Demineralization or de-ionisation process is known as ion-exchange method since all the cations and anions removed from the water completely. In this method resins are used as softening or ion-exchange material. They are insoluble cross-linked, long-chain organic polymers with functional groups capable of exchanging ions.
The resins are porous. They are of two types   :                                                                                                                                                             a) Cation exchange resin                                                                                                                                                                                                   b) Anion exchange resin

A)  Cation exchange resin:                                                                                                                                                                                                           They are obtained by co-polymerization of styrene and divinyl benzene followed by sulphonation or carboxylation of polymer.Such a polymer acquires capabilities of exchange their hydrogen ions with other cations in water. They contain acidic functional groups such  as   — COOH or — SO₃H and exchange their H ion  with other cations. They are simply represented as R-H where R- represents the insoluble polymeric material. It exchanges only cations from solution.

Ca⁺² + 2R^–H⁺                         →            R₂Ca + 2 H⁺ 

Mg⁺² + 2R^–H⁺     →            R₂Mg + 2 H⁺

Anion exchange Resin:                                                                                                                                                                                                        Resin containing basic functional groups such as –NH₂ = NH, etc are capable if exchanging their anions with other anions in water and therefore known as anion exchangers. They are resins obtained by co-polymerization of styrene and diviny benzene with quaternary ammoniurn groups. They become capable of exchanging their OH^– with other anions in water treatment with NaOH Solution.They are simply represented as R⁺ OH^– where R⁺ represented the insoluble organic matrix. Their exchange reactions with other anions may be written as.

R⁺OH + Cl^–              →        RCl + OH^–

2R + OH^– + SQ₄ ^2–   →        R2 SO₄ + 2OH^–                                                                                                                                                                                Both the resins are insoluble in water and mineral acids

Principle: -

a) Cation and anion exchange resins are placed separately in two vertical tanks (Fig 2) Demineralization of water they are connected through pipes as shown in the figure. Water is passed through cations exchange resins first then through anion exchange resin.

c)  In cation exchange resin tank all the cations are retained by the bed (H+ ions are replaced by Ca+, Mg2 ions) Reactions tanking place in the first tank are:   CaCl₂ + 2RH⁺       →      R₂Ca⁺² + 2HCl

          CaSO₄ + 2RH⁺      →      R₂Ca⁺² + H₂SO₄

          MgCl₂ + 2RH⁺       →      R₂Mg⁺² + H₂SO₄

          The acids so formed in the above reactions combine with        Ca (HCO₃) and Mg (HCO₃)₂ and produce CaCl₂ CaSO₄, MgCl₂ and MgSO₄. These substances will undergo exchange reactions as shown above.

i)  The water coming out from cations exchange resin will be free from hardness causing metal ions but contain acids like HCl and H₂SO₄

ii)  In anion exchange resin tanks all the anions are retained in the bed. The OH- ions replaced by SO₄ Reactions tanking place in second tank are    Hcl⁺R⁺OH^–                     →      RCl + H₂O

          H₂SO₄ + 2R + OH^–         →      R₂SO₄ + 2H₂O

         The H⁺ and OH^– ions released respectively from cat ion exchange and anion exchanger combine to give water.                                                           H⁺OH^–         →      H₂O
Water coming out from anion exchanger resin will be free from all types of anions. This ion-free water is called di- ionize water.

 Regeneration: After certain period of use, the resin beds will lose their exchange capacity. They are regenerated as follows                                           -The exhausted cat ion exchange resin is regenerated by passing through dil. HCl or Dil. H₂SO₄                                                                                           -The exhausted anion exchange resin is regenerated by passing through it a dilute solution of NaOH.

   OH^– + R₁C1^–                  →      R₁OH^– + Cl^–
   2OH^– + R₂ SO₄^2–            →      2R₂ OH^– + SO₄^2–

Advantages: -                                                                                                                                                                                                                                                 Water containing acids and alkalis can be treated.
-Water obtained is free from all types of ions                                                                                                                                                                            -Residue hardness is 2ppm. 100% pure water can also be obtained. 
Disadvantages:-
-Equipment is costly.                                                                                                                                                                                                                   -Water must be free from suspended impurities to avoid clogging problems.