The Effect of Oxygen Content in Molten. Tension between Molten Iron and Slag* By Kaxumi OGINO,** Shigeta HARA,** Takashi Shinji KIMOTO****

Transcription

1 The Effect of Oxygen Content in Molten Iron on the Interfacial Tension between Molten Iron and Slag* By Kaxumi OGINO,** Shigeta HARA,** Takashi Shinji KIMOTO**** MIWA*** and Synopsis The interfacial tension between molten iron and slag was measured by the sessile drop method with the transmission X-ray technique. Oxygen in iron remarkably decreased the surface tension of iron melt and the interfacial tension at the slag-metal boundary. It suggests that the oxygen dissolved in molten iron acts as a surface active solute at the slag-metal interface. Consequently, the interfacial tension depended, in most part, on the oxygen content in iron melt, but the composition of the slag had a minor effect on the interfacial tension. In order to clear the effect of slag composition on the interfacial tension, experimental results were discussed in terms of d a ( = Qm - ems) that indicated the difference between the surface and the interfacial tension at a constant content of sufrace active solute in iron melt. The values of 46 decreased with increasing silica content in slaps. The difference between oxygen and sulfur in molten iron as a surface active solute is that da for oxygen is approximately constant at a constant content of silica in slaps, while for sulfur it depends not only on the silica content but also on the sulfur content in molten iron. Therefore, it will be concluded that sulfur in molten iron and silica in molten slaps decrease the interaction between slag and iron melt. The work of adhesion between molten iron and slag, Wad.(= Um + 6s-6ms) depends on the slag composition. The values of Wad, for the ft uoride slag-iron system were smaller than that for the oxide slag-iron system. The values for the later system decreased with increasing silica content in slaps. I. Introduction There are many cases where the interface between molten metal and slag play an important role in the steelmaking process. The formation and aggregation of non-metallic inclusions during the deoxidation of steel, formation of metal-in-slag emulsion in the BOF process, and slag-metal reaction in the steelmaking process are typical examples. The interfacial tension is one of the most fundamental property at the slagmetal interface to analyze these phenomena. Although there are not so many determinations on the interfacial tension, disagreement among those results was observed because of the experimental difficulties. As oxygen and sulfur in molten iron are considered to be surface active, it is important to make clear the behavior of those elements at the slag-metal boundary in pyrometallurgical process. In this work, the interfacial tension at the slag-iron boundary was measured by the sessile drop method with the transmission X- ray technique and behavior of oxygen and sulfur at the interface was discussed. is usually measured by the direct determination from X-ray pictures of metal droplets immersed in molten slags,1-4) or by the measurement of the contact angle of a slag droplet on a molten metal surface.3'5-9) The later method has an advantage that the slag droplet is not in contact with a crucible material, but eventually contamination of the metal sample from gaseous phase affects the accuracy of the determination. Besides those methods, the drop detach method (modified drop weight method)1 ) and the drop pressure method11) have been reported, but they are not so general. The sessile drop method with the transmission X-ray used in this work has the advantages that it can be applied for various slag-metal systems and the measurement is not affected so much by the gaseous phase during the runs. 2. Measurement of the Surface Tension of Molten Metals The surface tension of molten metals has been carried out by the sessile drop method,l2-15) the pendant drop method,16) the maximum bubble pressure method,17) the capillary rise method18) and so on. In this work, the sessile drop method with the transmission X-ray technique was applied for the determination of the surface tension of iron-alloys. Recently, the oscillating drop method has been reported for the determination of the surface tension of iron alloys.19,20) 3. Principle of the Determination of the Surface and Interfacial Tension by the Sessile Drop Method A molten metal on a refractory substrate in contact with gaseous phase, or in a slag melt forms itself into a droplet as shown in Fig. 1. The shape of the droplet depends on the gravitational force and surface (or interfacial) tension. The surface (or interfacial) tension can be calculated from the shape of the droplet in accordance with Eq. (1), where, Q = g.