Which one of the following is used in the extraction of aluminium by electrolytic process?

Al₂O₃

The correct answer option 1. \(Al_2O_3\).

The extraction of aluminum from its ore, usually bauxite (which primarily consists of aluminum oxide, \(Al_2O_3\)), is achieved through the Hall-Héroult process, named after the American chemist Charles Martin Hall and the French engineer Paul Héroult, who independently developed it in the late 19th century.

Here is a brief explanation of the process:

Preparation of Aluminum Oxide \((Al_2O_3)\): The first step involves obtaining aluminum oxide from bauxite ore. This is typically done by the Bayer process, where bauxite ore is dissolved in hot sodium hydroxide solution \((NaOH)\) to form sodium aluminate \((NaAlO_2)\). Impurities are then removed, and aluminum hydroxide \((Al(OH)_3)\) is precipitated. This aluminum hydroxide is then calcined (heated), resulting in the formation of aluminum oxide \((Al_2O_3)\).

Electrolysis in Molten Cryolite: Pure aluminum oxide has a very high melting point (over 2000°C), making it impractical to use in conventional electrolysis. To lower the melting point, aluminum oxide is mixed with molten cryolite \((Na_3AlF_6)\), which acts as a solvent and reduces the melting point to around 950°C. This mixture forms the electrolyte.

Electrolysis: The molten mixture of aluminum oxide and cryolite is placed in a large electrolytic cell, which typically consists of a carbon-lined steel container acting as the cathode and a carbon anode suspended above the mixture.

• At the Cathode (Negative Electrode): Aluminum ions \((Al^{3+})\) migrate towards the cathode where they gain three electrons each and are reduced to form liquid aluminum metal \((Al)\).
• At the Anode (Positive Electrode): Oxygen ions \((O^{2-})\) from the aluminum oxide are attracted to the anode, where they lose two electrons each and combine to form oxygen gas \((O_2)\).

Collection of Aluminum: The liquid aluminum formed at the cathode is denser than the molten electrolyte, so it settles at the bottom of the cell and is periodically tapped off.

By this process, aluminum metal is extracted from aluminum oxide using electrolysis in a molten electrolyte, mainly composed of cryolite, which significantly lowers the melting point of the mixture, making the process feasible on an industrial scale.