Molten salt
Salt that has melted, often by heating to high temperatures
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Examples
As a reference, molten sodium chloride, table salt, has a melting point (m.p.) of 801 °C (1,474 °F). A variety of eutectic mixtures have been developed with lower melting points:
Chlorides
- Lithium chloride and potassium chloride, m.p. 450 °C (842 °F).
Nitrates
Alkali metal nitrates are relatively low melting and thermally stable. The least stable, LiNO3 (m.p. 255 °C (491 °F)) decomposes only at 474 °C (885 °F). At the other extreme, cesium nitrate melts at 414 °C (777 °F) and decomposes at 584 °C.
- 60:40 mixture of sodium nitrate and potassium nitrate is a liquid between 260 and 550 °C (500 and 1,022 °F). It has a heat of fusion of 161 J/g, and a heat capacity of 1.53 J/(g·K).
- 1:1 mixture LiNO3:KNO3, m.p. 125 °C (257 °F).
- 40:7:53 NaNO2:NaNO3:KNO3, m. p. 142 °C (288 °F), stable to 600 °C (1,112 °F).
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Uses
Molten salts have a variety of uses.
Production of magnesium and aluminium
One industrial application is the production of magnesium, which begins with production of magnesium chloride by chlorination of magnesium oxide:
- MgO + C + Cl2 → MgCl2 + CO
Electrolysis of the resulting molten magnesium chloride is conducted at 700 °C (1,292 °F):
- MgCl2 → Mg + Cl2
Aluminium metal is produced from aluminium oxides by electrolysis of a molten mixture of sodium hexafluoroaluminate and alumina at 950 °C (1,740 °F). This conversion is called the Hall-Héroult process.
Heat transfer
Molten salts (fluoride, chloride, and nitrate) can be used as heat transfer fluids as well as for thermal storage. This thermal storage is used in concentrated solar power plants.
Molten-salt reactors are a type of nuclear reactor that uses molten salt(s) as a coolant or as a solvent in which the fissile material is dissolved. Experimental salts using lithium can be formed that have a melting point of 116 °C while still having a heat capacity of 1.54 J/(g·K).
Other uses
Molten chloride salt mixtures are commonly used as quenching baths for various alloy heat treatments, such as annealing and martempering of steel. Cyanide and chloride salt mixtures are used for surface modification of alloys such as carburizing and nitrocarburizing of steel.
Cryolite (a fluoride salt) is used as a solvent for aluminium oxide in the production of aluminium in the Hall-Héroult process.
Fluoride, chloride, and hydroxide salts can be used as solvents in pyroprocessing of nuclear fuel.
Ambient-temperature molten salts
Ambient-temperature molten salts (also known as ionic liquids) are present in the liquid phase at standard conditions for temperature and pressure. Examples of such salts include N-ethylpyridinium bromide and aluminium chloride mix, discovered in 1951, and ethylammonium nitrate discovered by Paul Walden. Other ionic liquids take advantage of asymmetrical quaternary ammonium cations like alkylated imidazolium ions, and large, branched anions like the bistriflimide ion.