Question

The following questions refer to the extraction of aluminium and iron from their ores:
(a) Name the principle ore from which (i) iron, and (ii) aluminium are extracted.
(b) What is the most important chemical process in the extraction of any metal? State how this essential step is carried out in the extraction of: (i) iron, (ii) aluminium.
(c) Iron and aluminium ores both, contain impurities. Explain, briefly how these impurities are removed in each case.
(d) What is the major impurity present in iron when it is removed from the furnace?
Name an alloy of zinc is used in simple voltaic cells.

Answer 1

(a)

Metal	Principle ore
Iron	Haematite, Fe2O3
Aluminium	Bauxite, Al2O3.xH2O

(b) The most important chemical process in the extraction of any metal is the reduction of the converted oxide ore into metal.
(i) Being a moderately reactive metal, iron can be extracted using reducing agents like CO or hydrogen. Iron (III) oxide is reduced to metallic iron using CO as the reducing agent.
$$\begin{gathered} {\mathrm{Fe}}_2{\mathrm{O}}_3+3\mathrm{CO}\stackrel{\mathrm{Heat}}{\to }2\mathrm{Fe}+3{\mathrm{CO}}_2 \\ \mathrm{Iron}\left(\mathrm{III}\right)\mathrm{Oxide}\mathrm{Carbon}\mathrm{monoxide}\mathrm{Iron}\mathrm{Carbon}\mathrm{dioxide} \end{gathered}$$

(ii) Because aluminium is a reactive metal, it cannot be reduced by conventional reducing agents like C, CO, H, etc. Thus, an electrolytic reduction method called the Hall-Heroult's process is used. In this process, the electrolytic cell is made of steel with an inner layer of carbon. The inner gas carbon layer is made as the cathode and the thick carbon rods dipped in fused electrolyte act as the anode. Fused purified alumina (${\mathrm{Al}}_2{\mathrm{O}}_3$) mixed with cryolite (${\mathrm{Na}}_3{\mathrm{AlF}}_6$) and fluorspar (${\mathrm{CaF}}_2$) is taken as the electrolyte. When electricity is passed through the electrodes, the following reactions occur at the cathode and anode: Dissociation of alumina: $2{\mathrm{Al}}_2{\mathrm{O}}_3\to 4{\mathrm{Al}}^{3+}+3{\mathrm{O}}_{}^{2-}$
Aluminium ions accept electrons and get reduced to aluminium metal at the cathode. The equation denoting this reaction is
Reaction at cathode: ${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Al}$

Aluminium is formed in a molten state at the cathode and settles at the bottom of the cell. It is collected as and when it is formed. The aluminium thus obtained is 99% pure.
Oxygen ions lose electrons to form oxygen gas at the anode:
Reaction at anode:
$$\begin{gathered} {\mathrm{O}}^{2-}\to \mathrm{O}+2{\mathrm{e}}^{-} \\ \mathrm{O}+\mathrm{O}\stackrel{}{\to }{\mathrm{O}}_2 \end{gathered}$$

(c) Ores of both iron and aluminium contain impurities. To remove these impurities, iron ore is concentrated by applying the magnetic separation method. Powdered iron ore is passed through a conveyer belt. Being magnetic in nature, iron ore gets attracted to the magnetic wheel of the conveyer belt and is collected separately. The impurities that are non-magnetic are not attracted and thus, get separated from the ore.

To remove impurities from aluminium ore, it is concentrated by a chemical process. It is dissolved in a hot sodium hydroxide solution and forms sodium aluminate. The impurities that are not soluble are separated by filtration.
$$\begin{gathered} {\mathrm{Al}}_2{\mathrm{O}}_3.x{\mathrm{H}}_2\mathrm{O}+2\mathrm{NaOH}\stackrel{\mathrm{Heat}}{\to }2{\mathrm{NaAlO}}_2+(x+1){\mathrm{H}}_2\mathrm{O} \\ \mathrm{Aluminium}\mathrm{ore}\mathrm{Sodium}\mathrm{Sodium}\mathrm{aluminate} \\ (\mathrm{Bauxite}\mathrm{ore})\mathrm{hydroxide}\mathrm{Solu}. \end{gathered}$$
Sodium aluminate is then hydrolysed to form aluminium hydroxide which on heating decomposes to alumina.
$$\begin{gathered} {\mathrm{NaAlO}}_2+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{NaOH}+\mathrm{Al}{\left(\mathrm{OH}\right)}_3 \\ 2\mathrm{Al}{\left(\mathrm{OH}\right)}_3\stackrel{\mathrm{\Delta }}{\to }{\mathrm{Al}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O} \end{gathered}$$

(d) One major impurity present in iron (pig iron) is carbon and it causes iron to be hard and brittle. The other impurities present are phosphorous, silicon and manganese.
An alloy of zinc that is used in simple voltaic cells is brass.