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.     

Solubility of carbon in CaO-Al2O3 melts

Carbon distribution ratios between CaO-Al₂O₃ slags and Fe-0.0003 to 0.8 mass pct Al-0.2 to 5.6 mass pct C alloys were measured at 1873 K in an Al₂O₃, CaO, or graphite crucible. The carbon distribution ratios were dependent on the oxygen potential, determined by theAl/(Al₂O₃) equilibrium, not by theC/CO (P _co = 1 atm) equilibrium. The (mass pct C)/a _c ratios were proportional to the activity of Al in logarithmic form with a slope of 2/3, indicating that carbon in slag is dissolved as C^2? ion. Solubilities of carbon in CaO-Al₂O₃ slags were also measured at 1873 K under the CO-CO₂-Ar gas mixtures in an Al₂O₃ or graphite crucible. It was found that C^2? ion is present in the range of log $P_{O_2 } $ (atm) < ?15 and CO ₃ ^2? ion in the range of log $P_{O_2 } $ (atm) > ?7.     

Kinetics studies on the dissolution of nitrogen in the CaO-Al2O3-SiO2 and CaO-Al2O3-TiO x melts

The rate of nitrogen dissolution in CaO-Al₂O₃-SiO₂ and CaO-Al₂O₃-TiO _x melts was measured by ¹⁴N–¹⁵N isotope exchange reaction. The rate constant for the CaO-Al₂O₃-SiO₂ melts at the ratio of mass pct CaO/mass pct Al₂O₃ = 1 increases as SiO₂ content increases, whereas the rate constant for the same melts at the ratio of mass pct CaO/mass pct SiO₂ = 1 increases as Al₂O₃ content increases. The rate constant for the CaO-Al₂O₃-TiO _x melts at the ratio of mass pct CaO/mass pct Al₂O₃ = 1 decreases as the TiO _x content increases. The activation energies of nitrogen dissolution in CaO-Al₂O₃-SiO₂ melts are about 1.5 to 3 times larger than that of molten pure iron. Moreover, the rate constant of nitrogen dissolution is independent of the ratio of Ti³⁺/Ti⁴⁺.     

Rate of nitrogen desorption from CaO-Al2O3 melts to gas phase

There is a limit to lowering the nitrogen level in steel, because of pickup from the atmosphere during and after degassing. Therefore, the nitride capacity of various fluxes has been measured in order to examine the possibility of nitrogen removal by using slag. It is necessary to take into account the nitrogen reaction rate between the gas and slag phases for the efficient removal of nitrogen in practical steelmaking processes. In this study, the nitrogen desorption rate from CaO-Al₂O₃ melts to gas phase has been examined at 1873 K. The mass-transfer coefficient of nitrogen in the melts increases as the CaO content increases, and the dependence is related to the viscosity of the melts. The reaction-rate constant of nitrogen desorption is found to be proportional to the 3/2 power of the oxygen partial pressure and increases as the CaO content increases under constant oxygen partial pressure.     

Kinetic studies on the dissolution of nitrogen in CaO-Al2O3, CaO-SiO2, and CaO-CaF2 melts

The rate of nitrogen dissolution in CaO-Al₂O₃, CaO-SiO₂, and CaO-CaF₂ melts was measured by ¹⁴N-¹⁵N isotope exchange reaction. The rate constant of nitrogen dissolution in CaO-based oxide melts, which is defined as first order with respect to nitrogen partial pressure, was found to be much smaller than that in molten iron-based alloys investigated in our previous work. The activation energies for nitrogen dissolution in 40 mass pct CaO-60 Al₂O₃ and 50 CaO-50 SiO₂ melts are 224 and 581 kJ/mol, respectively. For CaO-Al₂O₃ and CaO-SiO₂, dependence of the rate constant on composition is very similar to that of nitride capacities. Moreover, it was confirmed that the rate constant was not affected by oxygen or nitrogen partial pressure.     

Thermodynamics of oxygen,nitrogen and sulfur in liquid iron equilibrated with CaO-TiOx and CaO-Al2O3-TiOx melts

Equilibria between CaO-TiO_x or CaO-Al₂O₃-TiO_x melts and liquid iron with respect to O, N and S were studied at 1873 K as a function of Ti (or Al) content in metal, using a CaO or an Al₂O₃ crucible. The Al-O and Ti-O relations in liquid iron were studied, and nitride and sulfide capacities defined by and , respectively, were obtained from nitrogen and sulfur distribution ratios, coupled with the analyzed oxygen content in liquid iron or the P_O2 determined by Ti/TiO₂ equilibrium. Based on these results, activities of Al₂O₃ and activity coefficients of TiO₂ and TiO_1.5 were evaluated.     

