Mechanism of hot metal dephosphorization by injecting lime base fluxes with oxygen into bottom blown converter.

An application of bottom blown converter (Q-BOP) has been devised to dephosphorize hot metal from [%P]i=0.140 to [%P]f=0.010 and desulfurize from [%S]i=0.020 to [%S]i=0.010 by injecting with O2 gas a lime fluorspar mixture of a low 23kg/t within only 2-4min. Experiments have been made to determine the contributions to the dephosphorization of permanent reactor reaction by top slag and of transitory reaction by lime powders injected with O2 gas from bottom tuyeres and ascending in the bath.The ratio of dephosphorization, 100 ([%P] i-[%P]f)/[%P]i, is reached 90-95% when fluorspar is added either to injected lime or to resulting top slag, whereas it decreased to 50-70% without the flourspar addition. Detailed analysis has shown that 50-70% of the above ratio of dephosphorization is shared by the transitory reaction, the rest of 0% (without fluorspar) to 25-40% (with fluorspar) by the permanent reactor reaction. Slag particles ascending in the bath have been sampled and found to contain 10-30% P2O5, confirming the considerable contribution of the transitory reaction.With the aid of additional laboratory experiments, the following sequence is found operative for the dephosphorization: Lime particle injected with O2 gas is transformed to liquid calcium ferrite. During ascending in the bath, calcium ferrite oxidizes [P] to P2O5, itself being reduced to CaO which combines with P2O5 to form 3CaO•P2O5 particles. The 3CaO• P2O5 particles ascend to be finaly assimilated in top slag to complete the sequence of dephosphorization.