Castner Process
The Castner process is a process for manufacturing sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

 metal by electrolysis
In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

 of molten sodium hydroxide at approximately 330°C. Below that temperature the melt would solidify, above that temperature, the metal would start to dissolve in the melt.

Process details

The diagram shows a ceramic crucible with a steel cylinder suspended within. Both cathode (C) and anode (A) are made of iron or nickel. The temperature is cooler at the bottom and hotter at the top so that the sodium hydroxide is solid in the neck (B) and liquid in the body of the vessel. Sodium metal forms at the cathode but is less dense than the fused sodium hydroxide electrolyte. Wire gauze (G) confines the sodium metal to accumulating at the top of the collection device (P). The cathode reaction is
2Na+ + 2e → 2Na

The anode reaction is
2OH → ½O2 + H2O + 2e

Despite the elevated temperature some of the water produced remains dissolved in the electrolyte. This water diffuses throughout the electrolyte and results in the reverse reaction taking place on the electrolized sodium metal:
Na + H2O → ½H2 + Na+ + OH

with the hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 gas also accumulating at (P). This, of course, reduces the efficiency of the process.


The Castner process for production of sodium metal was introduced in 1888 by Hamilton Castner
Hamilton Castner
Hamilton Young Castner was an American industrial chemist.-Biography:He was born in Brooklyn, New York and educated at the Brooklyn Polytechnic Institute, then at the Columbia University School of Mines. He left without a degree and in 1879 joined his brother, E. B. Castner, as a consulting chemist...

. At that time (prior to the introduction in the same year of 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:...

 for electrowinning
Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution or liquefied. Electrorefining uses a similar process to remove impurities from a metal. Both processes use electroplating on a large scale and are important techniques...

 aluminum) the primary use for sodium metal was as a reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

 to produce aluminum from its purified ores. The Castner process reduced the cost of producing sodium in comparison to the old method of reducing sodium carbonate
Sodium carbonate
Sodium carbonate , Na2CO3 is a sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Sodium carbonate is domestically well-known for its everyday use as a water softener. It can be extracted from the...

 at high temperature using carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

. This in turn reduced the cost of producing aluminum, although the reduction-by-sodium method still could not compete with Hall-Héroult. The Castner process continued nevertheless due to Castner's finding new markets for sodium. In 1926, however, the Downs cell
Downs cell
The Downs process is an electrochemical method for the commercial preparation of metallic sodium, in which molten NaCl is electrolyzed in a special apparatus called the Downs cell.-How it works:...

replaced the Castner process.
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