Manganese Equilibrium Distribution between Carbon-saturated Iron. Melts and Lime Based Slags Containing MnO, BaO, and Na20*

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1 Manganese Equilibrium Distribution between Carbon-saturated Iron Melts and Lime Based Slags Containing MnO, BaO, and Na20* By Simeon Ratehev SIMEONOV** and Nobuo SANO*** Synopsis Studies o f the manganese equilibrium distribution between lime-based slags containing MnO, BaO and Na20 and carbon-saturated iron have been made in the temperature range of to C under a CO atmosphere. The equilibrium manganese distribution was found to decrease with increasing basicity index CaO/SiO2 (wt io/wt%). The addition of Na20 to the CaO-CaF2-SiO2 MnO system and the replacement of CaO by BaO in the same slag decreases the equilibrium manganese distribution. The equilibrium manganese distribution decreases as the content of CaF2 in the CaO-CaF2-SiO2-MnO slags increases by the replacement of CaO by CaF2. Temperature dependence of equilibrium manganese distribution can be expressed as follows: 10g(MnO) _ [Mn] T The heat of reaction : Mn (1) + 1 /202 = MnO (1) is calculated as - 53 kcal/mol. The activity coefficient of manganese in carbon-saturated iron was experimentally determined to be 0.51 at C. For the purpose of utilization of BOF slaps, their applicability to the hot metal treatment was investigated by using a rocking furnace with carbon lining at C. I. Introduction External desiliconization, dephosphorization and desulfurization of hot metal priorr to the BOF process are a key step in the production of high-quality steel. In recent years many researches for the pretreatment of hot metal have been done'-3) from various viewpoints but a few studies3,4~ have been made of the manganese distribution between carbon-saturated iron melts and soda or lime-based slaps. The authors previously investigated the phosphorus equilibrium distribution between carbon-saturated iron containing % Mn and lime-based slags in the temperature range of to C.5 The purpose of the present work is to obtain the equilibrium manganese distribution for the CaO- CaF2Si02MnO system and to investigate the effects of Na20 and BaO on the equilibrium manganese distribution at C under a CO atmosphere for CaO-CaF2Si02MnO-BaO and CaO-CaF2Si02- MnO-Na20 systems. Because no information on the thermodynamic behavior of manganese in carbon-saturated iron at C was available in the literature, the activity coefficient of manganese in carbon-saturated iron manganese alloys has been determined experimentally in the present work, in order to calculate the activity coefficient of manganese oxide in Slags. II. Experimental Methods 1. Measurements of the Equilibrium Manganese Distribution between Carbon-saturated Iron Melts and Slag Systems : CaO-CaF2 Si02MnO, CaO-CaF2 Si02 MnO-Na20 and CaO-CaF2Si02-MnO-BaO The experimental apparatus was described in details elsewhere.5~ Slags containing Mn02, Na2CO3, BaCO3, CaO, CaF2 and Si02 weighing 2 g and carbon-saturated iron containing 5.5'.'6.5 % Mn weighing 2 g were charged in a graphite crucible of 10 mm in inner diameter and 31 mm in height. Four crucibles were equilibrated at the same time. The equilibration time was 24 h for the slag without Na20 and 3 h for the Slags containing Na20. Measurements were made at the temperature range of to C. 2. Determination of the Activity Coefficient YMn in Carbon-saturated Iron at C Experiments were carried out to measure the activity coefficient of manganese in carbon-saturated iron at C and the experimental conditions adopted are shown in Fig. 1. A graphite crucible of 26 mm in inner diameter and 30 mm in height was used for the experiment. Copper containing 0.9 to 3.8 wt% of manganese weighing 21 g and carbonsaturated iron with 1.2 to 5.5 wt% of manganese weighing 4 g were melted in two holes of the crucible at C. Lime-based slags of the CaO-CaF2 Si02MnO