A new technology for separation and recovery of materials from waste printed circuit boards by dissolving bromine epoxy resins using ionic liquid

Journal of Hazardous Materials 239–240 (2012) 270–278

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A new technology for separation and recovery of materials from waste printed circuit boards by dissolving bromine epoxy resins using ionic liquid P. Zhu a,∗ , Y. Chen a , L.Y. Wang a , G.Y. Qian a , M. Zhou b , J. Zhou c a Key Laboratory of Solid Waste Treatment and Resource Recycle (SWUST), Ministry of Education and College of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, People’s Republic of China b Semiconductor Manufacturing International (Shanghai) Corporation, 18 Zhangjiang Road, Shanghai 201203, People’s Republic of China c Institute of Microelectronics, Peking University, Peking University, No. 5 Yiheyuan Road, Haidian District, Beijing 100871, People’s Republic of China

h i g h l i g h t s  WPCBs were heated in [EMIM+ ][BF4 − ] for recovering solider at 240 ◦ C.  The bromine epoxy resins in WPCBs were all dissolved in [EMIM+ ][BF4 − ] at 260 ◦ C.  Used [EMIM+ ][BF4 − ] is treated by water to obtain regeneration.

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Article history: Received 8 April 2012 Received in revised form 11 August 2012 Accepted 29 August 2012 Available online 4 September 2012 Keywords: WPCBs Recycling Ionic liquid Melting Dissolution Separation

a b s t r a c t Recovery of valuable materials from waste printed circuit boards (WPCBs) is quite difficult because WPCBs is a heterogeneous mixture of polymer materials, glass fibers, and metals. In this study, WPCBs was treated using ionic liquid (1-ethyl-3-methylimizadolium tetrafluoroborate [EMIM+ ][BF4 − ]). Experimental results showed that the separation of the solders went to completion, and electronic components (ECs) were removed in WPCBs when [EMIM+ ][BF4 − ] solution containing WPCBs was heated to 240 ◦ C. Meanwhile, metallographic observations verified that the WPCBs had an initial delamination. When the temperature increased to 260 ◦ C, the separation of the WPCBs went to completion, and coppers and glass fibers were obtained. The used [EMIM+ ][BF4 − ] was treated by water to generate a solid–liquid suspension, which was separated completely to obtain solid residues by filtration. Thermal analyses combined with infrared ray spectra (IR) observed that the solid residues were bromine epoxy resins. NMR (nuclear magnetic resonance) showed that hydrogen bond played an important role for [EMIM+ ][BF4 − ] dissolving bromine epoxy resins. This clean and non-polluting technology offers a new way to recycle valuable materials from WPCBs and prevent environmental pollution from WPCBs effectively. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction The production of printed circuit boards (PCBs) is the foundation of the electronic industry as it is an essential component in virtually every electrical and electronic device. New technological innovations continue to accelerate the replacement of electronic devices, leading to a significant increase in waste printed circuit boards (WPCBs) [1,2]. WPCBs have a lot of electronic components (ECs) containing resistors, relays, capacitors, and integrated circuits. The dismantling of ECs from WPCBs is the first and the most important step in the chain of recycling WPCBs, which is helpful to conserve scarce resources, to establish a reuse of components, and to eliminate

∗ Corresponding author. Fax: +86 021 66137770. E-mail address: [email protected] (P. Zhu).

hazardous materials from the environment [3,4]. The solders, which find application in electronics component joining are researched to remove for the dismantling of ECs. Usually, chemical method and heating technology are used to remove solders. For the former, both the selection of suitable chemical reagents and their damage to components, prevent this application in practice. For the latter, there is a potential risk of releasing dioxin because the removal of the solder remaining on WPCBs by subjecting it to a temperature 40–50 ◦ C higher than the molten point of the solder, ranging from 220 to 280 ◦ C [4,5]. WPCBs without ECs are also known as bare boards, which are mixtures of polymer materials, glass fibers and multiple types of metals. WPCBs have approximately 28% metals containing Cu, Pb, Sn, Zn, Ni, Au, etc., and the content of their precious metals is 10 times more than that of rich-content mineral [6]. In addition, polymer materials in WPCBs consist of thermoplastic and thermosetting resins, such as bromine epoxy resins, which are used as flame retardants. The epoxy resins are intermingled

