Thermodynamics of oxygen and nitrogen in liquid nickel equilibrated with CaO-TiO x and CaO-TiO x -Al2O3 melts

Equilibrium between CaO-TiO _x or CaO-TiO _x -Al₂O₃ slags and liquid nickel with respect to oxygen and nitrogen has been studied at 1873 K as a function of Ti (or Al) content in metal, using lime and alumina crucibles. The Al-O, Ti-O, and Ti-Al relations were obtained for liquid nickel equilibrated with these slags. Nitride capacity,_{C (N)}, defined by (mass pct N) · , was evaluated from nitrogen distribution ratios,L _N {= (mass pct N)/[mass pct N], coupled with oxygen content in liquid nickel. Activities of Al₂O₃ and activity coefficients of TiO₂ and TiO_1.5 were estimated either from the content of Al, Ti, and O, using the compiled data for the free energy of formation of oxide and the respective interaction parameters or from the values forL _N andC _(N).     

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.     

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.     

Activities of SiO2 and Al2O3 and activity coefficients of FetO and MnO in CaO-SiO2-Al2O3-MgO slags

The activities of SiO₂ and Al₂O₃ in CaO-SiO₂-Al₂O₃-MgO slags were determined at 1873 K along the liquidus lines saturated with 2CaO · SiO₂, 2(Mg,Ca)O · SiO₂, MgO, and MgO · Al₂O₃ phases using a slag-metal equilibration technique. Based on these and previous results obtained in ternary and quaternary slags, the isoactivity lines of SiO₂ and Al₂O₃ over the liquid region on the 0, 10, 20, 30, and 40 mass pct Al₂O₃ planes and those on the 10 and 20 mass pct MgO planes were determined. The activity coefficients of Fe _t O and MnO, the phase boundary, and the solubility of MgO were also determined.     

Activities of MnO in CaO-SiO2-Al2O3-MnO (<10 Pct)-FetO(<3 pct) slags saturated with liquid iron

Activity coefficients of MnO and Fe,0 in CaO-SiO₂-Al₂O₃-MnO(<10 mass pct)-Fe,O(<3 mass pct) slags were determined at 1873 K in an Al₂O₃ or CaO crucible by using the reported values for the activities of Al₂O₃ and SiO₂ or the analyzed contents of oxygen. The activity coefficients of MnO and Fe_tO were found to be constant in the studied concentration range of MnO and Fe_tO. The former increased with an increase in the CaO content, while the latter increased with an increase in the SiO₂ content.     

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⁺.     

Activities of FetO in CaO-Al2O3-SiO2-FetO (<5 pct) slags saturated with liquid iron

The activity coefficients of Fe_tO in CaO-Al₂O₃ and CaO-Al₂O₃-SiO₂ slags with 0.01 to 5 mass pct Fe_tO were determined at 1873 K from the data obtained in the present and previous slag-metal experiments, using an alumina or lime crucible. It was found that the activity coefficients of Fe_tO obeyed a dilute solution law and increased with increasing the content of SiO₂. Based on the findings pertaining to the activity coefficient, the values for the activities of SiO₂ and Al₂O₃ in CaO-Al₂O₃-SiO₂ slags were assessed.     

Thermal Conductivity Measurements of Some Synthetic Al2O3-CaO-SiO2 Slags by Means of a Front-Heating and Front-Detection Laser-Flash Method

The thermal conductivities of some synthetic slags containing Al₂O₃, CaO, and SiO₂ have been determined in the temperature range between 1623?K (1350?°C) and 1823?K (1550?°C) by applying a front-heating front-detection laser-flash method. In this method, the temperature response curve is measured in the short initial time period immediately after irradiating a laser pulse. The resultant values obtained by this method are unaffected by the radiative heat transfer. The temperature dependence of the thermal conductivity values were found not to be significant for all slag samples currently investigated. The thermal conductivity (??) of samples is represented with standard deviation less than 2?pct of its value as a function of compositions as follows: $$ \lambda = - 0.48\left[ {{\text{Al}}_{ 2} {\text{O}}_{ 3} } \right] - 0.57\left[ {\text{CaO}} \right] - 0.55\left[ {{\text{SiO}}_{2} } \right] + 57.1{\text{ W m}}^{ - 1} {\text{ K}}^{ - 1} $$ where [Al₂O₃], [CaO], and [SiO₂] are molar percent of Al₂O₃, CaO and SiO₂, respectively. The equation is suggested to cover the region of the following compositions: 8.0?mol pct < Al₂O₃ <21.0?mol pct, 31.5?mol pct < CaO <41.5?mol pct and 43.0?mol pct < SiO₂ <58.1?mol pct. The addition of Al₂O₃ to slags with a constant CaO/SiO₂ molar ratio resulted in an increase in thermal conductivity. In contrast, the effects of addition of SiO₂ and CaO are found to be insignificant.     

