ALUMINUM
Saturday, October 25th, 2008Symbol Al; atomic number 13; atomic wt. 26.982; a Group III A (Group 13) metal; principal natural isotope 27Al; electronic config. [Ne]3s23p1; valence +3
Occurrence and Uses
Aluminum is the third most abundant element in the crust of the earth, accounting for 8.13% by weight. It does not occur in free elemental form in nature, but is found in combined forms such as oxides or silicates. It occurs in many minerals including bauxite, cryolite, feldspar and granite. Aluminum alloys have innumerable application; used extensively in electrical transmission lines, coated mirrors, utensils, packages, toys and in construction of aircraft and rockets.
Physical Properties
Silvery-white malleable metal, cubic crystal; melts at 660°C; b. p. 2520°C; density 2.70 g/cm3; insoluble in water, soluble in acids and alkalies.
Thermal, Electrochemical, and Thermochemical Properties
Specific heat 0.215 cal/g.°C (0.900 J/g.°C); heat capacity 5.81 cal/mol.°C (24.3 J/mol.°C); ΔHfus (2.54 kcal/mol (10.6 kJ/mol); ΔHvap 67.9 kcal/mol (284 kJ/mol); E° in aqueous soln. (acidic) at 25°C for the reaction
Al3+ + 3e– —› Al(s) , –1.66V; S°298 6.77 cal/degree mol. K (28.3 J/degree mol.K)
Production
Most aluminum is produced from its ore, bauxite, which contains between 40 to 60% alumina either as the trihydrate, gibbsite, or as the monohydrate, boehmite, and diaspore. Bauxite is refined first for the removal of silica and other impurities. It is done by the Bayer process. Ground bauxite is digested with NaOH solution under pressure, which dissolves alumina and silica, forming sodium aluminate and sodium aluminum silicate. Insoluble residues containing most impurities are filtered out. The clear liquor is then allowed to settle and starch is added to precipitate. The residue, so-called “red-mud”, is filtered out. After this “desilication,” the clear liquor is diluted and cooled. It is then seeded with alumina trihydrate (from a previous run) which promotes hydrolysis of the sodium aluminate to produce trihydrate crystals. The crystals are filtered out, washed, and calcined above 1,100°C to produce anhydrous alumina. The Bayer process, however, is not suitable for extracting bauxite that has high silica content (>10%). In the Alcoa process, which is suitable for highly silicious bauxite, the “red mud” is mixed with limestone and soda ash and calcined at 1,300°C. This produces “lime-soda sinter” which is cooled and treated with water. This leaches out water-soluble sodium alumnate, leaving behind calcium silicate and other impurites.
Alumina may be obtained from other minerals, such as nepheline, sodium potassium aluminum silicate, by similar soda lime sintering process.
Metal aluminum is obtained from the pure alumina at 950 to 1000°C electrolysis (Hall-Heroult process). Although the basic process has not changed since its discovery, there have been many modifications. Aluminum is also produced by electrolysis of anhydrous AlCl3.
Also, the metal can be obtained by nonelectrolytic reduction processes. In carbothermic process, alumina is heated with carbon in a furnace at 2000 to 2500°C. Similarly, in “Subhalide” process, an Al alloy, Al-Fe-Si-, (obtained by carbothermic reduction of bauxite) is heated at 1250°C with AlCl vapor. This forms the subchloride (AlCl), the vapor of which decomposes when cooled to 800°C.
Chemical Reactions
Reacts in moist air forming a coating of Al2O3; reacts with dilute mineral acids liberating H2,
2Al + 3H2SO4 ——›Al2(SO4)3 + 3H2↑
also reacts with steam to form H2; reduces a number of metals that are less active (in activity series), these include Fe, Mn, Cr, Zn, Co, Ni, Cu, Sn, Pb, etc.,
Al(s) + 3Ag+(aq) ——›Al3+(aq) + 3Ag(s)
Reactions, e.g., with alkyl halides in ether using Ziegler-Natta catalyst form alkyl aluminum halides, R3Al2X3, [R2AlX]2 and [RAlX]2; with bromine vapor forms anhydrous aluminum bromide,
2Al + 3Br2 ——› Al2Br6
Combines with iodine vapor forming aluminum iodide, AlI3; heating with HCl gas produces AlCl3,
Heating with Cl2 at 100°C also yields AlCl3,
When the metal is heated with AlCl3 at 1000°C it forms monovalent aluminum chloride, AlCl.
Produces aluminum carbide when the powder metal is heated with carbon at 2000°C or at 1000°C in presence of cryolite,
Heating the metal powder over 1000°C with sulfur, phosphorus, or selenium forms aluminum sulfide Al2S3, aluminum phosphide, AlP and aluminum selenide, Al2Se3, respectively,
Heating over 1100°C with N2 produces nitride, AlN; alkoxides are formed when the metal powder is treated with anhydrous alcohol, catalyzed by HgCl2
Reaction with CO at 1000°C produces the oxide Al2O3 and the carbide Al4C3.
Chemical Analysis
The metal may be analyzed by atomic absorption or emission spectrophotometry (at trace levels). Other techniques include X-ray diffraction, neutron activation analysis, and various colorimetric methods. Aluminum digested with nitric acid reacts with pyrocatechol violet or Eriochrome cyanide R dye to form a colored complex, the absorbance of which may be measured by a spectrophotometer at 535 nm.
Hazard
Finely divided aluminum dust is moderately flammable and explodes by heat or contact with strong oxidizing chemicals. Chronic inhalation of the powder can cause aluminosis, a type of pulmonary fibrosis. It is almost nontoxic by ingestion.
