Question

Write the chemical reaction of the Hall-Heroult process of bauxite ore and draw a labeled diagram of the electrolytic cell.

Answer 1

Hint: Hall -Heroult process is used to obtain pure aluminum from their ores(bauxite). The method to get aluminum from alumina by the electrochemical method was invented by Charles Hall and Paul Héroult.

Complete step by step answer:
Electric current cannot submit to the solid variety of alumina, and therefore the temperature of alumina is additionally very high. Hence, cryolite is added to alumina in an exceedingly bit to hold out electrolysis easily.
- This mixture acts as a decent conductor of electricity. The addition of feldspar also lowers the freezing point of alumina. Thus, a combination of alumina, cryolite, and feldspar is subjected to electrolysis to get pure aluminum.
- For electrolysis, an iron vessel with an inner layer coated with carbon-layered graphite is employed. This acts as a cathode.
- The carbon rods are joined with a copper clamp and immersed in an electrolyte. These rods act because of the anode.
- On passing electrical phenomenon, molten aluminium is deposited at the cathode and oxygen gas is released at the anode. The reactions are:
\[\begin{align}
 & at\,cathode:\,A{{l}^{3+}}+3{{e}^{-}}\to Al \\
 & at\,anode:\,2{{F}^{-}}\to {{F}_{2}}+2{{e}^{-}} \\
\end{align}\]
- Bauxite ore is fused with sodium carbonate, $A{{l}_{2}}{{O}_{3}}$, combines with sodium carbonate forming sodium metal aluminate. The fused mass is extracted with water where $F{{e}_{2}}{{O}_{3}}$ remains insoluble in the residue.
$A{{l}_{2}}{{O}_{3}}+N{{a}_{2}}C{{O}_{3}}\to 2NaAl{{O}_{2}}+C{{O}_{2}}$
The solution containing sodium meta aluminate is warmed to 50-60 degrees and $C{{O}_{2}}$ is circulated through it. Aluminum hydroxide separates out as a precipitate.

$2NaAl{{O}_{2}}+C{{O}_{2}}+3{{H}_{2}}O\to 2Al{{(OH)}_{3}}+N{{a}_{2}}C{{O}_{3}}$
The precipitate is filtered, washed, dried, and ignited to get $A{{l}_{2}}{{O}_{3}}$.

Note: Extraction of Aluminium
Aluminum being very reactive does not occur free in nature. Aluminum is obtained from bauxite ore.
The bauxite ore is generally contaminated with ferric oxide and silica. The extraction of aluminum from bauxite ore involves the following three steps
1. Concentration of the ore
2. Electrolytic reduction of aluminum oxide.
3. Electrolytic purification of aluminum