Thermodynamics of nitrogen in CaO-SiO2-AI2O3 slags and its reaction with Fe-Csat. melts

The solubility of nitrogen as the nitride ion in CaO-SiO₂-Al₂O₃ slags in equilibrium with N₂-CO gas mixtures and carbon was measured at 1823 K. The nitride capacity (C _N3-) was calculated to compare the nitrogen contents measured under different nitrogen and oxygen potentials.C _N3- decreased with increasing basicity and by replacing SiO₂ with A1₂O₃. The nitrogen partition ratio between carbon saturated iron and the slag was measured in CO gas at one atmosphere at 1823 K. By comparing the partition ratios with the corresponding nitride capacities measured by the gas-slag experiments, it was concluded that the oxygen partial pressure at the slag-metal interface was controlled by the Fe-FeO reaction. A new definition of nitride capacity was proposed based on the reaction between nitrogen and the network former,i.e., SiO₂ or A1₂O₃. This capacity could consistently explain the experimental results. Empirical equations were derived to estimate the activity coefficients of silicon and aluminum nitrides in the slags. On leave of absence from the Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai, Japan.     

Solubility of nitrogen and carbon in CaO-Al2O3 melts in the presence of graphite

CaO-Al₂O₃ slags were melted in graphite crucibles under N₂-CO-Ar gas mixtures at 1600°C. The contents of total nitrogen, cyanide and total carbon of the slags were determined by chemical analyses of quenched samples taken by suction from the melt. The nitrogen is present in the melt as nitride N^3- ion and cyanide CN^-1 ion, and carbon as cyanide and carbide C_2- ion. The equilibrium constants for the respective reactions were evaluated. It is found that the nitride capacity of the melt decreases whereas the cyanide and carbide capacities increase with increasing CaO/Al₂O₃ ratio.     

Activities in CaO-SiO2-Al2O3 slags and deoxidation equilibria of Si and Al

By using the data in previous and present slag-metal equilibrium experiments, the activities of SiO₂ along the liquidus lines in CaO-SiO₂-Al₂O₃ slags were determined at 1823 and 1873 K from the reaction Si+2O=SiO₂ (s), in which the oxygen activities were estimated from the measured oxygen contents or from the combination of nitrogen distribution ratios (L _N) and nitride capacities (C _N). The activities of Al₂O₃ were also determined from the reaction 2Al+3O=Al₂O₃ (s), in which the oxygen activities were estimated from the values for L _N and C _N, or from the reaction 3SiO₂ (s)+4Al=2Al₂O₃ (s)+3Si, in which the activities of SiO₂ and the contents of Al and Si along with the respective interaction coefficients were used. The activities of Al₂O₃ and CaO in the entire liquid region were estimated from the Rein and Chipman’s activities of SiO₂ by using the method of Schuhmann. On the basis of these activities, the deoxidation equilibria of Si and Al in steels were discussed.     

Sulfide capacity of CaO-CaF2-SiO2 slags

The sulfide capacityC _S ^2- = (pct S^2-) · (P _{O 2}/P _{S 2})^1/2) of CaO-CaF₂-SiO₂ slags saturated with CaO, 3CaO · SiO₂ or 2CaOSiO₂ was determined at 1200 °C, 1250 °C, 1300 °C, and 1350 °C by equilibrating molten slag, molten silver, and CO-CO₂ gas mixtures. Higher sulfide capacities were obtained for CaO-saturated slags. A drastic decrease was observed in those values when the ratio pct CaO/pct SiO₂ is less than 2. The sulfur partition between carbon-saturated iron melts and presently investigated slags was calculated by using the sulfide capacities obtained and the activity coefficient of sulfur in carbon-saturated iron, which was also experimentally determined. For slags saturated with CaO, partitions of sulfur as high as 10,000 were obtained at 1300 °C and 1350 °C. Correlations between the sulfide capacity and other basicity indexes such as carbonate capacity and theoretical optical basicity were also discussed. Formerly with the Department of Metallurgy, The University of Tokyo.     

