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Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathode-ray-tube funnel glass
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Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathode-ray-tube funnel glass Zhitong Yao a , Daidai Wu b,∗ , Jie Liu a , Weihong Wu a , Hongting Zhao a , Junhong Tang a,∗ a b
College of Materials Science and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
h i g h l i g h t s • An open-loop recycling method for CRT funnel glass was developed. • The influence of various factors on the resulting products were investigated. • Successive transformation among NaA, NaP1, Faujasite and hydroxysodalite was confirmed.
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Article history: Received 26 July 2016 Received in revised form 13 November 2016 Accepted 14 November 2016 Available online xxx Keywords: E-waste Cathode ray-tube Leaded glass Open-loop recycling Zeolite synthesis
a b s t r a c t The disposal of waste cathode ray-tubes (CRTs) from old televisions and discarded computer monitors has become a major environmental concern worldwide. In this work, an open-loop recycling method was developed to synthesize zeolites using CRT funnel glass as the raw material. The effects of hydrothermal temperatures and pressure, n(SiO2 /Al2 O3 ) molar ratios and hydrothermal time on the resulting products were investigated. The results indicated that hydrothermal temperatures and pressure played critical roles in zeolite synthesis. Amorphous phases were detected at lower temperatures (80–100 ◦ C) and pressure (0.47–1.01 bar) with n(SiO2 /Al2 O3 ) = 2.0. At the temperature of 110 ◦ C (pressure 1.43 bar), NaA formed with a mixture of NaP1 and Faujasite. With further increase in the temperature and pressure, the unstable NaA and Faujasite disappeared, and Hydroxysodalite developed. The influence of n(SiO2 /Al2 O3 ) ratios on resulting products revealed a single phase of NaA was formed at the ratio of 1.5 and a mixture of NaA and Faujasite at the ratio of 2.0. Prolonging hydrothermal time, however, could promote zeolite crystallization, and NaA gradually developed with an increase in the time from 2 to 6 h at n(SiO2 /Al2 O3 ) = 1.5. By comparison, crystallization phases were observed only when the time was longer than 8 h at n(SiO2 /Al2 O3 ) = 2.0. © 2016 Published by Elsevier B.V.
1. Introduction With the rapid innovations in electronic display technology during the last decade or so, conventional cathode ray-tubes (CRTs) have been gradually replaced by advanced display products, such as liquid-crystal displays (LCDs), plasma display panels (PDPs) and organic light-emitting diode (OLEDs) displays, resulting in a large number of waste CRTs that need to be disposed of. According to the waste electrical and electronic equipment (WEEE) collection and pretreatment market, approximately 50,000–150,000 tons/a of end-of-life CRTs are currently collected in Europe, and this volume is not expected to decrease any time in the next few years [1].
∗ Corresponding authors. E-mail addresses: [email protected] (D. Wu), tang [email protected] (J. Tang).
The recycling and dismantling amounts of waste electrical appliances in China came to 56 million units in 2010, of which 80% were CRT monitors [2]. CRTs are generally made of four different types of glass—panel, funnel, neck and frit junction—with varying chemical compositions and properties [3]. Among these glass types, funnel glass has been confirmed as having hazardous characteristics, because of its high level of lead content [4–7]. However, most discarded CRTs currently end up concentrated in landfills or e-waste recycling centers, and only a small portion of them are recycled. A long-range plan is needed, to find improved recycling methods for this waste. Generally, there are two principal ways of recycling CRT glass: closed-loop and open-loop recycling [8–10]. In closed-loop recycling, waste CRT glass is commonly used as raw material to manufacture new CRTs. This approach has the advantage of economic benefit and a high resource utilization rate. However, most
http://dx.doi.org/10.1016/j.jhazmat.2016.11.041 0304-3894/© 2016 Published by Elsevier B.V.
Please cite this article in press as: Z. Yao, et al., Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathoderay-tube funnel glass, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.11.041
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CRT manufacturers (e.g. Ancai Hi-tech Co., Ltd., and IRICO Group Electronics Co., Ltd., in China) have gradually ceased or restructured the funnel manufacturing facilities of their CRT operations, since the demand for new CRT monitors is rapidly shrinking. Consequently, attention has shifted to open-loop recycling. A number of projects have been undertaken to use CRT glass as raw material for the production of foam glass [11,12], glass ceramic [13,14], ceramic glazes [15], stoneware tile [16,17] and cement mortar [18,19]. Nevertheless, these approaches are problematic from the point of view of environmental and human safety, because the regenerated products still contain toxic heavy metals. Therefore, many attempts have been made to recover lead from leaded glass. Saterlay et al. [20] used ultrasound to facilitate lead leaching from CRT glass, achieving a removal rate of over 90% of the leachable lead. Yuan et al. [21] applied mechanical activation to pretreat CRT funnel glass, followed by diluted nitric acid leaching, and a high yield of 92.5% of the lead was achieved. Lu et al. [22] recovered lead from CRT funnel glass by thermal reduction with metallic iron, and 58 wt.% lead extraction was achieved. Xing and Zhang [23] extracted nano-lead particles by retrofitting the pyro-vacuum process; the leaded glass was thus converted into harmless glass. Erzat and Zhang [24] also used chloride volatilization to recover lead from CRT funnel glass. From the above literatures, it can be seen that lead can be effectively removed from leaded glass using recent advanced technologies. However, little attention has been paid to the comprehensive utilization of other fractions (e.g. silicon and aluminum) in the leaded glass. Hu et al. [25] prepared sodium silicate frit by melting leaded glass with sodium carbonate and then dissolving it and precipitating lead to obtain a sodium-silicate-rich solution. Okada and coauthors [26–29] treated funnel glass by a process combining reduction melting to recover lead from glass. The oxide phase generated by the thermal treatment was subjected to water and acid leaching for SiO2 purification. In this work, we attempted to prepare zeolites using CRT funnel glass as the raw material. The effects of hydrothermal temperatures and pressure, n(SiO2 /Al2 O3 ) molar ratios and hydrothermal time on zeolite synthesis were investigated. The aim was to develop an innovative technology for CRT funnel glass recycling and thus solve the significant environmental problems associated with leaded glass. 