Sodium chloride (NaCl), commonly known as salt, is an inorganic compound foundational to human health and a vast array of industrial processes. Beyond its familiar role in the kitchen, salt serves as a primary feedstock for the chlor-alkali industry, producing chlorine and caustic soda. Commercial production relies on methods ranging from harnessing natural forces to employing sophisticated industrial technology. The specific technique chosen depends on geological availability, climate, and the required purity of the final product.
Primary Sources of Sodium Chloride
All commercial salt is derived from two main geological sources: the world’s oceans and vast underground deposits of rock salt. Seawater and the brine of saline inland lakes are the most abundant source, containing an estimated 3.5% dissolved solids by weight. This dissolved salt is a continuous resource that requires only evaporation for extraction.
The second major source consists of solid underground layers of the mineral halite, or rock salt, formed millions of years ago. These deposits are remnants of ancient seas and salt lakes that evaporated, leaving behind thick, solid beds of sodium chloride. Over geological time, these formations were buried under layers of sediment and rock, creating massive reserves that can be hundreds of meters thick. The location of these reserves dictates whether the salt is extracted by physical mining or by dissolving it to create an artificial brine.
Solar Evaporation: Harnessing Sun and Wind
Solar evaporation is the oldest and most energy-efficient method of salt production, relying on the natural action of the sun and wind. This process is practical only in warm, dry climates where the annual evaporation rate significantly exceeds the precipitation rate. The production cycle begins by channeling seawater or natural brine into a vast, interconnected system of shallow earthen ponds.
The brine is progressively moved through a series of concentration ponds, where the sun and wind gradually increase the salinity. During this phase, less soluble compounds, such as calcium carbonate and gypsum, precipitate out first, a process known as fractional crystallization. Once the brine reaches near-saturation (typically around 25.6% sodium chloride concentration), it is transferred to the final crystallization ponds. Here, the sodium chloride precipitates and forms a solid, uniform layer, or “salt crop,” on the pond floor. This layer is then mechanically harvested using specialized equipment after the excess liquid, known as “bittern,” is drained off.
Dry Mining: Extracting Rock Salt
Dry mining extracts solid halite deposits from underground formations using techniques similar to those employed in coal mining. The most common method is the room-and-pillar technique, which involves excavating large chambers while leaving substantial columns of salt in place to support the mine roof. This structural requirement means that between 40% and 65% of the salt deposit is removed, with the rest remaining as permanent pillars.
The salt is extracted from the deposit face using either drilling and controlled blasting or continuous mining machines. These machines employ rotating cutting heads to shear the rock salt directly from the wall. After excavation, the rock salt chunks are transported by loaders and conveyors to an underground crushing station. The salt is reduced in size here before being hoisted to the surface, where it is further crushed, screened, and sized for applications such as road de-icing or industrial feedstock.
Brine Processing and High-Purity Refining
High-purity salt, required for food, pharmaceutical, or chemical manufacturing, is produced from a controlled brine source through specialized refining processes. The initial step for accessing deep, non-mineable underground deposits is solution mining. This involves injecting fresh water under pressure down a borehole to dissolve the salt, creating a saturated brine solution that is then pumped back to the surface through a second pipe.
This raw brine is chemically purified to remove dissolved impurities, such as calcium and magnesium ions, before crystallization. The purified brine is then fed into a vacuum evaporation system, consisting of a series of large, closed vessels called vacuum pans. Operating these pans under reduced atmospheric pressure allows the brine to boil and the water to evaporate at significantly lower temperatures. This rapid, controlled evaporation forces the sodium chloride to crystallize quickly, yielding fine, uniform crystals that can achieve a purity level exceeding 99.9%.