Distribution of P 2 O 5 and Na 2 O Between Solid Solution and Liquid Phase in the CaO-SiO 2 -Fe 2 O 3 -P 2 O 5 -Na 2 O S

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WITH the deterioration of iron ore quality caused by the increased mining and utilization of high phosphorus iron ores, the phosphorus content in hot metal is gradually increasing.[1] To meet the demand for low phosphorus steel and to ease the environmental burden, a highly efficient dephosphorization process with less slag is necessary. Hot metal dephosphorization with a multi-phase slag is well developed in Japan, which increases the efficiency of dephosphorization, and also reduces CaO consumption and slag emission. Dephosphorization slag mainly consists of a CaOSiO2-FetO-P2O5 system saturated with di-calcium silicate (2CaOÆSiO2). The 2CaOÆSiO2 forms a 2CaOÆ SiO2-3CaOÆP2O5 (C2S-C3P) solid solution with the dephosphorization product of 3CaOÆP2O5 over a wide composition range.[2,3] As there is a high distribution ratio of phosphorus between solid solution and liquid phase, Kitamura et al.[4] point out that the solid

CHUAN-MING DU is with the Graduate School of Engineering, Tohoku University, Sendai 980-8577, Japan. Contact e-mail: [email protected] XU GAO, SHIGERU UEDA, and SHIN-YA KITAMURA are with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted June 2, 2017.


solution acts as a sink for phosphorus and lowers the phosphorus content in the liquid phase. To improve the dephosphorization capacity of slag, the effect of Na2O addition on the distribution of phosphorus between the steelmaking slag and metal (LP) has been investigated by many researchers. Muraki et al.[5] found that the distribution ratio of phosphorus between slag and C-saturated iron increased significantly with a small Na2O addition to the CaO-CaF2SiO2 slag doubly saturated with CaO and 3CaOÆSiO2. Pak et al.[6] reported that the addition of Na2O to CaO-SiO2 slags significantly increased LP and the phosphate capacity. In steelmaking slags containing 40 mass pct of FeOt, the addition of 6 mass pct Na2O increased LP by a factor of 5. Suito et al.[7] and Kunisada et al.[8] evaluated the effect of Na2O on the activity coefficient of P2O5 in slag and found that it decreased with the addition of Na2O to steelmaking slag. These results indicate that a small addition of Na2O to steelmaking slag can greatly increase the efficiency of dephosphorization. It also means that a small amount of slag containing Na2O may achieve dephosphorization. On the other hand, the phosphorus in steelmaking slag, especially slag with high P2O5 content, is considered a potential phosphate resource. To recover phosphorus, the P2O5 should be further enriched in the solid solution and separated from slag. A series of studies on the distribution of P2O5 between the solid solution and liquid phase ðLP2 O5 Þ have been conducted. Ito et al.[9]

clarified that the distribution ratio of P2O5 increased with increasing total iron content in slag, regardless of the valency of iron. Shimauchi et al.[10] pointed out that the P2O5 content in the solid solution increased wi