Preparation of porous carbons from non-metallic fractions of waste printed circuit boards by chemical and physical activation

NEW CARBON MATERIALS Volume 28, Issue 2, Feb 2013 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2013, 28(2):108–114.

RESEARCH PAPER

Preparation of porous carbons from non-metallic fractions of waste printed circuit boards by chemical and physical activation KE Yi-hu, YANG Er-tao, LIU Xin, LIU Chun-ling*, DONG Wen-sheng Key Laboratory of Applied Surface and Colloid Chemistry (SNNU), MOE, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an

710062, China

Abstract: Non-metallic fractions of FR-3 type waste printed circuit boards were pyrolyzed at high temperatures. The resultant char at 600 oC was used to prepare activated carbons by physical and chemical activation. The influence of pyrolysis temperature on char yields, and activation conditions on the burn-off and porous properties of the activated carbons were investigated. Results show that char yields decrease with pyrolysis temperature. A granular activated carbon with a surface area of 1 019 m2·g-1 and a pore volume of 1.1 cm3·g-1 can be obtained by moulding, pyrolysis and physical activation using H2O as an activation agent. An activated carbon powder with a surface area of 3 112 m2·g-1 and a pore volume of 1.13 cm3·g-1 can be achieved by KOH activation. Key Words: Non-metallic fractions; Waste printed circuit boards; Activated carbon

1

Introduction

Printed circuit boards (PCBs) are the typical and fundamental component for almost all electronic products, which contain various metals e.g. Cu, Al, Fe, Sn, Sb, Pb, et al. and nonmetals e.g. thermosetting resins, reinforcing materials, brominated flame retardants (BFRs) and other additives. It has been reported that 20–50 million tones of waste electrical and electronic equipment (WEEE) are generated worldwide each year due to product replacement. As one of the most important branches of WEEE, waste PCBs have drawn much attention from the public and researchers because plenty of toxic materials including heavy metals and BFRs can cause huge damage to the environment if they are not treated properly [1–3]. Currently, the metallic fractions of waste PCBs can be effectively recycled through mechanical separation methods [3]. The non-metallic fractions (NMFs), which take up almost 70 mass% of waste PCBs, are generally treated by incineration or land filling. However, incineration of the NMFs could cause the formation of highly toxic polybrominated dibenzodioxins and dibenzofurans (PBDD/Fs) [4], while land filling of the NMFs leads to secondary pollution caused by heavy metal residues and BFRs leaching to the groundwater [5]. Therefore, recycling of the NMFs environmental friendly from waste PCBs remains a huge challenge. It has been reported that NMFs of waste PCBs can be recycled using physical or chemical method [6]. In the physical method NMFs are used as fillers or reinforcing fillers for

various products, such as construction materials, decorating agent, adhesives and insulating materials [7–9]. For example, Xu et al. [8] have prepared phenolic molding compound using NMFs from paper-based waste PCBs as fillers to replace wood flour. Shen et al. [9] found that both tensile and flexural properties of the NMFs/polypropylene composites can be significantly improved by adding the NMFs into the polypropylene. Whereas, NMFs are recycled by pyrolysis, gasification, depolymerization with supercritical fluids and hydrogenolytic degradation in the chemical method[10–14]. Among these methods, pyrolysis as a promising recycling method has been widely investigated [13,14]. Pyrolysis of organic materials contained in waste PCBs leads to the formation of gases, oils and chars, which can be used as chemical feedstocks or fuels. A process called “Hloclean” pyrolysis process has been developed by 10 European partners from industries, universities and research centers to transform materials like waste PCBs into fuel oils and to recover bromine from brominated flame retardants in PCBs. Although a number of researches have been carried out for handing NMFs in waste PCBs using pyrolysis, few research focused on the use of residue chars. In the present work activated carbons with high porosity were prepared from chars derived from pyrolysis of NMFs in waste paper-based PCBs by using chemical and physical activation methods. The optimum conditions for preparing high porosity activated carbons are recommended.