(b2//3)'(pm_p,) (1) s : the surface (or interfacial) tension b : radius of curvature at the top of the droplet II. Experimentals 1. Measurement of the Interfacial Tension The interfacial tension at the slag-metal boundary * ** *** Fig. 1. Profile of an iron droplet. Partly published in Tetsu-to-Hagand, 65 (1979), 2012, in Japanese. English version received October 17, ISIJ Faculty of Engineering, Osaka University, Yamadaoka, Suita 565. Formerly Graduate School, Osaka University. Now at Nagoya Works, Nippon Steel Corporation, Tokai-cho, Tokai 476. Formerly Department of Metallurgical Engineering, Osaka University. Now at Tsurumi Synthetic Refractories Co. Ltd. (522)

2 Transactions ISIJ, Vol. 24, 1984 (523) Ps : the density of gas (or slag) pn, : the density of metal j9 : a parameter for the shape of the droplet g: the gravitational constant. The values of b and ~3 can be calculated from the Bashforth-Adams' table.21~ 4. Preparation of Metal and Slag Samples The iron samples of higher oxygen content were prepared by melting an electrolytic iron and the addition of hematite to the molten iron. While the lower oxygen samples were obtained by melting the electrolytic iron in an alumina crucible under vacuum and deoxidation with carbon saturated iron. Ironsulfur alloys were prepared from the electrolytic iron and a chemically pure FeS by vacuum melting. Iron-manganese alloys were provided from the electrolytic iron and an electrolytic manganese by vacuum melting. Slag samples of CaO-A1203, CaO-5i02A12O3, CaO-Na2O-SiO2 and CaO-MgO-5i02A12O3 systems were synthesized by melting the mixtures of chemically pure reagents in an alumina crucible under air. Slags of CaF2 A12O3 system were prepared by melting the mixtures of chemically pure CaF2 and A12O3 in a molybdenum crucible under an argon atmosphere. The ANF-6 slag is an industrial one and its composition is CaF2 29.8%A12O3-1.3%SiO2. It was premelted in a molybdenum crucible before use. Slags of FeO-CaO-5i02 and FeO-MnO-5i02 systems were provided by melting the mixtures of chemically pure CaO, SiO2, Fe2O3 and MnCO3 in an iron crucible under air. " FeO " slag was made by fusion of Fe2O3 in an iron crucible under purified argon atmosphere at C. Slags of CaF2 CaO- FeO system were prepared by mixing " FeO ", CaF2 and CaO without premelting. MnO-SiO2 slaps were made by premelting MnCO3 and 5iO2 mixtures in an graphite crucible under air. 5. Experimental Apparatus The apparatus for the measurement of the interfacial tension between molten iron and slag was composed of an X-ray source, a Tamman furnace with a carbon heater and a gas purification system. The X-ray source was that for a medical use made by Shimadzu Co. (Yamashiro type C). A schematic illustration of the furnace was shown in Fig. 2. The distance between the X-ray source and the center of the furnace was 890 mm and that between the center and the X-ray film was 150 mm. A sintered alumina Tamman tube (T-4, 20 mm~b I.D.) was used as a crucible, while for the FeO-CaO-SiO2 and CaF2 CaO- FeO slaps, a sintered alumina substrate was set inside the Tamman tube. Temperature was measured by a W5 %Re-W26 %- Re thermocouple set on the bottom of the crucible. All of the experiments were carried out under a purified argon atmosphere. For the measurement of the surface tension, a sintered alumina tube having a closed end (46 mmo I.D., 55 mmo O.D., 500 mm L) was set inside the carbon heater to control the oxygen partial pressure in the tube. A sintered alumina funnel was set inside the tube as shown in Fig. 3. During the runs, Ar+H2, Ar+H2+H2O mixed gas ; or purified argon gas was flowed to maintain the desired oxygen partial pressure inside the tube. 6. Experimental Procedure After the temperature of the furnace was heated up to C under the purified argon atmosphere, Fig. 2. Apparatus for measurement of interfacial tension.

