Effects of Top Layer, Nozzle Arrangement, and Gas Flow Rate on Mixing Time in Agitated Ladles by Bottom Gas Injection
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TRODUCTION
THE large success of the ladle furnace is attributed to their ability to enhance both mass transfer and mixing phenomena. Many investigations have been carried out in the past to understand mixing phenomena during bottom gas injection; however, most of the previous research studies have focused on axisymmetrical gas injection as well as a slag-free system. The understanding of mixing phenomena involving the top slag layer and a nozzle configuration with eccentric gas injection is still very poor. Previous research involving a top layer has been carried out with one nozzle,[1–4] and more recently, including two nozzles at a fixed separation angle.[5,6] This work has concluded that the top layer always increases mixing time. A summary of the expressions reported to compute mixing time in the presence of a top layer, in the last three decades, is shown in Table I. These results indicate, in general, a small effect of the slag physical properties on mixing time and a detrimental effect of slag thickness on mixing time. Both these findings are an indication that mixing phenomena in agitated ladles are controlled by inertial and gravitational forces (i.e., viscous forces would be of secondary importance). Several mechanisms have been reported to explain flow retardation due to the presence of the top layer[8–12]: It has been suggested that the top layer decreases the average velocity of the underlying phase because part of the input energy is consumed in deformation of the slag/ metal interface which then leads to both slag emulsification and spout formation and subsequent steeper flow reversal as the slag thickness increases. In this case, the slag acts as a mechanical barrier that affects the radial energy dissipation to the underlying phase. Mazumdar A. N. CONEJO, Professor, is with the Graduate Program in Metallurgy, Morelia Technological Institute, Morelia, Mexico. Contact e-mail: [email protected] S. KITAMURA, Professor, N. MARUOKA, Associate Professor, and S.-J. KIM, Postdoctoral Student, are with the IMRAM-Tohoku University, Sendai, Japan. Manuscript submitted April 6, 2012. Article published online March 13, 2013. 914—VOLUME 44B, AUGUST 2013
and Guthrie[12] suggest that the top layer dissipates from 9 to 19 pct of the total stirring energy. The current study aims to improve the understanding of mixing phenomena in bottom gas-stirred ladles including a top-slag layer. Water modeling will be used to study the influence of the slag thickness, number of nozzles, nozzle location, and gas flow rate on mixing time.
II.
EXPERIMENTAL WORK
The experimental work was designed to simulate mixing phenomena in an industrial ladle of 210 tonnes of nominal capacity. This ladle is tapered with an average diameter of 3624 mm and a height of liquid of 2900 mm, which yields an aspect ratio (H/D) of 0.80. These dimensions correspond to the full-scale system and will serve as a basis to define the geometrical scale factor used in the water model. Five variables were investigated in the current study: gas flow rate, slag thickness, n
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