The Effect of Oxygen Potential on the Sulfide Capacity for Slags Containing Multivalent Species
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ON
1=2
SULFUR
removal has been an important research topic for metallurgists for many decades, since precise control of sulfur in steel is essential. A substantial amount of work has been carried out to understand the behavior of sulfur in slags in order to improve desulfurization. Richardson and Fincham introduced the sulfide capacity concept, which enabled the direct comparison of the desulfurization abilities of different slags.[1,2] They found that the sulfide capacity was only dependent on slag composition and temperature, but independent of oxygen and sulfur potentials over some ranges. Since its inception, the sulfide capacity concept has been widely adopted by both industry and academia. An impressive amount of work has been conducted measuring sulfide capacities for many different slag systems (it would be a huge task to include all references here). Recently Kang and Pelton compiled a database of sulfide capacities for systems including the oxides Al2O3, CaO, FeO, Fe2O3, MgO, MnO, SiO2, and TiOx.[3] Several models have also been developed based on the experimental data for the estimation of sulfide capacities.[4] The sulfide capacity can be derived according to the following gas–ion exchange reaction between sulfur gas and free oxygen ions in the slag: 1 1 2 S2 ðg) þ O2 Slag ¼ SSlag þ O2 ðg): 2 2
½1
On the basis of reaction [1], the sulfide capacity is defined as
CARL ALLERTZ, Ph.D. Student, and MALIN SELLEBY and DU SICHEN, Professors, are with the Department of Science and Engineering, Royal Institute of Technology (KTH), 100 44 Stockholm, Sweden. Contact email: [email protected] Manuscript submitted February 23, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
CS ¼ ðpct SÞSlag
p O2
1=2 p S2
¼
aO2 Kð1Þ ; fS2
½2
where (pct S) is the concentration of sulfur in the slag in mass pct, pO2 and pS2 the partial pressures of oxygen and sulfur, aO2 the activity of oxygen ions, and fS2 is the activity coefficient for sulfide ions, and K(1) is the equilibrium constant of reaction [1]. Two main techniques have historically been used to determine the sulfide capacities of slags, gas-slag equilibrium, and slag-metal equilibrium.[1,2,4–33] In both techniques, the oxygen and sulfur potentials are either controlled or measured. Note that the precondition for using the sulfide capacity is the assumption that aO2 and fS2 are constant for a specific slag composition, meaning that the sulfide capacity is independent of oxygen and sulfur potentials. Richardson and Fincham found that the sulfide capacity was only a function of temperature and slag composition for some slags systems in the oxygen and sulfur potential ranges relevant to ironmaking and steelmaking.[1,2] It is reasonable that this would be the case as long as the oxide components in the slag does not react with the gas phase, viz. the structure of the slag does not vary with the oxygen and sulfur potentials. A number of measurements have been performed for slags containing multivalent elements such as Cr and Fe[1,3,4,7–33] Even one of the present authors has made t
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