system were charged into the crucible so as Fig. 1. Schematic diagrams of crucible used for the measurement of the activity coefficient of manganese in carbon-saturated iron. * Based on the paper presented to the 109th ISIJ Meeting, April 1985, 5126, at Tokyo Institute of Technology in Tokyo. Manuscript received on March 1, 1985; accepted in the final form on May 10, ISIJ ** Department of Metallurgy, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113. On leave from Department of Metallurgy, Higher Institute of Chemical Technology, Sofia, Bulgaria. *** Department of Metallurgy, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113. (1116) Research Article

2 Transactions ISIJ, Vol. 25, 1985 (1117) to cover two metals. Temperatures were measured with a Pt-Pt/Rh10 thermocouple attached to the bottom of the crucible. The temperature of the sample was controlled with the aid of a PID controller within +2 C. Purified carbon monoxide was blown on to the surface of the slag with the flow rate of 100 ml/min and Pco was kept at 1 atm. After equilibrium was attained, the crucible was withdrawn and quenched in argon. Compositions of slag samples were determined by the chemical methods. Manganese and carbon contents in metal samples were determined by the atomic absorption method and the infrared ray absorption method, respectively. III. Experimental Results 1. Determination of the Activity Coefficient of Manganese in Carbon-saturated Iron-Manganese Alloys Experimental results are shown in Fig. 2, indicating that the manganese content in carbon-saturated iron is proportional to that in copper. Since carbonsaturated iron-manganese alloy is in equilibrium with copper-manganese alloy through the overlaying slag, Eqs. (1) and (2) hold: amnincu =amninfe-csatd.... ' ". " ""(1) 1. Effect of CaF2 on the Equilibrium Manganese Distribution for the CaO-CaF2-Si02-MnO System To investigate the influence of CaF2 on the equilibrium manganese distribution, experiments were carried out by the replacement of CaO by CaF2 in the CaO--CaF2 Si02 MnO system at C. The results are shown in Fig. 4. When the CaF2 content increases from 25 to 40 %, the equilibrium manganese distribution decreases from 0.63 to Influence of BaO on the Equilibrium Manganese Distribution in the CaO-CaF2-Si02-MnO-BaO System As can be seen from Fig. 5, the replacement of CaO by BaO in the CaO-CaF2 Si02 MnO-BaO system affects the equilibrium manganese distribution as follows : As the BaO content increases from 0 to 8.3 % the equilibrium manganese distribution decreases from 0.4 to Effect of Na20 Addition on the Equilibrium Manganese Distribution in the CaO-CaF2-Si02 MnO-Na20 System Figure 6 shows the relation between the equilibrium manganese distribution and the amount of Na20 added to the CaO-CaF2Si02MnO system at 1300 C. With increasing the Na20 content from 0.15 to 2.6 %, the equilibrium manganese distribution decreases from 0.42 to T Mn in Fe _ - TMn, in Cu [Mn] [Mn]Fe Cu (2) According to Hultgren and Desai6> TMn in copper at C is equal to By substituting the slope of straight line shown in Fig. 2, 1.07, into Eq. (2) the activity coefficient of manganese in carbon-saturated iron is calculated as at C. 2. Equilibrium Manganese Distribution Figure 3 shows the relation between the equilibrium manganese distribution and basicity index, defined as CaO/Si02 (wt%/wt%) for the CaO-CaF2-Si02 MnO system at C, with 30 % CaF2. As can be expected, the logarithmic value of equilibrium manganese distribution decrease linearly with increasing index of basicity. Fig. 3. Relation between the distribution of manganese and CaO/Si02 (wt%/wt%) for the CaO-CaF2-Si02- MnO system. Fig. 2. Relation between the mole fraction of manganese in copper and that in carbon-saturated iron equilibrated with CaO-CaF2 Si02-MnO melts at C. Fig. 4. Relation between the distribution of manganese and CaF2 content for the CaO-CaF2-Si02-MnO system.

