Sodium hexafluoroaluminate is an inorganic compound with formula . This white solid, discovered in 1799 by Peder Christian Abildgaard (1740–1801), "Zugleich theilte er … wie gefrorne Salzlauge schmilzt." (At the same time he also communicated a report on an especially white, spar-like mineral [that had been] brought a few years ago from Greenland to Denmark. According to one of the investigations undertaken regarding it, it consisted of alumina and hydrofluoric acid. A compound of which no similar example has yet occurred in the mineral realm. It has received the name "cryolite" because it melts like frozen brine before the [flame of a] blowpipe.) (He has named this Greenlandic stone cryolite or ice stone on account of its appearance, and because it melts so easily under a blowpipe.) occurs naturally as the mineral cryolite and is used extensively in the industrial production of aluminium. The compound is the sodium (Na+) salt of the hexafluoroaluminate () ion.
Production Most cryolite is manufactured by a variety of related pathways. One route entails combining sodium aluminate and hydrofluoric acid:
Other routes include:
Often the hexafluoroaluminic acid, which is recovered from phosphate mining, is the precursor in a two-step process beginning with neutralization with ammonia to give ammonium hexafluoroaluminate:
The mineral form of sodium hexafluoroaluminate, which is called cryolite, was mined at Ivigtût on the west coast of Greenland until the deposit was depleted in 1987.
Use The dominant 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.
Other uses include a whitener for enamels and an opacifier for glass.
Structure It adopts a perovskite-like structure. The centers are nearly idealized octahedra. Na+ occupy both six- and distorted 8-coordinate sites.
Safety The is 600 mg/kg for the comparable compound aluminium trifluoride. Cryolite is poorly soluble in water.
Related compounds Chiolite ( ), another sodium fluoroaluminate.
References
External links ESPI Metals CDC - NIOSH Pocket Guide to Chemical Hazards