Thermodynamic modeling of the NiO–SiO2, MgO–NiO,CaO–NiO–SiO2, MgO–NiO–SiO2, CaO–MgO–NiO and CaO–MgO–NiO–SiO2 systems

A complete literature review, critical evaluation and thermodynamic optimization of phase equilibrium and thermodynamic properties of all available oxide phases in the NiO–SiO₂, MgO–NiO, CaO–NiO–SiO₂, MgO–NiO–SiO₂, CaO–MgO–NiO and CaO–MgO–NiO–SiO₂ systems at 1 bar pressure are presented. The molten oxide phase is described by the modified quasichemical model, and the Gibbs energies of solid olivine and pyroxene solutions were modeled using the compound energy formalism. A set of optimized model parameters of all phases is obtained which reproduces all available and reliable thermodynamic and phase equilibrium data within experimental error limits from 25 °C to above the liquidus temperatures over the entire composition range. The unexplored ternaries and quaternary phase diagrams and activity of liquid phase in the CaO–MgO–NiO–SiO₂ system have been predicted for the first time. The database of the model parameters can be used along with software for Gibbs energy minimization to calculate all thermodynamic properties and any phase diagram section of interest.     

Phase equilibria studies in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 ratio of 0.9

Phase equilibria and liquidus temperatures in the CaO–SiO₂–Al₂O₃–MgO system at a CaO/SiO₂ weight ratio of 0.9 in the liquid phase have been experimentally determined employing high-temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms at 1573, 1623, 1673, and 1773 K were determined and the primary phase fields of wollastonite, melilite, olivine, periclase, spinel, and corundum have been located. Compositions of the olivine and melilite solid solutions were analyzed and discussed. Comparisons between the newly constructed diagram, existing data, and FactSage predicted phase diagrams were performed and differences were discussed. The present study will be useful for guidance of industrial practices and further development of thermodynamic modeling.     

Effect of varying Al2O3 contents of CaO–Al2O3–SiO2 slags on lumped MgO dissolution

This study utilized the single hot thermocouple technique to examine the dissolution behavior of lumped magnesium oxide (MgO) in CaO–Al₂O₃–SiO₂ ternary slags. The aluminum oxide (Al₂O₃) content in the slag (C/S = 1) varied from 10% to 30%; the MgO sphere with a diameter of 1 mm was placed in molten slags at 1,550 °C. Results showed that the dissolution rate decreased as the Al₂O₃ content increased up to 20%. Over 20% Al₂O₃, MgAl₂O₄ was formed at the interface of MgO and it did not fully melt at 30% Al₂O₃. The dissolution behavior and the formation of MgAl₂O₄ were analyzed by a phase diagram provided by Factsage 7.0 software. In the case of less than 20% Al₂O₃ content, apparent sphere radii were measured; the shrinking core model was then applied to understand the dissolution mechanism. The dissolution rate of both slags was controlled by boundary layer diffusion. The dissolution rate at 20% Al₂O₃ slag appeared to fit the behavior to the boundary layer diffusion, although it deviated during the middle stage of the dissolution because of MgAl₂O₄ formation. The 10% Al₂O₃ slag fitted well to the boundary layer diffusion curve; the obtained diffusion coefficient was 0.94 × 10⁻⁹ m²/s.     

Corrosion of spinel clinker by CaO–Al2O3–SiO2 ladle slag

Three different grades of sintered spinel clinker were used containing 47, 69 and 94 wt.% Al₂O₃, respectively, i.e. MgO-rich, stoichiometric and Al₂O₃-rich. Based on these clinkers, the corrosion mechanism of each spinel clinker by CaO–Al₂O₃–SiO₂ slag was investigated and the corrosion and penetration behavior of castables containing powdered spinel clinker examined. A layer of MgO·(Al, Fe)₂O₃ complex spinel formed at the slag-refractory interface was proportional to the MgO content of the spinel clinkers, and it depressed the slag corrosion. The free MgO and spinel minerals in each spinel clinker mainly trapped Fe₂O₃ from the slag. CaO–Al₂O₃ compounds were formed at the slag-clinker interface by the reaction between free Al₂O₃ in the Al₂O₃-spinel clinker and CaO from slag. Slag penetration into the spinel clinkers was retarded by these compounds. As a result of adding fine spinel powder to the matrix of Al₂O₃-based castables, it was observed that higher content of MgO in spinel clinker showed better resistance to slag corrosion but lower resistance to slag penetration.     

