Extraction of Aluminium — Revision Notes

⚡ 30-Second Revision

Bauxite: — $Al_2O_3 cdot xH_2O$ (ore)
Bayer's Process: — Purification of bauxite to alumina.

- Digestion: $Al_2O_3 cdot xH_2O + 2NaOH \rightarrow 2Na[Al(OH)_4] + (x-1)H_2O$ - Precipitation: $Na[Al(OH)_4] xrightarrow{Cooling, Seeding} Al(OH)_3 + NaOH$ - Calcination: $2Al(OH)_3 xrightarrow{1000-1200^circ C} Al_2O_3 + 3H_2O$

Hall-Héroult Process: — Electrolytic reduction of alumina.

- Electrolyte: $Al_2O_3$ dissolved in molten Cryolite ($Na_3AlF_6$) + Fluorspar ($CaF_2$). - Temperature: $950-1000^circ C$. - Cathode (Carbon lining): $Al^{3+} + 3e^- \rightarrow Al (l)$ - Anode (Graphite rods): $C (s) + 2O^{2-} (melt) \rightarrow CO_2 (g) + 4e^-$ - Role of Cryolite: Lowers melting point of alumina, increases conductivity. - Anode Consumption: Due to reaction with oxygen released at anode.

2-Minute Revision

Aluminium extraction is a two-stage process. First, bauxite, the ore, undergoes Bayer's Process for purification. Bauxite ($Al_2O_3 cdot xH_2O$) is digested with hot, concentrated $NaOH$ solution.

Aluminium hydroxide, being amphoteric, dissolves to form soluble sodium meta-aluminate ($Na[Al(OH)_4]$). Insoluble impurities like iron oxides and silica are filtered off as 'red mud'. The solution is then cooled, diluted, and seeded with $Al(OH)_3$ to precipitate pure aluminium hydroxide.

This is then calcined at high temperatures ($1000-1200^circ C$) to yield pure alumina ($Al_2O_3$).

The second stage is the Hall-Héroult Process, an electrolytic reduction. Pure alumina is dissolved in molten cryolite ($Na_3AlF_6$) and fluorspar ($CaF_2$) at $950-1000^circ C$. Cryolite lowers the melting point and increases conductivity.

In the electrolytic cell, the carbon lining acts as the cathode, where $Al^{3+}$ ions are reduced to molten aluminium ($Al^{3+} + 3e^- \rightarrow Al$). Graphite rods act as anodes, where $O^{2-}$ ions are oxidized and react with carbon to form $CO_2$ ($C + 2O^{2-} \rightarrow CO_2 + 4e^-$), leading to continuous anode consumption.

Molten aluminium collects at the bottom and is tapped off. This process is highly energy-intensive.

5-Minute Revision

The extraction of aluminium from its chief ore, bauxite ($Al_2O_3 cdot xH_2O$), is a critical industrial process due to aluminium's widespread applications. It proceeds in two main stages: purification of bauxite by the Bayer's process and electrolytic reduction of alumina by the Hall-Héroult process.

1. Bayer's Process (Purification of Bauxite):

This hydrometallurgical method aims to remove impurities (mainly iron oxides and silica) from bauxite to obtain pure alumina ($Al_2O_3$).

Digestion: — Crushed bauxite is treated with hot, concentrated sodium hydroxide ($NaOH$) solution at $150-200^circ C$ and high pressure. Aluminium hydroxide dissolves due to its amphoteric nature:

$Al_2O_3 cdot xH_2O (s) + 2NaOH (aq) \rightarrow 2Na[Al(OH)_4] (aq) + (x-1)H_2O (l)$

Filtration: — The insoluble impurities, known as 'red mud' (primarily $Fe_2O_3$, $TiO_2$), are filtered off.
Precipitation: — The clear sodium meta-aluminate solution is cooled, diluted, and seeded with fresh $Al(OH)_3$ crystals. This causes pure aluminium hydroxide to precipitate:

$Na[Al(OH)_4] (aq) xrightarrow{Cooling, Seeding} Al(OH)_3 (s) + NaOH (aq)$

Calcination: — The precipitated $Al(OH)_3$ is washed, dried, and heated strongly ($1000-1200^circ C$) to produce anhydrous alumina:

$2Al(OH)_3 (s) \rightarrow Al_2O_3 (s) + 3H_2O (g)$

2. Hall-Héroult Process (Electrolytic Reduction):

This electrometallurgical method reduces pure alumina to molten aluminium metal.

