Aluminum oxide

Description

The oxide of aluminum is Al2O3. The natural crystalline mineral is called corundum, but the synthetic crystals used for abrasives are designated usually as aluminum oxide or marketed under trade names. For other uses and as a powder, it is generally called alumina. It is widely distributed in nature in combination with silica and other minerals and is an important constituent of the clays for making porcelain, bricks, pottery, and refractories.

The crushed and graded crystals of alumina when pure are nearly colorless, but the fine powder is white. Off colors are due to impurities. American aluminum oxide used for abrasives is at least 99.5% pure, in nearly colorless crystals melting at 2050°C. The chief uses for alumina are for the production of aluminum metal and for abrasives, but it is also used for ceramics, refractories, pigments, catalyst carriers, and in chemicals.
Aluminum oxide crystals are normally hexagonal and are minute in size. For abrasives, the grain sizes are usually from 100 to 600 mesh. The larger grain sizes are made up of many crystals, unlike the single-crystal large grains of SiC. The specific gravity is about 3.95, and a hardness is up to 2000 Knoop.
There are two kinds of ultrafine alumina abrasive powder. Type A is alpha alumina with hexagonal crystals with particle size of 0.3 μm, density 4.0, and hardness 9 Mohs, and type B is gamma alumina with cubic crystals with particle size under 0.1 μm, specific gravity of 3.6, and a hardness 8. Type A cuts faster, but type B gives a finer finish. At high temperatures, gamma alumina transforms to the alpha crystal. The aluminum oxide most frequently used for refractories is the beta alumina and hexagonal crystals heat-stabilized with sodium.
Activated alumina is partly dehydrated alumina trihydrate, which has a strong affinity for moisture or gases and is used for dehydrating organic solvents, and hydrated alumina is alumina trihydrate.

History

The hexagonally closest packed α-Al2O3 modification is the only stable oxide in the Al2O3 –H2O system. Corundum is a common mineral in igneous and metamorphic rocks. Red and blue varieties of gem quality are called ruby and sapphire, respectively. The lattice of corundum is composed of hexagonally closest packed oxygen ions forming layers parallel to the (0001) plane. Only two-thirds of the octahedral interstices are occupied by aluminum ions. The structure may be described roughly as consisting of alternating layers of Al and O ions. The corundum structurewas determined in the early 1920s .

Physical properties

Regular aluminum oxide is white and has a chemical composition of approximately 95 % Al2O3, 1.5 % SiO2, less than 0.5 % Fe2O3, and 3 % TiO2. Because of very slow cooling in the Higgins furnace, the crystals of alumina are coarse, averaging 10 – 15mm in diameter.

Classifications

Four grades can be distinguished according to the content of calcined aluminium:
(i) Standard calcined aluminas with a sodium content of between 3000 and 7000 ppm wt. Na₂O.
(ii) Intermediate calcined alumina with a sodium content of between 1000 and 3000 ppm wt. Na₂O. The sodium content of this grade has been lowered by modifying the conditions of the precipitation of gibbsite or of the calcination.
(iii) Low-sodium calcined aluminas with a sodium content of between 300 and 1000 ppm wt. Na₂O. These aluminas are usually obtained by washing the precursor or by the extraction of sodium as a volatile compound with the mineralizer during calcination.
(iv) High-purity aluminas with an extra-low sodium content below 100 ppm wt. Na₂O. These aluminas obtained from an aluminum hydroxide produced by a process other than the Bayer process. The main applications for calcined aluminas are as feedstocks for refractories, glass and enamel, tiles and porcelain, and advanced ceramics. The diversity of applications for calcined aluminas can be explained by the wide range of properties: refractoriness, sinterability, chemical inertness in both oxidizing and reducing atmosphere and in both acid and alkaline media, hardness, wear and abrasion resistance, dimensional stability, high thermal conductivity, electrical resistivity, low dielectric loss and high permittivity, and high ionic conductivity in the case of beta-alumina.

Uses

Aluminum Oxide (Alumina) is the most widely used oxide, chiefly because it is plentiful, relatively low in cost, and equal to or better than most oxides in mechanical properties. Density can be varied over a wide range, as can purity-down to about 90% alumina-to meet specific application requirements. Alumina ceramics are the hardest, strongest, and stiffest of the oxides. They are also outstanding and with electrical resistivity and dielectric strength, are resistant to a wide variety of chemicals, and are unaffected by air, water vapor, and sulfurous atmospheres. However, with a melting point of only 2039°C, they are relatively low in refractoriness and at 1371°C retain only about 10% of room-temperature strength. In addition to its wide use as electrical insulators and its chemical and aerospace applications, the high hardness and close dimensional tolerance capability of alumina make this ceramic suitable for such abrasion-resistant parts as textile guides, pump plungers, chute linings, discharge orifices, dies, and bearings.

