The rate of formation of SiO by the reaction of CO or H 2 with silica and silicate slags
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I.
INTRODUCTION
S I L I C O N monoxide plays an important role in many metallurgical reactions, in particular silicon transfer in the blast furnace. As discussed in a recent publication by the authors j it has been proposed by Pompfret and Grieveson 2 and by Turkdogan et al. 3 that the reaction of silica in slags with carbon in iron, (SiO2) + 2 c = si + 2CO(g)
[1]
occurs by means of the gaseous intermediate SiO. The SiO is generated at a gas-slag interface by the reaction: (SiO2) 4- CO(g) = SiO(g) + CO:(g)
[2]
SiO then reacts at the gas-metal interface by the following reaction: SiO(g) + C = Si + CO
[3]
Recent work by Ozturk and Fruehan L has shown conclusively that the chemical kinetics for reaction [3] are fast and that the rate is controlled by gas phase mass transfer. This would indicate that reaction [2], the formation of SiO by the reaction of CO with silica dissolved in slags, could be the critical slow reaction in the proposed mechanism involving SiO. It is the purpose of the present investigation to determine the rate of formation of SiO(g) from silica containing slags by reaction [2]. In order to understand the rate mechanism fully the rate of reaction was also measured for pure silica spheres under conditions where mass transfer conditions are clearly defined. Schwerdtfeger 4 measured the rate of silica reduction to SiO in H2 and CO-CO2 gas mixtures at 1773 K. His results indicated that for H2 the rate was controlled by gas phase mass transfer of SiO away from the gas-solid interface. However, indications were that the rate of reduction by carbon monoxide was not controlled by gas phase mass transfer. To assist in our understanding of the present B. OZTURK, Senior Research Scientist, and R.J. FRUEHAN, Professor, are with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon Umversity, Pittsburgh, PA 15213. Manuscript submitted March 14, 1985. METALLURGICALTRANSACTIONSB
work the rate of reduction of pure silica or silica in slags to SiO by H 2 was also measured, SiO2 + H2(g) = SiO(g) + H 2 0 ( g )
[4]
Mizusaki et al. 5 have recently measured the rate of formation of SiO by the reaction of hydrogen with CaO-SiO2 slags. Their results indicated the rate was controlled by gas phase mass transfer under most conditions. When the activity of silica and the gas flow rates were high, there was an indication that the rate was also influenced by liquid phase mass transfer. Gas-Phase M a s s Transfer Relationships
Since gas phase mass transfer may play an important role in the reactions under consideration, it is important to review the mass transfer equations relevant to the experimental geometry. In general, gas phase mass transfer reactions are controlled by the counter current diffusion of the reactants to the interface and of the products away. However, for the present case, since the equilibrium constants for reactions [2] and [4] are much less than one, 4 x 10 -6 and 1.0 x 10 -5 at 1923 K for reactions [2] and [4], respectively, the flux equations simplify to Js,o = RTt
