Thermodynamics of lead and bismuth in CaO–CaF2 melts

In order to understand a new refining process for removing tramp elements from alloy steels under strongly reducing conditions, the authors previously reported the thermodynamic behaviour of P, Sn, Sb, As and Cu in CaO–CaF₂ melts. In this investigation, the thermodynamic behaviour of Pb and Bi in CaO–CaF₂ melts under oxygen partial pressures from 10^?16 to 10^?20 atm was studied at temperatures ranging from 1360 to 1550°C, using a similar technique. It was shown that Pb and Bi exist in slag mainly as Ca₂Pb and Ca_1.5Bi, respectively, under very low oxygen partial pressure although a part of Pb is soluble in slags as metal. The obtained results were extrapolated to lower oxygen partial pressures to investigate the possibility of removing these tramp elements from molten iron, causing Bi to be removed from molten iron below an oxygen partial pressure of 10^?23 atm, whereas Pb is removed under higher oxygen partial pressures.     

Thermodynamics of antimony and arsenic in BaO–BaF2 melts

Since the interest in removing tramp elements from steels is increasing, one of the authors has reported the thermodynamic behaviour of P, Sn, As, Sb, Bi, Pb and Cu in CaO–CaF₂ melts under strongly reducing conditions. In this investigation, the BaO–BaF₂ system, which is more basic than the CaO–CaF₂ system, was chosen as a flux and the thermodynamic behaviour of As and Sb in BaO–BaF₂ melts was studied at 1300°C under reducing conditions. The partition ratios of arsenic between Cu–As and BaO–BaF₂ melts and of antimony between Ag–Sb and BaO–BaF₂ melts were measured as functions of the flux composition and the partial pressure of oxygen. Both partition ratios increase with an increase in the BaO content of the flux and with a decrease in the partial pressure of oxygen. The reactions for the removal of arsenic and antimony have been identified. The partition ratios of arsenic and antimony between carbon saturated iron and BaO–BaF₂ melts are estimated and the possibility of the removal of tramp elements from molten iron by using a BaO–BaF₂ flux is discussed.     

Nitrogen solubility in metallurgical slags equilibrated with Fe–Al or Fe–Ca melts

Considerations are directed to the denitrogenation potential of metallurgical slags with respect to steel melts under reducing conditions. Experiments were made to determine partition ratios of nitrogen between molten slag and iron. The investigated systems were aluminate-based slags, containing CaO, MgO, SrO, BaO, CaF₂ or ZrO₂, in equilibrium with Fe–AI melts and Ca–CaO–CaF₂ slags equilibrated with Fe–Ca melts. Denitrogenation efficiency of aluminate-based slags is comparatively low and essentially determined by oxygen potential and basicity of the slag. Denitrogenation efficiency of Ca–CaO–CaF₂ slags is much higher and is dependent on calcium activity.     

Nitrogen solubilities in CaO—CaF2 melts

Nitrogen solubilities in the CaO—CaF₂ system were determined under controlled oxygen partial pressures as well as at carbon saturation. Nitrogen contents (as N^3?) were shown to strongly depend on oxygen partial pressure from 10^?11 to 10^?13 atm. At carbon saturation both nitride and cyanide contents are also dependent on oxygen partial pressure, i.e. CO partial pressure. The dependence of nitride and cyanide contents on nitrogen partial pressure was found to be compatible with the theory and is described by the equations proposed in this paper. The results are also discussed in terms of nitride and cyanide capacities for a better understanding of the dissolution of nitrogen in slags of different composition. The temperature dependence of nitride content showed an increase in nitride content with increasing temperature.     

