Sulfide capacity of CaO-SiO₂-FeO-Al₂O₃-Mg_Osatd.

The sulfide capacities of CaO-SiO₂-FeO-Al₂O₃-Mg_Osatd. slag were measured at 1 823 K over a wide range of compositions using the thermochemical equilibrating technique. The experimental results indicated that MgO had a marginal effect on desulfurization because of its low thermodynamic driving force for sulfur ion stabilization in slag, whereas Al₂O₃ decreases the sulfide capacity because of its acidic behavior in the present slag system. The effect of FeO on desulfurization can be changed by the slag basicity because the Fe²⁺ cation competes with the Ca and Mg cations, which is reflected in changes in the sulfide stability of FeS in the slag. The depletion of Ca²⁺ cations in an acidic slag electrically bound Ca²⁺ ions with near non-bridging oxygen to create electric neutrality because of its higher ionicity. Thus, the competitive affinity of Fe, Ca, and Mg with sulfur in the slag plays an important role in the effect of the basicity of the slag on the stability of sulfide in a moderate slag composition range. Nonetheless, S2 - ions form CaS instead of FeS in a basic slag because there are excess free Ca²⁺ ions.