Characterisation and properties of low-conductivity microporous magnesia based aggregates with in-situ intergranular spinel phases

Development of microporous magnesia based aggregates serving as working-line refractories have great significance in reducing energy loss and saving resource. Microporous magnesia-based aggregates were fabricated at 1780 °C by in-situ decomposition of magnesite with addition of nano-sized Al₂O₃. Intergranular MgAl₂O₄ phases formed in situ decreased the closed-pore size, thermal conductivity and improved the ceramic bonding and thermal shock resistance. Furthermore, the results suggested that pore size distribution was the dominate factor affecting thermal conductivity. Thermal contact resistance owing to networks of intergranular spinel in magnesia could improve thermal insulation performance effectively. The mismatch of thermal expansion coefficient between spinel and magnesia and the micro-scale closed pores enhanced thermal shock resistance by accommodating thermal stress and suppressing crack propagation. Microporous magnesia-based aggregates with 3 wt% nano-sized Al₂O₃ presented a mean pore size of 3.42 μm, thermal conductivity of 5.76 W m^?1 k^?1 (800 °C), a cold compressive strength of ~285 MPa, and a residual strength retention rate of 65.0% after thermal shock cycles. The low-conductivity microporous magnesia-based aggregates with excellent thermal shock resistance show promise for future application in working-lining lightweight refractories.     

Controllable Preparation of Al2O3‐MgO·Al2O3‐CaO·6Al2O3 (AMC) Composite with Improved Slag Penetration Resistance

Compact Al₂O₃‐MgO·Al₂O₃‐CaO·6Al₂O₃ (AMC) composite was obtained by melting technology using industrial alumina, light‐burned magnesia, and quick lime as raw materials based on the Al₂O₃‐MgO‐CaO ternary phase diagram. The results show that the phases of MgO·Al₂O₃ and Al₂O₃ are formed as the main framework with plate‐like CaO·6Al₂O₃ crystals mainly discontinuously embedded in MgO·Al₂O₃. The bulk density of AMC composite is up to 3.42 g/cm³, equivalent to 90.5% of the theoretical density. The synthesized compact AMC composite in the work also exhibits better slag penetration resistance than the castable based on tabular corundum due to the formation of liquid phase.     

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.     

Adsorption mechanism of oxide inclusions by microporous magnesia aggregates in tundish

The microporous refractory with low thermal conductivity shows a promising application prospect as tundish lining. In this study, the interactions between microporous magnesia aggregates and oxide inclusions in steel were explored. The experimental results and thermodynamic calculation show that the interaction process of microporous magnesia aggregates and oxide inclusions at high temperature can be divided into dissolution, reaction and post-reaction. The microporous magnesia aggregates can absorb the inclusions in steel by reaction and liquid phase penetration. The microporous magnesia aggregates have a strong adsorption for Al₂O₃ and TiO₂. Moreover, the microporous magnesia aggregates mainly absorb SiO₂ by penetration, but excessive SiO₂ will lead to the serious corrosion of microporous magnesia aggregates.     

A novel lightweight periclase-composite (Mg8-xFex+yAl16-yO32) spinel refractory material for cement rotary kilns

This study presents novel lightweight periclase-composite (Mg_8-xFe_x + yAl_16-yO₃₂) spinel refractories (LPSR) for the high temperature zone of cement rotary kilns. The LPSR was prepared by using microporous magnesia aggregates instead of sintered magnesia aggregates in traditional periclase-composite spinel refractories (TPSR). Hercynite-corundum composite aggregates, as well as microporous magnesia aggregates with a median pore size of 3.50 μm and a 20.1% lower bulk density than those of the sintered magnesia aggregates were used as raw materials. The microstructures, fracture behavior and strength of the LPSR in contrast with those of the TPSR were determined by SEM and three-point bending tests. After substituting the microporous magnesia aggregates for the sintered magnesia aggregates, a rougher surface of the microporous aggregates and wider transition-layer containing a solid solution spinel phase at the microporous magnesia aggregate/composite spinel aggregate interfaces were observed. Thus, a better bonding at the microporous magnesia aggregate/matrix interfaces as well as of the microporous magnesia aggregate/composite spinel aggregate interfaces was achieved. The wider transition-layer and better interfaces impeded crack propagation along the aggregate/matrix interface and increased the percentage of crack propagation within the aggregates. Thus, the mechanical strength of the LPSR was significantly enhanced. Compared with the TPSR, the LPSR had a lower bulk density of 2.56 g/cm³, but also a higher apparent porosity of 27.8% and a higher compressive strength of 46.4 MPa.     

