For the electrolytic refining of aluminium, the three fused layers consist of

Bottom Layer

Middle Layer

Upper Layer

• A

  Cathode of pure $Al$

  Cryolite and fluorspar

  Anode of $Al$ and $Cu$ alloy

• B

  Cathode of $Al$ and $Cu$ alloy

  Bauxite and cryolite

  Anode of pure $Al$

• C

  Anode of $Al$ and $Cu$ alloy

  Cryolite and barium fluoride

  Cathode of pure $Al$

• D

  Anode of impure $Al$

  Bauxite, cryolite and fluorspar

  Cathode of pure  $Al$