Question

Hoop’s process of purification of aluminium involves formation of layers during electrolysis. It involves _ _ _ _ _ _ _ _ _ _.
[This question has multiple correct options]
(A) The three layers have the same densities but different materials.
(B) The three layers have different densities.
(C) The upper layer is of pure aluminium which acts as a cathode.
(D) The bottom layer is of impure aluminium which acts as an anode and middle layer consists of cryolite and $Ba{F_2}$

Answer 1

Hint: The Hoop’s process purifies the aluminium obtained after the Hall-Heroult’s process. During this 3 layers are formed inside the cell: anode consisting of impure Al forms the bottom layer, the electrolyte forms the middle layer and the topmost layer made of molten aluminium forms the cathode.

Complete step by step answer:
First of all let us see what Hoop’s process is.
-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 ($Ba{F_2}$), cryolite ($N{a_3}Al{F_6}$) and aluminium fluoride ($Al{F_3}$).
Finally the top layer or the uppermost layer is usually made up of molten aluminium in the form of $A{l_2}{O_3}$ 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 $A{l^{ + 3}}$ 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 $A{l^{ + 3}}$ 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 $A{l^{ + 3}}$, $B{a^{ + 2}}$ and $N{a^ + }$.

Hence the correct options will be:
(B) The three layers have different densities.
(C) The upper layer is of pure aluminium which acts as a cathode.
(D) The bottom layer is of impure aluminium which acts as an anode and middle layer consists of cryolite and $Ba{F_2}$
So, the correct answer is “Option B, C and D”.

Note: 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.