Solubility of carbon in CaO-B2O3 and BaO-B2O3 slags

The solubility of carbon in molten CaO-B₂O₃ and BaO-B₂O₃ slags at high temperatures was measured to understand the dissolution mechanism of carbon into the slags. The B₂O₃-bearing slags, which have a wide range of liquids, at the temperature of interest have been applied to investigate the effect of basicity on the solubility of carbon from the saturation of acidic or basic components. The solubility of carbon, as a function of the composition of slags, shows a minimum value, and it is suggested that carbon dissolves by different mechanisms in the acidic and basic slags, respectively. From the infrared spectra measurements, the wave number indicating the B-C bond was found to be about 1150 cm⁻¹ in the acidic region of slags; hence, the incorporation of carbon into the borate network was confirmed qualitatively. The carbide capacity was compared to the nitride capacity, showing that the dissolutions of carbon and nitrogen into the slags are similar.     

Thermodynamics of oxygen and nitrogen in liquid iron equilibrated with CaO- SiO2-Al2O3 slags

Nitrogen distribution ratios, L_N ( = (mass pct N)/[mass pct N]), between CaO-SiO₂-Al₂O₃ slags and liquid iron were measured at 1823 and 1873 K as a function of Si (or Al) content in metal, using lime and alumina crucibles. Based on these results and the reported values for activities of SiO₂ (or Al₂O₃), nitride capacities, C_(N), defined by (mass pct N) · PO₂/3/4/PN₂/1/2 were evaluated. Activities of SiO₂ (or Al₂O₃) obtained by using the values for L_N and C_(N) obtained in present and previous gas-slag experiments were compared with previous results. FUMIHIKO TAMURA formerly Graduate Student with the Department of Metallurgical Engineering, Tohoku University.     

The nitrogen reaction between carbon saturated Iron and Na2O-SiO2 Slag: Part 1. Thermodynamics

The nitrogen partition ratio between Na₂O-SiO₂ slags and carbon saturated iron was measured for slags containing from 0.4 to 0.55 mole fraction of Na₂O and the temperature range 1200° to 1350 °C. The nitrogen is dissolved in the slag as the cyanide ion (CN⁻) and the partition ratio is proportional to the oxygen pressure to the −1/4 power as predicted for CN⁻ dissolution. The oxygen pressure for carbon saturated iron silicon alloys is controlled by the Si (metal)-SiO₂ (slag) equilibrium. The nitrogen partition ratio and the cyanide capacity increase with Na₂O content and temperature. Calculations indicate that Na₂O-SiO₂ slags will absorb three times more nitrogen than CaO-Al₂O₃ slags at the same basicity and temperature. Based on thermodynamic calculations it is estimated that for a typical Na₂CO₃ hot metal treatment, half of the nitrogen in the metal could possibly be removed. F.Tsukihashi is on leave of absense from Department of Metallurgical Engineering and Materials Science, Faculty of Engineering, The University of Tokyo, Bunkyo, Tokyo 113, Japan     

氮在Ca-SiO_2-Al_2O_3渣中的热力学研究

本文借助于光学碱度概念,对CaO-SiO_2-Al_2O_3熔渣中氮的稳定性进行了研究。结果表明,气相中氮不是与渣中自由氧离子作用,而是与网络结构作用而进入熔渣。根据氮与网络结构SiO_2或Al_2O_3之间的反应。提出了熔渣氮容量的新定义。根据渣中氮的行为,结合一系列的试验结果,得到了1823 K时CaO-SiO_2-Al_2O_3渣的氮容量计算公式IgC_N=-11.721gA-14.26和三元渣系的等氮容量曲线。     

