Removal of water hardness by ion-exchange process

Ion exchangers are chemicals which replaces the harmful calcium and magnesium ions with the harmless sodium ions. The ion exchangers may be minerals or synthetic polymers.

The mineral type ion exchangers are inorganic silicates namely sodium aluminium silicate (Na₂O 2Al₂O₃ 5SiO₂). These are called “Zeolite” and are represented by NaZ or Na₂Z, where Z stands for Zeolite. When the supply water is passed through the Zeolite, then the Na⁺ in the ion exchanger replaces the Ca²⁺ and Mg²⁺ in water and thus the zeolite removes hardness from the water,

Na₂Z  +  Ca²⁺  →  CaZ  +  2Na⁺

Na₂Z  +  Mg²⁺  →  MgZ  +  2Na⁺

The synthetic polymer ion exchange resin is called Permutit. These are basically highly cross-linked polymers with functional group such as sulphonic (-SO₃H), carboxyl (-COOH) or phenolic (-OH). The permutit is represented by R-SO₃^- or R-COO^- or R-O^- with cations H⁺ or Na⁺. If the cations are H⁺ then it is acidic form and if the cations are Na⁺ it is salt form. When treating water in a Dye house the synthetic ion exchange resin in salt form is used.

The polymer resin is made up of styrene and 2 to 10 % divinylbenzene. The crosslinking in the styrene makes the polymer thermoset and so the resin is stable. This works in the same way as Zeolites. The following reactions take place

2R-SO₃-Na⁺  +  Ca²⁺  →  Ca(R-SO₃-Na⁺)₂  +  2Na⁺

2R-SO₃-Na⁺  +  Mg²⁺  →  Mg(R-SO₃-Na⁺)₂  +  2Na⁺

After using the ion exchangers for some time Na⁺ ions get exhausted. So they can be easily replenished passing brine solution through the ion exchange resin.

Below is a simple representation of a Cation Exchange Process to remove water hardness in a Dye Industry