Effect of Al2O3 and CaO/SiO2 on the Viscosity of Calcium-Silicate–Based Slags Containing 10 Mass Pct MgO

The effect of Al₂O₃ and CaO/SiO₂ on the viscosity of the CaO-SiO₂-10 mass pct MgO-Al₂O₃ slags was studied at fully liquid temperatures of 1773 K (1500 °C) and below. At fixed CaO/SiO₂ between 0.8 and 1.3, higher Al₂O₃ content increased the slag viscosity due to the polymerization of the aluminate structures. At fixed Al₂O₃ of 15 and 20 mass pct, increasing the CaO/SiO₂ from 0.8 to 1.3 resulted in lower viscosity due to the depolymerization of the aluminate structure.     

A Reaction Between High Mn-High Al Steel and CaO-SiO2-Type Molten Mold Flux: Part I. Composition Evolution in Molten Mold Flux

In order to elucidate the reaction mechanism between high Mn-high Al steel such as twin-induced plasticity steel and molten mold flux composed mainly of CaO-SiO₂ during continuous casting process, a series of laboratory-scale experiments were carried out in the present study. Molten steel and molten flux were brought to react in a refractory crucible in a temperature range between 1713 K to 1823 K (1440 °C to 1550 °C) and composition evolution in the steel and the flux was analyzed using inductively coupled plasma atomic emission spectroscopy, X-ray fluorescence, and electron probe microanalysis. The amount of SiO₂ in the flux was significantly reduced by Al in the steel; thus, Al₂O₃ was accumulated in the flux as a result of a chemical reaction, 4[Al] + 3(SiO₂) = 3[Si] + 2(Al₂O₃). In order to find a major factor which governs the reaction, a number of factors ((pct CaO/pct SiO₂), (pct Al₂O₃), [pct Al], [pct Si], and temperature) were varied in the experiments. It was found that the above chemical reaction was mostly governed by [pct Al] in the molten steel. Temperature had a mild effect on the reaction. On the other hand, (pct CaO/pct SiO₂), (pct Al₂O₃), and [pct Si] did not show any noticeable effect on the reaction. Apart from the above reaction, the following reactions are also thought to happen simultaneously: 2[Mn] + (SiO₂) = [Si] + 2(MnO) and 2[Fe] + (SiO₂) = [Si] + 2(FeO). These oxide components were subsequently reduced by Al in the molten steel. Na₂O in the molten flux was gradually decreased and the decrease was accelerated by increasing [pct Al] and temperature. Possible reactions affecting the Al₂O₃ accumulation are summarized.     

Amphoteric behavior of alumina in viscous flow and structure of CaO-SiO<Subscript>2</Subscript> (-MgO)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> slags

The viscosity of CaO-SiO₂ (-MgO)-Al₂O₃ slags was measured to clarify the effects of Al₂O₃ and MgO on the structure and viscous flow of molten slags at high temperatures. Furthermore, the infrared spectra of the quenched slags were analyzed to understand the structural role of Al₂O₃ in the polymerization or depolymerization of silicate network. The Al₂O₃ behaves as an amphoteric oxide with the composition of slags; that is, the alumina behaves as a network former up to about 10 mass pct Al₂O₃, while it acts as a network modifier, in parts, in the composition greater than 10 mass pct Al₂O₃. This amphoteric role of Al₂O₃ in the viscous flow of molten slags at the Newtonian flow region was diminished by the coexistence of MgO. The effect of Al₂O₃ on the viscosity increase can be understood based on an increase in the degree of polymerization (DOP) by the incorporation of the [AlO₄]-tetrahedra into the [SiO₄]-tetrahedral units, and this was confirmed by the infrared (IR) spectra of the quenched slags. The influence of alumina on the viscosity decrease can be explained on the basis of a decrease in the DOP by the increase in the relative fraction of the [AlO₆]-octahedral units. The relative intensity of the IR bands for the [SiO₄]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO₄]-tetrahedra with high NBO/Si increased with increasing Al₂O₃ content greater than the critical point, i.e., about 10 mass pct in the present systems. The variations of the activity coefficient of slag components with composition indirectly supported those of viscosity and structure of the aluminosilicate melts.     

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.     

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.     

Silicon-Oxygen equilibrium and nitrogen distribution between CaO-SiO2 slags and liquid iron

Silicon-oxygen equilibria in an Fe-0.003 ~ 27 mass pct Si alloy in equilibrium with the CaO-SiO₂ slags were studied in the temperature range of 1823 to 1923 K using a lime crucible. At the same time, nitrogen distribution ratios, L_N, between slag and metal were measured, and from these results and the reported values for activities of SiO₂, nitride capacities, , defined by (mass pct N). were evaluated. It was found that the values for L_N decreased, whereas those for increased with an increase in temperature. Activities of SiO₂ were determined using the values for L_N and obtained in previous gas-slag experiments. These values were compared with the previous results.     

