Rate of nitrogen desorption from CaO-Al 2 O 3 melts to gas phase
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I. INTRODUCTION
IT is difficult to reduce nitrogen in steel to the desirable concentration level, because nitrogen is absorbed into metal by contact with air during and after degassing. Therefore, the nitride capacity of various fluxes has been measured to explore slags having a higher ability to absorb nitrogen and to investigate the possibility of removing nitrogen by using slag.[1–4] For the more efficient removal of nitrogen in practical steelmaking processes, it is also important to take into account the nitrogen desorption rate from slag to gas in consideration of removing nitrogen from the metal phase to the atmosphere through the slag phase. From this point of view, nitrogen adsorption and dissociation rates on the oxide melts have been previously investigated by the isotope exchange method,[5,6] but the measurements are limited and the relationship of the nitrogen adsorption and dissociation rates with the nitrogen desorption rate has not been clarified yet. Accordingly, the nitrogen desorption rate from CaOAl2O3 melts to gas phase has been examined at 1873 K, and the mechanism of the nitrogen reaction between the gas and slag phases has been discussed in this study. II. EXPERIMENTAL The experimental apparatus consisted of a vertical MoSi2 electric resistance furnace, which was controlled by a PID controller with a Pt-6 pct Rh/Pt-30 pct Rh thermocouple. A mullite tube (60-mm o.d., 52-mm i.d. 1000-mm long) was used as a furnace tube. The mixture of the specified quantities of reagent-grade CaO and Al2O3, weighing 25, 55, or 80 g, along with a small quantity of AlN, were put in an alumina (20-mm i.d.) or a molybdenum (28- or 38-mm i.d.) crucible, which was set in the furnace, and the mixture was melted at 1873 K. The sample compositions were adjusted at 36 to
HIDEKI ONO-NAKAZATO, Research Associate, and TATEO USUI, Professor, are with the Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, Osaka 565-0871, Japan. Contact e-mail: [email protected] SHUNSUKE MORISAWA, formerly Graduate Student, Department of Materials Science and Processing, Graduate School of Engineering, Osaka University, is with Sumitomo Metal Industries, Ltd., Hyogo 660-0891, Japan. Manuscript submitted January 9, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B
55 mass pct CaO. The alumina crucible was used in the experiments for the 36 mass pct CaO samples (Al2O3 saturation at 1873 K), and the molybdenum crucibles were used for the other experiments. The AlN was added for the purpose of dissolving nitrogen in the melt, and the AlN content corresponded to about 4 mass pct for the total sample weight. Argon was introduced from the upper part of the furnace and was blown onto the top of the sample at a fixed flow rate of 300 cm3/min STP using an alumina tube (6-mm i.d.). The oxygen partial pressure was controlled mainly by the argon and oxygen mixture. In several experiments conducted under lower oxygen partial pressures, the argon and hydrogen mixture was introduced into a water-v
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