Alkali manufacture

The lime process

Historically, sodium hydroxide was produced by treating sodium carbonate (usually from seaweed ash or "glasswort"-plant ash) with calcium hydroxide (aka lime) in a metathesis reaction. (Sodium hydroxide is soluble while calcium carbonate is not.) This process was called causticizing. :Ca(OH)2(aq) + Na2CO3(s) - CaCO3 v + 2 NaOH(aq)

The Leblanc process

Main article: Leblanc process

The Leblanc process, which was invented by Nicolas Leblanc around 1790, begins with the decomposition of sodium chloride by sulfuric acid, by which sodium sulfate and hydrochloric acid are produced. The sodium sulfate is afterwards fired with calcium carbonate and coal. Sodium carbonate can be extracted from this mixture by washing the mixture with water.

Until the rise of the ammonia-soda process, which has better economics, the Leblanc process was used extensively making the United Kingdom the lead in alkali production. By the early 20th century, the UK's production outstripped that of all other producers combined.

Most of the British alkali works are situated in South Lancashire and the adjoining part of Cheshire, near the mouth of the Tyne and in the West of Scotland.

The ammonia–soda process

Main article: Solvay process

In the 1870s the lime process was revolutionary improved to result in the ammonia-soda process, which is the most common method today. The overall reaction appears simple

: 2NaCl + CaCO3 - Na2CO3 + CaCl2

However, its actual implementation of this overall reaction is very inventive. A simplified description can be given using the four different, interacting chemical reactions illustrated in the figure. In the first step in the process, carbon dioxide (CO2) passes through a concentrated aqueous solution of sodium chloride (table salt, NaCl) and ammonia (NH3).

: NaCl + CO2 + NH3 + H2O - NaHCO3 + NH4Cl ---(I)

In industrial practice, the reaction is carried out by passing concentrated brine (salt water) through two towers. In the first, ammonia bubbles up through the brine and is absorbed by it. In the second, carbon dioxide bubbles up through the ammoniated brine, and sodium bicarbonate (baking soda) precipitates out of the solution. Note that, in a basic solution, NaHCO3 is less water-soluble than sodium chloride. The ammonia (NH3) buffers the solution at a basic (high) pH; without the ammonia, a hydrochloric acid byproduct would render the solution acidic, and arrest the precipitation. Here, NH3 along with ammoniacal brine acts as a mother liquor.

There are a number of reasons for this:

• The sodium used in the process is practically free.
• The fuel requirement is halved.
• Efficiency improvements have reduced 'waste' in the process to a minimum. -- The sodium in the latter costs next to nothing, being obtained from natural or artificial brine in which the sodium chloride possesses an extremely slight value. The fuel required is less than half the amount used in the Leblanc process.

Moreover, the ammonia process has been gradually elaborated into a very complicated but perfectly regularly working scheme, in which the cost of labor and the loss of ammonia are reduced to a minimum.

The only way in which the Leblanc process could still hold its own was by being turned in the direction of making caustic soda, to which it lends itself more easily than the ammonia-soda process; but the latter has invaded even this field. One advantage, however, still remained to the Leblanc process. All endeavours to obtain either hydrochloric acid or free chlorine in the ammonia-soda process have proved commercial failures, all the chlorine of the sodium chloride being ultimately lost in the shape of worthless calcium chloride. The Leblanc process thus remained the sole purveyor of chlorine in its active forms, and in this way the fact is accounted for that, at least in Great Britain, the Leblanc process still furnishes nearly half of all the alkali made, though in other countries its proportional share is much less. The profit made upon the chlorine produced has to make up for the loss on the alkali.

The ammonia-soda process was first patented on 30 June 1838 by Harrison Gray Dyar and John Hemming,

Other processes

• Chloralkali process
• Castner-Kellner process

References

References

1. (1925). "General Chemistry: An Elementary Survey Emphasizing Industrial Applications of Fundamental Principles". John Wiley & Sons, Inc..
2. {{EB1911
3. Speight, James. (2001). "Chemical Process and Design Handbook". McGraw Hill.
4. link. (2006-10-03, March 2004. Archived at WebCite from [http://www.cefic.be/files/Publications/ESAPA_Soda_Ash_Process_BREF3.pdf this original URL] {{Webarchive). link. (2008-04-07 on 2008-03-01.)
5. Moore, John T. Edd. (2005). "Chemistry Made Simple". Broadway Books.