Occurrence and recovery

Key points: Aluminium is highly abundant; thallium and indium are the least abundant of the Group 13 elements. Boron exists in several hard and refractory allotropes. The three solid phases for which crystal structures are available contain the icosahedral (20-faced) B12 unit as a building block (Fig. 13.1). This icosahedral unit is a recurring motif in boron chemistry and we shall meet it again in the structures of metal borides and boron hydrides. The icosahedral unit is also found in some in termetallic compounds and Zintl phases (Section 3.8c) of other Group 13 elements, such as Al₅CuLi₃, RbGa₇, and K3 Ga₁₃. Boron occurs naturally as borax, Na₂B₄O₅ (OH)₄ .8H₂O, and kernite, Na₂B₄O₅ (OH)4 .2H₂O, from which the impure element is obtained. The borax is converted to boric acid, B(OH)₃, and then to boron oxide, B₂O₃. The oxide is reduced with magnesium and washed with alkali and then hydrofluoric acid. Pure B is produced by reduction of BBr3 vapour with H₂:

Aluminium is the most abundant metallic element in the crustal regions of the Earth and makes up approximately 8 per cent by mass of crustal rocks. It occurs in numerous clays and alumino silicate minerals but the commercially most important mineral is bauxite, a complex mixture of hydrated aluminium hydroxide and aluminium oxide, from which it is extracted by the Hall Héroult process on an immense scale (Section 5.18). In this process the bauxite is dissolved in molten cryolite, Na3 AlF₆, the mixture is electrolysed and aluminium is deposited at the cathode. The process is very expensive but this expense is offset by the scale of production, the availability of the raw material, and the use of hydroelectric power. The oxide, alumina, occurs naturally as ruby, sapphire, corundum, and emery. Gallium oxide occurs as an impurity in bauxite and is normally recovered as a byproduct of the manufacture of aluminium. The process results in the concentration of gallium in the residues from which it is extracted by electrolysis. Indium is produced as a byproduct of the extraction of lead and zinc, and is isolated by electrolysis. Thallium compounds are found in flue dust, which is dissolved in dilute sulfuric acid; hydrochloric acid is then added to precipitate thallium(I) chloride and the metal is extracted by electrolysis.

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مواضيع ذات صلة

Extended silicon oxygen compounds

Compounds of boron with nitrogen

Boron oxygen compounds

Uses of the elements and their compounds

Boron clusters

Symmetry-adapted linear combinations

The assignment of molecular symmetry from vibrational spectra

Information from the symmetries of normal modes

The exclusion rule

Molecular vibrations

Chiral molecules

Polar molecules