The importance of salt
What is commonly referred to as salt is the compound sodium chloride. It has long been important in the food industry as a preservative and also to add flavour. Another common use is to treat icy roads in winter as salty water has a lower freezing point then pure water.
Salt can be obtained from the sea or from underground salt deposits.
Mining - Large earth moving equipment is used to extract rock salt. This method of extraction leaves the salt with insoluble impurities such as reddish clay. This kind of salt is used to put on roads during freezing weather and does not need to be pure.
Solution in water - when pure salt for industrial purposes is needed a different method is used. Water is forced down a borehole into rock. The salt dissolves making a solution of brine and this solution is withdrawn to the surface and pumped to a purification plant. Water is evaporated from the brine under a range of pressures making the process more efficient. The salt crystallises and is separated from any remaining brine by filtering or using a centrifuge.
Extracting salt in solution can create large underground caverns. This can lead to bedrock collapsing and as a consequence, cause the lowering of the Earth’s surface. This is known as subsidence. It can be avoided by sensibly spacing out the holes created so the surface is supported by a sufficient amount of rock underground.