Sodium chloride, commonly known as salt, is an ionic compound. This substance is necessary for biological life, playing a part in maintaining the body’s electrolytic balance, fluid levels, and nervous system function. Beyond its consumption as a dietary seasoning and preservative, salt is a foundational raw material for countless industrial processes, from manufacturing chlorine and caustic soda to de-icing roadways. Salt is found in dissolved forms in oceans and brine, and as solid mineral deposits called halite, necessitating specialized extraction techniques to meet massive global demand.
Solar and Mechanical Evaporation Methods
The oldest and most geographically restricted method for harvesting salt is solar evaporation, relying on the sun and wind. This technique is economically viable only in warm, arid coastal regions where the rate of water evaporation significantly exceeds the precipitation rate. Seawater or natural brine is channeled into a series of large, shallow, interconnected ponds called salterns.
In initial concentration ponds, the brine increases in salinity as water vaporizes, causing less-soluble impurities like calcium carbonate and gypsum to precipitate out first. The highly concentrated brine is then moved to crystallization ponds. Once saturated, pure sodium chloride begins to crystallize, forming a layer of salt on the pond floor. The raw salt is harvested mechanically, often once a year, and is generally used for industrial applications or further refined.
Mechanical evaporation, often termed vacuum evaporation, is a faster and more energy-intensive alternative. This process uses closed vessels called evaporator pans, which operate under reduced pressure to lower the boiling point of the brine. Applying heat under these vacuum conditions rapidly evaporates the water content. This method allows for a highly controlled and continuous crystallization process, yielding salt of high purity, which is ideal for food, pharmaceutical, and chemical industries.
Dry Mining of Underground Deposits
Dry mining extracts salt in its solid form, known as rock salt, from ancient underground deposits. These deposits are remnants of prehistoric seas that evaporated and were later covered by layers of earth and rock. This method requires sinking deep shafts into the earth to reach the salt layer, which can be hundreds of feet below the surface.
The most common technique used in these subterranean mines is the “room and pillar” method. Miners excavate large horizontal chambers, or “rooms,” while leaving massive columns of solid salt, the “pillars,” to support the mine’s roof and prevent structural collapse. Typically, between 45% and 65% of the deposit is extracted, with the remainder left as load-bearing pillars.
The process involves undercutting the salt face with large machines, drilling holes, and then blasting the material to dislodge hundreds of tons of rock salt at once. The coarse salt lumps are crushed underground and hoisted to the surface through the shaft. Due to natural mineral impurities like shale and anhydrite, this coarse, unrefined rock salt is primarily used for industrial applications, such as de-icing roads and highways.
Solution Mining from Brine Wells
Solution mining is used to retrieve salt from deposits that are too deep or structurally unstable for traditional dry mining. This technique involves dissolving the underground salt layer in water and pumping the resulting liquid, or brine, to the surface. The process begins by drilling a well down into the salt bed.
Fresh water is injected under pressure through one pipe, which dissolves the halite to create a concentrated salt solution. This saturated brine is then pumped back to the surface through a second, parallel pipe. Operators carefully control the injection and withdrawal rates to manage the brine concentration and determine the size and shape of the underground cavity.
The concentrated brine extracted from the wells is then sent to a surface processing facility. It must undergo a controlled evaporation process, usually using mechanical evaporation, to yield solid salt crystals. This method offers a continuous, clean extraction process that produces high-purity salt suitable for various uses.
Post-Extraction Processing and Final Products
Regardless of the extraction method, the raw salt must undergo refinement before it is ready for distribution. Initial processing involves washing the salt crystals with clean brine or water to remove residual insoluble matter and mineral impurities. After washing, the salt is dried to reduce its moisture content, which is particularly important for fine-grained products.
Screening involves passing the salt through mechanical sieves to grade it into various marketable sizes. This classification determines the final application, ranging from fine-powdered salt for food processing to coarse pellets for water softening. Additives are incorporated based on the intended use, such as anti-caking agents to ensure free flow, or iodine compounds to create iodized salt for public health purposes. The final products are broadly categorized as food-grade, industrial-grade for chemical production, and road salt for de-icing.