3 ( 524) Transactions Is", Vol. 24, 1984 Photo. 1. Typical radiographs of an iron droplet, (a) in Ca0- (50 %)-A1203(50 %) slag melt and (b) in Fe0- (70 %)-Ca0(10 %)-Si02(20 %) slag melt at C. Photo. 2. Typical radiograph of C. an iron droplet under argon at Fig. 3. Apparatus for measurement of surface tension. an iron sample of about 5 g was charged into the crucible through the charging hole at the top of the furnace (see Fig. 2) and then a slag sample was charged. In some cases, the contrarious charging method, in which a metal sample was charged into a molten slag, was tried, but for both methods, there was essentially no difference of the interfacial tension after the slag-metal equilibrium was established. Especially, for samples of higher oxygen content, the later method was adopted to prevent the reduction of oxygen in metal sample during its melting down. After the metal and the slag were melted down, the shape of the metal in the slag melt was recorded on an X-ray film (Fuji X-ray film I X 100) by the emission of the X-ray. Its conditions were as follows; X-ray voltage 115 kv, current 200 ma and exposure time 1.6 N 3.2 sec. Dimension of the metal droplet on the X-ray film was slightly larger than that of the real one. Therefore, the magnification of the photograph was checked by the comparison of the size on the film with that of steel ball known on size. The size on the film was converted to the real one by the measured magnification. Errors on the determination of the interfacial tension mainly depended on the accuracy of the determination of the dimension for the metal droplet on the film and they were estimated to be less than 50 dynf cm. Typical radiographs of a metal droplet on a molten slag are shown in Photo. 1. After cooling in the furnace, metal samples were analyzed on oxygen and nitrogen by Leco's gas analyzer. Setting-up samples for determination of oxygen and nitrogen were supplied by the Iron and Steel Institute of Japan. Sulfur in metal samples was analyzed by the gravimetric method of Japan Industrial Standard (JIS). The determination of the surface tension was carried out in the similar manner. Typical conditions of the X-ray photograph were as follows; X-ray voltage 115 kv, current 200 ma and exposure time 1.6 sec. Errors on the determination were same as that of the interfacial tension. A typical photograph is shown in Photo Density of Slags and Metals and Surface Tension of Slags In order to calculate the surface and interfacial tensions, informations on the densities of the molten slaps and iron-alloys are required (see Eq. (1)). The densities in Table 1 were used for the calculation. The numbers between parentheses show the weight percentage of component of the slag. Surface tension in Table 1 was used for the calculation on the work of adhesion of the metal to the slag. III. Results and Discussion 1. Surface Tension of Iron-Oxygen Alloy The effect of oxygen on the surface tension of molten iron was shown in Fig. 4 together with those re-

4 Transactions ISIJ, Vol. 24, 1984 (525) Table 1. Density and and slaps. surface tension of iron-alloys Fig. 4. Sur face tension of the iron-oxyg en system. Fig. 5. Sufrace tension of the iron-sulfur system, increasing oxygen content in the iron. The surface tension of Fe-0.15%Q alloy was about 930 dynf cm. Above 0.05 % oxygen, the effect of oxygen on the surface tension was not so high. The reduction of the surface tension with oxygen can be due to the surface active action of oxygen in iron melt. ported by many authors. In order to measure the surface tension of the system, Popel' and Konovalov,2} and Kozakevitch and Urbain13) applied the sessile drop method with the transmission X-ray, while Halden and Kingery,12) Esche and Peter,14> Ogino et al.15) and Mukai et a1.8 used the sessile drop method by optical photograph. Murarka et al.19) and Kasama et al.20) used the oscillating droplet method. Our results showed a good agreement with those reported by Popel' and Konovalov2) and Ogino et al.15) The values reported by Halden and Kingery12) were higher than those in this work. Their work was carried out under the reduced atmosphere (0.5 atm of He), therefore the higher values may be due to the existence of oxygen transfer from the metal to the gaseous phase. Higher values were observed by the oscillating drop method. The surface tension of molten iron decreased with 2. Surface Tension of Iron-Sulfur Alloy There are many determination on the surface tension of iron-sulfur alloy as shown in Fig. 5. The agreement of those values reported by many authors3,12'13,43,45) is excellent except those by Tsarevskii and Popel'.44) The solid line in the figure shows the surface tension of iron-sulfur alloy considered in terms of Langmuir's isotherm by Belton.46) It suggests that the surface tension of the system changes with Belton's equation. 3. Interfacial Tension between Molten Slag and Iron Melt The interfacial tension between molten slag and iron melt should be the values at the time when the equilibria between slag and metal has been established. Therefore, it was measured after a metal drop was kept in a slag melt enough time to achieve the slag-metal equilibria. After a metal sample was charged into slag melt, the interfacial tension changed with time as shown in Fig. 6. During the run, some amount of alumina from the crucible dissolved into the slag melt, but the effect was considered to be