Received date: 20 December 2012; Revised date: 1 April 2013 *Corresponding author. E-mail: [email protected] Copyright©2013, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(13)60069-4

KE Yi-hu et al. / New Carbon Materials, 2013, 28(2): 107–114

2 2.1

Experimental Materials

The NMFs of wasted PCBs in FR-3 type (kraft mat impregnated with epoxy resin) were provided by Vary Science and Technology, Ltd., corporation, Hunan, China with particle sizes less than 0.07 mm after separating metals. 2.2

Pyrolysis

A laboratory-scale fixed bed tubular reactor was used to carry out the pyrolysis of NMFs. The sample was heated in a nitrogen flow (20 mL·min–1) at a ramping rate of 5 ºC·min–1 from ambient temperature to the desired temperature i.e. 500, 600, 700, and 800 ºC, and held at the desired temperature for 2 h. Liquid products evolved during pyrolysis were collected by a cold trap maintained at 0 ºC by an ice/water bath. The resulting chars were collected for chemical and physical activation. The obtained samples were denoted as C500, C600, C700, and C800, where the numbers refer to pyrolysis temperatures. 2.3 Preparation of granular activated carbon by physical activation The granular activated carbon was prepared according to the following procedure. The char powder derived from the pyrolysis of the NMFs at 600 ºC for 2 h was formed in an extruder under a pressure of 20 MPa using coal-tar pitch as binder. The addition amount of the binder is 30 mass%. The resultant granules were heated at a rate of 5 ºC·min–1 to 800 ºC in nitrogen (20 mL·min–1) in the same tubular furnace, and carbonized at 800 ºC for 30 min. Then, the granular carbon was activated at 850 ºC in a mixed stream of N2 (20 mL·min–1) and H2O (2.8 mL·min–1) for different times i.e. 1, 2, 3, 4 h to achieve different burn-offs. The obtained activated carbons were denoted as PA1, PA2, PA3, and PA4, where the numbers refer to physical activation temperatures. 2.4 Preparation of powder activated carbon by chemical activation The same char as above was milled to 200 mesh size (US standard sieve) using a grinder. Then, the char powder was mixed with solid KOH (KOH/char = 4/1, w/w) and activated in N2 atmosphere at 600 and 900 ºC for 2 h. After activation, the sample was first washed with 1 mol/L HCl to dissolve ash in the samples, and then washed with distilled water until the pH value of the residual solution was 7. The

resultant products were collected and dried at 120 ºC overnight. The obtained samples were denoted CA600 and CA900, where the numbers refer to chemical activation temperatures. 2.5

Characterization

The surface areas and the pore size distributions (PSDs) of the activated carbons were obtained from N2 (77 K) adsorption measurement using a Micromeritics ASAP2020M system. The PSDs were calculated using density functional theory (DFT), and the surface area was calculated by BET method. The micropore surface area Smi was obtained by the t-plot method. The average pore diameter (D) was estimated from the SBET and total pore volume (V) according to the equation D = 4V/S. Powder X-ray diffraction (XRD) was performed on a Rigaku D/MAX-Ⅲ X-ray diffractometer (35 kV, 40 mA) using a Cu Kα source. Infrared spectra were recorded on a Bruker EQUINX55 FTIR spectrometer using KBr disc method. Thermogravimetric (TG) measurement was performed on a TGA analyzer (TA-Q600SDT, USA) with a heating rate of 10 ºC·min-1 under a flow of N2. The scanning electron microscopy (SEM) and energy dispersive analysis of X-rays (EDAX) were performed on a Philips-FEI Quanta 200 microscope at 20 kV.

3

Results and discussion

3.1

Characterization of the precursor

Table 1 shows the elemental compositions of the NMFs. FT-IR spectrum of the NMFs is shown in Fig. 1a. The bands in the spectrum can be assigned according to the literatures [15,16] . The peak at 3 440 cm−1 corresponds to O－H stretching. The peaks at 2 921 and 2 855 cm–1 correspond to C－H stretching in benzene ring skeleton. The peak at 1 773 cm–1 is assigned to C═O stretching in carboxymethyl cellulose of kraft paper. The characteristic stretching vibration of C═C double bonds in benzene ring skeleton appears at 1 605, 1 504, 1 454 cm-1, C－H deformation vibration of methyl appears at 1 375 cm-1. The band at 1 251 cm-1 corresponds to P－O－Ar stretching in triphenyl phosphate. The bands at 1 165, 1 109, 1 024 cm-1 correspond to C－O stretching vibration of aryl- and aryl ester-based ethers in epoxide resin. The peak at 813 cm-1 is due to the absorption of epoxy group. The peak at 609 cm-1 can be assigned to C－Br stretching. The results indicate that the NMFs mainly contain brominted epoxide resin, together with triphenyl phosphate as fire retardant.

