Salt, i.e. sodium chloride, is a mineral that naturally occurs in our seas and underground deposits. It is an ionic chemical compound with the formula NaCl. This means that for every gram of salt, almost 40 per cent (39.337%) is sodium (Na) and over 60 per cent (60.663%) is chlorine (Cl). Salt can vary in colour depending upon its level of purity.
There are two main sources of salt. It is harvested directly either from sea water or natural brine, or from rock salt deposits formed by the evaporation of earlier seas that left a layer of rock salt, otherwise known as halite. There are three types of salt extraction: solar evaporation, rock salt mining and solution mining. Each one involves specific technology and manufacturers select the most appropriate technique depending upon the particular topographic and socio-economic conditions in their area of operation.
Solar evaporation is probably one of the earliest methods used to produce salt. According to that process, sea water or natural brine evaporates up to the saturation point in open basins, thanks to the action of the sun and wind. Crystallisation occurs in dedicated open basins as well, where the saturated brine is finally poured. Once the salt crust is formed, the exceeding water is eliminated before harvest. The raw salt may be further processed, including washing, drying, sifting and grading, if necessary and depending upon the requirements.
Rock Salt Mining
In Europe, many rock salt deposits were formed over 200 to 250 million years ago as a result of the evaporation of earlier seas, including a basin that stretched from the UK to Poland but that has dried up since. For this reason and because of geological structures, mines vary in depth from 100 metres to 1 ½ kilometres. Within the mines, there are networks of pathways of sometimes up to hundreds of kilometres, formed by the areas from which salt has already been extracted.
Rock salt may be extracted using traditional shaft mining or via solution mining. As far as shaft mining is concerned, there are two main methods for extracting rock salt:
- 'Cut and Blast'mining consists in cutting slots at the base of the rock face using a machine. A series of carefully sited holes are then drilled into the rock face using an electro-hydraulic rotary drill. Subsequently, the holes are charged with explosives and the face is blasted. The resulting salt rocks are then repeatedly crushed into pieces,
- 'Continuous Mining' produces smaller lumps of rock using a machine similar to that used for building tunnels. It bores into the salt, extracting lumps that are then crushed into smaller pieces.
Both methods take care of ensuring that the mine is stable and safe by leaving substantial ‘pillars of salt’ to support the mine roof.
Depending on the geological and geographic structure and properties of the salt deposits, solution mining is sometimes the preferred method of extraction. In this process water is forced under pressure into a borehole drilled into an underground salt layer. The salt dissolves, turning the water into brine and creating a cavern in the salt layer. The saturated raw brine is then withdrawn and pumped out of the ground. It goes through a purification process where calcium, magnesium and other impurities are removed prior to the evaporation process. The latter may resort to a variety of processes to boil the brine, thus causing the water to evaporate and salt crystals to grow. Further to the crystallisation, the salt goes through centrifugation and is dried.