Interaction of Al2O3-rich slag with MgO–C refractories during VOD refining—MgO and spinel layer formation at the slag/refractory interface

The interaction mechanisms between a pitch-bonded MgO–C refractory and an Al₂O₃ rich (～15 wt%) stainless steelmaking slag were investigated by rotating finger tests in a vacuum induction furnace. A porous MgO layer (instead of a dense MgO layer) was observed at the hot face of the MgO–C bricks. This implies that under the present low oxygen pressure conditions, the oxygen supply from the slag is insufficient to meet the demand of reoxidising the entire amount of Mg vapor generated from the MgO–C reaction to form a fully dense MgO layer. A Mg(Al,Cr)₂O₄ spinel layer with zoning was found at the slag/brick interface in the top slag zone specimen of Test 3 (CHS3). Based on the thermodynamic analyses with and experimental data, a mechanism of Mg(Al,Cr)₂O₄ spinel formation is proposed. Initially, hot face periclase grains take up Cr₂O₃, and to a much lesser extent, Al₂O₃ from the slag. The further diffusion of Cr₂O₃ and Al₂O₃ from the slag establishes a spinel layer of three distinct compositions of the type MgAl_2(1?x)Cr_2xO₄, with x decreasing when moving from the interior to the exterior spinel layer. Due to the low oxygen pressures, the thermodynamically less stable, dissolved Cr₂O₃ in the hot face periclase decomposes and forms chromium-rich metal droplets.     

Effects of Sintering Aids on the Transparency and Conversion Efficiency of Cr4+ Ions in Cr: YAG Transparent Ceramics

Tetravalent chromium‐doped Y₃Al₅O₁₂ ceramics were fabricated by solid‐state reactive sintering method using high‐purity Y₂O₃, α‐ Al₂O₃, and Cr₂O₃ powders as the starting materials. CaO and MgO were co‐doped as the sintering aids. The effects of TEOS and divalent dopants (CaO and MgO) on the optical qualities, the conversion efficiency of Cr⁴⁺ ions, and the microstructure evolutions of 0.1 at.% Cr⁴⁺: YAG ceramics were investigated. Fully dense, dark brown colored Cr⁴⁺: YAG ceramics with an average grain size of 3.1 μm were achieved. The in‐line transmittance of the as‐prepared ceramic at 2000 nm was 85.3% (4 mm thick), and the absorption coefficient at 1030 nm (the characteristic absorption peak of Cr⁴⁺ ions) was as high as 3.7 cm^?1, which was higher than that of corresponding single crystals fabricated by Czochralski method.     

Thermodynamic Stability of Spinel Phase at the Interface Between Alumina Refractory and CaO–CaF2–SiO2–Al2O3–MgO–MnO Slags

The interfacial reaction between alumina refractory and CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was observed at 1873 K to estimate the stability of the spinel phase using computational thermodynamics under refining conditions of Mn‐containing steels. The concentration of MnO formed by the slag–steel reaction in the CaO–CaF₂–SiO₂–Al₂O₃–MgO melts generally increased by decreasing the CaO/SiO₂ ratio of the initial melts. No intermediate compounds were formed at the refractory–slag interface when the initial CaO/SiO₂ ratio was 0.5, whereas CaAl₁₂O₁₉ (CA6) and Mg(Mn)Al₂O₄ (spinel), identified from TEM analysis using EDS mapping and SAED patterns, were observed at the refractory–slag interface when the CaO/SiO₂ ratio was 1.0 or greater. The (at.%Mg)/(at.%Mn) ratio in the spinel solution increased by increasing the CaO/SiO₂ ratio, which originated from the fact that MgO activity continuously increased as the CaO/SiO₂ ratio increased. From thermodynamic analysis considering the equilibrium constant (K_SP) and activity quotient (Q_SP) of the spinel formation reaction at the slag–refractory interface and the bulk slag phase, the precipitation–dissolution behavior of the spinel phase was predicted, which exhibited good consistency with the experimental results. Hence, the dissolutive corrosion mechanism of alumina refractory into the CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was proposed.     

