Inorganic Chemicals

Preparation
Pure alumina, 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 out hydrated alumina, which is heated to remove water and produce Al2O3. These impurities are removed. Calcinations of bauxite produce alumina of abrasive and refractory grades. Activated aluminas of amorphous type, as well as the transition aluminas of γ, η, χ, and ρ forms, are obtained from various aluminum hydroxides, such as, α- and β-trihydrates, α-monohydrate and alumina gel. Such chemicals are obtained from bauxite by the Bayer process also.

Chemical Reactions
Alumina 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)

Alumina 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, alumina 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.

Analysis
Elemental composition: Al 52.91%, O 47.08%. Al may be anlayzed by atomic absorption or emission spectrophotometry or by colorimetric methods after acid digestion. Different forms of alumina may be identified by x-ray diffraction analysis. The X-ray crystallogaphic data for the mineral corundum are as follows:

crystal system: rhombohedral symmetry
space group     R3c
αο                    4.7591
χο                    12.9894
z                      6
x-ray density  3.9869 g/cm3

Toxicity
Chronic inhalation of Al2O3 dusts may cause lung damage.