molten salt

{{Short description|Salt that has melted, often by heating to high temperatures}}

Molten salt is salt which is solid at standard temperature and pressure but liquified due to elevated temperature. A salt that is liquid even at standard temperature and pressure is usually called a room-temperature ionic liquid, and molten salts are technically a class of ionic liquids.

Examples

As a reference, molten sodium chloride, table salt has a melting point (m.p.) of {{cvt|801|C|F}}. A variety of eutectic mixtures have been developed with lower melting points:

=Chlorides=

Lithium chloride and potassium chloride, m.p. {{cvt|450|C|F}}.{{cite book |doi=10.1002/0471238961.liqupagn.a01 |chapter=Liquid Salts for Reactions |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2012 |last1=Johnson |first1=Keith E. |last2=Pagni |first2=Richard M. |pages=1–35 |isbn=9780471484943}}

=Nitrates=

Alkali metal nitrates are relatively low melting and thermally stable. The least stable, lithium nitrate (m.p. {{cvt|255|C|F}}) decomposes only at {{cvt|474|C|F}}. At the other extreme, cesium nitrate melts at {{cvt|414|C|F}} and decomposes at 584 °C.{{Greenwood&Earnshaw2nd|page=469}}

60:40 mixture of sodium nitrate and potassium nitrate is a liquid between {{cvt|260-550|C|F}}. It has a heat of fusion of 161 J/g,"[http://www.archimedesolarenergy.it/molten_salt.htm Molten salts properties]" and a heat capacity of 1.53 J/(g·K).
1:1 mixture {{chem2|LiNO3}}:{{chem2|KNO3}}, m.p. {{cvt|125|C|F}}.{{Greenwood&Earnshaw2nd|page=90}}
40:7:53 {{chem2|NaNO2}}:{{chem2|NaNO3}}:{{chem2|KNO3}}, m. p. {{cvt|142|C|F}}, stable to {{cvt|600|C|F}}.

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:

:{{chem2|MgO + C + Cl2 → MgCl2 + CO}}

Electrolysis of the resulting molten magnesium chloride is conducted at {{cvt|700|C|F}}:{{cite book |doi=10.1002/0471238961.1301071423091219.a01.pub3 |chapter=Magnesium and Magnesium Alloys |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2010 |last1=Kramer |first1=Deborah A. |pages=1–55 |isbn=9780471484943}}

:{{chem2|MgCl2 → Mg + Cl2}}

Aluminium metal is produced from aluminium oxides by electrolysis of a molten mixture of sodium hexafluoroaluminate and alumina at {{cvt|950|C|F}}. This conversion is called the Hall-Haroult process.{{cite book |doi=10.1002/0471238961.0112211307180113.a01.pub3 |chapter=Aluminum Halides and Aluminum Nitrate |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2004 |last1=Grams |first1=G. W. |last2=Conley |first2=B. |last3=Shaikh |first3=T. |last4=Atwood |first4=D. A. |isbn=9780471484943}}

=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.{{cite web |title=Molten Salts systems other applications link to Solar Power Plants |publisher=National Renewable Energy Laboratory (NREL) |url=http://www.nrel.gov/csp/troughnet/pdfs/2007/koning_molten_salt_applications.pdf |access-date=2011-09-06 |url-status=dead |archive-url=https://web.archive.org/web/20111019111155/http://www.nrel.gov/csp/troughnet/pdfs/2007/koning_molten_salt_applications.pdf |archive-date=2011-10-19}}{{cite journal |last1=Bauer |first1=Thomas |last2=Odenthal |first2=Christian |last3=Bonk |first3=Alexander |date=April 2021 |title=Molten Salt Storage for Power Generation |url=https://onlinelibrary.wiley.com/doi/10.1002/cite.202000137 |journal=Chemie Ingenieur Technik |language=de |volume=93 |issue=4 |pages=534–546 |doi=10.1002/cite.202000137 |s2cid=233913583 |issn=0009-286X}}

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).Reddy, Ramana G. "[http://energy.gov/sites/prod/files/2014/01/f7/csp_review_meeting_042413_reddy.pdf Novel Molten Salts Thermal Energy Storage for Concentrating Solar Power Generation]" page 9 University of Alabama College of Engineering. Retrieved 9 December 2014.

=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,{{Cite journal |last1=Hurley |first1=Frank H. |last2=Wier |first2=Thomas P. |date=1951 |title=Electrodeposition of Metals from Fused Quaternary Ammonium Salts |url=https://iopscience.iop.org/article/10.1149/1.2778132 |journal=Journal of the Electrochemical Society |language=en |volume=98 |issue=5 |pages=203 |doi=10.1149/1.2778132}} 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.

References

Bibliography

C.F. Baes, [https://digital.library.unt.edu/ark:/67531/metadc1028644/m2/1/high_res_d/4470742.pdf The chemistry and thermodynamics of molten salt reactor fuels], Proc. AIME Nuclear Fuel Reprocessing Symposium, Ames, Iowa, USA, 1969 (August 25), {{doi|10.1016/0022-3115(74)90124-X}}