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Evaluation of Surface Tension and Adsorption for Liquid Fe–S Alloys

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Evaluation of Surface Tension and Adsorption for Liquid Fe–S Alloys ISIJ International, Vol. 42 (2002), No. 6, pp. 588–594 Evaluation of Surface Tension and Adsorption for Liquid Fe–S Alloys Joonho LEE and Kazuki MORITA1) Graduate Student, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656 Japan. E-mail: [email protected] 1) Department of Metallurgy, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656 Japan. (Received on December 11, 2001; accepted in final form on February 16, 2002) Surface tension of liquid iron is strongly influenced by the adsorption of sulfur. In this study, surface ten- sions of liquid Fe–S alloys at 1 823 K were measured by the sessile drop technique in a purified argon at- mosphere. Experimental results were compared with the model based on Butler’s equations considering the effect of size and interactions of the adsorbed elements assuming that the system was composed of Fe–“FeS” binary. The model could evaluate the surface tension and the adsorption of sulfur more reason- ably rather than a simple ideal adsorption model. In the calculations, excess free energy expanded by Margules’ series over the entire composition range of Fe–FeS system was used. KEY WORDS: surface tension; adsorption of sulfur; Butler’s equation; Margule’s series; sessile drop tech- nique. sorption for liquid iron containing surface active elements. 1. Introduction Bernard and Lupis6) indicated that differences in size of the Surface tension of liquid iron is considerably decreased adsorbed elements and interactions between them cause de- by the adsorption of the surface active elements which are viations of experimental results from an ideal adsorption known to be present in large concentrations at a surface. behavior, and Eq. (1) is only valid near the saturation stage, Thus, preferential adsorption of surface active elements at a since adsorbed elements may exclude adjacent sites from surface can be predicted by accurate surface tension values. occupancy. Belton7) developed a two-step isotherm model, Among the surface active elements, sulfur is one of the which considers repulsive interactions between surface ac- most important elements in steelmaking processes. There- tive elements on liquid iron. However, the surface tension fore, it is essential that the surface tension of liquid iron values obtained by the two-step model are indistinguishable containing sulfur should be evaluated properly. from those by the ideal adsorption model,1) because he used Usually, the surface tension of liquid iron containing sul- reported values scattered by more than 0.2 N/m. By some fur, s, has been given by the simple ideal adsorption model other groups, there have been several attempts to explain (so called Szyszkowski equation) proposed by Belton.1) the surface tension of liquid iron containing surface active elements based on Butler’s equations, considering the equi- sϭs ϪRTG ° ln(1ϩK a ) ...................(1) Fe S S S librium between a bulk phase and a monolayer at the sur- 8) where s Fe, G S°, KS and aS are surface tension of pure iron, face. Tanaka and Hara calculated the surface tension of surface adsorption of sulfur at saturation, adsorption coeffi- liquid Fe–O alloys with a regular solution model. In their cient and activity of sulfur with respect to 1 mass% as stan- calculations, the alloy was regarded as a Fe–“FeO” system. dard state, respectively. Most of the experimental results of Likewise, Hajra et al.9) calculated the surface tension of surface tension of liquid iron containing sulfur were ex- molten Fe–S alloy regarding the alloy as a Fe–“FeS” sys- pressed as an ideal adsorption isotherm of Eq. (1).2–4) tem. They assumed that the system follows a sub-regular 8,9) However, the derived value of KS varies from 185 to 365. model. These previous models agreed with the experi- Chung and Cramb4) asserted that surface tension curves mental results within Ϯ0.2 N/m. They, however, ignored in- may not be very sensitive to values of the adsorption coeffi- teractions or extrapolated thermodynamic data of interac- cient. Meanwhile, Lupis5) found that the adsorption of sur- tions for dilute solutions to high content regions, although face active elements such as sulfur in an ideal adsorption the surface may be highly occupied by sulfur and oxygen. model disagrees with the experimental observations which Recently, Hajra and Divakar10) have explained the surface yield a much steeper rise of the adsorption curve. There- tension of the liquid Fe–S–O system based on the modified fore, it is believed that Eq. (1) is not