5 ( 526 ) Transactions ISIJ, Vol. 24, 1984 small5~ and was neglected. Figure 7 shows the effect of oxygen in molten iron on the interfacial tension at the slag-iron boundary. The interfacial tension remarkably decreased with increasing the oxygen. At lower oxygen contents, the interfacial tension was affected by the composition of the slags, but its effect was smaller than that of the oxygen in molten iron. It is concluded that the interfacial tension is mainly controlled by the oxygen content in metal phase. The dotted line in Fig. 7 shows the result reported by Popel' and Konovalov2~ that used the sessile drop method with the transmission X- ray to measure the interfacial tension for iron-(fe0- Mn0-Si02) slag or iron-(ca0-mn0-a1203) slag system. The double-dotted line shows the result obtained by Mukai et a1.8~ that calculated the interfacial tension from the contact angle of the Ca0(50)-A1203- (50) slag droplet on various iron-oxygen melts. The broken line shows the result by Gaye et a1.3~ for the iron-(ca0-si02 A1203 Fe0) slag system in basis of the measurement of the contact angle mentioned above. Those results have the same tendency as in this work, although the composition of the slags was different from that in this work. When the oxygen content is less than 0.05 %, decrease of the interfacial tension with the oxygen bears a close parallel to that of the surface tension, but at the higher oxygen content (0 % >0.05), decrease of the interfacial tension with oxygen is larger than that of the surface tension. The lowest value on the interfacial tension was observed for the " Fe0 " slagoxygen saturated iron system to be 300 dynf cm. Assuming that the surface tension of the oxygen-saturated iron (0.21 % 0) is 910 dyn/cm by extrapolation of the result shown in Fig. 5 to 0.21 % 0, one can get the value of 610 dynf cm as the difference between the surface and the interfacial tension. The value of 610 dyn/cm nearly corresponds to that of the surface tension for the " Fe0 " melt (570 dyn/cm at 1580 C47)). It suggests that Antonov's equation (2) is approximately satisfied for the " Fe0 "-iron system, in other word, " Fe0 " melt spreads out completely over Fig. 6, Change C. of the interfacial tension with time at Fig. 8. Relation between logarithm of oxygen Fig. 7. Effect of oxygen content in molten iron on tension and the surface tension at C. the interfacial content in molten iron and the interfacial tension at C.

6 Transactions ISIJ, Vol. 24, 1984 (527) the molten iron. alms = 7m-6s...(2) where, ~ms : the interfacial tension between slag and metal as, ~m : surface tensions of the slag and the metal. Assuming that the application of the Gibbs' adsorption isotherm is valid for the adsorption of oxygen at the slag-iron or gas-iron interface, surface excess of oxygen on the iron surface can be calculated from the results shown in Figs. 4 and 7. The Gibbs' adsorption isotherm is as follows, 1 d o- (3) RTT d In ao... where, F : the surface excess quantity of oxygen (mol/cm2) R : gas constant T: absolute temperature a : surface (or interfacial) tension (dyn/cm) ao : activity of oxygen in the iron melt. On the assumption of the validity of Henry's law for the iron-oxygen system, the relation between the surface (or interfacial) tension and log [%0] was replotted in Fig. 8. Table 2 shows the comparison of the excess surface quantity of oxygen at the slag-iron interface with that of oxygen at the gas-iron boundary. Maximum surface excess quantity of oxygen at the slag-iron interface is slightly smaller than that at the gas-iron boundary. Similar tendency is observed for sulfur as shown in Table 2. mass transfer between two phases, very low apparent interfacial tension was observed.3,49-51) When a metal sample having higher oxygen content than that at the equilibrium state was charged into a molten slag, the apparent interfacial tension was slightly lower than that observed at the equilibrium state as shown in Fig. g, where the solid line shows the equilibrium values. In this case, analytical values of oxygen in metal samples were accompanied with some uncertainty, because it takes few minutes for the metal samples to be solidified and during the time some of oxygen in the metal samples may have a chance to transfer from the iron melt to the slag. However, it can be concluded that the interfacial tension even at the non-equilibrium state is changed in accordance with oxygen in the iron melt as well as that at the equilibrium state. Another evidence was observed when small amount of " FeO " was added into the CaO(5.0)-A1203(50) melt after the slag-metal equilibrium had been achieved. The typical examples were shown in Fig. 10. When the iron sample (5 g) was equilibrated with the CaO-A12O3 slag (15 g), the interfacial tension was about dynf cm. Subsequently, 1.1 g of " FeO " enough to increase the 4. Non-equilibrium Interfacial Tension during Mass Transfer Many authors have pointed out that during intense Table 2. Comparison of excess interfacial excess surface quantity for oxygen quantity with and sulfur. Fig. 9. The state. interfacial tension at the non-equilibrium Fig. 10. Variations of the interfacial tension between molten iron and Ca0-A1203 slag with the time after " Fe0 " addition at C.

