Salt (sodium chloride, NaCl) is a simple compound fundamental to human diet, chemical manufacturing, and civil engineering. Salt is not manufactured; the global supply relies entirely on the extraction and processing of massive, naturally occurring reserves found across the planet. The methods used for this extraction are diverse, depending on the source material and desired purity, dictating whether water or dry mining techniques are employed.
Geologic Origins and Natural Reservoirs
The two primary sources of commercially available salt are the oceans and thick underground layers of mineral deposits known as rock salt or halite. The vast ocean reservoir contains roughly 3.5% dissolved solids by weight, making it an almost inexhaustible source of sodium chloride. Geologic deposits, often found deep beneath the Earth’s surface, are remnants of ancient, landlocked seas or lakes that dried up millions of years ago.
These deposits formed through a natural evaporation process that required the original seawater volume to be reduced significantly before halite began to crystallize. Over geological time, these crystallized salt layers were buried under sediment and rock formations, creating large, stable beds of rock salt. In some regions, immense tectonic pressure has forced these buried deposits upward through surrounding rock, resulting in dome-like structures that are also exploited for their salt content.
Extraction from Liquid Sources
Salt extraction from liquid sources, or brine, involves two main industrial approaches: solar evaporation and solution mining followed by mechanical evaporation. Solar evaporation is the oldest known method, relying on natural energy sources like the sun and wind in climates where evaporation significantly outpaces rainfall. This process funnels seawater or lake brine into a series of shallow, interlocking ponds.
As the water moves through concentration ponds, it slowly evaporates, increasing the salinity until the brine reaches saturation. The highly concentrated liquid is then moved into crystallization ponds, where pure sodium chloride precipitates out as salt crystals, leaving behind other minerals. After a period of months, the resulting salt bed is mechanically harvested, washed, and dried, yielding a high-purity product.
In regions lacking the necessary arid climate, or where higher purity is required, salt is obtained through solution mining and mechanical evaporation. This technique involves drilling wells deep into underground salt deposits. Freshwater is injected under pressure, dissolving the subterranean halite to create a saturated brine solution.
The saturated brine is pumped back to the surface for processing. At the surface facility, the brine is chemically treated to remove impurities like calcium and magnesium, ensuring the final product’s quality. The purified brine is then introduced into vacuum pans where steam heat is applied under reduced pressure, forcing rapid evaporation and allowing pure salt crystals to precipitate. This energy-intensive method is favored for producing the fine-textured, high-purity salt required for food-grade and chemical applications. Careful control of temperature and pressure allows producers to create specific crystal shapes, such as the hollow flakes used in specialty salts.
Extraction from Solid Deposits
The final major method involves extracting solid rock salt, or halite, directly from underground beds using traditional mining techniques. This process, known as deep-shaft mining, is similar to the methods used for extracting coal or other hard minerals. Miners descend via shafts down to the salt bed, which can be thousands of feet below the surface.
The most common technique employed is the room-and-pillar method, where salt is removed in a checkerboard pattern. Substantial salt pillars are intentionally left in place to provide structural support for the mine roof, ensuring safety and stability. Extraction typically begins by undercutting the salt face, followed by drilling holes and using carefully sited explosives to blast the rock salt into large chunks.
Alternatively, continuous mining machines can grind the salt face directly, producing smaller, more consistent lumps without the use of explosives. Once the rock salt is collected, it is crushed and conveyed to the surface for screening. The salt is then screened into various marketable sizes, a process that determines its final application, such as de-icing roads or chemical manufacturing.