It has been known since antiquity thanks to the ” natron ” deposits, which consist of evaporitic rocks whose composition is close to Na 2 CO 3 , 10H 2 O and bicarbonate NaHCO 3 . They are evanescent white crystals resulting from the evaporation of water from lakes rich in sodium salts in several countries such as Egypt, Libya, Chad and the United States.
As a curiosity, mention should also be made of the sacred mountain of the Masai, Ol Doinyo Lengai, the only volcano in the world to emit carbonatite, a lava composed mainly of sodium carbonate, black when it is melting and white when it cools.
The Egyptians used natron for cleaning the body and mummification, not in solution, but in the form of desiccant powder as a salting technique. Its use, which goes back to Egyptian civilization, was already known for bleaching, cleaning, preserving meat and leather and, mixed with sand and lime (see Lime ), for obtaining glasses. of various colors.
The extensive use of glass, which initially depended largely on the supply of sodium salts from the Middle East and the exploitation of ash from algae or plants, encouraged chemists to find an industrial solution of preparation. from an abundant salt, sodium chloride (see sodium chloride ). It was Nicolas Leblanc, a French chemist, who in 1789 invented the revolutionary process that bears his name:
| 2 NaCl + H 2 SO 4 ——> Na 2 SO 4 + 2 HCl
Na 2 SO 4 + CaCO 3 + 2C ——> Na 2 CO 3 + CaS + 2 CO 2
In the Leblanc process, hydrochloric acid is produced (see hydrochloric acid) in the first stage, and then, during the reduction of sodium sulphate, Na2SO4, with limestone and coke, calcium sulphide production, Case. When the ” alkali act ” was published in the United Kingdom in 1863, these reaction products were considered waste, which was disposed of in rivers and nature, thus becoming outlawed. The search for new processes became necessary.
It is a Belgian chemist, Ernest Solvay, who in 1863 set up a first factory in Couillet with a new reaction scheme: the Solvay process was born, it supplanted the Leblanc process from 1870 and is still used today. The principle appears simple:
2 NaCl + CaCO 3 ——> Na 2 CO 3 + CaCl 2
In fact, the process steps are more complex. A sodium chloride brine is first formed on which ammonia is absorbed. A lime milk is then created after calcination of the calcium carbonate (see Calcite ) and the carbonation of the ammonia brine then leads to the precipitation of sodium bicarbonate, NaHCO 3 , which, by calcination, leads to sodium carbonate. ammonia is then recycled.
Products such as calcium chloride and CaCl 2 are used as a salt for snow removal (see Calcium) which is more environmentally friendly than sodium chloride. Calcined sodium bicarbonate also forms carbon dioxide that can be recycled to give an ammonium or calcium carbonate in the process loop.
2 NaHCO 3 ——> Na 2 CO 3 + H 2 O + CO 2
The discovery in 1950 of a huge deposit of natural carbonate whose ” Trona ” ore of composition Na 2 CO 3 , NaHCO 3 , 2H 2 O resulting from the drying up of lakes of Wyoming, Colorado and Utah revealed so-called geological veins of ” Green River ” which constitute a reserve of 22 Gt, with a simple process of calcination:
2 (Na 2 CO 3 , NaHCO 3 , 2H 2 O) ——> 3 Na 2 CO 3 + 5 H 2 O + CO 2
The carbonate is solubilized in water to separate the insoluble impurities and then again precipitated and dried. The cost is obviously lower than the Solvay process and the synthetic production plants that were 16 in the United States in 1950 disappeared one after the other until 1986.
Global production of soda ash is in the order of 45 Mt / yr, the largest producer being China since 2005 (18 Mt), followed by the United States (11 Mt). France produces about 1Mt. Solvay is the leader, followed by Tata Chemicals, FMC and OCI. In France, the Dombasle plant (Meurthe et Moselle), owned by Solvay, produces around 700,000 t / year.
The main uses of sodium carbonate are:
- the manufacture of glass, essentially soda-lime (Na-SiO 2 -CaO),
- use as an intermediate in industrial chemistry: phosphates, silicates (see Phosphoric acid , Silica ),
- use as an intermediate in the preparation of detergents, soaps and detergents (see Soaps ),
- the desulphurization of cast irons in metallurgy,
- the preparation of ceramics for enameling (see Enamels ),
- in the food industry as anticaking agent and pH regulator,
- in the agri-food industry as a yeasting agent. The carbonate is a constituent of the baking powder: mixed with tartaric acid (see tartaric acid ), it releases carbon dioxide which inflates the bread dough or that of pastries.