Reaction mechanism for the CaO-Al and CaO-CaF 2 desulfurization of carbon-saturated iron
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INTRODUCTION
THEORETICALLY, lime (CaO) is the most economical reagent that can be used for the desulfurization of hot metal from the iron blast furnace. However, in practice, the amount of CaO required far exceeds the theoretical minimum for desulfurization. The high CaO usage results in higher total reagent cost, more temperature loss, and more iron loss to the greater slag volume. The desulfurization reactions can be written as: CaO(s) + S + C = CaS(s) + CO(g)
[1]
or, in the presence of Si in the metal as: 5CaO(s) + 2_S + Si = 2CaS/s ) + Ca3SiOs(~)
[2]
Thermodynamic calculations indicate that for a Fe-C(,aturated)0.5 pct Si melt at 1450 ~ the equilibrium sulfur content for reaction [1] is about 8 ppm, while that for reaction [2] is even lower. In these calculations, it is important to take into account the effects of C and Si on the activity coefficient of sulfur. Therefore, there is no thermodynamic limitation to the desulfurization of carbon-saturated iron with CaO. The relatively low inefficiency of CaO is believed to be due to the formation of solid reaction products, such as CaS and Ca3SiO 5, on the CaO particles. Once these solid layers are formed, the diffusion of sulfur to the reaction surface is slow and the particle no longer desulfurizes effectively. One possible method of eliminating the solid reaction products is to add a flux such as CaF 2 with the CaO so that there is a liquid reaction surface. Landerfeld and Katz l~J have reported large increases in the rate of desulfurization of iron by the addition of about l0 pct CaF2. Several years ago, Fruehan I2~ showed that aluminum in liquid iron greatly increased the rate of desulfurization by CaO. Recently, Mitsuo e t a / . , 131 Leclercq et al.,t41 and J.C. NIEDRINGHAUS, formerly a Graduate Student with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, is with Armco, Inc., 703 Curtis Avenue, Middletown, OH 45042. R.J. FRUEHAN is Professor, Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted March 4, 1987.
METALLURGICALTRANSACTIONS B
Kossler and Koros r51 reported that the desulfurization of hot metal is greatly enhanced by the prior addition of aluminum. It is interesting to note that when CaO and Al are coinjected the effect of A1 is not as great as when A1 is added prior to the CaO injection. The addition of aluminum may aid desulfurization by lowering the activity of oxygen, thereby increasing the thermodynamic driving force. The presence of AI could also allow for the formation of a liquid calcium aluminate reaction product. Mitsuo et al. studied the reaction products formed on lump CaO immersed in Fe-C-Si melts and found Ca2SiO 4 and CaaSiOs, which are solid at ironmaking temperatures. In a recent experimental study, Oktay and Fruehan, t6J using CaO disks cemented into the bottom of graphite crucibles containing Fe-C-Si and Fe-C-Si-A1 alloys, found primarily CaS and CaESiO 4 layers when no A1 was present. In both studies, no p
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