0304-3894/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.071

P. Zhu et al. / Journal of Hazardous Materials 239–240 (2012) 270–278

with the glass fibers and metals [7]. As a result, it is very difficult for WPCBs to be separated into individual components. Many researches, such as pyrometallurgy [8], hydrometallurgy [9], combustion [10], pyrolysis [11], biometallurgy [12], supercritical fluid [13] and mechanical–physical process [14–17], have been carried out with regard to the recycling of WPCBs. These existing processes focus on the recovery of valuable materials from WPCBs. For example, mechanical–physical method is employed widely to treat WPCBs for the separation of both metal and nonmetal, which contains crushing, grinding, electrostatic separation, gravity separation, magnetic separation, etc. The nonmetal materials obtained from WPCBs using mechanical–physical method are used to prepare phenolic molding compound, but recovered metallic powders need further purification before they are utilized [6,14,18–20]. Nowadays, metallic powders recovered from WPCBs are purified usually using hydrometallurgical process, which is leached in aqueous solvents containing strong acid solutions or alkaline solutions. This process easily generates a large amount of waste acid solution, waste alkaline solution and sludge, which cause secondary environmental pollution. Thus, it is necessary to research continuously on the recycling of WPCBs. Ionic liquid as a nonaqueous, aprotic, and polar solvent is composed of large organic cation and small inorganic or organic anion. Ionic liquids have emerged as a new class of solvents for practical applications due to their unique properties: negligible vapor pressure, high conductivity, excellent thermal stability (<400 ◦ C), chemical stability, low flammability, ability to dissolve organic compounds (such as polymers and celluloses) and inorganic compounds and gases, wide electrochemical window, and tunable solvents [21–25]. In this work, a new process technology using 1-ethyl-3methylimizadolium tetrafluoroborate ([EMIM+ ][BF4 − ]) was employed to recycle WPCBs. At the first stage of this work, the WPCBs with ECs was heated to 240 ◦ C in [EMIM+ ][BF4 − ] as a heating medium to remove solders and ECs. At the second stage, the bare boards of the WPCBs were heated to 260 ◦ C in [EMIM+ ][BF4 − ] to separate and recover the coppers and glass fibers through dissolving polymer materials. The aim of this work was to achieve the separation of WPCBs and to obtain solders, coppers, glass fibers and bromine epoxy resins using [EMIM+ ][BF4 − ] as a solvent. 2. Materials and methods 2.1. Materials The WPCBs used in this study were collected from a solid waste disposal center of Shanghai, China. WPCBs were computer motherboards (the content of precious metals was low and main content was copper), which contained polymer materials, glass fibers, metals, and ECs. The WPCBs were cut with scissors into fragments of approximately 8–10 cm2 prior to conducting the experiment. [EMIM+ ][BF4 − ] reported in the literatures [26,27] was synthesized in the laboratory. All reagents and solvents used to synthesize [EMIM+ ][BF4 − ] were A.R. grade products from Shanghai Sinopharm Chemical Reagent Co., Ltd. (China). Water used was freshly deionized and distilled before used. 2.2. Methods [EMIM+ ][BF4 − ] was synthesized by ion-exchange reaction of sodium tetrafluoroborate (NaBF4 ) and 1-ethyl-3methylimidazolium bromide ([EMIM+ ][Br− ]), which are prepared by the reaction of N-methylimidazole and ethyl bromide. Above

