Global salt deposits

The word “salt” in the colloquial sense denotes the compound sodium chloride (NaCl), a combination of the two elements sodium and chlorine. Other commonly used terms such as “rock salt”, “evaporated salt”, “table salt”, “solar salt” or ”sea salt” (which refer to the production method) also denote that substance, without which there would be no life on Earth.

The fundamental importance of sodium chloride was pointed out by Justus von Liebig in the nineteenth century, who declared that “of all jewels, salt is the most precious”. Salt is a life-essential component of our diet, and is used for seasoning, colouring and preserving, as well as for softening water. It is one of the most important raw materials in the production of glass, paper and plastics and is indispensable for ensuring safety in winter road clearance.

Sodium chloride occurs naturally in large quantities. The oceans, by far the largest storehouse of salt, have contained different dissolved salts since they were formed several billions years ago, numbering more than 30 elements. By far the most prevalent of these is sodium chloride, which accounts for approximately 78% of dissolved salts. Assuming an average sodium chloride concentration of approximately 2.7% (27 g/litre), seawater contains the unimaginable quantity of 36 thousand billion tonnes of dissolved salt.

How rock salt deposits are formed

The formation of the rock salt deposits („Barrier theory“)

 

Underground deposits of solid rock salt are also of marine origin. They were formed in almost all geological systems in the Earth’s history. The oldest known salt deposits are in the Amadeus Basin in Australia, in the Bitter Springs Formation, thought to be 1.17 billion years old (Precambrian).

According to Ochsenius’ “bar theory” (1877), large inland seas existed for a time in the geological past, which were only connected to the ocean by narrow channels. These straits contained ridges (“bars”), which limited the influx of new salt-containing seawater. Because an arid climate with powerful sunlight frequently prevailed over large parts of the Earth in the different periods of its history, the salt-bearing seawater in the inland seas evaporated as if in a gigantic evaporation pan. As a result, the salt concentration of the water increased and the dissolved salts crystallised, which were then deposited in the order of their solubility – first rock salt and later potassium and magnesium salts.

This process was repeated over millions of years and resulted in layers of rock salt with a thickness of several hundred metres and potash seams several metres in thickness being formed on top of each other. Later in the Earth’s history, water impervious clay strata settled on the salt deposits (they were often blown over the dried-out salt pan by the wind), thus preventing the salts from being dissolved again.

 

The salt deposits worldwide. Source: K+S Käding/Beer

However, over the course of the geological eras, not all deposits retained their original flat form. Due to the pressure of the overburden and the effect of the Earth’s heat, the deposited salts were deformed and began to “flow”. In disturbed areas and other weak points, some of the deposits rose up to the Earth’s surface, creating vertical, mushroom-shaped or columnar salt domes or wall-like forms known as “salt walls”. Salt domes can be several dozen kilometres in diameter and reach to more than 10 kilometres in depth. Sometimes, the salt layers are arranged in complex folds, which are frequently accompanied by steep potash seams. An example of this is the deposits in the Hanover area.

Where rock salt deposits lying close to the surface came into contact with precipitation or groundwater, the rock salt dissolved over a period of thousands of years. As a result, underground brine (salt dissolved in water) deposits or saline springs were formed. Natural salt solutions can also result from erosion of salt-containing strata and volcanic rock by water. The discovery of natural brine discharges is often the starting point for searching for and finding salt deposits.

Global salt deposits

The underground deposits of solid rock salt that formed due to the sedimentation of primordial oceans amount to around 4 billion tonnes of sodium chloride. Like potash deposits, rock salt deposits are concentrated in the northern hemisphere. The world’s largest reserves are in North America, Europe, the European and Asian part of Russia, Kazakhstan, India, China, Iran and on the Arabian Peninsula.

Salt deposits which do not originate directly from the sea can be found in salt lakes, salt deserts or salt marshes. Salt lakes can originate from rising salt springs. However, they are usually non-draining, stationary inland waters or cut-off sea bays in arid areas which are supplied with less water than the sun evaporates. As the water level decreases, the originally low salt concentration in the lake water increases so sharply that the salt content exceeds that of seawater, in some cases many times over. The best-known salt lakes include the Great Salt Lake in the USA, which has an average sodium chloride content of approximately 15%, and the Dead Sea between Israel, the West Bank and Jordan, which has salt content of almost 8%. The water in both lakes also contains high concentrations of other salts and is used, among other things, for extracting potash. The saltiest bodies of water on Earth include Lake Assal in Djibouti, in one of the hottest regions in the world, and Garabogazköl, a vast bay on the east edge of the Caspian Sea in Turkmenistan.

The Salar Grande de Tarapacé in the Chilean Atacama Desert

 

Salt deserts are the remains of former salt lakes whose water was completely evaporated by powerful sunlight. The world’s largest salt deserts contain several billion tonnes of rock salt deposits and cover an area of several thousand square kilometres. With the exception of Europe, there are salt deserts on all the continents of the Earth. Most of them lie between the two cordilleras of the South American Andes at an altitude of several thousand metres.