proper to evaluate the Butler’s equations. However, this model still uses the first sulfur adsorption on liquid iron. order interaction coefficients of dilute solutions. According- Recently, attention has been paid to the development of a ly, in order to evaluate surface tension and adsorption of more general relationship between surface tension and ad- sulfur for Fe–S alloys more accurately, it is necessary to de- © 2002 ISIJ 588 ISIJ International, Vol. 42 (2002), No. 6 termine a moderate excess free energy equation across the composition range. In this paper, surface tensions of Fe–S alloys were mea- sured using the sessile drop technique. At the same time, a thermodynamic model calculation of surface tensions for Fe–S alloys based on Butler’s equations was presented. By comparing the model calculation with the experimental re- sults, the surface adsorption of sulfur on liquid iron could be deduced. 2. Theory 2.1. Butler’s Equation Fig. 1. Schematic diagram of surface tension measurements. Butler’s equations for A–B binary system can be ex- 11) pressed as Table 1. Chemical analysis of electrolytic iron used in the ex- periment. [mass%] RT 1Ϫ X s σσϭϩP ln B A Ϫ b SA 1 X B 1 ϩϪEx,s s Ex,b b ((,)GTXGTXA B A (,))B SA samples on a high purity (99.95 mass%) alumina boat with RT X s an alumina cover were placed at the center of a mullite re- ϭϩσ P ln B B b action tube. (I.D.: 0.040 m, O.D.: 0.050 m) The samples SB X B were prepared with high purity electrolytic iron (see Table 1 ϩϪEx,s s Ex,b b 1) and FeS (99.9 mass%), and concentrations of oxygen and ((,)GTXGTXB B B (,))B ...........(2) SB sulfur in the samples were analyzed by LECO analyzers after experiments. The temperature was well adjusted by P where R, T, s i , Si are the gas constant, the absolute temper- PID controller with a Pt–30%Rh/Pt–6%Rh thermocouple ature, the surface tension and molar surface area of the pure located by the heating element, which was preliminarily ¯Ex,s s element i, respectively. G i (T, Xi ) is the partial excess free calibrated by the temperature of the exact position where a s energy of i in the surface as a function of T and Xi (the liquid droplet was settled. ¯Ex,b b mole fraction of i in the surface). G i (T, Xi ) is the partial The experimental procedure was as follows. Cubes of b excess free energy of i in the bulk as a function of T and Xi alloy samples weighing about 1–2 g were ground on edges (the mole fraction of i in the bulk). Si is calculated from the and corners and polished on faces with a grinder, then molar volume Vi on the assumption that the elements occu- washed in acetone by a ultrasonic cleaner and dried before 5) ϭ py a close-packed configuration in a monolayer. (i A or being placed on ceramic substrates. Al2O3 (or graphite for B.) FeS) was used as a substrate in order to prevent the reaction of the molten alloy with the substrate. Contamination from S ϭ1.091·N 1/3V 2/3............................(3) i A i substrates was negligible in this study. After the specimen where NA is Avogadro’s number. Thus, in calculations based assembly was settled at the center of the furnace, the reac- on Butler’s equations, it is necessary to know the surface tion chamber was sealed and evacuated, and then purified tension and density of each component element of pure Ar gas was introduced at a flow rate of 0.3 l/min STP. Then, state and the excess free energy between the component el- the system was heated to the experimental temperature ements. However, if B is a surface active element such as (1 823 K). Once the sample was melted, measurements of sulfur, it is impossible to determine physical properties surface tensions were started. such as surface tension and density of the component B at The shape of the sessile drop of the liquid alloy on the such high temperatures. Hence, such system can be regard- ceramic substrate was monitored by a digital video camera ed as A–AB pseudo binary one.8,9) for 30–60 min. Several selected still images then could be directly converted into image files. The digitized dots were used to calculate the surface tension with a computer pro- 3. Experimental gram that has been developed by Jimbo et al.13) The sessile-drop method was employed in the present in- vestigation. This technique proved to be one of the most 4. Results and Discussion popular techniques to determine surface tension of molten metals after Bashforth and Adams empirical equation.12) 4.1. Surface Tension of Fe–S Recently, Jimbo et al.13) have developed more accurate way The experimental results of the surface tension for Fe–S of non-subjective and numerical determination of surface alloy were the same or slightly higher than the reported val- tension.
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