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all, N-methylimidazole had to be vacuum distillation for the collection of 94–95 ◦ C distillate, then sealed spare after adding anhydrous calcium chloride. Meanwhile, ethyl bromide had to be washed with concentrated sulfuric acid until the acid phase was colorless, then be vacuum distillation for the collection of 100–105 ◦ C distillate after adding NaHCO3 solution, other raw material must also dried and distilled before used. Based on reflux condenser, 300 ml dropping funnel and nitrogen gas line (20–30 ml/min) were connected to three-neck flask to keep the reactor dry. Stirring was done by magnetic stirrer. Ethyl bromide was dropped into N-methylimidazole for 3 h at 65 ◦ C with molar ratio of 1.5:1. After stirring for 5 h, the reaction solution was cooled to 0 ◦ C. The white precipitated crystal was filtered, then washed to purify by dehydrated acetonitrile and dehydrated ethylacetate repeatedly to ensure the bromides were removed completely. Whole operations were carried out under nitrogen atmosphere. [EMIM+ ][Br− ] intermediate was dried under vacuum to constant weight at 70 ◦ C. [EMIM+ ][Br− ] and NaBF4 with molar ratio of 1:1 were added into the reactor, and dehydrated acetone was as a solvent. The mixtures were reacted at 25 ◦ C for 10 h. Then, the mixed solution was vacuum filtration, and acetone was removed by rotary decompression evaporation. Then, the [EMIM+ ][BF4 − ] solution was washed to purify by anhydrous dichloromethane repeatedly. Finally, the [EMIM+ ][BF4 − ] was dried under vacuum to constant weight at 70 ◦ C. All the operations were also carried out under nitrogen atmosphere. Water content of the synthesized [EMIM+ ][BF4 − ] was determined by a Karl–Fisher titration. Experimental reactor of treating WPCBs was carried out in the self-made equipment, which contained stainless steel cylinder, stirrer, and temperature controlled furnace. [EMIM+ ][BF4 − ] was put into stainless steel cylinder with the volume of 1 L, and the WPCBs were submerged by [EMIM+ ][BF4 − ]. The solid to liquid mass ratio was 1:5. The [EMIM+ ][BF4 − ] solution was stirred at a speed of 150 rpm, heated from 25 ◦ C to 240 ◦ C at a rate of 5 ◦ C min−1 , and then maintained at 240 ◦ C for 10 min to ensure that the melting of the solders and the removal of the ECs from WPCBs went to completion. The melted solders were discharged from the stirred [EMIM+ ][BF4 − ] solution and fell to the bottom of the stainless steel cylinder. Solders, ECs and base plates were collected after the [EMIM+ ][BF4 − ] solution was cooled down. The above base plates were put into [EMIM+ ][BF4 − ] again, which was heated to 260 ◦ C and maintained at this temperature for 20 min to ensure that separation of WPCBs went to completion. Re-use of [EMIM+ ][BF4 − ] played an important role for the recycling of WPCBs. The used [EMIM+ ][BF4 − ] was treated by water at water and ionic liquid ratio of 2:1, which formed solid suspending liquid. The solid suspending liquid was filtered to obtain solid residues and filtrate which was vapored under rotary decompression to obtain the regenerated [EMIM+ ][BF4 − ]. Images were taken of treated WPCBs using a metallographic microscope (EPLPHOT 300) and a digital camera. The Fourier transform infrared (FT-IR) analyses (IFS 55, Bruker Company, Fällanden, Zurich, Switzerland) were conducted under the conditions of resolution of 4 cm−1 , scan time of 32 s, and scan range of 4000–400 cm−1 . Thermal analyses of [EMIM+ ][Br− ] and [EMIM+ ][BF4 − ] were examined by TG–DTG–DTA–MS (thermogravimetry–derivative thermogravimetry–differential thermal analysis–mass spectrum, STA 449C, Germany) under nitrogen atmosphere. The samples weighed 5000 mg and were placed in a Pt–Rh crucible, and heated from 25 to 800 ◦ C at a rate of 8 ◦ C min−1 . The NMR spectra (nuclear magnetic resonance) of samples were acquired on a Bruker AV 400 spectrometer with 16–32 scans for 1 H NMR measurements at room temperature. The used [EMIM+ ][BF4 − ] was analyzed with an inductively coupled plasma atomic emission spectrometer (ICP-AES,

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P. Zhu et al. / Journal of Hazardous Materials 239–240 (2012) 270–278 DTA/ DTG/ (uV/mg)(%/min)ionic flow/A Br 0.2 0

