Identification of Rate, Extent, and Mechanisms of Hot Metal Resulfurization with CaO-SiO 2 -Na 2 O Slag Systems
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IN blast furnace-based steelmaking, sulfur is considered as one of the main impurities in hot metal. The removal of sulfur is commonly conducted with powder injection. In powder injection, a desulfurization reagent is injected into hot metal with the help of a carrier gas through an immersed lance. Suitable desulfurization reagents include lime, calcium carbide, magnesium, soda ash, limestone, and zinc oxide. Hot metal desulfurization with powder injection consists of two main reactions[1]: (i) Transitory contact reaction (reagent–metal) (ii) Permanent contact reaction (metal–slag)
TERO VUOLIO, VILLE-VALTTERI VISURI, and TIMO FABRITIUS are with the Process Metallurgy Research Unit, University of Oulu, P.O. Box 4300, FI-90014, Finland. Contact email: tero.vuolio@oulu.fi TIMO PAANANEN is with the SSAB Europe Oy, Rautaruukintie 155, P.O. Box 93, FI-92101, Raahe, Finland. Manuscript submitted February 13, 2019. Article published online May 8, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS B
In the thermodynamic calculations conducted by Pal and Patil,[1] it was suggested that the majority of sulfur is extracted via the transitory contact reaction, but the equilibrium sulfur content is often higher for the permanent contact reaction.[1] This implies the existence of a thermodynamic driving force for the inverse reaction that is referred to as resulfurization. The kinetics of the desulfurization of hot metal via a permanent phase contact has been studied intensively for Na2O-containing slag systems in References 2 through 4. However, the resulfurization of hot metal has drawn less attention. Early observations on the resulfurization of hot metal in the case of the CaO-SiO2-Na2O system were made in the study of Schenck et al.[2] The authors conducted experiments on desulfurization in the Fe-C-S system and identified that the changes in the sulfur and iron oxide contents of the slag followed were found to be associated with each other, but the authors did not present the mechanisms behind the resulfurization reaction.[2] Liu et al.[5] studied the kinetics of resulfurization of steel in the CaO-FeO-SiO2-CaS slag system. In their study, it was established that resulfurization of steel follows 1st-order reaction kinetics, and is controlled by mass
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transfer in the slag phase. The authors also suggest that the increased liquid phase fraction in the slag phase could increase the rate of resulfurization due to the increased rate of mass transfer at the interface.[5] However, the authors did not consider the fact that the studied fluxes (FeO and CaF2) both decrease the sulfide capacity according to van Niekerk and Dippenaar[6] and Schenck et al.[2] This being so, the rate of resulfurization increases due to the increase in the thermodynamic driving force, which assumedly has a greater effect on the molar flux of sulfur through the metal–slag interface than the increased rate of mass transfer associated to the liquid phase portion. Also in their study, the sulfur content of the slag phase was only 0.
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