2. Experimental materials and methods 2.1. Materials Raw CRT funnel glass was obtained from Henan Ancai Hi-tech Co., Ltd. The glass block was first crashed into small pieces, ground with a P-7 planetary ball mill (Fritsch, Idar-Oberstein, Germany), and then sieved to achieve a particle size smaller than 200 m. Its chemical composition was determined using X-ray fluorescence spectroscopy (SXF-1200, Shimadzu, Japan) and the resulting contents were (wt.%): SiO2 53.90, PbO 23.10, K2 O 7.59, Na2 O 5.81, CaO 3.06, Al2 O3 3.03, and others 3.51. 2.2. Zeolite synthesis Zeolite synthesis includes three major procedures: leaching, purification and hydrothermal synthesis. 1) Before leaching, batch experiments were preliminarily carried out to determine the optimal conditions. Leaded glass powder and sodium hydroxide solution (6 mol/L), in a solid/liquid ratio of 1:8, were mixed and put into a 200-mL autoclave made of stainless steel. The reaction temperature and time were set as 220 ◦ C and 8 h, respectively. After treatment, the solid-liquid separation was performed by vacuum filtration and a filtrate was obtained. 2) The lead and silicon concentrations in filtrate were measured and the leaching rate of them
were determined as 85.8 and 90.1%, respectively. Sodium sulfide was gradually added according to the molar ratio of Pb:S = 1.0:1.5 to remove the hazardous lead. The dark brown precipitate was separated through centrifugation and lead removal rate reached 99.4%. The silicon-rich and lead-free liquid was thus obtained. 3) Silicon and aluminum in the concentrated liquid were measured and their molar ratios of SiO2 and Al2 O3 adjusted, using sodium silicate and sodium aluminate. The precursors were aged at 60 ◦ C in a water bath for 0.5 h and then put into the autoclave. The hydrothermal temperature and time were set as 80–210 ◦ C and 2–12 h, respectively. At the end of the process, the solid was separated by vacuum filtration with a Büchner funnel, washed five times with distilled water and dried overnight at 100 ◦ C. The dried products were then analyzed using X-ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR) and scanning electron microscopy (SEM). 2.3. Characterization and testing The obtained products were characterized by various conventional methods. XRD analysis was employed to determine the crystalline phases present in the zeolitic products. A Rigaku DMax/IIB X-ray power diffractometer was operated at 40 kV and 30 mA, with CuK␣ as the radiation source. The detector was scanned at a step scan of 0.02◦ and a scan speed of 4◦ /min. Scanning electron microscopy (SEM) investigations were conducted in an S-3400N scanning electron microscope (Hitachi, Japan) at an accelerating voltage of 15 kV to observe the microstructure of the samples. Transmission IR spectra were recorded with a Nicolet Nexus670 Fourier transform infrared spectrometer with a resolution of 2 cm−1 using the KBr disc method. The concentrations of lead, aluminum and silicon in the liquid were measured with a Perkin Elmer Optima 8300 DV (Perkin Elmer, Shelton CT) ICP-OES. 3. Results and discussion 3.1. The influence of hydrothermal temperatures and pressure The influence of varying hydrothermal temperatures and pressure on zeolite synthesis at n(SiO2 /Al2 O3 ) = 2.0 is plotted in Fig. 1. It was clear that lower temperatures (80–100 ◦ C) and pressure (0.47–1.01 bar) were not helpful for zeolite crystallization, and amorphous phases were detected. Increasing the temperature and pressure further resulted in the decrease of amorphous phases and promoted the growth of crystal zeolites, indicating the critical role of temperature and pressure in the zeolite crystallization [30,31]. At the temperature of 110 ◦ C (pressure 1.43 bar), the major phase NaA (NaAlSiO4 ·2.25H2 O, JPCDS card no. 39-0222) [32,33] developed with a mixture of NaP1 (Na3.6 Al3.6 Si12.4 O32 ·14H2 O, JPCDS card no. 39-0219) [34,35] and Faujasite (Na2 Al2 Si2.4 O8.8 ·6.7H2 O, JPCDS card no. 12-0246) [36,37]. Increasing the temperature still more, to 140 ◦ C (pressure 3.61 bar), caused the diffraction peak intensity of the three zeolites to increase, especially for the Faujasite. Yet increasing the temperature from 140 to 160 ◦ C (pressure 6.18 bar) resulted in even more development of zeolite NaP1, compared to Faujasite. When the temperature was increased to 180 ◦ C (pressure 10.02 bar), the product was still confirmed by a mixture of the three zeolites. However, the intensity of NaP1 reflections increased at the expense of NaA and Faujasite. The two unstable phases completely disappeared at 190 ◦ C (pressure 12.54 bar), accompanied by the formation of Hydroxysodalite (Na6 (AlSiO4 )6 ·4H2 O, JPCDS card no. 42-0216) [38,39]. Further increasing the temperature to 210 ◦ C (pressure 19.06 bar) caused the intensity of Hydroxysodalite reflections to increase at the expense of NaP1. This zeolite transformation coincided with Ostwald’s rule of successive transformation [40,41]. With the rise in hydrothermal temperature and pressure,
Please cite this article in press as: Z. Yao, et al., Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathoderay-tube funnel glass, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.11.041
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an increase in supersaturation was achieved as a result of the higher proportion of soluble species. The higher the supersaturation, the better the conditions to nucleate metastable phases [42], such as NaA, Faujasite and NaP1, which later recrystallized and were replaced by the more stable Hydroxysodalite. SEM investigations were conducted to explore the morphology evolution of the samples with hydrothermal temperature and pressure variations (Fig. 2). These revealed that at 130 ◦ C (pres-
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sure 2.70 bar) the produced particles took on a predominantly sharp cubic shape with a smooth surface texture. Some octahedral (red circle) and spherical particles with protuberant grains on the surface (blue circle) were also observed. According to the above XRD results, the product was a mixture of NaA, NaP1 and Faujasite. Hence, the cubic particles could be ascribed to NaA [43–45], the octahedral particles to Faujasite [46–48] and the spherical particles to NaP1 [49,50]. At 140–160 ◦ C (pressure 3.61–6.18 bar), the
Fig 1. XRD patterns of products at different temperature and pressure (synthesis conditions: hydrothermal time 12 h, n(SiO2 /Al2 O3 ) = 2.0).
