Molten salt
Encyclopedia
Molten salt refers to a salt that is in the liquid phase that is normally a solid at standard temperature and pressure (STP). A salt which is normally liquid at STP is usually called a room temperature ionic liquid
, although technically molten salts are a class of ionic liquids.
salt mixtures are commonly used as baths for various alloy heat treatment
s, 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
. Molten salts (fluoride, chloride, and nitrate
) can also be used as heat transfer fluids as well as for thermal storage. This thermal storage is used in for example solar thermal power plants.
. The first such salt, an N-ethylpyridinium bromide and aluminium chloride
mix, was discovered in 1951.
Ionic liquid
An ionic liquid is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ILs are largely made...
, although technically molten salts are a class of ionic liquids.
Uses
Molten salts have a variety of uses. Molten chlorideChloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
salt mixtures are commonly used as baths for various alloy heat treatment
Heat treatment
Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass...
s, such as annealing
Annealing (metallurgy)
Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating to above the recrystallization temperature, maintaining a suitable temperature, and...
and martempering
Martempering
Martempering is a heat treatment for steel involving austenitisation followed by step quenching, at a rate fast enough to avoid the formation of ferrite, pearlite or bainite to a temperature slightly above the martensite start point. Soaking must be long enough to avoid the formation of bainite...
of steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
. Cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
and chloride salt mixtures are used for surface modification of alloys such as carburizing and nitrocarburizing of steel. Cryolite
Cryolite
Cryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987....
(a fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
salt) is used as a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
for aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
in the production of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
in the Hall-Héroult process
Hall-Héroult process
The Hall–Héroult process is the major industrial process for the production of aluminium. It involves dissolving alumina in molten cryolite, and electrolysing the molten salt bath to obtain pure aluminium metal.-Process:...
. Fluoride, chloride, and hydroxide salts can be used as solvents in pyroprocessing
Pyroprocessing
Pyroprocessing is a process in which materials are subjected to high temperatures in order to bring about a chemical or physical change. Pyroprocessing includes such terms as ore-roasting, calcination and sintering...
of nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
. Molten salts (fluoride, chloride, and nitrate
Nitrate
The nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
) can also be used as heat transfer fluids as well as for thermal storage. This thermal storage is used in for example solar thermal power plants.
Ambient temperature molten salts
Ambient temperature molten salts are present in the liquid phase at standard conditions for temperature and pressureStandard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...
. The first such salt, an N-ethylpyridinium bromide and aluminium chloride
Aluminium chloride
Aluminium chloride is the main compound of aluminium and chlorine. It is white, but samples are often contaminated with iron trichloride, giving it a yellow colour. The solid has a low melting and boiling point. It is mainly produced and consumed in the production of aluminium metal, but large...
mix, was discovered in 1951.
See also
- Electromagnetic pumpElectromagnetic pumpAn electromagnetic pump is a pump that moves liquid metal using electromagnetism. A magnetic field is set at right angles to the direction the liquid moves in, and a current is passed through it. This causes an electromagnetic force that moves the liquid.Applications include pumping liquid metal...
- Molten salt batteryMolten salt batteryMolten salt batteries or liquid sodium battery are a class of primary cell and secondary cell high-temperature electric battery that use molten salts as an electrolyte. They offer both a higher energy density through the proper selection of reactant pairs as well as a higher power density by means...
- Molten salt reactorMolten salt reactorA molten salt reactor is a type of nuclear fission reactor in which the primary coolant, or even the fuel itself is a molten salt mixture...
- Parabolic troughParabolic troughA parabolic trough is a type of solar thermal energy collector. It is constructed as a long parabolic mirror with a Dewar tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the Dewar tube...
- Molten salt oxidationMolten salt oxidationMolten salt oxidation is a non-flame, thermal process that destroys all organic materials while simultaneously retaining inorganic and hazardous components in the melt...