Density Measurements of Low Silica CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Density measurements of a low-silica CaO-SiO₂-Al₂O₃ system were carried out using the Archimedes principle. A Pt 30 pct Rh bob and wire arrangement was used for this purpose. The results obtained were in good agreement with those obtained from the model developed in the current group as well as with other results reported earlier. The density for the CaO-SiO₂ and the CaO-Al₂O₃ binary slag systems also was estimated from the ternary values. The extrapolation of density values for high-silica systems also showed good agreement with previous works. An estimation for the density value of CaO was made from the current experimental data. The density decrease at high temperatures was interpreted based on the silicate structure. As the mole percent of SiO₂ was below the 33 pct required for the orthosilicate composition, discrete \textSiO₄^{4 -} {\text{SiO}}_{4}^{4 - } tetrahedral units in the silicate melt would exist along with O^2– ions. The change in melt expansivity may be attributed to the ionic expansions in the order of

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Concentration on Density and Structure of (CaO-SiO<Subscript>2</Subscript>)-xAl<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The effect of Al₂O₃ concentration on the density and structure of CaO-SiO₂-Al₂O₃ slag was investigated at multiple Al₂O₃ mole percentages and at a fixed CaO/SiO₂ ratio of 1. The experiments were conducted in the temperature range of 2154 K to 2423 K (1881 °C to 2150 °C) using the aerodynamic levitation technique. In order to understand the relationship between density and structure, structural analysis of the silicate melts was carried out using Raman spectroscopy. The density of each slag sample investigated in this study decreased linearly with increasing temperature. When the Al₂O₃ content was less than 15 mole pct, density decreased with increasing Al₂O₃ content due to the coupling of Si (Al), whereas above 20 mole pct density of the slag increased due to the role of Al³⁺ ion as a network modifier.     

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.     

Solubilities of Chlorine in CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO Slags: Correlation Between Sulfide and Chloride Capacities

To derive a correlation between sulfide and chloride capacities through our own systematic experimental studies by using a gas equilibrium technique involving Ar-H₂-H₂O-HCl gas mixtures, the solubilities of chlorine were determined for CaO-SiO₂-MgO-Al₂O₃ slags at temperatures between 1673 K and 1823 K (1400 °C and 1550 °C). As a formula to correlate sulfide and chloride capacities, the following equation that is the function of temperature only was obtainable;

Electrical conductivity of NaF-AlF<Subscript>3</Subscript>-CaF<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> melts

The electrical conductivity of NaF-AlF₃-Al₂O₃ melts with a CaF₂ concentration of 5 wt % is measured at a continuously varying cell constant when the molar cryolitic ratio CR = [NaF]/[AlF₃] changes from 1.2 to 2.0 [1, 2]. The experimental data are used to obtain a regression equation to describe the dependence of the electrical conductivity of the melts under study on CR, the alumina content, and temperature {χ] = f(CR, [Al₂O₃], T)}.     

The Thermodynamic Behavior of Copper in the CaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> and BaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag System

The Cu solubility was measured in the CaO-B₂O₃ and BaO-B₂O₃ slag systems to understand the dissolution mechanism of Cu in the slags. The Cu solubility had a linear relationship with oxygen partial pressure in the CaO-B₂O₃ slag system, which corresponds with previous studies. Also, the Cu solubilities in slag decreased with increasing the slag basicity, which value of slope was close to –0.5 in logarithmic form. From the results of experiment, the Cu dissolution mechanism established as follows:

Understanding the Relationship Between Structure and Thermophysical Properties of CaO-SiO<Subscript>2</Subscript>-MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Molten Slags

