A study of silicon carbide synthesis from waste serpentine

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1 Chemosphere 64 (2006) Technical Note A study of silicon carbide synthesis from waste serpentine T.W. Cheng *, C.W. Hsu Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei, Taiwan, ROC Received 28 July 2005; received in revised form 1 November 2005; accepted 2 November 2005 Available online 6 January Abstract There are tons of serpentine wastes produced in year 2004 in Taiwan. This is due to the well-developed joints in the serpentine ore body as well as the stringent requirements of the particle size and chemical composition of serpentine by iron making company. The waste also creates considerable environmental problems. The purpose of this study is reutilization of waste serpentine to produce a high value silica powder after acid leaching. These siliceous microstructure products obtained from serpentine would be responsible for high reactivity and characteristic molecular sieving effect. In this study, the amorphous silica powder was then synthesized to silicon carbide with the C/SiO 2 molar ratio of 3. The experiment results show that silicon carbide can be synthesized in 1550 C. The formed silicon carbide was whisker b type SiC which can be used as raw materials for industry. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Serpentine; Leaching; Amorphous silica; Silicon carbide 1. Introduction Serpentine, Mg 2 Si 2 O 5 (OH) 4, is a kind of hydrated magnesium silicate. In Taiwan, it was formed from gabbro and olivine during metamorphism. Almost all serpentine mines are located in Eastern Taiwan area. The major applications for serpentine are dimension stone and flux materials. There were 5966 m 3 of dimension stone produced (approximate value US$ ) in 2004, and most of them were exported for construction decoration. The production of serpentine for flux materials was tons (approximate value of US$ ) in the same year, and was mainly supplied to China Steel. According to the estimation of the Ministry of Economic Affairs (1996), the amount of waste serpentine produced in Taiwan amounted to more than tons in Such huge waste production can be attributed to the well-developed joints in the serpentine ore body and the stringent requirements of the particle size and chemical composition of serpentine by China * Corresponding author. Tel.: x2730; fax: address: twcheng@ntut.edu.tw (T.W. Cheng). Steel. The waste also creates considerable environmental problems. From the literature, serpentine waste can be used in various fields, such as fertilizer production, soil amelioration, extraction of amorphous silicate, carbon sequestration, and extraction of pure magnesium compounds; and recycling of serpentine waste has been widely investigated (Lackner et al., 1997; Goff and Lackner, 1998; Kanari et al., 1998; O Connor et al., 2001; Cheng et al., 2002; Kim and Chung, 2003; Pietrikova et al., 2004). However, little research has been made on its use for silica production. Because of its theoretical composition of SiO 2 (34.3%), MgO (44.1%), Fe 2 O 3 (6%), Al 2 O 3 (0.2%) and CaO (0.45%), serpentine can be a raw material for production of silica after acid treatment. It is well known that acid treatment of minerals results in formation of porous material. Therefore, serpentine treated by acid yields porous, reactive and amorphous silica of high purity, which can then be used for synthesis of silicon carbide (SiC). SiC is a useful technical ceramic. It has great mechanical properties, including high wear resistance, high temperature strength, thermal shock resistance, and it is used in abrasives, metallurgical and refractory industries. Generally, /$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi: /j.chemosphere