0

340.0 C

-2

-10

4.00E-010 -0.4 -12

2.00E-010

20

-14

100

200

0.00E+000

-0.6 -16

0

332.3 C

0

300

400

500

600

700

80

60 50 40 4000

0

Temperature/ C

850.11

70 1171.30

A

90

A B 3500

3000

2500

2000

1500

1055.14

40

6.00E-010

1575.32 1457.73

-0.2 -8

100

2357.47

8.00E-010

-6

3157.27 3113.28

0

140.7 C

110

1.00E-009

-4

3610.22

0.0

3417.78

80

60

120

1.20E-009

0

332.4 C

0

144.7 C

130

Transmittance%

TG/% 100

1000

500

-1

Wavenumbers(cm ) DTA/ DTG/ ionic flow/A (uV/mg) (%/min) F

TG/% 100

0.0

0

80

60

450.5 C

0

2.00E-008

-2

1.80E-008

-0.5 -4

1.60E-008

-6

1.40E-008

-8

1.20E-008

-10

1.00E-008

-12

8.00E-009

-2.0 -14

6.00E-009

-16

4.00E-009

-2.5 -18

2.00E-009

-1.0

B 0

485.0 C

40

-1.5

20 0

497.9 C 0

473.4 C

0 100

200

300

400

500

600

700

0

Temperature/ C Fig. 1. TG–DTG–DTA–MS curves of the [EMIM+ ][Br− ] (A) and [EMIM+ ][BF4 − ] (B).

Prodigy, USA) for metal ion concentration. The UV absorption spectrum (UV5300PC) was used to determine the amount of bisphenol A, which is a component of bromine epoxy resin, at  282 nm (the absorption maximum of bisphenol A).

Fig. 2. FT-IR spectra of [EMIM+ ][BF4 − ]. (A) Unused [EMIM+ ][BF4 − ] and (B) [EMIM+ ][BF4 − ] used for ten times.

3. Results and discussion 3.1. [EMIM+ ][BF4 − ] characteristics Thermal analyses of the [EMIM+ ][Br− ] and [EMIM+ ][BF4 − ] are shown in Fig. 1. It can be observed from Fig. 1(A) that the significant weight loss of [EMIM+ ][Br− ] was at temperatures of 300–400 ◦ C and the bromine ionic flow could be examined when the [EMIM+ ][Br− ] sample was heated to 280 ◦ C, and then its concentration reached a max value with the temperature increased to 332.4 ◦ C. Fig. 1(B) shows that initial weight loss was at 300 ◦ C and significant weight loss was at 473.4 ◦ C, and the complete decomposition of [EMIM+ ][BF4 − ] was at 497.9 ◦ C. It was not found characteristic peaks of [EMIM+ ][Br− ]. Fluorin ionic flow could be examined when the [EMIM+ ][BF4 − ] sample was heated to 420 ◦ C, and its concentration reached a max value with the temperature increased to 485.0 ◦ C. Meanwhile, bromine ionic flow had not been detected. So it was considered that bromide ion had not been contained in the final product of [EMIM+ ][BF4 − ] or that the content of bromide ion was too low to detect. The thermal analysis indicates that [EMIM+ ][BF4 − ] may be used safely below 300 ◦ C. Water

Fig. 3. Photographs of waste PCBs before and after treated by the [EMIM+ ][BF4 − ]. (A) Untreated WPCBs, (B) WPCBs without solder, (C) recovering solder, and (D) treated electronic components.

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Fig. 4. Photograph (A) and metallographic photograph (C) of cross section untreated WPCBs as well as photograph (B) and metallographic photograph (D) of cross section the WPCBs treated by the [EMIM+ ][BF4 − ].

