A comprehensive kinetic model for the CO-CO 2 reaction with iron oxide-containing slags
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I. INTRODUCTION
SOME 20 years ago, extensive research was initiated by Belton and his research group on the interfacial reaction kinetics between slag and reducing/oxidizing gases.[1–5] Our knowledge of the kinetics and mechanism of such reactions has grown significantly since then. In particular, the reaction between CO-CO2[2,6–18] and H2-H2O[19,20] with iron oxide in the slag have attracted considerable attention, owing to the significance of the reactions in a number of metallurgical processes including direct smelting of iron, electric steelmaking, and nonferrous metals refining. The reaction between the slag and CO-CO2 atmospheres in its simplest form can be written as CO2(gas) CO(gas) O(slag)
[1]
with a general agreement between different studies that the rate of oxygen transfer with melts follows the form → [2] n ka (PCO2 PCOaO) In this equation, ao is defined as the CO2/CO ratio of the gas in the local equilibrium with the slag (with the unit CO2/CO as the standard state), and the apparent rate constant (ka) has found to be a complex function of the bulk slag composition, ao, and temperature, rather than a simple temperature-dependent rate constant. For example, it has been well established that ka is a function of ao as → [3] ka ka°(aO)a where ka° and in this equation are both dependent on iron oxide content and melt basicity. The forward and backward rates of Reaction [1] can, thus, be expressed in the forms MANSOOR BARATI, former Ph.D. student, McMaster University, is currently with WP HG Engineering, Toronto, Ontario, Canada. KENNETH S. COLEY, Associate Professor, is with Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7. Contact e-mail: [email protected] Manuscript submitted July 6, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
→ na ka° (aO)a PCO2 ka PCO2 [4] ← 1a na ka° (aO) PCO ka PCO [5] ← → where ka and ka are the apparent rate constants of the forward and backward Reaction [1]. A great number of studies have been carried out to establish a consistent relationship between the apparent rate constant and slag chemistry. While several excellent studies have provided qualitative explanation of observations, very little has been achieved with respect to presenting a unified quantitative picture of the reaction kinetics. For example, the early works on the rate of the CO2 reaction with FeO,[7] FeO-CaO (sat),[7] and FeO-SiO2 (sat)[10] have reported a unit value for , while in other studies,[12,18] smaller values have been measured. Discrepancies are even more apparent with respect to the effect of iron oxide content on the rate constant. El-Rahaiby et al.[10] did not observe any dependency between the ‘FeO’ content and ka° in the iron oxide range from 5 to 100 wt pct for ‘FeO’-CaO-SiO2 melts with equimolar CaO and SiO2. Nagasaka et al.[21] have also reported that for slags with CaO/SiO2 1.0, the variation of iron oxide content between 35 to 100 wt pct did not affect the rate of FeO reduction by CO. In contrast, several r
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