Dynamics of the hot metal dephosphorization with Na₂O slags

Abstract

Dephosphorization reaction of hot metal by Na₂CO₃ has been studied experimentally to determine the reaction mechanism and thermodynamics. Most of the experiments were carried out at 1300 °C using Fe-C_sat.-Si-P-S alloys. The results indicate that the CO₂ gas released from Na₂CO₃ is important in the dephosphorization reaction as an oxidizer and increasing mass transfer by stirring the slag and metal. As the initial Si content in hot metal is increased, the degree of dephosphorization decreases significantly and the rephosphorization takes place earlier. The primary reason for the rephosphorization is that the activity of PO_2.5 increases in the slag because of the evaporation of Na₂O from the slag. The loss of Na₂O increases the activity coefficient of PO_2.5 and decreases the slag volume. At the later stage of Na₂CO₃ treatment, the reactions reach equilibrium with respect to phosphorus and sulfur, and the oxygen potential,P _o2, at the slag-metal interface is determined by the C-CO equilibrium (a _c=1 and 1 atm CO). The presence of sulfur in the metal increases the rate of the dephosphorization because of the electrochemical nature of the reaction; sulfur transfer to the slag accepts the electrons from phosphorus transfer.