Reaction mechanism between MgO crucible and AerMet100 steel during vacuum induction melting

AerMet100 steel has strict composition and inclusion requirements. Therefore, its reaction with MgO refractory during vacuum induction melting cannot be ignored. In this study, the reaction mechanism between the MgO refractory and AerMet100 steel during the refining stage was investigated using a MgO crucible. The influence of the MgO crucible on AerMet100 steel composition and inclusions under refining vacuum pressures of 50–100 and 5–10 Pa was compared. The results indicate that SiO₂, Al₂O₃, and MgO in the crucible decompose and are reduced by C in the liquid steel, which results in the increase of Si, dissolved Al (Als), and dissolved Mg (Mgs) content in the liquid steel. The increase of Ca content is due to the reduction of CaO in the crucible by C in the liquid steel. The reaction of Al₂O₃ inclusions and Mgs in the liquid steel is the primary generation method of MgO·Al₂O₃ spinel inclusions. As the Mgs content in the liquid steel increases, Al₂O₃ inclusions transform into MgO·Al₂O₃ spinel inclusions along the path Al₂O₃ + Mgs → Al₂O₃ with a small amount of MgO + Mgs → MgO·Al₂O₃ spinel. In contrast, the vacuum pressure of 50–100 Pa is more effective at controlling the composition and inclusions of AerMet100 steel and is a more appropriate choice for the refining vacuum pressure.     

Visible Light Transparency for Polycrystalline Ceramics of MgO·2Al2O3 and MgO·2.5Al2O3 Spinel Solid Solutions

Magnesium aluminate spinel solid solutions with the alumina‐rich compositions MgO·2Al₂O₃ and MgO·2.5Al₂O₃ have been prepared as polycrystalline ceramics with average in‐line transmissions at 550 nm of 85.5 ± 0.3% and 80.9 ± 0.4%, respectively. Starting powders are prepared from combinations of high purity Mg(OH)₂ and γ‐Al₂O₃ thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The optimum amount of LiF added varies with the alumina composition of the spinel solid solution. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa in Ar. Both compositions exhibit exaggerated grain growth with average sizes well over 500 μm. Knoop hardness measurements are 11.2 ± 0.3 GPa for MgO·2Al₂O₃ and 11.0 ± 0.4 GPa for MgO·2.5Al₂O₃.     

Slag resistance mechanism of CaO·6Al2O3 refractory and its effect on inclusions of aluminum deoxidized steel

In order to verify the advantage of CaO·6Al₂O₃ (CA₆)-based refractories on the inclusions of aluminum deoxidized steel, the five refractories, CA₆, alumina, spinel, and CA₆-alumina and CA₆-spinel composition refractories were prepared into crucibles, and then the laboratory smelting experiments were conducted. After experiment, the slag resistance of the crucible and the variation on inclusions in steel were characterized and discussed. A dense CaO·2Al₂O₃ (CA₂) layer, which was produced by CA₆ reacting with the slag, was distributed between the original bricklayer and the slag layer, improving the slag resistance of refractories. Meanwhile, the 12CaO·7Al₂O₃ (C₁₂A₇), generated by the reaction between CA₂ and refining slag, would release much Ca into the molten steel. The Ca would react with inclusions to produce low melting point substance to float up and remove, contributing to the reduction of the proportion of large size inclusions. In addition, typical inclusions in steel smelted with CA₆ crucible were small-sized MgO·Al₂O₃ inclusions, whereas those of other crucibles are MnS–MgO·Al₂O₃ composite inclusions with MgO·Al₂O₃ as the core, implying CA₆ may absorb sulfur during the smelting process.     