The effect of phase formation during use on the chemical corrosion of magnesia–chromite refractories in contact with a non-ferrous PbO–SiO2 based slag

One of the main factors limiting the lining lifetime in pyrometallurgical smelters is continuous refractory oxides dissolution in the slag bath. The overall wear is accelerated when the slag infiltrates the porous brick and the dissolution thus occurs in a larger part of the lining. This work investigates the possibility of preventing deep infiltration by sealing off the pores with newly formed phases. Static finger tests at constant temperature (1200 °C) were performed in contact with a synthetic non-ferrous PbO–SiO₂–MgO slag, showing the formation of forsterite (Mg₂SiO₄) throughout the refractory sample by the reaction between SiO₂ (slag) and MgO (refractory). This phase grows with time, eventually sealing off the pores near the interface with the bath. The phase grows too slow to prevent full infiltration of the refractory but creates an equilibrium state in the sealed off part of the sample, ceasing the chemical corrosion in that part of the sample.     

Structure and viscosity of CaO–Al2O3–B2O3–BaO slags with varying mass ratio of BaO to CaO

The structure of CaO–Al₂O₃–B₂O₃–BaO glassy slags with varying mass ratio of BaO to CaO has been investigated by Raman spectroscopy, ¹¹B and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy and atomic pair distribution function (PDF). ¹¹B MAS-NMR spectra reveal the dominant coordination of boron as trigonal. Both simulations on ¹¹B MAS-NMR spectra and Raman spectroscopy indicate the presence of orthoborate as the primary borate group with a few borate groups with one bridging oxygen and minor four-coordinated boron sites. ²⁷Al MAS-NMR and PDF show the Al coordination as tetrahedral. Raman spectral study shows that the transverse vibration of Al^IV–O–Al^IV and Al^IV–O–B^III, stretching vibration of aluminate structural units and vibration of orthoborate and pyroborate structural groups. A broader distribution of Al–O bond lengths in PDF also supports the enhanced network connectivity. Viscosity measurements show the increase in viscosity of molten slags with increasing mass ratio of BaO to CaO, which further attributes to the enhanced degree of polymerization of the aluminate network.     

Modification of glass network and crystallization of CaO–Al2O3–MgO–SiO2 based glass ceramics with addition of iron oxide

Modification of glass network and crystallization process of a CaO–Al₂O₃–MgO–SiO₂ (CAMS) based glass ceramic to form diopside through addition of iron oxide were investigated using differential thermal analysis (DTA), Raman spectrum, X-ray diffraction, SEM and EBSD techniques. The experimental results showed that addition of Fe₂O₃ led to remarkable reductions in both the glass transition temperature (Tg) and crystallization temperature (Tp) of the CAMS glass ceramic. At addition level below 5 wt%, the Tg and Tp temperatures were 651°C and 903°C, respectively, and the crystallization only occurred on the surface of the glass ceramic samples. Increasing the addition level to 10 wt% and 15 wt%, not only led to reduction in the Tg and Tp temperatures to 643-641°C and 892-876°C, respectively, but also promoted the formation of crystalline diopside throughout the CAMS samples. Based on the results of Raman spectrums, it was confirmed that Fe₂O₃ addition reduced the strength of glass connection as a result of chemical reactions between the isolated Si–O tetrahedron and Fe³⁺ ion, forming Fe³⁺O₄–SiO₄, which can be regarded as Q₂ unit. And this is the first experimental evidence that proving the approach of Fe³⁺ mending glass network. Microstructural examination also identified the formation of large numbers of spherical Fe-enriched regions within the CAMS glass matrix as a result of the amorphous phase separation due to the Fe₂O₃ addition. The interfaces between the Fe-enriched regions and the glass matrix acted as preferred nucleation sites for the diopside, facilitating the crystallization. Crystallographic analysis using EBSD technique determined the <001> as the most favorite growth direction for the diopside crystals in the CAMS based glass ceramic.     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

