Sodium hexafluoroaluminate is an inorganic compound with formulaNa3AlF6. This white solid, discovered in 1799 by Peder Christian Abildgaard (1740–1801),[4][5] 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.
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:
H3AlF6 + 3NH3 → (NH4)3AlF6
(NH4)3AlF6 + 3NaOH → Na3AlF6 + 3NH3 + 3H2O
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.[2]
Structure
It adopts a perovskite-like structure. The AlF3−6 centers are nearly idealized octahedra. Na+ occupy both six- and distorted 8-coordinate sites.[7]
Safety
The LD50 is 600 mg/kg for the comparable compound aluminium trifluoride.[2] Cryolite is poorly soluble in water.
Related compounds
Chiolite ( Na5Al3F14), another sodium fluoroaluminate.
^(Staff) (1799). "Norwegische Titanerze und andre neue Fossilien" [Norwegian titanium ores and other new minerals]. Allgemeine Journal der Chemie (in German). 2: 502. "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.)
^Abildgaard, P. C. (1800). "Om Norske Titanertser og om en nye Steenart fra Grönland, som bestaaer af Flusspatsyre og Alunjord" [On Norwegian titanium ores and on a new mineral from Greenland, which consists of hydrofluoric acid and alumina]. Det Kongelige Danske Videnskabers-Selskabs (The Royal Danish Scientific Society). 3rd series (in Danish). 1: 305–316. [From p. 312] Han har kaldt denne grönlandske Steen Kryolith eller Iissteen formedelst dens Udseende, og fordi den smelter saa meget let for Blæsröret. (He has named this Greenlandic stone cryolite or ice stone on account of its appearance, and because it melts so easily under a blowpipe.)
^F. C. Hawthorne; R. B. Ferguson (1975). "Refinement of the crystal structure of cryolite". The Canadian Mineralogist. 13: 377–382.((cite journal)): CS1 maint: multiple names: authors list (link)
