Editing Leblanc process

{{short description|Former industrial process for producing sodium carbonate from salt}}
{{Infobox industrial process
| name           = Leblanc process
| caption        =
| type           = Chemical
| sector         = Chlor-alkali industry
| technologies   =
| feedstock      = [[sodium chloride]], [[sulfuric acid]], [[coal]], [[calcium carbonate]]
| product        = [[soda ash]], [[hydrochloric acid]], [[calcium sulfide]], [[carbon dioxide]]
| companies      =
| facility       =
| inventor       = [[Nicolas Leblanc]]
| year           = 1791
| developer      = [[William Losh]], [[James Muspratt]], [[Charles Tennant]]
}}
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]].

== 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 13th century, [[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 ashes of [[glasswort]] plants (called [[barilla]] ashes in Spain), or imported from Syria.<ref>{{Cite journal|last1=Ashtor|first1=Eliyahu|last2=Cevidalli|first2=Guidobaldo|date=1983|title=Levantine Alkali Ashes and European Industries|journal=Journal of European Economic History|volume=12|pages=475–522}}</ref> The soda ash from glasswort plant ashes was mainly a mixture of sodium carbonate and potassium carbonate. In addition in Egypt, naturally occurring sodium carbonate, the mineral [[natron]], was mined from dry lakebeds. In Britain, the only local source of alkali was from [[kelp]], which washed ashore in Scotland and Ireland.<ref name="Clow52">
Clow, Archibald and Clow, Nan L. (1952). ''Chemical Revolution,'' (Ayer Co Pub, June 1952), pp. 65–90. {{ISBN|0-8369-1909-2}}.</ref><ref name="Kiefer">{{Cite journal |journal=Today's Chemist at Work |author=Kiefer, David M.|year=2002|url=http://pubs.acs.org/subscribe/journals/tcaw/11/i01/html/01chemchron.html|title=It was all about alkali|publisher=American Chemical Society |volume=11|issue=1|pages=45–6|accessdate=22 April 2007}}</ref>

In 1783, King [[Louis XVI of France]] and the [[French Academy of Sciences]] offered a prize of 2400 [[French livre|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, Seine-Saint-Denis|Saint-Denis]], and this began to produce 320 [[ton]]s of soda per year.<ref name='Aftalion11'>{{Cite book | first = Fred | last = Aftalion | title = A History of the International Chemical Industry | location = Philadelphia | publisher = University of Pennsylvania Press | year = 1991 | pages = 11–13 | isbn = 978-0-8122-1297-6}}</ref>   He was denied his prize money because of the [[French Revolution]].<ref name="Aftalion14"/>

''For more recent history, see [[#Industrial history|industrial history]] below.''

== Chemistry ==
[[File:Leblanc process reaction scheme.svg|thumb|250px|Reaction scheme of the Leblanc process (green = reactants, black = intermediates, red = products)]]
In the first step, sodium chloride is treated with [[sulfuric acid]] in the [[Mannheim process]].  This reaction produces [[sodium sulfate]] (called the ''salt cake'') and [[hydrogen chloride]]:
: 2 NaCl + H<sub>2</sub>SO<sub>4</sub> → Na<sub>2</sub>SO<sub>4</sub> + 2 HCl

This [[chemical reaction]] had been discovered in 1772 by the [[Sweden|Swedish]] 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]].<ref>{{cite encyclopedia|author=Christian Thieme|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley-VCH|location=Weinheim|year=2000|doi=10.1002/14356007.a24_299|isbn = 978-3527306732|chapter = Sodium Carbonates}}</ref>  This conversion entails two parts. First is the [[carbothermic reaction]] whereby the coal, a source of [[carbon]], [[Redox|reduces]] the [[sulfate]] to [[sulfide]]:
: Na<sub>2</sub>SO<sub>4</sub> + 2&nbsp; C → Na<sub>2</sub>S + 2&nbsp;CO<sub>2</sub>

In the second stage, is the reaction to produce [[sodium carbonate]] and [[calcium sulfide]]. This mixture is called ''black ash''.{{Citation needed|date=January 2021}}

: Na<sub>2</sub>S + CaCO<sub>3</sub> → Na<sub>2</sub>CO<sub>3</sub> + CaS
The soda ash is extracted from the black ash with water.  Evaporation of this extract yields solid sodium carbonate.  This extraction process was termed lixiviation.{{Citation needed|date=January 2021}}