Table 1 The elemental compositions of the samples obtained by EDAX Samples

C /mass%

O /mass%

Al /mass%

Cu /mass%

Ca /mass%

P /mass%

Br /mass%

Others /mass%

NMFs

72.29

17.06

0.18

0.83

0.11

0.42

6.56

1.33

C600

91.19

6.05

0.25

1.54

0.25

PA3

88.39

6.34

0.26

2.31

0.62

2.09

CA900

89.91

7.93

0.25

0.46

0.25

1.20

0.72

0.25

KE Yi-hu et al. / New Carbon Materials, 2013, 28(2): 107–114

Fig. 1 (a) FT-IR spectrum and (b) TGA curve of NMFs.

3.2

crystalline grows large and get ordered in the chars with increasing the pyrolysis temperature.

Pyrolysis

The thermal stability of the NMFs is very important for pyrolysis process. Fig. 1b shows TGA curve of the NMFs. It can be seen that the NMFs show a sharp weight loss in the temperature range of 250–325 ºC, followed by a relatively slow weight loss in the range of 325–500 ºC and much slower weight loss in the temperature interval of 500–800 ºC. Finally, it leaves 15% solid residue after finishing TG experiment. Fig. 2 shows XRD patterns of the chars derived from the pyrolysis of the NMFs at different temperatures. Besides some diffraction peaks at 43, 51, and 74o attributed to Cu residue, the broad diffraction peak near 23o corresponds to the (002) diffraction of carbons. From the position of the (002) peak (2θ) in the XRD patterns, the interplanar spacing d(002) was determined using Bragg equation [17], and the data is shown in Table 2. The char yields are also shown in Table 2. It is clear that the char yields decrease with pyrolysis temperature. The d(002) value is traditionally used to estimate the graphitization degree of carbons. In general, growing disorder in carbons is reflected by increased values of d(002) [18,19]. It can be found that C500 has the maximum d(002) value among all the samples, and the d(002) values decrease with increasing the pyrolysis temperature. From the position and full-width at half-maximum of the (002) peak, the average grain sizes Lc of the chars were estimated using the Scherrer equation [17], and the results are summarized in Table 2. It can be found that with increasing the pyrolysis temperature Lc increases continually from 0.355 4 nm at 500 oC to 0.760 2 nm at 800 o C. Combined the interplanar spacing d(002) with Lc values of the samples, it can be concluded that the micrographite Table 2

Lu et al. [20] have found that the precursors pyrolyzed at high temperatures are more difficult to be activated with KOH than those at low temperatures. Our results confirm that low pyrolysis temperature (500 oC) is insufficient to produce chars with effective surface area. Therefore, in the following study, the pyrolysis temperature was fixed at 600 oC. In comparison with the starting NMFs, C content in C600 increases significantly, the contents of O and Br element significantly decrease, the content changes of Al, Cu and Ca are negligible (see Table 1). This is due to the fact that during pyrolysis the resin and kraft mat in the NMFs decomposed, the bands of C—O, C—C and C－Br were broken, producing

Fig. 2 XRD patterns of samples derived from the pyrolysis of the NMFs at different temperatures.

Microcrystalline structural parameters and char yields of samples from the pyrolysis of the NMFs at different temperatures

Samples

Pyrolysis temperature t/ºC

Char yield w/%

2θ /(º)

d(002) /nm

Lc /nm

C500

500

36.2

23.31

0.381 6

0.355 4

C600

600

34.5

23.38

0.380 2

0.394 1

C700

700

32.2

23.48

0.378 6

0.446 8

C800

800

30.2

23.74

0.374 5

0.760 2

KE Yi-hu et al. / New Carbon Materials, 2013, 28(2): 107–114

CO2, CO, HBr, lower molecular weight hydrocarbons and aromatic hydrocarbons [13]. 3.3

