Leblanc process
598272
223290735
2008-07-03T12:35:37Z
Lightbot
7178666
Units/dates/other
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]].
== 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 [[United Kingdom|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 0836919092.</ref><ref name="Kiefer">{{cite online journal|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=Today's Chemist at Work|volume=11|issue=1|pages=45-6|accessdaymonth=22 April|accessyear=2007}}</ref>
In 1783, King [[Louis XVI of France]] and the [[French Academy of Sciences]] offered a prize of 2400 [[livre]]s 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 [[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 = 0-8122-1297-5}}</ref> Sadly, he never received his prize money, as a result of the [[French Revolution]].<ref name = 'Aftalion11'>{{cite book | title = A History of the International Chemical Industry }}</ref>
''For more recent history, see [[#Industrial history|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 [[hydrogen chloride|hydrochloric acid gas]] according to the [[chemical equation]]
:2 [[sodium chloride|NaCl]] + [[sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]] → [[sodium sulfate|Na<sub>2</sub>SO<sub>4</sub>]] + 2 [[hydrochloric acid|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 the salt cake was mixed with crushed [[limestone]] ([[calcium carbonate]]) and [[coal]] and fired. In the ensuing chemical reaction, the coal ([[carbon]]) was [[oxidation|oxidized]] to [[carbon dioxide]], [[Redox|reducing]] the [[sulfate]] to [[sulfide]] and leaving behind a solid mixture of sodium carbonate and [[calcium sulfide]], called ''black ash''.
:[[sodium sulfate|Na<sub>2</sub>SO<sub>4</sub>]] + [[calcium carbonate|CaCO<sub>3</sub>]] + 2 [[carbon|C]] → Na<sub>2</sub>CO<sub>3</sub> + [[calcium sulfide|CaS]] + 2 [[carbon dioxide|CO<sub>2</sub>]]
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.<ref name="lenn2">{{cite web|url=http://www.lenntech.com/Chemistry/Hydrochloric-Acid-Sodium-Sulphate.htm|publisher=Lenntech| title=Hydrochloric Acid and Sodium Sulphate|accessdaymonth=22 April|accessyear=2007}}</ref> <!-- <ref name="eb1911">[http://www.1911encyclopedia.org/Alkali_Manufacture Classic Encyclopedia: Alkali Manufacture]</ref> (classic encyclopedia removed this article as of 22 April 2007 -->
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.<ref name="lenn1">{{cite web|url=http://www.lenntech.com/Chemistry/Soda-%20industries.htm|publisher=Lenntech|title=The Soda Industries|accessdaymonth=22 April|accessyear=2007}}</ref>
The black-ash product of firing must be 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 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. <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 [[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 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 = 'Aftalion11'>{{cite book | title = A History of the International Chemical Industry }}</ref>
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.<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 = 0-8122-1297-5}}</ref> The first British soda works was built by [[John Losh]] 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, 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.<ref name = 'Aftalion14'>{{cite book | title = A History of the International Chemical Industry }}</ref> 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 [[hydrogen chloride|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 [[Egg (food)|egg]]s.
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 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.
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 by this time, the Leblanc process was becoming obsolete.
== Obsolescence ==
In 1861, the [[Belgium|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 [[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. 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.<ref name="Kiefer"/> 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 <ref name="shawhalton">Shaw, PJA & Halton W. (1998). Classic sites: Nob End, Bolton. ''British Wildlife'' 10, 13-17.</ref>
== References ==
<references/>
<!-- Categories -->
[[Category:Chemical processes]]
[[Category:Industrial processes]]
[[Category:Industrial Revolution]]
[[Category:French inventions]]
[[de:Leblanc-Verfahren]]
[[fr:Procédé Leblanc]]
[[ja:ルブラン法]]
[[pt:Processo Leblanc]]