Hot Metal Desulfurization by CaO–SiO₂–CaF₂–Na₂O Slag Saturated with MgO

Abstract

The effect of CaF₂ and Na₂O on the sulfide capacity of the CaO–SiO₂–CaF₂–MgO_satd.(–Na₂O) slags for hot metal desulfurization was investigated at 1623 and 1723 K by taking the role of MgO as an important refractory component into account. The liquidus line of the MgO-saturated system was similar to the CaO–SiO₂–CaF₂ ternary slag system. The silica content for the 2CaO·SiO₂-saturation was slightly decreased with MgO saturation. Although the sulfide capacity at MgO-saturation is lower than that of the CaO_satd.–SiO₂–CaF₂ slag system, it is greater than the ternary slag system saturated with 2CaO·SiO₂ phase. This suggests that MgO can increase the activity of free O²⁻ ions under conditions of relatively low CaO activity. Near the composition of 3CaO·SiO₂-saturation, MgO decreased the sulfide capacity of the slag because MgO is less basic than CaO in highly basic compositions. In addition, the addition of 5 mass% Na₂O significantly increases the sulfide capacity of the MgO-saturated slag especially in the composition of low ‘CaO+CaF₂’. This suggests that the input amounts of ‘CaO+CaF₂’ can be reduced from 95 to 75 mass% for maintaining the sulfide capacity of 0.01 at 1623 K, which is important considering the reduction of CaO and CaF₂ consumption in hot metal desulfurization process with MgO saturation.