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WHAT IS SALT?

Sodium chloride (salt, NaCL) is a chemical compound. It occurs in nature as a halite mineral and can also be found dissolved in seas and salt water lakes. It is produced in many different variations and forms depending on the mining method used and on its further use. Salt can come in the form of small or large crystals, fine powder, or it can be pressed into tablet or block form.

HOW DID SALT DEPOSITS DEVELOP?

Approximately 250 million years ago ocean water advanced into Central Europe and continued further toward the east. Then, the climate was dry and hot, similar to today's coastal climate. The ocean water then concentrated. Concentrated saline solution condensed and precipitated to the ocean floor of large basins. Later on, fresh ocean water poured on top of the deposits and continued to evaporate. Lime stone and dolomite gradually started to separate due to the increase in concentration and differences in solubility of the contained substances. Then, lime sulphate, or anhydrite, started to separate. One might say the deep sea water that was rich in salts (easily dissolved rock salt and potassium salts) first drained away from the deep waters of basins.

Tectonic movements caused sand deposits to surface very slowly, and frequency of thick sea water return gradually decreased. Due to this, as the concentration of salt increased, rock salt began to crystallize. When there was yet another increase in ocean water level due to geological movements, the process reoccurred several times.

Salt deposits proceeded to deeper layers of the ocean floor as they were being covered with heavy layers of sediments in the course of the Earth's existence. Higher temperatures and pressures developed at these depths and in the course of time tectonic movements, i.e. strata movements, disintegration, compression of layers, etc., took place, shedding light on the mystery question of why salt deposits show considerable differences in their composition and location.

TYPES OF SALT DIVIDED ACCORDING TO DIFFERENT MINING AND PROCESSING METHODS

Rock salt (halite = NaCl, from Greek halos = sea, salt) is the most common mineral in salt deposits. It has been an essential mineral for humans since primeval times. The exact chemical composition of rock salt was however determined in 1810 H. Davy, an English chemist and physicist.

Today we know that sodium chloride (NaCl) contains nearly 40% sodium and 60% chlorine, not to mention admixtures of calcium chloride and magnesium chloride that are frequently present. It usually occurs in a granular or fibrous form; however, it is also possible to find beautiful salt cubes in cavities. They can be relatively large, have perfect cleaving properties and a strong glassy lustre.

In pure form, salt is usually colourless; it can however have a grey tinge to it due to clay colouring, it can be red due to haematites, brown due to bituminous substances or blue due to sodium metal (changes in the crystal lattice). It is perfectly soluble in water. Salt can be recognized from other minerals by its salty taste. It colours flames a yellow colour (due to sodium content), Class 2 hardness. It crystallizes into a cubic pattern.

It naturally occurs in nearly all geological formations: in solid form or in the form of solutions, e.g. in sea water with a rock salt and other salt at a concentration of up to 3.7%. It usually forms heavy deposits (layers or lentils), and the deposits are often dome shaped. Such deposits formed due to the evaporation of water in hot and dry climates from former bays separated from the sea by a strip of land. In addition, salt effloresces on the shores of salt water lakes (the Dead Sea, the Great Salt Lake in Utah, or in the steppes). The United States of America, countries of the former Soviet Union, China, Germany and Great Britain are the main producers of rock salt.

Most salt deposits have a typical shape. The deposits form domes or plugs, and are capable of penetrating other layers. Sometimes they even protrude above ground level. The special shape of salt deposits is usually explained by the extreme plasticity of salt, which, due to various influences, mainly orogenetic ones, caused the displacement of the originally horizontal layers of salt into formation tops. The layers usually deformed without breaking into structures of several hundred to a thousand metres in height.

Solid salt is mined in most cases. Rock salt is sporadically also mined in open-pit mines. Room facing is usually used in salt mines. A main mineshaft is excavated and from it stem hallways that lead to salt chambers from which the salt is mined (they are 20 m wide, 20 m high and more than 100 m long). The excavated salt has the form of crystals of various sizes, from dust particles to large grains. Then, salt is further processed by means of milling, screening and cleaning.

Production of vacuum salt is based on the collection of salt brine and on the subsequent evaporation of water and crystallization of very pure common salt. Vacuum salt is mined from salt deposits that are contaminated to a greater extent by other minerals, clay, marl, etc. and have a low NaCl content (approximately 50%). In this case it is impossible to mine in the same manner as one would rock salt. A deep drill-well is drilled into a salt deposit, and a steel pipeline is inserted into it and fixed. Water is pumped into the salt deposit. After it becomes saturated with sodium chloride, it is pumped to the surface through another pipe and then to a salt processing plant. Salt brine is brought to the processing plant from several drill-wells and can be as many as 30 kilometres away from the plant.

The salt brine is first pumped into huge tanks where it is chemically clarified. Precipitations settle on the bottom of the tanks, the still-bottom is then discharged and pure saline is brought to a vacuum evaporation apparatus. The apparatus to which the salt brine is brought to is tall, vertical and conical, and made of steel. In the middle part of the apparatus, there is a pipe system into which steam is injected. In this part, the salt brine is heated up until it boils. Salt crystallizes into small crystals, and collect in the conical bottom. Then, the crystals are withdrawn together with the mother liquor to a centrifugal separator where the crystals separate from the mother liquor and so they only contain 2-3% water. Then, the salt is dried by means of hot air ventilation in a drum drier within which the contained air reaches temperatures of about 200°C. Salt of more than 99% NaCl is acquired in this manner.

