Base exchange process

ZEOLITE or BASE-EXCHANGE or CATION EXCHANGE PROCESS for REMOVING HARDNESS

• Zeolites are natural salts or clays that are hydrated silicates of sodium and aluminium with the general formula Na2O Al2O3 x.SiO2 y.H2O
• Commonly available zeolite is green in colour and is called green sand
• Commonly available synthetic resin is a white coloured substance, called Permutit which is manufactured from feldspar, kaolin, clay and soda
• Zeolites or resins have an excellent property of exchanging their cations and hence are used in softening operations where the sodium ions of the zeolite are exchanged with the calcium and magnesium ions present in hard waters
• The chemical reactions involved are shown below

Na2Z    +    Ca/Mg [(HCO3)2 / SO4 / Cl2]    ->    Na2[(HCO3)2 / SO4 / Cl2]    +    Ca/Mg[Z]

Sodium zeolite or active zeolite + Calcium or Magnesium salts in hard water ->Sodium salts that do not cause hardness + Calcium or Magnesium Zeolite (exchanged or used zeolite)

• Calcium and Magnesium zeolite can be regenerated by treating the exhausted zeolite with 5-10% solution of sodium chloride. The exchange reactions during regeneration are represented below

Ca / Mg [Z]    +    2NaCl    -->  Na2Z    +    Ca / Mg [Cl2]

Used zeolite + Sodium chloride solution -->  Regenerated zeolite  +  Calcium / Magnesium chloride

• A zeolite softener (cation exchange unit) resembles a sand filter in which a filtering medium is a zeolite instead of sand

Advantages of using zeolite softeners

• Water of zero hardness can be obtained which is very useful in textile industries, boilers, etc
• The plant is compact, automatic and easy to operate
• No sludge is formed
• The RMO cast is less
• It also removes ferrous iron and manganese from water
• It does not pose problems while treating water of varying quality
• The problem of incrustation of pipes in distribution system using lime-soda process is absent in this process

Disadvantages

• The process is not suitable for treating highly turbid waters
• The process leaves sodium bicarbonate which causes priming and foaming in industrial feed waters
• The process is expensive and cannot treat waters containing iron and manganese