Sodium hexafluoroaluminate

Sodium aluminium hexafluoride is an inorganic compound with formula Na3AlF6. This white solid, discovered by José Bonifácio de Andrada e Silva, occurs naturally as the mineral cryolite and is used extensively in the industrial production of aluminium metal. The compound is the sodium (Na+) salt of the hexafluoroaluminate (AlF63−) ion.

Sodium aluminium hexafluoride
Names
Other names
Sodium fluoroaluminate
Cryolite
Kryolite
Aluminate(3-), hexafluoro-, trisodium, (OC-6-11)-
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
Properties
Na3AlF6
Molar mass 209.94 g/mol
Appearance white powder
Density 2.9 g/cm3, solid
Melting point 950 °C (1,740 °F; 1,220 K)
Boiling point decomposes
0.04% (20°C)[1]
Vapor pressure essentially 0
Hazards
not listed
Lethal dose or concentration (LD, LC):
900 mg/kg (rabbit, oral)[2]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 2.5 mg/m3[1]
REL (Recommended)
 TWA 2.5 mg/m3[1]
IDLH (Immediate danger)
 250 mg/m3 (as F)[1]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Production

Most cryolite is manufactured from aluminium oxides, hydrofluoric acid, and sodium hydroxide or the equivalent reagent hexafluorosilicic acid:[3]

6 NaOH + Al2O3 + 12 HF → 2 Na3AlF6 + 9 H2O

The mineral cryolite was mined at Ivigtût on the west coast of Greenland until the deposit was depleted in 1987.

Use

The main application of synthetic cryolite is as a solvent (or flux) for electrolysis of aluminium oxides such as bauxite. The conversion of aluminium oxides into metallic aluminium requires that the metal ions be dissolved so that they can accept the electrons provided in the electrolysis cell. A mixture of cryolite and some aluminium trifluoride is used as that solvent. Unlike typical solutions, this one requires temperatures approaching 1000 °C to melt. Sodium aluminium hexafluoride is also used as a pesticide. Other uses include a whitener for enamels and an opacifier for glass.[4]

Solubility

Cryolite is poorly soluble in water. The LD50 is 600 mg/kg for the comparable compound aluminium trifluoride.[3]

References
  1. NIOSH Pocket Guide to Chemical Hazards. "#0559". National Institute for Occupational Safety and Health (NIOSH).
  2. "Fluorides (as F)". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  3. Aigueperse, Jean; Mollard, Paul; Devilliers, Didier; Chemla, Marius; Faron, Robert; Romano, René; Cuer, Jean Pierre (2000). "Fluorine Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_307.
  4. http://www.mineralszone.com/minerals/cryolite.html
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