Effect of Oxygen Partial Pressure on Liquidus for the CaO SiO 2 FeO x System at K

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1 , pp Effect of Oxygen Partial Pressure on Liquidus for the CaO SiO 2 FeO x System at K Hisao KIMURA, Shuji ENDO 1), Kohei YAJIMA 2) and Fumitaka TSUKIHASHI 2) Institute of Industrial Science, The University of Tokyo, Komaba, Meguro, Tokyo Japan. 1) Sumitomo Metal Mining Co., Ltd., 2) Graduate School of Frontier Science, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba Japan. (Received on May 10, 2004; accepted in final form on May 31, 2004) Phase diagrams for the CaO SiO 2 FeO x system at various oxygen partial pressures are necessary for the design of raw material for ironmaking, the analysis of smelting reaction and ore sintering process. In this study, liquidus lines for the CaO SiO 2 FeO x systems at various oxygen partial pressures were observed at K by using chemical equilibration technique. The liquid phase area changed with changing oxygen partial pressure from 10 3 to 10 2 Pa (10 8 to 10 3 atm). The effect of Fe 3 /Fe 2 ratio on the melting mechanism is discussed. KEY WORDS: phase diagram; CaO SiO 2 FeO Fe 2 O 3 system; oxygen partial pressure; thermodynamics; melting behavior. 1. Introduction Phase diagrams for the CaO SiO 2 FeO x system are important thermodynamic data for the analysis of melting mechanism of sinter in sintering process and smelting reaction of iron and copper ores. The phase relationship for the CaO SiO 2 FeO x and CaO SiO 2 FeO x Al 2 O 3 MgO systems is of fundamental importance for the design of new raw material having low melting point and softening and reducing properties for innovative ironmaking processes. The phase diagram for the CaO SiO 2 Fe 2 O 3 system in air and that for the CaO SiO 2 FeO system equilibrated with iron were reported as well known phase diagram. 1,2) The CaO FeO Fe 2 O 3 and SiO 2 FeO Fe 2 O 3 systems with changing partial pressure of oxygen were also reported and the effect of the oxygen partial pressure on the valence of iron in the oxide melts. 3) Recently, Pownceby et al. measured the phase equilibria for the Fe 2 O 3 rich part of the CaO SiO 2 Fe 2 O 3 system at to K in air by using rapid quench technique. 4,5) The chemical compositions and coexisting condensed phases were analyzed by combined optical microscope and electron probe microanalysis. The phase diagram observed by Pownceby et al. is at variance with that reported by Phillips and Muan. However, the effect of oxygen partial pressure on the liquidus for the CaO SiO 2 FeO x system has not been made clear and very little studies on the phase diagrams for the CaO SiO 2 FeO x system at the oxygen partial pressure between air and the oxygen partial pressure determined by Fe FeO equilibrium are available. Pownceby and Clout reported the phase relationship for the Fe rich part of the CaO SiO 2 FeO x system at to K with partial pressure of oxygen of Pa by using rapid quench technique. 6) On the other hand, Yazawa and Kongoli developed the quantitative model calculations and reported the calculated liquid surface of the CaO SiO 2 FeO x system with various oxygen partial pressures at around K. 7 9) Characteristics of sinter ore in ironmaking process depend on the formation of melts during sintering. It is necessary to control melting behavior for the development of pre-treatment technique of iron ore. Furthermore, the efficiency of operation of the copper smelting process depend on the formation of melts and the control of melting behavior of ore and formation of slag are important for the promotion of smelting processes. Therefore, knowledge of phase relationship for the CaO SiO 2 FeO x system at various oxygen partial pressures is required. The phase diagrams for the CaO SiO 2 FeO x system at various low oxygen partial pressures are observed in the present study by using chemical equilibration technique. 2. Experimental 2.1. Measurement of Phase Diagram A chemical equilibration technique was employed to measure the phase relationships for the CaO SiO 2 FeO Fe 2 O 3 system. About eight grams of mixture of CaO, SiO 2, FeO and Fe 2 O 3 powders melted in a platinum crucible (inner diameter 35 mm, height 40 mm) are equilibrated with a oxide pellet (2CaO Fe 2 O 3, Fe 2 O 3, 2CaO SiO 2, 3CaO 2SiO 2, CaO SiO 2 and SiO 2 ) in CO CO 2 or Ar air atmosphere (flow rate: about 150 cm 3 /min). The platinum crucible containing sample was put into a mullite tube (inner diameter 52 mm, outer diameter 60 mm, height mm) set in an electric furnace. The oxygen partial 2004 ISIJ 2040

