Foaming of slags under dynamic conditions
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I. INTRODUCTION
SLAG foaming is a common feature in steelmaking processes. It is found in most converters and is prevalent in the blast furnace, during pretreatment of hot metal, and in bath smelting processes. Foaming of slags is also common in the electric arc furnace (EAF). The foam has both desirable and undesirable effects on the process performance. Because of its relevance in metallurgical processes, a number of studies[1–8] were carried out by different investigators to characterize the foaminess of slag. It is well known that the presence of surface-active species is essential for the formation of stable foam. Most of the oxides present in steelmaking slag (e.g., Fe2O3 and Cr2O3) are surface active, causing the slag to foam.[9] Earlier studies have mostly concerned the foaming behavior of slag at steady state, where foam is generated by bubbling gas through a nozzle. Attempts to rationalize the foaming behavior have been made by introducing the foaming index,[10] which correlates the foam height (H) with the superficial gas velocity (U): H a U
[1]
The foaming index has empirically been related to physical properties of the slag such as the surface tension (), viscosity (), and density () of the liquid and the size of the bubbles (db). Consensus has, however, not been reached with respect to the magnitude of the influence of these parameters on the foaming index. Zhang and Fruehan[5] showed that the foaming index could, for a number of slags, adequately be represented as 115m1.2 a s 0.2 rdb0.9
[2]
A. KAPILASHRAMI, formerly with the Royal Institute of Technology, 10044 Stockholm, Sweden, is with Corus Research Development and Technology, 1970 CA Ijmuiden, The Netherlands. M. GÖRNERUP and S. SEETHARAMAN, Professor, are with the Department of Materials Science and Engineering, Royal Institute of Technology. Contact e-mail: raman@ kth.se A.K. LAHIRI, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore 560 012, India. Manuscript submitted August 23, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
while Kim et al.[7] showed that complex slags of the CaOSiO2-FeO and CaO-SiO2-FeO-MgO-X systems (X Al2O3, MnO, P2O5, and CaF2) follow the relationship Cm a (sr)0.5
[3]
where the constant C depends on the nature of the slag. Lahiri and Seetharaman[11] showed that, for a number of slags, the foaming index for foam of uniform bubble sizes can be expressed as Cm a rd b
[4]
The value of C is determined by the gas fraction in the foamy slag, the bubble shape, and the ratio of the bulk-tosurface viscosity of the slag. These authors further pointed out that the foaming index is independent of gas flow rate when the gas fraction of foam is constant, a condition that could be reasonably valid at steady state. The aforementioned studies were made for foaming under steady state, where both the gas flow rate and chemical composition of the slag were constant. In production processes, however, the gas generation rate is not constant. Oxidation and reduction reactions of the ox
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