Dissolution rate of Al2O3 into molten CaO–Al2O3–CaF2 flux

Abstract

Dissolution of Al₂O₃ into molten CaO–Al₂O₃–CaF₂, a base system of mould flux for continuous casting of high Al steel, has been investigated by employing a rotating cylinder method. The dissolution rate of an alumina rod into molten CaO–Al₂O₃–CaF₂ flux increased with increase in rotating speed and temperature. The apparent activation energy for mass transport of flux C was calculated to be 255·6 kJ mol^?1. The rate controlling step during the dissolution process of the alumina rod into molten CaO–Al₂O₃–CaF₂ flux was found to be the diffusion of the solute in the flux boundary layer. The dissolution rate of alumina into molten CaO–Al₂O₃–CaF₂ flux increased with increasing CaO/Al₂O₃, and it may be caused by the increase in thermodynamic driving force and the decrease in the viscosity of the flux. When the Al₂O₃ rod was immersed into molten flux, an intermediate compound of CaO.2Al₂O₃ formed firstly and then dissolved into molten flux.     

Reoxidation on the Surface of Molten Low‐Carbon Aluminum‐Killed Steel

The unwanted formation of oxide on the surface of molten steel is a process that plagues virtually all grades of steel and often results in unacceptable defect distributions in the finished product. The kinetic rates of the reaction remain largely unknown due to the difficulties in experimental measurements at high temperature involving reactive molten melts. This paper presents recent experimental work and analysis of oxide evolution on the surface of Al‐killed steel melts at oxygen partial pressures of Po₂= 1~5×10^?5atm, by using a Confocal Scanning Laser Microscope (CSLM) equipped with a gold image furnace. The effects of gas flow rate (170~300 cm³/min) and temperature (1580~1630°C) were investigated. The oxide phase formed on the melt surface was in all cases Al₂O₃ but not the thermodynamically stable FeAl₂O₄. It was found, under the range of experimental conditions in this study, that the rate controlling mechanism for oxide nucleation and growth was gas phase mass transfer of oxygen to the melt surface. The morphology of the oxide changed gradually from distinctly dendritic at low gas flow rates to aggregates as the flow rate was increased.     

Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

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.     

Molten salt synthesis of neodynium oxyfluoride in various fluoride media with different fluoride ion activities

Neodymium oxyfluoride has received much attention in the fields of anionic solid electrolytes, luminescent, catalytic and magnetic materials because of its structure combined advantages of rare-earth cations with F^– and O^2– anions. In this work, neodynium oxyfluoride was synthesized by the reaction between neodymium oxide and four fluoride media with different fluoride ion activities. The synthesis processes in molten LiF–CaF₂–NdF₃, LiF–NdF₃, NaF–CaF₂-NdF₃ and NaF–KF–NdF₃ are observed in situ by a confocal scanning laser microscope. The expansion of neodymium oxide particle is observed in the LiF–CaF₂–NdF₃, LiF–NdF₃, and NaF–KF–NdF₃ melts, and the growth of needle crystals on neodymium oxide particle is clearly observed in molten NaF–CaF₂–NdF₃. Based on scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analyses of products, neodynium oxyfluoride was successfully synthesized in the four fluoride media. The neodynium oxyfluoride generated in the LiF–CaF₂–NdF₃, LiF–NdF₃, and NaF–KF–NdF₃ melts is a tetragonal structure. However, in molten NaF–CaF₂–NdF₃, neodynium oxyfluoride with a rhombohedral structure is formed. It is suggested that the substitution of Na(I) and Ca(II) for Nd(III) can transform NdOF from tetragonal structure to rhombohedral structure. The growth rate of needle crystals generated in molten NaF–CaF₂–NdF₃ was calculated based on the result of a confocal scanning laser microscope, and it is found that the reaction kinetics of crystal formation is zero-order reaction. The effect of fluoride media on the structure and morphology of formed NdOF were evaluated by XRD, X-ray photoelectron spectroscopy (XPS) and SEM. The neodymium oxyfluoride prepared in the fluoride media with high fluoride ion activity has low binding energy of F 1s. The ratio of adsorbed oxygen to lattice oxygen for neodymium oxyfluoride prepared in molten LiF-NdF₃ is larger than those in the other three fluoride media, so it can have better catalytic performance.     

