Solvay process

The Solvay process, also referred to as the ammonia-soda process is the major industrial process used in the production of soda ash (sodium carbonate). Solvay-based chemical plants produce roughly three-fourths of the world's supply of sodium carbonate, the remainder being mined from natural deposits.

Chemistry

The Solvay process produces sodium carbonate from salt (sodium chloride) and limestone (calcium carbonate).

In neutral or basic solutions, sodium bicarbonate is less water-soluble than sodium chloride. When carbon dioxide passes through a concentrated solution of sodium chloride and ammonia, sodium bicarbonate precipitates according to the following chemical reaction:

NaCl + CO₂ + NH₃ + H₂O → NaHCO₃ + NH₄Cl

The ammonia is necessary to buffer the solution at basic pH. Without it, a hydrochloric acid byproduct would render the solution acidic, and in acidic solutions, the reaction cannot proceed. 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. It the second, carbon dioxide bubbles up through the brine and precipitates sodium bicarbonate. Carbon dioxide for this step is produced by heating calcium carbonate:

CaCO₃ → CO₂ + CaO

The solid sodium bicarbonate is then filtered out and converted to sodium carbonate by heating it, recovering some carbon dioxide in the process:

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Meanwhile, ammonia is recovered from the ammonium chloride byproduct by treating the ammonium chloride solution with the calcium oxide (a strong base) left over from carbon dioxide generation:

2 NH₄Cl + CaO → 2 NH₃ + CaCl₂ + H₂O

The recovered carbon dioxide and ammonia are recycled back to the initial brine solution. 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 and limestone, and its only major byproduct is calcium chloride.

History

Soda ash is important to the glass, soap, paper, and textile industries. Historically, it was extracted from the ashes of marine plants such as barilla or kelp or mined from dry lakebeds in Egypt. By the late 1700's however, these sources were insufficient to meet Europe's demand for the chemical, particuarly in Great Britain. In 1791, the French physician Nicolas Leblanc developed a method to manufacture soda ash using salt, limestone, sulfuric acid, and coal. Although the Leblanc process achieved widespread industrial use, the expense of its inputs and its polluting byproducts (including hydrochloric acid gas) made it apparent that it was far from an ideal solution.

In 1811, the French physicist Augustin Jean Fresnel discovered that sodium bicarbonate precipitates when carbon dioxide is bubbled through ammonia-containing brine—the chemical reaction central to the Solvay process. Over the next fifty years, several groups attempted to reduce this reaction to industrial practice, but none succeeded.

In 1861, the Belgian industrial chemist Ernest Solvay turned his attention to the problem. His solution, an 80-foot-tall gas absorption tower in which carbon dioxide bubbled up through a descending flow of brine, together with efficient recovery and recycling of the ammonia, proved effective, and by 1864, Solvay and his brother Alfred had acquired financial backing and constructed a plant in the Belgian town of Charleroi. The new process proved more economical and less polluting than the Leblanc method, and its use spread. In 1874, the Solvays expanded their facilites with a new, larger plant at Nancy. The same year, the British firm Brunner Mond began operation of the first Solvay-based plant in Great Britain, and in 1884, the Solvays opened a joint venture with the American engineer William Cogswell to produce soda ash in the United States. Ludwig Mond of Brunner Mond was instrumental in making the Solvay process a commercial success through several refinements between 1873 and 1880 that removed byproducts that could slow or halt the mass production of sodium carbonate through use of the process. By the 1890s, Solvay process plants produced the majority of the world's soda ash.

In 1938, large natural deposits of the mineral trona were discovered near the Green River in Wyoming. Sodium carbonate can be mined from this source less expensively than it can be produced by the Solvay process, and since 1986, there have been no Solvay-based plants operating in North America. Throughout the rest of the world, however, the Solvay process remains the major source of soda ash.

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