In the present work, the relationship between the microscopic structure and macroscopic thermophysical properties in a basic CaO-SiO₂-MgO-Al₂O₃ quaternary system was identified using Fourier transformation infrared, Raman and ²⁷Al magic angular spinning nuclear magnetic resonance (MAS-NMR) techniques. The Raman spectra quantitatively proved that with increasing Al₂O₃ content, the concentrations of the symmetric units of Q⁰(Si) and Q²(Si) decreased, while those of the asymmetric units of Q¹(Si) and Q³(Si) increased; consequently, the degree of polymerization of the networks increased, which resulted in an increase in slag viscosity. The ²⁷Al MAS-NMR spectra demonstrated that three structural units of Al atoms, namely, AlO₄, AlO₅, and AlO₆, mainly existed in the networks. With increasing Al₂O₃ content, the concentration of AlO₄ slightly decreased, while those of AlO₅ and AlO₆ increased; overall, Al₂O₃ acted as a network former in the present system. The increasing Al₂O₃ content led to additional AlO₆ and Si-NBO-Ca-NBO-Al frameworks, which replaced Si-NBO-Ca-NBO-Si in the networks (NBO: non-bridging oxygen) and induced a change in the primarily precipitated crystalline phase from Ca₂MgSi₂O₇ and Ca₂Al₂SiO₇ to MgAlO₄.     

Thermodynamics of MnO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling

The activities of MnO and MnS in a MnO-SiO₂-Al₂O₃(or AlO_1.5)-MnS liquid oxysulfide solution were investigated by employing the gas/liquid/Pt-Mn alloy chemical equilibration technique under a controlled atmosphere at 1773 K (1500 °C). Also, the sulfide capacity, defined as C _S = (wt pct S)(pO₂/pS₂)^1/2, in MnO-SiO₂-Al₂O₃ slag with a dilute MnS concentration was obtained from the measured experimental data. As X _SiO2/(X _MnO + X _SiO2) in liquid oxysulfide increases, the activity coefficient of MnO decreases, while that of MnS first increases and then decreases. As X(AlO_1.5) in liquid oxysulfide increases, the activity coefficient of MnS increases, while no remarkable change is observed for the activity coefficient of MnO. The behavior of the activity coefficient of MnS was qualitatively analyzed by considering MnO + A _x S _y (SiS₂ or Al₂S₃) = MnS + A _x O _y (SiO₂ or Al₂O₃) reciprocal exchange reactions in the oxysulfide solution. The behavior was shown to be consistent with phase diagram data, namely, the MnS saturation boundary. Quantitative analysis of the activity coefficient of the oxysulfide solution was also carried out by employing the modified quasichemical model in the quadruplet approximation.     

The Viscous Behavior of FeO<Subscript>t</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> Copper Smelting Slags

Understanding the viscous behavior of copper smelting slags is essential in increasing the process efficiency and obtaining the discrete separation between the matte and the slag. The viscosity of the FeO_t-SiO₂-Al₂O₃ copper smelting slags was measured in the current study using the rotating spindle method. The viscosity at a fixed Al₂O₃ concentration decreased with increasing Fe/SiO₂ ratio because of the depolymerization of the molten slag by the network-modifying free oxygen ions (O²⁻) supplied by FeO. The Fourier transform infrared (FTIR) analyses of the slag samples with increasing Fe/SiO₂ ratio revealed that the amount of large silicate sheets decreased, whereas the amount of simpler silicate structures increased. Al₂O₃ additions to the ternary FeO_t-SiO₂-Al₂O₃ slag system at a fixed Fe/SiO₂ ratio showed a characteristic V-shaped pattern, where initial additions decreased the viscosity, reached a minimum, and increased subsequently with higher Al₂O₃ content. The effect of Al₂O₃ was considered to be related to the amphoteric behavior of Al₂O₃, where Al₂O₃ initially behaves as a basic oxide and changes to an acidic oxide with variation in slag composition. Furthermore, Al₂O₃ additions also resulted in the high temperature phase change between fayalite/hercynite and the modification of the liquidus temperature with Al₂O₃ additions affecting the viscosity of the copper smelting slag.     