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Fig 1. (Continued)
sample particles were also predominantly cubic, however, dominately spherical accompanied with cubic and octahedral at higher temperature 170 ◦ C (pressure 7.92 bar) and 180 ◦ C (10.02 bar). The dissolution of NaA could be observed (orange circle), confirming Ostwald’s rule of successive transformation. At 190–210 ◦ C (pressure 12.54–19.06 bar), more ball-of-yarn-like particles and dense spherical particles with irregular blocks on the surface [51,52] were observed. The dissolution of NaP1 can also be observed (black circle). The above XRD results indicated that the materials were mixtures of NaP1 and Hydroxysodalite, so that the ball-of-yarn-like and dense spherical particles could be assignable to Hydroxysodalite. Fig. S1 shows the FTIR transmittance spectra of samples at temperatures of 130–210 ◦ C and pressure of 2.70–19.06 bar. The
spectrum of the material formed at 130 ◦ C (pressure 2.70 bar) possessed eight absorption bands. Those at 3460 and 1650 cm−1 were attributed to O H stretching and bending, respectively. The bands located at 987, 669 and 463 cm−1 were assigned to the asymmetric stretching vibration of internal tetrahedral, the symmetric stretching vibration and bending vibration modes of T O bonds in TO4 tetrahedra, respectively (T = Si or Al) [53]. The band at 555 cm−1 can be attributed to D4R, because NaA was formed at this temperature [54]. The band at 742 cm−1 was assigned to the symmetric stretching vibration of external linkages [55]. With the temperature increase, the intensity of peaks at 3460, 1650, and 987 cm−1 decreased and these bands steeply shifted to high wavenumbers [43,56,57]. It has been well recognized that the length of the AlO bond is longer than that of the Si-O bond, and the substitution
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Fig. 2. SEM images of products at different temperature and pressure (synthesis conditions: hydrothermal time 12 h, n(SiO2 /Al2 O3 ) = 2.0).
of tetrahedral Al for Si in aluminosilicates frameworks induces the low wave-number shift of the stretching vibration of the T-O band. Therefore, the high wavenumber shift was attributed to the decrease of Al concentration incorporated into the zeolites [58]. At 190–210 ◦ C (pressure 12.54–19.06 bar), new peaks at 1460 cm−1 were observed. The differences in structure between NaA, Faujasite and Hydroxysodalite were essentially the differences in the mode of linkage of the sodalite unit in building up the framework structure of the zeolites. Therefore, the new band detected could be attributed to the formation of Hydroxysodalite [59].
3.2. The influence of n(SiO2 /Al2 O3 ) ratios A series of experiments were undertaken to determine the influence of n(SiO2 /Al2 O3 ) ratios on the resulting products (see Fig. 3). It was found that larger n(SiO2 /Al2 O3 ) ratios were not helpful for
zeolite crystallization. A single phase of NaA was formed at the ratio of 1.5 and a mixture of NaA and Faujasite at the ratio of 2.0, which was consistent with a previous report [43]. To obtain further insight into the influence of n(SiO2 /Al2 O3 ) ratios, the XRD patterns were plotted as a function of larger ratios. It can be observed that no crystallization phases were detected and the amorphous phases increased when the ratio further increased from 2.2 to 5.5. To confirm the particle morphology, SEM observation of the products formed at various n(SiO2 /Al2 O3 ) ratios was carried out. According to the above XRD results, the zeolite phases developed only at the ratio of 1.5 and 2.0, and thus the two resulting products are included in Fig. 4. These results revealed that the individual zeolite crystals at the ratio of 1.5 were predominantly cubic in shape, interlocked and piling up on each other. As described before, a single phase of NaA was formed at this ratio. Hence, the cubic particles could be ascribed to NaA. The product at the ratio of 2.0
Please cite this article in press as: Z. Yao, et al., Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathoderay-tube funnel glass, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.11.041
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Fig. 3. XRD patterns of products at different n(SiO2 /Al2 O3 ) ratios (synthesis conditions: hydrothermal temperature 120 ◦ C, time 10 h).
