Unreactivity of aluminium oxide:
- Aluminium metal in itself is quite reactive and oxidises readily when brought in contact with oxygen to produce aluminium oxide.
- Aluminium oxide on the other hand is unreactive. It is a thin, hard layer that can actually protect iron from further oxidation, and eventually prevents rusting.
- Aluminium oxide appears as a powdery white or dull gray coating.
- As oxidation occurs, aluminium oxide hardens and creates a protective layer.
- This layer then halts the aluminium corrosion process, and the metal does not deteriorate unless the aluminium oxide is removed.
- And, unlike rust, which has a flaky and reddish appearance, aluminium oxide cannot be easily chipped off the metal surface.
- Additionally, this inert surface layer of aluminium oxide, termed as the “native oxide layer”, is also called a corrosion coating.
- Corrosion coating is basically a light coat of a protective material, such as metal oxide, to create a shell against corrosion.

Extraction of metals:
- For extraction of iron, iron oxide is reduced with carbon.
- Oxides of metals that lie below carbon on the reactivity series can be reduced with carbon for the extraction process.
- As for the very reactive metals, such as aluminium, magnesium, and calcium, which lie above carbon in the reactivity series, instead of reducing with carbon, electrolysis is required to extract the metals.

Extraction of aluminium metal:
- The process of electrolysis is used to extract aluminium.
- The main source of aluminium ore is called bauxite, which contains 30 to 60% aluminium oxide, and a mixture of silica, various iron oxides, and titanium dioxide.
- Extracting aluminium from bauxite is a two-step process including the Bayer process and the Hall–Héroult process.
- The Bayer process is used for extracting alumina, or aluminium oxide, from bauxite.
- After alumina is extracted, it is used in the Hall–Héroult process to extract metallic aluminium.

Hall–Héroult process:
- The Hall–Héroult process is used to smelt aluminium.
- Electrolysing pure molten alumina is impractical because aluminium oxide has a melting point of 2072°C.
- So, the Hall–Héroult process dissolves alumina in a molten mixture of synthetic cryolite (Na₃AlF₆) and calcium fluoride to lower its melting point for easier electrolysis.
  - By dissolving in this mixture of cryolite, the melting point drops to about 1000°C.
- Then, the mixture is electrolysed in a molten salt bath in a purpose-built cell.
- Aluminium fluoride (AlF₃) is usually added to the electrolyte to further reduce the melting point, so that the electrolysis can happen at temperatures between 940°C and 980°C.
- Electrolysis is done by passing a low voltage (under 5V) direct current between 100kA and 300kA through it.
- This causes the liquid aluminium to be deposited at the cathode, and carbon dioxide to be formed at the anode.
  - The anode used here is carbon anode.
  - Carbon dioxide after electrolysis is formed when the oxygen from the alumina combines with the carbon from the anode.

Equations of Hall–Héroult process at the electrodes:
- At the cathode, pure aluminium metal is produced.
  Al³⁺ + 3e⁻ → Al
- At the anode, oxygen reacts with the carbon anode to form carbon monoxide.
  O^2- + C(s) → CO(g) + 2e⁻
- Hence, the overall equation is as such:
  Al₂O₃(s) + 3C(s) → 2Al(l) + 3CO(g)
- In reality, much more carbon dioxide is formed at the anode than carbon monoxide.
  2Al₂O₃(l) + 3C(s) → 4Al(l) + 3CO₂(g)

Placement of electrodes:
- Cathode:
  - The cathode is placed at the bottom of the cell.
  - This is because aluminium has a higher density than the electrolyte and it’s easier to separate aluminium when the denser molten aluminium settles to the bottom.
  - Moreover, additives like lithium fluoride may be added to further decrease the density of the electrolyte and make it easier to extract the molten aluminium.
- Anode:
  - The carbon anodes are placed at the top of the cell.
  - As carbon dioxide gets formed, the carbon anode gets depleted.
  - Since the carbon anodes get used up gradually, they need to be replaced regularly, and this arrangement makes it easier to do so.
  - Also, carbon dioxide gas moves up to the top of the electrolytic cell, so placing the anodes at the top of the cell makes sense.

9.04 - Extraction of Aluminium

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