3 (1118) Transactions Is", Vol. 25, 1985 Fig. 5. Relation between the Ba0 content for the system. distribution of manganese and Ca0-CaF2 Si02-Mn0-Ba0 Fig. 7. Temperature dependence of log rmno between 1250 C and C for 53.5%Ca0-31.0%CaF2 12.5Si023.0%Mn0 melt. K = fmn2+ (%Mn2+) ao2-7 amn (1) Po22 From Eq. (7) the manganous capacity can be defined in the same manner as other capacities: Mn2+ amn (% Mn2+) (1) ' P12 f Mn2+ K a02-...(8) Fig. 6. Relation between the distribution of manganese and Na20 content for the Ca0-CaF2-Si02Mn0-Na20 system. 4. Temperature Dependence of the Equilibrium Manganese Distribution for the 53.5%Ca0-31.0%CaF2-11.5%- Si02-3.0%Mn0 System in the Temperature Range of to C The reaction of oxidation : Mn (1)+1/202 = Mn0 (1)...(3) dg = Tcal/mol (7)...(4) is exothermic so that the equilibrium manganese distribution is expected to decrease with increasing temperature as can be seen from Fig. 7. The value of log (Mn0)/[Mn] can be expressed as a function of temperature by Eq. (5) : to (Mn0) [M = (5) n] T Iv. Discussion Equilibrium of manganese between carbon-saturated iron-manganese alloys and slaps containing Mn0 can be expressed in terms of ionic species by the following equation: Mn (1)+ 1/202 = (Mn2+)+(02-)...(6) The equilibrium constant of reaction (6) is written: where, (Mn2+): wt% of manganese in slag amn (1): activity of manganese in carbonsaturated iron with reference to liquid manganese K: equilibrium constant of reaction (6) f Mn2+ : activity coefficient of manganese ion in slag Pot: oxygen partial pressure ao2- : activity of oxygen ion in slag. Since the middle term of Eq. (8) can be determined experimentally, CMn2+ can be calculated and the values obtained are plotted in Fig. 7, the heat of reaction (3) being estimated as -53 kcal/mol. This value is in good agreement with the value, kcal/mol, which was determined by Tsukihashi et al.3) for the Na20-5i02Mn0 system. 1. Determination of the Activity Coefficient of Mn0 in the Ca0-CaF2Si02Mn0 Slag System The equilibrium constant of reaction (3) is: K = amno (1)...(9) amn(1) P022 Consequently rmno can be expressed as follows : K rmn XMn P/2 TMnO = X MnO...(10) where, rmno : activity coefficient of Mn0 in slag amno (1): activity of Mn0 with reference to liquid Mn0 XMn. mole fraction of manganese in carbonsaturated iron-manganese alloys Tin : activity coefficient of manganese in carbon-saturated iron. From Eq. (10), the activity coefficient of manganese oxide in 53.5%Ca0-31.0%CaF2ll.5%Si023.0%

4 Transactions ISIJ, Vol. 25, 1985 (1119) MnO slag was calculated and is plotted in Fig. 8. As it is well known, log r is expressed by a linear function of the reciprocal of absolute temperature for a given composition and the slope of the line multiplied by gives the relative partial molar heat of solution of MnO in calorie.8~ For the reaction (3) : l HMnO MnO 1 og rmno = x T ^ { 0g MnO...(11) where, 4HMIO, JSMno: the partial molar heat of solution of MnO and the partial molar entropy of solution of MnO in the 53.5%CaO- 31.0%CaF211.5%Si02 3.0% MnO system, respectively. The relative partial heat of solution of MnO estimated from Eq. (11) is 31 kcal/mol. The positive sign indicates that the dissolution of MnO in CaO- CaF2Si02MnO melts is endothermic. 2. Dependence of the Activity Coefficient of MnO on the Slag Composition Figure 9 shows the relation between TMIO and basicity index CaO/Si02 at C, for the slag with 30 % of CaF2. The logarithmic value of 7Mno increases linearly with increasing basicity. Therefore, the behavior of MnO deviates positively from that of an ideal solution, as expected from the results of Smith and Davies9~ for the CaF2-MnO-CaO system. As the basicity increases, rmno increases, because MnO acts as a relatively basic oxide. Figure 10 shows the dependence of the activity coefficient of MnO on the CaF2 content at C. By the replacement of 20 % CaO by CaF2, the activity coefficient of MnO becomes about 4 times as large as initial one. Here again, the large positive deviation from the behavior of an ideal solution can also be seen. As the CaO content of slag increases, however, the activity coefficient decreases because the greater affinity between CaO and MnO is larger than that between CaF2 and MnO. 3. Comparison of the Present Data with the Literature The data obtained by the present work for manganese distribution in the CaO-CaF2-Si02-MnO, CaO- CaF2-Si02-MnO-Na20 and CaO-CaF2 Si02-MnO -BaO systems at C and the data of Inoue and Suito4~ for the CaO-Na20-Si02 system at C are shown in Fig. 11. As can be seen from Fig. 11, an increase in the content of basic oxides such as CaO, BaO, Na20 in slag melts results in a decrease of the manganese equilibrium distribution in case of limebased slags. Especially, when the content of BaO increases from 1 to 8.3 %, the equilibrium manganese distribution was strongly enhanced. For the experiment of the Na20 addition to lime-based slags, Pb-Na alloys were used as a reservoir to maintain the Na20 content constant to conpensate the vaporization loss of sodium due to the reduction by carbon. Sodium contents in lead after equilibriation were 0.24 % and 0.15 % for the slag containing 2.6 % and 0.95 % Na20, respectively. According to Tsukihashi et a1.10~ lead containing 0.24 % and 0.15 % sodium is in equilibrium with 50%Na20-Si02 and 46%Na20-Si02 for the Na20-Si02 system, respectively. This means that the activities of Na20 in the Fig. 9. The activity coefficient of MnO for the CaO-CaF2- Si02-MnO system as a function of CaO/Si02 (wt %/ wt%). Fig. 8. Temperature dependence of log (MnO)/[Mn] tween C and C for the 53.5%C 31.0%CaF2 12.5%Si02 3.0%MnO melt. be- ao- Fig. 10. Dependence of the activity coefficient of manganese oxide on the CaF2 content in the CaO-CaF2- Si02-MnO system.