The solubility of copper in lime-saturated and calcium ferrite-saturated liquid iron oxide

The solubility of copper in lime-saturated and calcium ferrite-saturated liquid iron oxide has been measured at 1300 °C by equilibrating copper-gold alloys with the melts in CO-CO₂ atmospheres of oxygen potentials 10^?7, 10^?8, 10^?9, and 10^?10 atm. It was found that copper exhibits Henrian behavior in the oxide melts and that the solubility is given by $$wt pct Cu = 17.6a_{CuO_{0.5} } $$ The copper capacity of the melts, 17.6, is approximately one-half of that of silica-saturated iron silicate melts at the same temperature. The results are compared with those of previous studies, and the difference between the solubilities of copper in silica-saturated and lime-saturated melts are discussed in terms of ionic interactions in the melts.     

Viscosity of niobium-containing oxide-fluoride melts

Influence of Nb₂O₅ on viscosity of the CaF₂-based and CaF₂-30% Al₂O₃-based melts was investigated using a vibration viscosimetry method in the range 1523–1873 K. It is found that viscosity of the melts rise and the crystallization range shifts to lower temperatures as the Nb₂O₅ concentration increases from 0 to 25 wt %. The activation energy of viscous flow increases from 34 to 110 kJ mol^?1 and from 56 to 148 kJ mol^?1 for the CaF₂-based and CaF₂-30% Al₂O₃-based melts, respectively. The obtained data indicate the complexing behavior of niobium in the oxide-fluoride melts.     

Removal of Boron from Silicon-Tin Solvent by Slag Treatment

To eliminate B effectively from Si for its use in a solar cell, a novel process involving the slag refining of molten Si with Sn addition was investigated. The partition ratio of B between CaO-SiO₂-24 mol pct CaF₂ slag and Si-Sn alloy at 1673 K (1400 °C) was determined by the chemical equilibrium technique. It was found that the partition ratio of B was remarkably increased with the increase in Sn content of alloy, which attributes to the increase in activity coefficient of B as well as the oxygen partial pressure. The partition function was accounted as much as 200 when the alloy composition was Si-82.4 mol pct Sn, which was much higher than the reported values in the range of 1 to 3. The required amounts of slag used for B removal from Si-30, 50, and 70 mol pct Sn melts were only 15.6 pct, 6.5 pct, and 1.2 pct of that used for the removal of B directly from MG-Si without Sn addition in a single slag treatment.     

Computer study of structure, thermodynamic, and electrical transport properties of Na3AlF6-Al2O3 and CaF2-Al2O3 melts

Structure, thermodynamic, and electrical transport properties of Na₃AlF₆-Al₂O₃ and CaF₂-Al₂O₃ melts were examined by molecular dynamics. Ionic models were constructed for Na₃AlF₆-Al₂O₃ and CaF₂-Al₂O₃ melts at 1283 and 2000 K, respectively. It was found that in the Na₃AlF₆-Al₂O₃ melts, stable aluminum-fluorine-oxygen groups are formed. Although bonds between F⁻ and Al³⁺ ions in the first coordination shell are weaker than between O²⁻ and Al³⁺ ions, very stable negatively charged AlF ₆ ³⁻ groups are formed at low oxygen concentrations in the Na₃AlF₆-Al₂O₃. This results in migration of aluminum to the anode in an external electric field. In the CaF₂-Al₂O₃ melts, positively charged aluminum-oxygen groups dominate. This results in migration of aluminum to the cathode at almost all Al₂O₃ concentrations. Therefore, in Na₃AlF₆-Al₂O₃ melts, the Al³⁺ ion as a component of the complex anion has a negative partial conductivity and the O²⁻ ion has positive partial conductivity; in CaF₂-Al₂O₃ melts, Al³⁺ has a positive transport number while O²⁻ has a negative transport number.     

