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Surface kinetics of nitrogen dissolution and its correlation to the slag structure in the CaO―SiO2, CaO―Al2O3, and CaO―SiO2―Al2O3 slag system
Journal of Non-Crystalline Solids 357 (2011) 2868–2875
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Surface kinetics of nitrogen dissolution and its correlation to the slag structure in the CaO―SiO2, CaO―Al2O3, and CaO―SiO2―Al2O3 slag system Seung-Min Han a, Jin-Gyun Park b, Il Sohn b,⁎ a b
Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 16 August 2010 Received in revised form 7 March 2011 Available online 16 April 2011
The kinetics of nitrogen dissolution into molten slag at 1873 K was investigated by an isotope exchange technique. Rate constants were correlated with the molten slag structure obtained from FT-IR spectra. The rate constant in the CaO―SiO2 binary system showed a maximum value at a specific slag composition followed by a decrease in the rate due to excess O2− blocking the surface sites for nitrogen adsorption. The rate constant in the CaO―Al2O3 binary system was comparatively constant within the experimental range of 45 mass% to 60 mass%. The rate constant in the CaO―SiO2―Al2O3 ternary slag system was measured within the boundary of the liquidus line and showed a close correlation with the slag structure. Furthermore, the rate constant in the CaO―SiO2―Al2O3 ternary system was found to be significantly higher compared to the binary system due to the correlated effect of lower binding energies of the Al―O bonds and the increased number of reaction sites available when smaller Si―O tetrahedral were simultaneously present with Al―O bonds. © 2011 Elsevier B.V. All rights reserved.
Keywords: Rate constant; slag structure; FT-IR; excess O2−; isotope exchange technique
1. Introduction Control of nitrogen pickup from the atmosphere through the slag layer is an essential factor in the production of interstitial free steels and inclusion control for high grade ultra clean steels. A fundamental understanding of the thermodynamics and kinetics of nitrogen dissolution in slags allows the design of optimum slag compositions and process parameters to achieve the required levels of nitrogen in molten steels. While many studies on the thermodynamics of nitrogen dissolution in slags using a gas-slag equilibration technique have been done [1–6], there are only a limited number of studies available on the kinetics as far as the authors knowledge. In particular, nitrogen dissolution on the surface of the slag and the effects of additives such as Al2O3 are still unclear. Thermodynamic evaluations have shown that nitrogen in molten slag can dissolve as N3− (free nitride), N0 (bridged nitride), or N− (non-bridged nitride) and expressed by the following [1–6]. Freenitride : N2 ðgÞ + 3ðO
2−
3−
Þ = 2ðN 0
Þ + 3 = 2O2 ðgÞ
0
Bridgednitride : N2 ðgÞ + 3ðO Þ = 2ðN Þ + 3 = 2O2 ðgÞ Non bridgednitride −
−
2−
: N2 ðgÞ + 4ðO Þ = 2ðN Þ + ðO
Þ + 3 = 2O2 ðgÞ
⁎ Corresponding author. Tel.: + 82 2 2123 5837; fax: +82 2 312 5375. E-mail address: [email protected] (I. Sohn). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.03.023
ð1Þ ð2Þ
ð3Þ
Where O2− is the free oxygen ion, O− is the non-bridged oxygen, and O0 is the bridged oxygen. Thus, the nitrogen dissolution in slags can be a function of temperature, activity of oxygen ions (O2−, O0, O−), activity of nitrogen ions (N3−, N0, N−), nitrogen partial pressure, and oxygen partial pressure. However, surface reactions of nitrogen is likely to be dissolved into slag as either N3− or N−. Min et al. [3] and Martinez et al. [4] described the amphoteric effect of basicity a2− O on the nitrogen solubility in slag, where the nitrogen solubility decreased with higher basicity in an acidic slag system and increased with higher basicity in a basic slag system. Furthermore, Min et al. [3] showed the nitride capacities in the B2O3 based binary slag system for free nitrides (N3−) and non-bridged nitrides (N−) to follow the thermodynamic expected slopes of 3/2 and −1/2, respectively, when plotted as a function of basicity. Ito et al. [1] reported that nitrogen in the CaO―SiO2―Al2O3 slag is incorporated into the silicate network structure as N− or N0 depending on the concentration of SiO2 and Al2O3. In the few kinetics studies done on nitrogen dissolution, Ono et al. [7] using the 28N2/30N2 isotope exchange technique measured the rate of nitrogen dissolution in the CaO―Al2O3 and CaO―SiO2 binary slag system showing the effects of Al2O3 and SiO2 additives on both the apparent rate constants(kapp) and the nitride capacity(C3− N ). It is widely known that the isotope exchange technique has an advantage of determining the kinetics of interfacial reactions, which is otherwise not possible using other methods. In particular, by measuring the isotope fraction of 29N2 in the off gas as a function of time during nitrogen dissolution, the rate constant of the interfacial reaction can be approximately obtained. The nitrogen dissolution into slags can be separated into five individual steps given by reactions (4) to (8). Using
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Table 1 Post-experiment composition using XRF and the rate constant for the slag compositions. Exp.#
Cao (mass%)
SiO2 (mass%)
Al2O3 (mass%)
Total slag weight (g)
kc × 105 (mol/cm2 s atm)
S-CS-01 S-CS-02 S-CS-03 S-CS-04 S-CSA-01 S-CSA-02 S-CSA-03 S-CSA-04 S-CSA-05 S-CSA-06 S-CSA-07 S-CSA-08 S-CSA-09 S-CSA-10 S-CSA-11 S-CSA-12 S-CSA-13 S-CSA-14 S-CSA-15 S-CSA-16 S-CSA-00 S-CSA-01 S-CSA-02 S-CSA-03
33 ± 0.005 42 ± 0.005 50 ± 0.005 56 ± 0.005 57⁎ ± 0.005 51.73 ± 0.005 50.68 ± 0.005 60.67 ± 0.005 62 ± 0.005 61.18⁎ ± 0.005 33.53 ± 0.005 32.3 ± 0.005 27.75 ± 0.005 24.48 ± 0.005 13.62 ± 0.005 7.63 ± 0.005 – 6.61 ± 0.005 13.6 ± 0.005 25.17 ± 0.005 40.8 ± 0.005 44.8 ± 0.005 50 ± 0.005 55.1 ± 0.005
67⁎ ± 0.005 58 ± 0.005 50 ± 0.005 44 ± 0.005 43 ± 0.005 33.79 ± 0.005 24.66 ± 0.005 11.86 ± 0.005 10 ± 0.005 – – 12.3 ± 0.005 15.83 ± 0.005 24.17 ± 0.005 45.86 ± 0.005 68.31 ± 0.005 94.41 ± 0.005 86.95 ± 0.005 80.5 ± 0.005 72.37 ± 0.005 – – – –
– – – – – 14.48 ± 0.005 24.66 ± 0.005 27.5 ± 0.005 28 ± 0.005 38.82 ± 0.005 66.47⁎ ± 0.005 55.4 ± 0.005 56.42 ± 0.005 51.38 ± 0.005 40.52 ± 0.005 24.06 ± 0.005 5.59 ± 0.005 6.44 ± 0.005 5.9 ± 0.005 2.71 ± 0.005 59.2 ± 0.005 55.2 ± 0.005 50 ± 0.005 44.9 ± 0.005
2.9970 ± 0.00005 3.0010 ± 0.00005 2.9998 ± 0.00005 2.0011 ± 0.00005 3.0033 ± 0.00005 3.0040 ± 0.00005 3.0020 ± 0.00005 3.0018 ± 0.00005 3.0018 ± 0.00005 3.0018 ± 0.00005 3.0045 ± 0.00005 3.0031 ± 0.00005 3.0098 ± 0.00005 3.0038 ± 0.00005 3.0006 ± 0.00005 3.0060 ± 0.00005 3.0028 ± 0.00005 2.3672 ± 0.00005 3.0008 ± 0.00005 2.8653 ± 0.00005 3.0011 ± 0.00005 3.0004 ± 0.00005 3.0004 ± 0.00005 3.0009 ± 0.00005
0.13 ± 0.007 1.10 ± 0.055 0.25 ± 0.013 0.01 ± 0.001 0.54 ± 0.027 6.36 ± 0.319 6.9 ± 0.345 5.15 ± 0.258 3.28 ± 0.164 4.59 ± 0.230 5.86 ± 0.293 4.94 ± 0.247 1.97 ± 0.099 1.37 ± 0.069 2.87 ± 0.144 2.00 ± 0.100 2.43 ± 0.122 3.36 ± 0.168 2.48 ± 0.124 3.52 ± 0.176 4.78 ± 0.239 4.78 ± 0.239 4.55 ± 0.228 4.22 ± 0.211
⁎Denotes the saturated component for the case of each binary system.
the isotope exchange technique the adsorption step can be distinguished from the dissolution step. N2 ðgÞ + □ = N2 ðadsÞ : adsorption
ð4Þ
N2 ðadsÞ + □ = 2NðadsÞ : dissociation
ð5Þ
NðadsÞ = NðinslagÞ : dissolution
ð6Þ
with the molten slag structure were investigated assuming a surface blockage model derived from the basic concepts of the Langmuir adsorption isotherm where the nitrogen adsorption sites are ideal and only one reactions can take place per reaction site [7,8]. To understand the dissolution behavior with respect to the slag structure, FT-IR analysis was done on as-quenched samples of slags and interfacial tension data on molten slags with pure Fe as a function of slag composition were also compared.
NðadsÞ + NðadsÞ = N2 ðadsÞ + □ : association
ð7Þ
2. Experimental techniques and materials
N2 ðadsÞ = N2 ðgÞ + □ : desorption
ð8Þ
Reagent-grade chemicals of SiO2, Al2O3, and CaCO3 were used. CaCO3 was calcined to CaO at 1273 K for 6 h. The chemicals were mixed and pre-melted in a graphite crucible at 1873 K. After premelting, the composition of the experimental slags was analyzed using the X-Ray Fluorescence (XRF) spectroscopy (S4 Explorer; Bruker AXS GmbH, Karlsruhe, Germany).
