Alumina, Al₂O₃

Aluminium sesqui-oxide or alumina, Al₂O₃, is found in the crystalline form in nature as the mineral corundum. It crystallises in the trigonal system (ditrigonal scalenohedral; a:c = 1:1.365) usually in double, six-sided pyramids and rhombohedra, with the basal plane. It is therefore isomorphous with ferric and chromic oxides. Hardness, 9; density, 3.9-4.1; lustre, vitreous. The dull and opaque varieties, or "common corundum," occur in India, China, Siberia, and the United States, and are largely used as abrading agents. The transparent varieties are highly valued as gem-stones. Clear, colourless stones are known as white sapphire; blue stones as sapphire; red stones as oriental ruby; yellow stones as oriental topaz or yellow sapphire; purple stones as oriental amethyst; and the rare, green stones as oriental emerald. These coloured varieties of corundum are pleochroic. Sapphires are found in Ceylon, Burma, Siam, and parts of India, and in the gold-bearing drifts of Victoria and New South Wales. Oriental rubies occur in Ceylon and also at Mogok, Upper Burma.

Impure granular or crystalline corundum, associated with magnetite, tourmaline, garnet, etc., occurs in nature as emery. It is obtained from Naxos in the Greek Archipelago, Asia Minor, and Massachusetts, and is used as an abrading agent.

Alumina is obtained as an amorphous white powder or gum-like mass by heating aluminium hydroxide or the aluminium salt of a volatile oxyacid, e,g. the nitrate, sulphate, etc. When molten alumina solidifies, when alumina vapour condenses to the solid state and when alumina separates from its solution in a suitable solvent, it assumes the crystalline form and characteristics of corundum. Rhombohedral crystals may be obtained by heating amorphous alumina with five times its weight of aluminium sulphide in the electric furnace, and treating the product with hydrochloric acid.

The crystallisation of alumina has been attempted by various chemists in the hope of preparing rubies and sapphires. The ruby owes its colour to a trace of chromic oxide. The first successful experiments on the production of rubies on a large scale were made by Fremy and Feil, who fused equal parts of alumina and litharge, plus 2 or 3 per cent, of potassium dichromate, in a fireclay crucible at % bright red heat. The product consisted of a layer of lead silicate and a vitreous layer in which crystals of ruby were embedded. Later, by replacing the litharge by barium fluoride and heating in a glass furnace, Fremy and Verneuil obtained beautiful rubies, which had arisen by the action of the furnace gases on aluminium fluoride vapour. Loyer obtained rubies by heating sodium aluminate (100 pts.) and potassium dichromate (1 pt.) to bright redness in chlorine.

At the present time, rubies are manufactured by a process devised by Verneuil. The material used is powdered alumina containing a little chromic oxide (2.5 per cent.), produced by precipitating with ammonia a solution of pure ammonium alum to which a little chrome alum has been added, and igniting the mixed hydroxides. The powder is fed through the oxygen tube of an inverted oxy-coal-gas blowpipe, falls as a molten drop on to the end of a small alumina rod, and crystallises as ruby. As the process is continued, the ruby grows upwards as a pear-shaped drop or " boule." These " boules " have a density of 4.01; although externally smooth, they have a crystalline structure and differ from natural stones only in one respect, viz., they contain microscopic air-bubbles and fine-curved internal streaks. The streaks in a natural stone are straight.

Sapphires are manufactured from alumina to which 1.5 per cent, of magnetic iron oxide and 0.5 per cent, of titanium oxide has been added, the mixture being fused and crystallised in an oxy-hydrogen reducing flame. The introduction of cobalt oxide into alumina can only be effected in the presence of a third oxide such as lime. The stones obtained are blue, but they are amorphous, and have not the tint of sapphire. The analyses of three natural sapphires gave the following results: -

Locality	Australia	India	Montana
Fe2O3	0.920	0.72	0.560
TiO2	0.031	0.04	0.058
SiO2	trace	nil	0.100
Al2O3 (diff)	...	...	...

It has been shown that synthetic and natural sapphires have identical properties.

About ten million carats of rubies and six million carats of sapphires were manufactured in 1913, and the demand is increasing.

Amorphous alumina is a white powder, insoluble in water. The density increases with the temperature at which it has been ignited, as follows: -

Temperature ° C	600°	700°	800°	900°	1200°
Density	2.82	2.83	3.39	3.53	3.92

When slowly heated it undergoes an exothermic change at 850°, and melts at 2010° to 2050°. In the electric furnace it melts and boils, and the vapour condenses to the crystalline form. The specific heat of ignited alumina increases with the temperature, as is shown by the following mean values: -

Temp, interval ° C.	15°-100°	15°-195°	15°-315°	15°-420°	15°-510°
Specific heat	0.2003	0.2195	0.2311	0.2400	0.2464

Alumina that has been dried at a low temperature is very hygroscopic and forms an excellent drying agent.

Alumina is soluble in mineral acids unless it has been strongly heated (above 850°), when it becomes extremely refractory. Calcined alumina must be brought into solution by fusion with potassium hydrogen sulphate or alkali hydroxide.

Alumina is unaffected by hydrogen or chlorine at a red heat, but is converted by fluorine into aluminium fluoride and oxygen. At a red heat it is converted into the sulphide by carbon disulphide. Alumina is reduced by carbon to the metal at temperatures above 2000°, at temperatures above which aluminium carbide is unstable. It was shown by Moissan that carbon reduces alumina vapour.

Alumina is used in the manufacture of aluminium; for this purpose it is prepared by igniting the hydroxide, prepared from bauxite. It has been proposed to prepare it from aluminium nitride, manufactured by Serpek's method. Another proposal, which may possibly develop into a successful commercial method, is to obtain the alumina from sodium aluminate, itself manufactured from china-clay.

The porous alundum laboratory utensils are composed mainly of alumina. Calcined bauxite is fused in a water-cooled electric arc furnace. The impurities in the bauxite are to a certain extent reduced and segregate at the bottom of the fused mass as an impure ferrosilicon. The cooled product consists essentially of a large mass of crystalline alumina. It is crushed, mixed with a ceramic binding material (ball-clay and felspar), moulded, dried, and fired in a porcelain kiln.