Give reasons for the following pertaining to Hall Herault's process.

(i) The fusion temperature of the electrolyte has to be lowered before conducting the electrolytic reduction.

(ii) The constituents of the electrolyte in addition to one part of fused alumina contains three parts of cryolite and one part of fluorspar.

(iii) A layer of powdered coke sprinkled over the electrolytic mixture, protects the carbon electrodes.

(iv) It is preferred to use a number of graphite electrodes as anode, instead of a single graphite electrode.

(i) Fused alumina has a melting point around 2050°C. Such high temperatures, create the below difficulties:

1. A large amount of electrical energy is required to liberate Al₂O₃ and maintain the electrolyte in molten state at that temperature.
2. The liberated Al metal [m.p. 660°C] may also tend to volatilize out and get wasted.

Hence the fusion temperature of the electrolytic mixture has to be lowered.

(ii) The reasons for addition of mainly Cryolite [in a higher ratio] and Fluorspar to the electrolytic mixture are:

1. They lower the fusion point of the mixture i.e., the mixture fuses [melts] around 950°C instead of 2050°C.
2. They enhance the mobility of the fused mixture by acting as a solvent for the electrolytic mixture. Thus cryolite in the molten state of subdivision dissolves aluminium oxide.
3. Addition of cryolite enhances the conductivity of the mixture. Since pure alumina is almost a non-conductor of electricity.

(iii) The layer of powdered coke is sprinkled over the electrolytic mixture as:

1. It prevents burning of carbon electrodes in air at the emergence point from the bath.
2. It minimizes or prevents heat loss by radiation.

(iv) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. [2C + O₂ ⟶ 2CO; 2CO + O₂ ⟶ 2CO₂] The carbon anode is hence consumed and renewed periodically after a certain period of usage. It is therefore preferable to use a number of graphite electrodes as anode instead of a single electrode.