also predominately exhibited as cubic shapes with rough surfaces. However, octahedral particles were observed (blue arrow) as well. The XRD result shown in Fig. 3 indicated that the material was composed of NaA and Faujasite, so that the octahedral particle could be assignable to Faujasite [60,61]. The FTIR transmittance spectra are presented in Fig. S2. The broad peaks located at 3440 and 1690 cm−1 for product at n(SiO2 /Al2 O3 ) = 1.5, 3450 and 1660 cm−1 at n(SiO2 /Al2 O3 ) = 2.0 were attributed to O H stretching and bending, respectively. The bands at 997 and 1000 cm−1 were assigned to the asymmetric stretching vibration of Si O Si bonds within SiO4 . The band located at 671 cm−1 was known to belong to Si O Si symmetric stretching vibration, while the bands 580 and 557 cm−1 were due to the bending vibration of Si O Al linkage.
3.3. The influence of hydrothermal time Figs. 5 and 6 illustrate the evolution of zeolite synthesis as a function of time at various n(SiO2 /Al2 O3 ) ratios in the thermal process. It was found that prolonging the hydrothermal time could promote zeolite crystallization. Prolonging the time from 2 to 6 h at n(SiO2 /Al2 O3 ) = 1.5 caused the amorphous phases to decrease. When the time was further prolonged from 8 to 10 h, NaA gradually developed, with its diffraction peaks increasing. For zeolite synthesis at n(SiO2 /Al2 O3 ) = 2.0, similar influences of time were observed. However, the crystallization phase was observed only when the time was longer than 8 h, and the zeolite product was a mixture of NaA and Faujasite.
Fig. 4. SEM images of products at different n(SiO2 /Al2 O3 ) ratios (synthesis conditions: hydrothermal temperature 120 ◦ C, time 10 h).
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4. Conclusion
Fig. 5. XRD patterns of products at different hydrothermal time (synthesis conditions: hydrothermal temperature 120 ◦ C, n(SiO2 /Al2 O3 ) = 1.5).
An open-loop recycling approach was proposed to prepare zeolites using CRT funnel glass as the raw material. This approach consisted of three procedures: leaching, purification and hydrothermal synthesis. The results indicated that hydrothermal temperature and pressure played an important role in zeolite synthesis. When the temperature increased to 90 ◦ C (pressure 0.70 bar), the amorphous phases decreased significantly and NaA developed at n(SiO2 /Al2 O3 ) = 1.5. At n(SiO2 /Al2 O3 ) = 2.0, NaA developed with a mixture of NaP1 and Faujasite at 110 ◦ C (pressure 1.43 bar). With a further increase in the temperature and pressure, NaA and Faujasite disappeared and the new phase Hydroxysodalite developed. SEM observation confirmed the dissolution of zeolite A and NaP1 at 180 ◦ C (10.02 bar) and 200 ◦ C (15.54 bar), respectively. Furthermore, the intensity of the peaks at 3460, 1650, and 987 cm−1 decreased and these bands steeply shifted to high wave numbers. At 190–210 ◦ C (12.54–19.06 bar), new peaks located at 1460 cm−1 were observed, which could be attributed to the formation of Hydroxysodalite. The influence of n(SiO2 /Al2 O3 ) ratios on the resulting products revealed that a single phase of zeolite A was formed at the ratio of 1.5, and a mixture of NaA and Faujasite at the ratio of 2.0. However, prolonging hydrothermal time could further promote zeolite crystallization, and zeolite A gradually developed when prolonging the time from 2 to 6 h at n(SiO2 /Al2 O3 ) = 1.5. By comparison, the crystallization phases were observed only when the time was longer than 8 h at n(SiO2 /Al2 O3 ) = 2.0. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant no. 41373121, 41101213 and 41271249). Appendix A. Supplementary data
Fig. 6. XRD patterns of products at different hydrothermal time (synthesis conditions: hydrothermal temperature 120 ◦ C, n(SiO2 /Al2 O3 ) = 2.0).
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2016.11. 041. References
From the above results, it can be seen that temperatures and pressure, n(SiO2 /Al2 O3 ) ratios, and hydrothermal time played critical roles in the zeolite synthesis. In most cases, the resulting products are mixtures of various zeolites, e.g. mixture of NaA, NaP1 and Faujasite, mixture of NaP1 and Hydroxysodalite. However, zeolite with a single phase can also be obtained, e.g. NaA. The synthesized zeolites can be applied in various fields, such as adsorbent in wastewater treatment [62], media for hydrogen storage [63], catalyst [64], etc. Before a zeolite being used for a certain application, it’s necessary to characterize it to see if it has the desired properties for that application. In this wok, the fate of lead was investigated. The total content of lead in the raw CRT funnel glass was 23.10 wt.%. The leaching rate of lead reached 85.8% and 99.4% of the lead in leachate could be precipitated by sodium sulfide. The silicon-rich liquid is almost lead-free, resulting in little lead remains in the synthesized products. The lead present in the synthesized products was below 0.3 wt.% and entangled tightly in the three-dimensional network of zeolite. Toxicity Characteristic Leaching Procedure (TCLP) experiment for the synthesized zeolites had been carried out to characterize lead content in the leachate. The results revealed that they were below 1.0 mg/L, which did not exceed the regulatory limit of 5.0 mg/L for TCLP lead [65]. It is worth noting that, the practical applications of zeolites depended on their characteristics and are future issue.
