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production

white salt - rock salt

white salt

While the natural energy of the sun can be used to evaporate brine in hot countries, in the UK, white salt is produced by evaporating ‘solution-mined’ brine in pressure vessels. In solution mining, water is forced under pressure into a bore-hole drilled into an underground salt bed or dome. The salt is dissolved, turning the water into brine and creating a cavern in the salt-bed. The saturated raw brine is then withdrawn and pumped to the purification plant where calcium, magnesium and other impurities are removed prior to the evaporation process.

solution mining:
graphic of solution mining:

 

vacuum evaporation

A typical vacuum plant consists of a series of closed cylindrical vessels, or 'effects', containing steam chambers, which in turn contain a number of tubes. There are several steps in the process:

  • Brine is circulated through these tubes with steam condensing on the outer surface
  • The first effect receives low-pressure steam into its steam chamber
  • The brine boils at a temperature dictated by the inlet steam pressure
  • As the brine boils in the first effect, water evaporates
  • This produces further steam and causes salt crystals to grow
  • As the brine boils and the water is “driven off”, a thick salt slurry of brine and salt crystals is formed
  • This is fed to the second effect and circulated through a second heater unit which utilises the exhaust steam from the first effect
  • Pressures (and boiling temperatures) become successively lower through the evaporators. The final ones operate under vacuum and enable the brine to boil at much lower temperatures, which is more fuel-efficient
  • The slurry from the final effect is fed into a centrifuge to extract more moisture
  • The resulting undried vacuum salt is then stored in bulk, for supply to the chemical industry.

For food and allied industries, a drier salt is required. Salt from the centrifuges is fed into fluid bed drier-coolers - rather like hair-driers - for further drying. The salt is then sieved and graded before being transferred into large storage hoppers ready for distribution.

a typical six effect evaporation process:
graphic of a typical six effect evaporation process:

 

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