Solvay Process

The Solvay process or ammonia-soda process is the major industrial process for the production of soda ash (valued primarily for its content of sodium carbonate). The ammonia-soda process was developed into its modern form by Ernest Solvay during the 1860s.
The ingredients for this process are readily available and inexpensive: salt brine (from inland sources or from the sea) and limestone (from mines). The worldwide production of soda ash in 2005 has been estimated at 42 billion kilograms, which is more than six kilograms per year for each person on Earth. Solvay-based chemical plants now produce roughly three-quarters of this supply, with the remainder being mined from natural deposits.

Chemistry.
The Solvay Process as an example of a cyclic process in chemical industry (green = reactants, black = intermediates, red = products)

(source : Wikipedia)

The Solvay process results in soda ash (predominantly sodium carbonate (Na2CO3)) from brine (as a source of sodium chloride (NaCl)) and from limestone (as a source of calcium carbonate (CaCO3)).

The overall process is:

The actual implementation of this global, overall reaction is intricate.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).

In industrial practice, the reaction is carried out by passing concentrated brine through two towers. In the first, ammonia bubbles up through the brine (salt water) 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 pH; without the ammonia, a hydrochloric acid byproduct would render the solution acidic, and arrest the precipitation.
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 °C. The calcium carbonate (CaCO3) in the limestone is partially converted to quicklime (calcium oxide (CaO)) and carbon dioxide:

The sodium bicarbonate (NaHCO3) that precipitates out in reaction (I) is filtered out from the hot ammonium chloride (NH4Cl) solution, and the solution is then reacted with the quicklime (calcium oxide (CaO)) left over from heating the limestone in step (II).

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 (NaHCO3) precipitate from reaction (I) is then converted to the final product, sodium carbonate (washing soda: Na2CO3), by calcination (160 - 230 C), producing water and carbon dioxide as byproducts:

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 sold as road salt.
In the modified Solvay process developed by Chinese chemist Hou Debang in 1930s, the first few steps are the same as the Solvay process. However, the CaCl2 is supplanted by ammonium chloride (NH4Cl). 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°C. Next, the solution is cooled to 10°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 CaCl2, 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.

Note:

- The Solvay process is less polluting and less expensive than the Leblanc process; chemical process which allows to obtain the sodium carbonate from sea salt. It is because it rejects polluting hydrochloric acid (HCl in the first step) and calcium sulfide, CaS (last step).

- Salt and chalk are plentiful and inexpensive, and the ammonia is recycled during the reaction.

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