Salt of the Earth: Part 1

Martin Pitt considers the chemical engineering history of common salt

SALT is necessary for most land animals (though carnivores get enough from meat), with migrating animals often moving between salt-rich areas. Early human hunters followed them, while herding often meant moving animals between grazing and salt sources. With fixed crop growing, salt had to be provided for both humans and animals, and it became a major item of trade. Salt also allowed fish, meat, and milk (as cheese) to be stored and transported, and an extensive trade developed across the world.

From 6000 BC, the Chinese harvested salt crystals from Lake Yungchen which partially dried in the summer sun. Many other lakes around the world have been used in the same way. The wealth produced by Lake Mata in Spain enabled the owners to finance Colombus’s journey, which was only possible because of food preserved with salt. The method was improved by scraping out flat pans on the shore, into which saltwater was swept to crystallise over a larger area in a thin layer. The ancient Egyptians did this on the salt marshes of the Nile. Today, 40% of world production is solar salt.

“Mankind can live without gold, but not without salt” Roman Statesman, Cassiodorus (485–505)

In areas not so blessed with sunshine, such as Stone Age Yorkshire in 3800 BC, crystals could not be picked up from the beach. Instead, in summer, seashore pans only concentrated the brine, which was separated from scum and sediment, carried to a higher point where it was boiled dry in ceramic pots over a wood fire, a more demanding and technological operation known as briquetage. Nearby was a cheeseworks.

The Chinese invented another method of production, solution mining. Instead of waiting for salt to appear in surface pools, they began to drill wells for salty water from 252 BC. As the technology advanced, they eventually had wells 100 m deep, with brine being lifted in bamboo buckets using a counterweight system. The water was then boiled to separate the salt. A side effect was evil spirits which sometimes arose from wells and resulted in illness or death for the workers, occasionally appearing as flames. By AD 100 the workers put bamboo pipes from the worst source and lit the gas which came out, using this to cook their food. By AD 200 they were boiling the brine in iron pans with it, the first industrial use of natural gas.

The word China comes from the Qin dynasty who defeated three other groups in 476 BC to obtain the Three Gorges brine springs and formed the Chinese Empire. Salt became a Chinese state monopoly in 221 BC, which lasted until 2014, the longest known monopoly. Today, 34% of salt comes from solution mining in which water is pumped down into a salt formation to erode out a cavern and the brine returned to the surface for processing.

In other areas, rock salt from ancient seas could be dug out of the ground. The earliest known salt mine was in 5000 BC, at a place in Austria now called Hallstatt, from the Greek halos, meaning salt. In many desert areas in North Africa, it was possible to dig down to a layer of salt which was dug out in large blocks. The introduction of camels in the third century enabled long-distance trade from poor arid areas to wealthy and productive but salt-lacking ones. An important route was to the River Niger where it was exchanged for gold from the Ghanaian Empire. Today, 26% of production is rock salt. The UK produces 1m t/y from a single mine in Cheshire, part of 2.8m annually (1% of global production).

The Romans (what have they done for us?)

In the 4th century BC, the city of Rome made its first settlement at Ostia, where salt pans were established, taking advantage of the natural tides and the hot southern sun to crystallise sea salt, and building warehouses to store it. The first Roman road was from Ostia to Rome in about 200 BC, and called the Via Saleria, or salt road. A perk of being a Roman soldier was the salarium a regular ration of salt for health and to add taste to food, from which we get the word salary. In addition, salted meat and fish (salsamenta) could be transported considerable distances, providing rations for an army and navy, and enabling goods to be traded further. It was also needed for tanning leather. In the first century AD, Romans established salt mines as far afield as Austria, England, Poland, Spain, Switzerland, and Tunisia. They were highly technical enterprises, overseen by a manager called the conductor salinarum, with skilled engineers, professional miners, and craftsmen, as well unskilled labourers and slaves.

Brine springs were directed with wooden pipes into briquetage containers (later iron pans) for boiling. They also used tanks in which salt-rich earths were stirred in water to leach salt, then settled and the leachate piped to boiling pans. In Roman Britain, salt was produced from sea water, boiled by burning peat from what is now the Norfolk Broads. They have since been flooded by sea level rise, as was Ostia, and many other salt production areas in France and the Netherlands. Most of medieval and later salt industry in Europe stems from the Romans, and names such as Salzburg (salt town) reflect the history.

Later Europe

As populations grew, there was more demand for salt. Sometime after 600 BC a new technique was invented in the Mediterranean. Instead of having one large lagoon, smaller pans were arranged in series. When the seawater in the first had been noticeably concentrated, it was moved to the second and so on, giving a much greater annual production rate. By the tenth century there were competing sets of pans on the north coast of the Adriatic, including one owned by Venice, which became a major supplier of salt, partly by destroying the saltpans of a rival. It later used its commercial muscle and powerful navy to take control of the trade in salt, creating a practical monopoly by importing rather than making all the salt it sold, at a hefty markup. It was the salt profit that created the beautiful city of today.

