Thermodynamic Assessment of the MgO-P2O5 and CaO-P2O5 Systems

The MgO-P2O5 and CaO-P2O5 systems have been thermodynamically assessed based on the available phase diagram and thermodynamic data using the Calculation of Phase Diagram (CALPHAD) method. The liquid phase is described by the modified quasichemical model with the pair approximation, which takes short-range ordering in liquid solution into account. The PO43- is considered as the basic building unit of P2O5 in the liquid solution since the maximum short-range ordering occurs at the M-3(PO4)(2) (M = Mg, Ca) composition. All intermetallic phases are treated as stoichiometric compounds and the phase transformations are considered. A set of self-consistent model parameters is obtained to describe the thermodynamic property of every phase in these two binary systems, by which the published phase diagram, enthalpy, entropy, and activity data are reproduced well within experimental error limits. The present study can be used as a basis for the development of a thermodynamic database of molten slag system for the steelmaking dephosphorus process.