Where □ is the vacancies for reactions to occur and “ads” is the surface adsorbed species. According to the results of Ono et al. [7], Iuchi et al. [8], and OnoNakazato et al. [9], the rate controlling step of the overall rate of nitrogen dissolution into molten slags were found to be either adsorption or the dissociation steps. The results of their studies showed both the nitride capacity and the apparent rate constants decreased with higher Al2O3 in the CaO―Al2O3 system. For the CaO―SiO2 system, both the nitride capacity and the rate constant showed a parabolic shaped curve with a maximum at 45 mass% CaO. Using the same isotope exchange method, Iuchi et al. [8] in the CaO―SiO2―Al2O3 ternary slag system showed the nitrogen dissolution to increase with higher SiO2 and Al2O3 concentration. However, the kinetics of nitrogen dissolution at the slag surface was not clearly distinguished from the bulk dissolution behavior. Furthermore, the surface kinetics of nitrogen dissolution was correlated with the thermodynamic nitride capacity and did not completely provide a direct cause of the correlation between one another. Considering that the nitride capacity is related only to the driving force for the total reaction rate of nitrogen dissolution and the rate constant is independent of the driving force for the reaction, it would seem a direct correlation between the rate constant and the nitride capacity to be difficult. In the present study, the surface kinetics of nitrogen dissolution in the CaO―SiO2, CaO―Al2O3, and CaO―SiO2―Al2O3 molten slag was investigated by using the isotope exchange technique. In particular, the kinetics of nitrogen adsorption and dissolution and its relationship
Fig. 1. Experimental composition to evaluate the rate constant of nitrogen dissolution in molten slags at 1873 K on ternary phase diagram of CaO―SiO2―Al2O3 [10].
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double check the seal, the outlet gas line of the sealed reaction chamber was also connected with the mass spectrometer resulting in negligible amounts of Ar detection. Thus, a relatively good seal could be verified. Additional details of the isotope exchange technique can be found elsewhere [11]. Using the results of a previous study [12], the apparent rate constant of nitrogen dissolution, ka (mol/cm2 s atm), was calculated using Eq. (9).
ka = −
ð29 Feq −29 Ff Þ V ln A × R × T ð29 F −29 F Þ eq
Fig. 2. Schematic diagram of experimental apparatus.
The composition of the slags is given in Table 1 and the experiments of the CaO―SiO2―Al2O3 ternary slag system are replicated on the ternary phase diagram in Fig. 1 [10]. A nitrogen isotope mixture of 99.999% 28N2 and 98% 30N2 (Cambridge Isotope Laboratories, Inc; Andover, MA) in the ratio of 97:3 was prepared by mixing individual containers of 28N2 and 30N2 in the volumetric ratio of 97 to 3 after excess moisture was removed using columns of CaSO4. The 28N2 and 30N2 was completely mixed in a quartz chamber filled with alumina balls and the mixture gas was then passed through heated Mg turnings at 773 K to remove oxygen impurities. A quadrupole type mass spectrometer (Hiden Analytical; model HPR 20, Warrington, UK) was used to analyze the fraction of each gas component. Preliminary calibration tests verified the detection capability of the mass spectrometer using the 3% mixture 30N2 isotope gas resulting in a standard deviation of ±2% using at least three consecutive measurements. In recent work published by Ono et al. [7], about 1% 30N2 isotope gas mixtures have also been used without significant issues in reproducibility and accuracy. The nitrogen partial pressure was fixed at unit atmospheric pressure. A schematic of the experimental apparatus is shown in Fig. 2. Approximately 3 g of slag (as listed in Table 1) was placed inside a graphite crucible and heated to the target temperature using an induction furnace. Smaller amounts of sample was used for the CS-04, CSA-14, and CSA-16 labeled in Table 1, but the slag sample amount was sufficient to completely cover the bottom of the crucible and provide comparable reaction surface areas for the experiments. A proportional integral differential (PID) controller was used to control the temperature within ±3 K. A dual alumina lance was inserted into the graphite crucible 5 mm above the surface of the molten slags and fixed with alumina cement. 0.2 l/min of isotope gas was blown through the lance. The outer and inner alumina double tube for isotope gas exchange was sealed with the carbon crucible using alumina cement. This sealed reaction chamber was placed inside the quartz containment tube purged with Ar gas. To ensure a reliable seal of the isotope gas reaction chamber and separate the Ar flowing quartz containment tube, a dual verification method was used. During the experiment, the outlet gas line of the quartz containment tube was connected with the mass spectrometer and from multiple measurements only Ar gas was detected within the analysis. To
ð9Þ
i
where Vis the volumetric flow rate or V/t (cm3/sec), A is the slag surface area (cm2), R is the gas constant (82.06 cm3 atm/K mol), T is the gas absolute temperature (K), 29Feq is the equilibrium fraction of 29 N2 species, and 29Fi and 29Ff are the fractions of 29N2 in the inlet and outlet gases, respectively. According to Eq. (9), there are four variables that may affect the accuracy of the rate constant measurements. The volumetric flowrate (V), the temperature measurements and control (T), the isotope fractions measured by the mass spectrometer (F), and the slag reaction surface area (A). Each individual measurement contains errors which can lead to deviations of the absolute values. The volumetric flow meter used in the present study has an accuracy of ±1% full scale and the temperature controller had an accuracy of ±0.19%. Although the mass spectrometer manufacturer did not provide accuracy values of the instrument, the standard deviation of at least three measurements using the calibration gas was found to be approximately ±2.3% and the reacting surface area was found to be at least 1.5% due to the curvature of the surface area and also the accuracy of the diameter of the graphite crucible. Thus, using a linear combination of the various errors inherent in the variables for the calculation of the rate constant in Eq. (9), the error was found to be approximately ±5.0%. To qualitatively analyze the slag structure, as-quenched slag samples from 1873 K were analyzed by Fourier Transform-InfraRed (FT-IR) spectrometer (Spectra100; Perkin Elmer, Shelton, CT) in the range of 4000 to 400 cm−1. It should be noted that significant changes in the transmittance peaks were observed in the wavenumber range between 1200 cm−1 and 400 cm−1 and the FT-IR spectra are focused within this region of interest. To obtain a representative sample for FT-IR analysis, the power of the high frequency induction furnace is turned off and a stream of high purity Ar gas was passed through the sealed reaction chamber. Using a separate Ar gas gun, the sealed reaction chamber is rapidly taken out of the external quartz containment tube and cooled.
Fig. 3. Dependence of the rate constant of nitrogen dissolution with mass % SiO2 in the CaO―SiO2 binary system at 1873 K with the nitride capacity of Martinez et al. [4] at 1823 K The lines have been drawn as guides.
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Fig. 4. IR spectra of the CaO―SiO2 binary system as a functional of wavenumbers at different CaO contents after Park et al. [14].
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Fig. 6. Rate constant of nitrogen dissolution as a function of Al2O3 (SiO2 + Al2O3) ratio for the silica based CaO―SiO2―Al2O3 ternary system at 1873 K. The lines have been drawn as guides.
3. Results 3.1. Surface kinetics of nitrogen dissolution in the CaO―SiO2 binary system To simplify the kinetics of nitrogen dissolution at the surface of molten ternary slag systems, binary slags of CaO―SiO2 and CaO―Al2O3 were first studied similar to the approach taken by Ono et al. [7] and Iuchi et al. [8]. Fig. 3 shows the rate constant of nitrogen dissolution as a function of mass% SiO2 in the CaO―SiO2 binary system at 1873 K with the results of Ono et al. [7] and the nitride capacity of Martinez et al. [4] at 1823 K. The rate constant of nitrogen dissolution at the surface of the CaO―SiO2 binary system shows a parabolic shape with a maximum at approximately 58 mass% SiO2 and subsequently decreases, but the cause of this decrease was not clearly understood. The nitride capacity showed a similar behavior to the rate constant and Ono et al. [7] attempted to correlate a direct relationship between the rate constant and the nitride capacity. The results of the present
study coincided with work done by both Ono et al. [7] and Iuchi et al. [8], but the interpretation of the nitride capacity and its apparent relationship with the rate constant is significantly different. As previously noted, plotting the nitride capacity with the rate constant should not have direct physical correlations since the nitride capacity is a thermodynamic function, which is fixed when the slag composition and temperature is determined. Thus, the nitride capacity provides a thermodynamics driving force for nitrogen dissolution reaction to proceed, but the rate constant is not directly correlated with the driving force of the nitrogen dissolution reaction. Therefore, the apparent correlation found from the results between the rate constant of nitrogen dissolution at the slag surface and the nitride capacity is likely a common superficial effect of the slag structure with both the nitride capacity and the rate constant. According to the aforementioned reactions of (1) to (3), both the kinetic rate of nitrogen dissolution and the thermodynamic nitrogen solubility in molten slags can be affected by the oxygen ion species and subsequently the structure of the Si―O bond. Park et al. [13] suggested the ionic species of oxygen in CaO―SiO2 binary slags to be a function of CaO content. Below 40.3 mass%, the majority of oxygen ions exist as a non-bridging oxygen (O−) and above 40.3 mass% the free oxygen (O2−) begins to prevail. Below 40.3 mass% CaO, the abundance of O− will increase the nitrogen incorporation into the network of silica as a non-bridging incorporated nitride (N-). The incorporation of nitrogen within the slag structure is clearly evident using the FT-IR spectra of Fig. 4 reported by Park et al. [14] According to Park et al. [15], the amount of monomers [SiO4]4− and dimers [Si2O7]6− located within the [SiO4]-tetrahedral stretching bands, which provides much of the reaction sites for nitrogen, increases with CaO additions and remains relatively constant above 42.4 mass% CaO. The increase in the fraction of dimers and monomers is evident from the stretching Si―O band (1180 ~ 760cm-1) after CaO is increased from 33.7 to 46.3 mass%. Beyond 46.3 mass% to 56.6 mass%, the amount of monomer and dimer seem similar. 3.2. Surface kinetics of nitrogen dissolution in the CaO―Al2O3 binary system
Fig. 5. The rate constant of nitorgen dissolution in CaO―Al2O3 binary system as a function of mass % Al2O3 at 1873 K plotted with the nitride capacity after Schwerdtfeger et al. [16]. The lines have been drawn as guides.
