The kinetics of the nitrogen reaction with liquid iron-Sulfur alloys
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INTRODUCTION
iNTEREST in the nitrogen absorption reaction in liquid iron alloys is two-fold. From a practical standpoint, the nitrogen absorption reaction must be included among the important gas-liquid reactions occurring in steelmaking and ladle metallurgy processes. Scientific interest stems from the fact that the nitrogen absorption reaction is, under certain conditions, one of the few high temperature heterogeneous reactions in which an elementary step in the chemical reaction mechanism, rather than a mass transfer step, is rate controlling. Fruehan et at. 1,2 and Byrne and Belton 3 have, most recently, reviewed the previous work on nitrogen absorption into liquid iron solutions. In the early 1960's. several groups of researchers456 noted a large decrease in the absorption rate with increasingly oxidizing conditions. Almost simultaneous with these studies, Pehlke and Elliott, 7 using a constant pressure Sieverts' apparatus, found the rate of nitrogen absorption into induction-stirred liquid iron alloys to be proportional to the square root of nitrogen pressure (equivalent to first order with respect to nitrogen concentration in the melt) and strongly retarded by the presence of sulfur and oxygen. They demonstrated that at low solute levels the rate was controlled by mass transfer in the liquid phase. A few years later, Inouye and Choh, 8 employing a sampling technique, found that at low solute concentrations the rate of nitrogen absorption was half order with respect to nitrogen pressure, in agreement with the work of Pehlke and Elliott and consistent with a liquid phase mass transfer controlled rate. However, at high solute levels, the rate was proportional to the nitrogen pressure. Fruehan and Martonik,~ using a modified Sieverts' technique with a highly sensitive pressure transducer, conclusively demonstrated that the rate of nitrogen absorption into iron alloys with surface active solute levels over 50 ppm followed a second order relationship, consistent with the P. C GLAWS, formerly with the Department of Metallurgical Engineering and Materials Science, Camegm-Mellon Umvers~ty, Pittsburgh. PA, is now with the Department of Metallurgy, University of Newcastle, Newcastle, New South Wales, Australia. R.J. FRUEHAN is Professor, Department of MetallurDcal Engineering and Materials Science. Carnegie-Mellon Umverslty, Pittsburgh, PA 15213. Manuscript submitted December 6, 1984.
METALLURGICALTRANSACTIONS B
rate being controlled by the dissociation of the nitrogen molecule on the surface. Recent research by Greenberg and McLean 9 on falling iron droplets is in accord with the results of Inouye and Choh and Fruehan and Martonik, as are several of the more recent studies on nitrogen desorption, m ~ In later work, Fruehan and Martonik ~2 noted that in the determination of the chemical rate constant for the nitrogen reaction in conventional experimental techniques, a correction should be made for liquid phase mass transfer effects. These effects are negligible at high sulfur concentrations, accounting for corre
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