China Metallurgical Encyclopedia: Refractories

Author: Metallurgical Industry Press
Publisher:
Publish Date: 1997-10-01
Features:
Fragment: AAl2O3jing×ingZhuanbanAl22O3 crystal transformation (transformation of Al2O3) The transformation between various crystal forms of Al2O3. The crystal forms of Al2O3 include α, γ, η, δ, θ, κ, χ, etc. When external conditions change, the crystal form will transform. Among these Al2O3 variants, only α-Al2O3 (corundum) is stable, while other crystal forms are unstable and will transform into α-Al2O3 upon heating. This is because the oxygen in α-Al2O3 is already closely packed. The density of α-Al2O3 is 3.99 g/cm3. Besides corundum, the common crystal forms of Al2O3 are γ-Al2O3. γ-Al2O3 has a spinel-type structure. However, in its structure, some tetrahedral voids are not filled, so the density of γ-Al2O3 is lower than that of corundum. The density of γ-Al2O3 is 3.65 g/cm3. When various Al(OH)3 are heated to dehydrate, γ-Al2O3 is formed at about 450°C. γ-Al2O3 transforms into corundum upon heating to higher temperatures. However, this transformation only occurs at a relatively high rate above 1000°C. Other unstable crystal forms of aluminum oxide are also formed from the dehydration of Al(OH)3 under different conditions. ρ-Al2O3 should be amorphous, but some also consider it a transition state between amorphous and crystalline forms. Since ρ-Al2O3 is the only form of Al2O3 that can spontaneously hydrate at room temperature, it can be used as a binder for refractory castables, and has received attention in recent years. β-Al2O3 (density 3.31 g/cm3) is not pure Al2O3 and does not belong to the Al2O3 monosystem; its chemical formula is Na2O·11Al2O3. Because Na2O was overlooked when β-Al2O3 was first discovered, it was mistakenly considered a variant of Al2O3, and the name β-Al2O3 was adopted and used to this day. When corundum is in a high-temperature, alkali metal atmosphere, it can transform into β-Al2O3. β-Al2O3 also transforms into corundum upon the escape of alkali metal oxides at high temperatures. (Chen Zhaoyou) Al2O3-SiC-C zhuan Al2O3-SiC-C brick (alumina-silicon carbide-carbon brick) This refractory product is fired using fused corundum (or sintered corundum, high-grade alumina clinker), silicon carbide, and flake graphite as main raw materials. This type of product has excellent slag resistance, thermal shock resistance, and hot metal erosion resistance. The performance of Al2O3-SiC-C bricks for torpedo cars (examples) is shown in the table. Al2O3-SiC-C bricks are made from fused corundum (or sintered corundum, high-grade alumina clinker), flake graphite containing 90%–95% fixed carbon, and silicon carbide as main raw materials, with phenolic resin as a binder, and are produced through processes such as mixing, forming, and heat treatment. Process requirements: The mixing error should be small, especially for components with an addition amount less than 50%, with a mixing error of ±0.2%; mixing should use a forced mixer, and the specified mixing time must be guaranteed; forming should use a large-tonnage friction press or hydraulic press with a forming pressure of about 150 MPa; the temperature control system in the heat treatment furnace should be easily adjustable. This product is mainly used as the inner lining material for hot metal pretreatment containers and ladle tuyeres. (Wu Xuezhen) Al2O3-SiO2 xiangtu Al2O3-SiO2 system phase diagram (phase diagram of Al2O3-SiO2 system) The phase equilibrium diagram of the system composed of Al2O3-SiO2. The Al2O3-SiO2 binary system phase diagram is very important for both ceramics and refractories. There have been some disagreements in the long-standing understanding of this binary system phase diagram. The main disagreements are: whether the formed mullite compound is monotectic or eutectic; whether mullite forms a solid solution and the range of solid solution. The reasons for these disagreements are related to experimental conditions, the presence of impurities, and the production of SiO from SiO2 gasification at high temperatures. Figure 1 shows two different Al2O3-SiO2 system phase diagrams commonly cited in the literature. In Figure 1a, mullite is a eutectic compound, while in Figure 1b, mullite is a monotectic compound. Although there are disagreements in the understanding of the Al2O3-SiO2 system phase diagram, there is little disagreement on the side of the phase diagram with a high SiO2 content. Additionally, the current consensus is that mullite can form a solid solution, with an Al2O3 content of 71.8%–77.4%, which is equivalent to the molecular formula 3Al2O3·2SiO2 (A3S2), and there is a solid solution region between mullite and corundum. From the Al2O3-SiO2 system phase diagram, we can know: (1) The composition range with a melting point below 1650°C is narrow, so most compositions of this binary system can be used as refractories. The approximate composition ranges of various named aluminosilicate refractories are shown in Figure 1a. (2) Near the SiO2 end member, where the Al2O3 content is 0–5.5%, the liquidus line is steep, indicating that the melting point will drop sharply when a small amount of Al2O3 is added to SiO2. When SiO2 contains 1% Al2O3, the liquid phase content reaches 18% at the eutectic temperature. This is why the Al2O3 content in silica bricks is required to be low, and why silica bricks cannot come into contact with semi-siliceous, clay, or high-alumina bricks during use. (3) When the Al2O3 content is between 2% and 15%, it cannot be used as a refractory due to its low melting point. (4) When the Al2O3 content is between 15% and 50%, the liquidus line is relatively flat, and a slight increase in temperature leads to a significant increase in liquid phase content. (5) When the Al2O3 content is less than 71.8%, the equilibrium mineral phase is mullite and quartz or tridymite, with a eutectic point. Therefore, Al2O3-SiO2 refractories with an Al2O3 content below this range typically have a load softening start temperature that does not exceed the eutectic point temperature. (6) Al2O3-SiO2 refractories with an Al2O3 content greater than 71.8% have higher refractory properties. Mullite has many excellent properties. Mullite bricks and mullite fibers are mainly composed of the mullite mineral, making them excellent refractory materials for some high-temperature furnaces.