Properties of liquid CaO–SiO2 and CaO–SiO2-‘Fe2O3’tot slags measured by a combination of maximum bubble pressure and rotating bob methods

Liquid slag properties are essential for understanding complex mass and momentum phenomena in metallurgical operations. The density, surface tension and viscosity were measured in six silicate-rich slags of the CaO–SiO₂ and CaO–SiO₂-‘Fe₂O₃’_tot systems by combining the maximum bubble pressure and rotating bob methods. The properties investigated were sensitive to the temperature, SiO₂ and Fe₂O₃ contents. Different experimental trends were derived due to the amphoteric properties of Fe₂O₃. The slags with ferric oxide were denser than the silicate melts. Surface tension gradually decreased with temperature and indicated firstly a rise and then decline with further Fe₂O₃ addition. Raman spectra were analyzed to provide structural information of the polymer melt and indicated an enhancement in the polymerization degree with Fe³⁺. The derived experimental trends and role of Fe³⁺ in the silicates were attributed to the interplay of complex factors: different bonding in the melt, cation interactions and the oxidation state of iron. The influence of Fe³⁺/Fe²⁺ on the melt properties was discussed.     

Glass-ceramics in the CaO–MgO–Al2O3–SiO2 system as potential dental restorative materials

This paper reports on development of novel alumina-containing glass-ceramics (GCs) with a high content of Al₂O₃ (12.5 wt.%) in the CaO–MgO–Al₂O₃–SiO₂ system aimed for dental restorations. The thermal properties of the parent glasses, the microstructure and the mechanical properties of the produced sintered and crystallized GCs along with bio-inertia performance were experimentally studied. Dense, white, and bio-inert GCs, comprised of melilite, either as a single-phase or with diopside, were produced. The values of flexural strength ranged between 120 and 171 MPa, the modulus of elasticity varied between 28 and 42 GPa, while the values of the hardness and the fracture toughness (measured by the indentation–Niihara equation) ranged from 6.3 to 7.0 GPa, and from 2.6 to 2.8 MPa m^0.5, respectively. The mechanical properties of the produced GCs, after being meticulously compared with the mechanical properties of GCs of various compositions reported in literature, including commercial ones, are a good match to the properties of dental hard tissues, and satisfy the requirements of the ISO 6872 “Dentistry-Ceramic Materials”.     

Dissolution behavior of spent MgO–C refractory in the CaO–SiO2–FeO slag system as a steelmaking flux

A large amount of spent MgO–C refractory is generated in steel plant every year. Because of the similarities in chemical and mineralogical composition of slag formers and MgO–C refractory, it is possible to reuse the spent MgO–C refractory as a steelmaking flux. To achieve this goal, it should promote the dissolution of MgO–C refractory during slag forming. In this study, the effect of slag composition on the dissolution behavior of spent MgO–C refractory in the CaO–SiO₂–FeO slag system and the dissolution kinetics were investigated. It showed that the dissolution rate of MgO–C refractory was controlled by surface chemical reaction. The dissolution of MgO–C refractory led to an increase in the MgO content in slag while the FeO content decreased because the graphite in refractory was oxidized by FeO. Increasing temperature significantly promoted the dissolution of MgO–C refractory. The MgO–C refractory was readily dissolved in the low-basicity slag. A higher FeO content in slag was beneficial for the oxidation of graphite in refractory, resulting in better dissolution. The dissolution thickness of MgO–C refractory could exceed 4.0 mm under these conditions and its dissolution supplied some MgO to slag.     