The most important deposits are those of Salars de Uyuni in Bolivia and the Chilean Atacama Desert with the Salar Grande de Tarapacá. The Great Salt Lake Desert with the Bonneville Flats in Utah (USA) also originated from a dried-up lake, the remains of which make up today’s Great Salt Lake.

Salt marshes are a combination of a salt lake and a salt desert. They form where a non-draining salt lake dries up temporarily rather than permanently. If there is further precipitation or periodic flows of river or meltwater, the depressions fill up and salt lakes are formed again. Examples of this are the Algerian and Tunisian chotts and shatts on the northern edge of the Sahara Desert and the ranns on the border between India and Pakistan.

The history of salt extraction

Salt’s preservative properties were already understood by hunters and gatherers in the Stone Age, who used it to make their food keep on long journeys. The regular use of salt as an ingredient of human food probably goes back more than 10,000 years, when there was a gradual transition from nomadic hunting to settled farming in the Middle East. As the economic form of agriculture changed, so too did eating habits. During the time of the hunters and gatherers, the raw or roasted meat of hunted game had been a natural source of salt. Now the consumption of salt became essential for survival. When communities were deciding where to establish a settlement, the existence of salt deposits was now just as important as water.

However, salt was scarce, as on the whole the valued mineral could only be obtained in large quantities where underground salt springs rose to the surface or climatic conditions allowed salt to be produced from seawater. In the warm Mediterranean regions of ancient Rome, Egypt and Greece, salt works in which salt was produced as a result of evaporating seawater by sunlight in “salt gardens” were widespread. This production method was also used in the coastal regions of the Black Sea, Africa, India and China. In the Chinese interior (according to Confucius), as early as 600 BC boreholes were drilled to a depth of more than 500 metres to open up underground brine deposits, from which highly sought-after salt could be obtained by evaporation in the sun.

In the northern parts of Europe, where the heat of the sun was insufficient for salt production, other ways of producing salt had to be found. Historical evidence indicates that the earliest salt production centres appeared in the sixth millennium BC, in the Neolithic period, when salt was obtained from salt springs. The salt water was collected and evaporated over an open fire in clay vessels to produce salt crystals.

The first large salt works were established in the first millennium AD, in which open evaporation pans were used to heat up brine and thus obtain evaporated salt.

 

Salt production in the Middle Ages: Salt crystals are skimmed out of boiling brine in open pans.
Source: Georgius Agricola De Re Metallica (1556), 12. Buch: Von den Salzen und vom Glas

The oldest evidence of solid rock salt mining goes back to the 15th century BC, i.e. the Middle Bronze Age. In Salzberg in Hallstatt near Salzburg, the Celts operated what is thought to be the world’s oldest salt mine. The shaft reached down to a depth of over 100 metres. The archaeological discoveries made in the salt mine and on Hallstatt Mountain were so important that the period of transition from the Bronze Age to the Iron Age in Europe was named the Hallstatt period and culture.

Despite the different methods used to produce salt, for centuries supply was unable to keep up with demand, and salt remained scarce and precious. Wars were waged around salt springs, salt was comparable with gold and used as a means of payment, and salt trading led to the appearance of trade routes (“salt roads”) and prosperous cities. For example, in addition to their wages Roman legionnaires received a salt ration or “salarium”, the origin of the word “salary”, which is still in use today. What is thought to be Rome’s first trade route was called “Via Salaria”. Numerous city names, such as Salzburg, Salzgitter, Salzdetfurth and Salzwedel, place names ending in “hall” (salt) such as Reichenhall, Schwäbisch Hall, Halle an der Saale and Hall in Tyrol, or including the element “sol”, as in Solikamsk in the Urals, remind us of the importance of salt.

With the advent of the industrial revolution, the demand for salt as a raw material increased rapidly, especially in the chemical industry. This led to a search for new ways of producing salt. The mid-nineteenth century brought a turning point, with salt mining becoming ever more prevalent. Due to technological advances and more efficient mining techniques, hitherto inaccessible rock salt deposits lying at a depth of several hundred metres could now be opened up for mining. At the same time, improved drilling procedures were introduced, which allowed greater quantities of salt to be extracted.

The four main forms of salt: rock salt, brine, evaporated salt and solar salt

Much has changed since the early days of conventional rock salt mining. Today, large underground rock salt deposits at a depth of hundreds of metres are mined using large machines, by blasting or cutting. For example, loaders with huge shovels are used to transport the excavated material to the crushers after blasting, where it is broken up into smaller fragments. It is then transported to the shaft by a conveyor belt and carried to the surface for further processing. The largest rock salt mines in the world extract several million tonnes of salt per year.

 

Loader with a scoop capacity of 20 tonnes in the rock salt mine Borth of esco - european salt company

In contrast, manual salt production is still widespread in salt deserts and salt marshes. In high temperatures and with great physical exertion, salt blocks or salt disks must be hewn out of the hard crust or crystallised salt removed. Large companies which exploit salt desert deposits which are many metres thick in open-cast mines use explosives to extract the salt.

Solid sodium chloride is also still obtained from brine or saline sea and lake water.