The rate constant of nitrogen dissolution in the CaO―Al2O3 binary system as a function of mass% Al2O3 at 1873 K is shown in Fig. 5 with the results of Ono et al. [7] and the nitride capacity calculated by
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The rate constants of nitrogen dissolution in the CaO―SiO2―Al2O3 ternary system at 1873 K have been studied in three major regions as previously shown in Fig. 1. The first region is the silica based CaO―SiO2―Al2O3 system beginning from the CaO―SiO2 binary
system following the liquidus line and ending in the CaO―Al2O3 binary system. The second region is the alumina based CaO―SiO2―Al2O3 system beginning from the CaO―Al2O3 binary system following the liquidus line and ending in the Al2O3―SiO2 binary system. The final region is the CaO―SiO2(satd.)―Al2O3 system beginning from the Al2O3―SiO2 binary system following the liquidus line and ending in the CaO―SiO2 binary system. The rate constant of nitrogen dissolution in the CaO―SiO2―Al2O3 slags is 10 to 100 fold faster than in the binary CaO―SiO2 slag previously reviewed. This could be due to the smaller binding energy of the Al―O bonds compared to the Si―O bonds, where nitrogen adsorption takes place and will be discussed later in detail. Fig. 6 shows the rate constant of nitrogen dissolution in the first region of the silica based CaO―SiO2―Al2O3 ternary slag system as a function of (mass% Al2O3)/[(mass% SiO2)+(mass% Al2O3)] ratio (or A/(S+A)). From the CaO―SiO2 binary slag system along the liquidus line through the CaO saturated region, the rate constant significantly increases with higher Al2O3 up to the (mass% Al2O3)/[(mass% SiO2)+(mass% Al2O3)] ratio of 0.3. Above 0.3, the rate constant of nitrogen dissolution was relatively constant. The FT-IR analysis for the silica based CaO―SiO2―Al2O3 as quenched slag samples is shown in Fig. 7. As the Al2O3 content increases, the stretching vibrations of the [SiO4]-tetrahedra band observed at 1160 to 760 cm−1 overlapped with the [AlO4]-tetrahedra band at 960 to 600 cm−1, which convolutes the IR spectra. In addition, the relative intensity of the [AlO6]9−-octahedral peak increases with higher (mass% Al2O3) / [(mass% SiO2) + (mass% Al2O3)] ratio [17–22]. The rate constant of nitrogen dissolution in the second region of the aluminate based CaO―SiO2―Al2O3 ternary system was also measured and shown in Fig. 8. As the (mass% CaO) / [(mass% CaO) + (mass% SiO2)] ratio (or C / (C + S)) increases appreciable change in the rate constant is not observed until the C / (C + S) ratio reaches 0.5. Beyond 0.5, the rate constant begins to show significant increase. Fig. 9 shows the corresponding FT-IR spectra of the alumina based CaO―SiO2―Al2O3 ternary slag for various C / (C + S) ratio. Above 0.64, the [AlO4]5−-tetrahedra band in the spectra widens indicating an increase in the depolymerization of Al―O network structure for incorporation of nitrogen. The final region studied for the CaO―SiO2―Al2O3 ternary slag system is the SiO2 saturated region. Fig. 10 shows the rate constant of nitrogen dissolution in the SiO2 saturated CaO―SiO2―Al2O3 ternary
Fig. 8. Rate constant of nitrogen dissolution as a function of CaO / (CaO + SiO2) ratio in alumina based CaO―SiO2―Al2O3 ternary system at 1873 K. The lines have been drawn as guides.
Fig. 9. FT-IR spectra of alumina based CaO―SiO2―Al2O3 slag samples as quenched from 1873 K for different CaO / (CaO + SiO2) ratio.
Fig. 7. FT-IR spectra of silica based CaO―SiO2―Al2O3 system in the liquidus composition at 1873 K.
Schwerdtfeger et al. [16]. The rate constant is relatively unaffected with Al2O3 additions between the ranges of 45 and 60 mass% Al2O3. The resulting trend of the rate constant is comparable to Ono et al. [7], but the once similar behavior of the nitride capacity with the rate constant observed for the CaO―SiO2 binary slag system in Fig. 3 no longer seems to match the results obtained in the CaO―Al2O3 binary slag system. It should also be noted that the present experimental rate constant values in the CaO―Al2O3 binary slag system are about 10 fold higher than the results of Ono et al. [7] 3.3. Surface kinetics of nitrogen dissolution in the CaO―SiO2―Al2O3 ternary system
S.-M. Han et al. / Journal of Non-Crystalline Solids 357 (2011) 2868–2875
Fig. 10. Rate constant of nitrogen dissolution as a function of CaO / (CaO―Al2O3) ratio in the SiO2 saturated CaO―SiO2―Al2O3 ternary system at 1873 K. The line has been drawn as a guide.
system as a function of (mass% CaO) / [(mass% CaO) + (mass% Al2O3)] ratio (or C / (C + A)). Results indicate that the rate constants was relatively independent of the (mass% CaO) / [(mass% CaO) + (mass% Al2O3)] ratio for this particular slag composition. This negligible change seems to correspond well with the negligible changes observed in the FT-IR analysis shown in Fig. 11. 4. Discussions According to the Langmuir adsorption model, vacant sites must be available for nitrogen adsorption to take place as described by reactions (4) to (8). If these reaction sites are occupied, the rate would be inhibited. The surface reaction sites in slag can be occupied by 2− 6− 6− 5− several different types of anions (SiO4− 4 , Si2O7 , Si3O9 , AlO4 , O etc.) and cations (Ca2+, Al3+, etc.), but an overall electronegative balance must be maintained near the surface. Silicate and aluminate slags can provide several different forms of anionic tetrahedra or larger species. However, these ionic structures are comparatively larger than the free oxygen ions (O2−), which can be easily supplied by the dissociation of CaO or basic oxides. As more of the free oxygen ions exist within the melt, the mobility, the size, and the shape of the O2− ions may allow the O2− to accumulate at the reaction sites inhibiting nitrogen dissolution. In the CaO―SiO2 binary slag observed in Fig. 3, the rate constant of nitrogen dissolution reaches a maximum and beyond a certain mass% CaO decreases. This may be related to the modification of the slag surface structure with CaO additions. With CaO additions in acidic slags, the O2− from CaO is consumed in order to break the bridged oxygen of the silicate network structures into non-bridged oxygen and increase the reaction sites for incorporated nitrides subsequently resulting in higher rate constants. At sufficiently high enough concentrations of CaO, excess O2− starts to accumulate at the slag surface and subsequently block the reaction sites of non-bridged oxygen for adsorption and incorporation. In the silica based CaO―SiO2―Al2O3 ternary slag system, the rate constant increased with higher A/(A + S) ratio up to approximately 0.5. Above 0.5, the rate constant was found to be constant. This can be qualitatively explained through the combination of the binding energy and the slag surface structure. In the aforementioned behavior of nitrogen dissolution at the surface of the slag, the binding energy of Si―O bonds (799.6 ± 13.4 kJ/mol) [23] are much higher than the
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Al―O bonds (511 ± 3 kJ/mol) [23] and thus nitrogen dissolution is significantly faster with non-bridged oxygen in the aluminate structure [8]. As Al2O3 is introduced into the CaO―SiO2 binary system and if O2− break some of the network bonds of alumina, O− sites are exposed and the nitrogen can be incorporated into the slag. With higher CaO, the free oxygen ions substitute the silicate and aluminatetetrahedrals that were once present for nitrogen to adsorb. In addition, the FT-IR spectra for the silica based CaO―SiO2―Al2O3 ternary slag showed the relative intensity of the [AlO6]9−-octahedral peak to increase with higher Al2O3 content, which suggest [AlO6]9− can also be one of the major incorporation sites for nitrogen adsorption. In the alumina based CaO―SiO2―Al2O3 system, the rate constant was unaffected as the C/(C + S) ratio increased. Not until C/(C + S) ratio reached 0.5 and above did the rate constant increase. The rate constant as a function of C/(C + S) ratio starting from the Al2O3―SiO2 binary system increases the relative number of O2−, but is not sufficient enough initially to make a significant impact on the depolymerization of aluminate or silicate structures. Beyond 0.5, the free oxygen ions start to significantly depolymerize the aluminate structure increasing the [AlO4]5− and [AlO6]9− incorporation sites. Above C/(C + S) of 0.5, the nitrogen dissolution would follow the free nitride (N3−) form as expressed by reaction (1). Thus, the amphoteric Al2O3 behaves as an acidic oxide and lowers the activity of the O2− resulting in decreased free nitrides. Furthermore, Al2O3 would provide AlO4-tetrahedrals to the surface, which may act as surface blocking sites for free nitride dissolution. Below C / (C + S) of 0.5, the overall slag composition would drive the amphoteric Al2O3 towards a basic behavior. Considering the incorporated nitride would be prevalent in an acidic slag system, the addition of a basic component to the slag would likely lower the dissolution of incorporated nitrides. In addition, if the Al2O3 behaves as a basic oxide in an acidic slag system, the incorporation sites for nitrogen dissolution may be eliminated by the supply of O2− to the slag system. This closely follows the slag structural information provided by the FT-IR analysis in Fig. 9. Above C / (C + S) ratio of 0.64 the [AlO4]5− tetrahedra band broadens indicating an increase in the depolymerization of Al―O network structure for incorporation of nitrogen. In this region, the amount of CaO additions is again not enough to provide significant adsorption sites for nitrogen and the rate constant is relatively constant as a function of C / (C + A) ratio.
