Reduction rate of SiO 2 in slag by carbon- saturated iron
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I.
INTRODUCTION
R E D U C T I O N of SiO: from slag by carbon dissolved in iron melts occurs in the lower levels of the slag layer in a cupola where the FeO content of the slag is relatively low. Prediction of the silicon content of tapped liquid iron during the cupola melting process could be affected by silicon pickup due to this reaction when an iron droplet falls through the slag layer. Accurate measurements of silica reduction rates are an important aspect in the computer simulation of the cupola process. As it is also one of most important reactions in blast furnace operation, the reduction of silica from CaO-A1203SiO2 slags by carbon in liquid iron has been studied by many investigators. EI-TJHowever, a considerable lack of agreement exists on the rate of the reaction. For example, the measured reaction rate, expressed as moles of SiO2 m -2 s -1, at 1600 ~ for carbon-saturated iron and an activity of silica in the slag of 0.30 are scattered from 9.17 • 10 -4[7] to 6.67 X 10-3. t4] Also, the rate-limiting step suggested by these investigations for silica reduction is not necessarily the same. However, it is generally agreed that the SiO2 reduction reaction is a very slow reaction compared with the rates of other slag-metal reactions under similar conditions and that the reaction rate is strongly dependent on the temperature. The primary objective of this study was to determine the reaction rates at various conditions similar to those in a cupola and to determine the step or steps which control the rate of silica reduction.
II.
EXPERIMENTAL PROCEDURES
An electric resistance furnace, as shown in Figure 1, was employed for heating and melting iron-carbon alloys in a graphite crucible. The reaction vessel was a vertical alumina tube of 60-mm ID and 1-m length. Both ends of the tube were closed with water-cooled brass caps, sealed with an O-ring, to prevent the oxidation of liquid iron by air. The temperature was measured with a HAIPING SUN, Visiting R e s e a r c h S c i e n t i s t , and ROBERT D. PEHLKE, Professor, are with the Department of Materials Science and Engineering, the University of Michigan, Ann Arbor, MI 48109-2136. KATSUMI MORI, Professor, is with the Department of Materials Science and Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan. Manuscript submitted June 11, 1992. METALLURGICAL TRANSACTIONS B
thermocouple placed just below the graphite crucible in the homogeneous temperature zone (15-cm long) and was controlled to within ---0.5 ~ About 200 g of iron-carbon alloy was first melted in a graphite crucible (30-mm ID) under an argon atmosphere. After the experimental temperature was attained, about 25 g of slag were placed on the surface of the liquid iron. The instant the slag was fully melted (which took about 1 minute) was taken as the starting point of the reaction. The drop in temperature due to the addition of slag was about 10 ~ but the melt recovered to the set experimental temperature within 2 to 3 minutes. The reaction time for most of the experiments was 6 hours. Some
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