Every year, the salt industry produces over 250 million tonnes of salt. In addition to food, salt has a wide range of uses, most notably preventing roads from becoming covered with ice. Salt can come from one of two sources: it may come from seawater, or be mined from underground deposits. Salt mined from the earth is called rock salt, and extracting it is a complicated and expensive process.
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Rock salt formation
Although rock salt and sea salt are produced in very different ways, rock salt also has its origins in the sea. Rock salt, or halite, forms when a body of salt water such as a lake or sea evaporates. Over time, the vanishing water can leave behind layers of minerals dozens or even hundreds of metres thick. In some places, these mineral deposits, known as evaporites, can be seen above ground, but in other locations they become buried under layers of rock as time passes.
Mining in history
Because of its vital role in preserving food, salt was a very valuable commodity in the ancient world. An early salt mine in Azerbaijan has been identified as being over 6,000 years old. However, salt mining was considered a dangerous occupation. Miners, many of them slaves or criminals, cut into the halite deposits using hand tools, working in lightless conditions, often far underground. Even today, "the salt mines" is a proverbial expression for thankless, difficult work.
Solution mining is one of the two major forms of salt mining. In solution method, a salt deposit is first located underground. Miners pump water into the deposit, allowing the salt to dissolve into it. The salt water, or brine, is then pumped to the surface. Once there, the water is evaporated, replicating the process that led to the formation of the halite in the distant past. It may also be processed to remove other impurities.
Other salt deposits are mined using more traditional methods, although the tools are modern. Modern salt mines often use what is known as a room-and-pillar method, in which large columns of rock salt are left untouched to support the ceiling of the area being mined. One such mine is at Winsford in Cheshire; the largest salt mine in the UK, it produces a large proportion of the £150 million worth of salt used to grit roads every year. Mining machines called continuous cutters work away at the salt deposits, breaking them down into loose salt crystals which are then collected and lifted to the surface.
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