Measurement of FeO activity and solubility of MgO in smelting slags
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INTRODUCTION
IN bath smelting, the rate of reduction is proportional to the FeO content of the slag in the smelter. The sulfur content of the metal is approximately proportional to the FeO activity in the slag.[1] In contrast, the phosphorous content of the metal is approximately inversely proportional to the FeO activity in the slag. Therefore, knowledge of the relationship between slag composition and FeO activity is necessary to control the reduction of FeO and the metal chemistry with respect to sulfur and phosphorus. Also, in order to obtain better control of the process operation, it would be useful to measure the FeO activity of the slags directly and rapidly. The typical composition of bath smelting slags[2] (2 to 5 wt pct FeO, 10 to 18 wt pct MgO, 1 to 2 wt pct MnO, and 7 to 12 wt pct Al2O3) is different from those in conventional steelmaking processes. Many studies have been carried out on the determination of the activity of FeO in multicomponent slag systems.[2–11] In these works, the FeO content of the slag was higher than that of the typical bath smelting slags. Information on the thermodynamics of CaO-SiO2Al2O3-MgOsat-FetO slags with low FeO content (⬍10 wt pct FeO) is limited.[2,12–17] Special reference should be made to the work of Iwase and co-workers, who measured the activity of FeO in a number of slags using an electromotive force (EMF) technique; their work is summaried by Ogura et al.[17] The thermodynamic relationships among components in the gas-slag-metal system of the bath smelter are too complicated to be calculated by the limited experimental data available. It is also not reliable to estimate the thermodynamic SHIH-HSIEN LIU, formerly Graduate Student, Materials Science and Engineering Department, Carnegie Mellon University, is Engineer, China Steel Corporation, Taiwan 81233, Republic of China. R.J. FRUEHAN, Professor, is with the Materials Science and Engineering Department, Carnegie Mellon University, Pittsburgh, PA 15213. A. MORALES, formerly Research Assistant, Materials Science and Engineering Department, Carnegie Mellon University, is Research Engineer, General Motors Corporation, Warren, MI 48090. B. OZTURK, formerly Research Faculty Member, Carnegie Mellon University, is Research Engineer, Allegheny Ludlum Corporation, Brackenridge, PA 15104. Manuscript submitted July 30, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
parameters by using existing slag models.[18–22] Therefore, direct measurements of the FeO activity of CaO-SiO2-Al2O3MgOsat-FetO slags, equilibrated with liquid iron, are required. In this study, the EMF technique was used to measure the FeO activity in the slags in equilibrium with iron. Oxygen EMF cells were used to measure the oxygen potential of the liquid iron that is in equilibrium with the liquid slags. Magnesia-stabilized zirconia was the solid electrolyte of the oxygen cell used in this study, and Cr-Cr2O3 mixed powder was used as the reference electrode. Based upon thermodynamic equilibrium of the reaction, Fe(l) ⫹ 1/2O2(g) ⫽ FeO(slag) the FeO activit
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