Question

Explain Hall-Heroult method of obtained pure aluminium from alumina with figure.

Answer 1

• The method to obtain aluminium from alumina by the electrochemical method was invented by Charles Hall and Paul Héroult.
  
• Electric current cannot pass through the solid form of alumina, and the melting point of alumina is also very high. Hence, cryolite is added to alumina in a small amount to carry out electrolysis easily.
  
• This mixture acts as a good conductor of electricity. Addition of feldspar also lowers the melting point of alumina. Thus, a mixture of alumina, cryolite and feldspar is subjected to electrolysis to obtain pure aluminium.
  
• For electrolysis, an iron vessel with an inner layer coated with carbon-layered graphite is used. This acts as a cathode.
  
• The carbon rods are joined with a copper clamp and immersed in an electrolyte. These rods act as the anode.
  
• On passing electric current, molten aluminium is deposited at the cathode and oxygen gas is released at the anode.
  

At the cathode : $2A{l}^{3+}\left(l\right)+6e^{-}\to 2Al\left(l\right)$

At anode : $6O^{2-}\left(l\right)\to 6O_2\left(g\right)+12e^{-}$