Research Note pubs.acs.org/IECR
Recycling of SiC−Si Sludge to Silicon Tetrachloride and Porous Carbon via Chlorination Kyun Young Park,*,† Hoey Kyung Park,† Bong Whan Ko,† Tae Won Kang,† and Hee Dong Jang‡ †
Department of Chemical Engineering, Kongju National University, 275 Budae-dong, Seobuk-gu, Cheonan, Chungnam 331-717, Republic of Korea ‡ Rare Metals Research Center, Korea Institute of Geoscience and Mineral Resources (KIGAM), Gwahang-no 124, Yuseong-gu, Daejeon 305-350, Republic of Korea ABSTRACT: A SiC−Si sludge composed of 79.4 wt % SiC, 14.2 wt % Si, and 5.06 wt % Fe was chlorinated in a tubular reactor with nitrogen as the carrier gas. The chlorination temperature was varied from 100 °C to 1200 °C, the time was varied from 1 h to 8 h, and the chlorine mole fraction was varied from 2.5% to 10.0%, while the flow rate of the carrier gas was fixed at 300 mL/ min. The Si was converted to SiCl4, the SiC was converted to SiCl4 and porous carbon, and the Fe was converted to FeCl3. The Fe and Si were more reactive to chlorination than the SiC. The conversion was nearly complete at 900 °C, 4 h, and 10% chlorine. The purity of the obtained SiCl4 was 99.7% and the surface area of the resulting carbon was 860 m2/g. The surface area was invariable as the temperature was further increased to 1200 °C. KEYWORDS: silicon sludge, chlorination, silicon tetrachloride, porous carbon
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INTRODUCTION
the conversion to SiCl4, and on the nature of the carbon left behind, were investigated.
A large amount of silicon sludge is produced during the slicing of silicon ingots into thin wafers used for integrated circuit (IC) chips and solar cells. Approximately 40% of a silicon ingot is lost as saw dust and included in the sludge.1 The other components of the sludge are the abrasives and the abrasivecarrying oils, usually polyethylene glycol (PEG), that were supplied through nozzles over the steel wire saw. SiC is known to be the most commonly used abrasive. Various methods have been proposed to recover the silicon, SiC, and oil in the sludge. The oil can be separated from the sludge by filtration,2 solvent extraction,3 or evaporation.4 The oil-free sludge, which is a mixture of silicon and SiC particles, can be further processed to separate the silicon from the SiC by centrifugation.5 The purities of the silicon and SiC concentrates, however, are not high enough for them to be recycling. The silicon concentrate can be leached with acid to improve the purity, for use as metallurgical- and solar-grade silicons.6,7 However, the SiC concentrate has rarely been studied for utilization, although it is more voluminous than the silicon concentrate. In the present study, a SiC concentrate derived from the silicon sludge (referenced hereafter as SiC−Si sludge) was chlorinated to produce silicon tetrachloride (SiCl4) and porous carbon. The SiCl4 can be used as raw material for fumed silica, optical fibers, and silica films in semiconductor industries. The porous carbon may find applications in adsorbents, hydrogen storage materials, and carbon electrodes.8 The SiC−Si sludge contains SiC, Fe, Si and other trace elements. Although the chlorination of pure SiC was published earlier,9 the chlorination of the materials containing Fe and Si other than SiC is new. The chlorination was carried out in a tubular reactor with varying chlorine concentration, gas flow rate, chlorination temperature, and time. The effects of the operating variables on © 2013 American Chemical Society
1. EXPERIMENTAL SECTION The SiC−Si sludge used for chlorination was derived from a waste provided by a silicon wafer manufacturer. It is particulate and composed of SiC (79.4 wt %), Si (14.2 wt %), Fe (5.06 wt %), and other minor components, as shown in Table 1. Figure 1 shows the particle size distribution of the SiC−Si sludge; the size is distributed in the range of 0.1−30 μm, with the volumeaverage diameter of 5.4 μm. Table 1. Chemical Analysis of SiC−Si Sludge component
