Study on the foaming of CaO-SiO 2 -FeO slags: Part II. Dimensional analysis and foaming in iron and steelmaking processe
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INTRODUCTION
S L A G foaming has been an important phenomenon in actual iron and steelmaking processes, such as BOF steelmaking and EAF processes, for a long time. For developing new ironmaking processes, in bath smelting, the control of slag foaming will be even more critical since foaming is extensive, and a stable foam may be used for post combustion and heat transfer. Therefore, a quantitative study which enables the prediction of slag foaming in practical processes is required. In a previous paper, m the foaming index (E) and foam life (~-) were newly defined and were confirmed in order to be used as characteristic parameters of slag foaming. The foam index E or gas retention time in the slag and foam life r are defined by Eqs. [1] and [2], respectively. Ah
:~
=
- -
r =
[l]
t
[2]
where h ( c m 3) is a foaming height, V~ (cm/s) is a superficial gas velocity, and t [s] is the time required to reduce the foam height from h ~ to a specified position h, measured by stopping the gas injection after the steady state was maintained (i.e., foam height is equal to h~ It was shown that for the slags studied, E is approximately equal to r. These parameters were measured as a function of the slag composition, the amount of secondphase particles, and temperature. In this study, an empirical equation for the foaming index E of a CaO-SiO2-FeO slag is obtained by dimenKIMIHISA ITO, Research Associate, formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is with the Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendal, Japan. R.J. FRUEHAN, Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted May 19, 1988 METALLURGICALTRANSACTIONSB
sional analysis. The anticipated foaming in BOF, EAF, and bath-smelting processes was estimated using the parameter E for various operating conditions and slag compositions. II.
D I M E N S I O N A L ANALYSIS
As pointed out in previous work, [2-5] the surface tension of the slag greatly affects foaming. The relation between the foaming index and the surface tension of slag for a CaO-SiO2-FeO system is shown in Figure 1. The surface tension of the slag was calculated from the data with Shiraishi e t a / . [6] taking into account the effects of Fe203 trl and A120 3.17] The logarithm of ~ varies approximately linearly and decreases with increasing the logarithm of surface tension. However, since SiO2 decreases surface tension and increases viscosity for a CaO-SiO2FeO system, the effect of the viscosity on the foaming index can be expected. Consequently, it is difficult to determine whether the increase in the foaming index with a SiO2 content is primarily due to the increase in viscosity or the decrease in surface tension. Therefore, measurements of slag foaming were carried out for a PbOSiO2 system at 1073 to 1273 K, because SiO2 increases both surface tension and viscosity in this system. The experimental
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