In 1556, an illustrated 12-volume encyclopaedia of mining, De re Metallica, by Georgius Agricola, was published. Volume 12 was all about chemical engineering, the extraction, purification, and conversion of chemical substances, remaining the practical textbook for centuries. Saltmaking throughout Europe essentially followed the methods in it.

In the 18th century, steam engines were used to pump brine and remove water from salt mines. However, brine crystallisation in the early 19th century would have been familiar to the Romans. Lead pans were 20 ft (6.1 m) across instead of 3 ft and were heated by coal fires (or steam after 1823) instead of wood. Rapid evaporation produced fine crystals for butter and cheese. Slower rates made large ones for salting meat and fish. Storing salt in covered piles for up to a year produced a better tasting product as repeated cycles of condensation and drying leached out contaminants.

The Americas

Europeans colonising the Americas found the natives already producing salt. The Caribbean island of St Martin was known as Soualiga “land of salt” to the native Arawaks, inhabited for 3,500 years. The Spanish, French, and Dutch fought over it, a pattern repeated elsewhere. Colonies were initially close to the sea for salt, which limited expansion into the interior. The discovery of the Great Salt Lake in desert Utah in 1824 made a big difference.

Elsewhere, drilling for brine was one way to get rich (or bankrupt). Reinventing Chinese techniques of centuries earlier, drilling rigs sprang up all over. It was this sort of rig which found the first commercial oil in 1859, provoking a different rush, which unfortunately was frequently unsuccessful. In 1866, at Goderich in Ontario, Canada, entrepreneur Sam Platt looked like another failure, having drilled 209 m through limestone and used up the US$10,000 investment capital without finding oil. The town gave him US$1,500 to drill to 1,000 ft. Instead, he hit clean white rock salt at 964 ft. Canada had little salt, so the Goderich Petroleum Co became the Goderich Salt Co, bought 50 boiling kettles and began solution mining in 1867. At the World’s Fair in Paris that year, the salt won a gold medal. In 1959, rock mining was added, and it is now the largest underground salt mine in the world, stretching 2.4 km x 3.2 km.

The presence of a salt dome was often indicated by the shape of the land above. Unfortunately, the brine product was not commercial, being contaminated with black muck. In 1901, self-taught US geologists Pattillo Higgins (1863–1955) and Anthony F Lucas (1855–1921) started drilling for oil in a Texas salt dome called Spindletop against the advice of professionals. This single well produced 145m barrels of oil over 65 years. This new geological understanding led to greater success in the US and the Persian Gulf, with huge economic and geopolitical consequences (see TCE 973/4 The Story of Oil).

Norbert Rillieux (1806–1894) was an American “free person of colour” who studied engineering at the École Centrale Paris and became an expert on steam engines. Returning to the US, he entered the sugar industry and in 1834 patented one of the greatest inventions of chemical engineering, a multistage vacuum evaporator. It replaced slaves ladling syrup from one pan to the next, like the Venetian series of salt pans. Working under vacuum allowed lower temperatures, and the steam was economised by using the vapour evaporated from one chamber to heat the next at a lower pressure and temperature (see the TCE feature The Breakfast Club¹). This was applied to brine evaporation in 1887 by US engineer Joseph M Duncan (1846–1904) at Salt Springs in New York. Duncan used three effects to pump concentrated brine into a circulating tank crystalliser in which the upflow of mother liquor suspended small crystals, which dropped to the bottom for removal when they reached a particular size. For the first time in history, salt grains of the same size were produced. He also used a centrifuge for drying. The technology was adopted at Goderich in 1910 and reached the UK in the 1930s.

Britain

19th century Britain was the world leader in salt exports. In 1875, the salt companies of Cheshire exported 150,000 t to Europe, 300,000 t to India, 250,000 t to North America, 25,000 t to Africa, 25,000 t to Australia, as well as supplying 700,000 t to the domestic market. India was a colony so was forced to buy instead of producing its own. China was independent and powerful, so did not.

From 1840 to 1879, the British Army constructed a 2,300-mile (3,701 km) thorn hedge across India, which was maintained by 12,000 troops. Its purpose was to control and tax locally produced salt, preventing it from undercutting imported salt. The high cost of salt was a major issue in calls for Indian independence with a 1930 salt march led by civil rights leader Mahatma Gandhi (1869–1948).

Today the UK’s largest producer, British Salt, is owned by the Indian company Tata Chemicals. One of the first Fellows of the IChemE was Kapilram Vakil (1884–1946), the author of the 1924 manual Salt – Technology and Manufacture of By-Products, and the technical director of Tata Chemicals at its formation in 1939.

In the next issue I will show how the salt industry gave birth to the British bulk chemical industry.

Reference

1. www.thechemicalengineer.com/features/the-breakfast-club/

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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