regia and ICP-MS analysis. The content organics in waste MFCs were examined by combustion method.27 Thermal gravimetric (TG) and differential scanning calorimetry (DSC) were conducted by a simultaneous TG-DSC instrument (Mettler Toledo, Shimadzu, Japan) under Argon. The pyrolysis oils and gases were analyzed by gas chromatography-mass spectroscopy (GC-MS, TurboMass, Perkin Elmer Corporation, US). The liquid and gaseous columns for GC-MC analysis used in this study were DB-5 (30 m×0.25 µm×0.25 mm) and HP-PLOTQ (30 m×0.32 mm×0.25 µm), respectively. The crystal structures of product were characterized by X-ray diffraction (XRD, D8 ADVANCE, BRUKER, Germany) with Cu Kα radiation. The morphology of product was analyzed by Field-emission scanning electron microscopy & energy dispersive spectrometer (SEM Sirion 200 & EDS INCA X-Act, FEI Company, America & Oxford Company, England). Results and discussion Pyrolysis TG-DTA analysis Before pyrolysis experiment, the brominated epoxy resin package and polystyrene dielectric were manual dismantled from MFCs and then analyzed by TG-DTA, and the results are shown in Fig. 5. As shown in Fig. 5a, resin package began to decompose at about 240 oC, and the maximum loss rate of 66.8 wt% occurred at 370.8 oC. Compared with resin package, as shown in Fig.5b, the polystyrene dielectric began to decompose at higher temperature of 350 oC, and the maximum weight loss attained to 62.7 wt% at 450.6 oC. Similar results were also 10


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observed in the pyrolysis of brominated epoxy resin and polystyrene, where the maximum loss rate occurred at 362 and 450 oC, respectively.28, 29

(a)

(b)

Fig. 5. TG-DTA curves of (a) epoxy resin package and (b) polystyrene dielectric of waste MFCs at a heating rate 20 oC/min. Influence factors on pyrolysis To obtain the optimal pyrolysis condition for organics, the effect of pyrolysis temperature and holding time on the organic decomposition efficiency were studied. 11



Fig.6a shows the relation between organic decomposition efficiency and temperature (holding time: 30 min). With increasing the temperature from 350 oC to 450 oC, the organic decomposition efficiency dramatically increased from 50.26% to 78.98%. When the temperature was above 500 oC, the organic decomposition efficiency remained a constant value. Actually, from the TG-DTA results, the maximum weight loss of resin package and polystyrene dielectric were obtained at 370.8 and 450.6 oC, respectively. Therefore, the pyrolysis temperature of 500 oC was sufficient to decompose the organics of waste MFCs. The relation between the organic decomposition efficiency and holding time is shown in Fig.6b (temperature: 500 oC). The organic decomposition efficiency increased with increasing the holding time from 10 to 30 min. After that, the organic decomposition efficiency remained almost the same (83.63%). The decomposition efficiency could not reach to 100%, which was attributed that some of the organics will be transformed into residual carbon under oxygen-free condition.30, 31 Therefore, the temperature of 500 oC and holding time for 30 min could be regarded as the optimal pyrolysis parameter for organics in this work. (b)

(a)

12


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Fig. 6. Effect of (a) pyrolysis temperature (holding time: 30 min) and (b) holding time (temperature: 500 oC) on the organic decomposition efficiency. Pyrolysis products analysis After the optimal conditions were confirmed, the pyrolysis oil, gas and residue were collected and analyzed. The pyrolysis residue was analyzed by ICP-AES, elemental analysis, and XRD. The ICP-AES and elemental analysis results suggested that the content of Zn, Al, Ag and C were about 69.26, 1.71, 0.02 and 24.49 wt.%, respectively. The other 4.52 wt.% may account for the oxygen, which could not be identified by ICP-AES and elemental analysis. The XRD result, as presented as in Fig. 8b, demonstrated that Zn, ZnO, Al and Al2O3 phases were observed. Since the pyrolysis were conducted under an inert atmosphere (Ar gas), some of organics would transform into char.8, 10 Besides, the formation of ZnO and Al2O3 may be attributed to the oxidization action by the pyrolysis vapors such as oxygen-containing substances. The pyrolysis oil and gas were analyzed by GC-MC and the results are listed in Table 2 and Table 3. As shown in Table 2, the oil was mainly composed of phenol (7.47 area %), phenol homologs (10.37 area %), bromophenol homologs (15.04 area %), biphenyl (13.77 area %), benzene homologs (19.6 area %) and so on. The generated phenol, phenol homologs, biphenyl and bromophenol homologs were mainly attributed to the decomposition products of brominated epoxy resin, while the benzene homologs (such as 1-methylethyl-benzene) mainly originated from polystyrene.28, 29 It was reported that the oils (phenol and benzene homologs) from the pyrolysis of WPCBs could be reused to synthesize resin.32, 33 Similarly, the pyrolysis 13



