Study of MnO Activity in CaO-SiO2-MnO-Al2O3-MgO Slags
Jun Tao1, Dongdong Guo1, Baijun Yan1, Longmei Wang2
1 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing
Beijing 100083, P.R. China
2 Central Iron & Steel Research Institute, Beijing 100081, P.R. China Keywords: High MnO slags, Activity, Regular solution model
Abstract
Liquid copper was used as reference metal phase to equilibrate with CaO-SiO2-MgO and CaO- SiO2-MnO-Al2O3-MgO slags under a controlled oxygen partial pressure (pO2=1.76´10-6Pa) at 1873K. Based on the activity of Mn in Cu-Mn melts, the activities of MnO in slags were determined. The activities of MnO in the two kinds of slags increase gradually with the increasing of MnO content. For the slags with the same MnO content, the activity of MnO in the ternary slags is smaller than that in the five components slags. Furthermore, according to the measured activities of MnO in slags, the conversion factor in the quadratic formalism based on regular solution model was corrected. And an expression was obtained; it can estimate the activities of MnO in the both slags satisfactorily.
Introduction
Twinning induced plasticity (TWIP) steels are one of the best choices for next generation automotive high strength steels due to its excellent mechanical properties. As the major alloying element, the concentration of manganese in TWIP steels is as high as 15wt. %-30wt. % [1-2]. From the perspective of economical producing of high manganese TWIP steels, the maximum use of manganese ore in the steelmaking process is more economic and reasonable.
Consequently, it is of fundamental and practical importance to understand the thermodynamic behavior of MnO in the steelmaking slags, especially in the high MnO slags.
Concerning the thermodynamic activity of MnO in high MnO slags, some investigations can be found in the literatures [3-10], but the data in CaO-SiO2-MnO-Al2O3-MgO system are scarce.
Therefore, in the present study, activities of MnO over a wide compositional range in the CaO- SiO2-MnO and CaO-SiO2-MnO-Al2O3-MgO system at 1873K were determined experimentally.
Furthermore, the applicability of the regular-solution model in these two systems was examined.
Experimental and Principle
The experimental setup is schematically shown in Figure 1. A resistance furnace with MoSi2
heating element was used. An alumina tube was employed as the reaction chamber. A water- Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International
Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
cooled quenching chamber was internally connected on the top of the reaction tube. The temperature of the sample was followed by a W-5%Re/W-26%Re thermocouple placed beneath the bottom of the crucible. Molybdenum crucible containing the sample was hung by molybdenum wire on the steel rod connected to a lifting system. The lifting unit could lift the sample very fast to the quenching chamber with 1-2 seconds.
To start the experiment, 3g slag was placed above 5g copper in a molybdenum crucible (16.4mm ID, 18mm OD and 32mm height). After the whole system was carefully sealed, the reaction tube was evacuated using a vacuum pump and then filled with Ar. This procedure was repeated at least three times. Thereafter, the CO/CO2 gas mixture with volume ratio of 99 (oxygen partial pressure 1.76×10-6 Pa at 1873K) was introduced, the flow rate was maintained 40 ml/min until the experiment was finished. The furnace was heated up at a rate of 5K/min to the 1873K, and the slag equilibrated with liquid copper at 1873K for 20h before quenching.
The quenched sample was taken out, and the slag was carefully separated from the solid copper.
Then, the content of Mn in copper was analyzed by ICP-AES. The morphology of the quenched slag was examined by SEM and its composition was determined by EPMA.
Figure 1 Schematic diagram of the experimental apparatus
The reaction took place between slag and liquid copper can be expressed as Eq. (1).
(MnO)(s)+CO(g)=[Mn](l)Cu+CO2(g) (1) DG1q= -120 000-0.96T(J/mol) [11] (2)
DrG1q= -RTlna[ Mn ]×(pCO2/pq)
a( MnO)×(pCO/pq) (3) Where, parentheses and square bracket denote the component in slags and liquid copper, respectively. From Eq. (3), the activity of MnO in slags can be determined from Eq. (4)
a(MnO)=x[Mn]×g[Mn ]×(pCO2/pCO)×exp(DrG1q/RT) (4)
The activity coefficient of Mn in liquid copper,
g
[Mn],have been reported in our previous paper[12]. And the relationship between g[ Mn ]and x[ Mn ] at 1873K can be expressed as Eq. (5).
112 . 9 ) 1 ( 052 . 10
lng[Mn]=- -x[Mn] 2+ (5) Combining Eq. (4) and Eq. (5), the activity of MnO in slag can be determined by using the concentration of Mn in the liquid copper equilibrated with slag.
Results and discussion 1. MnO activity in slags
In Figure 2, the typical microphotographs of slags of two different systems are presented. It can be seen that the slags investigated in the present study are single liquid phase at 1873K, which agrees well with the phase diagram. The compositions of the slags measured by EPMA are listed in Table I, the measured compositions are very close to the weighed-in compositions.
Figure 2 SEM microphotography of slag samples quenched from 1873K CaO-SiO2-MnO (1-2), CaO-SiO2-MnO-Al2O3-MgO (2-2)
The concentrations of Mn in the Cu-Mn melts equilibrated with various slags, as well as the activities of MnO in slags calculated using Eq. (4) and Eq. (5), are listed together in Table I.
Table I. Experimental Results and Activities of MnO in Slags
Slag System Sample No.