The effect of Na2O on dephosphorization by CaO-based steelmaking slags

The effect of Na₂O on the equilibrium phosphorous distribution ratio between slag and iron or iron alloys, L_P, has been measured for CaO-SiO₂, CaO-FeOr-SiO₂ (CaO or 2CaO·SiO₂ saturated), and CaO-Al₂-SiO₂ slags. The addition of Na₂O to CaO-SiO₂ slags significantly increases L_P and the phosphate capacity. A 25 pct CaO-25 pct Na₂O-SiO₂ slag has a distribution ratio nearly two orders of magnitude greater than a comparable binary 50 pct CaO-SiO₂ slag. For the CaO-saturated slags containing 40 wt pct FeO_T, the addition of 6 wt pct Na₂O increases L_P by a factor of 5. For the 2CaO·SiO₂-saturated CaO-FeO_T-SiO₂ slag, there is an optimum FeOr content (20 wt pct) for dephosphorization, and 10 wt pct Na₂O increases L_P by a factor of 2. For reducing slags typically used in ladle metallurgy for Al-killed steels (50 pct CaO-40 pct Al₂O₃-10 pct SiO₂), as little as 3 wt pct Na₂O increases L_P by a factor of 100. The present results indicate that small additions of Na₂O to conventional steelmaking slags can greatly improve dephosphorization. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Viscosity of blast furnace type slags

The effect of MgO, TiO₂, or Fe₂O₃ on the viscosity of 40CaO-40SiO₂-20Al₂O₃ (mass pct) slags has been measured by the rotating crucible viscometer. Viscosity of these quaternary slags decreased with an increase in the content of additive oxide. At the same content of additive oxide, the viscosity decreases from MgO, TiO₂ to Fe₂O₃. In addition, the effect of SiO₂ or Al₂O₃ on the viscosity of 26.1CaO-73.9Fe₂O₃ (mass pct) (CF) and 14.9CaO-85.1Fe₂O₃ (mass pct) (CF₂) slags has been measured. Viscosity of calcium ferrite slags increased with increasing SiO₂ or Al₂O₃ content. Al₂O₃ was found to be more effective for increasing the viscosity at the same content of the additive oxide. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

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.     

Experimental and Theoretical Studies on the Viscosity–Structure Correlation for High Alumina-Silicate Melts

Blast furnaces are encountering high Alumina (Al₂O₃ > 25 pct) in the final slag due to the charging of low-grade ores. To study the viscosity behavior of such high alumina slags, synthetic slags are prepared in the laboratory scale by maintaining a chemical composition of Al₂O₃ (25 to 30 wt pct) CaO/SiO₂ ratio (0.8 to 1.6) and MgO (8 to 16 wt pct). A chemical thermodynamic software FactSage 7.0 is used to predict liquidus temperature and viscosity of the above slags. Experimental viscosity measurements are performed above the liquidus temperature in the range of 1748 K to 1848 K (1475 °C to 1575 °C). The viscosity values obtained from FactSage closely fit with the experimental values. The viscosity and the slag structure properties are intent by Fourier Transform Infrared (FTIR) and Raman spectroscopy. It is observed that increase in CaO/SiO₂ ratio and MgO content in the slag depolymerizes the silicate structure. This leads to decrease in viscosity and activation energy (167 to 149 kJ/mol) of the slag. Also, an addition of Al₂O₃ content increases the viscosity of slag by polymerization of alumino-silicate structure and activation energy from 154 to 161 kJ/mol. It is witnessed that the activation energy values obtained from experiment closely fit with the Shankar model based on Arrhenius equation.     

Crystallization Behavior of the CaO-Al2O3-MgO System Studied with a Confocal Laser Scanning Microscope

The crystallization behavior of a calcium-aluminate system with various MgO content from 2.5 to 7.5?wt pct and CaO/Al₂O₃ ratios between 0.8 and 1.2 has been examined using a confocal laser scanning microscope (CLSM). CCT (continuous cooling transformation) and time temperature transformation (TTT) diagrams were constructed to identify the primary crystal phase of slag at different compositions and at cooling rates between 25 and 800?K/minutes. In the slag at a CaO/Al₂O₃ ratio of 1.0, crystallization temperature increased during isothermal and continuous cooling with higher MgO content, and the shortest incubation time was observed at 5?wt pct MgO. When MgO content was fixed to be 5?wt pct, crystallization temperature increased with lower CaO/Al₂O₃ ratio. According to the slag composition, cooling rates and temperature, the primary phase could be CA, or C₅A₃, or C₃A, or C₃MA₂, or MgO, and the crystal morphology changes from dendrites to faceted crystals to columnar crystals in this composition range.     

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.