Nitrogen solution in BaO-B2O3 and CaO-B2O3 slags

The nitrogen solubility and nitride capacity of the BaO-B₂O₃ and CaO-B₂O₃ systems were measured at 1698 and 1773 K, respectively, using a gas-liquid equilibration technique. These systems were chosen because they have wide liquidus regions at the temperature of interest and were expected to show two types of nitride solution behavior. The nitride capacities of both systems showed minimum values with changing composition. This was explained by considering that nitrogen can dissolve into slag as “free nitride” at high basicities and as “nitride incorporated” with the network for acidic slags. The change of the nitride capacity with optical basicity and the activity of the basic component followed the predicted behavior for the proposed solution mechanism. The nitride capacity of the BaO-B₂O₃ system is considerably greater than for the CaO-B₂O₃ system because BaO is a more basic oxide than CaO. The activity of BaO in the BaO-B₂O₃ system was also measured by equilibration with liquid silver, at known oxygen potentials, and the activities show large negative deviations from ideal behavior.     

Desulphurisation ability of blast furnace slag containing high Al2O3 and 5 mass% TiO2 at 1773 K

The sulphide capacities (C_S) of CaO–SiO₂–Al₂O₃–MgO–TiO₂ blast furnace slags were experimentally measured at 1773?K, and the percentages of free oxygen ions (O^2?) and bridge oxygen (O⁰) of molten slag were calculated using molecular dynamics at 1773?K. The measured and calculated results were closely correlated with the change in CaO/SiO₂, MgO, and Al₂O₃ contents in the slag. The results show that C_S increases with increasing CaO/SiO₂ and MgO contents in the slag, but decreases with increasing Al₂O₃ content. The effects of O^2? and O⁰ on C_S were analysed using multiple linear regression, and results show that sulphide increases with the increase in the mass percentage of free oxygen and decreases with the increase in the mass percentage of bridge oxygen.     

Thermodynamic Behaviour of Chlorine in CaO‐SiO2‐MgO‐Al2O3(‐CaF2) Slags

The solubility of chlorine in CaO‐SiO₂‐Al₂O₃‐MgO(‐CaF₂) slag was measured at 1673 ‐1823 K. By estimating the chloride capacity of slags, thermodynamic behaviour of chlorine in the molten slag was investigated. Chloride capacity increased with increasing CaO / SiO₂ ratio (C/S). An increase in MgO content decreased chloride capacity at C/S≥1.0 because it lowered the activity of Ca²⁺ which seemed to have strong affinity with Cl^? in molten slag. Also, the chloride capacity decreased with increasing Al₂O₃ content. The affinity between the Ca²⁺ and Cl^? ions was confirmed by measuring the infrared spectra of slags. The dissolution reaction of chlorine into slag was exothermic and its molar enthalpy was evaluated from the experimental results at 1673 ‐ 1823 K. Based on the result obtained in the present study, the quantitative prediction of chlorine distribution during the blast furnace process was performed. It was estimated that almost all chlorine in the blast furnace would be absorbed into molten slag even if the PCI ratio was increased or low quality coal with chlorine content less than 1.0 mass% was injected.     

Dephosphorization equilibria between pure molten iron and CaO-saturated FeOn–CaO–SiO2 and FeOn–CaO–Al2O3 slags

Metal-slag refining reactions have been investigated to determine dephosphorization equilibria in steelmaking using CaO-saturated slags, low in P₂O₅–content, based on the systems FeO_n–CaO–SiO₂ and FeO_n–CaO–Al₂O₃. Slag compositions have been optimized with respect to basicity and oxygen potential to achieve maximum partition ratios wt.%(P₂O₅)/wt.%[P] and minimum phosphorus contents in pure molten iron at 1550, 1600 and 1700°C. Both slag systems prove to be effective dephosphorizers. Optimal slag compositions are around 10 wt.% SiO₂ near the CaO–3CaO · SiO₂ double saturation in the case of FeO_n–CaO–SiO₂ slags and at Al₂O₃ contents tending to zero in the case of FeO_n–CaO–Al₂O₃ slags. Attempts were also made to present phosphate capacities C_PO4^3?, fractions of free oxygen ions x_O^2? and theoretical optical basicities Λ as a function of the FeO_n content of slags.     