2 T.W. Cheng, C.W. Hsu / Chemosphere 64 (2006) Table 1 The major chemical composition of serpentine raw material and serpentine residues after extracted in different temperatures (5 M H 2 SO 4 ; 48 h; liquid/solid ratio = 10 ml g 1 ) Composition % SiO 2 MgO Fe 2 O 3 CaO Al 2 O 3 Ig. loss Surface area (m 2 g 1 ) Raw serpentine C C C SiC is manufactured by fusing pure silica sand and finely ground coke in electric furnace at temperatures up to 2000 C or higher. In Taiwan, all SiC is imported from overseas, and the price ranges widely depending on the purity. The purpose of this research work is to treat waste serpentine by acid and the amorphous silica of high reactivity thus obtained is then used for making synthetic SiC at lower temperature. The treated and obtained products were characterized by X-ray diffractometry (XRD) for crystal structure determination, and scanning electron microscopy (SEM) for microstructure evaluation. This could be another way for potential usage of serpentine wastes in Taiwan. Table 2 The peaks used for the I n /I total calculation hkl 2h d (nm) Mineral phase SiC Cristobalite Carbon Other phase a-quartz b-quartz Experimental procedures 2.1. Materials Waste serpentine samples were collected from the undersized (less than 6 mm) raw materials where the oversized ones were supplied to China Steel. The serpentine sample was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and had the major chemical composition as shown in Table 1. After being ground with a hammer mill, the particle size of the waste serpentine ranged from 0.8 to 418 lm with D 50 of 21.8 lm. Nano-grade 98% carbon black of 24 nm particle size was used for SiC synthesis Methods The experimental procedures involved two steps. The first step was the extraction test. H 2 SO 4 was used for serpentine extraction to obtain amorphous silica powder of high purity and large surface area. The second step was SiC synthesis through mixing amorphous silica powder and carbon black at different molar ratios, and sintering at certain temperatures. Then 100 ml of H 2 SO 4 and waste serpentine of different concentrations were weighed and put them together into a glass tube. The acid/solid ratio was kept at 10 ml g 1. The glass tube was then put into a hybridization oven for extraction at various temperatures and for various durations. After extraction, the liquid and residue were separated by centrifugation at 5000 rpm for 15 min. The residues were then washed with deionized water until the ph value reached neutrality. The sample was dried at 105 C and mixed with carbon black at various molar Fig. 1. Preparation of amorphous silica by treating serpentine with acid at liquid/solid ratio of 10 ml g 1. (a) The effect of extraction temperature to the structures of the residue powders (5 M H 2 SO 4 ; 24 h); (b) the dependence of H 2 SO 4 concentration on the structures of the residue powders (90 C; 24 h); (c) the effect of extraction time to the serpentine structure (5 M H 2 SO 4 ;80 C).

3 512 T.W. Cheng, C.W. Hsu / Chemosphere 64 (2006) ratios using ball mill for 1 h. The mixed sample was pressed at 100 kgf cm 2 pressure without using any binder. The green body thus formed was subsequently sealed into a graphite crucible and sintered at different temperatures for synthesis of SiC. Following the high-temperature treatment process, the samples were ground to 74 lm for XRD (Rigaku D/MAX-VB diffractometer with CuKa radiation) determination and the phase transformation characteristics were evaluated (Cheng et al., 2002). The relative proportions of the phases as a function of C/SiO 2 molar ratio, sintering temperature, or sintering time were estimated from the ratio I n /I total, where I n is the intensity of a chosen X-ray peak for each phase and I total is the sum of the intensities of all peaks. The peaks used for calculating the I n /I total are shown in Table Results and discussion 3.1. Preparation of amorphous silica by treating serpentine with acid The effect of extraction temperature on the structures of the serpentine residue powders is shown in Fig. 1. As can be seen (Fig. 1a), after 60 C extraction, the structure of serpentine is still perfect. The peak intensity of antigorite can reach 2000 counts. However, when the extraction temperature increases up to 80 C, the serpentine structure was destroyed and showed partly amorphous phase. Fig. 1b shows the XRD result of the dependence of H 2 SO 4 concentration on the structures of the residue powders. When the H 2 SO 4 concentration increases, the peak intensity of antigorite and clinoclore decreases. When the H 2 SO 4 concentration exceeds 5 M, only small peaks can be found with the structure of serpentine destroyed and amorphous hydrated silica formed. Fig. 1c illustrates the effect of extraction time on the serpentine structure. As can be seen, the degree of serpentine structure destruction is timedependence. After 72 h of extraction, only a small amount of antigorite and magnetite peaks remain. The major chemical composition of serpentine residues after being extracted in 5 M H 2 SO 4 for 48 h at different temperatures is also displayed in Table 1. It shows that the amount of SiO 2 contained in the residues increases with increasing extraction temperatures. The increase in content of SiO 2 can be from 41% to 91%. Apart from the loss of ignition, the purity of SiO 2 in the residue can reach around 97%. SEM examination results reveal the microstructural evolution, as shown in Fig. 2. As can be seen, with increasing extraction time and temperature, the increase in serpentine surface corrosion leads to growth in the amount of amorphous silica. Table 1 shows that the surface area of serpentine particles decreases extensively with increasing extraction temperature. The reason could be the destruction and collapse of serpentine structure at higher extraction temperature, which is consistent with the XRD pattern shown above SiC synthesis from amorphous silica extracted from waste serpentine For SiC synthesis, the amorphous silica was obtained by extracting waste serpentine at 80 Cin5MH 2 SO 4 for 48 h. Fig. 2. SEM micrographs (100 K) for serpentine before and after 5 M H 2 SO 4 extracted at different temperature and time: (a) raw serpentine; (b) 60 C extracted for 24 h; (c) 80 C extracted for 24 h; (d) 80 C extracted for 72 h.

