A simplified model for the oxidation of disintegrated steel streams
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W H E N liquid s t e e l is poured into a ladle, tundish or mold, oxygen will be picked up from the surrounding air 1-8 by the following different mechanisms: 1) mass transfer into the m e t a l stream; 2) mass t r a n s fer through the agitated metal surface in the ladle or mold; 3) mass transfer to droplets ejected from the agitated surface; and 4) mechanical entrainment of air bubbles into the m e t a l pool and subsequent mass transfer from the bubbles to the metal. The mass transfer of oxygen into the m e t a l s t r e a m (Mechanism 1) can be predicted for smooth streams 6 and is found t o be s m a l l compared t o the oxidation by entrainmentP '~ In practice, however, s t e e l streams are often highly unsmooth, or more or less disintegrated, especially when an unsuitable nozzle design is used or when disturbances of the flow o c c u r in the nozzle. Oxidation by mass-transfer t o the disinteg r a t e d s t r e a m will be much l a r g e r than by mass t r a n s fer t o a smooth one. The p u r p o s e of the present communication is t o present a simple model for calculating the oxidation of completely disintegrated m e t a l streams. The completely disintegrated s t r e a m represents a limiting c a s e , another limiting case b e i n g the smooth stream. Oxygen pick up of a partially disintegrated s t r e a m will be between those for the two limiting c a s e s and can be computed by combining the relatively simple treatment of the smooth s t r e a m6 with that of the p r e s ent model. EXPLANATION
OF M O D E L
T h e completely disintegrated m e t a l s t r e a m will be considered which is failing downwards in a cylindrical space of radius R, Fig. 1. The oxygen is picked up by the s t e e l droplets inside of the cylinder. Consequently the oxygen concentration of the gas phase is lowered within the cylinder compared to that in the surrounding air. Because of the consumption of oxygen molecules and of the acceleration of both phases inside of the cylinder gas enters into the cylinder in r-direction at its periphery. In the fol-
KLAUS SCHWERDTFEGER, a Member of AIME, and WOLFGANG WEPNER are with Max-Planck-Institut fi~r Eisenforschung, GmbH, Di~sseldorrf, West Germany. Manuscript submitted July 30, 1976. METALLURGICALTRANSACTIONS B
lowing, macroscopical mass balances for oxygen and nitrogen are set up for a cylindrical s l i c e of height Az located at distance z from the nozzle. Transport of gaseous species in r a d i a l direction into the s l i c e will be both by bulk flow of air and by diffusion. Driving force for the bulk flow of air is the suction developed by the consumption of oxygen molecules and by the acceleration. Diffusion is caused by the concentration gradient established at r = R. On the other hand, concentration gradients are assumed t o be sufficiently s m a l l in z-direction so that transport in the z-direction is mainly by convective flow, and diffusion in z-direction can be neglected. In the macroscopical balances a v e r a g e values are taken for concentration and z-velocity ins
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