Question

In the extraction of aluminium ;
(i) Name the process of concentration of bauxite.
(ii) Write the cathode reaction in electrolytic reduction of alumina.
(iii) Write the function and chemical formula of cryolite.
(iv) Write a chemical equation for the action of heat on aluminium hydroxide.
(v) Why is it necessary to replace anodes from time to time?

Answer 1

Hint : Extraction of aluminium is done with the help of Bayer’s process followed by electrolysis, In Bayer’s process we obtain aluminium oxide, a white powder from which aluminium can be extracted. The extraction is done with the help of an electrolytic process which is named as Hall-Heroult’s process, and the ions in the aluminium oxide should be free enough to move so that electricity can pass through it easily.

Complete step by step solution :
(a) The process for concentration of bauxite ore is Bayer's process (In which an ore of aluminium which is bauxite is heated in a pressure vessel along with a solution of sodium hydroxide also known as caustic soda at a temperature of $150$ to ${200^o}C$ . At these temperatures, the aluminium is dissolved as primarily $Al{[{(OH)_4}]^ - }$ aluminate ion and further goes for the extraction process.
(b) The electrolytic reduction of alumina is done with the help of Hall-Heroult's process.
The reaction at cathode is :
$2A{l^{3 + }} + 6{e^ - } \to 2Al$
Thus, pure aluminium metal is obtained at the cathode in this process.

(c) Cryolite is also known as sodium aluminium fluoride and its chemical formula is $N{a_3}Al{F_6}$ ​. It is an ore of aluminium. The function of cryolite includes that it is added in hall-heroult’s process to act as a solvent and to decrease the melting temperature of alumina to approximately $1173K$ and to increase the electrical conductivity of the electrolytic cell.

(d) The chemical reaction for the action of heat on aluminium hydroxide involves the formation of aluminium oxide with water as a side product. The reaction is as follows;
$2Al{(OH)_3}\xrightarrow{{heat}}A{l_2}{O_3} + 3{H_2}O$

(e) There is a need to replace anodes from time to time as during the Electrolysis of the Alumina, Oxygen is deposited at the anode. Now, the Anode is made up of graphite (or Carbon), thus the carbon reacts with the Oxygen and forms its oxide releasing $C{O_2}$ and the anode is consumed with time. The reaction at anode for this case are:
$2{O^{2 - }} + 4{e^ - } \to {O_2}$
Now, the deposited oxygen reacts with the Carbon anode,
$C + {O_2} \to C{O_2}$
Note : The Bauxite ore which is used as the initial substrate consists of only $30 - 60\%$ of alumina rest are mixtures of silica, iron oxides and titanium dioxides.