Sodium Chloride

Sodium Chloride

Sodium chloride is the basic raw material for many chemical compounds such as NaOH, Na2CO3, Na2SO4, HCl, Na2PO4, Sodium Chlorate, Sodium Chlorite and its source of many other products through its derivatives. Practically all the chlorine products in the world is manufactured by electrolysis of Sodium Chloride (NaCl), a common salt is manufactured in three different ways;

  1. Solar evaporation of sea water
  2. Mining of rock salt
  3. From well brines

 

1. From saturated Brine by Multiple Effect Evaporator Process

Brine contains water 73.5%, sodium chloride 26.3%, calcium sulphate 0.12%, calcium chloride 0.003%, magnesium chloride 0.007%.

The flow sheet of process is given below;



Process:

  1. The Brine is first aerated to remove most of the H2S.
  2. Addition of chloride will remove H2S by displacement reaction.
  3. Brine is then pumped to settling tank where it is treated with caustic soda and soda ash to remove calcium, magnesium and ferric ions. Caustic soda and soda ash are blended in the miser to be taken to settling tank.
  4. In the Multiple Effect Evaporator (MEE) water is removed and salt crystals are removed as slurry.
  5. The slurry is sent to washer, where the salt crystals are washed with fresh brine.
  6. The washed slurry is filtered, mother liquor is return to the evaporators and salt crystals from the filter are dried and screened.
  7. Salt thus produced from the typical brine is 99.8% purity or even greater.
  8. The finest grade (some times made by grinding) is a flour salt, the next coarsest is table salt and finally the industrial salt.

The Iodine salt has the following composition;

  • Potassium Iodide (KI)    : 0.01%
  • Stabilizer Na2CO3         : 0.1%
  • Sodium Thio Sulphate    : 0.1%

 

2. From Saturated Brine by Open Pan Process

Process:

  1. Salt in the form of hopper-like crystal (grainer salt) is made by causing the salt crystal to form on the surface of brine held in an open pan.
  2. The grainer is a flat open pan 4.5 to 6.0 m width and 45 – 60 m long and about 60cm deep. Beneath the pan steam coils system provided for reciprocating the flakes for salt removal.
  3. The saturated brine mixed with circulating brine from the grinder is treated to 1200C at which temperature calcium sulphate is soluble and remove at that temperature.
  4. The precipitated calcium sulphate is removed from gravellier which consist of bed of stones.
  5. The purified brine is flash cooled to remove the remaining calcium sulphate.
  6. The slurry is then pumped to the grinders where evaporation takes place at 960C.
  7. A wet salt crystals obtain from the grinder are centrifuged, dried and screened.
  8. When the incoming brine has been treated salt of 99.98% sodium chloride can be obtained.

 

3. From Rock Salt Mining

About 35% of salt produced comes from mines of 8 different stages which are operated to produce rock salt. The salt deposits varying color from light reddish brown to half grey. The purity is 98.5%. After the rock is blasted loose they are crushed and then screened at the surface level. The remaining process is the series of grinding, screening to obtain the salft of crystal of various sizes.

 

4. From Sea Water by Solar Evaporation

Annual Evaporation exceeds precipitation, the statistics of 125mm of rain corresponding to 840mm evaporation.


By-products of Normal Salt (also called as value added products)

Manufacture of sodium sulfates from salt and sulfuric acid

2NaCl + H2SO4 ---> 2HCl + Na2SO4

Na2SO4 + 10H2O -> Na2SO4 + 2HCl

Hargreaves-Robinson Process

Sulphur Di-oxide, air, steam are passed over specially prepared porous common salt. The reaction is as follows;

2NaCl + SO2 + 1/2O2 + H2O ---> Na2SO4 + 2HCl

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