Salt of the Earth: Part 2

Martin Pitt continues his look at the chemical engineering history of common salt

BY USING various sources and careful crystallisation, salts other than NaCl could be extracted from brines and salt mines for different uses, though their chemical composition remained unclear.

It was not until 1648 that German apothecary and industrial chemist Johann Rudolf Glauber (1604–1670) described an industrial process to convert table salt into two new substances by heating it with sulfuric acid (then called oil of vitriol). One was “Glauber’s salt” (sodium sulfate Na₂SO₄) a mild and safe laxative, which he called sal mirable (miracle salt), now a major chemical commodity, used in detergents and paper making. The other was “spirits of salt” (hydrochloric acid) which was for him an interesting waste product, but later found uses where sulfuric acid was not appropriate and is now an even more important industrial chemical. His scientific description was only published after he had made his fortune from his miracle salt.

Going from scientific experiment and theory to a working and profitable industrial process gives him some claim to be the first modern chemical engineer.

Alkali industry

The British soap and glass makers needed alkali: either “soda ash” (Na₂CO₃) from burnt seaweed or expensive imports from some minerals, or “potash” (K₂CO₃) from burnt wood (see TCE 1,001 Glass in all its Glory Part 1). A way of producing it from salt was invented in 1791 by French chemist Nicolas Leblanc (1742–1806), as a second stage of Glauber’s process, and his plant produced 320 t/y. His intention to sell to Britain was thwarted by the French Revolution (1789–1799) and its aftermath. In 1816, English industrial chemist William Losh (1770–1861) set up a soda plant having read Leblanc’s published description and in 1802 visited his plant. This was the first of many. The nascent industry persuaded the UK government to end the salt tax (raised to pay for the Napoleonic War) in 1825 and a whole integrated industry bloomed – in 1850 becoming the first country to employ 10,000 people in the chemical industry. The companies made their own sulfuric acid from iron or copper sulfide ore, and sold the metal, got salt from Cheshire, lime from Derbyshire, coal from Tyneside, transported by canal. By 1875 the amount of salt used in soda production exceeded all other uses of NaCl itself.

However, it was very polluting. In 1840 a British Leblanc soda plant would only have one saleable product, with hydrochloric acid (HCl) being sent up a chimney, causing terrible damage to people, animals, and the environment. The sludge “black ash” (mainly CaS) was dumped on land, where it would continue to produce hydrogen sulfide for years by reacting with the local acid rain from HCl.

It took inventive practical chemist William Gossage (1799–1877) to resolve the HCl problem with an 1836 patent of a packed tower filled with coke or pebbles down which water was passed, absorbing nearly all the HCl, and meeting the emissions requirements of the 1863 Alkali Act (although the acid was then frequently discharged to the river). He tried but was unable to solve the CaS one. It was not chemistry but a cost-effective process which was needed – at the time there being no cost for harming the environment. It was German process chemist Ludwig Mond (1839–1909) who devised an economic process in 1862, which was installed at a Widnes works the year after. It produced elemental sulfur, which could be burned to make sulfuric acid and a byproduct, calcium chloride (CaCl₂), much less polluting.

The nascent industry persuaded the UK government to end the salt tax in 1825 and a whole integrated industry bloomed – in 1850 becoming the first country to employ 10,000 people in the chemical industry

Soda could also be converted into the more powerful alkali, caustic soda (NaOH), by reaction with quicklime (CaO) and water giving a solution of NaOH called lye, and insoluble CaCO₃ to be filtered off. Soapmakers had been using crude lye for centuries. In 1853 Gossage patented and began the first production of solid NaOH, with numerous improvements to the soda production process giving a pure lye which was evaporated down, and the monohydrate crystallised out. By 1900 NaOH was about 20% of the soda production. Its applications included aluminium production and the new artificial fibre Rayon, made by dissolving cellulose from wood with NaOH, then regenerating it as fibres by neutralisation. It was also used in the production of phenol for plastics from 1900 until replaced by the cumene process after World War Two.