Fig. 11. IR spectra of SiO2 saturated CaO―SiO2―Al2O3 as quenched slag samples from 1873 K with various CaO/(CaO―Al2O3) contents.
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Fig. 12. Schematic diagram of the adsorption site on the surface of molten slags for binary and ternary slag system.
When slags are usually depolymerized to polymeric ions such as rings, chains, dimers, and monomers, the oxygen ion is the outer most atom of these ionic structures and would likely be the atom exposed at the surface of the slag melts. Thus, non-bridged oxygen ions from the polymeric ion structures can act as the surface adsorption sites for nitrogen dissolution into molten slag. The depolymerized structures in the CaO―SiO2―Al2O3 exposing non-bridged oxygen of Si―O and Al―O are the adsorption sites for nitrogen dissolution and the smallest poly anions for example, [AlO4]5− and [SiO4]4− ions may compete as an adsorption site for nitrogen. The difference in the degree of nitrogen adsorption between Al―O and Si―O bond structures can be understood with the bond length and the bond enthalpy. The average bond length of the Al―O is longer than that of Si―O resulting in a larger Al―O structure than a Si―O structure. This suggests that the number of Al―O structures covering the surface is fewer than that of Si―O structures within a limited surface area of molten slags. However, the Al―O bond enthalpy is significantly smaller than the Si―O bond enthalpy indicating a lower activation energy required for nitrogen incorporation in the [AlO4]tetrahedra than the [SiO4]-tetrahedra. Thus, although the rate constant of nitrogen dissolution is easier for Al2O3 considering the activation energy for reaction, but due to the smaller volume of [SiO4]4− compared to [AlO4]5−. There are more numbers of Si―O adsorption sites than Al―O. As a result, the adsorption of nitrogen on the surface of molten slag which is occupied with a combination of both Al―O and Si―O bonds together is faster than just a single Al―O or Si―O bond
only due to the increase in the number of adsorption sites (or nonbridged oxygen) with SiO2 additions. Considering the above discussions, a schematic diagram of the adsorption sites on the surface of binary and ternary slag systems can be depicted as Fig. 12 where the CaO provides the necessary network breaking oxygen ions for incorporation of nitrogen to be possible, but beyond a certain threshold the amount of free oxygen ions may accumulate on the surface of the slag inhibiting the surface reaction of nitrogen adsorption and dissolution into molten slag. This effect of free oxygen ions on the slag surface is also supported by the interfacial tension data of Bretonnet et al. [24] in the CaO―Al2O3 binary slag system and Sun et al. [25] in the CaO―SiO2―Al2O3 ternary slag system shown in Figs. 13 and 14, respectively. The interfacial tension between the molten slag and pure Fe decreases with CaO additions and becomes relatively consistent beyond a certain mass% CaO. This limiting threshold is assumed to be the concentration where O2− significantly saturates the interface and impedes such reactions such as nitrogen dissolution at the slag surface. In the work of Tanaka et al. [26] and Nakamoto et al. [27], the surface tension of the molten silicate slags were greatly affected by the existence of anions (O2− 2+ and SiO4− , Na+) on the surface. Thus, the 4 ) and the cations (Ca surface tension is affected by the modification of the silicate network structure and also the existence of the ionic species causing surface relaxation, which lets the energetic state of the surface approach the bulk state.
S.-M. Han et al. / Journal of Non-Crystalline Solids 357 (2011) 2868–2875
Fig. 13. Interfacial tension of CaO―Al2O3 binary slag on pure Fe as a function of mass% CaO at 1873 K after Bretonet et al. [24]. The lines have been drawn as guides.
5. Conclusion The kinetics of nitrogen dissolution on the surface of CaO―SiO2, CaO―Al2O3, and CaO―SiO2―Al2O3 slags has been investigated by using the isotope exchange technique at 1873 K. The kinetic data were correlated with the slag structure and the following results were obtained. ▪ The rate constant of nitrogen dissolution for the binary CaO―SiO2 showed a maximum with CaO additions and followed by a decrease in the rate due to excess O2− blocking the surface sites for nitrogen adsorption. CaO additions in acidic slag compositions provide the necessary free oxygen ions to break the silicate and aluminate structures, which increases the number of non-bridging oxygen for nitrogen incorporation and subsequent increase in the rate constant. Beyond a certain CaO content in the basic slag compositions, the rate constant for nitrogen dissolution at the
Fig. 14. Interfacial tension of CaO―SiO2―Al2O3 ternary slag on pure. Fe as a function of Al2O3 / (SiO2 + Al2O3) ratio at 1853 K after Sun et al. [25]. The lines have been drawn as guides.
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surface of molten slags decrease due to the surface blockage of excess O2− ions. ▪ The CaO―Al2O3 rate constant is higher than the CaO―SiO2 rate constant due to the lower binding energy of the aluminates compared to the silicates. The rate constant is also faster with both the silicates and aluminates present due to the balanced effects of the lower binding energy of the Al―O bonds and the higher number of incorporation sites available when Si―O bonds exist. ▪ The rate constant in the CaO―SiO2―Al2O3 ternary system was found to be significantly higher compared to the binary system due to the correlated effect of lower binding energies of the Al―O bonds and the increased number of reaction sites available when smaller Si―O tetrahedral were simultaneously present with Al―O bonds. ▪ In the CaO―SiO2―Al2O3 ternary slag system, the nitrogen dissolution followed the free nitride (N3−) form when the C/(C+S) ratio was higher than 0.5. At high C/(C+S), Al2O3 behaved as an acidic oxide and provide AlO4-tetrahedrals to the surface, which acted as surface blocking sites for free nitride dissolution. Below C/(C+S) of 0.5, the nitrogen dissolution followed an incorporated nitride form and Al2O3 behaved as a basic component to the slag, which decreased the incorporation sites for nitrogen dissolution by the supplying of O2− to the system. The results of the binary and ternary slag systems suggest that additions of CaO has a limited effect and once a certain threshold is reached excess O2− ions can possibly inhibit the surface reaction of nitrogen adsorption and dissolution in molten slag system. Acknowledgments The present study was supported by POSCO, the Brain Korea 21 (BK21) Project of the Division of Humantronics Information Materials, the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. Additional financial support was provided by the Korean National Science Foundation under project number 2010-8-0581 and the Ministry of Knowledge Economy project number 2010-8-0972. References [1] K. Ito, R.J. Fruehan, Metall. Trans. B 19B (1988) 419–425. [2] N. Sano, Advanced Physical Chemistry for Process Metallurgy, Academic Press, London, 1997, pp. 60–64. [3] D.J. Min, R.J. Fruehan, Metall. Trans. B 21B (1990) 1025–1032. [4] E.R. Martinez, N. Sano, Metall. Trans. B 21B (1990) 97–104. [5] H. Suito, R. Inoue, Tetsu Hagane 73 (1987) S246. [6] W. Shin, H. Lee, ISIJ Int. 41 (2001) 239–246. [7] H. Ono, K. Morita, N. Sano, Metall. Trans. B 28B (1997) 633–638. [8] K. Iuchi, K. Morita, N. Sano, Metall. Trans. B 29B (1998) 1235–1237. [9] H. Ono-Nakazato, S. Morisawa, T. Usui, Metall. Trans. B 33B (2002) 393–401. [10] VereinDeutscher Eisenhuttenlente(VDEh): Slag Atlas, 2nd edition, Verlag Stahleisen GmgH, D-Dusseldorf, 1995, p. 105. [11] A. Ozaki, Isotopic Studies of Heterogeneous Catalysis, Kodansha Ltd. and Academic Press, Tokyo, 1977. [12] S.M. Han, J.H. Park, S.M. Jung, D.J. Min, ISIJ Int. 49 (2009) 487–494. [13] J.-H. Park, P.C.-H. Rhee, J. NonCrytal. Sol. 282 (2001) 7–14. [14] J.H. Park, D.J. Min, H.S. Song, ISIJ Int. 42 (2002) 344–351. [15] J.H. Park, D.J. Min, H.S. Song, ISIJ Int. 42 (2002) 38–43. [16] K. Schwerdtfeger, Schubert Hans George, Metall. Trans. B 8B (1977) 535–540. [17] B.O. Mysen, D. Virgo, C.M. Scarfe, Am. Mineral. 65 (1980) 690–710. [18] T. Tsunawaki, N. Iwamoto, T. Hattori, A. Mitsuishi, J. NonCryst. Solids 44 (1981) 369–378. [19] P. McMillan, Am. Mineral. 69 (1984) 622–644. [20] P. McMillan, Am. Mineral. 69 (1984) 645–659. [21] R.W. Luth, Am. Mineral. 73 (1988) 297–305. [22] G. Leekes, N. Nowack, F. Schlegelmilch, Steel Res. 59 (1988) 406–416. [23] J.A. Kerr, in: D.R. Lide (Ed.), A Ready-Reference Book of Chemical and Physical Data, CRC Handbook of Chemistry and Physics 1999–2000, 81st ed, CRC Press, Boca Raton, Florida, USA, 2000. [24] J.L. Bretonnet, L.D. Lucas, M. Olette, Circ. Inf. Techn. Cent. Doc. Sider 33 (1976) 105. [25] H. Sun, K. Nakashima, K. Mori, ISIJ Int. 46 (2006) 407–412. [26] T. Tanaka, T. Kitamura, I.A. Back, ISIJ Int. 46 (2006) 400–406. [27] M. Nakamoto, T. Tanaka, L. Holappa, M. Hamalainen, ISIJ Int. 47 (2007).
