.Ion exchange method ( Demineralisation)
- Here all the cations and anions are completely removed . It uses twp cylinders pf cation exchange cylinder and anion exchange cylinder filled with resins.
- 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
- The water is fed into cylinder –I where all the cations are replaced by RH2
RH2 + CaCl2 à R Ca + 2 HCl
- The cation free water is fed to cylinder II, where all the anions are replaced.
R’(OH)2 + 2 HCl à R’Cl2 + 2H2O
- So, the resultant water is free from all types of ions.
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:
ii) Fe, Mn cannot be removed as they form complexes with resins
iii) Cannot be used for turbid water as they clog the resins.