EXTERNAL TREATMENT METHOD:

.Ion exchange method  ( Demineralisation)
Working:
  1. Here all the cations and anions are completely removed .  It uses twp cylinders pf cation exchange cylinder and anion exchange cylinder  filled with resins.
  2. Resins are linear, insoluble, cross linked , organic polyers. There are 2 types.
i)                    Cation exchange resins – RH2   (e.g)  Sulphonated coals , RSO3H
ii)                  Anion exchange resins . R’(OH)2 (e.g)Urea-formaldehyde resin,Amines R-NH2
  1. The water is fed into cylinder –I  where all the cations are replaced by RH2  
Resins.
             RH2  + CaCl2  à  R Ca  +  2 HCl
  1. The cation free water is fed to cylinder II, where all the anions are replaced.
R’(OH)2  +  2 HCl  à  R’Cl2 + 2H2O
  1. So, the resultant water is free from all types of ions.

Regeneration:
            On prolonged use, as  all the resins are exhausted, there will be no H+ orOH – ions to exchange the unwanted ions. So, they have to be regenerated.
Cation resins are regenerated by HCl and anion resins by NaOH.
R Ca  +  2 HCl  à  RH2  + CaCl2
R’Cl2 + 2NaOH  à    R’(OH)2  +  2NaCl
Advantages of Ion exchange method:
i)                    Very good for use in high pressure boilers also.
ii)                  It can treat highly acidic or alkaline water.
iii)                We can get pure water as hardness as low of 2 ppm.

Drawbacks of Ionexchange method:
i)                    Expensive
ii)                  Fe, Mn cannot be removed as they form complexes with resins
iii)                Cannot be used for turbid water as they clog the resins.

Diagram: