A major industrial process used for the production of sodium carbonate known as soda ash. Also known as the ammonia-soda process, approximately three-quarters of all sodium carbonate is produced by this method with the remainder being mined from natural deposits. Developed by Ernest Solvay (1838–1922) in 1861, the process is based on the fact that when excess carbon dioxide is passed into a solution of brine containing ammonia, the ammonium bicarbonate, which is first formed, interacts with the sodium chloride to give a precipitate of sodium bicarbonate as the salt is only sparingly soluble in the brine due to the common ion effect. Sodium carbonate is then readily prepared from the bicarbonate by heating at up to 230^ºC producing carbon dioxide that can be used again. The sodium carbonate is finally ground to a powder. The process is carried out over several stages and starts by passing concentrated brine down two towers. In the first, ammonia gas is bubbled and absorbed in the brine liquid. In the second, carbon dioxide, which is produced by the calcination of limestone, is bubbled up through the ammoniated brine in which sodium bicarbonate precipitates out:

The sodium bicarbonate is the least soluble and is crystallized and filtered out from the hot ammonium chloride solution. The solution is then reacted with the quicklime (calcium oxide) remaining from the calcination of the limestone. As the ammonia is much more (p. 351) costly than the sodium carbonate, it is recovered by adding calcium hydroxide and recycled back to the initial brine solution:

The Solvay process is an improvement on the earlier Leblanc process since the materials are less costly; brine is cheaper than rock salt and no sulphuric acid is required. With no evaporation involved, less energy is required, there are no by-products produced, a purer product is obtained, and the process is continuous, and around 97 per cent of the carbon dioxide in the limestone is converted into sodium carbonate.