Effects of calcium ferrite slag on dephosphorization of hot metal during pretreatment in the BOF converter

A study was carried out to examine effects of calcium ferrite slag on dephosphorization of hot metal in the steelmaking BOF, with aims to pick up the speed of slag-forming, increase the slag dumping rate. Laboratory tests using a resistance furnace were performed to examine melting characteristics of calcium ferrite pellets, designed with the optimum ratio of iron oxides (total Fe 55%) to CaO of 3:1. The pellets were put in crucibles with furnace temperature held in the range of 1250–1350 °C for 10 min. The results showed that the melting of pellets was nearly completed at 1250°C and main mineral phases, detected by XRD, in the melted samples were CaO•Fe2O3 and 2CaO•Fe2O3. Based on results of the laboratory tests, experiments of full scale were performed using a converter of 120t, charging the hot metal with C 3.46–6.81%, Si 0.1–1.16%, Mn 0.12–0.30%, P 0.10–0.18%, S 0.017–0.13% and temperature of 1234–1430 °C. For the 13 test heats with 6 min of oxygen blowing, the rate of dephosphorization and phosphorus distribution ratio reached to 50–60% and 55.9, respectively, when the calcium ferrite pellets were charged. Averaged slag basicity 1.5–2.0, slag melting point 1280–1310℃, slag viscosity 0.5 pa.s were obtained for the BOF heats adding the calcium ferrite pellet, which ensured the slag foaming height controlled at 2.5–3.0 m during the early stages of the blow. Using above smelting process has increased the rate of slag dumping controlled at 50–70% and enhanced the rate of hot metal dephosphorization over 90%.