Solubility and thermodynamic properties of Y2O3 in LiF-YF3 melts

A systematic investigation was undertaken to measure the solubility of Y₂O₃ in several LiF-YF₂ melts, with the YF₃ composition ranging from 20 to 50 mole pct, in the temperature range of 998 to 1273 K. Experimental results showed that the solubility of Y₂O₄ in the melts increased with increase in temperature and also with increased YF₃ content. The activity of Y₂O₃ was calculated using the free energy of fusion of Y₂O₃, . The was deduced from the values of enthalpy, heat of fusion, and melting point of Y₂O₃. From the known values of activity, the activity coefficients of Y₂O₃ as a function of temperature and melt composition were calculated. Considering the ionic nature of the melt, activity coefficients were also calculated using Temkin’s ideal mixing and electrically equivalent fraction methods. The thermodynamic data, such as integral molar enthalpy, entropy, and free energy of formation, were calculated as a function of composition and temperature. The calculated thermodynamic data showed that the melt exhibited a negative deviation from ideal conditions.     

Investigations on the equilibria between Al-Ca-O containing iron melts and CaO-Al2O3-FeOn slags

The equilibria between liquid CaO-Al₂O₃-FeO_n slags and Al-Ca-O-containing iron melts in contact with pure lime (CaO) and pure calcium aluminate (CaO-2Al₂O₃) crucibles were investigated at 1600°C. The iron mass content of the metal phase was varied between 0 and almost 100 %. The results show a strong influence of the miscibility gap in the system Fe-Al-Ca arising from the Fe-Ca binary system on the phase equilibria. By these investigations the conditions of solid alumina reduction by dissolved calcium to liquid calcium aluminate were determined.     

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.     

Solubility and thermodynamic properties of Li2O in LiF-CaF2 melts

Solubility of Li₂O in LiF-CaF₂ melts was studied as a function of temperature, and it was found to increase from 10.6 wt pct at 1058 K to 14.8 wt pct at 1133 K. The liquidus temperature of 4LiF • CaF₂ + Li₂O (saturated) melt was determined to be 1004.5 ± 2.5 K. Activity of Li₂O in the melt was determined as a function of temperature using conventional and Temkin ap-proaches. The partial molar Gibbs energy, enthalpy, and entropy of formation were deduced at the given temperature and composition range studied. The results showed that the system ex-hibits a negative deviation from Temkin's ideality.     

Calcium deoxidation and nitrogen distribution in liquid nickel equilibrated with CaO-Al2O3 slags

The Wagner’s first-ordere _o ^Ca and second-order (r _o ^Ca ) interaction parameters between Ca and O in liquid nickel were determined as −1220 and 1.35 x 105, respectively, at 1873 K in the equilibrium experiments between liquid nickel and CaO-Al₂O₃ slags in an A1₂O₃ or CaO crucible. The values fore _Ca ^O , (−3060), r _Ca ^o (8.47 × 105), r _Ca ^O,Ca (6.76 x 10⁵), and r _Ca ^O,Ca (6.76 x 105) were also estimated. Nitride capacities, C(_N), defined by (mass pct N)·P ₂ ^3/4 PskN₂1/2 were obtained by using the present results for nitrogen distribution ratio, L_N =(mass pct N)/[mass pct N], and the reported values for the activity of Al₂O₃.     

Solubilities of carbon in CaO-Al2O3-SiO2 slags saturated with liquid iron

By equilibrating the CaO-Al₂O₃-SiO₂ slags with Fe - 0.0003～0.07% Al - 0.002～3.5% Si - 0.9～2.0% C alloys (mass contents in %) at 1873 K, the solubilities of carbon were measured as a function of Si contents in metal, using an alumina or lime crucible. The distribution ratios of carbon as a C^2- -ion were found to be linearly related to Si contents with a slope of 1/2 at a given slag composition. The carbide capacity, , defined by (% C) , increased with increasing SiO₂ contents at a given CaO/AlO_1.5 molar ratio in the CaO-AlO_1.5SiO₂ system.     

The effect of TiO2 additions and oxygen potential on liquidus temperatures of some CaO-Al2O3 melts

The effect of TiO₂ on the liquidus temperature of CaO-Al₂O₃ melts was determined in an argon atmosphere which had oxygen potentials of 0.21, 10⁻¹¹, 10⁻¹⁶, and 10⁻²¹ atm, using hot-wire microscopy. Compositions ranging from 44 to 52 wt pct A1₂O₃ and CaO and between zero and 10 wt pct TiO₂ were investigated. Experiments done in air show that TiO₂ lowers the liquidus temperature at low concentrations and raises the liquidus temperature at high concentrations. At low oxygen potentials, however, there is a continuous increase of the liquidus temperature with increasing TiO₂ concentrations. Liquidus temperature diagrams for the system at oxidizing (Po₂ = 0.21) and reducing (Po₂ = 10⁻¹⁶) conditions are presented. formerly a Research Assistant at the University of Kentucky     

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CaO-Al2O3, Al2O3-SiO2, and CaO-Al2O3-SiO2 systems

All available thermodynamic and phase diagram data have been critically assessed for all phases in the CaO-Al₂O₃, Al₂O₃-SiO₂, and CaO-Al₂O₃-SiO₂ systems at 1 bar pressure from 298 K to above the liquidus temperatures. All reliable data for the binary systems have been simultaneously optimized to obtain, for each system, one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasichemical model was used for the slag. With these binary parameters and those from the optimization of the CaO-SiO₂ system reported previously, the quasichemical model was used to predict the thermodynamic properties of the ternary slag. Two additional small ternary parameters were required to reproduce the ternary phase diagram and ternary activity data to within experimental error limits. The calculated optimized phase diagram and thermodynamic properties are self-consistent and are the most reliable currently available estimates of the true values.     