2 Table 1. Experimental results for the CaO SiO 2 FeO x system Pa at K. Fig. 1. Cross section of platinum crucibles. pressure of the system was controlled from 10 3 to 10 1 Pa by changing the CO/CO 2 ratio according to reaction (1). A high purity CO gas was purified by passing through columns of soda lime, H 2 SO 4 and Mg(ClO 4 ) 2. A high purity CO 2 gas was purified by H 2 SO 4 and Mg(ClO 4 ) 2. The oxygen partial pressure of Pa was controlled with mixing high purity argon gas and air. A sample melt was held for 20 h at K in an electric furnace. After equilibration, a platinum crucible was taken out from an electric furnace and sample melts in a platinum crucible were quenched in an argon flow. The composition of CaO, Fe 2, Fe 3 and SiO 2 were analyzed by titration and gravimetry, respectively. Reagent grade powders of CaO, SiO 2, and Fe 2 O 3 chemicals were used as samples. FeO powder was prepared by melting reagent grade of iron and Fe 2 O 3 powders in an iron crucible and quenched in water bath. X-ray diffraction was conducted to confirm the formation of FeO. Pellets of 2CaO Fe 2 O 3, Fe 2 O 3, 2CaO SiO 2, 3CaO 2SiO 2 and CaO SiO 2 were prepared by pressing and sintering a mixture of powder of reagent grade of oxides. Oxide melts were separated from a pellet of saturating oxide by platinum plate as shown in Fig. 1 to prevent the contamination of a pellet oxide into melts when the sample was quenched. In some experiments, since an oxide pellet such as 2CaO SiO 2 becomes powder during cooling, a platinum dish with an oxide pellet was taken out from a platinum crucible at first to prevent the contamination, and then a platinum crucible with molten sample was taken out from a furnace. 1 CO(g) O...(1) 2(g) CO 2(g) 2 DG T J/mol 10) 2.2. Direct Observation of Melting and Solidifying Behaviors The laser microscope was used to observe the melting and solidifying behavior with changing the oxygen partial pressure and to confirm the accuracy of the observed phase diagram. About 0.1 grams of slag samples put into a platinum crucible (inner diameter 5.0 mm, height 5.5 mm) were set in the laser microscope. The sample was heated by infrared image furnace to the experimental temperature in air atmosphere (Partial pressure of oxygen Pa). After the sample was heated to experimental temperature, the oxygen partial pressure was changed to Pa by using Ar air mixture gas and then the melting behavior was observed by laser microscope. After melting, the atmosphere was changed to air and the oxygen partial pressure was kept at Pa, then the solidifying behavior of slag was observed. This procedure was repeated to confirm the reproducibility of melting and solidifying behaviors. 3. Results and Discussion 3.1. Liquidus for the CaO SiO 2 FeO Fe 2 O 3 System The compositions of samples are shown in Tables 1, 2 and 3. The isothermal phase relationships for the CaO SiO 2 FeO Fe 2 O 3 system at K with the oxygen partial pressure of Pa ( atm), Pa ( atm) and Pa ( atm) are shown in Figs. 2, 3 and 4. The slag system of the present study is essentially the CaO SiO 2 FeO Fe 2 O 3 quaternary system and the phase diagrams should be shown on the quaternary system. However, since the experimental data points are not enough to show on the quaternary system, the phase diagram in the present study is shown as the CaO SiO 2 (FeO Fe 2 O 3 ) ternary system. Figure 5 shows the comparison of liquidus with the oxygen partial pressure of , and Pa at K. The liquid area at high (FeO Fe 2 O 3 ) content shrinks and it moves to the CaO rich side with increasing the oxygen partial pressure. At high CaO content area, the ISIJ

3 Table 2. Experimental results for the CaO SiO 2 FeO x system Pa at K. Fig. 3. Liquidus for the CaO SiO 2 FeO x system Pa at K. Table 3. Experimental results for the CaO SiO 2 FeO x system Pa at K. Fig. 4. Liquidus for the CaO SiO 2 FeO x system Pa at K. Fig. 2. Liquidus for the CaO SiO 2 FeO x system Pa at K ISIJ 2042 Fig. 5. Comparison of liquidus for the CaO SiO 2 FeO x system , and Pa at K.

