Salt (mineral)
Salt, scientifically known as sodium chloride (NaCl), is a vital mineral that plays a crucial role in both nature and human industry. It is widely found in seawater, salt lakes, and underground deposits, with significant reserves available in countries such as the United States, China, and India. Salt is integral to various industrial processes, including the production of chemicals, metallurgy, and food preservation, and it serves as an essential nutrient in human physiology.
Historically, salt has influenced trade routes and population migrations, even being used as a form of currency in ancient times. There are multiple methods for extracting salt, including traditional underground mining and solution mining, which involves dissolving salt with water to create brine. Despite its necessity for health and industry, excessive salt consumption is a concern in many developed nations due to its association with health issues such as high blood pressure.
Overall, salt's diverse applications range from culinary uses to significant industrial roles, making it a mineral of both historical and contemporary importance.
Salt (mineral)
Where Found
Salt (sodium chloride) is widely and abundantly distributed in nature. It is present in dissolved form in seawater, salt lakes, and groundwater in various parts of the world. There are also many substantial deposits of salt in solid form, notably in the United States, Great Britain, France, Germany, Russia, China, and India. In 2015, the primary forms of salt sold or used in the United States were salt in rock salt (44 percent), salt in brine (38 percent), solar salt (9 percent), and vacuum pan salt (9 percent). Also in 2015, the top salt-producing countries were China, with seventy million metric tons; the United States, with forty-eight million metric tons; and India, with seventeen million metric tons.
Primary Uses
Salt has numerous uses, chiefly in the chemical industry; in metallurgy; in ceramics, glass, and glaze manufacture; in agriculture; in medicine; in refrigeration; and in foods. In addition to its importance as an industrial raw material, salt is an essential nutrient, although its ubiquitous use in commercial food processing has made over-intake in industrialized nations a major health concern.
Technical Definition
"Salt," as it is commonly used, is a general term for naturally occurring sodium chloride (NaCl). (In chemistry, a salt is any polar-bonded ionic compound.) Synonyms include halite, common salt, and rock salt. Its average molecular weight is 58.448 grams per mole. Pure salt may be colorless or white; impurities may add a yellow, red, blue, or purple tint. Its hardness on the Mohs scale is 2 to 2.5. Salt usually occurs as cubic crystals. Its specific gravity is 2.17. It is readily soluble in water and is insoluble or only slightly soluble in most other liquids. It has a melting point of 801 degrees Celsius and a boiling point of 1,413 degrees Celsius.
Description, Distribution, and Forms
Sodium chloride is an important and abundant inorganic chemical. It is as essential to life as it is to modern industry. Human blood is composed of 90 percent water, 0.9 percent salt, and small amounts of proteins and other substances. As the salt is expended, it must be renewed. This fundamental need for salt has been a driving force behind exploration, commerce, and conflict throughout human history. Salt has long been a crucial industrial material as well, with approximately fourteen thousand different reported uses. Total world production of salt in 2015 was about 273 million metric tons; the United States accounted for about 18 percent of the total.
Salt is widely distributed throughout the world and the geologic column. Salt is produced by more than one hundred nations worldwide, most of which are able to fulfill their own consumption requirements from indigenous sources.
The world’s largest salt reserve is its oceans, which contain 2.5 percent dissolved salt by weight. The oceans are estimated to contain 44 × 1015 metric tons of salt, which would form a cube roughly 18.76 million cubic kilometers in volume. Dissolved salt is also present in salt seas and lakes, such as the Dead Sea in the Middle East, the Aral Sea in central Asia, and the Great Salt Lake in Utah. Subsurface brines are another source of dissolved salt. These brines can be ancient seawater that was entrapped in sediments at the time of deposition or saline waters that formed locally by solution of rock salt beds.
Extensive bedded deposits are also found in the form of rock salt. These sedimentary deposits occur interbedded with common strata and with other evaporite minerals, such as gypsum and anhydrite. The deposits were created as salts precipitated and accumulated on the floor of an ancient landlocked marine body of water. Extensive and widespread evaporation led to the formation of the deposits, which can reach thicknesses of up to 900 meters. Examples of bedded deposits can be found in Michigan, New York, Ohio, New Mexico, Canada, England, and central Europe. In North America bedded salt deposits occur mostly in Silurian, Permian, and Triassic formations.