On the Effect of Lithium on the Energetics and Thermal Stability of Nano‐Sized Nonstoichiometric Magnesium Aluminate Spinel

The thermal stability of Li‐doped nonstoichiometric nano‐sized magnesium aluminate spinel, synthesized using a combustion synthesis method, was studied using XRD, FTIR, and high‐temperature differential scanning calorimetry. Li content within the magnesium aluminate spinel was determined to be a function of crystallite size and stoichiometry. For smaller crystallite sizes and higher Mg deficits, a greater amount of lithium could be incorporated into the structure as a solid solution between LiAl₅O₈ and MgO·nAl₂O₃ spinel, where n is the ratio between Al₂O₃ and MgO. By assessing the intensities of the IR γ_1, γ_2, and γ₅ modes, the degree of structural disorder (i.e., the inversion parameter and lithium occupancy) was defined. The results indicated that the as‐synthesized materials were heavily disordered. The surface enthalpy of the MgO·1.06Al₂O₃, 1.51 ± 0.15 J/m², is in good agreement with the reported value for the same composition, 1.8 ± 0.3 J/m², measured using high‐temperature drop solution calorimetry_. The surface enthalpies of MgO·1.21Al₂O₃ and 0.20 at.% Li–MgO·1.21Al₂O₃ were 1.17 ± 0.15 and 1.05 ± 0.12 J/m, respectively.     

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al₂O₃ slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al₂O₃ slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.     

Charge distribution in nano‐scale grains of magnesium aluminate spinel

Charge distribution in magnesium aluminate spinel (MAS) results in the formation of a space‐charge region that plays a critical role in assigning functional properties. Significant theoretical advances explaining this phenomenon have been accomplished, even though quantitative experimental support from nano‐scale granular MAS is only indirect. In this work, the electrostatic potential distribution in nano‐scale grains of nonstoichiometric MAS (MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃) was measured by off‐axis electron holography (OAEH) and compared to the distribution of cations and defects in this material as measured by electron energy‐loss spectroscopy (EELS). In this manner, we studied the roles of composition, grain size, and applied electric field (EF) on the formation of a space‐charge region. We quantitatively demonstrated that regardless of grain size, the vicinity of MgO·0.95Al₂O₃ grain boundaries presented an excess of Mg⁺² cations, whereas the vicinity of MgO·1.07Al₂O₃ grain boundaries included an excess of Al⁺³ cations. The degree of structural disorder (ie, the inversion parameter, i) indicated that as‐synthesized MAS were significantly disordered (i between 0.37 and 0.41), with values decreasing toward equilibrium ordering values following annealing (i between 0.27 and 0.31). The application of an external ~150 V/cm EF during annealing further enhanced lattice ordering (i between 0.16 and 0.19). Such variations in the distribution of cations and defects should determine the space‐charged potential (SCP). However, using these measurements to calculate the SCP was not possible due to the wide range of values reported for formation energies of defects (0.82‐8.78 eV). Consequently, we correlated local ionic ordering with electrostatic potential in nonstoichiometric MAS. The magnitudes of the SCP in both MgO·0.95Al₂O₃ and MgO·1.07Al₂O₃ decreased following annealing from ?3.4 ± 0.3 V and 2.0 ± 0.2 V to ?2.0 ± 0.2 V and 1.6 ± 0.1 V, respectively.     

Effect of CaO on the optical quality and microstructure of transparent MgO·1.5Al2O3 spinel ceramics prepared by reactive sintering

Transparent MgO·1.5Al₂O₃ spinel ceramics were successfully prepared via reactive sintering of Al₂O₃ and MgO raw powders followed by hot isostatic pressing (HIP) using CaO as the sintering additive. The effects of CaO on the densification process, microstructure and optical quality of samples were investigated. It was found that the amount of CaO played an important role in the sintering process. By adding 0.05?wt% CaO, the sample with high transmittance (82.3% at 400?nm), small grain size (<5?μm) and high strength (228?±?15?MPa) was obtained after HIPing at 1550?°C. However, when the amount of CaO increased to 0.1?wt%, non-cubic and columnar-shaped grains generated at low HIP temperatures (1550–1650?°C), which severely reduced the optical quality of resulting samples. The grains were calcium aluminates, whose formation was closely related to the molar ratio of Al₂O₃/MgO, CaO amount and sintereing temperature.     