4 Fig. 6. Comparison of liquidus for the CaO SiO 2 FeO x system and Pa at K. liquid area enlarges with increasing the oxygen partial pressure. In comparison with the liquidus for the CaO SiO 2 FeO system equilibrated with iron and the CaO SiO 2 Fe 2 O 3 system in air, 1) these liquidus lines are shown in Fig. 6 together with that with the oxygen partial pressure of Pa at K. The liquid phase area is enlarged at high iron oxide region with decreasing oxygen partial pressure. The effects of the decrease of oxygen partial pressure on the liquidus of 2CaO SiO 2, 3CaO 2SiO 2, and CaO SiO 2 were small. The liquidus line of SiO 2 at high SiO 2 region is influenced by the oxygen partial pressure. The separated two liquid area observed in air connects to form one liquid area with the oxygen partial pressure of Pa at K. It is known that the change of oxygen partial pressure considerably affects the liquid area. Pownceby et al. observed the phase relationship for the CaO SiO 2 FeO x system in air and with the oxygen partial pressure of Pa at K. One liquid area is observed in air at iron oxide rich side at K in their results. This is different from the phase diagram observed by Muan et al. Their phase diagram observed with the oxygen partial pressure of Pa at K is compared with that measured in the present study with Pa as shown in Fig. 7. Both liquidus lines saturated with 2CaO SiO 2 agree well at approximately same partial pressures of oxygen, although the experimental methods are different. Yazawa and Kongoli 7 9) reported the calculated phase diagram for the CaO SiO 2 FeO Fe 2 O 3 system at low oxygen partial pressure. Their calculated phase diagram is compared with the results of the present study as shown in Fig. 8. Their calculated phase diagram is in very good agreement with the observed phase diagram in the present study Direct Observation of Melting and Solidifying Behavior for the CaO SiO 2 FeO x System The melting and solidifying behaviors of the CaO SiO 2 FeO x system were observed at K with changing the oxygen partial pressure from to Pa and vice versa. Two slag compositions were shown in Table 4. The slag Fig. 7. Liquidus for the CaO SiO 2 FeO x system with P O Pa compared with that measured by Pownceby et al Pa at K. Fig. 8. Comparison of observed liquidus with calculated one for the CaO SiO 2 FeO x system Pa at K. Table 4. Slag composition at K. with the oxygen partial pressure of Pa is solid and that of Pa is liquid. The slag compositions after experiments were not analyzed, since the amounts of sample were too small for chemical analysis. After changing the oxygen partial pressure from to Pa, melting of slag sample was observed in 2 min as shown in Fig. 9. For solidification, the solid phase was observed in 5 min after changing from to Pa. The observed melting and solidifying behaviors of slags are consistent with the observed phase diagrams shown in Fig. 6. Therefore, the accuracy of the observed phase diagram is confirmed ISIJ

