The Castner process is a process for manufacturing
sodium
Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
metal by
electrolysis
In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...
of molten
sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly corrosive base (chemistry), ...
at approximately 330 °C. Below that temperature, the melt would solidify; above that temperature, the molten sodium would start to dissolve in the melt.
History
The Castner process for production of sodium metal was introduced in 1888 by
Hamilton Castner. At that time (prior to the introduction in the same year of the
Hall-Héroult process) the primary use for sodium metal was as a
reducing agent
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are common reducing agents include hydrogen, carbon ...
to produce
aluminium
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
from its purified ores. The Castner process reduced the cost of producing sodium in comparison to the old method of reducing
sodium carbonate at high temperature using
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
.
[Manufacturer and builder / Volume 20, Issue 9, 1888](_blank)
/ref> This in turn reduced the cost of producing aluminium, 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, the Downs cell replaced the Castner process.[Caveman Chemistry:Dow Electrochemicals](_blank)
/ref>
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).[Newell, Lyman C. ''Descriptive Chemistry'' page 285; D. C. Heath and company, publisher] The cathode reaction is
:2 Na+ + 2 e− → 2Na
The anode reaction is
:4 OH− → O2 + 2 H2O + 4 e−
Despite the elevated temperature, some of the water produced remains dissolved in the electrolyte.
/ref> This water diffuses throughout the electrolyte and results in the reverse reaction taking place on the electrolyzed sodium metal:
:2 Na + 2 H2O → H2 + 2 Na+ + 2 OH−
with the hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
gas also accumulating at (P). This, of course, reduces the efficiency of the process.
See also
* Downs cell
References
Chemical processes
Electrolysis
Metallurgical processes
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