Corrosion resistance of Al–Cr-slag containing chromium–corundum refractories to slags with different basicity

Al–Cr slag is the solid waste generated by the smelting of Cr metal. It presents a range of environmental hazards. This study addressed the corrosion resistance of Al–Cr slag containing chromium–corundum refractories to slags with different basicity. Herein, we provide suggestions for the use of Cr–corundum of different basicity in kilns. Al–Cr slag, brown fused Al₂O₃, and chrome green were used as the raw materials, with pure calcium aluminate cement being used as a binder. The brick samples, prepared using different blends of chrome green and corundum, were fired at 1600?°C, and subsequently subjected to a slag corrosion test. After corrosion by slag of different basicity, the phase composition and microstructure of the sample were analyzed by X-ray diffraction, energy dispersive spectrometer and scanning electron microscopy. There were two major findings. First, Cr–corundum brick made from Al–Cr slag has a better slag corrosion resistance than that made from Cr₂O₃ and brown fused Al₂O₃. Second, Cr–corundum brick made from Al–Cr slag has superior corrosion resistance to slag with a CaO:SiO₂ ratio of 2:1.     

Cathodoluminescence imaging for rapid identification of low-melting CaO–MgO–SiO2 phases in MgO-based refractories involving the steelmaking process

For MgO–C refractories used in the steelmaking process, identifying low-melting CaO–MgO–SiO₂ phases is crucial because they accelerate the corrosion of the refractories. However, electron probe microanalysis, a conventional method for identifying such phases, is time-consuming. Herein, cathodoluminescence (CL) imaging is proposed for the rapid identification of low-melting CaO–MgO–SiO₂ phases at the reaction interface between MgO-based refractory and steelmaking slag. Monticellite, merwinite, and melilite were identified as the low-melting phases, emitting green, red, and violet luminescence, respectively, in the CL images. Other mineral phases emitted luminescence whose colors differed from those of the low-melting phases (3CaO·2SiO₂ and 2CaO·SiO₂) or no luminescence (magnesiowüstite, MgO·Al₂O₃, Ca₂Fe₂O₅, 3CaO·SiO₂, MnS, and FeS). The CL images (area: 0.5 ×0.3 mm²) were obtained in 30 s. Therefore, CL imaging is effective for the rapid identification of mineral phases, which limit the service life of MgO–C refractories during steelmaking.     

The role of MgO on the structural properties of CaO–Na2O(MgO)–P2O5–CaF2–SiO2 derived glass ceramics

The effect of increasing MgO/Na₂O replacements (on mole basis) on the crystallization characteristics of glasses based on the CaO–Na₂O(MgO)–P₂O₅–CaF₂–SiO₂ system were studied by using DTA, XRD, and SEM. The crystallization characteristics of the glasses, the type of crystalline phases formed and the resulting microstructure were investigated. The main crystalline phases formed after controlled heat-treatment of the base glass were diopside, wollastonite solid solution, fluoroapatite and sodium calcium silicate phases. The increase of MgO at the expense of Na₂O led to decrease the amount of sodium calcium silicate phase. The Vicker's microhardness values (5837–3362 MPa) of the resulting glass–ceramics were markedly improved by increasing the MgO-content in the glasses. The obtained data were correlated to the nature and concentration of the crystalline phases formed and the resulting microstructure.     