5 Fig. 9. Direct observation of melting and solidifying behavior at K. (a) P O Pa. (b) P O Pa. Fig. 11. Relationship between the (Fe 3 )/(Fe 2 ) ratio and the oxygen partial pressure at K for the CaO SiO 2 (Fe 2 O 3 FeO) system. Fig. 10. Relationship between the (Fe 3 )/(Fe 2 ) ratio and (mass%cao)/(mass%sio 2 ) on 2CaO SiO 2 liquidus , and Pa at K for the CaO SiO 2 (Fe 2 O 3 FeO) system Relationship between (Fe 3 )/(Fe 2 ) Ratio and Oxygen Partial Pressure The relationship between the (Fe 3 )/(Fe 2 ) ratios on 2CaO SiO 2 liquidus and the slag composition ((mass% CaO)/(mass%SiO 2 ) ratio) with different oxygen partial pressure of , and Pa at K is shown in Fig. 10. The (Fe 3 )/(Fe 2 ) ratios increase with increasing (mass%cao)/(mass% SiO 2 ) ratio at each oxygen partial pressure. Reaction between Fe 3 and Fe 2 ions in molten slags assumes to be expressed by Eq. (2), the equilibrium constant of reaction (2) is expressed by Eq. (3). If it is assumed that the activity of oxide ion does not change so much on the 2CaO SiO 2 liquidus and the ratio of the activity coefficients of Fe 3 and Fe 2 ions are constant with changing molten slag composition, log (Fe 3 )/(Fe 2 ) has a linear relationship with log P O2. The slope of line is estimated to be 0.25 according to Eq. (3). The values of (Fe 3 )/(Fe 2 ) ratio at (mass%cao)/(mass% SiO 2 ) 2 were observed from the data shown in Fig. 10 and the relationship between (Fe 3 )/(Fe 2 ) ratio and oxygen partial pressure is obtained as shown in Fig. 11. This shows a good linear relationship and its slope is calculated to be 0.22 that is in good agreement with the estimated value, 0.25, from Eq. (3). K 3 2n f 3 2n FeO (mass%feo n n ) 2 n f 2 (mass%fe ) a p Fe Fe n O O2 FeO n n 12 / 14 / 2 O O...(2)...(3) 3.4. Effect of Oxygen Partial Pressure on Liquid Area The change of liquid area with changing the oxygen partial pressure was observed as shown in Figs. 2, 3 and 4. For the practical sintering operation, the region of high iron oxide content is important to control the melting and sintering behavior. At high iron oxide region shown in Fig. 5, the liquidus lines move from iron oxide side to CaO side with increasing the oxygen partial pressure. Although the oxygen partial pressure of commercial sintering plant operation is not clearly observed, the consideration of selection of ore ISIJ 2044

6 and flux composition and the control of oxygen partial pressure by using the phase diagrams is important to control the melting and sintering behavior. 4. Conclusions The phase diagrams for the CaO SiO 2 FeO x at K with oxygen partial pressure of Pa ( atm), Pa ( atm) and Pa ( atm) were investigated by a chemical equilibration technique. The effects of oxygen partial pressure on the liquid area of the system were observed. The liquid area enlarges with decreasing the oxygen partial pressure. The (Fe 3 )/(Fe 2 ) ratio in molten slags increased with increasing (mass%cao)/(mass%sio 2 ) ratio at constant oxygen partial pressure of oxygen. REFERENCES 1) E. M. Levin, C. R. Robbins and H. F. McMurdie: Phase Diagrams for Ceramists, Vol. 1, American Ceramic Society, Inc., Columbus, Ohio, USA (1964), 204 (Fig. 656), 228 (Fig. 586). 2) B. Phillips and A. Muan: J. Am. Ceram. Soc., 42 (1959), ) Y. Takeda, S. Nakazawa and A. Yazawa: J. Mining Metall. Inst. Jpn., 97 (1981), ) M. I. Pownceby, J. M. Clout and M. J. Fisher-White: Trans. Inst. Min. Metall. C, 107 (1998), 1. 5) M. I. Pownceby and T. R. C. Patrick: Eur. J. Mineral., 12 (2000), ) M. I. Pownceby and J. M. F. Clout: Trans. Inst. Min. Metall. C, 109 (2000), 36. 7) A. Yazawa: Tetsu-to-Hagané, 86 (2000), ) F. Kongoli and A. Yazawa: Proc. 6th Int. Conf. Molten Slags, Fluxes and Salts, Royal Institute of Technology, Stockholm, Sweden and Helsinki University of Technology, Helsinki, Finland, (2000), No ) F. Kongoli and A. Yazawa: High Temp. Mater. Process., 20 (2001), ) E. T. Turkdogan: Physical Chemistry of High Temperature Technology, Academic Press, New York, (1980), ISIJ