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Surface kinetics of nitrogen dissolution and its correlation to the slag structure in the CaO―SiO2, CaO―Al2O3, and CaO―SiO2―Al2O3 slag system Seung-Min Han a, Jin-Gyun Park b, Il Sohn b,⁎ a b
Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 16 August 2010 Received in revised form 7 March 2011 Available online 16 April 2011
The kinetics of nitrogen dissolution into molten slag at 1873 K was investigated by an isotope exchange technique. Rate constants were correlated with the molten slag structure obtained from FT-IR spectra. The rate constant in the CaO―SiO2 binary system showed a maximum value at a specific slag composition followed by a decrease in the rate due to excess O2− blocking the surface sites for nitrogen adsorption. The rate constant in the CaO―Al2O3 binary system was comparatively constant within the experimental range of 45 mass% to 60 mass%. The rate constant in the CaO―SiO2―Al2O3 ternary slag system was measured within the boundary of the liquidus line and showed a close correlation with the slag structure. Furthermore, the rate constant in the CaO―SiO2―Al2O3 ternary system was found to be significantly higher compared to the binary system due to the correlated effect of lower binding energies of the Al―O bonds and the increased number of reaction sites available when smaller Si―O tetrahedral were simultaneously present with Al―O bonds. © 2011 Elsevier B.V. All rights reserved.
Keywords: Rate constant; slag structure; FT-IR; excess O2−; isotope exchange technique
1. Introduction Control of nitrogen pickup from the atmosphere through the slag layer is an essential factor in the production of interstitial free steels and inclusion control for high grade ultra clean steels. A fundamental understanding of the thermodynamics and kinetics of nitrogen dissolution in slags allows the design of optimum slag compositions and process parameters to achieve the required levels of nitrogen in molten steels. While many studies on the thermodynamics of nitrogen dissolution in slags using a gas-slag equilibration technique have been done [1–6], there are only a limited number of studies available on the kinetics as far as the authors knowledge. In particular, nitrogen dissolution on the surface of the slag and the effects of additives such as Al2O3 are still unclear. Thermodynamic evaluations have shown that nitrogen in molten slag can dissolve as N3− (free nitride), N0 (bridged nitride), or N− (non-bridged nitride) and expressed by the following [1–6]. Freenitride : N2 ðgÞ + 3ðO
2−
3−
Þ = 2ðN 0
Þ + 3 = 2O2 ðgÞ
0
Bridgednitride : N2 ðgÞ + 3ðO Þ = 2ðN Þ + 3 = 2O2 ðgÞ Non bridgednitride −
−
2−
: N2 ðgÞ + 4ðO Þ = 2ðN Þ + ðO
Þ + 3 = 2O2 ðgÞ
⁎ Corresponding author. Tel.: + 82 2 2123 5837; fax: +82 2 312 5375. E-mail address: [email protected] (I. Sohn). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.03.023
ð1Þ ð2Þ
ð3Þ
Where O2− is the free oxygen ion, O− is the non-bridged oxygen, and O0 is the bridged oxygen. Thus, the nitrogen dissolution in slags can be a function of temperature, activity of oxygen ions (O2−, O0, O−), activity of nitrogen ions (N3−, N0, N−), nitrogen partial pressure, and oxygen partial pressure. However, surface reactions of nitrogen is likely to be dissolved into slag as either N3− or N−. Min et al. [3] and Martinez et al. [4] described the amphoteric effect of basicity a2− O on the nitrogen solubility in slag, where the nitrogen solubility decreased with higher basicity in an acidic slag system and increased with higher basicity in a basic slag system. Furthermore, Min et al. [3] showed the nitride capacities in the B2O3 based binary slag system for free nitrides (N3−) and non-bridged nitrides (N−) to follow the thermodynamic expected slopes of 3/2 and −1/2, respectively, when plotted as a function of basicity. Ito et al. [1] reported that nitrogen in the CaO―SiO2―Al2O3 slag is incorporated into the silicate network structure as N− or N0 depending on the concentration of SiO2 and Al2O3. In the few kinetics studies done on nitrogen dissolution, Ono et al. [7] using the 28N2/30N2 isotope exchange technique measured the rate of nitrogen dissolution in the CaO―Al2O3 and CaO―SiO2 binary slag system showing the effects of Al2O3 and SiO2 additives on both the apparent rate constants(kapp) and the nitride capacity(C3− N ). It is widely known that the isotope exchange technique has an advantage of determining the kinetics of interfacial reactions, which is otherwise not possible using other methods. In particular, by measuring the isotope fraction of 29N2 in the off gas as a function of time during nitrogen dissolution, the rate constant of the interfacial reaction can be approximately obtained. The nitrogen dissolution into slags can be separated into five individual steps given by reactions (4) to (8). Using
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Table 1 Post-experiment composition using XRF and the rate constant for the slag compositions. Exp.#
Cao (mass%)
SiO2 (mass%)
Al2O3 (mass%)
Total slag weight (g)
kc × 105 (mol/cm2 s atm)
S-CS-01 S-CS-02 S-CS-03 S-CS-04 S-CSA-01 S-CSA-02 S-CSA-03 S-CSA-04 S-CSA-05 S-CSA-06 S-CSA-07 S-CSA-08 S-CSA-09 S-CSA-10 S-CSA-11 S-CSA-12 S-CSA-13 S-CSA-14 S-CSA-15 S-CSA-16 S-CSA-00 S-CSA-01 S-CSA-02 S-CSA-03
33 ± 0.005 42 ± 0.005 50 ± 0.005 56 ± 0.005 57⁎ ± 0.005 51.73 ± 0.005 50.68 ± 0.005 60.67 ± 0.005 62 ± 0.005 61.18⁎ ± 0.005 33.53 ± 0.005 32.3 ± 0.005 27.75 ± 0.005 24.48 ± 0.005 13.62 ± 0.005 7.63 ± 0.005 – 6.61 ± 0.005 13.6 ± 0.005 25.17 ± 0.005 40.8 ± 0.005 44.8 ± 0.005 50 ± 0.005 55.1 ± 0.005
67⁎ ± 0.005 58 ± 0.005 50 ± 0.005 44 ± 0.005 43 ± 0.005 33.79 ± 0.005 24.66 ± 0.005 11.86 ± 0.005 10 ± 0.005 – – 12.3 ± 0.005 15.83 ± 0.005 24.17 ± 0.005 45.86 ± 0.005 68.31 ± 0.005 94.41 ± 0.005 86.95 ± 0.005 80.5 ± 0.005 72.37 ± 0.005 – – – –
– – – – – 14.48 ± 0.005 24.66 ± 0.005 27.5 ± 0.005 28 ± 0.005 38.82 ± 0.005 66.47⁎ ± 0.005 55.4 ± 0.005 56.42 ± 0.005 51.38 ± 0.005 40.52 ± 0.005 24.06 ± 0.005 5.59 ± 0.005 6.44 ± 0.005 5.9 ± 0.005 2.71 ± 0.005 59.2 ± 0.005 55.2 ± 0.005 50 ± 0.005 44.9 ± 0.005
2.9970 ± 0.00005 3.0010 ± 0.00005 2.9998 ± 0.00005 2.0011 ± 0.00005 3.0033 ± 0.00005 3.0040 ± 0.00005 3.0020 ± 0.00005 3.0018 ± 0.00005 3.0018 ± 0.00005 3.0018 ± 0.00005 3.0045 ± 0.00005 3.0031 ± 0.00005 3.0098 ± 0.00005 3.0038 ± 0.00005 3.0006 ± 0.00005 3.0060 ± 0.00005 3.0028 ± 0.00005 2.3672 ± 0.00005 3.0008 ± 0.00005 2.8653 ± 0.00005 3.0011 ± 0.00005 3.0004 ± 0.00005 3.0004 ± 0.00005 3.0009 ± 0.00005
0.13 ± 0.007 1.10 ± 0.055 0.25 ± 0.013 0.01 ± 0.001 0.54 ± 0.027 6.36 ± 0.319 6.9 ± 0.345 5.15 ± 0.258 3.28 ± 0.164 4.59 ± 0.230 5.86 ± 0.293 4.94 ± 0.247 1.97 ± 0.099 1.37 ± 0.069 2.87 ± 0.144 2.00 ± 0.100 2.43 ± 0.122 3.36 ± 0.168 2.48 ± 0.124 3.52 ± 0.176 4.78 ± 0.239 4.78 ± 0.239 4.55 ± 0.228 4.22 ± 0.211
⁎Denotes the saturated component for the case of each binary system.
the isotope exchange technique the adsorption step can be distinguished from the dissolution step. N2 ðgÞ + □ = N2 ðadsÞ : adsorption
ð4Þ
N2 ðadsÞ + □ = 2NðadsÞ : dissociation
ð5Þ
NðadsÞ = NðinslagÞ : dissolution
ð6Þ
with the molten slag structure were investigated assuming a surface blockage model derived from the basic concepts of the Langmuir adsorption isotherm where the nitrogen adsorption sites are ideal and only one reactions can take place per reaction site [7,8]. To understand the dissolution behavior with respect to the slag structure, FT-IR analysis was done on as-quenched samples of slags and interfacial tension data on molten slags with pure Fe as a function of slag composition were also compared.
NðadsÞ + NðadsÞ = N2 ðadsÞ + □ : association
ð7Þ
2. Experimental techniques and materials
N2 ðadsÞ = N2 ðgÞ + □ : desorption
ð8Þ
Reagent-grade chemicals of SiO2, Al2O3, and CaCO3 were used. CaCO3 was calcined to CaO at 1273 K for 6 h. The chemicals were mixed and pre-melted in a graphite crucible at 1873 K. After premelting, the composition of the experimental slags was analyzed using the X-Ray Fluorescence (XRF) spectroscopy (S4 Explorer; Bruker AXS GmbH, Karlsruhe, Germany).