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Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathode-ray-tube funnel glass Zhitong Yao a , Daidai Wu b,∗ , Jie Liu a , Weihong Wu a , Hongting Zhao a , Junhong Tang a,∗ a b
College of Materials Science and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
h i g h l i g h t s • An open-loop recycling method for CRT funnel glass was developed. • The influence of various factors on the resulting products were investigated. • Successive transformation among NaA, NaP1, Faujasite and hydroxysodalite was confirmed.
a r t i c l e
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Article history: Received 26 July 2016 Received in revised form 13 November 2016 Accepted 14 November 2016 Available online xxx Keywords: E-waste Cathode ray-tube Leaded glass Open-loop recycling Zeolite synthesis
a b s t r a c t The disposal of waste cathode ray-tubes (CRTs) from old televisions and discarded computer monitors has become a major environmental concern worldwide. In this work, an open-loop recycling method was developed to synthesize zeolites using CRT funnel glass as the raw material. The effects of hydrothermal temperatures and pressure, n(SiO2 /Al2 O3 ) molar ratios and hydrothermal time on the resulting products were investigated. The results indicated that hydrothermal temperatures and pressure played critical roles in zeolite synthesis. Amorphous phases were detected at lower temperatures (80–100 ◦ C) and pressure (0.47–1.01 bar) with n(SiO2 /Al2 O3 ) = 2.0. At the temperature of 110 ◦ C (pressure 1.43 bar), NaA formed with a mixture of NaP1 and Faujasite. With further increase in the temperature and pressure, the unstable NaA and Faujasite disappeared, and Hydroxysodalite developed. The influence of n(SiO2 /Al2 O3 ) ratios on resulting products revealed a single phase of NaA was formed at the ratio of 1.5 and a mixture of NaA and Faujasite at the ratio of 2.0. Prolonging hydrothermal time, however, could promote zeolite crystallization, and NaA gradually developed with an increase in the time from 2 to 6 h at n(SiO2 /Al2 O3 ) = 1.5. By comparison, crystallization phases were observed only when the time was longer than 8 h at n(SiO2 /Al2 O3 ) = 2.0. © 2016 Published by Elsevier B.V.
1. Introduction With the rapid innovations in electronic display technology during the last decade or so, conventional cathode ray-tubes (CRTs) have been gradually replaced by advanced display products, such as liquid-crystal displays (LCDs), plasma display panels (PDPs) and organic light-emitting diode (OLEDs) displays, resulting in a large number of waste CRTs that need to be disposed of. According to the waste electrical and electronic equipment (WEEE) collection and pretreatment market, approximately 50,000–150,000 tons/a of end-of-life CRTs are currently collected in Europe, and this volume is not expected to decrease any time in the next few years [1].
∗ Corresponding authors. E-mail addresses: [email protected] (D. Wu), tang [email protected] (J. Tang).
The recycling and dismantling amounts of waste electrical appliances in China came to 56 million units in 2010, of which 80% were CRT monitors [2]. CRTs are generally made of four different types of glass—panel, funnel, neck and frit junction—with varying chemical compositions and properties [3]. Among these glass types, funnel glass has been confirmed as having hazardous characteristics, because of its high level of lead content [4–7]. However, most discarded CRTs currently end up concentrated in landfills or e-waste recycling centers, and only a small portion of them are recycled. A long-range plan is needed, to find improved recycling methods for this waste. Generally, there are two principal ways of recycling CRT glass: closed-loop and open-loop recycling [8–10]. In closed-loop recycling, waste CRT glass is commonly used as raw material to manufacture new CRTs. This approach has the advantage of economic benefit and a high resource utilization rate. However, most
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CRT manufacturers (e.g. Ancai Hi-tech Co., Ltd., and IRICO Group Electronics Co., Ltd., in China) have gradually ceased or restructured the funnel manufacturing facilities of their CRT operations, since the demand for new CRT monitors is rapidly shrinking. Consequently, attention has shifted to open-loop recycling. A number of projects have been undertaken to use CRT glass as raw material for the production of foam glass [11,12], glass ceramic [13,14], ceramic glazes [15], stoneware tile [16,17] and cement mortar [18,19]. Nevertheless, these approaches are problematic from the point of view of environmental and human safety, because the regenerated products still contain toxic heavy metals. Therefore, many attempts have been made to recover lead from leaded glass. Saterlay et al. [20] used ultrasound to facilitate lead leaching from CRT glass, achieving a removal rate of over 90% of the leachable lead. Yuan et al. [21] applied mechanical activation to pretreat CRT funnel glass, followed by diluted nitric acid leaching, and a high yield of 92.5% of the lead was achieved. Lu et al. [22] recovered lead from CRT funnel glass by thermal reduction with metallic iron, and 58 wt.% lead extraction was achieved. Xing and Zhang [23] extracted nano-lead particles by retrofitting the pyro-vacuum process; the leaded glass was thus converted into harmless glass. Erzat and Zhang [24] also used chloride volatilization to recover lead from CRT funnel glass. From the above literatures, it can be seen that lead can be effectively removed from leaded glass using recent advanced technologies. However, little attention has been paid to the comprehensive utilization of other fractions (e.g. silicon and aluminum) in the leaded glass. Hu et al. [25] prepared sodium silicate frit by melting leaded glass with sodium carbonate and then dissolving it and precipitating lead to obtain a sodium-silicate-rich solution. Okada and coauthors [26–29] treated funnel glass by a process combining reduction melting to recover lead from glass. The oxide phase generated by the thermal treatment was subjected to water and acid leaching for SiO2 purification. In this work, we attempted to prepare zeolites using CRT funnel glass as the raw material. The effects of hydrothermal temperatures and pressure, n(SiO2 /Al2 O3 ) molar ratios and hydrothermal time on zeolite synthesis were investigated. The aim was to develop an innovative technology for CRT funnel glass recycling and thus solve the significant environmental problems associated with leaded glass. 