When vertical or lateral stress is applied to stratified salt deposits, the lower-density salt flows plastically through the surrounding higher-density rock to form salt domes. These salt domes are usually cylindrical in shape and are often capped by anhydrite, gypsum, and calcite. Sulfur and hydrocarbons are frequently associated with salt-dome deposits. Salt domes are found in Texas, Mississippi, Louisiana, Mexico, Germany, Poland, Romania, Russia, and the Middle East. In arid regions salt occurs along with borax, potash, and other evaporite minerals as a surface deposit from desiccated salt lakes. Such playa deposits are important in California, Nevada, Utah, and India.
Salt occurs in nature as halite. It is often found interbedded with shale, limestone, dolostone, and rock-gypsum or rock-anhydrite in extensive beds and irregular masses. It is frequently associated with gypsum, anhydrite, calcite, sylvite, sand, and clay. In arid regions it can occur as a white powder, or efflorescence, on the soil surface. It can also be dissolved in the waters of salt springs, salt lakes and seas, and oceans.
History
Salt manufacture is one of the oldest chemical industries. Its availability influenced the locations of cities, the migration of populations, and the establishment of trade routes. Salt’s dietary importance led to its frequent use as a universal currency. Salt derives its name from sal, the Latin word for the substance. The word “salary,” which also comes from the Latin term, reflects the Roman practice of paying a portion of their soldiers’ wages with rations of salt.
Salt production in the United States began in 1614 with colonists in Virginia, who evaporated seawater to obtain the resource. Extraction of salt from subsurface brines began in the United States in 1788 in New York. In 1791, French chemist Nicolas Leblanc developed a commercial process that used salt to manufacture soda ash. The Solvay process, in which salt was also the chief raw material, supplanted the Leblanc process in the 1860s. In 1862, the first rock-salt mine in North America opened at Avery Island, Louisiana. In about 1882, the United States first employed solution mining methods. The 1887 invention of the vacuum pan was a significant contribution to the salt industry; applying a vacuum during evaporation made water boil at a lower temperature, thereby reducing the amount of fuel needed to heat the evaporation pans.
Obtaining Salt
Rock salt may be extracted from deposits using conventional underground mining or solution mining methods. Solution mining involves introducing pressurized and often heated freshwater into an injection well drilled into the salt deposit. The water dissolves the salt, and the resulting brine is pumped back to the surface for mineral recovery.
Whether brines are created by solution mining or obtained from the ocean, a sea, a lake, or another natural source, they must be evaporated for their salt contents to be harvested. Solar evaporation is effective in areas where the evaporation rate is high and the precipitation rate low. In many parts of the world, seawater or saline lake water is pumped into large, specially constructed ponds, where it is allowed to evaporate naturally. The brine passes through a series of these ponds during the solar evaporation process. In mechanical evaporation, brines are dehydrated in steam-heated vessels. This process is often augmented by applying a vacuum to make evaporation proceed at a lower temperature.
Desalination, the process of converting salt water into freshwater, produces salt as a by-product. Methods of desalination include distillation, membrane osmosis, freezing, and ion exchange. Some salt produced by desalination is used in industry.
Salt obtained through evaporation is not usually pure sodium chloride. Impurities are removed by aeration and chemical treatment. Small amounts of other substances, such as magnesium carbonate, hydrated calcium silicate, or tricalcium phosphate, may be added to prevent lumping. Iodized table salt usually contains small amounts of potassium iodide, sodium carbonate, and sodium thiosulfate.
Uses of Salt
The chief use of salt is as a raw material for the production of chlorine, sodium metal, and sodium hydroxide; it is also an ingredient in the Solvay process for manufacturing soda ash. Salt is used in making soaps, textile dyes, lacquers, cements, glass, ceramics, and glazes. It is employed in the treatment, smelting, and refining of ores and metals. While used as a refrigerating agent, it is also spread in large quantities to melt ice and snow on streets and highways. In agriculture, salt is a component of livestock feed, fertilizers, soil amenders, herbicides, and insecticides. In the medical field, salt is used in pharmaceuticals and specialty cleansers.
Salt is an essential part of human physiology. It is found in most body fluids, such as blood, sweat, and tears. It also provides chlorine for making hydrochloric acid, a small but vital part of human digestive fluid. Dietary intake of salt replaces the mineral as it is consumed through normal metabolism. The average per capita consumption of salt is approximately 5.44 kilograms a year. Salt is widely used as a seasoning for foods, a curing agent for meats, and a preservative for fish and other foods. While salt is an essential nutrient, excessive amounts in the diet can lead to health complications. Persons suffering from high blood pressure or heart disease often must restrict the amount of salt in their diets to avoid aggravating these conditions.
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