Distribution Ratios of Phosphorus Between CaO-FeO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Na<Subscript>2</Subscript>O/TiO<Subscript>2</Subscript> Slags and Carbon-Saturated Iron

In order to effectively enhance the efficiency of dephosphorization, the distribution ratios of phosphorus between CaO-FeO-SiO₂-Al₂O₃/Na₂O/TiO₂ slags and carbon-saturated iron (\( L_{\text{P}}^{\text{Fe-C}} \)) were examined through laboratory experiments in this study, along with the effects of different influencing factors such as the temperature and concentrations of the various slag components. Thermodynamic simulations showed that, with the addition of Na₂O and Al₂O₃, the liquid areas of the CaO-FeO-SiO₂ slag are enlarged significantly, with Al₂O₃ and Na₂O acting as fluxes when added to the slag in the appropriate concentrations. The experimental data suggested that \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the binary basicity of the slag, with the basicity having a greater effect than the temperature and FeO content; \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the Na₂O content and decrease in the Al₂O₃ content. In contrast to the case for the dephosphorization of molten steel, for the hot-metal dephosphorization process investigated in this study, the FeO content of the slag had a smaller effect on \( L_{\text{P}}^{\text{Fe-C}} \) than did the other factors such as the temperature and slag basicity. Based on the experimental data, by using regression analysis, \( \log L_{\text{P}}^{\text{Fe-C}} \) could be expressed as a function of the temperature and the slag component concentrations as follows:

$$ \begin{aligned} \log L_{\text{P}}^{\text{Fe-C}} & = 0.059({\text{pct}}\;{\text{CaO}}) + 1.583\log ({\text{TFe}}) - 0.052\left( {{\text{pct}}\;{\text{SiO}}_{2} } \right) - 0.014\left( {{\text{pct}}\;{\text{Al}}_{2} {\text{O}}_{3} } \right) \\ \, & \quad + 0.142\left( {{\text{pct}}\;{\text{Na}}_{2} {\text{O}}} \right) - 0.003\left( {{\text{pct}}\;{\text{TiO}}_{2} } \right) + 0.049\left( {{\text{pct}}\;{\text{P}}_{2} {\text{O}}_{5} } \right) + \frac{13{,}527}{T} - 9.87. \\ \end{aligned} $$

     

The effect of carbon in slag on steel reoxidation and carbon analysis by x-ray photoelectron spectroscopy in the CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO-MnO-Fe<Subscript><Emphasis Type="Italic">t</Emphasis></Subscript>O system

An equilibrium experiment was carried out at 1873 K to investigate the effect of carbon in CaO-SiO₂-Al₂O₃-MgO-MnO-Fe _t O slag systems on their Fe _t O and MnO activity coefficients, representing the slag’s thermodynamic potential for steel reoxidation. The activity coefficients of Fe _t O and MnO showed not only a sharp increment but also a simultaneous slow decrement with increasing carbon content in slag, suggesting opposite roles of the carbon in slag according to its stable forms. X-ray photoelectron spectroscopy (XPS) was introduced to determine the stable forms of carbon in slag. The XPS results proved that carbon dissolves in slag as carbonate, and carbide ions under oxidizing and reducing atmospheres, respectively. The simultaneous consideration of the activity coefficients of Fe _t O and MnO and stable carbon forms showed that carbonate ions increase the activity coefficients of Fe _t O and MnO, but that carbide decreases them. This article suggests an application method of the present results to actual ladle refining processes, in order to enhance steel cleanliness with maintaining (Fe _t O + MnO) in slag to some allowable amount.     

Effect of Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> on Structure,Viscosity, and Crystalline Phase of CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Li<Subscript>2</Subscript>O-Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Aiming at devising new mold flux for Ce-bearing stainless steel, a fundamental investigation on the effect of Ce₂O₃ on properties of the CaO-Al₂O₃-Li₂O-Ce₂O₃ slag was provided by the present work. The results show that adding Ce₂O₃ could decrease the viscosity of the slag due to its effects on decreasing the polymerization of the slag. The crystalline process was restrained by increasing the content of Ce₂O₃, and the crystalline phases also can be influenced by the slag structure. The crystalline phases were transferred from LiAlO₂ and CaO to LiAlO₂ and CaCeAlO₄ with the addition of Ce₂O₃ to the slag, which could be well confirmed by the structure of the unit cell of the crystals.     

