Salt is all around us. Underground and on the earth’s surface in the dried up residues of ancient seas. Some salt has even arrived from outer space in meteors. But our biggest source of salt is in our seas and oceans. With an average of 26 million tonnes per cubic kilometre, sea water offers a seemingly inexhaustible supply which if extracted, would cover the world’s total land mass to a depth of 35 metres.
There are many different types and grades of salt and a number of different methods of production. White salt is produced by evaporating ‘solution-mined’ brine in pressure vessels. The rock salt we use for gritting roads comes from mining ancient deposits. In some countries the natural energy of the sun is used to evaporate brine produced from sea water.
As with so many other things, China is the largest salt producer in the world followed closely by the United States. Salt is generally produced one of three ways: deep-shaft mining, solution mining or solar evaporation.
Deep-shaft mining is much like mining for any other mineral. Typically, the salt exists as deposits in ancient underground sea beds. Most salt produced this way is used as rock salt.
In solution mining, wells are erected over salt beds and fresh water is injected to dissolve the salt. Then the salt solution, or brine, is pumped out and taken to a plant for evaporation. Most common table salt are produced this way.
Salt is harvested through solar evaporation from seawater or salt lakes. Wind and the sun evaporate the water from shallow pools, leaving the salt behind. It is usually harvested once a year when the salt reaches a specific thickness. This only works in areas with low rainfall and a lot of sun - Mediterranean countries and Australia for example.
Underground corridor at Wieliczka salt mine in Poland
Workers transporting salt from the fields in Hon Khoi, Vietnam