Activities of Na₂O in CaO–based Slags Used for Dephosphorization of Steel

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The activity of Na₂O in CaO-based slags has been measured employing a galvanic cell technique with a β-Al₂O₃ solid electrolyte. The iso-activity lines for Na₂O were obtained over the liquidus composition range in the CaO–Na₂O–SiO₂ system at 1 400°C. The activity of Na₂O in CaO–based slag is strongly controlled by the SiO₂ content in slag and the activity coefficient of Na₂O increases as the CaO content increases. The activity of Na₂O in a 50%CaO–40%Al₂O₃–10%SiO₂ (typical ladle slag composition) is extremely high. At a Na₂O concentration of about 8 wt% in this slag, the activity of Na₂O is nearly equal to that of 50wt%Na₂O–SiO₂ slag at the same temperature. The Richardson solution model fits the experimental data well and this model was used to compute the activity of CaO in these systems. The addition of Na₂O significantly increases the activity of CaO.