aliphatic

Physical activation

The adsorption isotherms of N2 on the granular activated carbons derived from physical activation of C600 at 850 ºC for different times are shown in Fig. 3. The isotherms of PA1, PA2, PA3 and PA4 samples show that adsorbed volumes increase sharply when p/p0 is ≤ 0.05, and then gradual increase when p/p0 is ≥ 0.05. The sharp increase in adsorbed volume at low pressure indicates that these activated carbons mainly contain micropores. The linear increase in the medium-pressure region suggests the existence of larger micropores. As the activation proceeds the pores are widened, as inferred from opening knee of the isotherm and higher slope of the plateau [21]. The adsorption capacities of the activated carbons generally increase with the extent of burn-off, indicating that the gasification and pore development mainly occur inside the carbon particles. The burn-offs and porous properties of the activated carbons are shown in Table 3. It can be seen that the burn-off increases monotonously with increasing the activation time; while, the surface area, total pore volume, mesopore volume and average pore diameter increase with increasing the activation time up to 3 h. The results suggest that with increasing activation time the gasification of carbon results in the opening of closed pores as well as deepening and widening of the micropores [22]. When the activation time increases to 4 h (74.98% burn-off), a rapid decrease in the surface area, total pore volume, and mesopore volume is observed. This is due to that over-activation (4 h) causes the destruction of some pores, leading to an overall loss of surface area and pore volume [23]. The optimal physical activation time is then fixed at 3 h. The PSDs (see Fig. 4) indicate that all the samples contain two fractions of pores; one is near 0.5 nm and another near 1.3 nm, and a dominant pore size near 0.5 nm.

about 3.0 nm. The burn-offs and porous properties of the powder activated carbons are also shown in Table 3. It can be found that the burn-off, surface area, pore volume and pore size of the activated carbons increase with increasing activation temperature. However, the micropore volume decreases because an excessive carbon burn-off causes the widening of pores and even the loss of some walls between the pores, leading to an increased large mesopore volume [24].

Fig. 3 Nitrogen adsorption isotherms of the activated carbons.

Fig. 4 The pore size distributions of the activated carbons.

3.4

Chemical activation

Fig. 5 shows the adsorption isotherms of N2 on the powder activated carbons derived from KOH activation of C600 at 600 and 900 oC for 2 h. The isotherm of the CA600 shows typical characteristics of microporous carbons. The adsorbed volume increases sharply when p/p0 is ≤ 0.05, and then remains nearly constant after p/p0 is ≥ 0.1; the knee of the isotherm is sharp and the plateau is horizontal. Whereas, the isotherm of CA900 has typical characteristics of meso and microporous carbons. The adsorbed volume increases sharply when P/P0 is ≤ 0.05, and then increases continually after p/p0 is ≥ 0.05. Fig. 6 shows the PSDs of the obtained activated carbons. The CA600 contains mainly three fractions of pores; one is near 0.5 nm, another near 1.2 nm, and the third near 2.0 nm. The CA900 contains mainly three fractions of pores; one is near 0.5 nm, another near 1.2 nm, and the third centers at

Fig. 5 Adsorption isotherms of N2 on the activated carbons prepared by KOH activated at 600 and 900 oC

KE Yi-hu et al. / New Carbon Materials, 2013, 28(2): 107–114

Table 3 The burn-offs and porous properties of the activated carbons prepared by physical activation and chemical activation Samples

Burn-offs

BET surface areas

Pore volumes

Micropore volumes

Mesopore volumes

Average pore sizes

/mass%

SBET /m 2·g-1

v/cm3·g-1

v/cm3·g-1

v/cm3·g-1

D/nm

PA1

32.8

670

0.37

0.24

0.13

2.22

PA2

42.3

750

0.45

0.25

0.20

2.41

PA3

64.8

1019

0.71

0.21

0.46

2.81

PA4

74.9

726

0.50

0.21

0.29

2.75

CA600

46.9

1940

0.93

0.59

0.34

1.81

CA900

64.2

3112

1.13

0.30

0.83

2.74

4

Conclusions

It has been demonstrated that activated carbons with high porosities can be prepared from the chars derived from the pyrolysis of NMFs of waste PCBs using both physical and chemical activation. The granular activated carbon with a specific surface area of 1 019 m2·g–1 and a pore volume of 1.1 cm3·g–1 was obtained by physical activation using H2O as an activation agent. The powder activated carbon with a surface – – area of 3 112 m2·g 1 and a pore volume of 1.13 cm3·g 1 can be achieved by KOH activation. KOH activation has more power to destroy the turbostratic graphite microcrystallines than H2O activation. Fig. 6 The pore size distributions of the activated carbons.

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