CaO-activities and structure of CaO-Al2O3-melts

By using structure chemical results taken from IR-spectroscopic analyses, the CaO-activity was calculated. In liquid state the aluminates dissociate into Ca²⁺ and into the complexes and . By application of the Temkin-theory the CaO-acitivity can be calculated and compared with measured values.     

Iron redox equilibria in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags

Measurements have been made of the ratio of ferric to ferrous iron in CaO-Al₂O₃-SiO₂ and MgO-CaO-Al₂O₃-SiO₂ slags at oxygen activities ranging from equilibrium with pCO₂/pCO≈0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given by $\begin{gathered} \log {\text{ }}\left( {\frac{{Fe^{3 + } }}{{Fe^{2 + } }}} \right) = 0.3( \pm {\text{ }}0.02){\text{ }}Y + {\text{ }}0.45( \pm {\text{ }}0.01){\text{ }}\log \hfill \\ \left( {\frac{{pCO_2 }}{{pCO}}} \right) - 1.24( \pm {\text{ }}0.01) \hfill \\ \end{gathered} $ where Y=(CaO+MgO)/SiO₂, for melts with the molar ratio of CaO/SiO₂=0.45 to 1.52, 10 to 15 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and with 3 to 10 wt pct total Fe. Available evidence suggests that, to a good approximation, these redox equilibria are independent of temperature when expressed with respect to pCO₂/pCO, probably from about 1573 to 1873 K. Limited studies have also been carried out on melts containing about 40 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with $\frac{{Fe^{3 + } }}{{Fe^{2 + } }} \propto \left( {\frac{{pCO_2 }}{{pCO}}} \right)^{0.37} $ The nonideal behavior and the effects of basicity and Al₂O₃ concentration on the redox equilibria are discussed in terms of the charge balance model of alumino-silicates and the published structural information from Mössbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.     

Phase diagram for the system CaO-Al2O3-ZrO2

Phase equilibria in the system CaO-Al₂O₃-ZrO₂ were studied to optimize nozzle composition for tundishes that are used in continuous casters of steel, by identifying the composition that minimizes clogging. Mixtures of CaO, Al₂O₃, and ZrO₂ with various compositions were equilibrated at 1873 and 1773 K and then quenched into water. X-ray diffraction analysis was used for phase identification, and electron probe microanalysis was employed to measure chemical compositions. On the basis of these and previous results, the eutectic and peritectic lines have been estimated. Based on the phase diagram, the optimal nozzle composition is CaO-saturated CaZrO₃.     

Role of B2O3 on the Viscosity and Structure in the CaO-Al2O3-Na2O-Based System

The effect of B₂O₃ on the viscosity and structure in the calcium-aluminate melt flux system containing Na₂O was studied. An increase in the B₂O₃ content at fixed CaO/Al₂O₃ ratio lowered the viscosity. Higher CaO/Al₂O₃ ratio at fixed B₂O₃ content also decreased the viscosity. The alumino-borate structures were confirmed through Fourier transformed infrared (FTIR) and Raman spectroscopy and consisted of [AlO₄]-tetrahedral structural units, [BO₃]-triangular structural units, and [BO₄]-tetrahedral structural units, which could be correlated to the viscosity. At fixed CaO/Al₂O₃ ratio, B₂O₃ additions decreased the [AlO₄]-tetrahedral structural units and transformed the 3-D network structures such as pentaborate and tetraborate into 2-D network structures of boroxol and boroxyl rings by breaking the bridged oxygen atoms (O⁰) to produce non-bridged oxygen atoms (O^?) leading to a decrease in the molten flux viscosity. At fixed B₂O₃ contents and higher CaO/Al₂O₃ ratio, 3-D complex network structures become 3-D simple and 2-D isolated network structures, resulting in lower viscosities. The apparent activation energy for viscous flow varied from 132 to 249 kJ/mol according to the composition of B₂O₃ and CaO/Al₂O₃ ratio.     

Dissolution of carbon dioxide in CaO-CaF2-Al2O3 based melts containing Na2O or K2O oxide

The influence of Na₂O or K₂O on the solubility of CO₂ in CaO-CaF₂-Al₂O₃ based melts is measured by a thermogravimetric technique over a temperature range of 1250–1350°C. The values of the carbonate capacities (C'_c = wt. % ) are calculated by data on the dissolution and partial pressure of carbon dioxide. The replacement of CaO by Na₂O leads to an increase of the carbonate capacity to a maximum at 3–3.5 wt. % Na₂O and then to a decrease. The addition of Na₂O or K₂O up to 2 wt. %, to CaO-CaF₂-Al₂O₃ increases the carbonate capacity at 1300°C by 30%.