Effect of the bubble size and chemical reactions on slag foaming
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INTRODUCTION
THE growing interest in slag foaming rises in part from the worldwide activity in developing innovative iron making processes, particularly bath smelting. The bath-smelting process is designed to produce hot metal at a high specific productivity without using metallurgical coke. The process uses prereduced ore fines or pellets as the raw material, coal as the fuel and reductant, and oxygen or air injection for combustion. In this process, slag foaming needs to be controlled in order to achieve the optimum production rate without causing slopping of the liquid slag. On the other hand, a certain amount of foamed slag in the vessel is desired for promoting the transfer of heat generated by postcombustion to the bulk slag. Slag foaming is also of interest in electric are furnaces, basic oxygen steelmaking, and ladle processing, as discussed elsewhere. E~,21 Slag foaming in steelmaking has been investigated for many years. Early studiesI~,z,31described the stability of slag foams in terms of "foam life." The measured results signified the time of the decay of a certain volume of slag foam under static conditions but could not be directly applied to predicting the height of the dynamic foam in an iron or steelmaking vessel. Recently, Ito and Fruehan151 adopted a dynamic measurement of the foam-stability, the foam index, as the ratio of the foam height to the superficial gas velocity. In this case, the foam height is defined as the difference between the level of the surface of the foamed slag and that of the slag at rest. The foam height is usually obtained at different gas flow rates to ensure the accuracy of the experiment, and the foam index can be obtained by the slope of a plot of the foam height against the superficial gas velocity. Ito and Fruehan further measured the foam index for CaO-SiO2-FeO slags containing 30 pct FeO at Y. ZHANG, formerly Graduate Student and Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University, is now a Senior Process Engineer with Armco Advanced Materials Company, Butler, PA 16003-0832. R.J. FRUEHAN, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted February 9, 1993. METALLURGICALAND MATERIALSTRANSACTIONS B
temperatures from 1200 ~ to 1400 ~ They also examined the effects of P205, S, and CaF 2 contents in the slag on the foam index. As the result, the foam index was correlated with the physical properties of the liquid slag by dimensional analysis. In a later study, Jiang and Fruehan[6] measured the foam index for the bath-smelting type of slags (CaO-SiO2-A1203-FeO) containing less than 15 pct FeO. An improved correlation was obtained by using more accurate data for the slag viscosity, density, and surface tension in the dimensional analysis. The correlation developed is given by /z = 115 p ~
[11
where E is the foam index (seconds), /z is the viscosity (N's/m2), p is the density (kg/m3), and o- is the surface tension (N/m). A more rece
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