Phase Equilibria of the MnO-SiO2-Al2O3-MnS System

Abstract

Phase equilibria of the MnO-SiO2-MnS system in the temperature range of 1185 degrees-1400 degrees C and the MnS solubility in the MnO-SiO2-Al2O3-MnS system at 1200 degrees C under low oxygen partial pressure have been experimentally investigated using equilibration/quenching technique followed by electron probe X-ray microanalysis and X-ray diffraction analysis. Based on the experimental results, polythermal projection of the MnO-SiO2-MnS system was estimated. The system has two ternary eutectic points; namely, one at 1173 degrees +/- 3 degrees C where phases of MnS, MnO, "A" (67.0 mass% MnO-23.6 mass% SiO2-9.4 mass% MnS), and liquid coexist, and the other at 1187 degrees +/- 3 degrees C where phases of MnS, Mn2SiO4, MnSiO3, and liquid coexist. All invariant reactions observed above were discussed. The ternary compound "B" containing 74.4 mass% MnO-22.0 mass% SiO2-3.6 mass% MnS was observed at 1300 degrees C. In the MnO-SiO2-Al2O3-MnS system, the MnS solubility is strongly affected by Al2O3 content and also the MnO/SiO2 ratio. The solubility increased as MnO/SiO2 ratio increased at a constant Al2O3 content and decreased as Al2O3 content increased at a constant MnO/SiO2 ratio. In addition, depending on the Al2O3 content and MnO/SiO2 ratio, the liquid phase is possibly saturated with not only MnS but also other solid oxide phases. In view of inclusion utilization for free-cutting steel, it might have an advantage to decrease the Al2O3 content and increase the MnO/SiO2 ratio.