Sodium chloride, commonly known as salt, is a mineral compound fundamental to human civilization. Beyond seasoning food, salt is a major industrial raw material necessary for manufacturing chemicals like chlorine and caustic soda, and for de-icing roads. Global demand is met through the extraction of naturally occurring deposits found in seawater, underground rock formations, and concentrated brine sources. The choice of extraction technique depends primarily on the location, the source’s purity, and the intended final product.
Salt Production by Solar Evaporation
The most ancient and energy-efficient method of obtaining salt is through solar evaporation, a technique practical only in warm, arid climates with low rainfall. This process begins by channeling seawater or inland brine into a series of shallow, interconnected ponds called salinas. The movement of the water through these initial concentrating ponds allows the sun and wind to naturally evaporate the water, gradually increasing the salt concentration.
As the water evaporates, less soluble compounds like calcium sulfate precipitate out, leaving behind a highly concentrated sodium chloride solution, or brine. This saturated brine is then moved into the final, smaller areas known as crystallization ponds.
In these crystallization ponds, continued natural evaporation causes the sodium chloride to form salt crystals, which settle on the floor. The resulting layer of salt is collected using specialized mechanical harvesting machines, often after four to five months. This harvested “solar salt” is generally coarser and is further processed through washing and drying to meet purity standards. Solar evaporation is responsible for approximately one-third of the world’s total salt production, yielding a product with a purity between 99.7% and 99.8%.
Salt Production by Underground Mining
The dry extraction of halite, or rock salt, occurs in ancient underground deposits that are remnants of prehistoric seas. The extraction process is mechanically similar to conventional hard-rock mining, using drilling and blasting techniques to break apart the solid salt rock, which is then crushed and transported to the surface. This rock salt is typically less pure than evaporated salts because it naturally contains trace amounts of silica and other minerals. Due to its lower purity and coarser nature, a significant portion of mined rock salt is used for industrial applications, most notably for de-icing roads.
Salt Production by Brine Evaporation
Salt production requiring the highest degree of purity relies on controlled industrial evaporation, often starting with solution mining. This process involves injecting freshwater into underground salt deposits to dissolve the sodium chloride and create a saturated brine solution. The concentrated brine is then pumped back to the surface for purification and crystallization in a closed environment.
Once at the surface, the brine is chemically treated to remove impurities like calcium and magnesium salts before the final evaporation stage. The purified brine is then fed into large, sealed vessels called vacuum pans. These pans use artificial heat from steam and a vacuum to rapidly boil the water at lower temperatures, which conserves energy and accelerates crystallization.
The rapid crystallization within the vacuum pans yields extremely fine, high-purity salt crystals, often reaching purities between 99.8% and 99.95%. This fine-grained product is the source of table salt and is heavily used in the chemical and pharmaceutical industries. The precise control allows producers to meet stringent specifications for size and quality.
Post-Extraction Refining and Final Products
After extraction, the raw salt undergoes several mechanical and chemical processes to become a consumer-ready or industrial-grade product. The initial step is washing, often done with a saturated brine solution, to remove surface impurities and residual minerals. Following the wash, the salt is passed through dryers, which use hot air to reduce the moisture content.
Next, the dried salt is subjected to screening and milling, which separates the product into different grades based on crystal size. This sizing determines the final product category, such as the coarse grains for kosher salt or the fine grains for table salt. During this final stage, various additives are introduced depending on the salt’s intended use.
For human consumption, potassium iodide or sodium iodide is often added to create iodized salt, a measure used globally to prevent iodine deficiency. Anti-caking agents, such as tricalcium phosphate or silicon dioxide, are also mixed in to prevent the salt crystals from clumping together. These final steps of purification, sizing, and dosing ensure that the salt meets the diverse quality and functional requirements of end-users.