Aluminium is extracted from its chief ore, bauxite. The ore is first purified and then the metal is extracted from it by electrolytic reduction.
(a) Write three balanced equations for the purification of bauxite by Hall's process.
(b) Name a chemical used for dissolving aluminium oxide. In which state of sub-division is the chemical used?
(c) Write an equation for the reaction which takes place at the anode during the extraction of aluminium by the electrolytic process.
(d) Mention one reason for the use of aluminium in thermite welding. Which particular property of cast iron makes it unsuitable for the construction of bridges?

(a) Bauxite ore is purified by Baeyer's process. In this process, bauxite ore is powdered and mixed with a sodium hydroxide solution and heated strongly. This results in the formation of sodium aluminate.

$$\begin{gathered} {\mathrm{Al}}_2{\mathrm{O}}_3.2{\mathrm{H}}_2\mathrm{O}+2\mathrm{NaOH}\stackrel{\mathrm{heat}}{\to }2{\mathrm{NaAlO}}_2+{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Bauxite}\mathrm{ore}\mathrm{Sodium}\mathrm{hydroxide}\mathrm{solu}.\mathrm{Sodium}\mathrm{aluminate} \end{gathered}$$

The sodium aluminate solution is filtered and cooled. It is then passed through tanks called precipitators where its hydrolysis takes place and an insoluble aluminium hydroxide is formed.

$$\begin{gathered} {\mathrm{NaAlO}}_2+2{\mathrm{H}}_2\mathrm{O}\stackrel{}{\to }2\mathrm{NaOH}+\mathrm{Al}(\mathrm{OH}{)}_3 \\ \mathrm{Sodium}\mathrm{aluminate}\mathrm{Aluminium}\mathrm{hydroxide} \end{gathered}$$

The aluminium hydroxide recovered through filtration is dried and heated strongly at around 1100 to 1200 ^oC. This results in the decomposition of aluminium hydroxide to aluminium oxide (alumina).

$$\begin{gathered} 2\mathrm{Al}(\mathrm{OH}{)}_3\stackrel{1100-{1200}^{\mathrm{o}}\mathrm{C}}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Aluminium}\mathrm{hydroxide}\mathrm{Aluminium}\mathrm{oxide} \end{gathered}$$

(Note: For the purification of Bauxite ore, Baeyer's process is used. Hall's process is used for the extraction of aluminium from alumina by the process of electrolysis.)

(b) The chemical used for dissolving aluminium hydroxide is cryolite (${\mathrm{Na}}_3{\mathrm{AlF}}_6$). It is used during the extraction of aluminium from its purified ore by electrolysis, called the Hall-Heroult's process. Cryolite acts as a solvent and helps dissolve fused pure alumina, which allows the mixture to melt at ${950}^{\mathrm{o}}\mathrm{C}$ instead of ${2050}^{\mathrm{o}}\mathrm{C}$ (melting point of alumina). Thus, at this temperature, molten alumina mixture is obtained, which conducts electricity more easily than fused alumina.

(c) During the extraction of aluminium by the electrolytic process, oxygen gas is produced at the anode. The reaction that takes place at the anode is:
$$\begin{gathered} {\mathrm{O}}^{2-}\stackrel{}{\to }\mathrm{O}+2{\mathrm{e}}^{-} \\ \mathrm{O}+\mathrm{O}\stackrel{}{\to }{\mathrm{O}}_2 \end{gathered}$$
The oxygen produced at anode reacts with the carbon (graphite) anode, liberating carbon monoxide and carbon dioxide gas. Thus, the carbon rods at the anode get used up and should be replaced frequently.
$$\begin{gathered} \mathrm{C}+{\mathrm{O}}_2\stackrel{}{\to }{\mathrm{CO}}_2 \\ 2\mathrm{C}+{\mathrm{O}}_2\stackrel{}{\to }2\mathrm{CO} \end{gathered}$$

(d) Aluminium being reactive in nature has a high attraction towards oxygen. Thus, it acts as a very good reducing agent at high temperatures. It reduces many metallic oxides. This reducing property of aluminium makes it useful in thermite welding.
The weak tensile strength of cast iron makes it unsuitable for the construction of bridges.