Industrial Manufacture

Monocrystalline aluminum oxide is a very high purity abrasive, produced directly from bauxite in a single-stage fusion . The preferred method employs a furnace feed consisting of bauxite, pyrite (FeS2) or sulfur, carbon, and iron borings. When subjected to fusion in the Higgins batch furnace, two immiscible liquids are formed as with regular aluminum oxide. However, in the present case very slowcooling of the upper liquid results in essentially pure, individual crystals of Al2O3 in a matrix of sulfides. After the pig has cooled and been crushed, the matrix is removed chemically and mechanically. This treatment releases alumina crystals in the range of sizes required by the industry.

Description

Aluminum(III) oxide is also called aluminum oxide. In mineral form it is called corundum and is referred to as alumina in conjunction with mining and aluminum industries. Alumina exists in hydrated forms as alumina monohydrate, Al₂O₃?H₂O and alumina trihydrate Al₂O₃?3H₂O. The geologic source of aluminum is the rock bauxite, which has a high percentage of hydrated aluminum oxide. The main minerals in bauxite are gibbsite (Al(OH)₃), diaspore (AlO(OH)), and boehmite (AlO(OH).

Chemical Properties

Aluminum is a combustible, light, silverywhite, soft, ductile, malleable, amphoteric metal.Vary according to the method of preparation. White powder, balls, or lumps of various mesh.Insoluble in water, dif- ficultly soluble in mineral acids and strong alkali. Noncombustible.

Chemical Properties

Aluminum oxide occurs as a white crystalline powder. Aluminum oxide occurs as two crystalline forms: α-aluminum oxide is composed of colorless hexagonal crystals, and γ-aluminum oxide is composed of minute colorless cubic crystals that are transformed to the α-form at high temperatures.

Physical properties

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.

Physical properties

White and translucent hard material used as abrasive for grinding. Excellent electric insulator and also wear resistant. Insoluble in water and in strong mineral acids, readily soluble in strong alkali hydroxides, attacked by HF or NH₄HF₂. Owing to its corrosion resistance, in inert atmosphere, in molten metals such as Mg, Ca, Sr, Ba, Mn, Sn, Pb, Ga, Bi, As, Sb, Hg, Mo, W, Co, Ni, Pd, Pt, and U it is used as crucible container for these liquid metals. Alumina is readily attacked in an inert atmosphere by molten metals such as Li, Na, Be, Al, Si, Ti, Zr, Nb, Ta, and Cu. Maximum service temperature 1950°C

Occurrence

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.

Characteristics

Aluminum Oxide (Alumina) is the most widely used oxide, chiefly because it is plentiful, relatively low in cost, and equal to or better than most oxides in mechanical properties. Density can be varied over a wide range, as can purity — down to about 90% alumina — to meet specific application requirements. Alumina ceramics are the hardest, strongest, and stiffest of the oxides. They are also outstanding in electrical resistivity, dielectric strength, are resistant to a wide variety of chemicals, and are unaffected by air, water vapor, and sulfurous atmospheres. However, with a melting point of only 2039°C, they are relatively low in refractoriness, and at 1371°C retain only about 10% of room-temperature strength. In addition to its wide use as electrical insulators and its chemical and aerospace applications, the high hardness and close dimensional tolerance capability of alumina make this ceramic suitable for such abrasion-resistant parts as textile guides, pump plungers, chute linings, discharge orifices, dies, and bearings.

Uses

Aluminum oxide is known as the mineral bauxite. Its main use is for the production of aluminum metal by electrolysis. It is also used in many other chemical reactions.

Uses

• 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.

Uses

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.

Definition

The mineral corundum is natural aluminum oxide, and emery, ruby, and sapphire are impure crystalline varieties. The mixed mineral bauxite is a hydrated aluminum oxide.

Preparation

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.

Production Methods

The Bayer process begins by grinding the bauxite and mixing it with sodium hydroxide in a digester. The sodium hydroxide dissolves aluminum oxide components to produce aluminum hydroxide compounds. For gibbsite, the reaction is: Al(OH)₃ + NaOH → Al(OH)₄^- + Na⁺. Insoluble impurities such as silicates, titanium oxides, and iron oxides are removed from the solution while sodium hydroxide is recovered and recycled. Reaction conditions are then modified so that aluminum trihydroxide (Al(OH)₃) precipitates out. The reaction can be represented as the reverse of the previous reaction: Al(OH)₄^- + Na⁺ → Al(OH)₃ + NaOH. Aluminum trihydroxide is calcined to drive off water to produce alumina:
Al(OH)₃ Al₂O₃ + 3H₂O.

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.

General Description

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.

Air & Water Reactions

Insoluble in water.

Reactivity Profile

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.

Hazard

Toxic by inhalation of dust. Confirmed car- cinogen.

Health Hazard

The aluminas are considered to be nuisance dusts; their role in fibrogenic lung disease remains unclear.
Assessment of the toxicity of aluminas has been complicated by the chemical and physical variants of the compounds and inconsistencies in the nomenclature used to describe them.1 The group of compounds referred to as aluminas is composed of various structural forms of aluminum oxide, trihydroxide, and oxyhydroxide. 2 As these aluminas are heated, dehydration occurs, producing a variety of transitional forms; temperatures between 200 and 500°C result in low-temperature-range transitional aluminas characterized by increased catalytic activity and larger surface area.(Transitional aluminas include c, h, and g forms, which, taken together, were formerly termed “g.”)

