The nitrogen reaction between carbon saturated Iron and Na 2 O-SiO 2 Slag: Part 1. Thermodynamics
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I.
INTRODUCTION
THERE has been a growing emphasis on the production of clean steels low in residual elements. Processes have been developed to remove sulfur, phosphorus, carbon, and total oxygen to extremely low levels. Hydrogen can be reduced to less than 1.5 ppm by vacuum degassing. Some nitrogen will be removed during BOF steelmaking by the flushing action of the CO and 10 pct to 20 pct can be removed by vacuum degassing. However, a commercial chemical process for reducing the nitrogen content to low levels does not exist. Hot metal treatments have been developed in order to optimize the steelmaking process and to remove sulfur and phosphorus. The most common reactants used are CaO, CaO-Mg, Mg, CaC2, and Na2CO3. Soda ash (Na2CO3) is capable of simultaneously removing sulfur and phosphorus at the low oxygen potentials associated with carbon saturated iron. The Na2CO3 treatment has become more attractive since a refractory material for the process as well as a process for recycling the soda ash have been developed. When Na2CO3 is added to hot metal the resulting slag is Na20-SiO2. In recent work the solubility of nitrogen in Na20-SiO2 slags in equilibrium with carbon and CO-N: gas mixtures has been measured,l' 2 and the results indicate that the slags can absorb significant quantities of nitrogen. However, the equilibrium distribution partition ratio for nitrogen between carbon saturated iron and Na20-SiO2, (pct N)slag/ [pct N]me~, has not been measured. In addition, there is no information on the kinetics of the nitrogen reaction for this system. Exploratory plant studies indicated that Na2CO3 treatment can remove nitrogen from hot metal, but the mechanism and physical chemistry are not known. It is the purpose of the present work to determine the thermodynamics and kinetics of the nitrogen reaction between Fe-CsAT-Si-N alloys and NazO-SiO2 slags. In Part I the results of the investigation of the thermodynamics of the F. TSUKIHASHI, Research Associate, is with Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213 and is on leave of absence from Department of Metallurgical Engineering and Materials Science, Faculty of Engineering, The University of Tokyo, Bunkyo, Tokyo 113, Japan. R.J. FRUEHAN, Professor, is with Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted November 13, 1985. METALLURGICALTRANSACTIONSB
reaction as a function of slag composition, oxygen potential, and temperature are given.
II.
EXPERIMENTAL
A schematic of the experimental arrangements used is shown in Figure 1. In one set of experiments, five grams of a Na20-SiOz-NaCN slag was equilibrated with five grams of a carbon saturated Fe-Si-N alloy in a graphite crucible as shown in Figure l(a). With this experimental setup there is a possibility of Na20 loss for NazO-rich slag by the reaction Na20 + C = 2Na(v) + CO
[1]
In order to maintain a constant sodium potential, a second experimental arrangement w
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