Effects of Oxygen Pressure, Al₂O₃ and MgO on the Liquidus Surface of FeO_x-SiO₂-CaO System

Abstract

Various basic data are available for FeO_x-SiO₂-CaO system in coexistence with metallic iron, which are useful to reductive smelting, but available data are scarce on the effect of minor oxides, especially at higher oxygen potential. In the present paper, the liquidus surface data are reviewed precisely, and recent progress of the experimental and thermodynamic studies is introduced. On the olivine (fayalite) liquidus surface which spreads widely in reductive condition, the addition of CaO or Al₂O₃ decreases the liquidus temperature, but at higher oxygen pressure more than 10^-9 atm, olivine is replaced by spinel (Fe₃O₄). Above 10^-9P_O2, the silicate liquid region in the FeO_x-SiO₂-CaO system is limited by 4 primary surfaces of Fe₃O₄, SiO₂, CaSiO₃ and Ca₂SiO₄, and solid Fe₃O₄ surface extends substantially by increases in oxygen pressure, Al₂O₃, and CaO as well as decrease in temperature. However, addition of Al₂O₃ tends to enlarge the liquid region at SiO₂, CaSiO₃ and Ca₂SiO₄ surfaces. The addition of MgO increases the melting temperature substantially regardless the kinds of primary crystal surface. Besides conventional fayalite and calcium ferrite slags, the ferrous calcium silicate (FCS) slag is newly proposed. Instead of triangle FeO_x-SiO₂-CaO diagram, the phase diagram of CaO plotted against Fe/SiO₂ is useful especially for the expression of multi-component system at higher oxygen potential. Also to predict the liquidus surface of these complex systems, the computer aided thermodynamic assessments must be useful method.