Demineralizers

Demineralizers are ion-exchange-based water treatment units designed to remove dissolved mineral ions from feedwater to produce demineralized water. In a typical ion-exchange demineralizer, a cation exchange resin removes multivalent and monovalent cations (such as Ca2+, Mg2+, Na+, Fe2+) in exchange for hydrogen ions, while an anion exchange resin removes anions (such as Cl-, SO4 2-, HCO3-) in exchange for hydroxide ions. The exchanged H+ and OH- combine to form water, yielding very low-mineral water. Mixed-bed demineralizers combine cation and anion resins in a single bed for polishing to very low conductivity.

Typical configuration uses a sequence of vessels: a cation bed followed by an anion bed; a polishing mixed bed may be used after. Regeneration is performed when resins exhaust: cation resins regenerated with acid (commonly sulfuric or hydrochloric acid), and anion resins with caustic soda (sodium hydroxide). In large plants, some systems operate online with continuous regeneration or with separate standby units; resin beds can be offline during regeneration. Pretreatment includes filtration and sometimes degassing. Water quality is monitored by conductivity, resistivity, silica content, and total organic carbon.

Applications and considerations: Demineralizers are used to supply boiler feedwater, process water in electronics and pharmaceuticals, and in desalination pretreatment. They produce water with very low ionic content and high resistivity, but require ongoing chemical handling, resin regeneration waste streams, and periodic resin replacement. Limitations include fouling from organics and silica, boron if present, and higher maintenance costs compared to some membrane alternatives.