Sea salt is acquired from sea water in dry and hot climates. The sea water is poured in shallow large basins, and then it evaporates thanks to the wind and sun.

The concentrated salt brine settles in basins in layers of 25 to 40 cm, mechanical contamination settles, and the water evaporates. This eliminates the majority of insoluble matter, calcium carbonate, ferric carbonate and calcium sulphate. Then, the salt brine is taken to crystallizer tanks, and becomes more concentrated at the same time as crystals of common sea salt start to separate. The crystallizer tanks are large, and the brine is kept in layers of 10 to 15 cm at the beginning, and in layers of 20 - 25 cm at the end of the process. Salt is raked out from the tanks and piled up so that the mother liquor drains out of it. It is then distributed in the resulting form or is milled in mills. Sometimes the concentrated sea water from the tanks is refined in salt refineries, or the salt that was raked out from the tank crystallizers is dissolved again and purified in salt processing plants. Sea salt has natural iodide content ranging from around 0.5 to 5 mg/kg of salt, depending on the salt deposit.

Seas are the biggest deposits of salt. The average salt content of (i.e. all substances dissolved in sea water) is 3.5% (35 g in 1,000 g of water). However, salt content is not identical in all seas. For instance, there is a mere 1% salt content in the centre of the Baltic Sea, and nearly a 4% concentration in the Mediterranean Sea. The salt content is considerably higher in inland seas. For instance, the Great Salt Lake in Utah, United States of America, has a salt concentration of 27%, and the Dead Sea in Israel is more than 30% salt.

SALT UTILISATION

Salt has an indispensable role in human nutrition. Common salt serves for the flavouring of foods, and is also a source of sodium and chloride ions; essential elements for the human body. However, approximately 4% of salt is consumed in households as cooking salt, or used for the production or preservation of foods.

It is also used in animal nutrition -where it is added to feed, or used directly as salt licks or salt blocks for game or livestock. Salt and salt licking provide animals' bodies with the quantity of salt necessary, promote digestion, not to mention improve the taste of feed.

More than 70% of salt is used for the purposes of the chemical industry. Sodium chloride, crude oil, coal, lime and water are the most important materials for chemical production. The reason is that there are many chemicals and products that contain sodium and chlorine, i.e. the elements contained in salt. For instance, salt is a primary material for production of sodium metal, sodium sulphate, soda, chlorine, salt acid, sodium hydroxide, ammonium chloride, chlorinated lime, etc. These substances are then primary or auxiliary materials for production of an endless series of other chemical products, e.g. artificial fibres, plastic materials, paper, pulp, dyes or pigments, soaps, medicaments, chlorides, disinfection means, organic solvents etc.

Sodium chloride is also used as industrial salt in other branches of industry. It is for instance used for regeneration of ion resins for softening of water in industrial plants. It is also used for the preservation of skins and casings, in the leather industry, for the dyeing of clothes, in cooling systems, in the stoneware industry as well as in the production of cosmetic preparations.

A large quantity of salt is used in the form of deicing salts to melt snow and ice in winter.

IMPORTANCE OF SALT FOR PEOPLE

Salt is one of the most common and known cooking materials. Literally each of us knows the taste of salt, and the Czech fairy tale Salt above Gold showed the value of salt. However, only few are really aware of the importance of salt and consequences of excessive salt intake for the human body.

Common (cooking) salt (NaCl) is a chemical compound composed of sodium (Na) and chlorine (Cl). Common salt also contains 2-3% of other substances, e.g. chlorides and sulphates. We cannot do without salt, or, more accurately, without sodium. It is essential for quality of conduction of nerve impulses and muscle activity; it regulates blood pressure and helps to maintain the balance of body fluids within cells and in intercellular spaces. These irreplaceable functions then suggest which problems develop when there is too much salt in the body. On the other hand, chlorine is contained in the acid of gastric juices, and it is important for digestion.

Salt is part of most organic substances, as it is contained in plants and animal organisms (in meat, blood plasma, cartilaginous and bone tissues of these organisms). It influences diffusion processes in the body, and promotes digestion, metabolism and it also influences the process of cellular formation. It maintains the balance of fluids in the body, and it is indispensable for the creation of hydrochloric acid (HCl) in the stomach.

Salt is also part of the blood as it is dissolved in blood serum. Serum contains approximately 0.9% of sodium chloride. In the case of blood transfusions, a solution of sodium chloride (normal saline solution) is often used as a quick medical aid. Normal saline solution (0.9%) is not only used in medicine in cases of blood loss, but also in the field of biology for preparation of zoological specimens and slide preparations.

Sodium chloride even controls gastric juice secretion. It directly stimulates nerves in the gastric mucosa, supports the degradation of proteins into amino acids as well contributes to their absorption from the lumen of the digestive tract into the body's fluid compartments.

More information about the influence of salt on the human body is available in the menu "On Health".