Editing Solvay process (section)

== Chemistry ==
[[File:Solvay Process.PNG|thumb|right|Chemistry of the Solvay Process. Each circle represents a reaction.]]
[[File:Solvay process reaction scheme.svg|thumb|The Solvay Process as an example of a cyclic process in chemical industry (green = reactants, black = intermediates, red = products)]]
The Solvay process results in soda ash (predominantly [[sodium carbonate]] (Na<sub>2</sub>CO<sub>3</sub>)) from [[brine]] (as a source of [[sodium chloride]] (NaCl)) and from [[limestone]] (as a source of [[calcium carbonate]] (CaCO<sub>3</sub>)).<ref name="Kiefer">

{{cite journal
 |author=Kiefer, David M. 
 |url=http://pubs.acs.org/subscribe/journals/tcaw/11/i02/html/02chemchron.html 
 |title=Soda Ash, Solvay Style 
 |journal=Today's Chemist at Work 
 |volume=11 
 |issue=2 
 |date=February 2002 
 |pages=87–88, 90 
 |url-status=unfit 
 |archive-url=https://web.archive.org/web/20030120041011/http://pubs.acs.org/subscribe/journals/tcaw/11/i02/html/02chemchron.html
 |archive-date=January 20, 2003
}} Online version archived at WebCite from [http://pubs.acs.org/subscribe/journals/tcaw/11/i02/html/02chemchron.html this original URL] on 2008-03-12.</ref> The overall process is:

: <chem>2NaCl + CaCO3 -> Na2CO3 + CaCl2 </chem>

The actual implementation of this global, overall reaction is intricate.<ref>{{cite book |last=Speight |first=James |title= Chemical Process and Design Handbook |year=2001 |publisher=McGraw Hill | isbn = 978-0-07-137433-0 }}</ref><ref name="ESAPA">[https://web.archive.org/web/20080407001754/http://www.cefic.be/files/Publications/ESAPA_Soda_Ash_Process_BREF3.pdf "Process Best Practices Reference Document (BREF) for Soda Ash,"] report produced by the [http://www.cefic.be/templates/shwAssocDetails.asp?NID=473&HID=27&ID=60 European Soda Ash Producer's Association] {{Webarchive|url=https://web.archive.org/web/20061003115649/http://www.cefic.be/Templates/shwAssocDetails.asp?NID=473&HID=27&ID=60 |date=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|url=https://web.archive.org/web/20080407001754/http://www.cefic.be/files/Publications/ESAPA_Soda_Ash_Process_BREF3.pdf |date=2008-04-07 }} on 2008-03-01.</ref><ref>{{cite book |last=Moore |first=John T. Edd |title=Chemistry Made Simple |year=2005 |publisher=Broadway Books |isbn=978-0-7679-1702-5 |pages=[https://archive.org/details/chemistrymadesim00moor/page/190 190] |url-access=registration |url=https://archive.org/details/chemistrymadesim00moor/page/190 }}</ref> 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]] (CO<sub>2</sub>) passes through a concentrated [[aqueous solution]] of sodium chloride (table salt, NaCl) and [[ammonia]] (NH<sub>3</sub>).

: <chem>NaCl + CO2 + NH3 + H2O -> NaHCO3 + NH4Cl</chem> ---(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 [[base (chemistry)|basic]] [[Solution (chemistry)|solution]], NaHCO<sub>3</sub> is less water-soluble than sodium chloride. The ammonia (NH<sub>3</sub>) [[buffering agent|buffers]] the solution at a basic (high) [[pH]]; without the ammonia, a [[hydrochloric acid]] byproduct would render the solution [[acid]]ic, and arrest the precipitation. Here, NH<sub>3</sub> along with ammoniacal brine acts as a [[mother liquor]].

The necessary ammonia "catalyst" for reaction (I) is reclaimed in a later step, and relatively little ammonia is consumed. The carbon dioxide required for reaction (I) is produced by heating ("[[calcination]]") of the limestone at 950–1100&nbsp;°C, and by calcination of the sodium bicarbonate (see below). The calcium carbonate (CaCO<sub>3</sub>) in the limestone is partially converted to [[quicklime]] (calcium oxide (CaO)) and carbon dioxide:

: <chem>CaCO3 -> CO2 + CaO</chem> ---(II)

The sodium bicarbonate (NaHCO<sub>3</sub>) that precipitates out in reaction (I) is filtered out from the hot ammonium chloride (NH<sub>4</sub>Cl) solution, and the solution is then reacted with the [[quicklime]] (calcium oxide (CaO)) left over from heating the limestone in step (II).

: <chem>2 NH4Cl + CaO -> 2 NH3 + CaCl2 + H2O</chem> ---(III)

CaO makes a strong basic solution. The ammonia from reaction (III) is recycled back to the initial brine solution of reaction (I).

The sodium bicarbonate (NaHCO<sub>3</sub>) precipitate from reaction (I) is then converted to the final product, sodium carbonate (washing soda: Na<sub>2</sub>CO<sub>3</sub>), by [[calcination]] (160–230&nbsp;°C), producing water and carbon dioxide as byproducts:

: <chem>2 NaHCO3 -> Na2CO3 + H2O + CO2</chem> ---(IV)

The carbon dioxide from step (IV) is recovered for re-use in step (I).  When properly designed and operated, a Solvay plant can reclaim almost all its ammonia, and consumes only small amounts of additional ammonia to make up for losses. The only major inputs to the Solvay process are salt, limestone and [[thermal energy]], and its only major byproduct is [[calcium chloride]], which is sometimes sold as [[road salt]].

After the invention of the [[Haber process|Haber]] and other new ammonia-producing processes in the 1910s and 1920s its price dropped, and there was less need in reclaiming it. So in the modified Solvay process developed by Chinese chemist [[Hou Debang]] in 1930s, the first few steps are the same as the Solvay process, but the CaCl<sub>2</sub> is supplanted by [[ammonium chloride]] (NH<sub>4</sub>Cl). Instead of treating the remaining solution with lime, carbon dioxide and ammonia are pumped into the solution, then sodium chloride is added until the solution saturates at 40&nbsp;°C. Next, the solution is cooled to 10&nbsp;°C. Ammonium chloride precipitates and is removed by filtration, and the solution is recycled to produce more sodium carbonate. Hou's process eliminates the production of calcium chloride. The byproduct ammonium chloride can be refined, used as a fertilizer and may have greater commercial value than CaCl<sub>2</sub>, thus reducing the extent of waste beds.

Additional details of the industrial implementation of this process are available in the report prepared for the European Soda Ash Producer's Association.<ref name="ESAPA" />