1250°C liquidus for the CaO–MgO–SiO2–Al2O3–TiO2 system in air

Ti-bearing blast furnace slags have been regarded as an important secondary material in modern society, and the efficient recycling of Ti oxides from it is of key interest. For this reason, more thermodynamic data is needed regarding the phase relations in different composition ranges and sections. Therefore, the equilibrium phase relations of CaO–MgO–SiO₂–Al₂O₃–TiO₂ system in a low w(CaO)/w(SiO₂) ratio of 0.6–0.8 at 1250 °C in air and fixed concentrations of MgO and Al₂O₃, were investigated experimentally using a high temperature equilibration and quenching method followed by SEM-EDS (Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer) analyses. The equilibrium solid phases of perovskite (CaO·TiO₂), a pseudo-brookite solid solution (MgO·2TiO₂, Al₂O₃·TiO₂)_ss, and anorthite (CaO·Al₂O₃·2SiO₂) were found to coexist with the liquid phase at 1250 °C. The calculated results of Factsage and MTDATA were used for comparisons, and significant discrepancies were found between predictions and the experimental results. The 1250 °C isotherm has been constructed and projected on the CaO–SiO₂–TiO₂-8 wt.% MgO-14 wt% Al₂O₃ quasi-ternary plane of the phase diagram. The obtained results provide new fundamental data for Ti-bearing slag recycling processes, and they add new experimental features for thermodynamic modeling of the high-order titanium oxide-containing systems.     

Blackening of magnesia grains in MgOC refractory

MgO grains in MgOC refractory usually blacken after being used at high temperature. In this paper, the cause and mechanism of blackening of the MgO grains were investigated. Fused MgO crystal grains, which are generally used in MgOC refractory, contain many micropores below 1 μm in size. When the MgO grain with micropores is heated in an atmosphere with high partial pressure of Mg, i.e. with low partial pressure of O₂, Mg diffuses from the grain surface to the micropores, and excess Mg condenses during cooling. Blackening of the MgO grain is caused mainly by condensation of Mg in the microdefects — in particular, in the micropores.     

VISCOSITY STUDIES OF SYSTEM CaO–MgO–Al2O3–SiO2: 1, 40% SiO2*

The measurement of glass viscosities at elevated temperatures with an oscillating cylinder-type viscometer is described. Viscosity data are presented covering those compositions in the CaO–MgO–Al₂O₃–SiO₂ system which contain 40% SiO₂ and which are liquid at 1500°C. The pattern of the system of isokoms presented indicates that viscosity is closely related to the ratio of basic to acidic components. Theoretical aspects of the variation of viscosity with composition in silicate-aluminate systems are discussed in terms of silica-alumina polymers.     

Thermodynamics and crystallization kinetics of REEs in CaO–SiO2–Ce2O3 system

Rare earth elements (REEs) have become increasingly important as ceramic materials. The RE-bearing slags contain massive REEs resources, whereas the lack of thermodynamic and kinetic data of REEs has brought great difficulties to efficient recovery of REEs from RE-bearing slags and the application in ceramics. According to the compositions of the RE-bearing slags in industrial production, the isothermal phase equilibria of CaO–SiO₂–Ce₂O₃ system at 1500°C and 1300°C were constructed by means of liquid-quenching method combined with a series of analyses, which provides the thermodynamic data for the equilibria of REEs. On this basis, the crystallization behaviors of the RE phase (Ce_9.33−xCa_x(SiO₄)₆O_2−0.5x) was investigated, and the temperature range in which the RE phase crystallized singly in RE-bearing slags with a selected compositions was acquired. CCT and TTT diagrams for CaO–SiO₂–Ce₂O₃ system were established to characterize the crystallization kinetics of the RE phase, and the favorable conditions for its crystallization and growth in RE-bearing slags were determined. In this study, the complete thermodynamic and kinetic basic data of REEs in CaO–SiO₂–Ce₂O₃ system are provided for RE-bearing slags.     

Preparation of glass–ceramic glazes in the SiO2–Al2O3–CaO–MgO–K2O–Na2O–ZnO system by variable content of ZnO

This paper is focused on glass–ceramic glazes from the SiO₂–Al₂O₃–CaO–MgO–K₂O–Na₂O system with ZnO additions (2.5, 5, 10, 15, 15, 20 and 25 wt%). The compositions were designed based on constant molar ratio of SiO₂/Al₂O₃. In the resulting glazes diopside (CaMg[Si₂O₆]), willemite (Zn₂SiO₄) and vitreous phase were identified by X-ray diffraction. Morphological and structural date of these glazes were supplementary determined by EPMA, FTIR and Raman Spectroscopy. DSC analysis was carried out to characterize thermal properties of the materials.