WHAT IS SALT?
No one can imagine how much salt is an essential element to life and has always been inextricably linked to the history of humanity and its development. Initially used for food and food storage, today is present in the composition of most of what surrounds us.
Salt, or sodium chloride, is found abundantly in nature. Most of this element is dissolved in water, to form seawater; while the remaining part is located in the solid state in underground reservoirs. The manufacturing processes vary depending on the form in which sodium chloride is available, and can be divided mainly into three types of cultivation:
- SOLAR EVAPORATION It’s the oldest form of production, made in establishments such salt pans. The sea water is collected in tanks waterproofed of great extension and low depth in which, as a result of solar radiation, the brine is concentrated. Since the concentration occurs during a precipitation of salts other than sodium chloride, the brine itself is transferred, with the increase of the concentration, in different tanks. The first tanks, in which there is no precipitation, are said “evaporator”; those in which precipitate calcium salts, if they exist, are called “descaling”. When the concentration of about 300g / l of sodium chloride is reached, the brine is passed into crystallizing tanks, in which there is precipitation of sodium chloride. The solid sodium chloride deposited on the bottom of the crystallizing tanks is then collected with machines known as “growers” and sent to the next stage of refining. The purpose of the refining is the elimination of salts different from sodium chloride. This elimination is achieved by washing in “countercurrent” with water as pure as possible: thus obtaining a sodium chloride with NaCl title more than 99.5%, which is dried and marketed as sea salt.
If solar evaporation has the advantage of not requiring sources becoming from different energy except from the sun, except for the functions of pumping and a few others, it has the serious limitation of being only possible where the difference between evaporated water and fall of rainwater is positive where that water evaporates more during the year than it falls with rain. This is possible in a Mediterranean-type climate, but very difficult in monsoon-type climates, or in cold climates. It can be partially overcome this by exploiting the seasonal differences, but, clearly, there are limits to production.
- MINING In nature there are large deposits of sodium chloride, residues of ancient seas, from which the salt, better known as mine salt (or halite), is extracted already in solid form. In this case, the production is made for cultivation of the mine, typically by mechanical excavation equipment; a sodium chloride in large blocks is obtained, and these are then milled to bring them to marketable granulometries. Very often it’s available in solid form mixed with large amounts of foreign compounds, especially clays, magnesium salts and others. In these cases, as also in the case of deep reservoirs, it is convenient to carry out an indirect cultivation of the mine, by injecting into the ground water which then rises to the surface as saturated brine, leaving depth in most of the impurities. In almost all cases, this method of cultivation is wet followed by evaporation, to produce the so-called salt under vacuum.
- INDUCED EVAPORATION It differs from solar evaporation because the source of energy is no longer the sun, but the vapor of water or electricity. Given that the energy cost is proportional to the amount of water to be evaporated, it’s used exclusively with saturated brines or almost saturated ones. Considering that, it’s used for the treatment of wet cultivation and, recently, of brines from treatment of reverse osmosis (which, however, are less concentrated and less pure than the other). The incoming brine is usually chemically treated to remove the calcium salts and magnesium which would be harmful to the heat exchange, and reduce the “title” of the salt produced. The treated brine is then concentrated in vaporizers-crystallizers and from this brine an hyper-pure salt in a very uniform particle size is produced. The purity reaches 99.9% for the edible salt and 99.99% for the salt used in electrolysis plants. This process is called improperly “under vacuum” as the first plants of this type operated at lower pressures of the atmosphere; today is more frequent the functioning at atmospheric pressure to the heat pump, also said “to mechanical recompression”, while the pressure is variable in the so-called “multiple effect” plants.