amount (wt %)
SiC Si Fe Al Cr Cu others
79.4 14.2 5.06 0.17 0.11 0.07 0.99
The chlorination system consists of two mass flow meters, a dehumidifier (Alltech, Model 60015), an alumina tube (2.4 cm in diameter and 32 cm in length), an electric heater surrounding the tube, a membrane filter, a condenser, and a NaOH flask. One gram (1 g) of the SiC−Si sludge was spread on the bottom of a ceramic boat to form a shallow bed ∼2 mm in depth. The boat loaded with the sludge was then pushed into the alumina tube and placed at the midpoint between the inlet and outlet. The tube was heated to a set temperature in a flow of nitrogen. When the set temperature was reached, a volume Published: February 20, 2013 3943
dx.doi.org/10.1021/ie302699g | Ind. Eng. Chem. Res. 2013, 52, 3943−3946
Industrial & Engineering Chemistry Research
Research Note
the mass. At higher temperatures, the FeCl3 is removed as a vapor from the boat, decreasing the mass. The residual mass after a 4 h of chlorination was measured with reactor temperature being varied from 100 °C to 1200 °C. The gas flow rate was held at 300 cm3/min; a higher flow rate elutriated particles in the boat. The chlorine content in the gas was maintained at 10 vol %. The conversion (X) can be determined from the residual mass if all the metal chlorides formed by chlorination are volatile: m −m X= 0 mM,0 (4) where m0 is the initial mass of the SiC−Si sludge charged to the boat, m the residual mass, and mM,0 the mass of metallic elements initially present in the charge.
Figure 1. Particle size distribution of SiC−Si sludge. The parameter dM/M along the vertical axis represents the mass fraction of the particles at a given size.
mM,0 = m0(x Fe + xSi + 0.7xSiC)
Here, xFe, xSi, and xSiC represent the mass fractions of Fe, Si, and SiC, respectively, in Table 1. The constant of 0.7 in eq 5 is the mass fraction of Si in SiC. The contribution of the minor components to the conversion is neglected. The conversions determined by eq 4 are shown in Figure 2. A lower limit in temperature was set at 400 °C; above this temperature, any of the produced metal chlorides is volatile enough to make eq 4 valid.
of chlorine was introduced into the nitrogen at the inlet of the dehumidifier. The flow rate of the chlorine was controlled to match the mole fraction of chlorine in the gas entering the reaction tube. Any particles in the gas exiting the reaction tube were collected in the filter. The SiCl4 vapor produced was condensed with dry ice. The excess chlorine was captured in the NaOH flask. The chlorination was terminated by closing the chlorine flow valve and turning off the heater simultaneously. The tube was then cooled to ambient temperature in a flow of nitrogen gas. The residue in the boat was taken out, weighed, and analyzed. Scanning electron microscopy coupled with energy-dispersive spectroscopy (Tescan, Model Mira LMH) was used for morphology and semiquantitative elemental composition, transmission electron microscopy (TEM) (JEOL, Model JEM3000) was used for imaging internal structures, and X-ray diffraction (XRD) (SCINCO, Model SMD 3000) was used to examine the crystalline structure. The porous properties of the residues left behind after chlorination were investigated with a Brunauer−Emmett−Teller (BET) analyzer (Quantachrome Instruments, Model Quadrasorb SI) and the ignition loss was measured using a thermogravimetric balance (TA Instruments, Model SDT2960).
Figure 2. Conversion of SiC−Si sludge after 4 h of chlorination at temperatures higher than 400 °C. The gas flow rate and the chlorine mole fraction were fixed at 300 cm3/min and 0.1, respectively.