Cr7C3: A potential antioxidant for low carbon MgO–C refractories

Magnesia carbon (MgO–C) refractory, one of the most commonly used refractories in the steelmaking system, relies on graphite to improve the thermal shock resistance and slag corrosion resistance. The oxidation of graphite carbon in a MgO–C brick usually leads to the destruction of the carbon network in the brick, which causes the structure of the brick to become loose and easily eroded. At present, metal powders, carbides, and borides are used as antioxidants to prevent the oxidation of carbon in MgO–C bricks. The metal carbide Cr₇C₃ can be prepared from aluminum chromium slag through a simple synthetic process and at a low cost. In this work, we investigated the oxidation resistance of low carbon MgO–C refractories with different amounts of Cr₇C₃ powder (1, 2, 3, and 4 wt%). The refractories with 3 wt% Cr₇C₃ powder showed optimal resistance to oxidation. The microstructure indicated that oxygen reacts with Cr₇C₃ preferentially over carbon to form chromium oxide and magnesium chromium spinel, blocking the pores and hindering oxygen diffusion. Carbon arising from the reduction of carbon monoxide by Cr₇C₃ can act as a supplementary carbon source. The better oxidation resistance also contributed to the improvements in slag corrosion and thermal shock resistance of the refractories.     

Radical reaction-induced Turing pattern corrosion of alumina refractory ceramics with CaO–Al2O3–SiO2–MgO slags

Quasi-volcanic corrosion occurs at the triple-phase interface of alumina refractory ceramics and MgO-containing CaO–Al₂O₃–SiO₂ slags in the air, causing severe damage to ceramics. To address this limitation, in this study, a slag corrosion experiment is performed on alumina refractory ceramics using CaO–Al₂O₃–SiO₂–MgO slags. Various spectroscopic techniques, including electron paramagnetic resonance spectroscopy, are used to investigate the influence of slag structures with varied MgO contents on the corrosion peaks and mechanism. The results show large quantities of reactive radicals, including superoxide radicals, in the slags. Free-radical reactions between refractory ceramics and slags lead to Turing pattern corrosion. An increase in the amount of non-bridged oxygen in the slag structure decreases the amount of original superoxide radicals. Consequently, the intensity of the free-radical reactions of alumina dissolution increases, thereby increasing the height of the corrosion peaks.     

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.     

Anionic effect of chloride,fluoride, and sulfide ions on the viscosity of slag melt

The effect of the addition of CaX (X=Cl₂, F₂ and S) on the viscous behavior and structure of CaO–SiO₂–Al₂O₃–MgO–CaX slag was investigated by measuring its viscosity. The viscosity of the slag without CaX gradually decreased with an increase in the C/S ratio because of the depolymerization of the silicate groups in the slag. While the viscosity of the CaX‐bearing slag decreased with an increase in the CaX content, depolymerization was not observed in this case. Three distinct compositional regions for the activation energy of the viscous flow were observed because of the effect of the equilibrium of the polymeric silicate groups. The relaxation effect of the CaX groups on the activation energy was also observed. Raman spectroscopic analysis indicated that the relaxation in the viscosity and activation energy by CaX addition stemmed from the breaking of the NBO‐M²⁺‐NBO linkage to form NBO‐M²⁺‐F^?, NBO‐M²⁺‐Cl^?, or M²⁺‐S^2?. All these results are discussed in detail with the help of a viscous flow model based on the ionic interactions.     