content of the synthesized [EMIM+ ][BF4 − ] was below 100 ppm, which has little effect on [EMIM+ ][BF4 − ] separating WPCBs. Fig. 2(A) shows FT-IR spectrum of [EMIM+ ][BF4 − ] synthesized in the Lab, which is in good agreement with FT-IR spectrum reported in the literature [28]. Fig. 2(B) shows FT-IR spectrum of the [EMIM+ ][BF4 − ] used ten times. Comparing Fig. 2(A) to (B), it was found that the FT-IR spectrum of the used [EMIM+ ][BF4 − ] was the same as that of the unused [EMIM+ ][BF4 − ], which indicates that the chemical properties of the regenerated [EMIM+ ][BF4 − ] did not change. The regenerated process of used [EMIM+ ][BF4 − ] will be discussed in detail in the final section of the manuscript. 3.2. Recovering solder from WPCBs The recovery of solder is very important in the process of the recycling of WPCBs. It separated early from WPCBs not only eliminates the adverse influence in the recycling process of other metals especially precious metals but also avoids secondary environmental pollution of heavy metals. The following section will discuss the recovering solder from WPCBs using [EMIM+ ][BF4 − ].

Fig. 5. Concentrations of Sn and Cu in [EMIM+ ][BF4 − ].

Fig. 3(A) and (B) shows photographs of untreated WPCBs and WPCBs treated in [EMIM+ ][BF4 − ] at 240 ◦ C, respectively. It can be observed from Fig. 3(B) that all solders were separated from WPCBs and exposed surfaces were copper foils. The recovered solder is shown in Fig. 3(C). During separating the solder process, there are three main forces acting on melting solder, including centrifugal force, gravity force, and surface conglutinate force [29]. Although the surface conglutinate force prevents melted solder separating from WPCBs, the stirring process is propitious to remove melted solder from WPCBs. This indicates that the stirring produces a resultant force produced by both gravity and centrifugal force to increases collision probability of melting solders. As a result, melted solder discharged from WPCBs and fell to the bottom of stainless steel cylinder. After the [EMIM+ ][BF4 − ] solution was cooled down, the solders, the ECs, and the base plates were collected. However,

Fig. 6. TG–DTG–DTA–MS curves of the [EMIM+ ][BF4 − ] dissolving the WPCBs.

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Fig. 7. Photographs of WPCBs treated at 260 by the [EMIM+ ][BF4 − ].

the liquid photo solder resists were still affixed to the surfaces of the copper foils, as shown in Fig. 3(B). Fig. 4(A) and (B) shows cross-section photographs of untreated and treated WPCBs, and their metallographic photographs are shown in Fig. 4(C) and (D). It was found from Fig. 4(B) and (D) that there was a initial delamination for WPCB treated at 240 ◦ C. Fig. 5

shows the changes of Cu and Sn concentrations in [EMIM+ ][BF4 − ] with increases in time. The concentrations of Sn and Cu increased slowly with an increase in time when WPCBs were treated at 240 ◦ C, and Cu concentrations also increased when the temperature increased to 260 ◦ C. However, the trace metal ions dissolved

WPCBs

Cutting WPCBs

Cutting WPCBs/[EMIM+][BF4 ] Regeneration Solid residues

Filtration

Recycle

[EMIM+][BF4 ]

H2O

Wash, dry Recovering

Evaporation

Fig. 8. A flowchart for dissolution of epoxy resin and regeneration of [EMIM+ ][BF4 − ].

Fig. 9. Photograph of the solid residues from [EMIM+ ][BF4 − ].

P. Zhu et al. / Journal of Hazardous Materials 239–240 (2012) 270–278

B A

30

20

4000

831.17

1232.83 1170.58 1047.49

1735.66 1610.95 1508.98 1458.30

40

3436.58

Transmittance%

50

275

3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers(cm ) Fig. 10. FT-IR spectra of epoxy resin base material (A) and solid residue (B).

into [EMIM+ ][BF4 − ] can be recovered in situ using electrodeposited process because [EMIM+ ][BF4 − ] has the characteristic of high conductivity and wide electrochemical window [21,26].

