Reduction of Quartz to Silicon Monoxide by Methane-Hydrogen Mixtures
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UCTION
METALLURGICAL silicon and ferrosilicon alloys are produced in submerged electric arc furnaces by carbothermal reduction of silicon-bearing oxides (typically quartz) with carbonaceous materials. Coal, coke, charcoal, and woodchips are commonly used as reductants in these processes.[1] Carbothermal reduction requires high temperatures and is energy intensive. The contents of trace elements in both quartz and reductants determine the purity of the produced silicon.[2] SiC is also produced by the carbothermal reduction of quartz. Silicon monoxide (SiO) vapor is one of the major intermediate species in the production of SiC and metallurgical silicon.[3,4] SiO2 is reduced by carbon to SiO according to the following reaction: SiO2 þ C ¼ SiOðgÞ þ COðgÞ
½1
SiO further reacts with carbon to form SiC: SiOðgÞ þ 2C ¼ SiC þ COðgÞ
½2
XIANG LI, formerly Ph.D. Student with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, Wollongong, NSW 2522, Australia, is now Postdoctoral Fellow with the University of Science and Technology (NTNU), Trondheim, Norway. Contact e-mail: [email protected] GUANGQING ZHANG, Senior Lecturer, is with the School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong. RAGNAR TRONSTAD, Director R&D, is with Elkem AS, Drammensveien 169, Skøyen, P.O Box 334, 0213 Oslo, Norway. OLEG OSTROVSKI, Emeritus Professor, is with the School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia. Manuscript submitted November 9, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
The primary reaction for production of Si is the following: SiC þ SiOðgÞ ¼ 2SiðlÞ þ COðgÞ
½3
Methane has the advantage of high purity over conventional carbonaceous reductants such as coal or coke which always contain impurities.[5–7] The gas-phase reaction between SiO and methane with or without a catalyst has been used for synthesis of SiC whiskers.[8–12] However, no reports were found in the literature on the direct reduction of SiO2 to SiO by the methane-containing gas. The purpose of the present study is to investigate the possibility of reduction of quartz to SiO via the gas–solid reaction by methane-hydrogen gas mixture, as described by Reaction [4]. Natural gas contains low harmful impurities of solar silicon, and provides high carbon activity for the reduction of quartz, which makes it an attractive reductant over conventional solid carbon. SiO2 þ CH4 ðgÞ ¼ SiOðgÞ þ COðgÞ þ 2H2 ðgÞ
½4
At temperatures higher than 1729 K (1456 C), Reaction [4] has a negative Gibbs free energy change under standard conditions (DG); at 1873 K (1600 C), DG = 63.2 kJ (calculated using HSC Chemistry 6.1). However, methane is unstable at temperatures higher than 823 K (550 C); it cracks with deposition of solid carbon (Reaction [5]) at temperatures higher than 1273 K (1000 C).[13] Solid carbon deposits on the sample surface, hindering further progress of the reduction by blocking the quartz particle surface. CH4 ðgÞ ¼ C þ 2H2 ðgÞ
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