Thermodynamics of Ca-CaF₂ and Ca-CaCI₂ systems for the dephosphorization of steel

Calcium is soluble in halide salts which can be used to remove phosphorus from steel as a phosphide ion. The activity and activity coefficient of calcium phosphide, and the equilibrium phosphorus distribution ratio between Ca-CaF₂ and Ca-CaCl₂ fluxes and pure solid iron were measured as a function of the Ca composition in the flux at 1350 °, 1400 °, and 1450 °. The Ca-Ca halide fluxes were equilibrated with pure solid iron and a Ag-Ca alloy in an iron crucible under an Ar atmosphere. The Ag-Ca alloy was used to maintain a constant chemical potential of calcium. Phosphorus distribution between between these fluxes and solid pure iron increased with increasing calcium activity and decreasing temperature. The activity coefficient of γ_{Ca 1.5 P} was calculated to be 36.6 at 1350 ° and 11.0 at 1450 ° for a calcium activity of 0.2 (wt pct Ca = 2.5) in the Ca-CaF₂; the activity coefficient increases with increasing Ca in the flux. In addition, the activity of Ca in the Ca-Ca halide fluxes was determined. The equilibrium phosphorus distribution ratio between Ca-Ca halide systems and molten chromium steel was calculated as functions of Cr and C contents of the metal and calcium activity in the flux at 1600 °C by using γ_{Ca 1.5 P} obtained in the present work. This ratio was found to be about 20 for 18 pct Cr stainless steel at 1600 °.