Solvay Process - Chemistry

Chemistry

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:

2 NaCl + CaCO₃ → Na₂CO₃ + CaCl₂

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 (NaCl) and ammonia (NH₃).

NaCl + CO₂ + NH₃ + H₂O → NaHCO₃ + NH₄Cl (I)

In industrial practice, the reaction is carried out by passing concentrated brine 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 (NaHCO₃) 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:

CaCO₃ → CO₂ + CaO (II)

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).

2 NH₄Cl + CaO → 2 NH₃ + CaCl₂ + H₂O (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 (NaHCO₃) precipitate from reaction (I) is then converted to the final product, sodium carbonate (Na₂CO₃), by calcination (160 - 230 C), producing water and carbon dioxide as byproducts:

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (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 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.