7 ( 528 ) Transactions ISIJ, Vol. 24, 1984 " FeO " content in the slag up to 7 % was charged into the slag melt. In this case, the interfacial tension was immediately decreased and then gradually increased. By the observation with transmission X-ray, " FeO " passed through the molten slag layer and contacted directly to the molten metal because of its higher density than the slag melt. Consequently, the oxygen in the metal firstly increased and then decreased with time by solution of " FeO " into the slag. For Run 1 in Fig. 10, the sample was quenched at 17 min after " FeO " was added. While for Run 2 and Run 3, samples were cooled at 2 min after the addition of " FeO ". Run 4 shows the change of the interfacial tension with time after a metal sample was charged into the CaO (46.5%)-A1203 (46.5%)-FeO- (7%) slag. From those results, the relation between the interfacial tension and the oxygen content in the iron was plotted in Fig. 11, where the solid line indicated the values at the equilibrium state as shown in Fig. 7. Those results suggest that the interfacial tension is mainly controlled by the oxygen content in the iron melt not only at the equilibrium state but also at the non-equilibrium state. oxygen in the metal 4a(-c -6~ns) could be considered to be the reduction of the surface tension with the presence of the slag phase. Figure 12 shows the relation between the value 46 and the oxygen content. It suggests that the value dr depends on the slag composition. The value 4 r for fluoride based slags was smaller than that for oxide slags. For CaO- Si02 A1203 slags, the value doi decreased with increasing the silica content in the slag as shown in Fig. 13. Similar relation was observed for FeO- CaO-Si02 slaps as shown in Fig. 14. They suggest that the interaction between slag and metal decreased with increasing the silica content in the slags, in other words, the silica in the slag contributes to the inter- 5. Effect of Slag Composition on the Interfacial Tension As shown in the previous section, the interfacial tension at the slag-metal boundary mainly depends on the oxygen content in the metal. Therefore, in order to discuss the effect of slag composition on the interfacial tension, it is necessary to consider the two factors; oxidizing power of the slag and its composition. Since the surface excess quantity of oxygen at the slag-metal interface is nearly equal to that for the gas-metal interface at a constant content of oxygen in the metal, it will be assumed that the structure of the slag-metal interface is resemble to that of the gasmetal interface. The difference between the surface and the interfacial tension at a constant content of Fig. 12. Variation of 4j iron at C. with oxygen content in molten Fig. 13. Variation of da with silica content in CaO A1203 slaps at C. -Si02 Fig. 11. Effect of oxygen content in molten iron in nonequilibrium interfacial tension for the iron-(cao- A1203 FeO) slag system at C. Fig. 14. Variation of 46 with silica content in FeO -CaO- Si02 slags at C.

