Modeling of slag eye formation over a metal bath due to gas bubbling
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DURING ladle metallurgy operations in steelmaking, gas is bubbled through molten steel to promote the flotation of inclusions and to enhance slag-metal reactions. The bubbles rise in a gas-liquid plume that creates a raised area or ‘‘spout’’ at the bath surface.[1,2,3] The upwelling flow from the bubble plume turns horizontally at the bath surface and pushes the slag layer to the periphery of the ladle. If the slag layer is sufficiently thin, this process exposes an open area of liquid metal to the atmosphere, termed the ‘‘eye.’’ While the stirring is desirable, unwanted phenomena may occur, such as slag entrainment and oxygen and nitrogen pickup, which are detrimental to steel quality. The hydrodynamics and the associated transport phenomena of these regions are of great practical significance for the production of clean steel. The phenomenon of slag eye formation has been the subject of some recent work,[4–8] but many aspects require further investigation. The current study is aimed at obtaining a better fundamental understanding of the formation of the eye region. To this end, room-temperature model experiments have been carried out under a variety of experimental conditions, and a mechanistic model to explain the results has been developed. II.
APPARATUS AND PROCEDURE
A cylindrical acrylic vessel, having an inner diameter of 42 cm and a height of 50 cm, was used to represent a 1/10scale steel ladle. Air was injected into the bath through a central 3-mm nozzle flush on the bottom of the vessel. A schematic diagram of the apparatus is shown in Figure 1. Due to the difference in color between the oil and the aqueous solution, clear identification of the eye boundary could be made with a video technique. Digital video recordings were made from the top of the vessel with proper illumination on the bath surface. The dynamics of the eye movement were recorded for each condition over 250 sec-
KRISHNAKUMAR KRISHNAPISHARODY, Graduate Student, and GORDON A. IRONS, Dofasco Professor of Ferrous Metallurgy and Director, are with the Steel Research Centre, McMaster University, Hamilton, ON, Canada. Contact e-mail: [email protected] Manuscript submitted August 17, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
onds with a frequency of 30 Hz. The recorded images were transferred to a personal computer and processed by an image analysis program to extract the area of the eye. The properties of the liquids used in the experiments are shown in Table I. Three liquid-liquid systems were used: (1) a water-paraffin oil system with a low density ratio of 1.15; (2) an aqueous CaCl2 solution-paraffin oil system with a density ratio of 1.61 (this system has a density ratio closer to those of steel-slag systems, approximately 2.5); and (3) water-heavy motor oil with a density ratio of 1.14 (this system was chosen to simulate a higher viscosity in the slag-simulating layer). Measurements were conducted with three different bath heights and a range of oil heights at various gas flow rates. The ranges of experimental variables and conditions are
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