Compositions of slags and the Cu-Mn melts
aMnO
Slags Mn in Cu
(mass %) CaO SiO2 MnO Al2O3 MgO
MnO-CaO- SiO2
1-1 38.22 50.36 11.42 - - 0.398 0.009
1-2 32.52 46.82 20.66 - - 0.408 0.022
1-3 27.11 41.55 31.34 - - 0.424 0.043
1-4 21.96 37.22 40.82 - - 0.446 0.074
1-5 16.79 33.31 49.90 - - 0.487 0.135
1-6 12.24 30.39 57.37 - - 0.523 0.193
Table I continued. Experimental Results and Activities of MnO in Slags
Slag System Sample No.
Compositions of slags and the Cu-Mn melts
aMnO
Slags
Mn in Cu (mass %) CaO SiO2 MnO Al2O3 MgO
MnO-CaO- SiO2-Al2O3-
MgO
2-1 31.91 38.82 6.48 17.74 5.05 0.088 0.010 2-2 29.69 36.76 10.69 18.16 4.70 0.174 0.021 2-3 23.81 30.45 21.36 19.77 4.61 0.340 0.042 2-4 17.77 27.63 34.36 15.94 4.30 0.798 0.109 2-5 11.89 23.42 43.85 16.60 4.24 1.253 0.190 2-6 6.56 19.44 52.08 17.67 4.25 1.547 0.250 From the activity data of MnO in slags shown in Table I, it can be seen obviously that the activity of MnO in both systems increase gradually with the increasing of MnO content in slags.
Comparing the activities of MnO in the ternary system with that in the five components system, it can be found that the activities of MnO in CaO-SiO2-MnO-Al2O3-MgO slags are larger than those in CaO-SiO2-MnO slags. This indicates that the addition of Al2O3 and MgO to the CaO- SiO2-MgO slags is beneficial to the increasing of MnO activity.
2. Thermodynamic model of MnO activity in slags
For the silicate melts that is not strictly regular solution, the activity coefficient of components can be approximately expressed with the Eq. (6):
RTlngi= aijx2j+ (aij+aik-ajk)xjxk
k
å
j
å
+I'j
å
(6)where aijdenote the interaction energy between the cation, i.e. (i cation)-O-(j cation), I' is the conversion factor of activity coefficient between hypothetical regular solution and the real solution. And all the parameters of the interaction energy and the conversion factor can be found in the literature [13]. Then, the expression shown as Eq. (7) can be obtained to predict the activity coefficient of MnO in silicate melts.
16496
49370 53550
31380
70290 20920
33470
61920 83680
75310 92050
ln RT
MgO AlO MgO
SiO AlO
SiO
MgO CaO AlO
CaO SiO
CaO
2 MgO 2
AlO 2
SiO 2
CaO MnO
5 . 1 2
5 . 1 2
5 . 1 2
5 . 1 2
+
+ +
-
+ -
-
+ -
- -
= g
x x x
x x
x
x x x
x x
x
x x
x x
(7)
Using Eq. (7), the activities of MnO in the slags were calculated, and the calculated values were compared with the experimental results. It can be seen from Figure 3 that the calculated activities of MnO in the slags are larger than the experimental measured values. However, a good linear relationship exists between the calculated values and the experimental values. This implies that the quadratic formalism based on the regular solution model can predict the trend of activity coefficient of MnO in slags perfectly.
Figure 3 Comparison of MnO activities calculated by RS model ([Eq. 7]) with the measured values in CaO- SiO2-MgO and CaO-SiO2-MnO-Al2O3-MgO slags at 1873K
In order to make the quadratic formalism shown as Eq. (7) can estimate the activity of MnO in slags satisfactorily, the conversion factor, I', was corrected based on the measured activities of MnO in slags. Then, the expression shown as Eq. (8) was obtained.
3406
49370 53550
31380
70290 20920
33470
61920 83680
75310 92050
ln RT
MgO AlO MgO
SiO AlO
SiO
MgO CaO AlO
CaO SiO
CaO
2 MgO 2
AlO 2
SiO 2
CaO MnO
5 . 1 2
5 . 1 2
5 . 1 2
5 . 1 2
-
+ +
-
+ -
-
+ -
- -
= g
x x x
x x
x
x x x
x x
x
x x
x x
(8)
By using the Eq. (8), the activities of MnO were calculated again and compared with the measured values. The new comparing result is present in Figure 4. In Figure 4, it can be seen clearly that the calculated values show good agreement with the measured values.
Figure 4 Comparison of MnO activity calculated by modified RS model ([Eq. 8]) with measured data in CaO- SiO2-MgO and CaO-SiO2-MnO-Al2O3-MgO slags at 1873K
Conclusions
The activities of MnO in the liquid CaO-SiO2-MnO-Al2O3-MgO and CaO-SiO2-MgO slags were measured at 1873K by equilibrating the slags with Cu-Mn melts. The oxygen partial pressure of the reaction system was controlled using CO/CO2 gas mixture. The obtained results could be summarized as follows:
1. The activity coefficients of manganese in Cu-Mn melts at 1873K could be expressed as:
112 . 9 ) 1 ( 052 . 10
lng[Mn]=- -x[Mn] 2+
2. For the CaO-SiO2-MnO-Al2O3-MgO and CaO-SiO2-MnO slags with the same MnO contents, the activity of MnO in the ternary system is smaller than that in the five components system. Therefore, the addition of Al2O3 and MgO to the CaO-SiO2-MgO slags is believed to increase the activity of MnO.
3. The corrected quadratic formalism based on the regular solution model can estimate the activity of MnO in the slags satisfactorily.
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