Briefly describe the stepwise process of extraction of aluminium from its ore. [9]

Al₂O₃·2H₂O+Na₃AlF₆Molten state (950–980°C), Electrolysis (Hall–Héroult ProcessAl+NaF+O₂

Extraction of Aluminium from its Ore (Bauxite)

Aluminium is the most abundant metal in the Earth’s crust, but it is never found in its pure form. It is extracted from its chief ore, bauxite (Al₂O₃·2H₂O), through a series of well-defined steps. The extraction process involves three main stages:

1. Purification of bauxite (Bayer’s process)
2. Electrolysis of pure alumina (Hall–Héroult process)
3. Refining of crude aluminium (Hoopes process)

Each step is carefully designed to obtain pure aluminium metal efficiently.

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Step 1: Purification of Bauxite (Bayer’s Process)

Bauxite is the primary ore of aluminium, but it contains impurities such as iron oxide (Fe₂O₃), silica (SiO₂), and titanium oxide (TiO₂). To obtain pure alumina (Al₂O₃), the Bayer’s process is used.

Process Steps:

1. Crushing and Grinding

   • The bauxite ore is crushed and ground into fine powder to increase surface area for chemical reactions.

2. Digestion with Sodium Hydroxide (NaOH)

   • The powdered bauxite is mixed with concentrated sodium hydroxide (NaOH) solution and heated under pressure (150–200°C, 35–50 atm) in a digester.
   • Aluminium oxide reacts with NaOH to form sodium aluminate (NaAl(OH)₄): $${\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{NaOH}+3{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaAl}(\mathrm{OH}{)}_4$$
   • Impurities like iron oxide, silica, and titanium oxide do not dissolve in NaOH and settle as red mud (a waste product).

3. Filtration and Precipitation

   • The solution is filtered to remove red mud.
   • Seed crystals of Al(OH)₃ are added to the filtrate, which causes sodium aluminate to decompose and form aluminium hydroxide (Al(OH)₃): $$\mathrm{NaAl}(\mathrm{OH}{)}_4\to \mathrm{Al}(\mathrm{OH}{)}_3+\mathrm{NaOH}$$
   • The precipitated Al(OH)₃ is filtered, washed, and then calcined (heated strongly at 1200°C) to remove water and obtain pure alumina (Al₂O₃): $$2\mathrm{Al}(\mathrm{OH}{)}_3\to {\mathrm{Al}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}$$

Key Points:

• The Bayer’s process removes impurities and produces 99% pure alumina.
• The red mud is disposed of carefully as it contains toxic substances.

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Step 2: Electrolysis of Alumina (Hall–Héroult Process)

Pure alumina has a very high melting point (~2072°C), making electrolysis impractical. Instead, it is dissolved in molten cryolite (Na₃AlF₆), which lowers the melting point to ~950–980°C.

Setup of Electrolysis Cell:

The electrolysis is carried out in a steel tank lined with carbon (graphite) acting as the cathode. A carbon anode is dipped into the molten mixture. The overall reaction is:

$$2{\mathrm{Al}}_2{\mathrm{O}}_3+3\mathrm{C}\to 4\mathrm{Al}+3{\mathrm{CO}}_2$$

Electrochemical Reactions:

1. At the Cathode (Reduction):

   • Aluminium ions (Al³⁺) gain electrons and deposit as molten aluminium at the cathode: $${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\to \mathrm{Al}(\mathrm{liquid})$$

2. At the Anode (Oxidation):

   • Oxide ions (O²⁻) lose electrons and react with carbon anodes to form carbon dioxide (CO₂): $$2{\mathrm{O}}^{2-}+\mathrm{C}\to {\mathrm{CO}}_2+4{\mathrm{e}}^{-}$$
   • The carbon anodes gradually burn away, requiring frequent replacement.

Key Points:

• The process consumes large amounts of electricity (15–20 kWh per kg of Al).
• Cryolite acts as a solvent and improves conductivity.
• Molten aluminium sinks to the bottom and is tapped periodically.

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Step 3: Refining of Crude Aluminium (Hoopes Process)

The aluminium obtained from electrolysis contains impurities like silicon, iron, and copper. To obtain 99.9% pure aluminium, the Hoopes process (electrolytic refining) is used.

Al (impure)+Na₃AlF₆Molten state (700–750°C), Electrolysis (Hoopes Process)Al (pure)+AlF₃

Process Steps:

1. Impure aluminium is made the anode, and pure aluminium is used as the cathode.
2. Both are immersed in a molten fluoride electrolyte (e.g., AlF₃·NaF).
3. When an electric current is passed:
   • Aluminium ions (Al³⁺) move to the cathode and deposit as pure aluminium.
   • Impurities either remain in solution or settle as anode sludge.

Key Points:

• This process ensures high-purity aluminium for industrial use.
• The refined aluminium is used in aerospace, electrical wiring, and packaging.

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Summary of the Extraction Process

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Economic and Environmental Considerations

• Energy Consumption: The Hall–Héroult process is highly energy-intensive, accounting for ~2% of global electricity use.
• Anode Consumption: Carbon anodes are consumed, releasing CO₂ (a greenhouse gas).
• Waste Management: Red mud from the Bayer’s process is toxic and requires safe disposal.
• Recycling: Aluminium is highly recyclable, reducing the need for fresh extraction.

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This stepwise extraction ensures that aluminium is obtained in a pure, efficient, and economically viable manner.