Phosphorus Distribution between CaO-CaF₂-SiO₂ Melts and Carbon-saturated Iron

Abstract

The information about thermodynamic properties of phosphorus in lime bearing slags at relatively low temperatures is required in connection with the recent progress of hot metal treatment.
In the present study, the equilibrium distribution ratio of phosphorus (L_P) between CaO-CaF₂-SiO₂ melts saturated with CaO, 3CaO•SiO₂ and/or 2CaO•SiO₂ and carbon-saturated iron has been measured under a CO atmosphere at temperatures ranging from 1200 to 1400°C, keeping in mind that the industrial flux always contains some undissolved solids.
The results are summarized as follows:
(1) The distribution ratio, L_P increases with increasing CaO content and has a maximum when the slag is doubly saturated with CaO and 3CaO•SiO₂.
(2) The phosphate capacity is confirmed to be independent of the partial pressure of oxygen in the case that iron-carbon melts are equilibrated with austenite and CO at 1300°C. The phosphate capacity in basic regions of this system is much greater than that of other lime bearing slaps.
(3) CaF₂ is more effective for dephosphorization than CaCl₂, probably because the interaction between fluorine and phosphorus and between fluorine and calcium is stronger than that between chlorine and phosphorus and between chlorine and calcium.
(4) The distribution ratio, L_P increases significantly with a small Na₂O addition to the CaO-CaF₂-SiO₂ system, indicating that the costly soda ash treatment can be replaced by lime-based flux containing a little soda ash.