4. Draw and label the diagram of electrolysis of alumina.
Answers
Answer:
If Bauxite contains Fe²O3³ as impurity, it is purified by Baeyer's process.
Electrolysis of alumina (Hall-Heroult process): Electrolysis of alumina is done in an iron tank fitted with a lining of gas carbon which acts as cathode. Graphite rods act as anode and a bulb is fitted in parallel as indicator for alumina content. The electrolyte is a mixture of alumina and cryolite (Na³AlF⁶). The melting point of alumina is very high (2050⁰C). But, in presence of cryolite (Na³AlF³) and fluorspar (CaF³), it melt at 870⁰C. Then pure alumina is added to the tank which dissolve in cryolite mixture.
Reaction: Reaction take place as below
Na³AlF³→3NaF+AlF³
AlF³→Al³+3F−
3F−→3e−+3F (oxidation) at anode
Al⁴+3e−→Al (reduction) at cathode
2Al²O³+12F→4AlF³+3O²↑
