Leblanc process
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The Leblanc process was the industrial process for the production of soda ash (sodium carbonate) used throughout the 19th century.
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Background
Soda ash and potash (potassium carbonate), collectively termed alkali, are vital chemicals in the glass, textile, soap, and paper industries. The traditional source of alkali in western Europe had been potash obtained from wood ashes. However, by the 1700s, deforestation had rendered this means of production uneconomical, and alkali had to be imported. Potash was imported from North America, Scandinavia, and Russia, where large forests still stood. Soda ash was imported from Spain and the Canary Islands, where it was produced from the marine plant barilla, whose ashes contained up to 30% soda ash, or from Egypt, where the mineral natron was mined from dry lakebeds. Especially in Britain, the only local source of alkali was from kelp, which washed ashore in Scotland and Ireland.
In 1783, King Louis XVI of France and the French Academy of Sciences offered a prize of 2400 livres for a method to produce alkali from sea salt (sodium chloride). In 1791, Nicolas Leblanc, physician to Louis Philip II, Duke of Orléans, patented a solution.
Chemistry
The Leblanc process was a batch process in which sodium chloride was subjected to a series of treatments, eventually producing sodium carbonate. In the first step, the sodium chloride was heated with sulfuric acid to produce sodium sulfate (called the salt cake) and hydrochloric acid gas according to the chemical equation
This chemical reaction had been discovered in 1772 by the Swedish chemist Carl Wilhelm Scheele. Leblanc's contribution was the second step, in which the salt cake was mixed with crushed limestone (calcium carbonate) and coal and fired. In the ensuing chemical reaction, the coal (carbon) was oxidized to carbon dioxide, reducing the sulfate to sulfide and leaving behind a solid mixture of sodium carbonate and calcium sulfide, called black ash.
Because sodium carbonate is soluble in water, but neither calcium carbonate nor calcium sulfide is, the soda ash was then separated from the black ash by washing it with water. The wash water was then evaporated to yield solid sodium carbonate.
Industrial history
Leblanc established the first Leblanc process plant in 1791 in St. Denis. However, the French Revolution seized the plant, along with the rest of Louis Philip's estate, in 1794, and publicized Leblanc's trade secrets. Napoleon I returned the plant to Leblanc in 1801, but lacking the funds to repair it and compete against other soda works that had been established in the meantime, Leblanc committed suicide in 1806.
By the early 1800's, French soda ash producers were making 10,000 - 15,000 tons annually. However, it was in Britain that the Leblanc process became most widely practiced. The first British soda works was built by John Losh on the River Tyne in 1807, but steep British tariffs on salt production hindered the economics of the Leblanc process and kept such operations on a small scale until 1824. Following the repeal of the salt tariff, the British soda industry grew dramatically, and the chemical works established by James Muspratt in Liverpool and Charles Tennant near Glasgow became some of the largest in the world. By the 1870s, the British soda output of 200,000 tons annually exceeded that of all other nations in the world combined.
Pollution issues
The Leblanc process plants were decidedly not environmentally friendly. The process of generating salt cake from salt and sulfuric acid released hydrochloric acid gas, and because this acid was industrially useless in the early 1800s, it was simply vented into the atmosphere. In addition, for every 8 tons of soda ash, the process produced 7 tons of calcium sulfide waste. This solid waste had no economic value, and was piled in heaps and spread on fields near the soda works, where it weathered to release hydrogen sulfide, the gas responsible for the odor of rotten eggs.
Because of their noxious emissions, Leblanc soda works became targets of lawsuits and legislation. An 1839 suit against soda works alleged, "the gas from these manufactories is of such a deleterious nature as to blight everything within its influence, and is alike baneful to health and property. The herbage of the fields in their vicinity is scorched, the gardens neither yield fruit nor vegetables; many flourishing trees have lately become rotten naked sticks. Cattle and poultry droop and pine away. It tarnishes the furniture in our houses, and when we are exposed to it, which is of frequent occurrence, we are afflicted with coughs and pains in the head ... all of which we attribute to the Alkali works."
In 1863, the British Parliament passed the first of several Alkali Acts, the first modern air pollution legislation. This act allowed that no more than 5% of the hydrochloric acid produced by alkali plants could be vented to the atmosphere. To comply with the legislation, soda works passed the escaping hydrogen chloride gas up through a tower packed with charcoal, where it was absorbed by water flowing in the other direction. The chemical works usually dumped the resulting hydrochloric acid solution into nearby bodies of water, killing fish and other aquatic life.
By the 1880s, methods for converting the hydrochloric acid to chlorine gas for the manufacture of bleaching powder and for reclaiming the sulfur in the calcium sulfide waste had been discovered, but by this time, the Leblanc process was becoming obsolete.
Obsolesence
In 1861, the Belgian chemist Ernest Solvay developed a more direct process for converting producing soda ash from salt and limestone through the use of ammonia. The only waste product of this Solvay process was calcium chloride, and so it was both more economical and less polluting than the Leblanc method. Throughout the 1870s and 1880s, Solvay-based soda works on the European continent provided stiff competition to the Leblanc-based British soda industry, and by 1900, 90% of the world's soda production was through the Solvay method, or on the North American contintent, through the mining of trona, discovered in 1938, which caused the closure of the last North American Solvay plant in 1986. The last Leblanc-based soda ash plant closed in the early 1920s.