Aluminum hydroxides are separated from bauxite by the Bayer process, in which these hydroxides are dissolved in sodium hydroxide to separate them from the other unwanted constituents of the bauxite. The dissolution reactions are carried out at about 285°C and 200 atm. pressure, and are:
Al(OH)3(s) + NaOH(soln) = NaAl(OH)4(soln) (1)
AlOOH(s) + H2O(soln) + NaOH(soln)= NaAl(OH)4(soln) (2)
in which (s) stands for solid and (soln) for solution. The solution containing NaAl(OH)4 is separated from the unwanted solid impurities by sedimentation and filtration, and the solute is cooled to about 55°C. The aluminum hydroxides precipitate from the solution, aided by the addition of gibbsite seeds. The dried precipitated alumina or aluminum hydroxides can be used directly or further purified by resolution and reprecipitation. Other methods for preparing alumina and aluminum hydroxides from bauxite are described in [1, 2].
The stable phase of alumina at all temperatures and ambient pressure (one atm, or (1.01) 105 Pa) is corundum or a-Al2O3 (see Table 2). In single-crystal form, corundum is called sapphire. No phase transformation of corundum up to 175 GPa pressure has been observed experimentally [5, 6]; however, a calculation predicts that corundum should transform to the Rh2O3 (II) structure at about 78 GPa, and to a cubic perovskite structure at 223 GPa . The Rh2O3 (II) structure has an X-ray pattern close to the corundum structure, so the transformation may have been missed in experimental studies.
Solid polycrystalline alumina is made from alumina powder by sintering. The traditional sintering methods for ceramics involve forming a powder into “green” ware, partially drying it at low temperatures, possibly “calcining” (heating) it at intermediate temperatures (perhaps 900°-1,100°C), and firing it to a dense solid at high temperature, for alumina above 1,400°C.
The time and temperature required to form the desired degree of porosity in the dense solid depend mainly on the particle size of the alumina powder. The usual sintering sequence is imagined to be: (1) neck formation between powder particles, (2) formation of open porosity with a continuous solid phase (intermediate stage), and (3) removal of closed pores imbedded in the dense solid. In the usual practical sintering of alumina, stage one is rapid, and the final density or porosity is determined mainly by stage three.
Various other oxides have been added to alumina to reduce the porosity of the final sintered solid. An especially valuable finding by Coble  was to add MgO to pure alumina powder; the resulting sintered alumina can be translucent (partial transmission of light). Usually sintered ceramics are opaque because of light scattering from residual pores, but in the translucent alumina, called Lucalox™, the porosity is low enough to reduce this scattering, so that Lucalox tubes are used in street lamps for containing a sodium arc. Because the lamp can be operated at high temperature, it is quite efficient.
Dense alumina can also be made by melting, but the high-melting temperature of 2,054°C makes this process expensive and difficult to control. High-value materials such as gem stones and laser hosts can be made by adding various colorants such as chromium, titanium, iron, cobalt, and vanadium to the melt.
In the sintering of alumina powders, the desired shape is formed in the green state before drying and firing. Various other constituents can be added to the starting powder. The density of the final product can be increased by hot-pressing, that is, by carrying out the firing under pressure. This method is expensive, so it is used only for high value polycrystalline products.
Alumina refractories for use in high temperature applications such as glass melting furnaces are usually made by the fusion-cast process. Various other oxides, such as SiO2, MgO, Cr2O3, and ZrO2 are added to the alumina powder to lower its melting point, and the resulting mixture is melted in an electric furnace and cast into the desired shapes for refractory applications. See the section on phase diagrams for the melting temperatures and compositions of a few mixtures of alumina with other oxides.