2. Experimental materials and methods 2.1. Materials Raw CRT funnel glass was obtained from Henan Ancai Hi-tech Co., Ltd. The glass block was first crashed into small pieces, ground with a P-7 planetary ball mill (Fritsch, Idar-Oberstein, Germany), and then sieved to achieve a particle size smaller than 200 m. Its chemical composition was determined using X-ray fluorescence spectroscopy (SXF-1200, Shimadzu, Japan) and the resulting contents were (wt.%): SiO2 53.90, PbO 23.10, K2 O 7.59, Na2 O 5.81, CaO 3.06, Al2 O3 3.03, and others 3.51. 2.2. Zeolite synthesis Zeolite synthesis includes three major procedures: leaching, purification and hydrothermal synthesis. 1) Before leaching, batch experiments were preliminarily carried out to determine the optimal conditions. Leaded glass powder and sodium hydroxide solution (6 mol/L), in a solid/liquid ratio of 1:8, were mixed and put into a 200-mL autoclave made of stainless steel. The reaction temperature and time were set as 220 ◦ C and 8 h, respectively. After treatment, the solid-liquid separation was performed by vacuum filtration and a filtrate was obtained. 2) The lead and silicon concentrations in filtrate were measured and the leaching rate of them
were determined as 85.8 and 90.1%, respectively. Sodium sulfide was gradually added according to the molar ratio of Pb:S = 1.0:1.5 to remove the hazardous lead. The dark brown precipitate was separated through centrifugation and lead removal rate reached 99.4%. The silicon-rich and lead-free liquid was thus obtained. 3) Silicon and aluminum in the concentrated liquid were measured and their molar ratios of SiO2 and Al2 O3 adjusted, using sodium silicate and sodium aluminate. The precursors were aged at 60 ◦ C in a water bath for 0.5 h and then put into the autoclave. The hydrothermal temperature and time were set as 80–210 ◦ C and 2–12 h, respectively. At the end of the process, the solid was separated by vacuum filtration with a Büchner funnel, washed five times with distilled water and dried overnight at 100 ◦ C. The dried products were then analyzed using X-ray diffraction (XRD), Fourier transform IR spectroscopy (FTIR) and scanning electron microscopy (SEM). 2.3. Characterization and testing The obtained products were characterized by various conventional methods. XRD analysis was employed to determine the crystalline phases present in the zeolitic products. A Rigaku DMax/IIB X-ray power diffractometer was operated at 40 kV and 30 mA, with CuK␣ as the radiation source. The detector was scanned at a step scan of 0.02◦ and a scan speed of 4◦ /min. Scanning electron microscopy (SEM) investigations were conducted in an S-3400N scanning electron microscope (Hitachi, Japan) at an accelerating voltage of 15 kV to observe the microstructure of the samples. Transmission IR spectra were recorded with a Nicolet Nexus670 Fourier transform infrared spectrometer with a resolution of 2 cm−1 using the KBr disc method. The concentrations of lead, aluminum and silicon in the liquid were measured with a Perkin Elmer Optima 8300 DV (Perkin Elmer, Shelton CT) ICP-OES. 3. Results and discussion 3.1. The influence of hydrothermal temperatures and pressure The influence of varying hydrothermal temperatures and pressure on zeolite synthesis at n(SiO2 /Al2 O3 ) = 2.0 is plotted in Fig. 1. It was clear that lower temperatures (80–100 ◦ C) and pressure (0.47–1.01 bar) were not helpful for zeolite crystallization, and amorphous phases were detected. Increasing the temperature and pressure further resulted in the decrease of amorphous phases and promoted the growth of crystal zeolites, indicating the critical role of temperature and pressure in the zeolite crystallization [30,31]. At the temperature of 110 ◦ C (pressure 1.43 bar), the major phase NaA (NaAlSiO4 ·2.25H2 O, JPCDS card no. 39-0222) [32,33] developed with a mixture of NaP1 (Na3.6 Al3.6 Si12.4 O32 ·14H2 O, JPCDS card no. 39-0219) [34,35] and Faujasite (Na2 Al2 Si2.4 O8.8 ·6.7H2 O, JPCDS card no. 12-0246) [36,37]. Increasing the temperature still more, to 140 ◦ C (pressure 3.61 bar), caused the diffraction peak intensity of the three zeolites to increase, especially for the Faujasite. Yet increasing the temperature from 140 to 160 ◦ C (pressure 6.18 bar) resulted in even more development of zeolite NaP1, compared to Faujasite. When the temperature was increased to 180 ◦ C (pressure 10.02 bar), the product was still confirmed by a mixture of the three zeolites. However, the intensity of NaP1 reflections increased at the expense of NaA and Faujasite. The two unstable phases completely disappeared at 190 ◦ C (pressure 12.54 bar), accompanied by the formation of Hydroxysodalite (Na6 (AlSiO4 )6 ·4H2 O, JPCDS card no. 42-0216) [38,39]. Further increasing the temperature to 210 ◦ C (pressure 19.06 bar) caused the intensity of Hydroxysodalite reflections to increase at the expense of NaP1. This zeolite transformation coincided with Ostwald’s rule of successive transformation [40,41]. With the rise in hydrothermal temperature and pressure,
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an increase in supersaturation was achieved as a result of the higher proportion of soluble species. The higher the supersaturation, the better the conditions to nucleate metastable phases [42], such as NaA, Faujasite and NaP1, which later recrystallized and were replaced by the more stable Hydroxysodalite. SEM investigations were conducted to explore the morphology evolution of the samples with hydrothermal temperature and pressure variations (Fig. 2). These revealed that at 130 ◦ C (pres-
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sure 2.70 bar) the produced particles took on a predominantly sharp cubic shape with a smooth surface texture. Some octahedral (red circle) and spherical particles with protuberant grains on the surface (blue circle) were also observed. According to the above XRD results, the product was a mixture of NaA, NaP1 and Faujasite. Hence, the cubic particles could be ascribed to NaA [43–45], the octahedral particles to Faujasite [46–48] and the spherical particles to NaP1 [49,50]. At 140–160 ◦ C (pressure 3.61–6.18 bar), the
Fig 1. XRD patterns of products at different temperature and pressure (synthesis conditions: hydrothermal time 12 h, n(SiO2 /Al2 O3 ) = 2.0).