Where □ is the vacancies for reactions to occur and “ads” is the surface adsorbed species. According to the results of Ono et al. [7], Iuchi et al. [8], and OnoNakazato et al. [9], the rate controlling step of the overall rate of nitrogen dissolution into molten slags were found to be either adsorption or the dissociation steps. The results of their studies showed both the nitride capacity and the apparent rate constants decreased with higher Al2O3 in the CaO―Al2O3 system. For the CaO―SiO2 system, both the nitride capacity and the rate constant showed a parabolic shaped curve with a maximum at 45 mass% CaO. Using the same isotope exchange method, Iuchi et al. [8] in the CaO―SiO2―Al2O3 ternary slag system showed the nitrogen dissolution to increase with higher SiO2 and Al2O3 concentration. However, the kinetics of nitrogen dissolution at the slag surface was not clearly distinguished from the bulk dissolution behavior. Furthermore, the surface kinetics of nitrogen dissolution was correlated with the thermodynamic nitride capacity and did not completely provide a direct cause of the correlation between one another. Considering that the nitride capacity is related only to the driving force for the total reaction rate of nitrogen dissolution and the rate constant is independent of the driving force for the reaction, it would seem a direct correlation between the rate constant and the nitride capacity to be difficult. In the present study, the surface kinetics of nitrogen dissolution in the CaO―SiO2, CaO―Al2O3, and CaO―SiO2―Al2O3 molten slag was investigated by using the isotope exchange technique. In particular, the kinetics of nitrogen adsorption and dissolution and its relationship
Fig. 1. Experimental composition to evaluate the rate constant of nitrogen dissolution in molten slags at 1873 K on ternary phase diagram of CaO―SiO2―Al2O3 [10].
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double check the seal, the outlet gas line of the sealed reaction chamber was also connected with the mass spectrometer resulting in negligible amounts of Ar detection. Thus, a relatively good seal could be verified. Additional details of the isotope exchange technique can be found elsewhere [11]. Using the results of a previous study [12], the apparent rate constant of nitrogen dissolution, ka (mol/cm2 s atm), was calculated using Eq. (9).
ka = −
ð29 Feq −29 Ff Þ V ln A × R × T ð29 F −29 F Þ eq
Fig. 2. Schematic diagram of experimental apparatus.
The composition of the slags is given in Table 1 and the experiments of the CaO―SiO2―Al2O3 ternary slag system are replicated on the ternary phase diagram in Fig. 1 [10]. A nitrogen isotope mixture of 99.999% 28N2 and 98% 30N2 (Cambridge Isotope Laboratories, Inc; Andover, MA) in the ratio of 97:3 was prepared by mixing individual containers of 28N2 and 30N2 in the volumetric ratio of 97 to 3 after excess moisture was removed using columns of CaSO4. The 28N2 and 30N2 was completely mixed in a quartz chamber filled with alumina balls and the mixture gas was then passed through heated Mg turnings at 773 K to remove oxygen impurities. A quadrupole type mass spectrometer (Hiden Analytical; model HPR 20, Warrington, UK) was used to analyze the fraction of each gas component. Preliminary calibration tests verified the detection capability of the mass spectrometer using the 3% mixture 30N2 isotope gas resulting in a standard deviation of ±2% using at least three consecutive measurements. In recent work published by Ono et al. [7], about 1% 30N2 isotope gas mixtures have also been used without significant issues in reproducibility and accuracy. The nitrogen partial pressure was fixed at unit atmospheric pressure. A schematic of the experimental apparatus is shown in Fig. 2. Approximately 3 g of slag (as listed in Table 1) was placed inside a graphite crucible and heated to the target temperature using an induction furnace. Smaller amounts of sample was used for the CS-04, CSA-14, and CSA-16 labeled in Table 1, but the slag sample amount was sufficient to completely cover the bottom of the crucible and provide comparable reaction surface areas for the experiments. A proportional integral differential (PID) controller was used to control the temperature within ±3 K. A dual alumina lance was inserted into the graphite crucible 5 mm above the surface of the molten slags and fixed with alumina cement. 0.2 l/min of isotope gas was blown through the lance. The outer and inner alumina double tube for isotope gas exchange was sealed with the carbon crucible using alumina cement. This sealed reaction chamber was placed inside the quartz containment tube purged with Ar gas. To ensure a reliable seal of the isotope gas reaction chamber and separate the Ar flowing quartz containment tube, a dual verification method was used. During the experiment, the outlet gas line of the quartz containment tube was connected with the mass spectrometer and from multiple measurements only Ar gas was detected within the analysis. To
ð9Þ
i
where Vis the volumetric flow rate or V/t (cm3/sec), A is the slag surface area (cm2), R is the gas constant (82.06 cm3 atm/K mol), T is the gas absolute temperature (K), 29Feq is the equilibrium fraction of 29 N2 species, and 29Fi and 29Ff are the fractions of 29N2 in the inlet and outlet gases, respectively. According to Eq. (9), there are four variables that may affect the accuracy of the rate constant measurements. The volumetric flowrate (V), the temperature measurements and control (T), the isotope fractions measured by the mass spectrometer (F), and the slag reaction surface area (A). Each individual measurement contains errors which can lead to deviations of the absolute values. The volumetric flow meter used in the present study has an accuracy of ±1% full scale and the temperature controller had an accuracy of ±0.19%. Although the mass spectrometer manufacturer did not provide accuracy values of the instrument, the standard deviation of at least three measurements using the calibration gas was found to be approximately ±2.3% and the reacting surface area was found to be at least 1.5% due to the curvature of the surface area and also the accuracy of the diameter of the graphite crucible. Thus, using a linear combination of the various errors inherent in the variables for the calculation of the rate constant in Eq. (9), the error was found to be approximately ±5.0%. To qualitatively analyze the slag structure, as-quenched slag samples from 1873 K were analyzed by Fourier Transform-InfraRed (FT-IR) spectrometer (Spectra100; Perkin Elmer, Shelton, CT) in the range of 4000 to 400 cm−1. It should be noted that significant changes in the transmittance peaks were observed in the wavenumber range between 1200 cm−1 and 400 cm−1 and the FT-IR spectra are focused within this region of interest. To obtain a representative sample for FT-IR analysis, the power of the high frequency induction furnace is turned off and a stream of high purity Ar gas was passed through the sealed reaction chamber. Using a separate Ar gas gun, the sealed reaction chamber is rapidly taken out of the external quartz containment tube and cooled.
Fig. 3. Dependence of the rate constant of nitrogen dissolution with mass % SiO2 in the CaO―SiO2 binary system at 1873 K with the nitride capacity of Martinez et al. [4] at 1823 K The lines have been drawn as guides.
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Fig. 4. IR spectra of the CaO―SiO2 binary system as a functional of wavenumbers at different CaO contents after Park et al. [14].
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Fig. 6. Rate constant of nitrogen dissolution as a function of Al2O3 (SiO2 + Al2O3) ratio for the silica based CaO―SiO2―Al2O3 ternary system at 1873 K. The lines have been drawn as guides.
3. Results 3.1. Surface kinetics of nitrogen dissolution in the CaO―SiO2 binary system To simplify the kinetics of nitrogen dissolution at the surface of molten ternary slag systems, binary slags of CaO―SiO2 and CaO―Al2O3 were first studied similar to the approach taken by Ono et al. [7] and Iuchi et al. [8]. Fig. 3 shows the rate constant of nitrogen dissolution as a function of mass% SiO2 in the CaO―SiO2 binary system at 1873 K with the results of Ono et al. [7] and the nitride capacity of Martinez et al. [4] at 1823 K. The rate constant of nitrogen dissolution at the surface of the CaO―SiO2 binary system shows a parabolic shape with a maximum at approximately 58 mass% SiO2 and subsequently decreases, but the cause of this decrease was not clearly understood. The nitride capacity showed a similar behavior to the rate constant and Ono et al. [7] attempted to correlate a direct relationship between the rate constant and the nitride capacity. The results of the present
study coincided with work done by both Ono et al. [7] and Iuchi et al. [8], but the interpretation of the nitride capacity and its apparent relationship with the rate constant is significantly different. As previously noted, plotting the nitride capacity with the rate constant should not have direct physical correlations since the nitride capacity is a thermodynamic function, which is fixed when the slag composition and temperature is determined. Thus, the nitride capacity provides a thermodynamics driving force for nitrogen dissolution reaction to proceed, but the rate constant is not directly correlated with the driving force of the nitrogen dissolution reaction. Therefore, the apparent correlation found from the results between the rate constant of nitrogen dissolution at the slag surface and the nitride capacity is likely a common superficial effect of the slag structure with both the nitride capacity and the rate constant. According to the aforementioned reactions of (1) to (3), both the kinetic rate of nitrogen dissolution and the thermodynamic nitrogen solubility in molten slags can be affected by the oxygen ion species and subsequently the structure of the Si―O bond. Park et al. [13] suggested the ionic species of oxygen in CaO―SiO2 binary slags to be a function of CaO content. Below 40.3 mass%, the majority of oxygen ions exist as a non-bridging oxygen (O−) and above 40.3 mass% the free oxygen (O2−) begins to prevail. Below 40.3 mass% CaO, the abundance of O− will increase the nitrogen incorporation into the network of silica as a non-bridging incorporated nitride (N-). The incorporation of nitrogen within the slag structure is clearly evident using the FT-IR spectra of Fig. 4 reported by Park et al. [14] According to Park et al. [15], the amount of monomers [SiO4]4− and dimers [Si2O7]6− located within the [SiO4]-tetrahedral stretching bands, which provides much of the reaction sites for nitrogen, increases with CaO additions and remains relatively constant above 42.4 mass% CaO. The increase in the fraction of dimers and monomers is evident from the stretching Si―O band (1180 ~ 760cm-1) after CaO is increased from 33.7 to 46.3 mass%. Beyond 46.3 mass% to 56.6 mass%, the amount of monomer and dimer seem similar. 3.2. Surface kinetics of nitrogen dissolution in the CaO―Al2O3 binary system
Fig. 5. The rate constant of nitorgen dissolution in CaO―Al2O3 binary system as a function of mass % Al2O3 at 1873 K plotted with the nitride capacity after Schwerdtfeger et al. [16]. The lines have been drawn as guides.