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oil in this study could also be potentially used as chemical feedstock. The separation and utilization of the pyrolysis oils are under study. As shown in Table 3, the gaseous products

were

most

some

hydrocarbon,

consisted

of

ethylene,

ethane,

2-methyl-1-propene, 2-butene 1,3-butadiene, and so on. The gaseous product can be directly incinerated or recycled as gas fuel. Table 2. Main composition of pyrolysis oil product from the waste MFCs ret. Time (min)

components

area%

9.127

(1-methylethyl)-benzene

11.36

9.439

Phenol

7.47

10.492

(2-methyl-2-propenyl)-benzene

8.24

11.367

2-methyl-phenol

4.22

11.766

Acetophenone

3.61

11.925

p-cresol

4.02

13.817

2,4-dimethyl-phenol

2.13

14.836

Naphthalene

1.98

16.147

1-Cyclohexene-1-carboxylic acid

1.08

17.268

4-bromophenol

6.58

18.658

2,6-dibromophenol

8.46

19.476

Biphenyl

13.77

20.059

4-methyl-cyclohexene

1.65

20.566

Diphenylmethane

1.71

26.172

Stilbene

2.08

Table 3. Main composition of pyrolysis gaseous product from the waste MFCs ret. Time (min)

components

area%

4.267

Ethylene

25.5

5.141

Ethane

24.22 14


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17.862

2-methyl-1-Propene

7.68

17.967

2-Butene

13.74

18.214

1,3-Butadiene

9.45

18.648

2-Butene, (Z)-

6.95

18.918

2-Butene, (Z)-

4.51

24.322

Isoprene

1

24.532

2-Pentene, (Z)-

1.1

In general, the fracture and combination of chemical bonds are the main chemical reactions of polymer during pyrolysis process.34,

35

Based on the bond

energy and the GC-MC results, the pyrolysis mechanism for organics of waste MFCs are shown in Fig. 7. Firstly, the chemical bonds with low bond energy of macromolecular monomers in organics can be broken under certain temperature. The bond length and bond energy of some covalent bond in polymer are shown in Table S1 of SI. The possible bond breaking positions are flagged as red dashed lines. Then, the macromolecular monomers are decomposed to small molecular monomers and various free radicals. The free radicals can produce new compounds through recombination/conjugation. As a result, the main pyrolysis products such as phenol, phenol homologs, bromophenol homologs, benzene homologs, ethylene and ethane are formed. Moreover, it should be noted that some metals or metal oxides may have a catalytic effect on the pyrolysis of organics.36-38 Shie et al. reported that the additives might increase the conversion of oil sludge by the following order Fe2O3 > AlCl3 > Fe2SO4·7H2O >Al2O3 > FeCl3 > Al > Fe > no additives.36 Li et al. demonstrated that metal oxide (CuO, Fe2O3 and ZnO) might increase the combustion rate and burnout of 15



the fixed carbon.37 Some other studies also suggested that the addition of metals or metal oxides could affect the composition of pyrolysis gas and oil.39-42 Generally, it was considered that the metals or metal oxides additives could give rise to catalytic cracking of the pyrolysis vapors, and some macromolecular monomer vapors could be cracked into monomeric compounds.40-42 In this study, some Zn and Al in waste MFCs were transformed to ZnO and Al2O3 during the pyrolysis process, indicating that the metals reacted with pyrolysis vapors. Consequently, Zn and Al as well as the formed ZnO and Al2O3 would further play a catalytic role on the pyrolysis of organics in waste MFCs.

