Molten salt
A molten salt is formed when a salt is heated above its melting point. Molten salts include a variety of liquids. They are largely unknown in everyday life, in science and technology, e.g. B. in manufacturing, but they are used in many processes. Classic molten salts have a temperature of 150 ° C to 1300 ° C. For some time now, low-melting salts ( ionic liquids ) have also been used in molten salts below 80 ° C to replace organic solvents. However, these ionic liquids generally have different properties than molten salts of classic ionic compounds.
properties
| material | Molecular formula | T m |
|---|---|---|
| Lithium chloride | LiCl | 610 ° C |
| Sodium chloride | NaCl | 801 ° C |
| Potassium chloride | KCl | 772 ° C |
| sodium | Na 2 CO 3 | 851 ° C |
| Potassium carbonate | K 2 CO 3 | 901 ° C |
| Nitrate mixture | 7% NaNO 3 , 40% NaNO 2 , 53% KNO 3 | 142 ° C |
| Sodium thiosulfate pentahydrate | Na 2 S 2 O 3 • 5H 2 O | 45-50 ° C |
Many molten salts are stable over a wide temperature range. They have low vapor pressure and viscosity, which makes them easy to handle. Molten salts conduct electricity very well, as the concentration of charge carriers in them is very high.
Applications
As a heat exchanger and storage
Heat treatment and hardening technology
One of the main applications of molten salts is the heat or thermochemical treatment of metals. Due to their temperature uniformity, they are often the medium of choice for annealing and hardening processes. The temperature of these melts can be selected within a wide range. The metal can be heated or cooled in a salt bath. Mainly melting salts such as cyan salt are used.
Heat storage
In solar technology, the crystallization energy of thiosulfates is used to store heat. The salt is melted using water that has been heated in a solar collector on the roof. Later, the energy released during recrystallization can be used to heat the domestic water.
Heat transfer
Liquid salts are also used to transport heat at temperatures well above 100 ° C. The pipe system does not need to be under pressure, as with water vapor. Their area of application lies between the liquids water, (thermal) oils and liquid metals. For example, mixtures of alkali nitrite and nitrate with a melting point of approx. 140 ° C are used.
For example, the salt bath is in the vulcanization ( Salzbadvulkanisierung used). With her, the rubber profile is pressed directly from the press into the molten salt. Window seals, wiper blades and drive belts with smooth, dense surfaces are produced. Here one works with molten alkali nitrates and nitrites or with molten nitrates at temperatures between 240 ° C and 280 ° C.
cleaning
Molten salts are also suitable for cleaning that is difficult to carry out ( thermochemical cleaning of components ). They are often used in mass production to remove oils, greases, paintwork, coatings, oxides, waxes, glasses, plastics or molding sand, mainly from metallic parts. In the case of gas turbine blades, for example, deposits are removed from operation with the help of molten salt.
Flux used in welding or soldering
In brazing , salts are used so that the liquid metal is better distributed.
As an electrolyte
In metal extraction
Some metals are extracted from the molten salt by means of fused salt electrolysis . In this case, the metal oxide or salt is dissolved in the melt and broken down with the help of electricity. The molten salt keeps the metal fluid and prevents immediate oxidation of the molten metal as it sinks into the melt.
Examples are magnesium and aluminum production .
In fuel cells
In molten carbonate fuel cells ( MCFC ), a molten salt of 580 ° C - 660 ° C forms the electrolyte . Alkali carbonates (Li 2 CO 3 / K 2 CO 3 ) are used, for example .
In analytical chemistry
Preliminary trials
Some heavy metal salts give characteristic colors in a phosphate or borate melt ( phosphor salt pearl , borax pearl ). This can be used as a preliminary test for the analysis.
Outcrops
By means of suitable salt melts, the digestion , compounds that are insoluble in aqueous solutions including strong acids or alkalis can be converted into soluble substances.
Chemical reactions in molten salts
Oxidation and reduction
Coal and cyanide act as reducing agents in molten salts; thereby to form carbon monoxide or cyanates . Nitrates , chlorates and peroxides act as oxidizing agents, whereby nitrite , chloride or oxide is formed.