3.3. Separation of WPCBs in [EMIM+ ][BF4 − ] Fig. 6 shows TG–DTG–DTA–MS curves of [EMIM+ ][BF4 − ] dissolving WPCBs. It can be observed that the bromine epoxy resin started to decompose when the temperature was heated to 280 ◦ C, and its max value of decomposition was at temperatures of 330–440 ◦ C. This indicates that the initial temperature of bromine epoxy resin pyrolysis is of 270–280 ◦ C, corresponding to the results of reported references [30–35]. As mentioned in Section 2, the WPCBs produced a initial delamination when treated at 240 ◦ C in [EMIM+ ][BF4 − ]. When the temperature increased to 260 ◦ C and was maintained at this temperature for 20 min, the separation of WPCBs was completed as shown in Fig. 7(A). It can be observed from Fig. 7(B) that the liquid photo solder resists were removed completely from the surfaces of copper foils. It can be observed clearly from Fig. 7(C) that the coppers filled in via of the WPCBs still existed. Therefore, the higher temperature is, the easier delamination of WPCBs is.

Fig. 12. TG–DTA curve of the solid residue (A) and WPCB (B) (data of WPCB (B) from literature [36]).

3.4. Regeneration of [EMIM+ ][BF4 − ] and analysis of solid residues From the view point of protecting environment and decreasing cost, it is very important that [EMIM+ ][BF4 − ] used to treat WPCBs can be regenerated. Fig. 8 depicts a schematic flowchart for the process involving the dissolution of WPCBs using [EMIM+ ][BF4 − ] and regeneration of [EMIM+ ][BF4 − ]. Water as an anti-solvent was poured into the used [EMIM+ ][BF4 − ] to form a solid suspending liquid. This may be explained that [EMIM+ ][BF4 − ] with strong polarity is transferred into water, but the polymer materials dissolved in [EMIM+ ][BF4 − ] cannot be dissolved into water to form solid residues. By filtration, the solid residues were separated from

0

260 C 0 240 C

concentration(ug/ml)

10000 8000 6000 4000 2000 0 0

Fig. 11. FT-IR spectra of oil for vacuum pyrolysis WPCB at 400 ◦ C. From literature [36].

20

40

60

80

100

time (min)

120

140

160

Fig. 13. The change over time in bromine epoxy resin concentrations from dissolving WPCBs at 240 ◦ C and 260 ◦ C, respectively.

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Fig. 14. The 1 H NMR spectrum of [EMIM+ ][BF4 − ] in DMSO-d6 (A) and ([EMIM+ ][BF4 − ] + bromine epoxy resin) in DMSO-d6 (B).

the solid suspending liquid, as shown in Fig. 9. The filtered liquids are evapored under the rotary decompression to obtain the regenerative [EMIM+ ][BF4 − ]. Fig. 10(A) and (B) shows the FT-IR spectra of the base materials in WPCBs and the solid residues. Comparing two figures, it was found that the FT-IR spectrum of the solid residues was same as that of the WPCB base materials. This indicates that molecular structure of the solid residues was similar to that of the WPCB base material, which contained a hydroxyl group in molecules chain, a phenyl group, the middle of the chain molecule and an ether group. Fig. 11 shows the FT-IR spectrum of pyrolysis oil collected from WPCBs by vacuum pyrolysis at 400 ◦ C (from literature [36]). Comparing Fig. 10(B) to Fig. 11, it can be observed obviously that the FT-IR spectrum of pyrolysis oil was not the same as that of the solid residues. Fig. 12(A)

shows TG–DTA–DTG curves of the solid residues. It can be observed obviously that the significant weight loss for the solid residues was at temperature of 300–380 ◦ C, corresponding to the result of vacuum pyrolysis of WPCBs as shown in Fig. 12(B) (from literature [36]). Therefore, it can be considered from the above analysis that the solid residues are bromine epoxy resins in WPCBs dissolved by [EMIM+ ][BF4 − ]. Therefore, the changes in bromine epoxy resin concentrations at 240 ◦ C and 260 ◦ C with increases in time were used to examine the process of separating WPCBs. As shown in Fig. 13, the temperature of start time was at 25 ◦ C and the solutions of [EMIM+ ][BF4 − ] submerging WPCBs were heated to 240 ◦ C and 260 ◦ C, respectively. It can be seen that the concentrations of the bromine epoxy resin increased with an increase in time. The concentrations of bromine

