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A few readers have emailed in their questions about how salt is made.
Salt is found mostly underground in rocks and also dissolved in sea water. To get the salt from the sea, you have to collect sea water in shallow ponds and let the wind and sun dry out the water naturally, leaving only the salt behind. This is called solar evaporation, and the salt can form in layers up to 30 cm thick.
However, as the concentration of salt in the sea is about 3%, a great deal of energy is needed to produce dry salt, and it is only feasible in hot, dry areas.
In northern regions, salt is usually produced from solid deposits of rock salt. These deposits were formed from the evaporation of shallow seas in hot, dry climate, around 200 million years ago and occur in many regions of the world. In the North of England we have deposits in mid-Cheshire (in Northwich and Winsford).
The extraction process involves injecting hot water into these underground deposits of salt, which dissolves the salt, and the resulting concentrated solution (brine) is pumped out. The saturated brine (26% NaCl or sodium chloride) is then boiled in large vessels called effects.
Steam is used to heat the brine in chambers called calandrias. To save energy, there are lots of chambers linked together at different pressures. As the boiling point of water decreases with decreasing pressure, you only need to heat the first chamber, and the steam from the first will heat the second chamber, and so on.
This means that the final evaporation chamber is under vacuum. For this reason, the salt made in this process is called vacuum salt. This is what happens at the INEOS salt plant in Runcorn.
The first product of the evaporation process is a mixture of salt in brine. This is then centrifuged (spun round very fast) to give a slightly damp salt called Undried Vacuum Salt which is largely used by the chemical and water treatment industries.
Some of this salt is further dried to give Pure Dried Vacuum Salt (PDV). This product is used for industrial water softening, the cosmetics industry, food and many other applications.
Did you know?
The word “salary” comes from the Roman times, when soldiers were given money to buy salt!