White Salt Production
The table salt in your saltshaker and the salt used in food production is the end product of a two-step process of extraction and refinement – called Solution Mining and Brine Evaporation.
In solution mining, salt is extracted by forcing water under pressure into a bore-hole drilled into an underground salt bed or dome.
The salt dissolves, turning the water into brine and creating a cavern in the salt-bed. The brine is then pumped back to the surface and on to the purification plant where calcium, magnesium and other impurities are removed before beginning the brine evaporation process.
Vacuum Evaporation Refinement
While in hot countries the sun can be used to evaporate brine, here in the UK, white salt is produced by evaporating ‘solution-mined’ brine in pressure vessels – a process known as 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 tubes with steam condensing on the outer surface.
- The first effect or vessel receives low-pressure steam into its steam chamber and the brine boils at a temperature dictated by the inlet steam pressure. As the brine boils in the first effect, water evaporates producing further steam and causing salt crystals to grow. As the brine boils and the water starts to evaporate, a thick salt slurry of brine and salt crystals is formed.
- This is fed to the second effect and circulated through a second heat exchange unit that utilises the exhaust steam from the first effect to evaporate further moisture from the brine to produce further crystals. 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 energy-efficient.
- The slurry from the final effect is fed into a rotating centrifuge which spins off more moisture and the resulting undried vacuum salt is stored in bulk. This salt is usually for supply to the chemical industry.
- For food and related 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.
(courtesy of Chemical Industry Education Centre)