He also produced “soluble glass” (sodium silicate (Na₂O)_x·(SiO₂)_y) by heating soda ash with clean sand. In 1855 he opened a soap factory including this to give a smooth semi-crystallised soap, which proved very popular. Gossages became the largest soapmaker in the UK.

In 1874 a Solvay Plant (see TCE 1,002/1,003 Glass In all its Glory: Part 2) was introduced in Winnington, Cheshire, using salt and limestone (CaCO3), with solid calcium chloride (CaCl₂) as the waste. The quantities of CaCl₂ inspired the Solvay corporation to find uses for it, which are now substantial. The solid is an effective de-icer, used on roads and coal. It also keeps surfaces moist, suppressing dust, and is an accelerant for concrete. Its brine can be used as a refrigerant down to -40°C, and if sufficiently pure, it can be used in pickling. It is also used inside tractor tyres to add weight and thus traction, even in hard frosts. Solvay is now the main way of producing both Na₂CO₃ and CaCl₂ and threatened to put the Leblanc plants out of business. The reason it did not is bleach.

Bleach

In 1774, Swedish chemist Carl Wilhelm Scheele (1742–1786) found that spirits of salt (then called muriatic or marine acid) could be oxidised by the mineral pyrolusite (MnO2) to give a greenish gas we now know as chlorine, which decolorised many substances. French chemist Claude Louis Berthollet (1748–1822) studied its properties and in 1784, when he became the government inspector of dyeworks, he promoted its use dissolved in water for textiles, giving details for large-scale production of the gas. He also told his friend, the Scottish inventor James Watt (1736–1819). He in turn introduced it to his father-in-law’s factory, bleaching 1,500 cloths at once. However, the fumes were harmful to workers and many materials of construction. Berthollet discovered that adding the right amount of potash produced a liquid bleach (solution of KOCl, potassium hypochlorite) which was much more manageable, but too expensive for bulk use.

In 1789, English bleacher Charles Tennant (1768–1838) produced what he called bleach liquor, made by dissolving chlorine in milk of lime (aqueous suspension of Ca(OH)₂) producing a solution of calcium hypochlorite Ca(ClO)₂. In 1799, he patented bleaching powder, the same chemical made by absorbing chlorine on slightly moist Ca(OH)₂. Lime was vastly cheaper than potash. The industry was able to expand because of the HCl now available from the Leblanc process, which in turn survived despite the Solvay process. NaOCl or KOCl could be prepared from Ca(ClO)₂ by treatment with their chloride or sulfate and precipitating out insoluble CaCl₂ or CaSO₄.

The ability to bleach quickly transformed the textile industry. Outstanding German chemist (and the inventor of Marmite – see TCE 982) Justus von Liebig (1803–1873) said: “But for this new bleaching process it would scarcely have been possible for cotton manufacture in Great Britain to have attained the enormous extent which it did during the 19th century, nor could it have competed in price with France and Germany.”

The initial chlorine producers simply discarded the MnCl2 product. This expense was reduced by the remarkable Walter Weldon, (1832–1885), a British fashion journalist who took an interest in chemistry at the age of 33 and did so well that he published 35 patents and served as president of the Society of Chemical Industry. A papermaker friend had complained of the high cost of bleaching powder, and he put his mind to it, with a result within a year. His process fully recycled the catalyst, cutting the price of bleaching powder by £6 (US$30) a ton. It used air to oxidise and the “Weldon blowers” became a feature of bleachworks. In competition from 1870 was a simple process by prolific British chemical inventor Henry Deacon (1822–1876) in which HCl was oxidised by air over copper chloride catalyst on ceramic beads, producing only water as a byproduct. It was revived in the 1950s for some processes producing byproduct HCl to convert it to the more valuable chlorine, but with oxygen instead of air. Its sustainability has attracted a lot of catalyst research this century.