5 (1120) Transactions ISIJ, Vol. 25, 1985 Table 1. The thermodynamic behavior of phosphorus, sulfur and manganese in modified BOF slags with the CaF2 addition. Fig. 11. Logarithms slag system. of log (MnO)/[Mn] in the various above mentioned lime-based slags containing 2.6 % and 0.95 % Na20 are equal to those of the binary sodium silicates with 50 % and 46 % Na20. For this CaO-CaF2-Si02 MnO-Na20 system, having such a high activity of Na20, the equilibrium manganese distribution are 0.37 and 0.32, respectively, being in great contrast to the values of 50 and 65 for the sodium silicate slags containing 50 % and 45 % Na20.1o> Therefore, Na20 in the CaO-CaF2-Si02-MnO- Na20 system seems to have comparatively small influence on the equilibrium manganese distribution. In general the sodium silicate system may have much larger capability to hold MnO than those used in the present work. On the other hand, the addition of a small amount of Na20 to the lime-based slags was found to enhance the dephosphorization capability of lime-based slag ten times as large as the slag without Na20.5~ V. Dephosphorization and Desulf urization of Silicon-free Hot Metal by BOF Slags For the purpose of utilizing BOF slags, their applicability to the hot metal treatment was investigated. By using the method mentioned before (Chapter II), the equilibrium phosphorus, sulfur as well as manganese distribution between carbon-saturated iron and BOF slag, containing 20, 30 and 40 % CaF2 were determined. The initial composition of BOF slag is as follows : CaO = %, Si02 =14.20 %, MnO =15.90 %, MgO = 5.65 %, A1203 =1.90 %, P205=1.22 %, FeO =10.69 %. The obtained equilibrium phosphorus, sulfur and manganese distribution at C under a CO atmosphere at carbon saturation are given in Table 1 together with the values for the slaps of different CaF2 contents. The equilibrium phosphorus distribution increased from 1.25 to 1.50, when the initial content of CaF2 increased from 10 to 30 % except for 25 % CaF2. The manganese equilibrium distribution decreased from 0.8 to 0.41 but sulfur equilibrium distribution was nearly constant in the range of CaF2 content studied. The phosphate and manganese capacities are also shown in Table 1, demonstrating that the values for modified fluxes studied here are nearly equal to those for synthesized basic slags (Ref. 5) and the present work). As is seen from Table 1, the most suitable composition is with 30 % CaF2. On the basis of fundamental thermodynamic data tests were carried out using a rocking furnace10 with 4 kg of molten carbon-saturated iron charged in a carbon cylinder at C. In this experiment, 400 g of fluxes were charged. As the preliminary experiment, carbon-saturated iron containing 0.63 % Mn, % P and % S was treated by BOF slag containing 30 % CaF2 and 10 % Fe203. In this case, 55 % of phosphorus and 45 % of sulfur were removed in 10 min, indicating that the modified BOF slag can still remove sulfur and phosphorus, as expected. Since the phosphate and sulfide capacities increase significantly by the addition of Na20 to the lime-based flux, some attempts have been made to enhance the dephosphorization and desulfurization by the addition of a small amount of Na20 to the BOF slag-caf2 mixture. The initial composition is as follows : 1) Carbon-saturated iron: % P, % S, 0.77 % Mn 2) Flux: BOF slag with the composition given before is mixed with 30 % CaF2. The total amounts of Na2CO3 and Fe203 added were 7 % and 10 %, each half was added at 5 and 15 min after the beginning of the treatment. Change in the composition of metal was examined by taking the samples intermittently up to 30 min. Experimental results obtained by the second test are summarized in Fig. 12, showing the variations in phosphorus, sulfur and manganese contents of hot metal with time. It is evident that dephosphorization and desulfurization are enhanced by the addition of a small amount of Na20 and the contents of phosphorus and sulfur were lowered down to % and %, within 7 to 10 min after the beginning of treatment. Rephosphorization occurs towards the end of treatment. Manganese content of the hot metal increased from 0.77 to 1.20 %. The findings reported recently by Shiomi and Sano11~ have been confirmed by the present experimental results, indicating that BOF slaps, which are considered as an industrial waste, can be effectively utilized in the hot metal treatment.