The sulfur partition ratio with Fe-CSAT melts and the sulfide capacity of CaO-SiO2-Na2O- (Ai2O3) slags

The sulfur partition ratio between slag and carbon saturated iron and the sulfide capacity of CaO-Na₂O-SiO₂ slags and a 48 pet CaO-45 pet Al₂O₃-7 pet SiO₂-(Na₂O) slag have been mea-sured at 1400 °C. The addition of Na₂O to a CaO-SiO₂ slag increases the sulfur partition ratio and the sulfide capacity; however, Na₂O at low concentrations has no measurable effect on the sulfide capacity of a CaO-Al₂O₃-SiO₂ slag. To convert the sulfur partition ratio to the sulfide capacity, the oxygen potential was calculated assuming equilibrium between iron in the alloy and FeO in the slag with the activity of FeO calculated via a regular solution model. The optical basicity may be used to correlate the data, but at high Na₂O contents the data do not adhere to the correlation previously developed for CaO-based slags. Formerly Graduate Student at Carnegie Mellon University     

Removal of nitrogen from steel using novel fluxes

The solubility of nitrogen and the nitride capacity of CaO-Al₂O₃-TiO₂ and CaO-BaO-Al₂O₃-TiO₂ slags were measured at 1873 K using a gas-slag-metal equilibration technique with carbon-saturated iron and gas mixtures of CO and N₂. The nitride capacity increased with increasing the TiO₂ and BaO content and is significantly higher than the nitride capacity for normal ladle slags. The activity coefficient of TiO₂ in CaO-Al₂O₃-TiO₂ and CaO-BaO-Al₂O₂-TiO₂ systems were measured. This is necessary to know in order to estimate the possible pickup of titanium in the metal when an aluminum-killed steel is treated with these slags. Also, the activity coefficient of Ti in carbon-saturated iron was measured. The kinetics of the nitrogen reaction between slag and metal is influenced by the oxygen potential in the metal and is primarily controlled by liquid-phase mass transfer of nitrogen in the metal. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

A study of nitrogen solubility in Na2O‐B2O3 slag

The nitrogen solubility and nitride capacity of a Na₂O‐B₂O₃ slag system were investigated in a concentration range of Na₂O mass contents up to 40 % at 1373K. The solubility of nitrogen in Na₂O‐B₂O₃ slag was found to be proportional to the oxygen partial pressure to the power of ?3/4, and it was also found to increase with increasing reaction temperature. The nitride capacity of the slag decreased with increasing Na₂O content. From the experimental results it was concluded that nitrogen dissolution into Na₂O‐B₂O₃ slags (Na₂O mass contents < 40 %) occurs by its reaction with both free oxygen ions and with oxygen ions incorporated with the network.     

thermodynamics of nitrogen in BaO-TiOx melts

Using a gas-slag-metal equilibration technique, nitrogen contents in BaO-TiO^ slags and nitrogen and titanium contents in liquid Cu were measured at 1823, 1873, and 1923 K under controlled partial pressures of oxygen(@#@ P_{O 2} = 10^-11.5 ≈ 10^-13.7 atm) and nitrogen(@#@ P_{N 2} = 0.9 atm). The nitride capacity, C_(N) [=(mass pct N) · PO₂/3/4, (mass pct Ti³⁺)/(mass pct Ti⁴⁺) ratio, and solubility of TiN in BaO-TiO₂-TiO_1.5 slags were obtained as a function of slag com-position(@#@ X_BaO = 0.20 = 0.43) and temperature. Activity coefficients of TiN were estimated, using the values for activity coefficients of Ti in liquid Cu which were calculated from the results of a TiN saturation experiment. Free energy of dissolution of nitrogen into liquid Cu was derived as °GG_N ^o = 32,400 + 46.17 ± 1400 (J/g · atom).     

