Influence of slag and foam characteristics on reduction of FeO-containing slags by solid carbon
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I.
INTRODUCTION
IN recent years, the smelting reduction of iron oxide in molten slags has drawn considerable attention from researchers. Faster kinetics and the use of noncoking coal have been projected as the major advantages of the smelting reduction processes. Reduction of FeO in the slag may be achieved by solid carbon or carbon dissolved in a molteniron bath. A good review on the subject[1] has highlighted kinetics and the mechanism of such processes. However, further studies are necessary for clarification of the reaction mechanism and recognition of factors influencing the reaction rate. The present work is concerned with analyzing the effect of slag characteristics and consequent foaming behavior on the rate of reduction of FeO by solid carbon. There is extensive literature on the subject of reduction of FeO in molten slag by solid carbon.[2–12] The reduction reaction is believed to take place in the following two steps.[5] Step 1: (FeO) 1 CO (g) → [Fe] 1 CO2 (g) Step 2: CO2 (g) 1 C (s) → 2CO (g) Overall: (FeO) 1 C (s) → [Fe] 1 CO (g)
[1a] [1b] [1c]
Here, the parentheses denote the liquid slag phase and the brackets denote the liquid metal phase. The following four possible rate-limiting steps may be visualized for this reaction between FeO in molten slag and solid carbon. (1) mass transfer of FeO (i.e., Fe2+ and O22 ions) in the slag phase; (2) gas-slag reaction (Eq. [1a]); (3) gas diffusion in the gas layer separating slag and solid carbon; and (4) CO2-carbon reaction (Eq. [1b]). Different rate-controlling mechanisms have been postulated by different authors. Step (4) was proposed to be the ratecontrolling step by Sasaki and Soma,[9,10] whereas, Borgianni[8] proposed the nucleation of iron to be the rate-controlling mechanism. A good number of other studies[2–4,7,11] have indicated that mass transfer in the slag phase
may be rate limiting. In a previous article, the authors have shown[12] that the reaction rate is directly proportional to the FeO concentration in the initial stages and the activation energy value is about 90 kJ/mole. Based on these observations, they postulated the FeO transport in the slag phase (i.e., step (1)) to be the rate-controlling step. Fun[4] interpreted their kinetic data on the basis of convective agitation due to evolution of CO gas bubbles. The present authors[12] have also proposed a correlation between Morton number and Reynolds number to demonstrate the effect of such convective agitation. The reduction reaction is undoubtedly a complex one and is characterized by a gas halo which forms quickly around solid carbon particles. This gas layer surrounding the reductant particles plays an important role in the reaction. The gas generated in the reduction, besides affecting the kinetics, also causes foaming of the slag. First, foaming agitates the bath and improves the transport phenomena associated with it. Second, foaming provides a large interface between the slag and gas to facilitate the reaction between FeO and CO. Therefore, a number of studies have been undertaken
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