Leblanc process

The Leblanc process was the industrial process for the production of soda ash (sodium carbonate) used throughout the 19th century, named after its inventor, Nicolas Leblanc. It involved two stages: Production of sodium sulfate from sodium chloride, followed by reaction of the sodium sulfate with coal and calcium carbonate to produce sodium carbonate. The process gradually became obsolete after the invention of the Solvay process.

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Background

Soda ash (sodium carbonate) 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 coastal plants called barilla, 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.^[1][2]

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. That same year he built the first Leblanc plant for the Duke at Saint-Denis, and this began to produce 320 tons of soda per year.^[3] Sadly, he never received his prize money, as a result of the French Revolution.^[3]

For more recent history, see #Industrial history below.

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

2 NaCl + H₂SO₄ → Na₂SO₄ + 2 HCl

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.

Na₂SO₄ + CaCO₃ + 2 C → Na₂CO₃ + CaS + 2 CO₂

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. This extraction process was termed lixiviation.

Process detail

The sodium chloride is initially mixed with sulfuric acid solution and the mixture exposed to low heat. Much of the hydrogen chloride gas (and virtually all of the economically recoverable HCl gas) escapes at this point. This continues until all that is left is a fused mass. This mass still contains enough chloride to contaminate the later stages of the process. The mass is then exposed to direct flame, which evaporates nearly all of the remaining chloride.^[4]

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.^[5]

The black-ash product of firing must be lixiviated right away to prevent oxidation of sulfides back to sulfate.^[5] 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 been already been through prior stages. The liquor from that stage is used to leach an earlier stage of the black-ash, and so on. ^[5]

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.^[5]

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.^[3]

By the early 1800s, 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.^[6] The first British soda works was built by John Losh on the River Tyne in 1816, 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 1852, annual soda production had reached 140,000 tons in Britain and 45,000 tons in France.^[6] 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 quite damaging to the local environment. 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. Also, an insoluble, smelly solid waste was produced. For every 8 tons of soda ash, the process produced 5.5 tons of hydrogen chloride and 7 tons of calcium sulfide waste. This solid waste (known as galligu) 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 toxic 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.

Obsolescence

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. From the late 1870s, Solvay-based soda works on the European continent provided stiff competition in their home markets to the Leblanc-based British soda industry. Additionally the Brunner Mond Solvay plant which opened in 1874 at Winnington near Northwich provided fierce competition nationally. By 1900, 90% of the world's soda production was through the Solvay method, or on the North American continent, 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.^[2] However, because of solubility of the bicarbonate, the Solvay process does not work for the manufacture of potassium carbonate, and the Leblanc process continued in limited use for its manufacture until much later.

Biodiversity

Ironically there is a strong case for arguing that Leblanc process waste is the most endangered habitat in the UK, since only 4 sites have survived the new millennium; 3 are protected as local nature reserves of which one (the biggest, at Moses Gate country park) is an SSSI largely for its sparse orchid-calcicole flora, most unusual in an area with acid soils. This alkaline island contains within it an acid island, where acid boiler slag was deposited, which now shows up as a zone dominated by heather Calluna vulgaris ^[7]

References

1. ^ Clow, Archibald and Clow, Nan L. (1952). Chemical Revolution, (Ayer Co Pub, June 1952), pp. 65-90. ISBN 0836919092.
2. ^ ^{a b} Kiefer, David M. (2002) "It was all about alkali" Volume 11 (1), pages 45-6. Retrieved on 22 April 2007.
3. ^ ^{a b c} Aftalion, Fred (1991). A History of the International Chemical Industry. Philadelphia: University of Pennsylvania Press, 11-13. ISBN 0-8122-1297-5. 
4. ^ Hydrochloric Acid and Sodium Sulphate. Lenntech. Retrieved on 22 April 2007.
5. ^ ^{a b c d} The Soda Industries. Lenntech. Retrieved on 22 April 2007.
6. ^ ^{a b} Aftalion, Fred (1991). A History of the International Chemical Industry. Philadelphia: University of Pennsylvania Press, 14-16. ISBN 0-8122-1297-5. 
7. ^ Shaw, PJA & Halton W. (1998). Classic sites: Nob End, Bolton. British Wildlife 10, 13-17.