Salt Production by Solar Evaporation of Sea Water

Raw materials are the substances that are supplied from outside for the chemical industry and which take part in the production process. Let's understand the composition of seawater and the availability of seawater on the earth. Water covers about 70% of the earth's surface. Out of this, nearly 98.3% is seawater. As seawater is largely available on earth, many industries are starting from seawater as a raw material.

Written by: S.D. Udani Punsadee, BSc Eng Hons (Peradeniya), last update: 2021.06.20

Requirements for salt manufacturing plant

The traditional salt manufacturing process is happening in the coastline area. Followings are the features of the places where Saulters are established to produce salt.

  • Plane land nearby sea or Lagon
  • Less rainfall
  • Dry and more sunlight
  • Water impervious clay land

Evaporation - sedimentation process of manufacturing sodium chloride

Traditional salt production ponds can be illustrated below in Figure.

Evaporation-sedimentation process of manufacturing sodium chloride

Following steps are followed in manufacturing salt.

  • Initially, seawater feeds into the first crystallizing tank as raw material. The concentration of the seawater rises due to the sunlight and the dry air. When the concentration rises from a certain point, calcium carbonate (CaCO3) gets deposited.
  • The remaining solution is transferred into the second tank & evaporated by dry air and sunlight. when the concentration further increases, Calcium sulfate (CaSO4) gets deposited & allowed to settle down.
  • The remaining solution is transferred into the 3rd tank of the saltern and let to evaporated by sunlight and dry air. Once it has been concentrated, Nacl gets deposited as in the last pan. The salt deposit on this pan floor is piled up next to the pond and left to dry.

To achieve successful salt production, The densities of seawater in every above-mentioned pond in the salt production process will be regularly monitored during the entire process. The purer the sea salt the much more valuable it is; When impurities present in salt, it decreases its value and reduces the potential end uses. Apart from insoluble, impurities in salt are largely from seawater.

The harvested salt contains highly hygroscopic impurities such as Ca2+, Mg2+ and S042- . Pure NaCl is not hygroscopic. To remove impurity irons, the collected salt from 3rd tank is stored outside for nearly 6 months. During this period salt is purified as Ca2+ and Mg2+ ions absorb water and become a solution while the pure NaCl remains still.

As previously mentioned, the solution leaving the 3rd tank that remains after evaporation and crystallization of sodium chloride (salt) rich in compounds of magnesium, potassium, chloride, and sulfates. This solution is called a bittern solution since the Magnesium concentration in the bittern can reach 20 - 30 times of seawater, this also is called as Mother solution since is a major source for many other products. As bittern contains significant amounts of magnesium ions, it is more suitable to use bittern to extract magnesium.

By-products of salt production

The bittern contains significant amounts of MgSO44, MgCl2, MgBr2 , Mg(OH)2 and solid CaSO4 can be produced by the reaction of bittern and milk lime (aqueous Ca(OH)2) as in the below reaction. This reaction deposits the CaSO4 which uses for gypsum ( CaSO4.2H2O) & (Paris) plaster (2 CaSO4.H2O)

CaSO4 preparation as a by-product from salt production

The remaining liquid contains Br- ions which can be used to produce bromine gas by reacting the remaining liquid with chlorine gas as in the below reaction.

produce bromine gas by reacting NaBr with chlorine gas

The remaining liquid from the above step contains a significant amount of Mg(OH )2, which can be used to extract MgCl2 by resting with HCl as in the below reaction.

extract MgCl2 by resting with HCl from salt manufacturing

The extracted MgCl2 can be used to produce magnesium by electrolysis of the anhydrous MgCl2 in higher temperatures.