Thermodynamic stability of SFCA (silico-ferrite of calcium and aluminum) and SFCA-I phases

Silico-ferrite of calcium and aluminum (SFCA) and SFCA-I phases form in iron ore sintering operations. Their behavior in blast furnaces, with sinter being a significant component of blast furnace burden, is of critical importance to the ironmaking process. They have complex disordered crystal structures and form over a range of composition. In the present work, enthalpies of formation from oxides for five different SFCA compounds and one SFCA-I phase were measured by oxide melt solution calorimetry in molten lead borate solvent at 800°C. The enthalpies of formation from binary oxides are zero within experimental error for SFCA phases and slightly endothermic for SFCA-I, confirming that SFCA phases are stabilized, not by energetics, but by their configurational entropies resulting from atomic site disorder. In addition, enthalpies of drop solution into a molten slag at 1450°C were measured for SFCA phases and show good agreement with values predicted using the heats of formation and previously measured drop solution enthalpies for the binary oxides. This agreement confirms data consistency and shows that the presence of variable amounts of SFCA relative to binary oxides in a sinter will have negligible effect on the heat balance in the blast furnace.