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Fig 1. (Continued)
sample particles were also predominantly cubic, however, dominately spherical accompanied with cubic and octahedral at higher temperature 170 ◦ C (pressure 7.92 bar) and 180 ◦ C (10.02 bar). The dissolution of NaA could be observed (orange circle), confirming Ostwald’s rule of successive transformation. At 190–210 ◦ C (pressure 12.54–19.06 bar), more ball-of-yarn-like particles and dense spherical particles with irregular blocks on the surface [51,52] were observed. The dissolution of NaP1 can also be observed (black circle). The above XRD results indicated that the materials were mixtures of NaP1 and Hydroxysodalite, so that the ball-of-yarn-like and dense spherical particles could be assignable to Hydroxysodalite. Fig. S1 shows the FTIR transmittance spectra of samples at temperatures of 130–210 ◦ C and pressure of 2.70–19.06 bar. The
spectrum of the material formed at 130 ◦ C (pressure 2.70 bar) possessed eight absorption bands. Those at 3460 and 1650 cm−1 were attributed to O H stretching and bending, respectively. The bands located at 987, 669 and 463 cm−1 were assigned to the asymmetric stretching vibration of internal tetrahedral, the symmetric stretching vibration and bending vibration modes of T O bonds in TO4 tetrahedra, respectively (T = Si or Al) [53]. The band at 555 cm−1 can be attributed to D4R, because NaA was formed at this temperature [54]. The band at 742 cm−1 was assigned to the symmetric stretching vibration of external linkages [55]. With the temperature increase, the intensity of peaks at 3460, 1650, and 987 cm−1 decreased and these bands steeply shifted to high wavenumbers [43,56,57]. It has been well recognized that the length of the AlO bond is longer than that of the Si-O bond, and the substitution
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Fig. 2. SEM images of products at different temperature and pressure (synthesis conditions: hydrothermal time 12 h, n(SiO2 /Al2 O3 ) = 2.0).
of tetrahedral Al for Si in aluminosilicates frameworks induces the low wave-number shift of the stretching vibration of the T-O band. Therefore, the high wavenumber shift was attributed to the decrease of Al concentration incorporated into the zeolites [58]. At 190–210 ◦ C (pressure 12.54–19.06 bar), new peaks at 1460 cm−1 were observed. The differences in structure between NaA, Faujasite and Hydroxysodalite were essentially the differences in the mode of linkage of the sodalite unit in building up the framework structure of the zeolites. Therefore, the new band detected could be attributed to the formation of Hydroxysodalite [59].
3.2. The influence of n(SiO2 /Al2 O3 ) ratios A series of experiments were undertaken to determine the influence of n(SiO2 /Al2 O3 ) ratios on the resulting products (see Fig. 3). It was found that larger n(SiO2 /Al2 O3 ) ratios were not helpful for
zeolite crystallization. A single phase of NaA was formed at the ratio of 1.5 and a mixture of NaA and Faujasite at the ratio of 2.0, which was consistent with a previous report [43]. To obtain further insight into the influence of n(SiO2 /Al2 O3 ) ratios, the XRD patterns were plotted as a function of larger ratios. It can be observed that no crystallization phases were detected and the amorphous phases increased when the ratio further increased from 2.2 to 5.5. To confirm the particle morphology, SEM observation of the products formed at various n(SiO2 /Al2 O3 ) ratios was carried out. According to the above XRD results, the zeolite phases developed only at the ratio of 1.5 and 2.0, and thus the two resulting products are included in Fig. 4. These results revealed that the individual zeolite crystals at the ratio of 1.5 were predominantly cubic in shape, interlocked and piling up on each other. As described before, a single phase of NaA was formed at this ratio. Hence, the cubic particles could be ascribed to NaA. The product at the ratio of 2.0
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Fig. 3. XRD patterns of products at different n(SiO2 /Al2 O3 ) ratios (synthesis conditions: hydrothermal temperature 120 ◦ C, time 10 h).
also predominately exhibited as cubic shapes with rough surfaces. However, octahedral particles were observed (blue arrow) as well. The XRD result shown in Fig. 3 indicated that the material was composed of NaA and Faujasite, so that the octahedral particle could be assignable to Faujasite [60,61]. The FTIR transmittance spectra are presented in Fig. S2. The broad peaks located at 3440 and 1690 cm−1 for product at n(SiO2 /Al2 O3 ) = 1.5, 3450 and 1660 cm−1 at n(SiO2 /Al2 O3 ) = 2.0 were attributed to O H stretching and bending, respectively. The bands at 997 and 1000 cm−1 were assigned to the asymmetric stretching vibration of Si O Si bonds within SiO4 . The band located at 671 cm−1 was known to belong to Si O Si symmetric stretching vibration, while the bands 580 and 557 cm−1 were due to the bending vibration of Si O Al linkage.
