The Rate of Hot Metal Desulfurization by Na₂O-CaO Flux

Abstract

Study was made on the effect of Na₂O content and phosphorus content of hot metal on the desulfurization rate by Na₂O-CaO mixed flux at 1623K under argon gas atmosphere.
In the case of high phosphorus content of hot metal, the optimum Na₂O content was observed. Na₂O may act not only as a desulfurization material but also as a oxidizer at slag-metal interface by decomposition reaction. The optimum Na₂O content appeared as a compromise between oxygen activity and sulfide capacity. Higher degree of desulfurization was obtained when hot metal of higher phosphorus content was treated. This result was ascribed to lowered oxygen activity at slag-metal interface caused by reduction of Na₂O activity and suppression of Na₂O decomposition by P₂O₅, formed by dephosphorization reaction. The highest degree of desulfurization could be achieved by Na₂O-SiO, flux, and this result support the mechanism described above.
Through the experiment, large amount of unknown sulfur was observed and the rate of unknown sulfur increased with increase in Na₂O content in flux. To reveal this reaction mechanism, coupled reaction model calculation was performed. Although no evidence of gas formation at slag-metal interface could be obtained directly from this analysis, thermo-dynamic considerations suggested the possibility of SO₂ gas formation reaction in slag bulk phase.