Solvay process

The Solvay process or ammonia-soda process is a chemical process for the production of sodium carbonate ( Na ₂ C O ₃ ). It was developed by Ernest Solvay in 1860 and replaced the Leblanc process used up until then .

The world's most important producer is the chemical company Solvay with seven million tonnes of sodium carbonate per year (2010), the majority of which is converted into the most important detergent component sodium percarbonate .

Material balance

Soda production according to Solvay as a chemical cycle process ( Educts , Intermediate products , Products )

The process is based on the cheap raw materials lime (" calcium carbonate ") and table salt ("sodium chloride"), which are available in large quantities, and their reaction to form soda and calcium chloride

${\displaystyle \mathrm {\ CaCO_{3}+2\ NaCl\rightarrow Na_{2}CO_{3}+CaCl_{2}} }$

However, it does not take place voluntarily, as the reverse reaction to the starting materials calcium carbonate and sodium chloride

${\displaystyle \mathrm {\ CaCl_{2}+Na_{2}CO_{3}\rightarrow CaCO_{3}+2\ NaCl} }$

which is thermodynamically preferred. In order to achieve the forward reaction, the Solvay process therefore brings another substance, ammonia , into play, which is then missing in the final mass balance: The Solvay process works as a chemical cycle in which the added ammonia is in a closed cycle remains and environmental pollution of the order of magnitude of the Leblanc process can be avoided.

However, the Solvay process also has disadvantages: It consumes large amounts of water and still produces approx. 1 kg of unusable calcium chloride per kilogram of soda, which is released into the wastewater and thus into rivers and seas. To reduce water pollution, there are also sedimentation basins (lime ponds), e.g. B. in Latdorf near Bernburg, where the surface water is fed back into the Saale river. The production of 1 million tons of soda annually also produces around 1 million tons of calcium chloride. This amount corresponds to three 1000-tonne freight trains every day.

The procedure in detail

The individual steps of the process are:

1. The lime is burned:

${\displaystyle \mathrm {\ CaCO_{3}\rightarrow CaO+CO_{2}\uparrow } }$

Calcium carbonate decomposes in the heat to calcium oxide ("burnt lime") and carbon dioxide.

2. Carbon dioxide is introduced into a concentrated saline solution together with ammonia . Here falls sodium from that of the emerging, ammonium chloride solution is separated.

${\displaystyle \mathrm {\ NaCl+CO_{2}+NH_{3}+H_{2}O\rightarrow NaHCO_{3}\downarrow +NH_{4}Cl} }$

Table salt reacts with carbon dioxide, ammonia and water to form sodium hydrogen carbonate ("soda") and ammonium chloride ("salmiac").

The individual sub-steps:

${\displaystyle \mathrm {\ CO_{2}+H_{2}O\rightarrow H_{2}CO_{3}} }$

Carbon dioxide and water react to form carbonic acid ...

${\displaystyle \mathrm {\ H_{2}CO_{3}+NH_{3}\rightarrow NH_{4}HCO_{3}} }$

... which in turn with ammonia to form ammonium hydrogen carbonate ...

${\displaystyle \mathrm {\ NH_{4}HCO_{3}+NaCl\rightarrow NaHCO_{3}\downarrow +NH_{4}Cl} }$

... that in turn reacts with common salt to form sodium hydrogen carbonate ("soda") and ammonium chloride ("salmiak").

3. The sodium hydrogen carbonate is heated to approx. 200  ° C , whereby water and carbon dioxide escape. What is left is the product soda. This process is known as calcination ( calcining ).

${\displaystyle \mathrm {\ 2\ NaHCO_{3}\rightarrow Na_{2}CO_{3}+H_{2}O+CO_{2}} }$

Sodium hydrogen carbonate splits off water and carbon dioxide in the heat, creating soda.

4. In the fourth step the ammonia is recovered and added to step 2:

${\displaystyle \mathrm {\ 2\ NH_{4}Cl+CaO\rightarrow 2\ NH_{3}+CaCl_{2}+H_{2}O} }$

The ammonium chloride from reaction 2 is reacted with the quick lime from reaction 1 to form ammonia, calcium chloride and water.

Individual evidence

1. ^ Solvay Germany ( Memento from September 27, 2011 in the Internet Archive )
2. ↑ Solvay Germany - Soda and Percarbonat
3. ↑ bbglive.de: "Solvey plant in Bernburg puts new lime pond into operation"

Web links

• Process scheme (PDF file; 966 kB)