In response to the [[Alkali Act]], the noxious calcium sulfide was converted into calcium carbonate:
:{{chem2|CaS(s) + CO2(g) + H2O(l) -> CaCO3(s) + H2S(g)}}

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:
:{{chem2|4 HCl(g) + O2(g) -> H2O(g) + Cl2(g)}}

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]].{{Citation needed|date=October 2024}}

== Process detail ==
The sodium chloride is initially mixed with concentrated sulfuric acid and the mixture exposed to low heat. The hydrogen chloride gas bubbles off and was discarded to atmosphere before gas absorption towers were introduced. 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.<ref name="lenn2">{{Cite web|url=http://www.lenntech.com/Chemistry/Hydrochloric-Acid-Sodium-Sulphate.htm|publisher=Lenntech| title=Hydrochloric Acid and Sodium Sulphate|accessdate=22 April 2007}}</ref><ref name="EB1911">{{Cite EB1911 |wstitle=Alkali Manufacture|volume=1|pages=674–685}}</ref>

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&nbsp;°C.<ref name="lenn1">{{Cite web|url=http://www.lenntech.com/Chemistry/Soda-%20industries.htm|publisher=Lenntech|title=The Soda Industries|accessdate=22 April 2007}}</ref> Sometimes the reverberatory furnace rotated and thus was called a "revolver".<ref>{{Cite book|last=Museum|first=Victoria and Albert|url=http://worldcat.org/oclc/608086021|title=Catalogue of the mechanical engineering collection in the Science Division of the Victoria and Albert Museum, South Kensington, with descriptive and historical notes.|date=1908|publisher=H.M.S.O|volume=2|pages=107|oclc=608086021}}</ref>

The black-ash product of firing must be [[Leaching (chemistry)|lixiviated]] right away to prevent oxidation of sulfides back to sulfate.<ref name="lenn1" /> 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.<ref name="lenn1" />

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 H<sub>2</sub>S 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.<ref name="lenn1" />

== Industrial history ==

Leblanc established the first Leblanc process plant in 1791 in [[Saint-Denis, Seine-Saint-Denis|St. Denis]].  However, [[French Revolution]]aries seized the plant, along with the rest of Louis Philip's estate, in 1794, and publicized Leblanc's [[trade secret]]s. [[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.<ref name="Aftalion14"/>

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.<ref name='Aftalion14'>{{Cite book | first = Fred | last = Aftalion | title = A History of the International Chemical Industry | location = Philadelphia | publisher = University of Pennsylvania Press | year = 1991 | pages = 14–16 | isbn = 978-0-8122-1297-6}}</ref>   The first British soda works using the Leblanc process was built by [[William Losh|the Losh family of iron founders]] at the [[Losh, Wilson and Bell]] works in Walker on the [[River Tyne]] in 1816, but steep British [[tariff]]s 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,<ref>{{Cite journal|last=Ronalds|first=B.F.|date=2019|title=Bonnington Chemical Works (1822-1878): Pioneer Coal Tar Company|journal=International Journal for the History of Engineering & Technology|volume=89|issue=1–2|pages=73–91|doi=10.1080/17581206.2020.1787807|s2cid=221115202}}</ref> and the chemical works established by [[James Muspratt]] in [[Liverpool]] and [[Flint, Flintshire|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.<ref>Peter Reed, Acid Rain and the Rise of the Environmental Chemist in Nineteenth Century Britain, (2014), p. 94</ref> By 1852, annual soda production had reached 140,000 tons in Britain and 45,000 tons in France.<ref name="Aftalion14"/>  By the 1870s, the British soda output of 200,000 tons annually exceeded that of all other nations in the world combined.{{Citation needed|date=January 2021}}

=== Obsolescence ===
In 1861, the [[Belgium|Belgian]] chemist [[Ernest Solvay]] developed a more direct process for 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 [[Europe]]an 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. Leblanc producers were unable to compete with Solvay soda ash, and their soda ash production was effectively an adjunct to their still profitable production of chlorine, bleaching powder etc. (The unwanted by-products had become the profitable products).  The development of electrolytic methods of [[chlorine]] production removed that source of profits as well, and there followed a decline moderated only by "gentlemen's' agreements" with Solvay producers.<ref>Reader W J ''Imperial Chemical Industries; A History Volume 1 The Forerunners 1870-1926'' Oxford University Press 1970 SBN 19 215937 2</ref> 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 in the West closed in the early 1920s,<ref name="Kiefer" /> 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.<ref>{{Cite book |last=Reardon-Anderson |first=James |url=https://books.google.com/books?id=o_XxL0f-AWMC&q=Leblanc |title=The Study of Change: Chemistry in China, 1840-1949 |date=1991 |publisher=Cambridge University Press |isbn=978-0-521-53325-6 |language=en}}</ref>

However, the Solvay process does not work for the manufacture of [[potassium carbonate]], because it relies on the low solubility of the corresponding [[bicarbonate]].

