Several Factors Affecting the Performance of Alumina-Magnesia Refractory Castables for Steel Ladles
Alumina-magnesia castables—a type of refractory castable—are widely used as linings for steel ladles. Refractory specialists both in China and abroad have conducted extensive research on these materials, focusing on how various components—such as aggregates, magnesia, corundum fines and ultrafines, and silica fume—influence their performance.
Selection of Aggregates
Aggregates serve as the structural framework within refractory materials. The alumina-based aggregates used in alumina-magnesia castables primarily include fused alumina, sintered alumina, and natural bauxite clinker.
Studies comparing the effects of high-alumina bauxite clinker, high-purity fused corundum, and electro-fused bauxite-corundum on the properties of alumina-spinel castables have shown that castables utilizing fused high-alumina corundum exhibit the best flowability, while those based on bauxite-spinel demonstrate the poorest thermal shock stability.
Research into the selection of aggregates for steel ladle alumina-magnesia castables—specifically comparing fused white corundum, brown corundum, and premium-grade bauxite clinker—indicates that samples containing brown corundum aggregates offer the best flowability. In terms of thermal shock stability, samples with white corundum aggregates perform best, followed by those with brown corundum, while samples using bauxite clinker perform the worst.
An analysis of replacing “Grade 88” high-alumina material with “Grade 88” homogenized material, or with “Grade 85” homogenized high-alumina material, suggests that substituting higher-grade high-alumina materials with “Grade 85” homogenized material—specifically in the 3–1 mm and 1–0 mm aggregate fractions as well as the fine powder fraction—offers greater economic viability and feasibility.
Selection of Corundum Fines
The Al₂O₃ present in corundum fines can react with CaO in molten slag to form high-melting-point phases, specifically CA₂ and CA₆. Furthermore, alumina-rich spinel—which contains cationic vacancies—can capture FeO and MnO from the slag, forming (Mg, Fe, Mn)O·Al₂O₃. This process increases the SiO₂ content and the viscosity of the slag, thereby enhancing the material’s resistance to slag erosion and penetration.
Research into the effect of alumina fines on spinel formation in alumina-magnesia castables has shown that when a castable contains reactive alumina, the magnesia acts as a substrate for spinel formation; the reactive alumina provides an abundant supply of Al₂O₃, which diffuses to form magnesium-rich spinel. Conversely, because sintered alumina possesses lower reactivity, the interdiffusion between alumina and magnesia requires more time. Given the nature of the contact interface during this diffusion process, alumina-rich spinel is formed, which offers superior slag resistance. Moreover, as sintered alumina is more cost-effective than reactive alumina, it is the more advantageous choice. Additionally, incorporating an appropriate amount of reactive α-Al₂O₃ micro-powder into alumina-magnesia castables increases the density of the material, further improving its resistance to slag. A high-purity alumina-magnesia castable bonded with ρ-Al₂O₃ was also developed; it was found that, compared to castables bonded with pure calcium aluminate cement, the matrix of the ρ-Al₂O₃-bonded castable is significantly purer, which extends the service life of the material.
Proportioning for Spinel
The chemical formula for magnesium-aluminum spinel is MgO·Al2O3, with a theoretical composition of 28.3% MgO and 71.7% Al2O3. The proportioning used to synthesize magnesium-aluminum spinel—specifically the molar ratio of MgO to Al2O3—directly influences the properties of the resulting material. Magnesium-rich spinels, which typically have an MgO/Al2O3 molar ratio greater than 1, exhibit a relatively dense structure. In contrast, aluminum-rich spinels contain a higher density of intra-crystalline pores; this is likely due to the differing growth mechanisms occurring on the MgO-rich and Al2O3-rich sides during the spinel formation process. In industrial practice, the MgO/Al2O3 ratio for synthetic spinel is determined by its intended application. Magnesium-rich spinel is commonly used for the kiln linings of large-scale cement kilns. Aluminum-rich spinel is frequently combined with Al2O3-based raw materials to produce castables or precast shapes for large- and medium-sized steel ladles, as well as specialized components such as porous plugs and seating blocks. Meanwhile, bauxite-based spinel castables—synthesized from natural raw materials—are primarily used for small-scale steel ladle applications.
Magnesia Content
As the primary raw material for the in-situ formation of spinel in alumina-magnesia castables, magnesia (MgO) significantly influences the strength, volume stability, slag resistance, and thermal shock resistance of the specimens. Furthermore, because MgO is prone to hydration, the magnesia content also has a notable impact on the workability (construction properties) of the castables.
Research into the effect of fine magnesia powder content on the slag resistance of alumina-magnesia castables indicates that as the amount of added magnesia increases, the material’s resistance to slag erosion decreases, while its resistance to slag penetration improves. However, an excessive amount of magnesia leads to a loose material structure, which ultimately results in a decline in overall slag resistance.
Studies on the influence of fused magnesia content on the properties of unfired alumina-magnesia materials have shown that the reaction to form spinel begins at 1050°C. By adjusting the particle size and the quantity of added magnesia, it is possible to effectively prevent issues such as spalling, swelling, and slag penetration. Leveraging China’s abundant high-alumina bauxite resources—and taking into account the practical operational capabilities of small and medium-sized steel mills—refractory castables in the Al2O3-MgO system have been developed using premium-grade calcined high-alumina bauxite and fused magnesia as the primary raw materials. Research demonstrates that increasing the magnesia content within the matrix enhances the matrix’s resistance to erosion, though it simultaneously reduces the castable’s resistance to penetration.
Silica Fume
Silica fume has a significant impact on the properties of alumina-magnesia castables. Specifically, its inclusion can substantially reduce the required water content, inhibit the hydration of magnesia (periclase), enhance fluidity, and improve workability during installation. On one hand, silica fume can react with MgO, CaO, and Fe2O3 present in the material to form low-melting-point phases—such as anorthite, gehlenite, forsterite, and merwinite—which adversely affect high-temperature performance. On the other hand, it can be used to regulate the volume expansion associated with spinel formation and accelerate the spinel-forming reaction itself.
Research into the relationship between silica fume content and the phase composition and properties of high-purity alumina-magnesia castables indicates that an excessive amount of silica fume leads to the infiltration of a liquid phase—formed at high temperatures—into the spaces between corundum, periclase, and spinel grains. This causes “over-firing” of the castable, resulting in cracking and spalling. Conversely, the addition of silica fume can enhance the cold strength of the castable across various temperature ranges and reduce its residual expansion. In the development of bauxite-based ladle castables—utilizing premium-grade bauxite and fused magnesia as primary raw materials and employing an MgO-SiO2-H2O bonding system—the amount of silica fume added should be minimized, with the limit being the point at which it would otherwise cause a significant increase in the water required for the mix.