Reactive synthesis of <Emphasis Type="Italic">in-situ</Emphasis> ZrAl<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al composites

In this work, a reactive synthesis process is proposed to obtain ZrAl₃-Al₂O₃ particulate-reinforced aluminum matrix composites. The process involves the in-situ formation of Al₂O₃ and ZrAl₃ from Al-ZrO₂ green compacts. Upon compact heating, it is found that reduction of ZrO₂ by molten aluminum occurs at temperatures above 750 °C, leading to the development of ZrAl₃ and Al₂O₃ phases. Thermodynamically, it is found that the reduction of zirconium oxide is driven mainly by the dissolution of Zr in molten aluminum. Because the solubility of Zr in liquid aluminum is extremely small, the formation of ZrAl₃ is favored after relatively small Zr dissolutions. The first Zr-Al intermetallics to form at the lowest temperatures seem to be metastable, as infered from the measured atom ratios for Al : Zr of 2.83 : 1. At increasing temperatures, the reaction comes into completion, resulting in the formation of equilibrium intermetallic ZrAl₃ phases. The results obtained from differential scanning calorimetry (DSC) indicate that by increasing the scanning rates, both the reaction temperature and the exothermic peak intensity also increase. Alternatively, it is found that by reducing the amount of ZrO₂ in the green compact, the in-situ reaction temperatures also shift toward higher values.     

Experimental Investigation and Thermodynamic Modeling of the B<Subscript>2</Subscript>O<Subscript>3</Subscript>-FeO-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript> System for Recycling of NdFeB Magnet Scrap

NdFeB magnet scrap is an alternative source of neodymium that could have a significantly lower impact on the environment than current mining and extraction processes. Neodymium can be readily oxidized in the presence of oxygen, which makes it easy to recover neodymium in oxide form. Thermochemical data and phase diagrams for neodymium oxide containing systems is, however, very limited. Thermodynamic modeling of the B₂O₃-FeO-Fe₂O₃-Nd₂O₃ system was hence performed to obtain accurate phase diagrams and thermochemical properties of the system. Key phase diagram experiments were also carried out for the FeO-Nd₂O₃ system in saturation with iron to improve the accuracy of the present modeling. The modified quasichemical model was used to describe the Gibbs energy of the liquid oxide phase. The Gibbs energy functions of the liquid phase and the solids were optimized to reproduce all available and reliable phase diagram data, and thermochemical properties of the system. Finally the optimized database was applied to calculate conditions for selective oxidation of neodymium from NdFeB magnet waste.     

Effects of an α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> thin film on the oxidation behavior of a single-crystal Ni-based superalloy

A ∼150-nm-thick coating layer consisting of α-Al₂O₃ as the major phase with a minute amount of Φ-Al₂O₃ was deposited on the surface of a single-crystal Ni-based superalloy by chemical vapor deposition (CVD). Within 0.5 hours of oxidation at 1150°C, the resulting thermally grown oxide (TGO) formed on the coated alloy surface underwent significant lateral grain growth. Consequently, within this time scale, the columnar nature of the TGO became established. After 50 hours, a network of ridges was clearly observed on the TGO surface instead of equiaxed grains typically observed on the uncoated alloy surface. Comparison of the TGO morphologies observed with and without the CVD-Al₂O₃ layer suggested that the transient oxidation of the alloy surface was considerably reduced. Also, the CVD-Al₂O₃ layer significantly reduced the growth rate of the TGO and improved its spallation resistance, while slowing the internal oxidation of Ta-rich areas that were present in the superalloy as-casting defects. These results demonstrated that this thin α-Al₂O₃ coating could be used as a means of favorably altering the TGO morphology and growth kinetics for no bond coat thermal barrier coating (TBC) applications. Y.-F. SU, formerly Doctoral Candidate