Distribution of P₂O₅ between Solid Solution of 2CaO.Si0₂-3CaO.P₂O₅ and Liquid Phase

It is desirable to decrease the phosphorous content of steel and the amount of slag in steel refining. For satisfying this requirement recently, the importance of multiphase slag in steel refining has been given considerable attention. Normally, hot metal dephosphorization slag consists of the CaO-Si0₂-FeO-P₂0₅ system, and the industrial operation is mainly carried out in the dicalcium silicate (C₂S) saturated region. It is well known that C₂S forms a solid solution with the main product of dephosphorization-tricalcium phosphate (C ₃P)-at the treatment temperature over a wide composition range, and a high distribution ratio of P₂0₅ between the solid solution and the liquid phase has been reported. In order to determine the ruling factors on the distribution ratio, the influences of MgO, MnO, and A1₂0₃ were investigated for various slag compositions in the case of FeO or Fe₂0₃ as the iron oxide. First, a mixture of a standard regent was heated to the melting temperature in order to produce homogenous liquid slag. Next, it was cooled down to a semisolid state: during cooling, the solid solution of C₃P-C₂S was precipitated under the equilibrium condition. For clarifying the influence of slag composition on the distribution ratio, the ruling factors on the activity coefficient of P₂0₅ in the solid solution and liquid slag phase were evaluated. It is the contention of this research that the activity coefficient of P205 in the solid solution was largely influenced by P ₂0₅ and the total solved oxide content of the solid solution. (log γp₂,O,_5SS)_cal was obtained empirically in order to represent the activity coefficient in the solid solution. On the other hand, the activity coefficient of P₂0 ₅ in the liquid phase was strongly influenced by the CaO content of the liquid phase. On the basic of these correlations, it was shown that the CaO content of the liquid phase and (log γp₂,O,_5SS) _cal are the ruling factors on distribution ratio.