Characterization of a Si-SiO 2 Mixture Generated from SiO(g) and CO(g)
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INTRODUCTION
SiOðgÞ þ 2CðsÞ ! SiCðsÞ þ COðgÞ
METALLURGICAL silicon is produced by the reduction of silica by carbonaceous materials in electric arc furnaces. The thermodynamics of silicon production are well known. The kinetics, which are slow and difficult to predict, have, however, not been studied to the same extent. Process understanding is limited by the large number of reactions happening simultaneously in the silicon production process. Most of them involve intermediate products, such as SiO(g) and SiC. These two compounds are the main intermediate species in silicon production. As this gas mixture is entering the low-temperature area in the furnace (1300 K to 2000 K), it will generate condensates and SiC according to Reactions [1 through 3][1–4]: 2SiOðgÞ ! Siðs,lÞ þ SiO2 ðs,lÞ
½1
3SiOðgÞ þ COðgÞ ! 2SiO2 ðs,lÞ þ SiCðsÞ
½2
ANDREA BROGGI and MERETE TANGSTAD are with the Department of Materials Science and Engineering, University of Science and Technology (NTNU), Alfred Getz vei 2b, 7034 Trondheim, Norway. Contact e-mail: [email protected] ELI RINGDALEN is with the SINTEF, Materials and Chemistry, Alfred Getz vei 2b, 7034 Trondheim, Norway. Manuscript submitted December 5, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
½3
SiO(g) and SiC contribute to the formation of silicon in the inner high-temperature zone (2100 K to 2400 K),[5] by Reaction [4]. SiOðgÞ þ SiCðsÞ ! 2SiðlÞ þ COðgÞ
½4
The mass of condensates in an industrial system can be estimated with the help of thermodynamics and mass balance calculations. Quantitating the mass of condensation products is unfeasible in an industrial oven. Condensates will stick firmly to any surface, and mechanical removal is challenging at high temperatures. The condensate crust is located either at the stoking area, below the electrode or in the central area of the oven.[6–9] In other words, it is very difficult to foresee where condensation would occur, or to confine it in a defined temperature zone. In industrial- and pilot-scale excavations of silicon and ferrosilicon furnaces,[6–16] the condensate crusts found are of two types. The mixture generated from Reaction [2] is referred to as white condensate. This compound will not be the main focus of this paper. The compound generated through Reaction [1] is called brown condensate. This crust is often found in industrial excavations, but it has never been characterized thoroughly. The only compositional analysis found in literature for industrial samples of brown condensate comes from an industrial excavation performed at Elkem Thamshavn, Norway[6] in 2014. Results show
that the compound contains porous SiO2 and SiC. No quantitative chemical analysis was performed. Si was expected, as a product of Reaction [1]. Vangska˚sen[2] hypothesized that the pores in the condensate are generated by liquid silicon ‘‘bleeding’’ from the silica matrix. Laboratory-scale experiments reproducing the reaction between SiO(g) and CO(g)[2,3,17–20] revealed also the presence of a solid brown deposit. Its microstructure consists of micr
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