Desulfurizing Ability of the CaO satd. -CaCl 2 -CaF 2 Slags
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TION
IN the conventional hot metal desulfurization process, excessive solid lime is used as a desulfurizing agent and flux agent such as CaF2 is added to promote CaO dissolution. The solid CaO does not directly participate in the desulfurization reaction, which is only due to the liquid slag coexisting with solid CaO.[1,2] For the desulfurizing ability of this liquid slag, two key factors should be considered: (1) sulfide capacity from a thermodynamic point of view and (2) solubility of CaO which ensures more chances for sulfur reacting with CaO from a kinetic point of view. Over the years, in the practical hot metal pretreatment, an excellent desulfurization has been achieved using the CaO-CaF2-based slags. However, it is strongly required to reduce the amount of CaF2 in terms of environmental concern. Taniguchi et al.[3,4] proposed to directly use the carryover of a certain amount of blast furnace slag in the hot metal ladle as a flux agent and measured the sulfide capacity of the formed CaOAl2O3-SiO2-based slags. However, the value obtained is much lower than that of CaO-CaF2-SiO2 slags.[5] Besides that, it is well known that CaO-Al2O3-SiO2 slags have limited solubilities of CaO and CaS;[6] therefore, fluxes having higher ability are required. As a sort of flux agent, CaCl2 is a candidate to replace CaF2. Addition of CaCl2 into CaO-based slags can
effectively promote the melting of CaO, as reported elsewhere.[7] The research of Sakai et al.[8] confirmed that CaO-CaCl2 slags have high sulfide capacity in the temperature range from 1273 K to 1523 K (1000 °C to 1250 °C). However, the sulfide capacity and CaO solubility in CaO-CaCl2 based slags have not been clarified at hot metal desulfurization temperature which is usually over 1573 K (1300 °C), because the traditional experimental methods are not feasible due to the evaporation of CaCl2 at higher temperature. Therefore, the present work aims to establish a new experimental method to finally investigate the effect of replacement of CaF2 by CaCl2 on the sulfide capacity and CaO solubility of CaO-CaCl2-CaF2 slags saturated with CaO in the temperature range from 1573 K (1300 °C) to 1673 K (1400 °C).
II.
A. Experimental Principles The sulfide capacity of molten slags was originally defined on the basis of the gas/slag reaction (Eq. [1]) as shown in Eq. [2]. 1=2S2 ðgÞ þ ðO2 Þ ¼ 1=2O2 ðgÞ þ ðS2 Þ
CS2 ¼ JIAZHAN LIU and YOSHINAO KOBAYASHI are with the Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology 152-8550, Tokyo, Japan. Contact e-mail: liu.j.ah@ m.titech.ac.jp Manuscript submitted July 19, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
EXPERIMENTAL
K1 aO2 ¼ ðpct SÞ ðPO2=PS Þ1=2 ; 2 fS2
½1
½2
where K1 ; aO2 ; fS2 ; Pi, and (pct S) are the equilibrium constant of Reaction [1], the activity of oxide ion, the Henrian activity coefficient of sulfide ion, the partial
pressure of i, and the sulfur content on mass pct basis in slag, respectively. As shown in the definition, the sulfide capacity solely depends on slag composition and temperature.
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