Q. Describe the ion exchange process of softening hard water. What are its advantages?
Determination of ion exchange or Deionization process: In this process ion exchange resins are used to remove all minerals for water. It is a cross linked organic polymer network having some ionisable group. It can be divided into two types on the basis of the nature of the ionisable group.
1. Cationic exchange materials (RH+): Synthetic resin like sulphonated copolymers of styrene and diving benzene and sulphonated phenol aldehyde polymers act as cations exchangers. They react with water containing salts which contains hardness, the cationic portion of the minerals or salts are replaced by hydrogen ion. It can also be regenerated by the action of hydrochloric acid.
2. Anionic exchange Materials (R'-OH): Synthetic resins which are produced (a) by the interaction of polyamines phenol and formaldehyde. (b) By the nitration and subsequent reduction of copolymers of styrene and diving benzene act as anion exchangers.
Process: The raw water is first passed through cations exchange react with water containing salts which causes hardness, the cationic portion of the minerals or salts are replaced by hydrogen ion. It can be also regenerated by the action of HCl.
H2R + CaSO4 = CaR + H2SO4
H2R + MgSO4 = MgR + H2SO4
H2R + Ca (HCO3)2 = CaR + 2H2O + 2CO2
H2R= Cations resin exchanger.
After cations exchange column, the hard water is passed through anion exchange column. They exchange the anionic portion of the materials from water by OH- ions of anion exchanger.
R-OH + HCl = RCl + H2o
2R-OH + H2SO4 = R2SO4 + 2H2O
Here R-OH = Anionic resinous exchanger.
Thus, the water coming out from the exchange is free from cations as well as anions, is known as deionised or demineralised water.
1. Highly acidic or alkaline water can be softened.
2. Water of very low hardness is obtained. So it is very good for treating water for use in high pressure boilers.
3. Coloured ions and turbidity is removed.