16


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Fig. 7. Proposed mechanisms for pyrolysis of organics in waste MFCs. Vacuum metallurgy separation (VMS) Feasibility of VMS. After pyrolysis, the resin package was destroyed, and the pyrolysis residue was easily ground. The pyrolysis residue (about 4.87 g) was ground by a mortar and all the Cu-Fe electrodes were collected by screening using a sieve (pore size: 0.6 mm). Finally, VMS was used to separate Zn from the residues. The principle of separating metals by VMS is based on the different vapor pressures of various metals at the same 17



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temperature. The metal with high vapor pressure and low boiling point can be separated from the other metals by distillation or sublimation, and then be recovered through condensation.18, 19 The relation between vapor pressure and temperature can be calculated by the following Clausius-Clapeyron eq. 3,

+ + +

(3)

Where PM is the vapor pressure of pure metal; T is temperature; A, B, C, and D are constants determined for each metal element. The saturated vapor pressure calculations of the metals are presented in Table S2 of SI and the relation between the vapor pressure and temperature for metals is shown in Fig.8a. It is clear that the saturated vapor pressure of Zn is much higher compared with other composition. Therefore, Zn can be easily separated from the residues. In addition, the triple phase equilibrium graph of Zn is shown in Fig. 8b. The O is the triple point of Zn and the curve OB represents the evaporation curve. Under atmospheric pressure, the boiling point of Zn is about 900 oC (the D point in curve OB). When the pressure is 100 Pa, the boiling point of Zn will decrease to about 440 oC, which means that the low pressure can significantly lower the Zn evaporation temperature. However, considering the limitation of experimental condition and economic cost (when pressure is 10 Pa, boiling point of Zn is about 380 oC), we choose 100 Pa and 500 oC as the lowest pressure and temperature, respectively, in the following optimized experiments. In addition, during the pyrolysis process, some of Zn were oxidized to ZnO. As is known, the boiling point of ZnO is much higher than that of Zn. Thus, the 18


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formation of ZnO will not benefit for the Zn vaporization. However, the char generated during the pyrolysis of organics will act a reducing agent. It was reported that the carbon formed in the pyrolysis process of liquid crystals could in-situ reduce indium oxide.10 Besides, under vacuum, the carbon reduction reaction will more easily occur because vacuum could significantly decrease the Gibbs free energy values.10, 43, 44 To further investigate the carbon reduction reaction during the VMS, the possible reactions between ZnO and C, and the Gibbs energies (△G) were calculated by HSC 6.0 Software. Fig. 8c and d show the relation between △G and temperature for reduction of ZnO under different vacuum degree. As shown in Fig. 8c, the reduction reaction of ZnO generating Zn and CO could be occurred above 1250 K under atmospheric pressure. However, with decreasing the system pressure, the temperature of the reduction reaction decreased. When the system pressure reached 100 Pa, the reduction reaction could occur at 950 K. For the reduction reaction of ZnO generating Zn and CO2, as shown in Fig. 8d, the reduction reaction could occur at 1150 K. Therefore, the reduction reaction of ZnO generating Zn will occur above 950 K under 100 Pa, and the reaction generating CO seem more easily happen.

19



(a)

(b)

(c)

(d)

Fig. 8. (a) Relation between vapor pressure of metals and temperature; (b) The triple phase equilibrium graph of Zn; (c) and (d) The relation between △G and temperature for reduction of ZnO under different vacuum degree VMS factors on Zn recovery To obtain the optimal VMS conditions, the three influencing factors of temperature, vacuum pressure and holding time on the Zn recovery rate were studied. Fig. 9a shows the relation between Zn recovery rate and pressure (temperature: 700 o

C, holding time: 1 h). It shows that the effect of pressure on the recovery rate of Zn is

significant. The recovery rate of Zn dramatically increased from 50.15 to 92.28% with decreasing the system pressure from 10000 to 500 Pa, which indicates that high vacuum degree facilitates the recycling of Zn. A higher Zn recovery rate of 95.12 % could be obtained when the pressure decreased to 100 Pa. The effect of temperature 20


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on the Zn recovery rate is shown in Fig. 9b (pressure: 100 Pa, holding time: 1h). It shows that the Zn recovery rate sharply increased with increasing the temperature from 500 to 650 oC. The recovery rate could reach 94.28% when the temperature was 650 oC, and then its rise began to slow. In general, the metal evaporation rate (recovery rate) in vacuum can be described by the mean free path of gas molecules. The larger mean free path of gas molecules usually means the higher metal recovery rate.25 The relation between the mean free path of gas molecules and pressure as well as temperature can be obtained by Sutherland eq. 4,

λ 2.33 10" /$ "

