Modeling Energy Dissipation in Slag-Covered Steel Baths in Steelmaking Ladles
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E physical and mathematical model studies of gas-stirred ladle systems have been carried out and reported in the literature.[1,2] Most of these were concerned primarily with the injection of a gas through a liquid in a cylindrical vessel in the absence of any upper buoyant phase (e.g., slag). As melts contained in refining ladles are typically covered with a protective slag layer (i.e., to minimize radiation heat losses, prevent reoxidation, and to capture harmful nonmetallic inclusions), slag-covered steel baths rather than bare steel baths are more typical of industrial ladle systems. Consequently, most model studies fall short of representing industrial ladle refining operations realistically. DIPAK MAZUMDAR, Professor and Head, is with the Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur 208016 India. Contact e-mail: [email protected] RODERICK I.L. GUTHRIE, Professor and Director, is with the Department of Mining and Materials Engineering, McGill Metals Processing Centre, Macdonald Professor of Metallurgy, M.H. Wong Eng. Building, McGill University, Montreal H3A 2B2, Quebec, Canada. Manuscript submitted October 2, 2009. Article published online July 13, 2010. 976—VOLUME 41B, OCTOBER 2010
Despite the difficulty in representing molten steel and slag system exactly using room temperature analogs, several studies[3–13] have been reported over the years, in which, the role of a slag has been investigated via an oil– water or similar room temperature system. These studies, in general, have indicated that flows and mixing in gasstirred vessels become ‘‘sluggish’’ in the presence of an overlying second-phase liquid. Furthermore, mixing times tend to increase as the thickness of the upper phase liquid increases. Similarly, although axial gas volume fractions, bubble rise velocities, etc., are noted to be somewhat smaller in the presence of an overlying liquid, bubble and liquid flow characteristics, by contrast, were found to be largely independent of the physical properties of the upper phase liquid, such as surface tension, density, and viscosity (within the operational ranges of interest). It is now generally acknowledged that the upper phase liquid will dissipate a part of the input energy, and this will slow down liquid steel turnover rates and affect bath homogenization adversely. In an earlier study, the relative contributions of energy dissipation processes in a slag-free ladle were analyzed by Kishimoto et al.[14] through differential modeling. However, many gaps still exist in the literature as far as understanding the interactions between an METALLURGICAL AND MATERIALS TRANSACTIONS B
upper phase and lower bulk liquid, and on their quantification. For example, the mechanism governing the dissipation of input energy because of the presence of an overlying slag phase is still not known with any certainty. Understanding two fluid interactions and the associated energy dissipation mechanisms at a fundamental level is vital before macroscopic models applicable to slag-covered refining la
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