Nitrogen solubility in CaO-CaF2-SiO2 melts

The preceding paper^[5] demonstrated that nitrogen dissolves in silicate melts either as a free ion or complex anion, being incorporated into silicate networks. In the present study, the influence of CaF₂ addition to CaO-SiO₂ melts on the nitrogen solubility was investigated along the liquidus at 1573 K and within the liquidus at 1723 K at constant CaF₂ levels. In the latter case, as the SiO₂ content increases from CaO saturation, the total nitrogen content decreases to reach the minimum and then starts to increase up to the SiO₂ saturation. This is in accord with the abovementioned mechanism of nitrogen dissolution, which is supported by the changing behavior of free and incorporated nitrogen contents with the slag composition. The role of CaF₂ is complicated through the formation of fluorosilicates. The CaF₂ seems not to function simply as a diluent but to enhance the dissolution of nitrogen by releasing oxygen from silicate networks, promoting the formation of free nitride ions. Formerly Graduate Student, Department of Metallurgy, The University of Tokyo.     

The Thermodynamics of Some Transition Metals in Molten Slags

In the present paper, the thermodynamic behaviour of transition metals, such as Cr, Ti, Nb and V in molten slags is systematically analysed based on a literature survey. These metals exist in molten slags with multi valences. Oxygen partial pressure, slag basicity, total content of each transition metal, content of each component in slag, and temperature are the influential factors on their thermodynamic properties in molten slags. Higher basicity and strong oxidative atmosphere are generally favourable for the stable existence of transition‐metal ions with higher oxidation states. Temperature is a factor that is less influential than the above‐mentioned ones. For a transition metal in molten slag, the concentration ratio of ions of different valences depends on the activity coefficient ratio of their oxides. The present paper summarizes the activity studies regarding the transition metal oxides in various molten slags. For chromium and titanium oxides, information on CaO? SiO₂ based systems is involved. For titanium oxides, its thermodynamic behaviour in MnO? SiO₂ based slags is introduced. For niobium and vanadium, the information in Na₂O? SiO₂, CaO? CaF₂? SiO₂ systems is provided. Thermodynamic studies are described for Nb₂O₅? MnO? SiO₂ molten slag equilibrated with liquid iron at 1828 K.     

Interfacial Tension between CaO‐Al2O3 Slag and Iron Containing Nonmetal or Tramp Elements

The interfacial tension between CaO‐Al₂O₃ slag and iron was studied for the iron containing nonmetal or tramp elements. The sessile drop technique was used to measure interfacial tensions under argon atmosphere at 1853K. Oxygen in liquid iron markedly reduced the interfacial tension. This effect was taken into consideration when analysing the interfacial tension between the slag and Fe‐S, Fe‐C, Fe‐Sn, Fe‐Sb and Fe‐Cu alloys. When adding S, Sn, Sb, or Cu to the iron, the interfacial tension was observed to decrease while carbon had no significant effect.     

Evaluation of B2O3 as replacement for CaF2 in CaO–Al2O3 based mould flux

In order to develop low fluoride or fluoride free CaO–Al₂O₃ based mould flux for casting high aluminium steel, an investigation was carried out to study the effect of substituting CaF₂ with B₂O₃ on heat transfer and crystallisation behaviour of CaO–Al₂O₃ based mould flux by employing a heat transfer simulator of mould flux and a single hot thermocouple technique. The results showed that addition of CaF₂ promoted heat transfer of CaO–Al₂O₃ based mould flux, which was opposite to the effect of CaF₂ on heat transfer in conventional CaO–SiO₂ based mould flux. Addition of CaF₂ inhibited crystallisation of CaO–Al₂O₃ based mould flux by lowering the start crystallisation temperature and prolonging the incubation time of crystallisation. B₂O₃ showed similar effects to CaF₂ on heat transfer and crystallisation of CaO–Al₂O₃ based mould flux, but its ability to promote heat transfer and suppress crystallisation was stronger than CaF₂. Ca₃B₂O₆ (melting temperature 1480°C) was found as the primary crystalline phase in fluoride free CaO–Al₂O₃ based mould flux compared with the primary crystalline phase Ca₂Al₃O₆F (melting temperature 1507°C) in fluoride bearing (20% CaF₂) CaO–Al₂O₃ based mould flux.     