While in earlier centuries brine collection was limited to natural brine springs, today it is usually carried out by means of controlled solution mining. This modern procedure involves accessing rock salt deposits from the surface via one or more boreholes, through which fresh water is pumped into the deposits. The water dissolves the solid rock salt and is then pumped to the surface as concentrated brine (26% NaCl). The brine is either used directly as a raw material in the chemical industry or is processed into solid evaporated salt. This involves feeding the brine into closed evaporation vessels, where it is evaporated in temperatures of between 50 and 150°C and crystallised as sodium chloride. The centuries-old practice of evaporating natural brine in open evaporation pans is now only carried out in a few small plants.

The heat of the sun is still used for producing salt when climatic conditions are suitable, as in pre-Christian times. Essentially, this process for evaporating salt solutions (solar evaporation) has not changed, although the evaporation plants, some of which have an annual capacity of several hundred thousand tonnes, are now incomparably larger and cover many square kilometres.

Evaporation ponds of Morton Salt at the Bahamas

 

Solar evaporation is used to evaporate seawater and salt lake water, as well as natural brine or brine obtained through boreholes. The first step is to pump the salt solutions into separate evaporation ponds.After several months of exposure to sunlight, the crystallised sodium chloride can be harvested as a layer of salt several centimetres in thickness.In large salt works, this is done by modern machines. In smaller or specialised plants, the salt is also collected traditionally by hand, like the well-known fleur de sel. Solid sodium chloride obtained through solar energy is known as solar salt, which includes sea salt obtained from evaporated seawater.

Salt is produced, whether in solid or liquid form or in smaller or greater quantities, in almost every country on Earth. Due to the high share of transport costs in production costs, markets are generally regionally limited to the area around the production centres.

Around 70% of worldwide salt production (more than 250 million tonnes) is obtained from rock salt mining and solution mining. Approximately 30% of production is obtained from seawater and salt lakes.

K+S Group – the world’s leading salt supplier

 

The main salt suppliers worldwide
Capacity in million tonnes (crystallized salt and salt in brine; excl. captive use)
Source: Roskill, K+S

In terms of production capacity, the K+S Group, with its subsidiaries esco – european salt company (Hanover), the Chilean K+S Chile (Santiago de Chile) and the US company Morton Salt (Chicago), which was acquired on 1 October 2009, is the world’s largest salt producer. The total annual capacity of the Group amounts to almost 30 million tonnes. Broken down into production method, 70% or 21 million tonnes is attributable to mined rock salt, 4.7 million tonnes to evaporated salt, 2.4 million tonnes to solar salt and 1.7 million tonnes to salt contained in brine.

esco, the leading supplier in Europe, operates three rock salt mines, three solution mines and several evaporated salt plants in Germany, France, the Netherlands, Portugal and Spain. Rock salt mining is concentrated in Germany, where deposits with a thickness of several hundred metres are mined at a depth of between 400 and 800 metres. While the rock salt mined in Borth (North Rhine-Westphalia) and Bernburg (Saxony-Anhalt) are almost horizontal, the deposits mined in the Lower Saxony field in the Brunswick-Lüneburg plant are positioned almost vertically to the salt domes due to deformation of the Earth’s crust. The deposits were formed in the Upper Permian era more than 250 million years ago, when the so-called Zechstein Sea, which at that time covered a broad area of Central Europe, evaporated due to powerful sunlight.

In Harlingen on the North Sea coast of Holland, esco operates the deepest solution mine in the world. By a controlled process of drilling boreholes, Upper Permian rock salt layers at a depth of 3,000 metres are dissolved and brought to the surface, where they are processed into evaporated salt. esco has a total annual production capacity of 8 million tonnes of crystallised salt and 1.7 million tonnes of brine.

 

Mining chamber in the rock salt mine Bernburg of esco - european salt company

The Chilean company K+S Chile, the largest salt producer in South America, mines rock salt in Salar Grande de Tarapacá, a part of the Atacama Desert. The origin of Salar Grande has not yet been completely explained. However, it is thought to be the remains of a vast lake which covered the area until the end of the last ice age approximately 12,000 years ago. The 100-metre thick deposits, which reach to the surface, are mined by the cost-effective open-cast method. Production capacity is around 6.5 million tonnes per year. The salt reserves in these deposits are sufficient to supply the world’s population with salt for the next 200 years. K+S Chile’s subsidiary Salinas Diamante Branco also operates a sea salt plant on the east coast of Brazil, in which 0.5 million tonnes of salt can be produced by solar evaporation per year.

 

Open-cast mining of K+S Chile in the Chilean Atacama Desert

Morton Salt, one of the leading salt suppliers in North America, operates a total of six rock salt mines, seven solar plants and ten evaporated salt plants. The mines and evaporated salt plants are concentrated in the eastern half of the USA and in south-east and south-west Canada. The origin of the rock salt deposits goes back to the Silurian era 415 to 445 million years ago. The solar salt production plants are located on the east and west coasts of the USA, by the Great Salt Lake, and in the Bahamas (where the climate is suitable). Morton Salt has a total annual production capacity of approximately 13 million tonnes of salt.