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Cryolite

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Cryolite

Summary: Cryolite (Na₃AlF₆) is a mineral used in the extraction of aluminium. It dissolves aluminium oxide and lowers its melting point from ~2000°C to ~950°C, significantly reducing the energy cost of the Hall-Héroult electrolysis process. Tags: igcse chemistry definition metals extraction Created: 2026-07-14 Last Updated: 2026-07-23

Cryolite is a mineral with the chemical formula Na₃AlF₆ (sodium hexafluoroaluminate). Its primary industrial use is as a solvent for aluminium oxide in the Hall-Héroult process for aluminium extraction.

Role in Aluminium Extraction

In the electrolysis cell:

  1. Pure aluminium oxide (Al₂O₃) from bauxite has a melting point of ~2000°C
  2. Melting pure Al₂O₃ directly would require enormous energy → very expensive
  3. Al₂O₃ is instead dissolved in molten cryolite
  4. The cryolite-Al₂O₃ mixture melts at approximately 950°C
  5. This dramatically reduces energy costs and makes the process economically viable

Why It Works

Cryolite acts as a solvent:

  • It dissolves the Al₂O₃ (like water dissolving salt)
  • The Al³⁺ and O²⁻ ions become free to move in the solution
  • Electrolysis can occur at a much lower temperature
  • The cryolite itself is NOT consumed in the overall process

The Electrolysis Cell

In the cell containing molten cryolite + Al₂O₃ at ~950°C:

  • Cathode (−): Al³⁺ + 3e⁻ → Al(l) — molten aluminium collects at the bottom
  • Anode (+): 2O²⁻ → O₂(g) + 4e⁻ — oxygen gas produced
  • The oxygen reacts with the carbon anodes: C(s) + O₂(g) → CO₂(g)
  • This is why carbon anodes need regular replacement — they are consumed

Key Facts

  • Cryolite = Na₃AlF₆ (sodium hexafluoroaluminate)
  • Used in aluminium extraction (Hall-Héroult process)
  • Dissolves Al₂O₃ and lowers the melting point from ~2000°C to ~950°C
  • Reduces energy costs significantly
  • Not consumed in the overall electrolysis reaction
  • Enables the only commercially viable method for aluminium production

See Also

This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.

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