The Solubility of MgO-Cr₂O₃ in MgO-Al₂O₃-SiO₂-CaO Slag and the activilies of chromium oxide in the slags were measiredat 1 600°C under Reducing conditions. For MgO-SiO₂ melts which were saturated with 2MgO·SiO₂, the solubility of MgO·Cr₂O₃ increased with decreasing oxygen partial pressure, because of decrease in γ_CrO1.5.When CaO is added to the melts mentioned above, soluble total chromium content decreased because γ_CrO1.5 and γ_CrO increased with increasing CaO content in the melts.
Under a fixed oxygen partial pressure, activities of chromium oxides in the chromite MgO·(Cr_xAl_1-x)₂O₃ were also measured by equilibrating them with Ni-Cr alloys for which the activity coefficient γ°_Cr(at 1 600°C) was determined as 0.421 in this study. It was found that the free energy change of the reaction : MgO(s)+Cr₂O₃(s) = MgO·Cr₂O₃(s) is expressed by-126 800+47.3 T (J/mol) and that MgO·Cr₂O₃ and MgO·Al₂O₃ form a solid solution which exhibits negative deviation from ideal behavior at 1600°C.