Question

Explain the zeolite method for softening of water giving suitable diagram and reactions. What are the limitations of this method. 

Answer 1

Zeolite is chemically hydrated sodium alumino silicate Na₂0 . Al₂03. xSi0₂. yH₂0  where x  = 2 to 10 and y= 2 to 6 . there are two types of zeolite (1) Natural zeolite (2} Synthetic zeolite.
PROCESS:-
Zeolite softener is made up of a cylinder in which there is a bed of zeolite. Hard water is percolated through the bed of zeolite at a specific rate. The hardness producing ions like Ca^{2 +},Mg²⁺ etc are retained by the zeolite forming CaZe and MgZe. The outgoing water contains sodium salts. 

The reactions taking place during the softening process are:-
Na₂Ze + Ca(HCO₃)₂ → CaZe + 2NaHC0₃ 
Na₂Ze + Mg( HCO₃)₂ → MgZe + 2NaHC0₃

Na₂Ze + CaCl₂ (or CaSO₄) →  CaZe + 2NaCI (or Na₂SO₄) 

Na₂Ze + MgCl₂ (or MgS0₄) →  MgZe + 2NaCl (or Na₂SO.₄)

 

REGENERATION OF ZEOLITE:-
During softening of water Ca²⁺ and Mg²⁺ ions are retained by zeolite forming GaZe and MgZe i.e.,calcium zeolite and magnesium zeolite. Once all the zeolite gets completely converted to GaZe and MgZe it is said to be exhausted. At this stage zeolite is unable to soften more of the hard water. The supply of hard water is then stopped. They exhausted zeolite which is GaZe and MgZe is treated with concentrated brine solution f. e., NaG/ solution.

CaZe + 2NaCl  → Na₂Ze + CaCl₂
MgZe + 2NaCl →  Na₂Ze + MgCl₂

The washing containing CaCl₂ and MgCl₂ are discarded and the regenerated zeolite is used again for softening of water.
LIMITATIONS OF ZEOLITE:-
1. If the supply of water is turbid, the suspended matter must be removed before the water is admitted to the zeolite bed. Otherwise the turbidity will clog the pores of zeolite bed thereby making it inactive.

2. If water contain large quantities of coloured ions such as Mn²⁺ and Fe²⁺, they must be removed first because these ions produce manganese and iron zeolites, which cannot be easily regenerated.

3. Mineral acids, if present in water destroy the zeolite bed and therefore they must be neutralised with soda before admitting the water to the zeolite softening plant.