8 Transactions ISIJ, Vol. 24, 1984 (529) facial structure at the slag-metal boundary. Similar consideration can be applied for the silicate slag-iron-sulfur alloy system. From the results reported by Gaye et al.3~ for the system, the relation between the value 4a and the sulfur activity in the iron melt is replotted and shown in Fig. 15. In this case, the value 4~ depends not only on the silica content in the slag phase but also the sulfur activity in the metal phase. It suggests that sulfur in the metal decreases the interaction between slag and iron. Similar phenomena were observed on the study on the wettability of alumina by molten iron alloys reported by Ogino et a1.48> They pointed out that sulfur, selenium and tellurium in molten iron decreased the interaction between molten iron alloy and aluminum oxide, while oxygen increased it. Therefore, it will be concluded that sulfur in molten iron is adsorbed on the iron surface at the slag-metal boundary and the adsorption of sulfur decreases the interaction of the metal to the oxide slag. <2 %) system was lower than that for the MnO free slag-iron system. Figure 17 shows the results on the interfacial tension for the iron-slag containing manganese oxide system by Popel' and Konovalov2~ for (FeO-MnO-Si02 MgO) slaps, by Gaye et a1.3> for (MnO-Si02A1203) slaps, by Shinozaki et a1.4~ for (FeO-MnO-CaO-MgO-Si02) slags and our results for (FeO-MnO-Si02) slags, together. The numbers between parentheses in the figure correspond to the weight percentage of manganese oxide in the slaps. These results support the finding by Gaye et al.,3~ but it is not clear yet why manganese oxide in the slags decreases the interfacial tension. While the addition of manganese in the iron decreased the interfacial tension. In our works, the content of oxygen in the Fe-Mn alloy was less than wt% after the equilibrium between the CaO- (50)-A1203(50) slag and the Fe-Mn alloy had been established. In our previous works5'9 shown in Fig. 6. Work of Adhesion between Molten Slag and Iron Melt The work of adhesion is energy required for the separation of slag from metal. It is defined by the following equation. Wad. = 61+6s-a,...(4) The work of adhesion for the various slag-iron system is shown in Fig. 16 as a function of oxygen in the iron melt. At lower content of oxygen (less than 0.05 % Q), the work of adhesion hardly depends on the oxygen content in the metal phase, but only depends on the composition of the slag phase. The highest values were observed for the (CaO-A1203) slag-iron system. The work of adhesion for fluoride slags showed considerably lower values than that for oxide slaps. It is one of the reason why the excellent separation between the fluoride slag and steel ingot is observed during the electroslag remelting process. Increase of silica content for oxide slaps reduces the work of adhesion. 7. Effect of Manganese on the Interfacial Tension Recently, Gaye et al.3~ reported that the interfacial tension for the (MnO-Si02 A1203) slag-iron (Mn Fig. 16. Work of adhesion, Wad. for tems at C. various iron-sl ag sys- Fig. 15. Variation of 4a with sulfur content in molten iron at C. (the values of the interfacial tension from Gaye et al.3~) Fig. 17. Interfacial tension between iron ing high manganese oxide. and sla g contain-

9 (530) Transactions ISIT, Vol. 24, 1984 Fig. 18. Effect of manganese content in molten iron on the interfacial tension at C. 18, the interfacial tension for the system was calculated from the contact angle of a slag droplet on the Fe-Mn melt. In the method, oxygen content in the metal phase was mainly controlled by the oxygen partial pressure in the gaseous phase, therefore, lower values shown in Fig. 18 may attribute to higher oxygen content in metal phase than those in this work. From the extrapolation of the results to Mn % = 0, the interfacial tension between the Ca0-A1203 slag and pure iron (0(0.001 wt%) is estimated to be about 1600 dynf cm. This value corresponds to about dyn/ cm of the surface tension for the pure iron, because, as mentioned above, the value 4a for the (Ca0-A1203) slag and the iron (0<0.05 %) was approximately constant to be 400 dyn/cm. It is concluded that the decrease of the interfacial tension by the addition of manganese was caused by the reduction of the surface tension of metal with the addition. l v. Summary From the determination of the interfacial tension for the slag-iron system, the following results were obtained : (1) The interfacial tension was mainly controlled by the oxygen content in the iron melt, but composition of slaps had an additional effect on the interfacial tension. (2) Oxygen and sulfur dissolved in iron melt act as the surface active solutes at the slag-metal boundary as at the gas-metal boundary. However, maximum surface excess quantities of oxygen and sulfur at the slag-metal boundary are slightly less than those at the gas-metal boundary. (3) To evaluate the effect of slag composition on the interfacial tension, the value of 4a(=cm-a,, ) which indicates the difference between the surface tension and the interfacial tension at a constant oxygen content in metal was introduced. 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