The reaction efficiency of hot metal desulfurization by lime powder injection with nitrogen gas is improved by prior addition of aluminum. That is, the rate of desulfurization is higher in a Fe–C_sat.–Si–Al–S melt than in a Fe–C_sat.–Si–S melt. The reaction layers of lime lumps immersed in the melts were examined by electron probe microanalysis and X-ray diffraction. The investigation revealed the mechanism by which the addition of aluminum improves the reaction efficiency of the subsequent lime injection.
In the Fe–C_sat.–Si–S melt, solid 2CaO·SiO₂ and 3CaO·SiO₂ layers are formed on the lime surface, and thereby retards the transfer of sulfur to its internal part. On the other hand, a fusible CaO–Al₂O₃–FeO layer is formed on the lime surface in the Fe–C_sat.–Si–Al–S melt, promoting the inward transfer of sulfur. Furthermore, the layer dissolves as much sulfur as up to approximately 50%.
The aluminum concentration needed to obtain this improvement of desulfurization in a 250 ton torpedo ladle was the aluminum content of (0.005+ΔAl)%, where ΔAl is the aluminum lost during the desulfurization process.