== 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 [[hydrogen chloride|hydrochloric acid gas]], and because this acid was industrially useless in the early 19th century, 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.{{Citation needed|date=January 2021}}

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."<ref name="Council1840">{{cite book|author=Newcastle upon Tyne (England). Town Council|title=Newcastle Council Reports|url=https://books.google.com/books?id=4hZMAQAAMAAJ&pg=RA2-PA19|year=1840|pages=2–}}</ref>

In 1863, the British Parliament passed the [[Alkali Act 1863]], the first of several [[Alkali Act]]s, 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.{{Citation needed|date=January 2021}}

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.<ref>Russell. Colin Archibald, Chemistry, society and environment: a new history of the British chemical industry, Royal Society of Chemistry, 2000. {{ISBN|0-85404-599-6}}</ref>
Sometimes, workmen cleaning the reaction products out of the reverberatory furnace wore cloth mouth-and-nose [[gag]]s to keep dust and [[aerosol]]s out of the lungs.<ref>Described, and called a "gag", in a recorded commentary in [[Catalyst (museum)|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.</ref><ref>{{cite web |work=Dig This! The Newsletter of 3D Archaeological Society |volume=11 |issue=3 |page=7 |date=September 2017 |title=What did our Ancestors do? |url=https://3darchaeology.co.uk/wp-content/uploads/2017/06/Volume-11-Issue-3.pdf |archive-url=https://web.archive.org/web/20190417141629/https://3darchaeology.co.uk/wp-content/uploads/2017/06/Volume-11-Issue-3.pdf |archive-date=2019-04-17 |last=Beeby |first=Ann |access-date=2022-08-11 }}</ref>

This improved somewhat later as processes were more heavily mechanised to improve economics and uniformity of product.{{Citation needed|date=January 2021}}

By the 1880s, methods for converting the hydrochloric acid to [[chlorine]] gas for the manufacture of [[Bleach (chemical)|bleach]]ing 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.{{Citation needed|date=January 2021}}

== Biodiversity ==

There is a strong case for arguing that Leblanc process waste is the most endangered habitat in the UK, since the waste weathers down to [[calcium carbonate]] and produces a haven for plants that thrive in lime-rich soils, known as [[calcicole]]s. Only four such sites have survived the new millennium; three are protected as local nature reserves of which the largest, at [[Nob End]] near [[Bolton]], is an [[SSSI]] and [[Local Nature Reserve]] - 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]]''.<ref name="shawhalton">Shaw, PJA & Halton W. (1998). Classic sites: Nob End, Bolton. ''British Wildlife'' 10, 13-17.</ref>

== References ==

{{Reflist}}

== External links ==
* [http://www.ebooksread.com/authors-eng/t-howard-deighton/the-struggle-for-supremacy--being-a-series-of-chapters-in-the-history-of-the-le-ala/page-2-the-struggle-for-supremacy--being-a-series-of-chapters-in-the-history-of-the-le-ala.shtml T. Howard Deighton. ''The struggle for supremacy : being a series of chapters in the history of the leblanc alkali industry in Great Britain'': 32:Wealth from Waste] "...this abominable refuse tipped by millions of tons upon the banks of the .. Tyne"
* {{cite book|author=Newcastle upon Tyne (England). Town Council|title=Newcastle Council Reports|url=https://books.google.com/books?id=4hZMAQAAMAAJ&q=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&pg=RA2-PA19|year=1840|page=2}}
* http://cavemanchemistry.com/oldcave/projects/chloralkali/index.html

== See also ==
* [[Sodium carbonate]]
* [[Alkali Act]]
* [[Deacon Process]]
* [[Soda-lime glass]]


{{DEFAULTSORT:Leblanc Process}}
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