Removal mechanism of permanent hardness

Permanent (non-carbonate) hardness is caused by the chlorides, sulphates and nitrates of calcium and magnesium (CaSO$_4$, MgSO$_4$, CaCl$_2$, MgCl$_2$). It is not removed by boiling, because these salts remain dissolved. It is removed by chemical precipitation or ion exchange.

1. Lime-soda process

Soda ash (Na$_2$CO$_3$) is added; it converts the soluble calcium salts into insoluble calcium carbonate:

$$CaSO_4 + Na_2CO_3 \rightarrow CaCO_3\downarrow + Na_2SO_4$$ $$CaCl_2 + Na_2CO_3 \rightarrow CaCO_3\downarrow + 2NaCl$$

Magnesium carbonate is fairly soluble, so magnesium salts are first precipitated with lime as Mg(OH)$_2$:

$$MgSO_4 + Ca(OH)_2 \rightarrow Mg(OH)_2\downarrow + CaSO_4$$

and then the CaSO$_4$ formed is removed by soda ash as above. The precipitates are removed by sedimentation and filtration. The remaining hardness is about 30 to 50 mg/l.

2. Zeolite (base-exchange) process

Water passes through a bed of sodium zeolite (Na$_2$Z). Ca and Mg exchange with sodium:

$$CaSO_4 + Na_2Z \rightarrow CaZ + Na_2SO_4$$ $$MgCl_2 + Na_2Z \rightarrow MgZ + 2NaCl$$

When the zeolite is exhausted, it is regenerated with strong brine (NaCl): $CaZ + 2NaCl \rightarrow Na_2Z + CaCl_2$. It gives almost zero hardness water.

3. Demineralisation (ion exchange)

Water passes through a cation exchange resin (which replaces Ca$^{2+}$ and Mg$^{2+}$ with H$^+$) and then an anion exchange resin (which replaces SO$_4^{2-}$ and Cl$^-$ with OH$^-$), removing practically all dissolved ions.