Plume characteristics and liquid circulation in gas injection through a porous plug
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I.
INTRODUCTION
THE design
and performance of porous plugs for the injection of purging and homogenizing gases into molten metals have recently been reviewed. [1,2~ A particularly important application of the plug is in the ladle treatment of steel, where porous refractory elements are assembled into the bottom wall of the ladle as a means of introducing inert gases into the melt for flushing out inclusions and homogenizing temperature and composition prior to casting. [3-s~ Similar gas injection arrangements exist for desulfurization and recarburization of iron t9'1~ and also for steelmaking by the lance bubbling equilibrium (LBE) process. [6] In the treatment of nonferrous metals, porous plugs are commonly used in the gaseous refining of anode copper [12,13j and for the degassing and stirring of aluminum melts for improved casting, t141 In metal refining practice, the porous plug has been shown to be more efficient than the tuyere in gas utili-
P.E. ANAGBO, Associate Professor, is with the Department of Metallurgical Engineering, University of Lagos, Lagos, Nigeria. J.K. BRIMACOMBE, Stelco/NSERC Professor and Director, is with the Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC V6T 1W5, Canada. Manuscript submitted October 18, 1989. METALLURGICAL TRANSACTIONS B
zation [151 and more effective than the lance in ladle operations, t4] Nevertheless, not very much is known about the physical interaction of gas and liquid in porous plug injection to properly account for the observed superior refining performance of the porous plug. In their work on gas bubble beds, Houghton et al. [161 related the average pore diameter, 6, of a glass or steel porous plug to the pressure drop, Ap, across the plug and the surface tension, or, of the liquid in the form 4o-
6 aP
[1]
- aP,
where APy is the frictional component of the pressure drop. A more practical index of the resistance to gas flow through the plug is the permeability, r which can be determined from the expression t81 vQh r -
AAP
[21
where v is the kinematic viscosity of the gas, h is the height of the plug of cross-sectional area A, and Ap is the pressure drop across the plug under a gas flow rate, Q. It has been shown in laboratory model studies that the gas flow rate through a porous plug increases linearly VOLUME 21B, AUGUST 1990--637
with increasing injection pressure but only for low values of permeability.[16] Houghton e t a / . tl6l also investigated the dispersion of gas injected into liquid by means of a porous plug and described the formation of fine bubbles, which were oblate spheroidal in shape, with the bubble size well correlated by means of a probability function. No influence of plug permeability on the size and density distributions of the bubbles was reported, but in a subsequent study, Koide et al. [m demonstrated the effects of plug porosity (e), pore size (~), liquid density (p), and surface tension (o-) on the average bubble diameter, in the form of a correlation: /
Fr \ " [ ~ r 6 \ '/3
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