In 2006, well over 200 million tons of salt were mined in the world. China is the largest producer with 48 million tons, followed by the United States with 46 million tons. How salt is mined? Salt is generally mined in one of three ways: deep mining, solution mining, or solar evaporation.
How is salt mined by the deep mine method?
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In many ways, it is similar to the extraction of any other mineral. Typically, salt exists as deposits in ancient subterranean marine formations that have been buried by tectonic changes over thousands of years. Many salt mines use the “room and pillar” mining system. Salt mines are among the safest mines.
They are also the most comfortable to work in. Although the temperature of the mine varies depending on the depth, the average temperature remains around 21 °C (69.8 °F) all year round. Salt can appear in veins, just like coal. Veins are primary layer deposits of salts.
Salt can also be found in domes that were formed when Earth’s pressure forced salt through cracks in bedrock from depths of 9,000 or 12,000 meters; they resemble traffic jams of almost circular shape with a diameter of several hundred meters to two kilometers. Some domes are close to the surface.
After the salt is mined and crushed, a conveyor belt pulls it to the surface. Most of the salt produced in this way is used as rock salt. Ground processing of rock salt consists of sorting the mined salt into different marketable sizes by calibrating through mechanical sieves. After separation, each size is transported to a separate storage bunker where it awaits packing for shipment or loading as bulk salt into rail cars, trucks, river barges for shipment to customers.
In solution mining, wells are constructed over salt beds or domes (deposits of salt pushed out of the ground by tectonic pressure) and water is pumped in to dissolve the salt. Then the salt solution, or brine, is pumped out and fed to the unit for evaporation. At the plant, the brine is cleaned to remove minerals and pumped into vacuum tanks, sealed containers where the brine is boiled and then evaporated until no salt remains. Then it is dried and refined. Depending on the type of salt, iodine is added to the salt. Most table salt is produced this way.
When solution mines are located near chemical plants, they are called brines, and the salt is used for chemical production. After the salt is removed from the salt mine, other substances, such as natural gas or industrial waste, are often stored in the resulting cavity.
How salt is extracted by the method of solar evaporation. This is the oldest method of salt extraction. Its use is appropriate only in warm climates where the rate of evaporation exceeds the rate of precipitation, either annually or over a long period, and constant wind currents are present.
Salt is harvested through solar evaporation from seawater or salt lakes. Wind and sun evaporate water from shallow bodies of water, leaving behind salt. It is usually collected once a year when the salt reaches a certain thickness. After harvesting, the salt is washed, drained, cleaned and refined. Two types of ponds are typically used. First, it is a concentration pond, where salt water from the ocean or a salt lake is concentrated. The second is called a crystallization pond, where salt is actually produced.
During the salting season, which lasts four to five months, crude oil flows continuously through these ponds. It is a saturated salt solution that contains as much salt as it can dissolve in itself, so pure salt crystallizes from the oil when the water evaporates.
This is the purest way to collect salt, often resulting in nearly 100 percent sodium chloride. Only in regions with low annual precipitation and high levels of evaporation – Mediterranean countries and Australia, for example – can solar evaporation plants be successful. This crop is harvested by machines, but in some areas it is still done manually.
A little history about salt
Before industrialization, it was expensive and time-consuming to collect the large quantities of salt needed for food preservation and seasoning. This made salt an extremely valuable commodity. The goals of the economy were based on the extraction and trade of salt.
In the Iron Age, Britons evaporated salt by boiling seawater or oil from salt springs in small clay pots over an open fire. Roman salting involved boiling seawater in large lead pots. Salt was used as currency in ancient Rome, and the roots of the words “soldier” and “salary” can be traced back to Latin words related to giving or receiving salt. In the Middle Ages, salt was transported along roads built specifically for this purpose. One of the most famous of these roads is the Old Salt Road in Northern Germany, which ran from the salt mines to the seaports.
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