Reducing dephosphorization of simnalloy by CaO-CaF2 slag and the stability of Ca3P2 in slag under atmospheric cooling conditions

Abstract

This study consists of experimental measurement of the phosphide capacity of the CaO-CaF2 (SiO2) slag in equilibrium with SiMn alloy at 1823 K, and includes the thermodynamic analysis for the stability of reducing refining slags under various cooling conditions. Because Ca3P2, which is a reaction product of the reducing dephosphorization mechanism, is very active, the phosphine gas (PH3), which is hazardous to environment as well as to human being, is evolved when it contacts to moisture. The phosphide capacity increased with increasing CaO concentration in the slag, followed by a constant value. The composition for the saturating capacity is in good accordance to the saturation content of CaO in the CaO-CaF2 slag at 1823 K. When the slag disintegrated into fine powders not only due to the formation of dicalcium silicate but also due to the hydration of lime in the highly basic slags, the evolution of phosphine gas during cooling increased. When the Vee ratio (=CaO/SiO2) of the dephosphorization slag is greater than about 1.35, lime and dicalcium silicate phases appeared during cooling cycle based on the CaO-SiO2-CaF2 phase diagram, resulting in an increase in the evolution rate of PH3 gas due to an increase in the reaction area. However, when the Vee ratio of the slag is lower than about 1.35, the CaF2, Ca4Si2F2O7 (cuspidine), and CaSiO3 (wollastonite) phases appeared from the phase diagram, resulting in negligible amount of PH3 evolution during cooling because the reaction between Ca3P2 and H2O was restricted to the surface of bulk slag. Alternatively, the Ca3P2 phase was successfully transformed to the stable 3 CaO. P2O5 phase by oxygen injection into the molten dephosphorization slag at high temperatures. © 2020 INFACON.All rights reserved.