Key features of alumina/magnesia/graphite refractories for steel ladle lining

The corrosion resistance of resin bonded alumina/magnesia/graphite refractories containing different kinds of aggregates were investigated when submitted to the action of slags of several CaO/SiO₂ ratios. The laboratory testing was performed by means of the rotary slag attack test. Specifically evaluated was the influence of alumina/carbon ratio and magnesia and silica contents on the refractories corrosion resistance. It was found that this property could be improved by increasing the refractory Al₂O₃/SiO₂ ratio as well as by choosing the appropriate Al₂O₃/C ratio.     

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.     

Microstructures and strengths of microporous MgO-Al2O3 refractory aggregates using two types of magnesite

Six microporous MgO-Al₂O₃ refractory aggregates were prepared with Al(OH)₃ and two different types of magnesite powders via an in-situ decomposition pore-forming technique. One magnesite powder contained more CaO (Magnesite C), the other one contained more SiO₂ and Al₂O₃ impurities (Magnesite SA). Effects of magnesite powder type and content (11 wt.%, 36 wt.% and 77 wt.%) on the phase compositions, microstructures and mechanical strengths of the prepared microporous aggregates were investigated by X-ray diffractometry (XRD), mercury porosimetry measurements, and scanning electron microscopy (SEM), etc. With the addition of 11 wt.% and 77 wt.% magnesite, the type of magnesite had only a small effect on the microporous corundum-spinel and periclase-spinel aggregates, while great influence on the microporous spinel aggregates was observed with 36 wt.% addition. This was mainly because of the sintering process with liquid phase. The best microporous MgO-Al₂O₃ refractory aggregates, which had an apparent porosity of 39.1%, a median pore size of 3.38 μm and a compressive strength of 66.3 MPa, were prepared by using 36 wt.% Magnesite C. This work has practical significance for the efficient utilization of magnesite and the development of energy-saving lightweight MgO-Al₂O₃ refractories.     

The development of a thermodynamic model for Al2O3–MgO refractory castable corrosion by secondary metallurgy steel ladle slags

Alumina magnesia in situ spinel castables are used as ladle refractory lining in the steel industry. In contact with slag, they suffer degradations which limits their performance. The purpose of this article is to predict the thermochemical attack of a slag on alumina magnesia refractory using Factsage^® thermodynamic modeling. To evaluate the reliability of the thermodynamic results, a validation step was carried out, which supported that the database was well adapted to the alumina magnesia spinel system. The corrosion phenomenon was then computed for a simple to a complete system to understand the mechanism and the influence of specific oxides. The model was also compared to corroded microstructures from a steel ladle to evaluate the contribution of each constituent in the castable. The aggregates of alumina react with slag to produce monomineral layers of lime aluminates (CA6 and CA2), while complex spinels (Mg, Fe, Mn)O (Fe₂, Al₂)O₃ are formed from the reaction of the slag with the matrix of the castable. Several oxides (MnO, FeO, Fe₂O₃) from the slag contribute to the formation of the spinel structures. The microstructures of refractories used in steel ladles confirm the main conclusions and the thermodynamic approach.     

Interactions of MgO·Al2O3 spinel with Cu,Cu2O and copper matte at high temperature

Magnesia-chrome refractory has been used in the copper industry for decades, and the chromium in the used refractory has been proved to have the risk to harm the environment. A reliable chromium-free refractory is urged to replace the chromium-containing refractory in the future. In the present study, MgO·Al₂O₃ spinel was systematically tested with Cu, Cu₂O, and industrial matte respectively at 1300?°C. All samples were directly quenched into the water after the experiments and examined by electron probe X-ray microanalysis. The molten Cu and MgO·Al₂O₃ spinel did not react and showed a low wettability, while limited reactions between the spinel and matte were detected. However, severe reactions occurred between the spinel and Cu₂O at 1300?°C. The possible applications of the spinel in the copper-making industry are discussed based on the experimental results.     