Flammability and Explosibility

Not classified

Pharmaceutical Applications

Aluminum oxide is used mainly in tablet formulations.It is used for decoloring powders and is particularly widely used in antibiotic formulations. It is also used in suppositories, pessaries, and urethral inserts. Hydrated aluminum oxide is used in mordant dyeing to make lake pigments, in cosmetics, and therapeutically as an antacid.

Industrial uses

Fused aluminum oxide was the second synthetic abrasive to be developed. Synthetic aluminum oxide (alumina) is made as a white powder and can be somewhat harder than corundum (natural alumina) because of its purity. However, corundum has a Mohs hardness of approximately 9 (on a scale of 1 to 10. Alumina can be processed with different properties by slight alteration of the reactants in the manufacturing process. Several grain sizes of alumina are available, and alumina has largely replaced emery for several abrasive uses. Aluminum oxide is widely used to make bonded abrasives, coated abrasives, and air-propelled grit abrasives for dental applications.
Sintered aluminum oxide is used to make white stones, which are popular for adjusting dental enamel and finishing metal alloys, resin-based composites, and ceramic materials.
Pink and ruby variations of aluminum oxide abrasives are made by adding chromium compounds to the original melt. These variations are sold in a vitreous-bonded form as noncontaminating mounted stones for the preparation of metal– ceramic alloys to receive porcelain. Remnants of these abrasives and other debris should be removed from the surface of metals used for metal–ceramic bonding so as not to prevent optimal bonding of porcelain to the metal alloy. A review by Yamamoto (see Selected Reading) suggests that carbide burs are the most effective instruments for finishing this type of alloy because they do not contaminate the metal surface with entrapped abrasive particles.

Safety Profile

Suspected carcinogen with experimental neoplastigenic and tumorigenic data by implantation. Inhalation of finely divided particles may cause lung damage (Shaver's disease). Exothermic reaction above 200℃ with halocarbon vapors produces toxic HCl and phosgene. See also ALUMINUM COMPOUNDS

Safety

Aluminum oxide is generally regarded as relatively nontoxic and nonirritant when used as an excipient. Inhalation of finely divided particles may cause lung damage (Shaver's disease).
HUMAN HEALTH RISK ASSESSMENT FOR ALUMINIUM, ALUMINIUM OXIDE, AND ALUMINIUM HYDROXIDE

Potential Exposure

Most hazardous exposures to aluminum occur in smelting and refining processes. Aluminum is mostly produced by electrolysis of Al2O3 dissolved in molten cryolite (Na3AlF6). Aluminum is alloyed with copper, zinc, silicon, magnesium, manganese, and nickel; special additives may include chromium, lead, bismuth, titanium, zirconium, and vanadium. Aluminum and its alloys can be extruded or processed in rolling mills, wire works, forges, or foundries; and are used in the shipbuilding, electrical, building, aircraft, automobile, light engineering, and jewelry industries. Aluminum foil is widely used in packaging. Powdered aluminum is used in the paints and pyrotechnic industries. Alumina, emery, and corundum has been used for abrasives, refractories, and catalysts; and in the past in the first firing of china and pottery.

storage

Aluminum oxide should be stored in a well-closed container in a cool, dry, place. It is very hygroscopic.

Shipping

UN1309 Aluminum powder, coated, Hazard Class: 4.1; Labels: 4.1-Flammable solid. UN1383 Pyrophoric metals, n.o.s. or Pyrophoric alloys, n.o.s., Hazard Class: 4.2; Labels: 4.2-Spontaneously combustible material, Technical Name Required. UN1396 Aluminum powder, uncoated, Hazard Class: 4.3; Labels: 4.3-Dangerous when wet material. NA9260 (North America) Aluminum, molten, Hazard class: 9; Labels: 9-Miscellaneous hazardous material.

Purification Methods

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.]

Incompatibilities

Aluminum oxide should be kept well away from water. It is incompatible with strong oxidizers and chlorinated rubber. Aluminum oxide also reacts with chlorine trifluoride, ethylene oxide, sodium nitrate, and vinyl acetate. Exothermic reactions above 2008℃ with halocarbon vapors produce toxic hydrogen chloride and phosgene fumes.

Waste Disposal

Consult with environmental regulatory agencies for guidance on acceptable disposalpractices. Generators of waste containing this contaminant (≥100 kg/mo) must conform with EPA regulations governing storage, transportation, treatment, and waste disposal of Aluminum Oxide-Disposal in a sanitary landfill. Mixing of industrial process wastes and municipal wastes at such sites is not encouraged however. Aluminum powder may be recovered and sold as scrap. Recycling and recovery is a viable option to disposal for aluminum metal and aluminum fluoride (A-57).

Regulatory Status

Included in the FDA Inactive Ingredients Database (oral tablets and topical sponge). Included in nonparenteral medicines licensed in the UK.