(4)

where λ is the mean free path of gas molecules (cm); T, P and ξ are temperature (K), pressure (Pa) and atomic diameter of metal (cm), respectively. It is obvious from the eq. 4 that increasing the temperature and decreasing pressure are beneficial to the Zn recovery. However, as shown in Fig. 9b, when the pressure was 100 Pa, the further increase of temperature from 650 to 700 oC would not significantly improve the Zn recovery rate. In addition, during the pyrolysis process, some of Zn were oxidized to ZnO, which was not benefit for the Zn recovery. However, from the Gibbs energy (△G) results (Fig. 8c), ZnO can be reduced with the pyrolysis char to generate Zn above 950 K (677 oC) under 100 Pa. Therefore, the high Zn recovery rate obtained at 650-700 oC may be also related to the ZnO reduction reaction with pyrolysis char. Fig. 9c shows the effect of holding time on the Zn recovery rate (temperature 650 oC, pressure: 100 Pa), showing that the recovery rate 21



of Zn could reach 95.66% at 2 h and then the recovery rate almost unchanged with further increasing the holding time. According to the above results, 650 oC, 100 Pa, 2 h were chosen as the optimal VMS parameters. (a)

(b)

(c)

Fig. 9. Effects of (a) vacuum pressure, (b) temperature, and (c) holding time on the Zn recovery rate. Characterization of products The product under optimal parameters was analyzed. Fig. 10a shows the image of the condensed product, presenting light grey deposits on the inner wall of the quartz tube. The deposits were analyzed by XRD, as shown in Fig. 10b, suggested that the product was Zn. Besides, no other phases were observed, indicating that the recovered Zn had a high purity. The ICP-AES result suggested that the purity of Zn 22


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was over 99.87%. Moreover, the microstructure of the Zn product was observed by SEM. As shown in Fig. 10c and d, uniform Zn powder with micrometer level and hexagonal flake shape was observed. The high magnification SEM images (inset in Fig. 10d) reveal that the particle size was about 15 µm. (a)

(b)

(d)

(c)

(e)

Fig. 10. (a) Image of condensed product in one end of quartz tube and deposits scraped off from the quartz tube; (b) XRD patterns for pyrolysis residue and recovered Zn product after VMS (PDF cards: Zn 65-5973; ZnO 76-0704, 77-0191; Al 23



89-2837; Al2O3 86-1410, 70-3321); (c) SEM and (d) high magnification SEM of the deposits on the quartz tube; (e) schematic illustration of hexagonal flake-shape micron Zn particles growth process. Generally, two main stages account for the fine particles formation -nucleation and growth, as shown in Fig 10e. Firstly, Zn was evaporated into the gas phase under certain temperature and pressure. Then, the Zn vapor flows from high temperature zone to the relatively cool condensation zone, and Zn vapor becomes supersaturated there. When the temperature of these Zn atoms decreased to a certain value, nucleation will occur, and small Zn droplets emerged. The growth stage usually involves in agglomeration because of the random collision between these Zn droplets . Consequently, Zn with controllable morphology can be obtained by controlling the condition of the growth stage.21 To avoid agglomeration, these Zn droplets should be migrated to the cold region and solidified as quickly as possible. Therefore, large temperature gradient during VMS process can facilitate

preparation of nano-sized

particles.21 The study about controlling the morphology of Zn during VMS is underway. Conclusions This study provided an integrated and efficient process for recycling waste MFCs, as shown in Fig. 11. The technological process contained pyrolysis, grinding and screening, and vacuum metallurgy separation (VMS). The pyrolysis temperature of 500 oC and holding time for 30 min were determined as the optimal parameters for decomposing the organics of waste MFCs. The organics were decomposed to oils 24


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(mainly phenol homologs, bromophenol homologs and benzene homologs) and gases (some hydrocarbon), which could be potentially recycled as chemical feedstock and gas fuel. After pyrolysis, Cu-Fe electrodes were separated by grinding and screening. After the VMS process, the recovery rate and purity of zinc could reach 95.66 and 99.87%, respectively, under 650 oC, 100 Pa and 2 h. Moreover, the recovered zinc exhibited hexagonal flake-like powder with particle size of 15 µm. This study demonstrates an efficient and promising process for the maximum recycling of waste MFCs.

Fig. 11. Schematic illustration of the recycling process for waste MFCs. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The bond length and bond energy of some common covalent bond in polymer; saturated vapor pressure calculation of Zn, Al and Ag. Acknowledgments This work was supported by the National Natural Science Foundation of China (51534005). 25



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For Table of Contents Use Only

Synopsis This study proposed an integrated process for recycling waste MFCs, which is significant for e-waste treatment and resource utilization.

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From waste metallized film capacitors to valuable materials

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