Question

For each of the substances listed below, describe the role played in the extraction of aluminium:
(1) Cryolite
(2) Sodium hydroxide
(3) Graphite

Answer 1

Bauxite ore is the major source for the aluminium extraction. To obtain aluminium, bauxite is purified, a white powder form in which aluminium can be extracted. Electrolysis is mainly used for this process. The listed compounds have important roles in the extraction process.

Complete step by step answer:
(1)Cryolite: In the extraction of aluminium, the metal has to be isolated from alumina ($A{{l}_{2}}{{O}_{3}}$) by carrying out its electrolysis(electrolytic reduction). The melting point of alumina is very high($2323K$). Therefore, Cryolite ($N{{a}_{3}}Al{{F}_{6}}$) is mixed with it, to decrease the melting point into $1173K$. Also, cryolite increases the electrical conductivity of alumina.

(2)Sodium hydroxide: Here the sodium hydroxide($NaOH$) is used as a leaching agent. The ore of aluminium usually contains impurities like silica, iron oxide and titanium oxide. Hence the concentration of bauxite ore is modified by mixing the powdered ore with concentrated solution of sodium hydroxide at $473-523K$ and $35-36bar$ pressure. By this procedure alumina is leached out as sodium aluminate leaving behind the impurities. The insoluble impurities are called red mud. The following reactions are given below;
$A{{l}_{2}}{{O}_{3}}.2{{H}_{2}}O+2NaOH\xrightarrow{473-523K}2NaAl{{O}_{2}}+3{{H}_{2}}O$
$NaAl{{O}_{2}}+2{{H}_{2}}0\to NaOH+Al{{(OH)}_{3}}$
$2Al{{(OH)}_{3}}\to A{{l}_{2}}{{O}_{3}}+3{{H}_{2}}O$

(3)Graphite: We have known that the extraction of aluminium is an electrolysis process as it contains anode and cathode. Here graphite rod is used as anode and graphite lined iron is used as cathode during the extraction of aluminium. During the process aluminium is liberated at the cathode and oxygen at the anode. So the graphite reacts with the oxygen at anode to form $CO$ and $C{{O}_{2}}$. Hence this process prevents oxygen from oxidizing aluminium.

Note: Other than bauxite, corundum and feldspar are the other ore of the aluminium. It is rarely found in nature. About four tonnes of bauxite is required to make one tonne of aluminium metal.