What is corundum?

Corundum (Al2O3) has abundant raw material reserves, accounting for about 25% of the weight of the earth’s crust. It is inexpensive and has many excellent properties. There are many different crystals of Al2O3, and there are more than ten kinds of variants reported, but there are three main ones, namely α-Al2O3, β-Al2O3, and γ-Al2O3.

Tabular corundum

γ-Al2O3 is a spinel structure, which is unstable at high temperatures and is rarely used as a single material. β-Al2O3 is essentially an aluminate containing alkali metals or alkaline earth metals. Its chemical composition can be approximated by RO·6Al2O3 and R2O·11Al2O3, hexagonal lattice, density 3.30~3.63g/cm3, 1400~1500 It starts to decompose at ℃ and transforms into α-Al2O3 at 1600℃. α-Al2O3 is a high-temperature form, with a stable temperature as high as the melting point, and a density of 3.96~4.01g/cm3, which is related to the impurity content. The unit cell is a sharp prism, which exists in the form of natural corundum, ruby and sapphire in nature. α-Al2O3 has compact structure, low activity, good electrical properties, and excellent mechanical properties. The Mohs hardness is 9. α-Al2O3 belongs to hexagonal crystal system, corundum structure, a=4.76, c=12.99.

Al2O3 has high mechanical strength. The purer the Al2O3 composition, the higher the strength. The mechanical strength can be used to make device porcelain and other mechanical components. The resistivity of Al2O3 is high, the electrical insulation performance is good, the resistivity at room temperature is 1015Ω·cm, and the dielectric strength is 15kV/mm. Using its insulation and strength, it can be made into substrates, sockets, spark plugs, circuit shells, etc. Al2O3 has high hardness, Mohs hardness of 9, plus excellent wear resistance, so it is widely used to manufacture tools, grinding wheels, abrasives, drawing dies, bearings, bearing bushes and artificial gems. Al2O3 has a high melting point and corrosion resistance. It has a melting point of 2050°C. It has good resistance to the erosion of molten metals such as Be, Sr, Ni, Al, V, Ti, Mn, Fe, CO and sodium hydroxide, glass, and slag. It also has high resistance; it does not interact with Si, P, Sb, Bi in an inert atmosphere, so it can be used as refractory materials, furnace tubes, glass drawing crucibles, hollow balls, fibers and thermocouple protective covers, etc.

Al2O3 has excellent chemical stability. Many complex sulfides, phosphides, arsenides, chlorides, nitrides, bromides, iodides, dry fluorides, sulfuric acid, hydrochloric acid, nitric acid, and hydrofluoric acid do not interact with Al2O3. Therefore, it can be made into pure metal and single crystal growth crucibles, human joints, artificial bones, etc. Al2O3 has optical properties and can be made into light-transmitting materials to make Na vapor lamp tubes, microwave fairings, infrared windows and laser oscillation components. The ionic conductivity of Al2O3 is used as a material for solar cells and storage batteries. Al2O3 is also commonly used in ceramic surface metallization technology.

The main crystalline phase of alumina-based fused corundum is the corundum phase with a size of 1.0-1.5mm and interlaced crystals. The rest are trace amounts of rutile, alumina and aluminum titanate, and are located inside the corundum phase or between the crystal phases. A small amount of glass phase. In China, after more than ten years of unremitting efforts, the smelting process of bauxite-based fused corundum has made great progress, with an annual production capacity of more than 110,000 tons. Bauxite-based fused corundum has been successfully used as a raw material for various fired bricks and unshaped refractory materials. For example, it can partially replace dense corundum in blast furnace castables, and is used as a matrix material and granular material to produce low creep. High alumina bricks are used to replace white corundum in other Al2O3-SiO2 refractories to prepare high-performance products.

Brown corundum smelting is based on the basic principle that aluminum has a greater affinity for oxygen than iron, silicon, titanium, etc. By controlling the amount of reducing agent, the main impurities in bauxite are reduced by reduction smelting, and the reduced impurities form ferrosilicon alloys. It is separated from the corundum melt to obtain brown corundum with crystal quality meeting the requirements and Al2O3 content greater than 94.5%. Fe2O3 is reduced to produce ferrosilicon alloy and removed during the smelting process, but a small amount of iron oxide and alumina produced spinel is still left in the product. TiO2 is partially reduced into the ferrosilicon alloy during the smelting process, and a considerable part of it remains in the brown corundum, which is the main factor in the coloring of the brown corundum. CaO and MgO are difficult to reduce during the smelting process, and most of the CaO and MgO in the raw materials still exist in the product. Although Na2O and K2O can volatilize at high temperature during the smelting process, they cannot be reduced and remain in the brown corundum, which has a great impact on the quality.

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The raw material of brown corundum is composed of α-alumina crystal grains and a small amount of glass phase, α-alumina crystals are composed of Al2O3 solid solution containing Ti2O3, and the glass phase is mostly composed of titanium dioxide and silicon dioxide and other trace oxidation existing in the electric arc furnace.物组合。 Material composition. These oxides constitute the glass phase, and they have only low solubility in the crystal structure of the alumina grains. Ti2O3 is the only oxide that Ti can dissolve in the alumina grains. TiO2 is the thermodynamically stable oxide of Ti. During the smelting and reduction of brown corundum, part of TiO2 is reduced to sub-oxidation of titanium. (Ti2O3), above 1000℃, oxygen can diffuse into the Ga-alumina grains, oxidize Ti2O3 into more stable TiO2 and then wrap it in the α-alumina grains, so most of the titanium dioxide is α-alumina A solid solution of crystal grains exists.

The excessive TiO2 in brown corundum cannot remain in the glass phase, but reacts with alumina to form aluminum titanate (TiO2·Al2O3). Aluminum titanate is the third phase at the interface between α-alumina grains and the glass phase; The toughness of brown corundum increases with the growth of TiO2 crystal nuclei. The TiO2 phase uniformly dispersed in the α-alumina crystal grains toughens the α-alumina particles. Brown corundum solid solution Ti2O3 causes brown corundum to appear blue.

Ti2O3 is the only oxide that Ti can dissolve in the alumina grains. TiO2 is the thermodynamically stable oxide of Ti. During the smelting and reduction of brown corundum, part of TiO2 is reduced to sub-oxidation of titanium. (Ti2O3), above 1000℃, oxygen can diffuse into the Ga-alumina grains, oxidize Ti2O3 into more stable TiO2 and then wrap it in the α-alumina grains, so most of the titanium dioxide is α-alumina A solid solution of crystal grains exists. The excessive TiO2 in brown corundum cannot remain in the glass phase, but reacts with alumina to form aluminum titanate (TiO2·Al2O3). Aluminum titanate is the third phase at the interface between α-alumina grains and the glass phase; The toughness of brown corundum increases with the growth of TiO2 crystal nuclei. The TiO2 phase uniformly dispersed in the α-alumina crystal grains toughens the α-alumina particles. Brown corundum solid solution Ti2O3 causes brown corundum to appear blue.