Model study on mixing and mass transfer in ferroalloy refining processes
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I.
INTRODUCTION
THE mixing in many metallurgical reactors is carried out by injection of a gas either from the top or bottom of the vessel in order to accelerate the rates of chemical reactions and to promote bath homogenization. Inefficient stirring leads to larger blowing periods, adversely affecting the process economy. Therefore, a vital issue in the processes involving liquids is the relationship between mixing and the rate controlling processes of the reactions. The results of several studies showed that the rate of mixing in metal processing operations depends on the rate of energy input or energy dissipation.[1,2] Most of the workers related the mixing time (tmix) with the energy input (E).[3,4,5] Empirical relationships of the type tmix 5 mE2n have been proposed, and various values of n have been reported. The effect of the presence of a slag layer in gasstirred ladles on the mixing time was investigated.[4,5] From these studies, the mixing time was found to increase with the presence of slag. Most of the previous studies were concerned with ladles where the gas injection rates are much less than those of high-strength bottom, top, or combined blown metallurgical vessels. However, among some of the studies related to vessels where high gas flow rates are used, Paul and Ghosh,[6] in an attempt to investigate the differences in mixing and mass transfer rates between LD and Q-BOP furnaces on the basis of cold model experiments, related the mixing time with gas flow rate and number of tuyeres. They also reported that the mass transfer for bottom blowing was higher than that for top blowing. Many investigators have worked on the bath mixing conditions in top and bottom blown steelmaking by using cold model simulations in which the time required to homogenize a single phase was measured without considering the G. AKDOGAN, Lecturer, and R.H. ERIC, Professor and Head, are with the School of Process and Materials Engineering, University of the Witwatersrand, Johannesburg, WITS 2050, South Africa. Manuscript submitted January 27, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
mass transfer resulting from mixing.[7,8,9] Very little was reported about two-phase mixing caused by gas jet during the steelmaking cycle and ferroalloy refining process. In a model study on the mixing condition in combined blown steelmaking bath, Koria and Pal[10] found that the rate constant for the process of mass transfer was dependent on the top gas injection rate, top lance distance, and bottom gas injection rate. Singh and Ghosh[11] argued that the mass transfer rate of combined blowing process was about twice that of the LD process and almost equal to that of the QBOP process. In his study of a BOF scale model using various tuyere arrangements, Matway[12] discussed that the mass transfer rate was found to increase as the mixing time decreased. In gas-stirred metallurgical vessels, the turbulence caused by a gas/liquid plume rising through the bath facilitates the chemical reactions at the interface between two immiscible liquid ph
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