Cationic effect of charge compensation on the sulfide capacity of aluminosilicate slags

The effect of CaO on the sulfide capacity of CaO-Al₂O₃-SiO₂ slags was studied from the viewpoint of the ionic structure of alumina in slag. The aluminum coordination number was analyzed using ²⁷Al 500-MHz solid nuclear magnetic resonance spectroscopy and the results were compared with those of the sulfide capacity analysis. The sulfide capacity of slag, in the peralkaline region (X_CaO/X_Al2O3 > 1.0), exhibited a linear relationship with respect to basicity (ao^2-) as excess free Ca²⁺ formed a 4-coordinated aluminum unit structure (^[IV]Al; AlO^5-₄) and stabilized the sulfide ions (γ^S2-). However, sulfide capacity in the peraluminous region (X_CaO/X_Al2O3 < 1.0) exhibited a nonlinear relationship with respect to basicity (ao2-) owing to the structure of higher-coordinated aluminum units (^[V]Al, ^[VI]Al; Al³⁺) and the relative lack of Ca²⁺. Therefore, the sulfide capacity of high Al₂O₃-bearing slags strongly depended on the basicity (aO2-) and stability of sulfide ions (γ^S2-), which depended on the competitive behavior of Ca²⁺ owing to the structural changes in Al₂O₃. The effect of the aluminum coordination number on the sulfide capacity was discussed in detail using an analysis of the slag structure and thermodynamics model.