Demineralisation removes most of the mineral content of raw water. In general, demineralisation of water is carried by first passing water onto a cation exchanger in hydrogen form, followed by an anion exchanger in hydroxyl form, in a two-column process.
Broadly, demineralisation can be categorised into two types:
Demineralisation without silica removal
In this process, the weak base anion exchange resin, in hydroxide form, is used. It takes up strong mineral acids like Hydrochloric acid (HCl) and Sulphuric acid (H2S04). Weak acids like Carbonic acid (H2C03) and Silicic acid pass through without being exchanged.
A degasser placed after the weak base anion exchanger, removes H2C03. This process removes most of the salts from raw water except SiO2.
The weak base anion exchanger is regenerated after exhaustion by 1-5% of Sodium hydroxide (NaOH), Sodium carbonate (Na2CO3) or Ammonium hydroxide (NH4OH).
Demineralisation with silica removal
In this process, a strong acid cation exchange resin and strong base anion exchange resin in hydroxide form is used. Water, after passing through strong acid cation exchanger, is passed through strong base anion exchanger, which removes weak acids like Carbonic acid (H2C03) and Silicic acid along with strong mineral acids like Sulphuric acid (H2S04), Hydrochloric acid (HCl), etc.
The degasser is generally placed between cation and anion exchangers for economical removal of Carbon dioxide (C02). This also reduces the load on anion exchanger. The treated water quality of conductivity is less than 30 Micro Mhos, SiO2 less than 0.5 ppm and pH between 7.5 to 9.5 is achievable.
The strong base anion exchanger is regenerated after exhaustion by 4–5% Sodium hydroxide (NaOH) solution.