- As adsorbent, desiccant, abrasive,thickening and anti-caking agent;
- As filler for paints and varnishes;
- In manufacture of alloys,refractories, ceramic materials, electrical insulators and resistors, dental cements, glass, steel, artificial gems; in coatings for metals, etc.;
- As catalyst for organic reactions.
- As an insoluble carrier for mineral pigment, and is frequently mixed into mineral powder makeup. Because of its abrasive texture, many use these crystals to exfoliate and resurface the skin-particularly with Microdermabrasion.
- As a chromotagraphic matrix; originally called Brockmann aluminum oxide when used for this purpose.
- The minerals corundum (hardness = 9) and Alundum (obtained by fusing bauxite in an electric furnace) are used as abrasives and polishes;
- In manufacture of cosmetic products like blush, powder foundation, lipstick and facial cleanser.
Stir the oxide with hot 2M HNO3, either on a steam bath for 12hours (changing the acid every hour) or three times for 30minutes, then wash it with hot distilled water until the washings have pH 4, and follow by three washings with hot MeOH. The product is dried at 270o [Angyal & Young J Am Chem Soc 81 5251 1959]. For the preparation of alumina for chromatography see Chapter 1. [For , and Al2O3 see Becher in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol I p 822-823 1963 and Wagner in Handbook of Preparative Inorganic Chemistry (Ed. Brauer) Academic Press Vol II p 1662 1965.]
Occurs in nature in abundance; the principal forms are bauxites and laterites. The mineral corundum is used to produce precious gems, such as ruby and sapphire. Activated aluminas are used extensively as adsorbents because of their affinity for water and other polar molecules; and as catalysts because of their large surface area and appropriate pore sturcture. As adsorbents, they are used for drying gases and liquids; and in adsorption chromatography. Catalytic properties may be attributed to the presence of surface active sites (primarily OH– , O2– , and Al3+ ions). Such catalytic applications include sulfur recovery from H2S (Clauss catalysis); dehydration of alcohols, isomerization of olefins; and as a catalyst support in petroleum refining.
Pure Aluminum oxide, needed to produce aluminum by the Hall process, is made by the Bayer process. The starting material is bauxite (Al2O3 • nH2O). The ore contains impurities, such as, SiO2, Fe2O3, TiO2, and Na2O. Most impurities are removed following treatment with caustic soda solution. Bauxite is dissolved in NaOH solution. Silica, iron oxides and other impurities are filtered out of the solution. CO2 is then bubbled through this solution. This precipitates are heated to remove water and produce Al2O3. These impurities are removed. Calcinations of bauxite produce Aluminum oxide of abrasive and refractory grades. Activated Aluminum oxide of amorphous type, as well as the transition Aluminum oxides of γ, η, χ, and ρ forms, are obtained from various aluminum hydroxides, such as, α- and β-trihydrates, α-monohydrate and Aluminum oxide gel. Such chemicals are also obtained from bauxite by the Bayer process.
White odorless crystalline powder. Water insoluble. Properties (both physical and chemical) vary according to the method of preparation; different methods give different crystalline modifications. The variety formed at very high temperature is quite inert chemically.
Aluminum oxide is chemically amphoteric (behaves as a weak acid in the presence of base and as a weak base in the presence of acid). May act catalytically. May cause the exothermic polymerization of ethylene oxide. May cause the vigorous polymerization of vinyl chloride [MCA SD-75, 1970]. The degree of subdivision of the Aluminum oxide may affect the vigor of such reactions.
Aluminum oxide exhibits amphoteric behavior. It is soluble both in acids and bases. With acids, it produces their corresponding salts. It froms Al2(SO4)3, Al(NO3)3 and AlCl3 upon reactions with H2SO4, HNO3, and HCl, respectively. In acid medium, it exists as a solvated aluminum ion, in which water molecules are hexacoordinated to trivalent Al3+, as shown below:
Al2O3 + 6H3O+3H2O ——› 2[Al(H2O)6]3+
(Rollinson, C. L., 1978., Aluminum Compounds. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed. Vol 2, pp 188-97. NY,: Wiley Interscience)
Aluminum oxide forms hydroxide in aqueous alkaline solution. The reaction is slow. The products, aluminum hydroxides (hydrated aluminas), contain hexacoordinated aluminohydroxide anion:
Al2O3 + 2OH– + 7H2O → 2[Al(OH)4(H2O)2]–
In its dry state, Aluminum oxide exhibiting basicity reacts with silica, forming aluminum silicate
Al2O3 + 3SiO2 → Al2(SiO3)3
Similarly, with basic CaO or MgO aluminate salts are formed
MgO + Al2O3 → Mg(AlO2)2 CaO + Al2O3 → Ca(AlO2)2
It forms aluminum nitride, AlN when heated with coal in a stream of nitrogen; and aluminum borate, Al2O3 •B2O3 when heated with B2O3 at 1000°C.
Al2O3 Colorless hexagonal crystal; refractive index 1.768; density 3.965 g/cm3 (at 25°C); mp 2072°C; bp 2980°C; insoluble in water
α-Al2O3 Colorless rhombic crystal; mp between 2005 to 2025°C ; density 4.022 g/m3 ; hardness 9Moh
γ-Al2O3 white microscopic crystal
Al2O3•H2O colorless rhombic crystal; refractive index 1.624; density 3.014 g/cm3
Al2O3•3H2O white monoclinic crystal; refractive index 1.577; density 2.420 g/cm3
All forms are insoluble in water.
Air & Water Reactions
Insoluble in water.