Surface segregation of calcium oxide in wustite and its effects on the reduction
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I.
INTRODUCTION
THE addition of a small amount of calcium oxide to wustite facilitates the reduction of wustite in the mode in which a porous type of iron forms, resulting in accelerated reduction of the wustite, t~-s~ Such effects of added calcium oxide reach the maximum at small amounts as low as about 1 mass pct, followed by a slight weakening of the effect with any further addition, t~,3) Therefore, the effect might be caused by its segregation to the surface of wustite and the reduced iron, which are in contact with each other, and/or by its segregation to the wustite/reduced-iron interface. There have been several studies concerning the segregation of calcium oxide at grain boundaries of magnesia [6,7"81and alumina, jT,9-J~) Since wustite is of the same structure as magnesia, that is, a rock salt structure, and forms a continuous solid solution with it, it would be reasonable to expect calcium oxide to segregate at the surface of wustite as well as at the surface of magnesia. On the other hand, the presence of a very small amount of hydrogen-sulfide in hydrogen, especially in hydrogen containing water vapor, can change the morphology of reduced iron from a dense to a porous layer in almost the same mode as was observed for the reduction of wustite doped with calcium oxide, t~2,13~ Since it is well known that sulfur is a typical surface-active element, tlal sulfur may affect the morphology of iron by segregating at the surface of metallic iron 1~5)and wustite, or at their interface. Thus, analyzing the surface should be truly essential to elucidate the mechanism by which calcium oxide gives rise to the effect already mentioned. In this study, the equilibrium surface concentration of calcium oxide was YOSHIAKI IGUCHI, Professor, and SHOJI HAYASHI, Research Assistant, Department of Materials Science and Engineering, and KEISUKE GOTO, Professor, Department of System Engineering, are with Nagoya Institute of Technology, Nagoya 466, Japan. Manuscript submitted June 9, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
measured in situ on the wustite and on the iron in contact with the wustite at the reduction temperatures by means of Auger electron spectrometry, using a specially designed hot stage. In addition, a reduction experiment was performed with wustite plates containing calcium oxide to examine the relationship between the reduction rate and the surface concentration of calcium oxide.
II.
MATERIALS AND PROCEDURES
A reagent grade of ferric oxide (Kanto Chemicals Ltd., Tokyo, Japan) was used as the starting material. The ferric oxide powder was pelletized, sintered, reduced with a 50 pet CO-50 pet CO2 gas mixture at 1273 K, and pulverized to wustite powder, the chemical composition of which is shown in Table I. Then calcium oxide and wustite were weighed and mixed to give molar fractions of calcium oxide to total moles o f Fe0.9240 and calcium oxide of 0.0001, 0.001, 0.003, 0.005, and 0.01. By arc melting the powder mixture using a nonconsumable electrode of tungsten in a high-purity argon atmosphere deo
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