ALUMINUM, or ALUMINIUM, the most abundant of all metallic elements in the earth's crust, and ranking third among all the known elementary substances, following the non-metals oxygen and silicon. It is a large constituent of all common rocks, excepting only the sedi mentary rocks — the sandstones and limestones —and forms about 15 per cent of the solid substance of the world. Aluminum is never found native, but in the form of oxide, fluoride, phosphate, silicate, sulphate, etc. Its most im portant ore, and that from which the whole commercial supply is produced, is known as bauxite (q.v.). This mineral is a hydrated silicate of aluminum, containing when pure, 74 per cent of alumina —39 per cent of the metal. As it comes from the mines, bauxite is a mix ture, containing not more than 50 to 60 per cent of alumina. For several years the State of Arkansas has produced more than 80 per cent of the bauxite mined in the United States, the remainder being supplied by Alabama, Georgia and Tennessee. The Tennessee prod uct, however, being of especially high grade, is used in the manufacture of the chemical salts of aluminum, and very little, if any, reaches the furnaces which produce the metallic aluminum.
Practically all the metal produced in the United States is obtained by the Hall process. Alumina is first prepared by the Bayer process. Bauxite, very finely ground, is digested with caustic soda under steam pressure of 70 to 80 pounds, sodium aluminate being formed. This solution is filtered, diluted and run into tanks where it is agitated with freshly precipitated alumina. About 70 per cent of the alumina present is thrown down. This is washed free of sodium and the moisture evaporated. When dry, it is charged into an iron tank lined with carbon, containing a fused bath of cryolite, with some other admixtures, such as fluorite of sodium and fluorite of calcium, or similar ingredients, the nature of which is guarded as a trade secret. Above the vat are suspended a series of carbon anodes which may be lowered into the bath. The alumina being stirred into the fused cryolite, the anodes are lowered and the current turned on. The alumina is de composed and the metallic aluminum collects in the bottom of the bath and is drawn off from time to time. The bath remains unaltered, and as the dissolved alumina is exhausted, more is stirred in, and the operation continues as long as the current is provided. The most scrupu lous care is taken that the materials used shall be free from silica and sodium. The first reduces resistance to corrosion, and the second produces a yellowish coating particularly ob jectionable in aluminum cooking utensils. Plants for the economical production of alu minum have to be very large. The metal can not be produced cheaply on a small scale.
Characteristics and Properties.— As found
in the market, aluminum varies from 98 per cent to 99.75 per cent pure, the impurities being chiefly iron, silicon, copper and sometimes sodium. It is grayish white in color with a bluish cast, quite distinct from silver, nickel and tin. It has a satiny lustre and is practically unaffected by pure air, though in regions where the atmosphere is permeated with mixed gases from factory chimneys, sheet aluminum soon becomes perforated and eventually falls to powder. Owing to its great affirmity for oxygen, aluminum is almost instantly covered with a very thin pellicle of oxide which acts as a protection to the metal beneath. In hardness it closely resembles silver, and this may be considerably increased by working. In elas ticity it compares with copper as three to four. Its thermal conductivity is very high, exceeded only by copper, and being double that of tin and three times that of iron. Its electrical conductivity is 61 per cent that of copper, so that wire to carry an equal charge must be 64 per cent larger in sectional area; that is, 28 per cent greater in diameter. Aluminum is sonorous to a high degree, and this quality is improved by the addition of a little silver or German silver. Aluminum is extremely ductile, and may be drawn into wire 1/250 of an inch in diameter, which has about half the strength of copper wire of the same size. In malleabil ity, aluminum ranks next to gold. It may be rolled into sheets 17/10,000 of an inch in thick ness, and these may be hammered into foil 1/10,000 of an inch in thickness. Its tensile strength is ordinarily from 6.5 to 15.5 tons per square inch, with a top limit for selected specimens of 29 tons. Weight for weight it has a tensile strength equal to that of 80,000 pound steel. Thus the weight of steel in an eight-inch girder would make a 15-inch girder of aluminum, which would carry 80 per cent more than the steel. Aluminum is non-magnetic and therefore exceedingly useful in much elec trical work. Like lead, aluminum has the property of extnidability, and when warm may be pressed into a mold of any shape through a small opening. If the mold is of polished steel the aluminum will have a surface as per fect as if machined. By virtue of this qualir; it may be pressed into continuous bars and tubing of any length, and into engineering shapes of great strength, fancy moldings, cor nices, etc. This property is also possessed IT some of its hard alloys. Its atomic weight i; 27; its atomic volume, 10.1; its specific gravity is 2.56, which is increased by rolling to 2.67; its specific heat ranges from 0.2120 to 0.21P: Its melting point is 1215° F. In fusing it expands about 5 per cent and its specific gravity falls to 2.54. Above 1300° F. the metal °burns', that is, it unites very rapidly with the oxygen of the air.