1. Performance and main influencing factors of high purity aluminum magnesium castables
High purity aluminum-magnesium castables are developed based on aluminum and magnesium castables. The purpose is to improve the corrosion resistance and high temperature performance of aluminum castables, as well as the permeability resistance and thermal shock stability of magnesium castables. Its batching point falls on the aluminum side of the MgO-Al2O3 binary phase diagram. The main component of the castable, Al2O3, reacts with MgO at high temperature to form spinel, accompanied by a volume expansion of about 7%. In order to suppress the spalling damage caused by this expansion stress during use, the experimental study selected two different raw materials, fused magnesia and magnesium-aluminum spinel, to study the influence of the material's slag resistance. The results show that when a certain amount of magnesia is added, the castable is lubricated in a small amount of liquid phase, especially when it is used under the static pressure of molten steel, the reaction sintering is advanced, and the loose body formed by the expansion of spinel becomes more compact. Magnesia with appropriate particle size can make the castable still show slight expansion at high temperature, maintain integrity, and is also beneficial to reduce corrosion. However, the coarser the critical particle size of magnesia or the amount added exceeds 4C, the corrosion tends to increase due to excessive expansion, organizational degradation, and deeper slag penetration.
Presynthesized spinel is introduced to replace fused magnesia. The study believes that the more theoretical spinel content, the better the corrosion resistance of the castable, while the slag penetration depth is the smallest when the spinel content is 10% to 30%, and the spinel content exceeds 50%, and it increases with the increase of spinel content. The spinel particle size with uniform distribution of micro powder is most effective in blocking the structural peeling caused by slag penetration. The study found that the spinel component plays a decisive role in the slag resistance of the spinel clinker itself and the castable made of spinel mixed with corundum. The ideal MgO content in spinel is 3% to 5%. Silicon powder is also effective in inhibiting the expansion stress of spinel. Studies have shown that at low temperatures, silicon powder and MgO powder generate M-S-H substances, which can prevent periclase hydration, improve the fluidity of castables, and increase the density of castables. At high temperatures, M-S-H dehydrates and reacts with CaO to generate low-melting products to produce plastic deformation and absorb high-temperature expansion stress. However, as the amount of silicon powder increases, the amount of liquid phase generated at high temperatures increases, and the high-temperature creep resistance decreases. Under the conditions of molten steel pressure, the material is prone to over-sintering and cracking, with more cracks, wider cracks, and deeper peeling. Cement and silicon powder are generally used as a composite binder.
In short, aluminum-spinel castables and aluminum-magnesium castables have good organizational uniformity, high-temperature creep resistance, thermal shock stability, and slag erosion and penetration resistance. The main difference between the two is that the former introduces pre-synthesized spinel, and the strength after firing at different temperatures is low, the high-temperature flexural strength is large, the volume stability is good, and the linear change rate is small; the latter reacts to form spinel when used at high temperature, and the strength after firing at different temperatures is large, the high-temperature creep resistance is strong, the density is dense, and the linear change rate is large.
2. Damage of high purity aluminum-magnesium castables
Aluminum-spinel castables and aluminua magnesium castables are essentially the same system at high temperatures, and the main crystal phases are corundum and aluminum-rich spinel. The factors affecting the slag resistance of castables are very complex, such as steel type, slag composition, smelting conditions, etc., but are mainly controlled by the mineral composition and microstructure of the castables. The FeO and MnO in the aluminum-rich spinel captured slag first occupy the cation vacancies and replace part of the MgO to form a composite spinel solid solution with a typical composition of Mg0.70Mn0.08Fe0.21Al2.00O4. Electron probe analysis shows that the solid solubility of Fe and Mn in fine-grained spinel in the same area is roughly the same, while the content of Fe and Mn elements at the edge of larger spinel particles is much higher than that in the interior of the particles. Chemical analysis and X-ray diffraction analysis of the samples after slag corrosion also show that the lattice constant of the spinel gradually decreases from the working surface to the inside, which is consistent with the change of Fe2O3 content in each layer. As the solid solubility of Fe2O3 in each layer decreases, the lattice constant and diffraction intensity of the spinel become closer to the spinel of the original layer. Corundum absorbs CaO in the slag to generate calcium aluminate minerals and solidify. Optical microscope observation shows that there are plate-like calcium aluminate reaction circles on the edges of the corundum particles in the sample penetration layer, and there are a large number of needle-like CA6 minerals in the matrix. SiO2 promotes the crystallization and growth of CA6, which makes the pores finer and forms a denser barrier layer. The residual slag is rich in SiO2 and becomes viscous and difficult to penetrate. Unlike aluminum-spinel castables, although aluminum-magnesium castables form more liquid phases at high temperatures, the newly formed spinel grains of MgO and Al2O3 are fine, have many defects, and have a small lattice constant. SiO2 makes the spinel more fragmented, promotes the solid solution of Al2O3 in the spinel, and forms aluminum-rich spinel with a higher lattice defect concentration. In addition, the castable is also denser, so the slag resistance, especially the resistance to slag penetration, is superior.