Dissolution behavior of silica in molten CaO–SiO2–Fe2O3–MgO–MnO slag

The modification of basic oxygen furnace (BOF) slag by adding silica can improve the properties of BOF slag for applications in the cement industry. The rapid dissolution of silica is essential to hot slag modification. In this work, the dissolution behavior of silica in the molten CaO–SiO₂–Fe₂O₃–MgO–MnO system as synthetic BOF slag was investigated by using the traditional rotating cylinder technique. Effects of rotation speed, temperature, immersion time, and slag basicity on the silica dissolution were studied. Scanning electron microscopy equipped with energy dispersive spectrometer (SEM-EDS) and FactSage simulations were employed to reveal the dissolution mechanism. It was found that the dissolution of the silica rod was affected by both the thermodynamic driving force and the slag viscosity. The silica dissolution rate in molten CaO–SiO₂–Fe₂O₃–MgO–MnO slag increased with increasing the rotation speed and temperature, but first increased and then decreased when decreasing the slag basicity from 2.5 to 1.5. A linear correlation between the logarithm of the dissolution rate and the logarithm of cylinder periphery velocity with a slope of 0.44 was observed, indicating the mass transfer within the boundary layer as the dissolution rate determining step. A direct dissolution way was found during the dissolution of silica in molten CaO–SiO₂–Fe₂O₃–MgO–MnO 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.     

Effect of surface properties of MgO on its dissolution and spinel growth in CaO–SiO2–Al2O3 slag

The dissolution behavior of MgO in CaO–SiO₂–Al₂O₃ ternary slag at the interface of single-crystal, dense poly-crystal, and porous poly-crystal MgO was investigated to evaluate the effect of the surface properties of the MgO. The experimental results revealed that a detached spinel layer formed at the MgO interface due to the change in thermodynamic condition of the slag, which was independent of the surface properties. On the other hand, it was also confirmed that the growth rate and morphology of the detached spinel layer strongly depended on the surface properties, such as porosity and curvature of MgO. During the formation of the spinel layer at the interface during MgO dissolution, a kinetic approach adopting parabolic relation theory was employed to determine the correlation between the surface properties and the spinel growth mechanism.     

Influence of calcium concentration on formation of tetravalent chromium doped Y3Al5O12 ceramics

Yttrium Aluminium Garnet (YAG) ceramics doped with chromium were prepared by solid-state reactive sintering in a vacuum. The influence of the charge compensator Ca²⁺ concentration on microstructure, optical properties and efficacy of Cr³⁺ oxidation to Cr⁴⁺ under air annealing was investigated. A non-monotonic dependence of these features on the amount of CaO as an additive was found. The changes in ceramic transparency and microstructure were explained considering the interaction between CaO and Cr₂O₃ at the ceramic grain boundaries, which leads to a different pore evolution in distinct samples during sintering. The efficacy of the oxidation of Cr³⁺ to Cr⁴⁺ strongly depends on the concentration of Ca dissolved in the YAG. The calcium solubility decreases due to the higher oxygen partial pressure of the extra phases on the grain boundaries that decreases the amount of generated Cr⁴⁺ ions. Such phenomenon explains the lower concentration of Cr⁴⁺ ions in the sample with 0.8% of Ca against the one with 0.5%. The experiment shows that the ceramic with 0.5% of Ca has a better in-line transmission and a higher concentration of Cr⁴⁺ ions in comparison with samples with a different Ca concentration.     

Interfacial reaction between magnesia refractory and “FeO”-rich slag: Formation of magnesiowüstite layer

This study investigated the reaction between CaO-SiO₂-Al₂O₃-xFeO-MgO-MnO (CaO/SiO₂?=?1.2, x?=?20–50?wt%) slag and magnesia refractory. Using SEM-EDS analysis, we confirmed the formation of a (Mg,Fe)O_{ss(solid_solution)}, called magesiowüstite (MW), intermediate layer at the slag-refractory interface. MgO dissolved from refractory and reacted with the bulk slag to form MW layer at the interface. Simultaneously, slag penetrated through micro-pores and reacted with the refractory to form MW layer. In other words, the MW layer built up in both directions from initial refractory-slag interface. The thickness of the MW layer increased as the FeO content in the slag increased, and using EDS line scanning, a Mg and Fe concentration gradient was confirmed within the MW layer. The slag, which penetrated into the refractory, had a chemical composition of the CaO-SiO₂-Al₂O₃-MgO system without FeO, indicating that FeO was consumed by forming a MW layer at the refractory hot face. The slag-refractory interfacial reaction was simulated using thermochemical software, FactSage?7.0. The results predicted a MW monoxide composed of MgO and FeO. A spinel phase was formed when FeO was greater than 40?wt%. These thermochemical computations were comparable to our experimental findings.     