2. RESULTS AND DISCUSSION 2.1. Conversion. The chlorination of the SiC−Si sludge can be represented by SiC + 2Cl 2 = SiCl4 + C (1) Si + 2Cl 2 = SiCl4
(5)
The conversion increased as the reactor temperature increased, reaching 30% at 400 °C, and it remained nearly constant between 400 °C and 600 °C. A steep increase started at 600 °C and ended at 900 °C, beyond which the conversion leveled out. The plateau between 400 °C and 600 °C implies that the materials composing the SiC−Si sludge differ in reactivity. In the plateau zone, the more-reactive material has been nearly exhausted and the less-reactive one is not yet active enough. A pure SiC similar in size to the SiC−Si sludge under study was chlorinated. The pure SiC remained nearly intact until 600 °C. This indicates that the conversion prior to the inception of the plateau zone is due to the silicon, iron, and other minor components that are more reactive than SiC. There is a recent report that silicon is more reactive to chlorination than silicon carbide.10 The selective removal of Si and Fe in the early stage is validated by XRD, which will be discussed later. The ultimate residual mass with the SiC−Si
(2)
3 Cl 2 = FeCl3 (3) 2 The boiling points of SiCl4 and FeCl3 are 57.7 °C and 315 °C, respectively. Under the reactor temperature, ranging from 100 °C to 1200 °C, the chlorination of Si or SiC would bring a loss in mass of the SiC−Si sludge as the silicon is removed in the form of silicon tetrachloride vapor. The chlorination of Fe results in a gain or a loss, depending on the reactor temperature. At reactor temperatures lower than the boiling point of FeCl3, the chlorinated product (FeCl3) is a solid higher in molecular weight than Fe and resides in the boat to increase Fe +
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Research Note
sludge was ∼23%, which is 7% less than that with the pure SiC. This lower ultimate mass is due to the metals in the concentrate; the metals are removed by chlorination without forming any residue, while the SiC leaves carbon behind. Figure 3 shows the variation of conversion as the mole fraction of
behind the chlorination of SiC. No significant loss can be seen thereafter. A white powder weighing 0.38 mg (or 2.1 wt % of that charged to the pan) remained at the end. The powder was assigned to silica by energy-dispersive spectroscopy. Figure 4 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the carbon
Figure 4. Electron microscopic images of the porous carbon obtained with reactor temperature at 900 °C: (a) scanning electron microscopy (SEM) image and (b) transmission electron microscopy (TEM) image.
Figure 3. Conversion of SiC−Si sludge at 900 °C with varying chlorine concentration. The reaction time and the gas flow rate were fixed at 4 h and 300 cm3/min, respectively.
obtained by chlorination at 900° for 4 h. The morphology and size of the particles of the SiC−Si sludge charged to the reactor was nearly preserved. No pores can be seen in the SEM image. However, a TEM image of the edge of a particle shows the presence of nanosized pores inside the particle that were formed as the silicon was extracted from SiC by chlorination. X-ray diffraction (XRD) spectra of the residues were investigated for three reactor temperatures (500, 900, and 1100 °C), with the spectra of the virgin SiC−Si sludge as a reference. The Si and Fe peaks present in the SiC−Si sludge disappeared at 500 °C and the SiC peak disappeared at 900 °C. This indicates that Si and Fe are more reactive than SiC. As the temperature was further increased to 1100 °C, new peaks assigned to SiO2 and Si3N4 appeared. The Si3N4 peak disappeared as the carrier gas was changed from nitrogen to argon. The appearance of a silica (SiO2) peak is probably due to the SiO2 layer that may have been present initially on the silicon and to the SiO2 that was formed later via hydrolysis or oxidation of the silicon by oxygen or water vapor present as an impurity in the carrier gas. The characteristic peaks of graphite in the vicinity of 26° and 44° that were reported for carbons derived from pure SiC11 were absent, indicating that the carbon resulting from the SiC in the SiC−Si sludge has poor crystallinity. Figure 5 shows the variation of the BET surface area of the residue with reactor temperature ranging from 300 °C to 1200 °C. The surface area was 5.6 m2/g at 300 °C and increased to 14 m2/g at 600 °C. The increase between 300 °C and 600 °C was probably