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aluminum oxide:see aluminaalumina
or aluminum oxide,
Al2O3, chemical compound with m.p. about 2,000°C; and sp. gr. about 4.0. It is insoluble in water and organic liquids and very slightly soluble in strong acids and alkalies. Alumina occurs in two crystalline forms.
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alumina, A1203; a compound of aluminum with oxygen; a component of clays and a raw product for the production of aluminum. Colorless crystals with a melting point of 2,050°C and a boiling point above 3,000°C.
Aluminum oxide is known in two modifications, α and γ. Of these two, α-Al2O3 is found in nature in the form of the colorless mineral corundum. Crystals of α-Al2O3 which have been stained red or blue by the oxides of other metals are the precious stones ruby and sapphire. Corundum crystallizes in a hexagonal system with a density of 3,960 kg/m3. It is possible to obtain α-Al203 artificially by heating aluminum hydroxide or its salts above 900°C. In heating aluminum salts within the limits of 600–900°C, γ-Al2O3 is formed; it is a cubic modification which changes irreversibly into α-Al2O3 when heated above this temperature. Hydrated (hydrous) forms of Al2O3 of varying composition are also known. Among the aluminum hydroxides are hydrargillite (gibbsite) Al(OH)3, which is found in many bauxites, and the artificially produced unstable form of Al(OH)3, bayerite. There is also an incomplete aluminum hydroxide, AlOOH which exists in two modifications—α (diaspore) and γ (boehmite).
Aluminum oxide and its hydrated forms are insoluble in water and possess amphoteric properties; that is, they react with both acids and alkalies. In the air, natural corundum is chemically inert and nonhygroscopic. Aluminum oxide reacts intensively with alkalies at approximately 1,000°C, forming water-soluble aluminates of the base metals. It reacts more slowly with Si02 and acid slags, with the formation of aluminosilicates, and it decomposes in an alloy with KHSO4.
Bauxites, nephelites, kaolins, and other raw materials containing aluminum are used as the stock for obtaining aluminum oxide. Bauxites are always contaminated with iron oxides or silicic acid. In order to obtain pure aluminum oxide, the bauxites are processed by heating with CaO and Na2CO3 (the dry method) or with caustic soda in autoclaves (the Bayer process). In both methods the aluminum oxide in the form of aluminates is converted into a solution, which is then broken down by running it through carbon dioxide or adding prepared aluminum hydroxide. In the first case, the decomposition occurs according to the equation
2[Al(OH)4]− + CO2→2Al(OH)3 + C032− + H20
Decomposition according to the second method is based on the fact that the aluminate solution obtained by autoclaving is metastable. The added aluminum hydroxide accelerates the decomposition of the aluminate:
[AL(OH)4]−→Al(OH)3 + OH−
The aluminum hydroxide obtained is roasted at 1,200°C and pure alumina is obtained as a result.
The basic use of aluminum oxide is in aluminum production. Corundum is widely used as an abrasive material (corundum wheels and emery), as well as for manufacturing ceramic cutting tools and extremely fire-resistant materials—in particular the “fused alumina” which is used for lining cement kilns. Bearing stones in precision mechanisms and jewelry are made from corundum monocrystals obtained by melting powdered aluminum oxide with the addition of chromium, iron, titanium, and vanadium oxides.
“Hairs” (filamentary crystals) of aluminum oxide are formed by the distillation of pure aluminum at 1,650°C in a hydrogen atmosphere containing water vapor. These “hairs” possess enormous strength, close to the theoretical. Sapphire “hairs” (α-Al2O3) 2–3 microns in diameter possess a strength of 16 henrys per square meter (H/m2), and those with a diameter of 10 microns have a strength of 11 H/m2. “Hairs” of larger diameter have a strength of 6.5–7 H/m2 (1 H/m2 = 100 kilogram forces per square meter). The introduction of these “hairs” into structural materials, even under the condition of a partial maintenance of their strength, makes it possible to obtain valuable materials for missile construction. Metals reinforced with such fibers have greater strength at high as well as low temperatures.
A specially prepared aluminum oxide, known as active aluminum oxide, is used in the form of a fine crystallic powder as an adsorbent and catalyst. Its adsorbent and catalytic properties depend to a great degree upon the quality and processing of the initial materials and upon the method of preparation. As an adsorbent, active aluminum oxide is widely used for chromatographic analysis of all types of organic and (more rarely) inorganic substances. Aluminum hydroxides are also used for producing all possible aluminum salts. Careful drying of gelatinous hydroxide produces alumogel; this is a porous, porcelain-like substance which is sometimes transparent. Alumogel is used in catalysis and serves as one of the most important industrial adsorbents.
REFERENCESLainer, A. I. Proizvodstvo glinozema. Moscow, 1961.
Carroll-Porczynski, C. Materialy budushchego. Moscow, 1966. (Translated from English.)
IU. I. ROMAN’KOV