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January 2002
Vol. 11, No. 1,
pp 45–46, 49.
Chemistry Chronicles
David M. Kiefer
It was all about alkali

The world’s industrial chemical enterprise developed from the Leblanc process.

During the latter part of the 18th century, French authorities became increasingly concerned about the availability of alkali. Most types of that chemical, which was vital to the country’s blossoming glass, textile, and soap industries, had to be imported. Because France was at odds at the time with Great Britain and much of the rest of Europe, it feared that its supplies might be cut off.

In 1783, Louis XVI ordered the French Academy of Sciences to offer a prize of 2400 livres “to discover the simplest and most economical method of decomposing sea salt on a large scale, in order to secure from it the alkali.” Several scientists proposed plans to accomplish this feat. A French physician, Nicolas Leblanc, devised the only scheme that proved of practical use in 1791. During his lifetime, Leblanc won neither fame nor fortune from his discovery. The process that bore his name, however, would provide the basis for the development of the world’s industrial chemical industry, becoming the most important method of producing chemicals for almost a century.

Previously, alkalies had been obtained largely from plant materials. A chief source had been wood ashes, from which potash (potassium carbonate) was leached by hot water. During the 18th century, in fact, potash (or the more refined pearl ash) was one of the leading exports of Britain’s American colonies. But adequate supplies of wood ash—located in Scandinavia, Russia, and North America—were distant from users in western Europe. A better and more convenient source was barilla, a marine plant found primarily along the Mediterranean coasts, especially in Spain, and on the Canary Islands. Its ash contained as much as 25–30% soda ash (sodium carbonate). The finest alkali came from naturally occurring Egyptian trona (sodium sesquicarbonate) deposits, but hauling it from its desert site was expensive.

In the 18th century, British industry increasingly depended on soda ash derived from kelp, a large brown seaweed harvested along the western coast of Scotland and on the Scottish islands. Kelp was only about half as rich a source of alkali as barilla, but it was not subject to import duties. During the late 18th and early 19th centuries, it was a mainstay of the rural economy of Scotland. Collecting the seaweed was arduous labor, but during the summer months Scottish crofters would neglect their farms to gather the coastal plant. Annual harvests grew to as much as 25,000 tons, providing seasonal employment for up to 100,000 workers.

But the quality of potash from wood ashes and soda ash from barilla or kelp varied widely, and prices were subject to sharp fluctuations. By the late 18th century, with industrial demand for alkali rising, the time was ripe for a supply that was more consistent and dependable and less costly.

Pursuing the Prize
Nicolas Leblanc, born in 1742, pursued a degree in medicine and surgery during the 1760s. In 1780, he was employed as a physician by the Duc d’Orléans. His duties for d’Orléans were not onerous, and chemistry struck his interest. He won modest renown for his investigations of crystallography. When the French academy announced its prize, Leblanc (with financial backing from d’Orléans) began studying the problem of preparing synthetic soda ash. After about five years of work, with the help of a young chemist named J. J. M. Dizé, he finally came up with a practical method.

The first step involved heating common salt with sulfuric acid to produce salt cake (sodium sulfate) and hydrochloric acid:

2NaCl + H2SO4 right arrow Na2 SO4 + 2HCI

The salt cake was then roasted with a mixture of limestone or chalk and coal or charcoal. The resulting solid, known as “black ash”, consisted of soda ash, calcium sulfide, and unburnt coal:

Na2SO4 + CaCO3 + 2C right arrow Na2CO3 + CaS + 2CO2

The soda ash could be washed out of black ash with water.

This procedure did not spring full-blown from Leblanc’s mind. The preparation of salt cake by reacting salt with sulfuric acid was already well known. And several investigators had worked on ways to produce soda ash from salt cake. After the Swedish chemist Carl Wilhelm Scheele discovered in 1772 that salt brine heated with litharge (lead oxide) yielded small amounts of caustic soda, several others modified this approach. For example, mixing salt cake with iron filings and coal made soda ash. Another approach involved mixing salt or salt cake with slaked lime. However, these methods produced only small amounts of soda ash of poor quality that were unsuitable for large-scale output.

The success of Leblanc’s process hinged on his addition of limestone (calcium carbonate) to the reaction. Again with the backing of his patron the Duc d’Orléans, by 1791 he was ready to build a full-size soda works at St. Denis, a small town near Paris. He began production at a rate of 500–600 lb/day. Because France was at war with Spain, barilla supplies had been short-circuited, and Leblanc’s soda ash was in high demand.