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.     

Effect of Ca2+ and Mg2+ ions on the sintering and spectroscopic properties of cr-doped yttrium aluminum garnet ceramics

In this work, we investigated the effects of Ca²⁺ and Mg²⁺ ions and annealing temperature on the spectroscopic parameters of chromium-doped yttrium aluminum garnet ceramics (Cr:YAG). Samples were obtained with either a separate or a simultaneous addition of calcium and magnesium oxides. To achieve this, aqueous suspensions were prepared using Y₂O₃, Al₂O₃, Cr₂O₃, MgO, and CaO high-purity powders as raw materials. The obtained suspensions were freeze-granulated, pressed into pellets, debinded, and subjected to reactive sintering in vacuum at 1715°C for 6 h. Each material was annealed in air with temperatures between 1300 and 1700°C. Samples were also compared to Cr:YAG ceramics with the addition of silica as a sintering aid. All the materials obtained were then exposed to 445 nm excitation, and emission spectra in the visible and infrared wavelengths were recorded. The results showed that the emission spectra of Cr:YAG ceramics varied according to the annealing conditions: as-sintered samples exhibited strong emissions of around 680 nm and, after air annealing, of around 1400 nm. This phenomenon is attributed to the Cr³⁺→Cr⁴⁺ transition. Samples doped solely with MgO exhibited the highest emission intensity in the infrared region. Thus, Mg²⁺ ions provided the best conversion efficiency of chromium ions.     

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.     

Effect of electromagnetic field on slag corrosion resistance of low carbon MgO–C refractories

Using MgO–C refractories containing 6% carbon and the slag with a basicity (CaO/SiO₂) of around 0.8, the melting slag resistance experiments of low carbon MgO–C refractories were carried out in induction furnace and resistance furnace, respectively. The microstructure of low carbon MgO–C refractories corroded by slag under the different conditions was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDAX). The results show that in induction furnace having electromagnetic field (EMF), there are MgFe₂O₄ spinel with a little of Mn ions generated in the interfacial layer. Part of the solid solution is monticellite [CaMgSiO₄] containing a little MnO and FeO. While under the condition of EMF free, there is not MgFe₂O₄ spinel in the interfacial layer and the solid solution is monticellite (CaMgSiO₄). At a high temperature, EMF increases the diffusion coefficient of Fe^2+/3+ ions, which displaces Mg²⁺ and forms MgFe₂O₄ with a little of Mn ions. There are MgAl₂O₄ spinel in the penetration layers under the conditions of both EMF and EMF free. EMF speeds up corrosion of low carbon MgO–C refractories.     

Preparation of CaCO3 coated corundum aggregates by dip-coating and heat treatment and its effects on the properties and microstructures of Al2O3–MgO castables

The CaCO₃ coated corundum aggregates were prepared by impregnating tabular corundum aggregates with sizes of 1–5 mm in calcium hydrogen citrate solution and heat treatment at 430 °C, which were also used in Al₂O₃–MgO castables. The effects of Ca²⁺/Cit^3? mole ratio in precursor solution on coating characteristics of CaCO₃ coated corundum aggregates as well as the effects of CaCO₃ coatings on properties and microstructure of castables were investigated. It is found that the thickness and continuity of CaCO₃ coating is increased and the size of CaCO₃ particles in coatings decreases first and then increases as Ca²⁺/Cit^3? mole ratio is decreased. High-temperature properties of castables are improved by in-situ formation of calcium hexaaluminate (CA₆) layer at aggregate/matrix interface after sintering at 1600 °C. The Al₂O₃–MgO castables exhibit the best thermal shock resistance when Ca²⁺/Cit^3? mole ratio is 1/3. It is contributed by deflections of cracks and consumptions of fracture energy in a continuous platelet CA₆ layer with thickness of 10 μm, which is in-situ formed through reaction between Al₂O₃ and CaO derived from CaCO₃ coatings. The present investigation provides a novel approach to enhance thermal shock resistance of the Al₂O₃–MgO castables.     