governed by the chlorination of the silicon particles. The decrease in particle diameter due to the removal of silicon by chlorination increased the specific surface area. There is no creation of pores with the chlorination of silicon particles, which resulted in only a moderate increase in surface area. The slope of increase in surface area turned remarkably steeper at 600 °C, at which point the chlorination of the SiC particles came into play to form microporous structures via the extraction of silicon from the SiC matrix. The surface area reached 860 m2/g at 900 °C, and then remained nearly insensitive to further increases in reactor temperature. The flattening in excess of 900 °C indicates no further creation of pores as the chlorination was nearly completed, as shown
chlorine in the feed gas was varied from 0 to 0.1, holding the reactor temperature at 900 °C and the reaction time at 4 h. The conversion increased as the chlorine mole fraction increased: 66% conversion was observed at 0.05 mole fraction chlorine and nearly 100% conversion was observed at 0.1 mole fraction chlorine. The reaction order, with respect to chlorine, is indeterminable from the data in Figure 3; a separate kinetic study may be necessary to obtain the kinetic parameters, including the reaction order and the activation energy. 2.2. Characterization of Products. The purity of the SiCl4 obtained at a reactor temperature of 900 °C was 99.7%. The impurities are Fe, Al, and Cr, as shown in Table 2. The Fe Table 2. Chemical Analysis of SiCl4 as Produced component
amount (wt %)
Si Fe Al Cr others
99.71 0.26 0.06 0.03 0.03
content in the SiCl4 is much smaller than that in the SiC−Si sludge. This is because a majority of the iron chlorinated into FeCl3 was removed as solid particles in the filter prior to the SiCl4 condenser. The as-prepared SiCl4 can be further purified by fractional distillation taking advantage of the boiling point of SiCl4 being considerably lower than those of the impurities. An amount of 17.8 mg of the residue after reaction at 900 °C was placed in a pan and heated in an air atmosphere to 1200 °C at a rate of 10 °C/min and held at that temperature for 1 h. The mass loss was 5.0% until the temperature reached 100 °C. This loss is probably due to desorption of the materials that were adsorbed on the residue from the air after the residue was taken out of the reactor. No significant change in mass was observed between 100 °C and 500 °C. The mass loss resumed at 500 °C and progressed rapidly to bring the incremental loss of 90% after 20 min. This loss is attributed to burning of the carbon left 3945
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Research Note
increased up to 900 °C, and it was invariable as the temperature was further increased to 1200 °C. The pore size of the carbon obtained at 900 °C was distributed between 1.1 nm and 2.1 nm. The nitridation due to the carrier nitrogen was minimal up to the reactor temperature of 900 °C, but turned significant at 1100 °C. This work will be useful as a stepping stone for further studies on scaleup for industrial application.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the R&D Center for Valuable Recycling (Global-Top Environmental Technology Development Program), funded by the Ministry of Environment (Project No. 11-A08-IR).
Figure 5. Effect of chlorination temperature on BET surface area of the residue. The residue was obtained with a reaction time of 1 h, a gas flow rate of 300 cm3/min, and a chlorine mole fraction of 0.1.
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earlier. The highest surface area of the carbon obtained from the present study is comparable to that derived from a chlorination of pure SiC in an argon atmosphere,12 but is lower than 1558 m2/g that was derived from a SiC infiltrated with free silicon to create secondary porosity.10 In the chlorination of pure carbides, the surface area increased as the reaction temperature decreased.13 By analogy, the specific surface area of the carbon from the SiC−Si sludge may be further increased by reducing the temperature and increasing the reaction time for compensation. However, an increase in reaction time results in a reduction in production capacity. Further studies may be necessary to obtain optimal tradeoff between reaction temperature and time. The pores in the carbon obtained at 900 °C are distributed in the range of 1.1−2.1 nm with the peak at 1.5 nm, as shown in Figure 6.