4 T.W. Cheng, C.W. Hsu / Chemosphere 64 (2006) Fig. 3a shows that SiC synthesis is a function of C/SiO 2 molar ratio at 1550 C for 5 h. At C/SiO 2 molar ratio of 1, only a small amount of SiC was formed. The overdosed silica formed a-quartz and cristobalite. With increasing C/SiO 2 molar ratio, the amount of SiC increased sharply. The maximum amount of SiC was obtained at C/SiO 2 molar ratio of 3, above which, the amount of SiC decreased while that of carbon and other phases increased. This result conforms the equation of SiO 2 +3C! SiC + 2CO. Fig. 3b shows the plot of I n /I total versus sintering temperature. The sintering time was 5 h and the C/SiO 2 molar ratio was 3. It is obviously that higher sintering temperature will improve SiC formation. When the sintering temperature was 1450 C, only a small amount of SiC was formed. Most of the silica was transferred to the cristobalite phase. With increasing sintering temperature, the peak intensity of SiC increased while that of other phases decreased. When the temperature reached 1550 C, almost all the phases were transferred to form SiC. The theoretical reaction temperature for SiC is around 1700 C. However, in this study, SiC can be synthesized at 1550 C because of the high reactive amorphous silica used. The effect of sintering time on transformation of mineral phases is shown in Fig. 3c. As can be seen, SiC formation is directly proportional to the sintering time. When the sintering time increased from 1 to 5 h, the peak intensity of SiC increased sharply and its phase became more prominent while that of other mineral phases decreased. Therefore, the synthesis of SiC requires sufficient sintering time. The results discussed above revealed that SiC can be synthesized by mixing carbon black and amorphous silica at C/SiO 2 molar ratio of 3, and sintering the green body at 1550 C for at least 5 h. The synthesized SiC was whisker b type SiC. The XRD and SEM determination are shown in Figs. 4 and 5, respectively. Fig. 3. XRD results for SiC synthesis from amorphous silica. (a) Plot of I n /I total versus C/SiO 2 molar ratio at temperature 1550 C for 5 h; (b) plot of I n /I total versus sintering temperature for 5 h with C/SiO 2 molar ratio of 3; (c) plot of I n /I total versus sintering time with C/SiO 2 molar ratio 3 in 1550 C. Fig. 4. XRD pattern for synthesized SiC in this study. Fig. 5. SEM micrographs of synthesized SiC in this study.

5 514 T.W. Cheng, C.W. Hsu / Chemosphere 64 (2006) Conclusions This study demonstrated that it is feasible to produce SiC using amorphous silica extracted from serpentine waste. High-reactivity amorphous silica can be obtained by extracting serpentine in high concentration of H 2 SO 4 at temperature above 80 C for 48 h. Apart from the loss of ignition, the purity of SiO 2 in the amorphous silica can reach around 97%. Experimental results shows that SiC can be synthesized at 1550 C for 5 h by mixing carbon black and amorphous silica at C/SiO 2 molar ratio of 3. The SiC thus formed was whisker b type SiC that can be used as a raw material for industrial applications. References Cheng, T.W., Ding, Y.C., Chiu, J.P., A study of synthetic forsterite refractory materials using waste serpentine cutting. Miner. Eng. 15, Goff, F., Lackner, K.S., Carbon dioxide sequestering using ultramafic rocks. Environ. Geosci. 5, Kanari, N., Menad, N., Gaballah, I., Some aspects of the reactivity of olivine and serpentine towards different chlorinating gas mixtures. Thermochim. Acta 319, Kim, D.J., Chung, H.S., Synthesis and characterization of ZSM-5 zeolite from serpentine. Appl. Clay Sci. 24, Lackner, K.S., Butt, D.P., Wendt, C.H., Magnesite disposal of carbon dioxide. In: Proceedings of the 22nd International Technical Conference on Coal Utilization and Fuel Systems, Clearwater, Florida, Ministry of Economic Affairs, The application of limestone and serpentine wastes. Ministry of Economic Affairs, Republic of China, Taipei. O Connor, W.K., Dahlin, D.C., Nilsen, D.N., Rush, G.E., Walters, R.P., Turner, P.C., Carbon dioxide sequestration by direct mineral carbonation: Results from recent studies and current status. First national conference on carbon sequestration, National Energy Technology Laboratory (NETL), US Department of Energy. Pietrikova, A., Bugel, M., Golja, M., Preparation of SiO 2 powder through leaching of serpentine. Metalurgija 43,