Understanding the solidification and leaching behavior of synthesized Cr-containing stainless steel slags with varying Al2O3/SiO2 mass ratios

This study investigated the effect of Al₂O₃/SiO₂ mass ratios on the equilibrium crystallization behavior of synthesized CaO–SiO₂–MgO–Al₂O₃–Cr₂O₃ stainless steel slags to understand the selective concentration behavior of Cr into a primary Mg(Cr,Al)₂O₄ spinel phase during slag solidification and to determine the leaching stability of Cr-containing slags. The spinel solid solution was precipitated within the temperature range of 1600-1400 °C, where the Cr/(Cr+Al) mole ratio in the Mg(Cr,Al)₂O₄ spinel phase gradually decreased for slags with higher Al₂O₃/SiO₂ mass ratios. When the Al₂O₃/SiO₂ mass ratio increased from 0.125 to 0.5, the Cr content in the amorphous glass phase gradually decreased, with a subsequent increase in the Cr content in the crystalline phase. For slags with a unit Al₂O₃/SiO₂ mass ratio and MgO mole percent comprising less than the combined sum of the Cr₂O₃ and Al₂O₃ mole percents, the Cr content in the amorphous glass phase increased, which was correlated with the enhanced substitution of Cr³⁺ with Al³⁺ in the spinel. The trend of the amount of Cr-related ions in the leachate was consistent with the trend of Cr in the amorphous glass phase: the amount decreased for slags with Al₂O₃/SiO₂ mass ratios from 0.125 to 5 and then increased for slags with an Al₂O₃/SiO₂ mass ratio of 1. The results suggest that the addition of appropriate amounts of Al₂O₃ to stainless steel slags could be conducive to stabilizing Cr into the primary spinel phase to minimize Cr leaching into the environment.     

Effect of soluble Cr2O3 on the silicate network,crystallization kinetics,mineral phase,microstructure of CaO-MgO-SiO2-(Na2O) glass ceramics with different CaO/MgO ratio

Cr₂O₃ is often used as a glass additive to prepare glass ceramics. Chromium element exists mainly in two parts in the glass ceramics: chromium-containing spinel and soluble chromium in glass matrix. Herein, effect of soluble chromium on the CaO-MgO-SiO₂-(Na₂O) glass system is researched. Glass and glass ceramics were characterized by Raman spectrum, X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry. It is found that the addition of Cr₂O₃ increased the Q²si structure unit in glass networks, especially in glass systems with high MgO content. The crystallization temperatures of the systems were increased with the addition of Cr₂O₃. Soluble chromium reduced the crystallization activation energy of the glass system slightly, but did not alter its crystallization behavior (surface crystallization). With the increase of MgO content, the mineral phases of the glass ceramics gradually changed from wollastonite to diopside. Cr₂O₃ reduced the lattice parameters of the mineral phases. The addition of Cr₂O₃ has a significant effect on grain refinement and structural compactness of the glass ceramics system with high MgO content.     

Volumetric nucleation of crystals catalyzed by Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in glass based on furnace slags

The features of the volumetric nucleation of crystals in glass obtained by melting furnace slags with the additive of SiO₂, chromium sesquioxide Cr₂O₃, are studied by the methods of differential thermal and Xray phase analysis and optical microscopy. Upon the introduction of Cr₂O₃ as the catalytic additive, two phases are sequentially formed in the glass: magnesiochromite (MgO · Cr₂O₃) and diopside (CaO · MgO · 2SiO₂). The characteristics of homogeneous and heterogeneous crystallization are determined: the stationary nucleation rate, nonstationary nucleation time, crystal growth rate, and their temperature dependences are obtained. Practical recommendations on the use of the obtained glass are given.