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The Leblanc process was an early industrial process for making soda ash (sodium carbonate) used throughout the 19th century, named after its inventor, Nicolas Leblanc. It involved two stages: making sodium sulfate from sodium chloride, followed by reacting the sodium sulfate with coal and calcium carbonate to make sodium carbonate. The process gradually became obsolete after the development of the Solvay process.
In 1783, King Louis XVI of France and the French Academy of Sciences offered a prize of 2400 French livre 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. That same year he built the first Leblanc plant for the Duke at Saint-Denis, and this began to produce 320 of soda per year. He was denied his prize money because of the French Revolution.
For more recent history, see industrial history below.
This chemical reaction had been discovered in 1772 by the Sweden chemist Carl Wilhelm Scheele. Leblanc's contribution was the second step, in which a mixture of the salt cake and crushed limestone (calcium carbonate) was reduced by heating with coal. This conversion entails two parts. First is the carbothermic reaction whereby the coal, a source of carbon, Redox the sulfate to sulfide:
In the second stage, is the reaction to produce sodium carbonate and calcium sulfide. This mixture is called black ash.
In response to the Alkali Act, the noxious calcium sulfide was converted into calcium carbonate:
The hydrogen sulfide can be used as a sulfur source for the lead chamber process to produce the sulfuric acid used in the first step of the Leblanc process.
Likewise, by 1874 the Deacon process was invented, oxidizing the hydrochloric acid over a copper catalyst:
The chlorine would be sold for bleach in paper and textile manufacturing. Eventually, the chlorine sales became the purpose of the Leblanc process. The inexpensive chlorine was a contributor to the development of the chloralkali process.
The coal used in the next step must be low in nitrogen to avoid the formation of cyanide. The calcium carbonate, in the form of limestone or chalk, should be low in magnesia and silica. The weight ratio of the charge is 2:2:1 of salt cake, calcium carbonate, and carbon respectively. It is fired in a reverberatory furnace at about 1000 °C. Sometimes the reverberatory furnace rotated and thus was called a "revolver".
The black-ash product of firing must be lixiviated right away to prevent oxidation of sulfides back to sulfate. In the lixiviation process, the black-ash is completely covered in water, again to prevent oxidation. To optimize the leaching of soluble material, the lixiviation is done in cascaded stages. That is, pure water is used on the black-ash that has already been through prior stages. The liquor from that stage is used to leach an earlier stage of the black-ash, and so on.
The final liquor is treated by blowing carbon dioxide through it. This precipitates dissolved calcium and other impurities. It also volatilizes the sulfide, which is carried off as H2S gas. Any residual sulfide can be subsequently precipitated by adding zinc hydroxide. The liquor is separated from the precipitate and evaporated using waste heat from the reverberatory furnace. The resulting ash is then redissolved into concentrated solution in hot water. Solids that fail to dissolve are separated. The solution is then cooled to recrystallize nearly pure sodium carbonate decahydrate.
By the early 19th century, 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 using the Leblanc process was built by William Losh at the Losh, Wilson and Bell works in Walker on the River Tyne in 1816, but steep British 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. The Bonnington Chemical Works was possibly the earliest production, and the chemical works established by James Muspratt in Liverpool and Flint, and by Charles Tennant near Glasgow became some of the largest in the world. Muspratt's Liverpool works enjoyed proximity and transport links to the Cheshire salt mines, the St Helens coalfields and the North Wales and Derbyshire limestone quarries.Peter Reed, Acid Rain and the Rise of the Environmental Chemist in Nineteenth Century Britain, (2014), p. 94 By 1852, annual soda production had reached 140,000 tons in Britain and 45,000 tons in France. By the 1870s, the British soda output of 200,000 tons annually exceeded that of all other nations in the world combined.
The last Leblanc-based soda ash plant in the West closed in the early 1920s, but when during WWII Nationalist China had to evacuate its industry to the inland rural areas, the difficulties in importing and maintaining complex equipment forced them to temporarily re-establish the Leblanc process.
However, the Solvay process does not work for the manufacture of potassium carbonate, because it relies on the low solubility of the corresponding bicarbonate.
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 Alkali Act 1863, the first of several , 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.
The Leblanc process also meant very unpleasant working conditions for the operators. It originally required careful operation and frequent operator interventions (some involving heavy manual labour) into processes giving off hot noxious chemicals.Russell. Colin Archibald, Chemistry, society and environment: a new history of the British chemical industry, Royal Society of Chemistry, 2000. Sometimes, workmen cleaning the reaction products out of the reverberatory furnace wore cloth mouth-and-nose to keep dust and out of the lungs.Described, and called a "gag", in a recorded commentary in the Catalyst chemical industry museum in Runcorn (Cheshire, England), to keep alkali dust out of workers' lungs in the early years of the chemical industry in Britain.
This improved somewhat later as processes were more heavily mechanised to improve economics and uniformity of product.
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 the Leblanc process remained more wasteful and more polluting than the Solvay process. The same is true when it is compared with the later electrolytical processes which eventually replaced it for chlorine production.
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