Molten salts have a variety of uses. Molten chloride salt mixtures are commonly used as 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. 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 solar thermal power plants.[1]
A commonly used thermal salt is the eutectic mixture of 60% sodium nitrate and 40% potassium nitrate, which can be used as liquid between 260-550°C. It has a heat of fusion of 161 J/g,[2] and a heat capacity of 1.53 J/(g K).[3]
Experimental salts using Lithium may have a melting point of 116°C while still having a heat capacity of 1.54 J/(g K).[3] Salts may cost $1,000 per ton, and a typical plant may use 30,000 tons of salt.[4]
Regular table salt has a melting point of 800°C and a Heat of fusion of 0.52 kJ/g.[5][6] It is not used in solar power stations.