Removal of Permanent Hardness

It is not easy to remove permanent hardness of water as temporary hardness. Permanent hardness can be removed by following methods.

a) Treatment with washing soda: 
       Soluble salts of calcium and magnesium turn into insoluble salts when washing soda (sodium carbonate) is mixed with it.

Fig: Washing soda

CaCl₂ + Na₂CO₃ = CaCO₃ + 2NaCl

MgSO₄ + Na₂CO₃ = MgCO₃ + Na₂SO₄

b) Permutit process: 
        Permutit is the trade name for an artificially prepared sodium zeolite or sodium aluminium orthosilicate obtained by fusing together sodium carbonate, silica and alumina. Its composition corresponds to Na₂Al₂Si₂O₈ or Na₂Z where Z stands for zeolite = Al₂Si₂O₈. It possesses remarkable property of reacting with calcium and magnesium ions forming insoluble calcium or magnesium zeolite. By passing hard water through a cylinder packed with permutit all the ions responsible for the hardness of water are drawn and replaced by sodium ions.

Ca-salt + Na-permutit = Ca-permutit + Na-salt

Mg-salt + Na-permutit  = Mg-permutit + Na-salt

In due course, however, the permutit is exhausted by having given up all its sodium ions. This can be regenerated by soaking in a 10% solution of common salt when the reverse change take place and calcium and magnesium ions pass into solution and sodium zeolite is formed again.

CaZ + 2NaCl = Na₂Z + CaCl₂

MgZ + 2Nacl = Mg₂Z + MgCl₂

Fig: Permutit process

The resulting calcium and magnesium chlorides are washed out and the regenerated permutit used over again.