Sea salt

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Dissolved ions in seawater over 50mg/L
Cation Molarity Anion
0.535 Cl-
Na+ 0.459
0.009 SO4
Mg2+ 0.051
Ca2+ 0.009
K+ 0.009
0.002 HCO3-
0.00082 Br-

Seawater contains just about every element in some small quantity. The materials over 50mg/L is a shorter list.


For seawater and many brines, the order of deposition is:[1]

  1. calcium carbonate
  2. calcium sulfate
  3. sodium chloride
  4. magnesium sulfate
  5. potassium magnesium chloride hexahydrate (KCl•MgCl2•6H2O)
  6. magnesium chloride
Once it has been concentrated, the brine is run through a series of crystallizing pans, usually four in number, where the salt is deposited as evaporation proceeds. In the first crystallizing pan, the brine is concentrated to a specific gravity of 1.23 and remains partly contaminated with calcium sulfate. The specific gravity of the solution in the pan increases slowly during crystallization of the salt, reaching 1.24 in the second pan. In the third pan the specific gravity of the solution reaches 1.25, and the salt deposited there contains small amounts of magnesium sulfate as an impurity. The final solution, termed bitterns, has a specific gravity of 1.25–1.26 and is used in some countries (United States and Israel) in the manufacture of potash, bromine, epsom salts (magnesium sulfate), and magnesium chloride.[1]


Removal of sulfate and bicarbonate

To remove the 0.011 mol of these, add a small amount soluble calcium. (0.5g/L calcium oxide or 0.75g/L of calcium hydroxide) This will cause calcium carbonate and calcium sulfate to precipitate.

CaO + NaHCO3 CaCO3 + NaOH
Ca(OH)2 + NaHCO3 CaCO3 + NaOH + H2O
CaO + Na2SO4 + H2O CaSO4 + 2 NaOH
Ca(OH)2 + Na2SO4 CaSO4 + 2 NaOH

Removal of calcium and magnesium

With Calcium and Magnesium combined for 0.06 mol/L, adding 0.06mol of soluble ash (7-8g/L) should precipitate most of the alkali earth metal content.

(Ca/Mg)Cl2(aq) + (Na/K)2CO3(aq) (Ca/Mg)CO3(s) + 2 (Na/K)Cl(aq)

This leaves sodium/potassium chloride/carbonate in solution, which can be more easily separated by crystallization

See Also


  1. 1.0 1.1 Hills, John M.; Wood, Frank Osborne (2019) "Salt: Salt Manufacture"
    Encyclopedia Britannica 
    link courtesy Encyclopedia Britannica.