Solubility of nitrogen and carbon in CaO-Al2O3 melts in the presence of graphite

CaO-Al₂O₃ slags were melted in graphite crucibles under N₂–CO–Ar gas mixtures at 1600°C. The contents of total nitrogen, cyanide and total carbon of the slags were determined by chemical analyses of quenched samples taken by suction from the melt. The nitrogen is present in the melt as nitride N⁻³ ion and cyanide CN⁻¹ ion, and carbon as cyanide and carbide C ₂ ²⁻ ion. The equilibrium constants for the respective reactions were evaluated. It is found that the nitride capacity of the melt decreases whereas the cyanide and carbide capacities increase with increasing CaO/Al₂O₃ ratio.     

Viscosities of the Quaternary Al2O3‐CaO‐MgO‐SiO2 Slags

Viscosities of some quaternary slags in the Al₂O₃‐CaO‐MgO‐SiO₂ system were measured using the rotating cylinder method. Eight different slag compositions were selected. These slag compositions ranging in the high basicity region were directly related to the secondary steel making operations. The measurements were carried out in the temperature range of 1720 to 1910 K. Viscosities in this system and its sub‐systems were expressed as a function of temperature and composition based on the viscosity model developed earlier at KTH. The iso‐viscosity contours in the Al₂O₃‐CaO‐MgO‐SiO₂ system relevant to ladle slags were calculated at 1823 K and 1873 K for 5 mass% MgO and 10 mass% MgO sections. The predicted results showed good agreement with experimental values and the literature data.     

Thermodynamic study on the effect of CaF2 on the nitrogen and carbon solubility in the CaO-Al2O3-CaF2 slag system

The nitrogen and carbon solubilities in the CaO-Al₂O₃-CaF₂ slag system at 1723K were measured to understand the effect of CaF₂ on the nitrogen and carbon dissolution behaviour using thermochemical equilibration technique. The nitride capacity in the CaO_satd.-Al₂O₃-CaF₂ system was constant until X_Al2O₃ / X_CaF2 = 0.23, followed by decrease with an increase in X_Al2O₃ / X_CaF2 ratio, and then showed a constant value. The carbide capacity in the CaO_satd.-Al₂O₃-CaF₂ system also showed the similar behaviour to the nitride capacity, however, the effect of CaF₂ on the activity coefficient of nitride and carbide ions showed some differences in case of X_CaF2 = 0.2. The effect of CaF₂ on the activity coefficient of carbide and nitride ions would not be significant in case of CaO-rich region of X_CaO / X_CaF2 = 0.31. However, CaF₂ could be expected to affect the activity coefficient of each ion in case of the Al₂O₃-rich region, causing compensating effect of the incorporated nitride (carbide) mechanism. The relationship between nitride and carbide capacity was also discussed.     

The solubility of water vapour in CaO‐SiO2‐Al2O3‐MgO slag system

The solubility of water vapour in the CaO‐SiO₂‐Al₂O₃‐MgO quaternary slag system was measured using an inert gas fusion technique with thermal conductivity detection. The slags were equilibrated with argon‐water vapour mixture corresponding to 0.157 bar of water vapour pressure at 1873 K. The slag solubility of water vapour is proportional to the square root of vapour pressure. Since the hydroxyl capacity of slag, C_OH shows an independence on the relative amount of CaO or MgO in slag, the contributions of CaO and MgO on the hydroxyl capacity are equivalent on a molar basis. Whereas, Al₂O₃ shows a better effect on the hydroxyl capacity than SiO₂. A linear relationship between hydroxyl capacity and slag basicity in logarithmic scale was obtained with the slope of 1/2, confirming the water vapour dissolution reaction into a basic slag as (O^2‐) + H₂O(g) = 2(OH^?). The correlation between hydroxyl capacity and slag components was derived in terms of their contributing weight factors. The measured values of C^'_OH agree well with the calculated ones using the interaction energies of α_H‐Al = ?38300 and α_H‐Mg = ?22700 J determined with the aid of the regular solution model. In addition, the correlation between hydroxyl capacity and sulphide capacity was empirically derived as a formula of logC_OH = 1/2logC_S + (4.38 ± 0.25) through the thermodynamic expression of both capacities by virtue of the common oxygen ion activity.