“But for this new bleaching process it would scarcely have been possible for cotton manufacture in Great Britain to have attained the enormous extent which it did during the 19th century”

The electric age

In 1800, Italian physicist Alessandro Volta (1745–1827) invented the electric battery, and many chemists tried putting silver electrodes in many solutions. In 1851, Charles Watt got a patent for electrolysing brine to produce NaOH, Cl₂ and NaOCl, though he did not separate the electrode solutions, so the products reacted. It only became practical in 1870 with the advent of industrial-scale electric power from dynamos. In 1890, German chemical engineer Ignatz Stroof (1838–1920) at the Chemische Fabrik Greisheim-Elektron began commercial production of Cl₂ and KOH in a cell with a salted cement diaphragm for the dye industry. Hydrogen from this process was used in Zeppelins and for oxyhydrogen welding which the company pioneered.

In 1892, American chemist Hamilton Y Castner (1858–1899) working in the UK, and Austrian chemist Carl Kellner (1851–1905) separately patented a brine cell in which a pool of mercury reacted with the sodium to form an amalgam. By periodically rocking it, the mercury was brought into contact with a separate section containing salt-free water and the sodium reacted to form NaOH and H2. Instead of competing, the two chemists got together, exchanged patents, and formed the Castner-Kellner company in 1895. By 1900 there were 30 plants across Europe.

In the US, chemist Herbert H Dow (1866–1930) had developed a more economic process for separating bromine from the brine liquor remaining after salt crystallisation from sources. He thought that an electrochemical process might be even better. By controlling the voltage, bromine was preferentially released directly from the unprocessed brine. The debrominated brine was sent to waste. He then built a bigger plant, to electrolyse the waste brine with cells of tarred wood and electrodes of reject quality carbon pencils for arc-lights. The chlorine was immediately passed onto quicklime to make bleaching powder, with first production in 1898. Other UK and US electrolytic designs followed. Electrolytic production of chlorine finally ended the Leblanc process. Sodium alkalis could be produced by the Solvay process and HCl by Glauber’s.

Sodium metal was first prepared in 1807 by the electrolysis of molten NaOH by British pioneer electrochemist Humphrey Davy (1778–1829), and a commercial version produced by Castner in 1888. However, it was in 1924 that American electrical engineer James Cloyd Downs (1885–1958) patented equipment for preparing it from molten NaCl (with CaCl₂, which reduces the melting point), which is still the standard today. The octane improver for gasoline tetraethyl lead (TEL) was produced by the reaction of sodium-lead alloy with chloroethane (C₂H₅Cl) supplied by Du Pont from 1923. For this reason, Du Pont took over Downs’ company to gain the patent and technology. The cost reduction contributed to the commercial success of leaded petrol, and the unfortunate health damage which followed (see TCE 981 A Short History of Unintended Consequences).

In 1934, Downs received a medal of the American Chemical Society, noting the important contribution of sodium in production of TEL, sodium cyanide for heat treatment (surface hardening) of steel, dyes, perfumes, and pharmaceuticals. The process also produces an equivalent amount of chlorine, but not enough to meet the demand for chlorine, which mainly comes from brine electrolysis.

Large-scale production of chlorine allowed it to be used to disinfect water supplies (see TCE 1,000 Drink it in). It has also become the starting point for a huge number of organic chemicals, which is now the major use, the biggest one being the plastic polyvinyl chloride (PVC) patented by Griesheim Elektron in 1912 and used for water piping from the 1930s. Plasticised PVC was first used instead of rubber for insulating electric wires in 1952, and became popular in the 1960s, being now the commonest insulation. A country’s use of PVC has been used as a measure of economic development.

Chlorine is essential in the production of titanium metal and titanium oxide providing whiteness for paints, paper, and plastics by its action on crude ore to form TiCl₄, which is purified by distillation, then reduced to the metal, or oxidised to TiO₂.

Sodium, sodium hydroxide, chlorine and hydrogen chloride are also essential reagents in many chemical syntheses with products which do not contain either sodium or chlorine.

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Martin Pitt CEng FIChemE is a regular contributor. Read other articles in his history series: https://www.thechemicalengineer.com/tags/chemicalengineering-history