REFERENCES
(1) Wang, T. Y.; Lin, Y. C.; Tai, C. Y.; Sivakumar, R.; Rai, D. K.; Lan, C. W. A Novel Approach for Recycling of Kerf Loss Silicon from Cutting Slurry Waste for Solar Cell Applications. J. Cryst. Growth 2008, 310, 3403. (2) Zavattari, C.; Fragiacomo, G. A Method for Separating and Regenerating Polyethylene Glycol and Silicon Carbide Abrasive Material to Enable Re-Use Thereof. Eur. Patent EP 0968801, 2000. (3) Lin, Y.; Tai, C. Y. Recovery of Silicon Powder from Kerfs Loss Slurry Using Phase-Transfer Separation Method. Sep. Purif. Technol. 2010, 74, 170. (4) Heinle, E.; Grimm, A.; Rubenbauer, H.; Kurth, R. Process for Reclaiming a Grinding Suspension. U.S. Patent 6,010,010, 2000. (5) Nishijima, S.; Izumi, Y.; Takeda, S.; Suemoto, H.; Nakahira, A.; Horie, S. Recycling of Abrasives from Wasted Slurry by Superconducting Magnetic Separation. IEEE. Trans. Appl. Supercond. 2003, 13, 1596. (6) Kim, J.; Kim, U.; Hwang, K.; Cho, W.; Kim, K. J. Recovery of Metallurgical Silicon from Slurry Waste. J. Kor. Ceram. Soc. 2011, 48, 189. (7) Sahu, S. K.; Asselin, E. Effect of Oxidizing Agents on the Hydrometallurgical Purification of Metallurgical Grade Silicon. Hydrometallurgy 2012, 121−124, 120. (8) Oschatz, M.; Kockrick, E.; Rose, M.; Borchardt, L.; Klein, N.; Senkovska, I.; Freudenberg, T.; Korenblit, Y.; Yushin, G.; Kasel, S. A Cubic Ordered, Mesoporous Carbide-Derived Carbon for Gas and Energy Storage Applications. Carbon 2010, 48, 3987. (9) Lee, A.; Zhu, R.; McNallan, M. Kinetics of Conversion of Silicon Carbide to Carbide Derived Carbon. J. Phys.: Condens. Matter 2006, 18, 1763. (10) Schmirler, M.; Knorr, T.; Fey, T.; Lynen, A.; Greil, P.; Etzold, B. J. M. Fast Production of Monoclinic Carbon-Derived Carbons with Secondary Porosity Produced by Chlorination of Carbides Containing a Free Metal Phase. Carbon 2011, 49, 4359. (11) Smorgonskaya, E. A.; Kyutt, R. N.; Shchukarev, A. V.; Gordeev, S. K.; Grechinskaya, A. V. X-ray Studies of Nanoporous Carbon Powders Produced from Silicon Carbide. Semiconductors 2001, 35, 661. (12) Rufino, B.; Mazerat, S.; Couvart, M.; Lorrette, C.; Maskrot, H.; Pailler, R. The Effect of Particle Size on The Formation and Structure of Carbon Derived Carbon on β-SiC Nanoparticles by Reaction with Chlorine. Carbon 2011, 49, 3073. (13) Presser, V.; Heon, M.; Gogotsi, Y. Carbide-Derived Carbons From Porous Networks to Nanotubes and Graphene. Adv. Funct. Mater. 2011, 21, 810.
Figure 6. Pore size distribution of the carbon obtained with a reactor temperature of 900 °C, a reaction time of 4 h, a gas flow rate of 300 cm3/min, and a chlorine mole fraction of 0.1.
3. CONCLUSION A SiC−Si sludge composed of 79.4 wt % SiC, 14.2 wt % Si, 5.06 wt % Fe, and other minor components was chlorinated in a nitrogen atmosphere to produce 99.7% SiCl4 and a porous carbon with a BET surface area of 860 m2/g. The silicon and iron were more reactive to chlorination than the SiC. At a reaction time of 4 h, the conversion was nearly complete at 900 °C. The surface area increased as the reactor temperature 3946
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