Sodium Chloride (NaCl) plays a fundamental role in human biology and global industry. It is an essential nutrient, providing the body with the sodium and chlorine ions necessary for nerve function, muscle contraction, and fluid balance. Beyond its use as a common food seasoning and preservative, salt is an industrial commodity used in the production of caustic soda, chlorine, and in processes like water softening and de-icing. The world’s supply of this compound comes from three main sources: the vast expanse of the oceans, ancient salt deposits buried deep underground, and natural inland brine sources.
Harvesting Salt from Seawater
The most traditional method of acquiring salt involves harnessing the power of the sun and wind to evaporate seawater. This process, known as solar evaporation, relies on a series of shallow ponds called salterns. These are typically constructed in coastal regions with high evaporation rates and low annual rainfall. Seawater is first channeled into large concentration ponds, where solar heat and wind progressively remove the water, causing the brine’s salinity to increase significantly.
As the brine becomes more concentrated, less soluble minerals like calcium carbonate and gypsum precipitate out, effectively purifying the solution. The highly saturated brine is then moved to smaller, final crystallization ponds, where the sodium chloride begins to precipitate and form solid layers on the pond floor. Once the salt layer reaches a sufficient thickness, it is mechanically harvested using specialized machinery that scrapes the crystals from the clay base. This solar-evaporated salt is highly pure.
Extracting Salt from Ancient Underground Deposits
Salt is also extracted from deep geological formations known as halite or rock salt deposits, which are remnants of ancient, evaporated seas. These underground sources are accessed through two distinct, large-scale industrial methods: shaft mining and solution mining.
Shaft Mining
Shaft mining is similar to the excavation of other minerals, where vertical shafts are sunk hundreds or even thousands of feet below the surface to reach the salt bed. Miners use the “room-and-pillar” technique, blasting and cutting the solid rock salt to create large underground caverns supported by pillars of unmined salt. The extracted rock salt is then crushed and transported to the surface via conveyor belts for processing. This type of mined salt often contains minor impurities like shale and anhydrite, which makes it suitable for non-food applications such as road de-icing.
Solution Mining
Solution mining involves drilling a well into the underground salt formation and injecting fresh water to dissolve the halite. This creates a saturated brine solution, which is then pumped back to the surface. The recovered brine is treated to remove impurities and is then fed into vacuum evaporators, where the brine is rapidly boiled under reduced pressure. This energy-intensive process quickly evaporates the water, forcing the salt to crystallize into fine, high-purity grains. This method is the primary source for manufacturing most table salt and the pure sodium chloride required for many chemical manufacturing processes.
The Geological History of Salt Formation
The vast underground salt deposits that are mined today are the result of a geological process that unfolded over millions of years, transforming ancient oceans into solid mineral layers. These deposits are a type of sedimentary rock known as evaporites, which form in arid environments where water loss through evaporation greatly exceeds water input. During various periods of Earth’s history, large, shallow seas became partially isolated from the main body of the ocean. The hot, dry climate of these regions caused the seawater within the restricted basin to evaporate continuously.
As the water level dropped, the dissolved salts became increasingly concentrated, reaching the point of supersaturation. Minerals then began to precipitate in a specific sequence based on their solubility. Calcium carbonate precipitates first, followed by calcium sulfate minerals like gypsum and anhydrite, and finally, sodium chloride, or halite, begins to crystallize. This process led to the accumulation of massive layers of halite on the basin floor.
Over geological time scales, these salt layers were buried by subsequent deposits of sediment and rock, protecting them from dissolution. Tectonic movements further compressed and deformed these layers, sometimes forcing the salt upward into dome-like structures called salt domes.
Salt from Inland Brine and Salt Lakes
Significant quantities of salt are sourced from terrestrial bodies of water, such as inland salt lakes and naturally occurring brine springs. These salt lakes, like the Great Salt Lake in Utah or the Dead Sea, are terminal basins that have no outlet to the ocean. This lack of an outlet causes dissolved minerals carried in by rivers to concentrate over time.
The composition of the brine in these lakes is often distinct from seawater, containing higher concentrations of minerals like magnesium and potassium salts alongside sodium chloride. The extraction method used at these inland sources closely mirrors the solar evaporation technique employed in coastal regions. Brine is pumped from the lake or spring into a series of shallow, interconnected ponds to allow for natural evaporation and concentration. As the water evaporates, the different salts precipitate in sequence, allowing for the targeted harvesting of sodium chloride and other commercially valuable minerals.