With complete demineralisation, all salts dissolved in the water are removed through a combination of strongly acidic cation exchanger and strongly basic anion exchanger.

In the first demineralisation stage, all cations are exchanged for hydrogen ions. The concentrations of anions remain unchanged, i.e. strong acids such as hydrochloric acid HCl, nitric acid HNO3 and sulphuric acid H2SO4, in addition to CO2, arise. This process is also called decationisation for this reason (see Abb.1).

 

Regeneration of cation exchangers primarily occurs with hydrochloric acid.

The acidic discharge from the cation exchanger is passed over an anion exchanger in the second stage.
If a weakly basic exchanger is used, only mineral acids (HCl, H2SO4, HNO3) are removed (see Abb.2).

If, on the other hand, a strongly basic exchanger is used, silicon acid (SiO2) and carbon dioxide can also be removed.
The result is demineralised water (see Abb.3).
The regeneration of these exchangers occurs with strong bases, e.g. sodium hydroxide.