Modeling of metallurgical emulsions
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I.
INTRODUCTION
EMULSIFICATION is a process by which one liquid is dispersed into another immiscible liquid, the resulting liquid-liquid dispersion usually being termed an emulsion. The formation of a slag-metal emulsion is of considerable practical importance for many metalprocessing operations. It is generally believed that the large interfacial area caused by the dispersion of one liquid phase into the other is responsible for the very fast overall reaction rates found in "in-bath" smelting, in the basic oxygen furnace (BOF), and in the Q-BOP processes. Figure 1 shows the metal droplets entrained in a slag sample taken from a pilot trial of the AISI Direct Steelmaking Project. Similar dispersions of liquid iron in the slag phase have also been documented in Nippon Steel's pilot experiments of the direct iron ore smelting (DIOS) process, m In the AISI and the DIOS processes, two sources of iron droplet generation in the slag phase can be identified. First, iron reduced from the iron ore dissolved in the slag must be dispersed into droplets by the slag before they settle down to the iron bath. Second, iron droplets can also be induced by bottom blowing, which disperses liquid iron in the iron bath into the upper slag phase. It should be noted, unlike the BOF process, that a thick layer of slag is usually employed in the AISI and DIOS smelting operations to shield the iron bath from direct contact with the top-blown oxygen jet. Consequently, the top-blown oxygen jet does not contribute to the droplet generation in these particular cases. In the process of smelting reduction of iron ores, the total rate of iron ore reduction may be expressed as 12J R = [k,_,,Ab + k~_o4c + k~.,,(A,~ + A,~)I(pct FeO)
[1]
where k,.c and k,_,, are the rate constants for the slag-char ZUOHUA LIN, formerly Graduate Student, Department of Mining and Metallurgical Engineering, McGill University, is Assistant to Director, China Operations Iron and Steel, Hoogovens Technical Services, Hong Kong Office, Central, Hong Kong. R.I.L. GUTHRIE, MacDonald Professor, Department of Mining and Metallurgical Engineering, and Director, McGill Metals Processing Center, is with McGill University, Montreal, PQ, H3A 2A7 Canada. Manuscript submitted June 23, 1993. METALLURGICALAND MATERIALSTRANSACTIONSB
and slag-metal reactions; Ab and Ac are the bath and char areas; and Ad and A~ are the metal droplet areas created by bottom blowing and iron ore reduction, respectively. Based on Eq. [1], the dependence of the total rate of reduction on the interracial areas of dispersed metal droplets is readily appreciated. Nevertheless, because of the experimental difficulties involved, little work has yet been done that allows one to directly estimate the interfacial area generated by the dispersed metal droplets. In order to predict the rate of slag-metal reaction, an apparent mass-transfer coefficient, k,.m (A~ + A~), is often defined for quantifying the reaction rate. However, such a simplification gives no insight into emulsification behavior or any of the other f
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