6 Transactions ISIJ, Vol. 25, 1985 (1121) CaF2 11.5%Si02-3.0%MnO slag and carbon-saturated iron in the temperature range of to C is expressed as follows : log (Mn0) [M _ 3.87 n] T VI. Conclusions The equilibrium manganese distribution between carbon-saturated iron and lime-based slags containing MnO, Na20 and BaO has been investigated in the temperature range of to C. The results are summarized as follows : (1) The activity coefficient of MnO in carbonsaturated iron was experimentally determined is at C. (2) The equilibrium manganese distribution decreases with increasing basicity index which is defined as CaO/Si02 in the CaO-CaF2Si02-MnO slag system. Fig. 12. Relation between P, S and Mn (wt%) and time during the treatment in a rocking furnace (CaF2 =30 wt%). (3) The equilibrium manganese distribution decreases both by the addition of Na20 to the CaO- CaF2-Si02MnO slag system and by the replacement of CaO by BaO. (4) When the CaF2 content in slag of the CaO- CaF2-SiO2 MnO system increases from 25 to 40 % the equilibrium manganese distribution decreases from 0.63 to (5) Temperature dependence of the equilibrium manganese distribution between 53.5%CaO-31.0% (6) The heat of reaction (3) and the relative partial heat of solution of MnO was found to be -53 kcal/mol and 31 kcal/mol, respectively. (7) It was experimentally proved that a large portion of flux for the hot metal treatment can be replaced by BOF slags. Acknowledgements One of authors (S.R.S.) gratefully acknowledges a Japanese Government (Monbusho) Scholarship, which enabled him to take part in the present work. Appreciation is also extended to Dr. M. Maeda for his help to prepare the manuscript. REFERENCES 1) H. Hirahara, K. Marukawa and Y. Shirota : Tetsu-to- Hagane, 66 (1980), 58; Trans. ISIJ, 20 (1980), B358. 2) K. Ito and N. Sano: Tetsu-to-Hagane, 69 (1983), 51. 3) F. Tsukihashi, M. Yukinobu, T. Hyodo, A. Werme and N. Sano : Tetsu-to-Hagane, 69 (1983), S945. 4) R. Inoue and H. Sui.to : Trans. ISIJ, 24 (1984), ) S. Simeonov and N. Sano: Trans. ISIJ, 25 (1985), ) R. Hultgren and P. D. Desai : Selected Thermodynamics Value and Phase Diagrams for Copper and Some of Its Binary Alloys, Monograph Ser. No. 1, Internat. Copper Res. Assoc., New York, (1971). 7) E. T. Turkdogan: Physical Chemistry of High Temperature Technology, Academic Press, New York, (1980), 15. 8) H. E. McGannon: The Making, Shaping and Treating of Steel, 9th ed., U.S. Steel Corp., Pittsburgh, (1971), ) P. H. Smith and M. W. Davies: Trans. Inst. Mining Met., Sec. C, 80 (1971), ) F. Tsukihashi, A. Werme, F. Matsumoto, A. Kasahara, M. Yukinobu, T. Hyodo, S. Shiomi and N. Sano: Proceeding of Second International Symposium of Metallurgical Slags and Fluxes, AIME, New York, (1984), ) S. Shiomi and N. Sano : Tetsu-to-Hagane, 71 (1985), 1504.