Thermodynamics of Fluoride Vaporisation from Slags containing CaF2 at 1773 K

The changes in the composition of the CaO‐SiO₂‐CaF₂ and CaO‐Al₂O₃‐CaF₂ slags were measured after an equilibration at 1773 K to deduce the vaporisation loss of fluoride species. Furthermore, the volatilisation phenomena were discussed based on the thermodynamic fundamentals. In the CaO‐SiO₂‐CaF₂ system, the SiO₂ and CaF₂ contents simultaneously decreased at basicity B(■(%CaO)/(%SiO₂)) ≤ 1.0, while the content of CaF₂ only decreased at basicity of the slags greater than about unity. On the other hand, the content of CaF₂ only decreased in the entire composition range in the CaO‐Al₂O₃‐CaF₂ system. The volatilisation behaviour of fluoride species from the slags could be explained from the thermodynamic assessment by taking the activity and the activity coefficient of CaF₂ into account. From the measured results and the thermodynamic calculations, it could be concluded that the major volatilisation species in the relatively acidic (B ≤ 0.8~1.0) silicate melts would be SiF₄ (g), while the CaF₂ (g) would be a dominant vaporisation species in the relatively basic (B ≥ 1.0) silicate and in the aluminate melts.     

The equilibrium partitioning of titanium between Ti3+ and Ti4+ valency states in CaO-SiO2-TiO x slags

The redox behavior of titanium in CaO-SiO₂-TiO _x melts was investigated using a slag-gas equilibrium technique. Titanium partitioning between Ti³⁺ and Ti⁴⁺ valency states and the ratio of activity coefficients of TiO_1.5 and TiO₂ were determined as functions of oxygen partial pressure, temperature, and slag composition. The equilibrium experiments were carried out at temperatures between 1783 and 1903 K under CO-CO₂-Ar gas atmosphere with oxygen partial pressure ranging from 10⁻¹² to 10⁻⁷ atm (1.01×10⁻¹⁰ kPa to 1.01×10⁻⁵ kPa). The slags had CaO/SiO₂ ratios between 0.55 and 1.35 and total titanium oxide concentrations from 7 to 50 mass pct. Experimental results showed that the Ti³⁺/Ti⁴⁺ ratio in CaO-SiO₂-TiO _x slags, containing up to 50 mass pct TiO _x , increased with decreasing oxygen partial pressure and decreased with increasing CaO/SiO₂ ratio and decreasing temperature. Measured variation of the redox ratio Ti³⁺/Ti⁴⁺ with oxygen partial pressure closely followed the ideal behavior. Increasing the CaO/SiO₂ ratio increased the ratio of activity coefficients of TiO_1.5 and TiO₂. The effect of total titania content on this ratio was more complex and in accord with Raman spectroscopy data.     

The dependence of the oxidation state of vanadium on the oxygen pressure in melts of VOx,Na2O-VOx,and CaO-SiO2-VOx

The oxidation state of vanadium has been determined as a function of oxygen pressure in pure VO_x melts at 808 °C to 1000 °C, in Na₂O-VO_x melts with the initial molar ratios Na₂O/V₂O₅ = 0.2, 0.5, and 1.0 at 1000 °C, and in CaO-SiO₂-VO_x melts with the molar ratios CaO/SiO₂ = 0.71, 1.00, and 1.36 at 1600 °C. In the VO_x melts,x is close to 2.5 in the range of oxygen pressure fromP _{O 2} to 0.94 atm. The deviation, δ, from stoichiometric V₂O₅ (δ = 2.5-x) varies approximately proportionally toP _{O 2} ^-1/4, indicating an equilibrium between V⁴⁺ and V⁵⁺ ions. In the Na₂O-VO_x melts, and in the CaO-SiO₂-VO_x melts,x varies with logP _{O 2} according to an S-shaped function, withx approaching 1.5 at low and 2.5 at high oxygen pressures. At given oxygen pressures,x increases with Na₂O or CaO content, respectively. Hence, these oxides stabilize the higher valent vanadium ions. For the CaO-SiO₂-VO_x system, the determinedx-P _{O 2} dependence can be interpreted quantitatively in terms of V⁴⁺/V⁵⁺ and V³⁺/V⁴⁺ equilibria.