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at this temperature for 10 min, meanwhile, metallographs showed that WPCBs produced initial delaminating phenomenon. When the temperature increased to 260 ◦ C and maintained at this temperature for 20 min, WPCBs were separated completely. Water was poured into the used [EMIM+ ][BF4 − ] to form a solid suspending liquid, and both solid and liquid are separated by filtration. The FT-IR and thermal analyses verified that the filtered solid residues were the dissolving bromine epoxy resins. 1 H NMR analyses indicated that the hydrogen bond played an important role for the bromine epoxy resins dissolved into [EMIM+ ][BF4 − ]. The filtered liquids were vaporized rotatably under the decompression to obtain regenerated [EMIM+ ][BF4 − ]. This complies with the principle of sustainable development by decreasing the recycling cost of WPCBs to achieve complete recovery of reusable resources. Fig. 15. Hydrogen bonding and ␲–␲ bonding between [EMIM+ ] and bromine epoxy resin.

Acknowledgments

epoxy resin increased slowly before 40 min because of low solution temperature, then, the increase was rapid because of reaching the set temperature, especially 260 ◦ C curve. It indicates that temperature is the key of separating WPCBs.

The authors are grateful for support of key personnel in the Shanghai Municipality (S30109), the Opening Project of Key Laboratory of Solid Waste Treatment and Resource Recycle (SWUST), Ministry of Education (12zxgk09) and Shanghai Science and Technology Commission (10dz1205302).

3.5. NMR analysis of bromine epoxy resin dissolved into [EMIM+ ][BF4 − ]

References

Ionic liquids are intriguing solvents and their potential to replace organic solvents in different areas of chemistry has been firmly established. Ionic liquids are considered as “green solvents”, frequently displaying low vapor pressure, wide liquidus range and efficient dissolution power. Their characteristics reported to be very important in the dissolution of cellulose, lignin, and hemicelluloses are viscosity, melting point, dipolarity and hydrogen bond [21]. The following section will discuss the mechanism of the bromine epoxy resin dissolved into [EMIM+ ][BF4 − ]. Fig. 14(A) shows that the 1 H NMR spectrum of [EMIM+ ][BF4 − ] in DMSO-d6 , which exhibited sufficient resolution to clearly identify the distinct proton signals of [EMIM+ ][BF4 − ] ranging from 1 to 10 ppm. The assignment has been done referring to the literature [37–39]. When the bromine epoxy resins in WPCBs were added into [EMIM+ ][BF4 − ]/DMSO-d6 solution, two peaks of four proton resonances in site 4 of [EMIM+ ] were split into four peaks (Fig. 14(B)). Meanwhile, the change in the value of the chemical shift was observed at 4 site proton resonances in [EMIM+ ]. 1 H chemical shifts changed from 7.7530 and 7.6634 ppm at site 4 in [EMIM+ ] of Fig. 14(A) to 7.0914, 6.9881, 6.9724, 6.8424 ppm at site 4 in [EMIM+ ] of Fig. 14(B) with the addition of bromine epoxy resins. Therefore, the upfield shift of proton resonances was ascribed to the formation of hydrogen bond between hydrogen atoms of [EMIM+ ] and the bromine of the epoxy resins, as shown in Fig. 15. In addition, the imidazolium ring of [EMIM+ ][BF4 − ] is similar to the benzene ring, which could interact with the benzene of the bromine epoxy resin to enhance the dissolution of the bromine epoxy resin (␲–␲ bonding) (in Fig. 15). The characteristic of strong dipolarity of [EMIM+ ][BF4 − ] favored the dissolution of the bromine epoxy resin, on the contrary, liquid photo solder resists (acrylate oligomer) were not dissolved into [EMIM+ ][BF4 − ] [37–44]. 4. Conclusion A new process without having a negative impact on the environment was built to recover value materials from WPCBs. TG–DTA indicated that [EMIM+ ][BF4 − ] could be used safely below 300 ◦ C, and the significant weight loss for WPCBs was at temperature of 280–380 ◦ C. The solder materials in WPCBs were recovered when the [EMIM+ ][BF4 − ] solution was heated to 240 ◦ C and maintained

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