Dephosphorization in Double Slag Converter Steelmaking Process at Different Temperatures by Industrial Experiments

In the present work, the effect of dephosphorization endpoint temperature on the dephosphorization of hot metal was studied for the double slag converter steelmaking process under the conditions of low temperature and low basicity by the industrial experiments. In the temperature range of 1350–1450 °C, with an increasing dephosphorization endpoint temperature, the dephosphorization ratio and phosphorus distribution ratio first increase and then decrease. The phosphorus content in hot metal first decreases and then increases at the end of dephosphorization. At the dephosphorization temperature range of 1385–1410 °C, the dephosphorization ratio is higher than 55%, the P2O5 content in the dephosphorization slag is 3.93–4.17%, the logLP value is 1.76–2.09, the value of PCOP-C of the selective oxidation reaction of carbon and phosphorus is 53–80 Pa, and the aFeO value is 0.284–0.312. The path of phosphorus in hot metal entering the P-rich phase of dephosphorization slag can be reasonably inferred as: hot metal → Fe-rich phase → P-rich phase. Under the present industrial experimental conditions, the dephosphorization and rephosphorization reactions are in dynamic equilibrium at 1413 °C. Considering the experimental results and thermodynamic calculation results of industrial experiments by the double slag dephosphorization process, the optimal temperature range for intermediate deslagging is about 1400–1420 °C.

New trends in basic oxygen furnace dephosphorization

Except for special grades of steel where it is used as an alloying element, phosphorus is regarded as an impurity that must be removed. Considering the conventional integrated iron and steelmaking, there are primarily two processes for phosphorus removal. The first is a hot metal dephosphorization (DeP) process that is applied to a blast furnace for hot metal before the steelmaking process. The second is the basic oxygen furnace steelmaking (BOS), a unique method primarily used for steelmaking, with the exception of stainless steels. Hot metal phosphorus content has a direct impact on BOS. An increase of phosphorus in hot metal is mainly related to the use of high P2O5 containing iron ores. In the current literature review, new trends of phosphorus removal in converter steelmaking are outlined. The double-slag practice was reported to be successful when hot metal P content was larger than 0.100%. It was indicated that the tapping temperature was critical for the production of low-phosphorus grades for which maximum allowable P content was 0.007% and that high tapping temperatures should be avoided. The tap-to-tap time for the double-slag process was slightly longer than the conventional converter steelmaking. It was further reported that the double-slag practice would be more economical than an establishment of a separate hot metal dephosphorization unit, if low-phosphorus grades did not have a significant share in the product mix of a steelmaking company. Endpoint phosphorus prediction was one of the important recent trends of converter steelmaking. A mixed injection of CO2-O2 to a basic oxygen furnace was applied to enhance dephosphorization, and promising results were reported. Unfortunately, a successful process for recycling of BOS dephosphorization slag has not been reported yet.

Dephosphorization by Double-Slag Process in Converter Steelmaking

The double-slag converter steelmaking process can smelt low- and ultra-low-phosphorus steel and reduce lime and dolomite consumption and the amount of final slag simultaneously. Industrial steelmaking tests on a 150-metric ton converter at the Tangsteel Company were carried out to study this principle and its effect on the dephosphorization ratio and material consumption. The results showed that low-temperature stage could be used with a reduced amount of slag in the double-slag steelmaking process to achieve rapid and efficient dephosphorization. A low-basicity slag (~1.5–2.0) in the dephosphorization stage is required in double-slag process. The dephosphorization ratio reached a maximum of 71 % when the slag basicity was 1.7. The end-point phosphorus content after the smelting process was reduced from an average of 0.018 mass% to an average of 0.011 mass% and the dephosphorization efficiency increased by more than 6 %. The dephosphorization slag could be poured out rapidly when the FeO content was controlled at ~16–20 mass% in the double-slag smelting process. Based on key factors such as an efficient dephosphorization and a rapid iron–slag separation, the production efficiency was improved and the smelting cycle increased by only four minutes over the conventional process.