The rate constant of nitrogen dissolution in the CaO―Al2O3 binary system as a function of mass% Al2O3 at 1873 K is shown in Fig. 5 with the results of Ono et al. [7] and the nitride capacity calculated by
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The rate constants of nitrogen dissolution in the CaO―SiO2―Al2O3 ternary system at 1873 K have been studied in three major regions as previously shown in Fig. 1. The first region is the silica based CaO―SiO2―Al2O3 system beginning from the CaO―SiO2 binary
system following the liquidus line and ending in the CaO―Al2O3 binary system. The second region is the alumina based CaO―SiO2―Al2O3 system beginning from the CaO―Al2O3 binary system following the liquidus line and ending in the Al2O3―SiO2 binary system. The final region is the CaO―SiO2(satd.)―Al2O3 system beginning from the Al2O3―SiO2 binary system following the liquidus line and ending in the CaO―SiO2 binary system. The rate constant of nitrogen dissolution in the CaO―SiO2―Al2O3 slags is 10 to 100 fold faster than in the binary CaO―SiO2 slag previously reviewed. This could be due to the smaller binding energy of the Al―O bonds compared to the Si―O bonds, where nitrogen adsorption takes place and will be discussed later in detail. Fig. 6 shows the rate constant of nitrogen dissolution in the first region of the silica based CaO―SiO2―Al2O3 ternary slag system as a function of (mass% Al2O3)/[(mass% SiO2)+(mass% Al2O3)] ratio (or A/(S+A)). From the CaO―SiO2 binary slag system along the liquidus line through the CaO saturated region, the rate constant significantly increases with higher Al2O3 up to the (mass% Al2O3)/[(mass% SiO2)+(mass% Al2O3)] ratio of 0.3. Above 0.3, the rate constant of nitrogen dissolution was relatively constant. The FT-IR analysis for the silica based CaO―SiO2―Al2O3 as quenched slag samples is shown in Fig. 7. As the Al2O3 content increases, the stretching vibrations of the [SiO4]-tetrahedra band observed at 1160 to 760 cm−1 overlapped with the [AlO4]-tetrahedra band at 960 to 600 cm−1, which convolutes the IR spectra. In addition, the relative intensity of the [AlO6]9−-octahedral peak increases with higher (mass% Al2O3) / [(mass% SiO2) + (mass% Al2O3)] ratio [17–22]. The rate constant of nitrogen dissolution in the second region of the aluminate based CaO―SiO2―Al2O3 ternary system was also measured and shown in Fig. 8. As the (mass% CaO) / [(mass% CaO) + (mass% SiO2)] ratio (or C / (C + S)) increases appreciable change in the rate constant is not observed until the C / (C + S) ratio reaches 0.5. Beyond 0.5, the rate constant begins to show significant increase. Fig. 9 shows the corresponding FT-IR spectra of the alumina based CaO―SiO2―Al2O3 ternary slag for various C / (C + S) ratio. Above 0.64, the [AlO4]5−-tetrahedra band in the spectra widens indicating an increase in the depolymerization of Al―O network structure for incorporation of nitrogen. The final region studied for the CaO―SiO2―Al2O3 ternary slag system is the SiO2 saturated region. Fig. 10 shows the rate constant of nitrogen dissolution in the SiO2 saturated CaO―SiO2―Al2O3 ternary
Fig. 8. Rate constant of nitrogen dissolution as a function of CaO / (CaO + SiO2) ratio in alumina based CaO―SiO2―Al2O3 ternary system at 1873 K. The lines have been drawn as guides.
Fig. 9. FT-IR spectra of alumina based CaO―SiO2―Al2O3 slag samples as quenched from 1873 K for different CaO / (CaO + SiO2) ratio.
Fig. 7. FT-IR spectra of silica based CaO―SiO2―Al2O3 system in the liquidus composition at 1873 K.
Schwerdtfeger et al. [16]. The rate constant is relatively unaffected with Al2O3 additions between the ranges of 45 and 60 mass% Al2O3. The resulting trend of the rate constant is comparable to Ono et al. [7], but the once similar behavior of the nitride capacity with the rate constant observed for the CaO―SiO2 binary slag system in Fig. 3 no longer seems to match the results obtained in the CaO―Al2O3 binary slag system. It should also be noted that the present experimental rate constant values in the CaO―Al2O3 binary slag system are about 10 fold higher than the results of Ono et al. [7] 3.3. Surface kinetics of nitrogen dissolution in the CaO―SiO2―Al2O3 ternary system
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Fig. 10. Rate constant of nitrogen dissolution as a function of CaO / (CaO―Al2O3) ratio in the SiO2 saturated CaO―SiO2―Al2O3 ternary system at 1873 K. The line has been drawn as a guide.
system as a function of (mass% CaO) / [(mass% CaO) + (mass% Al2O3)] ratio (or C / (C + A)). Results indicate that the rate constants was relatively independent of the (mass% CaO) / [(mass% CaO) + (mass% Al2O3)] ratio for this particular slag composition. This negligible change seems to correspond well with the negligible changes observed in the FT-IR analysis shown in Fig. 11. 4. Discussions According to the Langmuir adsorption model, vacant sites must be available for nitrogen adsorption to take place as described by reactions (4) to (8). If these reaction sites are occupied, the rate would be inhibited. The surface reaction sites in slag can be occupied by 2− 6− 6− 5− several different types of anions (SiO4− 4 , Si2O7 , Si3O9 , AlO4 , O etc.) and cations (Ca2+, Al3+, etc.), but an overall electronegative balance must be maintained near the surface. Silicate and aluminate slags can provide several different forms of anionic tetrahedra or larger species. However, these ionic structures are comparatively larger than the free oxygen ions (O2−), which can be easily supplied by the dissociation of CaO or basic oxides. As more of the free oxygen ions exist within the melt, the mobility, the size, and the shape of the O2− ions may allow the O2− to accumulate at the reaction sites inhibiting nitrogen dissolution. In the CaO―SiO2 binary slag observed in Fig. 3, the rate constant of nitrogen dissolution reaches a maximum and beyond a certain mass% CaO decreases. This may be related to the modification of the slag surface structure with CaO additions. With CaO additions in acidic slags, the O2− from CaO is consumed in order to break the bridged oxygen of the silicate network structures into non-bridged oxygen and increase the reaction sites for incorporated nitrides subsequently resulting in higher rate constants. At sufficiently high enough concentrations of CaO, excess O2− starts to accumulate at the slag surface and subsequently block the reaction sites of non-bridged oxygen for adsorption and incorporation. In the silica based CaO―SiO2―Al2O3 ternary slag system, the rate constant increased with higher A/(A + S) ratio up to approximately 0.5. Above 0.5, the rate constant was found to be constant. This can be qualitatively explained through the combination of the binding energy and the slag surface structure. In the aforementioned behavior of nitrogen dissolution at the surface of the slag, the binding energy of Si―O bonds (799.6 ± 13.4 kJ/mol) [23] are much higher than the
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Al―O bonds (511 ± 3 kJ/mol) [23] and thus nitrogen dissolution is significantly faster with non-bridged oxygen in the aluminate structure [8]. As Al2O3 is introduced into the CaO―SiO2 binary system and if O2− break some of the network bonds of alumina, O− sites are exposed and the nitrogen can be incorporated into the slag. With higher CaO, the free oxygen ions substitute the silicate and aluminatetetrahedrals that were once present for nitrogen to adsorb. In addition, the FT-IR spectra for the silica based CaO―SiO2―Al2O3 ternary slag showed the relative intensity of the [AlO6]9−-octahedral peak to increase with higher Al2O3 content, which suggest [AlO6]9− can also be one of the major incorporation sites for nitrogen adsorption. In the alumina based CaO―SiO2―Al2O3 system, the rate constant was unaffected as the C/(C + S) ratio increased. Not until C/(C + S) ratio reached 0.5 and above did the rate constant increase. The rate constant as a function of C/(C + S) ratio starting from the Al2O3―SiO2 binary system increases the relative number of O2−, but is not sufficient enough initially to make a significant impact on the depolymerization of aluminate or silicate structures. Beyond 0.5, the free oxygen ions start to significantly depolymerize the aluminate structure increasing the [AlO4]5− and [AlO6]9− incorporation sites. Above C/(C + S) of 0.5, the nitrogen dissolution would follow the free nitride (N3−) form as expressed by reaction (1). Thus, the amphoteric Al2O3 behaves as an acidic oxide and lowers the activity of the O2− resulting in decreased free nitrides. Furthermore, Al2O3 would provide AlO4-tetrahedrals to the surface, which may act as surface blocking sites for free nitride dissolution. Below C / (C + S) of 0.5, the overall slag composition would drive the amphoteric Al2O3 towards a basic behavior. Considering the incorporated nitride would be prevalent in an acidic slag system, the addition of a basic component to the slag would likely lower the dissolution of incorporated nitrides. In addition, if the Al2O3 behaves as a basic oxide in an acidic slag system, the incorporation sites for nitrogen dissolution may be eliminated by the supply of O2− to the slag system. This closely follows the slag structural information provided by the FT-IR analysis in Fig. 9. Above C / (C + S) ratio of 0.64 the [AlO4]5− tetrahedra band broadens indicating an increase in the depolymerization of Al―O network structure for incorporation of nitrogen. In this region, the amount of CaO additions is again not enough to provide significant adsorption sites for nitrogen and the rate constant is relatively constant as a function of C / (C + A) ratio.
