Experiment procedure
Use tabular corundum, fused corundum, sintered aluminum-magnesium spinel, fused magnesia, α-Al2O3 micropowder, silica micropowder and pure calcium aluminate cement, etc. Drying at 110℃, firing at 1000℃×3h and 1500℃×3h.
Determine the apparent porosity, bulk density, compressive strength, flexural strength, linear change rate and 1400℃×1h thermal flexural strength of the sample after treatment at different temperatures according to GB; test the linear change rate of the sample under load, the pressure is 0.196 MPa, the heating rate is 10℃/min, the maximum temperature is 1500℃, and the temperature is kept for 3h; the slag resistance experiment adopts the crucible method, and the final slag of the converter (wCaO36.84%, wSiO214.77%, wAl2O328.17%, wFeO7.95%, wMnO4 .58%) 150g was put into the crucible, and after being treated in a MoSi2 rod electric furnace at 1650℃×3h, it was cut along the central plane of the crucible to measure the corrosion and penetration depth of the crucible by slag; chemical analysis, optical microscope, x-Ray diffraction, The samples after slag corrosion are analyzed by means of electronic probes.
Results and analysis
3.1 Properties 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 improve its permeability resistance and thermal shock stability. 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 to form spinel at high temperature, accompanied by a volume expansion of about 7%. In order to suppress the spalling damage caused by this expansion stress, the effects of two different raw materials, fused magnesia and magnesia-alumina spinel, on the slag resistance of the material were studied experimentally. 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, it is subjected to the hydrostatic pressure of molten steel, the reaction sintering is advanced, and the loose body of spinel expansion is promoted to be more densified. Magnesia can make the castable still show micro-expansion at high temperature, maintain the integrity, and is also beneficial to reduce corrosion loss. However, the coarser the critical particle size of magnesia, or the addition of more than 4C, the larger the expansion, the deterioration of the structure, the deepening of the slag penetration, and the tendency of the corrosion loss to increase.
The introduction of pre-synthetic spinel to replace fused magnesia, the research believes that the more theoretical spinel content, the better the corrosion resistance of the castable, and the slag penetration depth is the smallest when the spinel content is 10% to 30%, and the spinel content is 10% to 30%. When the content exceeds 50%, it shows an upward trend with the increase of spinel content. The spinel particle size with uniform distribution of fine powder is the most effective for blocking the structural spalling caused by slag infiltration. The study found that the spinel component plays a decisive role in the slag resistance of the spinel clinker itself and the castable mixed with corundum, and the MgO in the spinel is ideal at 3% to 5%. The silica micropowder is also effective in inhibiting the formation of spinel expansion stress. Studies have shown that at low temperature, silicon micropowder and MgO powder form MSH substance, which can prevent periclase hydration, improve the fluidity of castables, and increase the density of castables. Absorb high-temperature expansion stress, however, the amount of silicon micropowder added increases, the liquid phase formation increases at high temperature, and the high-temperature creep resistance decreases. As shown in Figure 2, the material is prone to over-sintering and cracking under the molten steel pressure. increase, cracks widen, and spalling deepens. Generally, cement and silica fume composite binder are used.
Appropriate amount of high alumina cement hydrate is dehydrated to form CA series of highly active substances, which are easy to chemically react with the added Al2O3 powder from about 1000 °C.
In conclusion, both Al-spinel castables and Al-Mg castables have good microstructure uniformity, high temperature creep resistance, thermal shock stability, and resistance to slag erosion and penetration. The main difference between the two is that the former introduces pre-synthesized spinel, which has low strength after firing at different temperatures, high high-temperature flexural strength, good volume stability, and small linear change rate; the latter reacts to form spinel when used at high temperature, and different It has high strength after burning at high temperature, strong creep resistance at high temperature, compactness and large linear change rate.
3.2 Damage of high-purity aluminum-magnesium castables
The aluminum-spinel castable and the aluminum-magnesium castable are essentially the same system at high temperature, and the main crystalline phases are corundum and aluminum-rich spinel. The factors affecting the slag resistance of castables are very complex, such as steel grade, slag composition, smelting conditions, etc., but they are mainly controlled by the mineral composition and microstructure of the castable. The FeO and MnO of the aluminum-rich spinel capture slag first occupy the cation holes, 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 in the same region fine The solid solubility of Fe and Mn in the spinel of the particles is roughly the same, while the content of Fe and Mn elements in the edge of the larger spinel particles is much higher than that in the inside of the particles. The analysis also shows that the lattice constant of spinel gradually decreases from the side of the working face to the inside, which is consistent with the change of Fe2O3 content in each layer. The strength is closer to the spinel of the original layer.
Corundum absorbs CaO in the slag to form calcium aluminate minerals and solidifies. Optical microscope observation shows that there is a plate-like calcium aluminate reaction circle at the edge of the corundum particles in the permeable layer of the sample, and there are a large number of needle-like CA6 minerals in the matrix. SiO2 promotes CA6 When the crystal grows, the pores become finer, forming a denser barrier layer, and the residual slag is rich in SiO2 and becomes viscous and difficult to penetrate.
Different from the aluminum-spinel castable, although the aluminum-magnesium castable forms more liquid phases at high temperature, the spinel newly formed by the reaction of MgO and Al2O3 has fine grains, many defects and small lattice constants. The spinel is more finely divided, which promotes the solid solution of Al2O3 into the spinel, forming an aluminum-rich spinel with a larger concentration of lattice defects, and the castable is also denser. Therefore, the slag resistance, especially the resistance to slag penetration ability is better. Microscopic observation shows that the composite spinel grains in the altered layer of the sample are completely developed and are euhedral, with a grain size of about 15-40 mm, and some are up to 120 mm. The solid solubility of FeO and MnO in the spinel is greatly increased. Composition Mg0.68Mn0.17Fe0.47Al1.79O4.
In conclusion
(1) Reasonable selection of the mixing amount of spinel, magnesia, silica micropowder and cement, and control of the ideal microstructure are essential to obtain high-purity aluminum-magnesium castables with stable performance.
(2) Although the developed high-purity Al-Mg castables have different properties, they all have good microstructure uniformity, high temperature creep resistance, thermal shock stability, and slag erosion and penetration resistance.
(3) Anti-slag mechanism of high-purity aluminum-magnesium castables: Spinel captures FeO and MnO in slag to occupy its cation holes, replaces MgO to form composite spinel, corundum absorbs CaO to generate CA2, CA6, SiO6 promotes CA6 crystal The grains grow up to form a denser barrier layer, and the residual slag is rich in SiO2 and thickens, thereby improving the resistance to slag penetration and erosion. Due to the reaction of MgO and Al2O3, the newly formed spinel in the aluminum-magnesium castable has fine grains and many defects. Aluminum spinel, therefore, its slag resistance is stronger than that of aluminum-spinel castables.