3.3. The influence of hydrothermal time Figs. 5 and 6 illustrate the evolution of zeolite synthesis as a function of time at various n(SiO2 /Al2 O3 ) ratios in the thermal process. It was found that prolonging the hydrothermal time could promote zeolite crystallization. Prolonging the time from 2 to 6 h at n(SiO2 /Al2 O3 ) = 1.5 caused the amorphous phases to decrease. When the time was further prolonged from 8 to 10 h, NaA gradually developed, with its diffraction peaks increasing. For zeolite synthesis at n(SiO2 /Al2 O3 ) = 2.0, similar influences of time were observed. However, the crystallization phase was observed only when the time was longer than 8 h, and the zeolite product was a mixture of NaA and Faujasite.
Fig. 4. SEM images of products at different n(SiO2 /Al2 O3 ) ratios (synthesis conditions: hydrothermal temperature 120 ◦ C, time 10 h).
Please cite this article in press as: Z. Yao, et al., Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathoderay-tube funnel glass, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.11.041
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4. Conclusion
Fig. 5. XRD patterns of products at different hydrothermal time (synthesis conditions: hydrothermal temperature 120 ◦ C, n(SiO2 /Al2 O3 ) = 1.5).
An open-loop recycling approach was proposed to prepare zeolites using CRT funnel glass as the raw material. This approach consisted of three procedures: leaching, purification and hydrothermal synthesis. The results indicated that hydrothermal temperature and pressure played an important role in zeolite synthesis. When the temperature increased to 90 ◦ C (pressure 0.70 bar), the amorphous phases decreased significantly and NaA developed at n(SiO2 /Al2 O3 ) = 1.5. At n(SiO2 /Al2 O3 ) = 2.0, NaA developed with a mixture of NaP1 and Faujasite at 110 ◦ C (pressure 1.43 bar). With a further increase in the temperature and pressure, NaA and Faujasite disappeared and the new phase Hydroxysodalite developed. SEM observation confirmed the dissolution of zeolite A and NaP1 at 180 ◦ C (10.02 bar) and 200 ◦ C (15.54 bar), respectively. Furthermore, the intensity of the peaks at 3460, 1650, and 987 cm−1 decreased and these bands steeply shifted to high wave numbers. At 190–210 ◦ C (12.54–19.06 bar), new peaks located at 1460 cm−1 were observed, which could be attributed to the formation of Hydroxysodalite. The influence of n(SiO2 /Al2 O3 ) ratios on the resulting products revealed that a single phase of zeolite A was formed at the ratio of 1.5, and a mixture of NaA and Faujasite at the ratio of 2.0. However, prolonging hydrothermal time could further promote zeolite crystallization, and zeolite A gradually developed when prolonging the time from 2 to 6 h at n(SiO2 /Al2 O3 ) = 1.5. By comparison, the crystallization phases were observed only when the time was longer than 8 h at n(SiO2 /Al2 O3 ) = 2.0. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant no. 41373121, 41101213 and 41271249). Appendix A. Supplementary data
Fig. 6. XRD patterns of products at different hydrothermal time (synthesis conditions: hydrothermal temperature 120 ◦ C, n(SiO2 /Al2 O3 ) = 2.0).
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2016.11. 041. References
From the above results, it can be seen that temperatures and pressure, n(SiO2 /Al2 O3 ) ratios, and hydrothermal time played critical roles in the zeolite synthesis. In most cases, the resulting products are mixtures of various zeolites, e.g. mixture of NaA, NaP1 and Faujasite, mixture of NaP1 and Hydroxysodalite. However, zeolite with a single phase can also be obtained, e.g. NaA. The synthesized zeolites can be applied in various fields, such as adsorbent in wastewater treatment [62], media for hydrogen storage [63], catalyst [64], etc. Before a zeolite being used for a certain application, it’s necessary to characterize it to see if it has the desired properties for that application. In this wok, the fate of lead was investigated. The total content of lead in the raw CRT funnel glass was 23.10 wt.%. The leaching rate of lead reached 85.8% and 99.4% of the lead in leachate could be precipitated by sodium sulfide. The silicon-rich liquid is almost lead-free, resulting in little lead remains in the synthesized products. The lead present in the synthesized products was below 0.3 wt.% and entangled tightly in the three-dimensional network of zeolite. Toxicity Characteristic Leaching Procedure (TCLP) experiment for the synthesized zeolites had been carried out to characterize lead content in the leachate. The results revealed that they were below 1.0 mg/L, which did not exceed the regulatory limit of 5.0 mg/L for TCLP lead [65]. It is worth noting that, the practical applications of zeolites depended on their characteristics and are future issue.
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Please cite this article in press as: Z. Yao, et al., Recycling of typical difficult-to-treat e-waste: Synthesize zeolites from waste cathoderay-tube funnel glass, J. Hazard. Mater. (2016), http://dx.doi.org/10.1016/j.jhazmat.2016.11.041