Effect of CaF 2 on Phosphorus Refining from Molten Steel by Electric Arc Furnace Slag using Direct Reduced Iron (DRI) as
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UCTION
USE of direct reduced iron (DRI) as an alternative iron source in electric arc furnace (EAF) steelmaking has grown in popularity because DRI has no tramp elements, a uniform density and shape, a higher yield and purity of carbon, charging convenience, lower cost, and low CO2 emissions.[1–9] Unfortunately, phosphorus level in commercially available DRI is relatively high, which adversely affects the properties of steel and increases the likelihood of phosphorus contamination.[10,11] Phosphorus should, therefore, be strictly controlled during the EAF process. However, Heo and Park recently reported that the degree of dephosphorization strongly depends on DRI mixing ratio: the higher is the DRI mixing ratio, the lower is the degree of dephosphorization because of the variety of gangue oxides (e.g., SiO2 and Al2O3) in DRI.[12] These oxides reduce the dephosphorization efficiency by lowering the basicity and the stability of phosphate ion in the slag. In addition, use of DRI increases absolute slag volume, which increases electric
MIN KYO OH and TAE SUNG KIM are with the Department of Materials Science and Chemical Engineering, Hanyang University, Ansan 15588, Korea. JOO HYUN PARK is with the Department of Materials Science and Chemical Engineering, Hanyang University and also with Department of Materials Science and Engineering, KTH Royal Institute of Technology, 100 44, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted May 22, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
power consumption for refining, due to an increase in consumption of basic oxide fluxes such as burnt lime.[12,13] Therefore, in an EAF process using DRI, slag must be designed to minimize the amount used and to compensate for decreased dephosphorization efficiency. Dephosphorization in steelmaking process involves oxidizing refining, which is affected by basicity of slag and oxygen potential. The partial pressure of oxygen at the slag-metal interface is determined by FetO activity in the slag. In addition, basicity of slag is generally determined by CaO activity. Therefore, the thermodynamic behavior of both FetO and CaO in the slag should be inevitably considered.[14–16] The effects of small amounts of oxides and fluorides on the thermodynamic behavior of phosphorus in slag containing CaO and FetO have been widely studied. Dephosphorization potential depends on the activity coefficient of P2O5 in the slag. The lower is the activity coefficient, the higher is the stability of phosphate ions in the slag. Kor reported that activity coefficient of P2O5 in the CaO-MgOsat-FetO-SiO2 slag decreased with increasing content of CaF2 (1 to 5 mass pct) at 1823 K (1550 °C) in an argon atmosphere,[17] which possibly originated from the fact that CaF2 reduced the activity coefficient of P2O5 due to the formation of fluorapatite in the slag. Similar results were also confirmed by Suito and Inoue in the same slag system at 1823 to 1923 K (1550 to 1650 °C) in an argon atmosphere.[18,19] Shirota et al.[20] reported dephosphorization of levitated li
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