Carbothermal Reduction of Quartz with Carbon from Natural Gas
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RODUCTION
IN practice, mixtures of reducing agents are used in the silicon furnace. According to their origin, it can be divided into two groups: fossil carbon materials (coal, coke/char, petroleum coke) and biological carbon materials (charcoal, woodchips). Higher purity of silicon and environment issues are some of the important areas focused today. Introducing natural gas in the silicon process is helpful for extending the accessible carbon base and maybe positive to the purity and environmental problem.[1–4] Searching for new carbon sources for quartz reductant takes many years. In 1984, Hunt et al.[5] from the U.S.A and Norway started a trail on using rice hulls as raw material for silicon producing. Rice hulls were mixed with water as a lubricant and sucrose as a binder, and then extruded into pellets. Results show that the pellet has very high reactivity compared to existing carbon materials. In the year of 1995, Martin[6] tried to use sugar as the reductant to reduce SiO2. Silica sol and sugar were mixed, and then freeze dried. Obtained powder was heated to 2023 K (1750 °C) in a tube furnace. The sugar decomposes into carbon and water at temperatures between 473 K and 773 K (200 °C and 500 °C). At temperatures of more than 1573 K (1300 °C), a carbothermic reaction occurs and the carbon reduces the silica generating carbon monoxide and silicon carbide. Temperature in this experiment is much lower than Acheson process FEI LI and MERETE TANGSTAD are with the Department of Material Science and Engineering, Norwegian University of Science and Technology, Trondheim, 7491, Norway. Contact e-mail: [email protected] Manuscript submitted April 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
[more than 2273 K (2000 °C)]. Also, bamboo[7] and coconut shell[8] have been tested to reduce SiO2 in different research work. However, industrially still only charcoal, coal, and coke are used as carbon raw materials. As one of the promising reducing agents, natural gas was also used in the research for Cr, Fe, Mn, Ni, Ti, Zn, and Co production processes[9] recently. In the field of silicon production process, Monsen et al.[10] investigated the reaction when cold methane [298 K (25 °C)] meets hot SiO gas 1923 K (1650 °C) directly in reaction chamber. Thermodynamic calculations show that maximum SiC production occurs at CH4/SiO = 1.3 at 1653 K (1380 °C), while the lowest temperature for SiC production by reacting SiO gas with solid C is around 1785 K (1512 °C). According to experimental results, a large amount of SiC was formed. Maximum SiC was formed close to CH4/SiO = 1.5. Ksiazek et al.[11] studied the reduction of silica by the use of natural gas directly. Cracking of methane occurred and the solid carbon deposited on the edge and inside of the quartz particles. Two different kinds of SiC were produced during the experiment at 1673 K and 1773 K (1400 °C and 1500 °C). At 1673 K and 1773 K (1400 °C and 1500 °C), compacted-needle-shape SiC around SiO2 particles was found as the main SiC formed. Another kind of SiC formati
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