Question

Draw the diagram for demineralization process and write suitable reactions involved in the process .What are the advantages and disadvantages of the method .

Answer 1

Demineralization is also called as Ion exchange or de-ionization process .This softening process is carried out with help of organic substances called as ion-exchange resins .

Principle : Ion-exchange resins are insoluble , crosslinked organic polymers with a microporous structure and the functional groups attached to chains are responsible for exchanging ions .Ion exchange resins are of two types : 

a) Cation exchange resins (RH⁺) : Resins which contain acidic functional groups (-COOH ,-SO₃H ,etc ) are capable of exchanging their H⁺ ions with other cations .These are mainly styrenedivinyl benzene copolymers which on sulphonation/carboxylation become capable of exchanging their H⁺ ions with other cations in water . 
Eg . Amberlite IR-120 ,Dowex-50 .
2RH⁺ + Ca²⁺ → R₂Ca²+ + 2H⁺ 

b) Anion exchanger resin (ROH- ):Those resins containing basic functional group (-NH2 ,= NH as hydrochloride ) are capable of exchanging their anions with other anions in contact .The styrene-divinyl benzene or amine formaldehyde copolymers ,which contain amino/quaternary ammonium/quaternary sulphonium groups ,on treatment with dil NaOH becomes capcble of exchanging their OH- ions with any other anion .
Eg .Amberlite 400 ,Dowex-3 .
ROH^- + Cl^- → R⁺Cl^- + OH^-

Diagram : The schematic diagram of the unit used for this purpose is as shown in the figure .

Process : The hard water first passes through the cation exchange column when all the cations like Ca²⁺, Mg²⁺ ,etc are removed (taken up by the resin) and an equivalent amount of H⁺ is released from resin to water .Subsequently this water is passed through the anion exchange column when all the anions like Cl^- ,SO4^2- ,etc. The H⁺ and OH^- released respectively from cation exchanger and anion exchanger combine to give water .

 H⁺ + OH^- → H²O 

Thus water flowing out of the anion exchange column is free from all the cations and anions and becomes ion-free or deionized or demineralized .
When capacities of cation and anion exchangers to exchange H⁺ and OH^- ions respectively are lost ,they are said to be exhausted . 
These columns are regenerated by respective acid and alkali solutions as stated before .
The cation exchanger is regenerated by diluted H₂SO₄ and then washed with deionised water and washing (containing Ca²⁺ ,Mg²⁺ and Cl^- , SO4^2- ions) is passed to the sink . 

R₂Ca²⁺ + 2H⁺→ 2RH⁺ + Ca²⁺ 

The anion exchanger is regenerated by diluted NaOH and then washed with deionised water and washing (containing Na⁺ , Cl^- , SO4^2- ions ) is passed to the sink .The regenerated column is used again . 

R’₂SO4^2- + 2OH^-→ 2R’OH^- + SO4^2-

Advantages of the method :
1) The process can be used to soften highly acidic or alkaline waters .
2) It produces water of very low hardness (upto 2 ppm ) .
3) The water softened by this process is good for high pressure boilers .
Disadvantages of the method :
1) The equipment is costly and more expensive chemicals are needed .
2) If water contains turbidity then the output of the process is reduced .