A thermodynamic study of BaO-BaF 2 -Cr 2 O 3 system fluxes used for dephosphorization of chromium-containing iron melts
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I. INTRODUCTION
APPROPRIATE processing conditions and optimum fluxes are very important for stainless steel refining. For high-alloy stainless steel, dephosphorization under oxidizing condition results in chromium loss, so appropriate oxygen potential must be ensured. Alkaline or alkaline-earth metal oxide should be included in fluxes to dephosphorize. However, the former is not suitable because of rephosphorization, high volatility, environmental pollution, and refractory erosion. Many fluxes have been developed for dephosphorization and chromium retention for chromium containing iron melts, such as BaO-BaCl2-Cr2O3[1] and CaO-BaO-CaF2-Cr2O3[2] systems, from which satisfactory effects have been attained. It has been found that the phosphate capacities of BaOBaF2 base fluxes are higher than most of the other dephosphorization fluxes, because of the higher basicity of BaO and the decreasing melting point of the fluxes from the BaF2.[3,4] Moreover, BaO-BaF2-Cr2O3 fluxes are suitable for dephosphorization and chromium retention because of Cr2O3 addition reducing chromium loss and controlling oxygen potential, in addition to the preceding two causes. However, since thermodynamic studies on the fluxes have not been carried out yet, it is necessary to do more theoretical and experimental work in this field. There have been many studies on thermodynamic properties of the fluxes such as activities and phase diagrams. Mohanty and Kay[5] determined the activities of Cr2O3 in CaO-CaF2-Cr2O3 slags by a chemical equilibrium process, and Inouye et al.[6] determined activities of Cr2O3 in BaOBaCl2-Cr2O3 system fluxes by an electromotive force (EMF) process. In the present study, BaO-BaF2-Cr2O3 fluxes and LIAN-FU LI, Lecturer, MAO-FA JIANG and WEN-ZHONG WANG, Professors, and CHENG-JI JIN, Graduate Student, are with the Department of Ferrous Metallurgy, School of Materials and Metallurgy, Northeastern University, Shenyang 110006, People’s Republic of China. ZHAO-PING CHEN, formerly Associate Professor, Department of Ferrous Metallurgy, School of Materials and Metallurgy, Northeastern University, is Senior Engineer with the Technology Center, Baosteel, Shanghai, China 201900. Manuscript submitted April 29, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
{Cu-Cr} alloy are used in equilibrium, and a solid electrolyte cell, 2Mo.Cr 1 Cr2O3|ZrO2(MgO)|{Cu-Cr}alloy 1 (Cr2O3)fluxes.Mo1
[1]
was used to determine the partial pressure of oxygen in the melting bath, so that activities of Cr2O3 can be acquired and isothermal phase diagrams can be inferred. II. EXPERIMENTAL ASPECTS As shown in Figure 1, the experimental system included four subsystems: gas cleaning, temperature controlling, furnace, and data acqusition. The EMF and temperature were acquired by the HP3497A* Data Acquisition/Control Unit *HP is a trademark of Hewlett-Packard, Colorado Springs, CO.
and HP9000 serials computer. The furnace subsystem included a SiC resistance furnace, alumina reaction tube, alumina crucible, magnesia reaction crucible, and solid electrolyte cell.
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