How Is Salt Produced

How Is Salt Produced? Salt is produced using three methods: Rock salt mining, solar evaporation, and vacuum evaporation. In rock salt mining, salt is mined from large underground deposits called beds or domes. It is crushed into manageable chunks and hauled to the surface, where it is screened, bagged, and shipped for further processing into commercial products and food-grade salt. Solar evaporation is the oldest method of salt production known to man. Seawater is captured in shallow ponds and allowed to evaporate by sun and wind. According to the Salt Institute, seawater contains on average 2.7% salt by weight. Cargill's solar evaporation process takes five years to complete. During that time, salty bay water is transferred into progressively saltier ponds and finally into crystallizer beds. There the brine becomes so concentrated that salt crystals drop to the bottom, forming a hard salt layer 12 inches thick. This "solar salt" is scraped up by machine and trucked to a wash house where it is rinsed with a brine solution. Since the brine solution is very salty, the salt crystal do not melt when rinsed. The rinsed salt is transferred to large outdoor storage stacks until ready for further processing. This "raw salt" is about 99.5% pure sodium chloride. Much of the raw salt is processed for use in agriculture, industry, and water conditioning. Cargill re-washes the raw salt to remove dust that may have collected while in the outdoor storage stack. It is then dried and sterilized in a gas-fired dryer to a temperature of 250°F. The resulting salt is 99.8% pure. It is shaken through a series of screens to sort the salt by size and packaged according to coarseness. For home use and commercial food producers, Cargill takes raw salt from the outdoor storage stack and refines it using a vacuum evaporation process (described below). The result is salt that is 99.99% pure sodium chloride. Vacuum evaporation is the process of dissolving rock salt or solar salt in water to create a brine solution, then boiling off the water to leave behind pure salt. Cargill dissolves raw solar salt in pure drinking water, removing dust and some trace minerals that cling to the salt crystals. The resulting brine is pumped into a vacuum evaporator—essentially a large vessel from which the air is removed—and it is boiled. Since water boils at a lower temperature in a vacuum, less energy is needed to evaporate the water than if an open-air vessel was used. Once most of the water has boiled away, the remaining salt slurry is dried, filtered, and air-cooled. A series of vibrating screen sort the salt crystals by size for packaging. Solution mining is the process of injecting water into underground salt layers and pumping out the resulting brine solution and processing it via vacuum evaporation. Solution mining is more commonly used by chemical companies in the production of chlorine, an important industrial chemical.