Thermodynamics of the Si-C-O system for the production of silicon carbide and metallic silicon
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I.
INTRODUCTION
THE invention of predominance
diagrams by Kellogg and Basu 1 marked a giant leap in the pyrometallurgical understanding of the metal-sulfur-oxygen systems. The predominance diagrams permit description of thermodynamic stability conditions of various substances and their interrelations as functions of 02 and SO2 partial pressures as well as temperature. Many process improvements and new process developments have resulted therefrom. The electric furnace production of silicon carbide and metallic silicon with silica and carbon as raw materials have been practiced commercially for many decades. 2'3'4 In both cases, the reactions are of the Si-C-O system. It is known that heating a mixture of SiO2 + 2C in a laboratory furnace fails to yield metallic silicon. Thus, the thermochemistry for metallic silicon production remains a mystery today despite the successful industrial operation. 5 Although the predominance diagrams of the Si-C-O system, analogous to those of the metal-sulfur-oxygen systems, are felt most useful for metallurgical practice, such diagrams are yet to be published. Several attempts were made in the past to draw a coherent thermodynamic picture of the Si-C-O system, but without much progress in theory. 5'6'7 Although the Si-C-O system may appear to be a simple ternary at first glance, further approach reveals its rather complex nature due especially to the formation of SiO gas. There is also the presence of liquid (amorphous) silicon monoxide, as described in detail by Kubaschewski and Alcock, 8 which was, strangely enough, never included in the previous studies of the Si-C-O system. In all the previous works, the SiC- and Si-making systems were not properly recognized as being governed by two independent variables of carbon and oxygen activities at a given temperature, as seen in the frequent errors of looking upon the SiO partial pressure as an independent parameter. 5'6'7 Also in all of the so-called stoichiometric models M. NAGAMORI, I. MALINSKY, and A. CLAVEAU are with the Centre de Recherches minrrales, Quebec Government, 2700 Einstein, SteFoy, Quebec G1P 3W8, Canada. Manuscript submitted July 11, 1985. METALLURGICALTRANSACTIONS B
published hitherto, the effect of temperature has never been included nor defined thermodynamically. The present work has been undertaken to construct the predominance diagrams of the Si-C-O system at electric furnace temperatures such as 1350 to 2200 ~ with the inclusion of liquid as well as gaseous SiO. The work has been extended to cover the equilibrium mass-balance analyses of various silica-carbon feeds with the aim of laying a thermodynamic foundation for the practical control of electric furnace operations.
II.
THERMODYNAMIC DATA
The selected thermodynamic data required for calculating the predominance diagrams are listed in Table I. It appears that some thermodynamic properties of silicon monoxide, such as melting point, remain unsettled as yet. According to the detailed analyses by Kubaschewski and Alcock,8 however, SiO(s, 1) is stable betwe
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