Sodium hexafluoroaluminate

Sodium hexafluoroaluminate, commonly known as cryolite, is an inorganic compound with the formula Na₃AlF₆. This white crystalline solid is used primarily in the production of aluminum metal. It occurs naturally as a rare mineral, found primarily in Greenland, but has been synthesized industrially due to the limited availability of natural deposits.

Properties

Sodium hexafluoroaluminate is a complex fluoride of sodium and aluminium. It is non-flammable and has a low toxicity but is slightly soluble in water. Its solubility increases with temperature, which is a relevant factor in its use in aluminum smelting processes. The compound's melting point is approximately 1012 °C, which makes it an excellent solvent for aluminum oxide (Al₂O₃) in the Hall-Héroult process, the major industrial process for aluminum extraction.

Production and Synthesis

The majority of sodium hexafluoroaluminate used industrially is produced synthetically. The synthetic process involves the reaction of aluminum fluoride (AlF₃) with sodium fluoride (NaF) at high temperatures. Natural cryolite is mined in limited quantities, primarily in Greenland, but the natural reserves are insufficient to meet industrial demand.

Applications

The primary use of sodium hexafluoroaluminate is in the Hall-Héroult process for the extraction of aluminum from its oxide. In this process, it acts as a solvent for the aluminum oxide and lowers the melting point of the mixture, which significantly reduces the energy consumption of the aluminum smelting process. Additionally, it increases the conductivity of the molten solution, which further improves the efficiency of the process.

Beyond its application in aluminum production, sodium hexafluoroaluminate is also used in the manufacture of glass, ceramics, and enamel. It serves as a flux in the production of these materials, lowering the melting point and thus reducing the energy required for their production.

Health and Environmental Impact

Sodium hexafluoroaluminate is considered to have low toxicity, but it can be hazardous if ingested in large quantities. In the workplace, exposure to dust or fumes containing the compound should be controlled to prevent respiratory irritation. Environmental concerns primarily relate to the mining and processing of the compound, which can lead to the release of fluoride compounds into the environment. Proper handling and disposal practices are essential to minimize these impacts.

See Also

• Aluminum fluoride
• Sodium fluoride
• Hall-Héroult process
• Aluminum production