Hoop’s process is used in the refining of :

It is basically a metallurgical process which is used to extract aluminium of very high purity. Although the aluminium which is obtained at the end of the Hall-Heroult’s process is 99% pure it can be further purified by an electrolytic method known as Hoop’s process.
In this process we use a cell which consists of an iron box and is lined with carbon.
 It basically contains 3 layers.
The bottom layer contains impure aluminium, molten alloy of copper and silicon which forms the anode. 
The middle layer consists of an electrolyte which is made up of Sodium fluoride (NaF), Barium fluoride BaF2, cryolite Na3AlF6 and aluminium fluoride AlF3. 
Finally the top layer or the uppermost layer is usually made up of molten aluminium in the form of Al2O3 which acts as the cathode. 
The upper layer of the cell has the lowest specific gravity while the lowermost layer has the highest specific gravity.
When electrolysis occurs, current passes through the middle layer and the Al+3 ions migrate from the middle layer towards the cathode where they get reduced to form Al. 
The lowermost layer or the anode produces an equal number of Al+3 ions, which move to the middle layer and then to the upper layer where they form Al.
In this way all the pure aluminium gets deposited at the uppermost layer and the impurities are left behind. The aluminium obtained at the end was 99.99% pure.   From the above discussion we can now conclude that all the three layers are made up of different materials and so will have different densities, the topmost layer of pure molten aluminium forms the cathode while the lowermost layer of impure aluminium will form the anode and the middle layer is of electrolytes like fluorides of Al+3, Ba+2 and Na+.
The electrolyte contains cryolite and is also used in the Hall-Heroult’s process because it increases the conductivity and also lowers the melting point which helps to bring down the extraction cost. Also due to the high temperatures the carbon present at the anode reacts with oxygen and burns, hence it needs to be regularly replaced.

Electrolysis

At anode:

At cathode:

The overall reaction is