Fig. 11. IR spectra of SiO2 saturated CaO―SiO2―Al2O3 as quenched slag samples from 1873 K with various CaO/(CaO―Al2O3) contents.
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Fig. 12. Schematic diagram of the adsorption site on the surface of molten slags for binary and ternary slag system.
When slags are usually depolymerized to polymeric ions such as rings, chains, dimers, and monomers, the oxygen ion is the outer most atom of these ionic structures and would likely be the atom exposed at the surface of the slag melts. Thus, non-bridged oxygen ions from the polymeric ion structures can act as the surface adsorption sites for nitrogen dissolution into molten slag. The depolymerized structures in the CaO―SiO2―Al2O3 exposing non-bridged oxygen of Si―O and Al―O are the adsorption sites for nitrogen dissolution and the smallest poly anions for example, [AlO4]5− and [SiO4]4− ions may compete as an adsorption site for nitrogen. The difference in the degree of nitrogen adsorption between Al―O and Si―O bond structures can be understood with the bond length and the bond enthalpy. The average bond length of the Al―O is longer than that of Si―O resulting in a larger Al―O structure than a Si―O structure. This suggests that the number of Al―O structures covering the surface is fewer than that of Si―O structures within a limited surface area of molten slags. However, the Al―O bond enthalpy is significantly smaller than the Si―O bond enthalpy indicating a lower activation energy required for nitrogen incorporation in the [AlO4]tetrahedra than the [SiO4]-tetrahedra. Thus, although the rate constant of nitrogen dissolution is easier for Al2O3 considering the activation energy for reaction, but due to the smaller volume of [SiO4]4− compared to [AlO4]5−. There are more numbers of Si―O adsorption sites than Al―O. As a result, the adsorption of nitrogen on the surface of molten slag which is occupied with a combination of both Al―O and Si―O bonds together is faster than just a single Al―O or Si―O bond
only due to the increase in the number of adsorption sites (or nonbridged oxygen) with SiO2 additions. Considering the above discussions, a schematic diagram of the adsorption sites on the surface of binary and ternary slag systems can be depicted as Fig. 12 where the CaO provides the necessary network breaking oxygen ions for incorporation of nitrogen to be possible, but beyond a certain threshold the amount of free oxygen ions may accumulate on the surface of the slag inhibiting the surface reaction of nitrogen adsorption and dissolution into molten slag. This effect of free oxygen ions on the slag surface is also supported by the interfacial tension data of Bretonnet et al. [24] in the CaO―Al2O3 binary slag system and Sun et al. [25] in the CaO―SiO2―Al2O3 ternary slag system shown in Figs. 13 and 14, respectively. The interfacial tension between the molten slag and pure Fe decreases with CaO additions and becomes relatively consistent beyond a certain mass% CaO. This limiting threshold is assumed to be the concentration where O2− significantly saturates the interface and impedes such reactions such as nitrogen dissolution at the slag surface. In the work of Tanaka et al. [26] and Nakamoto et al. [27], the surface tension of the molten silicate slags were greatly affected by the existence of anions (O2− 2+ and SiO4− , Na+) on the surface. Thus, the 4 ) and the cations (Ca surface tension is affected by the modification of the silicate network structure and also the existence of the ionic species causing surface relaxation, which lets the energetic state of the surface approach the bulk state.
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Fig. 13. Interfacial tension of CaO―Al2O3 binary slag on pure Fe as a function of mass% CaO at 1873 K after Bretonet et al. [24]. The lines have been drawn as guides.
5. Conclusion The kinetics of nitrogen dissolution on the surface of CaO―SiO2, CaO―Al2O3, and CaO―SiO2―Al2O3 slags has been investigated by using the isotope exchange technique at 1873 K. The kinetic data were correlated with the slag structure and the following results were obtained. ▪ The rate constant of nitrogen dissolution for the binary CaO―SiO2 showed a maximum with CaO additions and followed by a decrease in the rate due to excess O2− blocking the surface sites for nitrogen adsorption. CaO additions in acidic slag compositions provide the necessary free oxygen ions to break the silicate and aluminate structures, which increases the number of non-bridging oxygen for nitrogen incorporation and subsequent increase in the rate constant. Beyond a certain CaO content in the basic slag compositions, the rate constant for nitrogen dissolution at the
Fig. 14. Interfacial tension of CaO―SiO2―Al2O3 ternary slag on pure. Fe as a function of Al2O3 / (SiO2 + Al2O3) ratio at 1853 K after Sun et al. [25]. The lines have been drawn as guides.
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surface of molten slags decrease due to the surface blockage of excess O2− ions. ▪ The CaO―Al2O3 rate constant is higher than the CaO―SiO2 rate constant due to the lower binding energy of the aluminates compared to the silicates. The rate constant is also faster with both the silicates and aluminates present due to the balanced effects of the lower binding energy of the Al―O bonds and the higher number of incorporation sites available when Si―O bonds exist. ▪ The rate constant in the CaO―SiO2―Al2O3 ternary system was found to be significantly higher compared to the binary system due to the correlated effect of lower binding energies of the Al―O bonds and the increased number of reaction sites available when smaller Si―O tetrahedral were simultaneously present with Al―O bonds. ▪ In the CaO―SiO2―Al2O3 ternary slag system, the nitrogen dissolution followed the free nitride (N3−) form when the C/(C+S) ratio was higher than 0.5. At high C/(C+S), Al2O3 behaved as an acidic oxide and provide AlO4-tetrahedrals to the surface, which acted as surface blocking sites for free nitride dissolution. Below C/(C+S) of 0.5, the nitrogen dissolution followed an incorporated nitride form and Al2O3 behaved as a basic component to the slag, which decreased the incorporation sites for nitrogen dissolution by the supplying of O2− to the system. The results of the binary and ternary slag systems suggest that additions of CaO has a limited effect and once a certain threshold is reached excess O2− ions can possibly inhibit the surface reaction of nitrogen adsorption and dissolution in molten slag system. Acknowledgments The present study was supported by POSCO, the Brain Korea 21 (BK21) Project of the Division of Humantronics Information Materials, the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. Additional financial support was provided by the Korean National Science Foundation under project number 2010-8-0581 and the Ministry of Knowledge Economy project number 2010-8-0972. References [1] K. Ito, R.J. Fruehan, Metall. Trans. B 19B (1988) 419–425. [2] N. Sano, Advanced Physical Chemistry for Process Metallurgy, Academic Press, London, 1997, pp. 60–64. [3] D.J. Min, R.J. Fruehan, Metall. Trans. B 21B (1990) 1025–1032. [4] E.R. Martinez, N. Sano, Metall. Trans. B 21B (1990) 97–104. [5] H. Suito, R. Inoue, Tetsu Hagane 73 (1987) S246. [6] W. Shin, H. Lee, ISIJ Int. 41 (2001) 239–246. [7] H. Ono, K. Morita, N. Sano, Metall. Trans. B 28B (1997) 633–638. [8] K. Iuchi, K. Morita, N. Sano, Metall. Trans. B 29B (1998) 1235–1237. [9] H. Ono-Nakazato, S. Morisawa, T. Usui, Metall. Trans. B 33B (2002) 393–401. [10] VereinDeutscher Eisenhuttenlente(VDEh): Slag Atlas, 2nd edition, Verlag Stahleisen GmgH, D-Dusseldorf, 1995, p. 105. [11] A. Ozaki, Isotopic Studies of Heterogeneous Catalysis, Kodansha Ltd. and Academic Press, Tokyo, 1977. [12] S.M. Han, J.H. Park, S.M. Jung, D.J. Min, ISIJ Int. 49 (2009) 487–494. [13] J.-H. Park, P.C.-H. Rhee, J. NonCrytal. Sol. 282 (2001) 7–14. [14] J.H. Park, D.J. Min, H.S. Song, ISIJ Int. 42 (2002) 344–351. [15] J.H. Park, D.J. Min, H.S. Song, ISIJ Int. 42 (2002) 38–43. [16] K. Schwerdtfeger, Schubert Hans George, Metall. Trans. B 8B (1977) 535–540. [17] B.O. Mysen, D. Virgo, C.M. Scarfe, Am. Mineral. 65 (1980) 690–710. [18] T. Tsunawaki, N. Iwamoto, T. Hattori, A. Mitsuishi, J. NonCryst. Solids 44 (1981) 369–378. [19] P. McMillan, Am. Mineral. 69 (1984) 622–644. [20] P. McMillan, Am. Mineral. 69 (1984) 645–659. [21] R.W. Luth, Am. Mineral. 73 (1988) 297–305. [22] G. Leekes, N. Nowack, F. Schlegelmilch, Steel Res. 59 (1988) 406–416. [23] J.A. Kerr, in: D.R. Lide (Ed.), A Ready-Reference Book of Chemical and Physical Data, CRC Handbook of Chemistry and Physics 1999–2000, 81st ed, CRC Press, Boca Raton, Florida, USA, 2000. [24] J.L. Bretonnet, L.D. Lucas, M. Olette, Circ. Inf. Techn. Cent. Doc. Sider 33 (1976) 105. [25] H. Sun, K. Nakashima, K. Mori, ISIJ Int. 46 (2006) 407–412. [26] T. Tanaka, T. Kitamura, I.A. Back, ISIJ Int. 46 (2006) 400–406. [27] M. Nakamoto, T. Tanaka, L. Holappa, M. Hamalainen, ISIJ Int. 47 (2007).