Effect of spinel content on hot strength of alumina-spinel castables in the temperature range 1000–1500°C

Hot modulus of rupture of Al₂O₃-spinel castables containing 5–15 wt% alumina-rich magnesia alumina spinel and 1·7 wt% CaO generally increases with increase in spinel content and temperature from 1000 to 1500°C. The magnitudes of hot modulus of rupture of castables containing 15 wt% spinel and 1·7 wt% CaO are 14·3 MPa at 1400°C and 15·6 MPa at 1500°C, while those of castables containing 20 wt% spinel and 1·7 wt% CaO are 12·5 MPa at 1400°C and 14·7 MPa at 1500°C. The former castables contained 15 wt% spinel of −75 μm size, while the latter contained 10 wt% spinel of +75 μm size and another 10 wt% spinel of −75 μm size. The bond linkage between the CA₆ and spinel grains in the matrix is believed to cause both the spinel content and temperature dependence of hot strength of Al₂O₃-spinel castables, as well as fine grain spinel even in amount less than coarser grain spinel to be more effective for enhancing hot strength. The trend of the magnitude of thermal expansion under load (0·2 MPa) above 1500°C of the castables is not necessarily indicative of the magnitude of hot modulus of rupture at 1400 or 1500°C. ©     

Mechanism of in-situ formation of spinel and its effect on the mechanical properties of Al2O3–C refractories

This work looked at the in-situ formation mechanism of magnesia alumina spinel in Al₂O₃–C refractories with magnesia addition at different firing temperatures. A comprehensive study on the mechanical properties of Al₂O₃–C refractories was performed in comparison to traditional analogs. The magnesia alumina spinel was in-situ formed at the firing temperature of 1150 °C in Al₂O₃–C refractories. With the increase of the firing temperature, the Al₂O₃ phase was gradually dissolved in spinel phase to form aluminum rich spinel phase, resulting in a decrease in its lattice constant due to the defects formation. The formed spinel phase was homogenously distributed and bonded well with corundum, improving the interfacial bond, load transferring capacity and crack propagation resistance. The formation of spinel phase also enhanced the sintering of the alumina matrix owing to the solid solution of alumina in the spinel. Therefore, the mechanical properties such as cold modulus of rupture and hot modulus of rupture in Al₂O₃–C refractories achieved a substantial enhancement compared with traditional refractories.     

Pore evolution of microporous magnesia aggregates with the introduction of nano-sized MgO

Microporous refractories applied in the working-lining of metallurgical furnaces have been rapidly developed in recent years owing to the outstanding mechanical properties, thermal insulation performance and slag resistance, the pore structure of which plays a critical role in the variation of service performance. Meanwhile, the microporous magnesia aggregates were prepared in our previous research with the introduction of nano-sized particles to overcome the shortcomings of high thermal conductivity, poor thermal shock resistance and slag penetration resistance, however, the pore evolution during sintering still remains to be investigated. Hence, in this study, the pore evolution of microporous magnesia aggregates is explored specifically and the effect of nano-sized MgO on pore structure and sintering is simultaneously discussed. The sintering model of microporous magnesia was built for analyzing the pore structure evolution process. The results revealed that a micro-nano double-scale sintering model developed by the introduction of nano-sized MgO dramatically promoted the sintering kinetic force and boundary migration velocity. The sintering pressure discrepancy and free energy change per unit mole of specimens were respectively increased by ～43 times and ～48 times, which effectively improved the closed porosity and pore distribution homogeneity, while reduced the pore size. Meanwhile, the high sintering diving force lead to the significant improvement of direct bonding degree and grain size of microporous magnesia. With the addition of 3 wt% nano-sized MgO, the optimal sintering properties with closed porosity of 6.4%, bulk density of 3.23 g/cm³ and median equivalent pores diameters of 4.07 μm were achieved. The exploration of pore evolution in microporous magnesia aggregates contributed to the fabrication and industrialization development of microporous refractories.