By then, France was in the throes of revolution. The property of the Duc d’Orléans, including the soda works, was seized by the revolutionary government. Although he professed sympathy for the new Republic, d’Orléans was guillotined in 1793. Leblanc was forced to disclose all the details of his patented process to the government, which published them for anyone to use.

Over the next few years, Leblanc held several poorly paying government positions. Finally, after numerous petitions, Leblanc’s works at St. Denis were returned to him in 1801. The plant had not operated since 1794 and was dilapidated. He started it up but could not compete against other producers at plants in better condition. His pleas for compensation from the government for having lost his plant were ignored. Despondent and impoverished, he put a bullet through his brain on January 6, 1806.

Postmortem Payment
Leblanc never received the prize offered for a soda ash process. The Academy of Sciences had been dissolved before he completed its development. In 1855, however, Napoleon III bestowed a payment on his heirs in lieu of the earlier award. A statue of him was later unveiled in Paris.

Several plants based on the Leblanc process operated in France in the early 19th century, producing 10,000–15,000 tons of soda ash a year. It was in Britain, however, that the process took hold on a large scale. Early on, the use of Leblanc’s technology was hobbled by steep British import duties on salt. When these were eliminated at the end of 1824, works based on the process sprang up rapidly. One of the first of these was put into operation in 1823 by James Muspratt on the banks of the Mersey River in Liverpool. A second was started up about the same time by Charles Tennant at St. Rollox, near Glasgow, where Tennant had earlier produced bleaching powder (calcium hypochlorite) for sale to textile mills and papermakers. By the 1830s, Tennant’s works were the largest chemical establishment in all of Europe. Another center of Leblanc soda ash output arose along the River Tyne in northeastern England, where the country’s first such small-scale works were built in 1816.

By mid-century, Britain was producing more than 70,000 tons of soda ash a year. Consequently, demand for sulfuric acid greatly increased as well (see TCAW, September 2001, p 57), and large lead-chamber facilities to make the acid were a part of most Leblanc operations. Soda ash made Britain the world’s dominant manufacturer of chemicals in the mid-19th century.

The rise of the alkali industry had a major impact on Britain’s landscape and economy. Scotland’s kelp collectors, for one thing, faced ruin, although small amounts of kelp continued to be harvested to extract other chemicals, especially iodine and bromine. The Leblanc process was not environmentally friendly. The hydrochloric acid fumes emitted in the first step, essentially worthless at the time, were passed into the atmosphere, wreaking havoc on vegetation, buildings, metalware, and fabrics. Producers were harried by frequent litigation regarding the damage they inflicted on their neighborhoods. The British Alkali Act of 1863 (later strengthened) required soda ash plants to cut acid gas emissions by 95%. By the 1860s, acid gases were controlled by passing them through a tower filled with coal or coke down which water trickled. The dilute acid at the bottom was often poured into a nearby stream, killing aquatic life. Later, hydrochloric acid was oxidized with manganese dioxide to form chlorine, which could be reacted with quicklime to make bleaching powder.

The spent black ash, mostly composed of calcium sulfide and unburnt coal, was dumped into the sea, abandoned clay pits, and mines or heaped up in ugly piles on vacant land around the plants. Various attempts were made to recover sulfur from the calcium sulfide, although with little success until the 1880s, when it was found that hydrogen sulfide is liberated if carbon dioxide is blown through the waste.

The Solvay Solution
The heyday of Britain’s Leblanc soda ash industry was in the 1860s and 1870s, with output climbing to more than 200,000 tons a year. However, its days were numbered. In the early 1860s, two Belgian brothers, Ernest and Alfred Solvay, developed an ammonia–soda process for manufacturing soda ash that was less expensive and less complicated.

By the 1870s, the Solvay technology was the preferred method in much of western Europe. Most British soda ash firms, with their heavy investment in Leblanc operations, stubbornly stuck with their traditional business. But they found it more and more difficult to compete with the newer method. In fact, their profits increasingly depended on making bleaching powder or caustic soda rather than soda ash. In 1890, most of the British firms combined to form United Alkali Co., which would become part of Imperial Chemical Industries when it was formed in 1926. By the 20th century, the Leblanc process was obsolete. The last British plant shut down in the early 1920s.

No Leblanc plant ever operated in the United States. U.S. industry, in fact, provided a large, lucrative outlet for British exports of soda ash in the mid-19th century. A modified version of the Leblanc process was used on a small scale, however, to prepare alkali from sodium sulfate mined in Wyoming.


David M. Kiefer is a consulting editor for Today’s Chemist at Work. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office 1155 16th St N.W., Washington, DC 20036.

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