Electrolyte: — Pure alumina has a very high melting point. To overcome this, it is dissolved in a molten mixture of cryolite ($Na_3AlF_6$) and a small amount of fluorspar ($CaF_2$) at $950-1000^circ C$. Cryolite acts as a solvent, lowering the melting point, and increases electrical conductivity.
Electrolytic Cell: — A large steel tank lined with carbon acts as the cathode. Large graphite rods suspended in the electrolyte act as anodes.
Reactions: — When a strong direct current is passed:

* At Cathode: $Al^{3+}$ ions (from dissolved $Al_2O_3$) gain electrons and are reduced to molten aluminium. $Al^{3+} (melt) + 3e^- \rightarrow Al (l)$ * At Anode: $O^{2-}$ ions (from dissolved $Al_2O_3$) lose electrons and react with the carbon anode to form $CO_2$ gas. This leads to continuous consumption of the carbon anodes. $C (s) + 2O^{2-} (melt) \rightarrow CO_2 (g) + 4e^-$

Product: — Molten aluminium, being denser, collects at the bottom of the cell and is periodically tapped off. This process is highly energy-intensive due to the large electrical energy required for reduction.

Prelims Revision Notes

Extraction of Aluminium: Key Facts for NEET

1. Ore: Bauxite ($Al_2O_3 cdot xH_2O$).

2. Two Main Stages:

a. Purification of Bauxite (Bayer's Process) b. Electrolytic Reduction of Alumina (Hall-Héroult Process)

3. Bayer's Process (Purification):

* Principle: Utilizes the amphoteric nature of aluminium hydroxide to selectively dissolve it in strong base. * Steps & Reactions: 1. Digestion: Bauxite + Hot, conc. $NaOH$ ($150-200^circ C$, high pressure).

$Al_2O_3 cdot xH_2O (s) + 2NaOH (aq) \rightarrow 2Na[Al(OH)_4] (aq) + (x-1)H_2O (l)$ 2. Filtration: Removes insoluble impurities (red mud: $Fe_2O_3$, $TiO_2$). 3. Precipitation: Cooling, dilution, and seeding with $Al(OH)_3$ crystals.

$Na[Al(OH)_4] (aq) \rightarrow Al(OH)_3 (s) + NaOH (aq)$ 4. Calcination: Heating $Al(OH)_3$ at $1000-1200^circ C$. $2Al(OH)_3 (s) \rightarrow Al_2O_3 (s) + 3H_2O (g)$ * Product: Pure alumina ($Al_2O_3$).

4. Hall-Héroult Process (Electrolytic Reduction):

* Principle: Electrolysis of molten alumina to produce aluminium metal. * Electrolytic Cell: Steel tank lined with carbon (cathode), graphite rods (anodes). * Electrolyte: Molten mixture of: * **Alumina ($Al_2O_3$):** Source of $Al^{3+}$ ions.

* **Cryolite ($Na_3AlF_6$): Crucial role:** Lowers melting point of alumina from $2072^circ C$ to $950-1000^circ C$, increases electrical conductivity. * **Fluorspar ($CaF_2$):** Further lowers melting point, improves fluidity.

* **Reactions ($950-1000^circ C$):** * At Cathode (Carbon lining): Reduction of $Al^{3+}$ ions. $Al^{3+} (melt) + 3e^- \rightarrow Al (l)$ * At Anode (Graphite rods): Oxidation of $O^{2-}$ ions and reaction with carbon.

$C (s) + 2O^{2-} (melt) \rightarrow CO_2 (g) + 4e^-$ (Anodes are continuously consumed, producing $CO_2$ gas). * Product: Molten aluminium (collected at bottom). * Energy: Highly energy-intensive process due to high electrical energy required for reduction.

5. Key Concepts to Remember:

* Aluminium is highly reactive, hence not extracted by carbon reduction. * Amphoteric nature of $Al(OH)_3$ is key to Bayer's. * Cryolite is a solvent and conductivity enhancer, not a reactant. * Carbon anodes are consumed, leading to $CO_2$ emissions. * High energy demand is a characteristic of the Hall-Héroult process.

Vyyuha Quick Recall

To remember the steps of Aluminium Extraction:

Bright Aluminum Yearns Electrolysis Really Soon!

Bauxite (Ore)
Amphoteric (Alumina's nature in Bayer's)
Yes, NaOH (Reagent for digestion)
Eliminate Red Mud (Filtration)
Re-precipitate (Al(OH)3 with seeding)
Strong Heat (Calcination to Al2O3)

Heavy Aluminum Loves Lots of Current Really Yummy Oxygen Leaves In Tanks Everywhere!

Hall-Héroult (Process name)
Aluminum (Product)
Liquid (Molten state)
Low Temp (with Cryolite)
Current (Electricity for electrolysis)
Reduction (at Cathode)
Yummy (Cryolite - solvent)
Oxygen (at Anode)
Leaves (CO2 gas)
In (Carbon Anodes)
Tanks (Electrolytic cell)
Everywhere (Consumed anodes)