Pure de-mineralised water can be obtained by:

Pure de-mineralized water can be obtained by ${\mathrm{H}}^{+}$cation exchanger and  ${\mathrm{OH}}^{-}$ anion exchanger

Demineralized water is water that lacks cations and anions. It is made by passing hard water  through cation exchange resin and subsequently anion exchange resin ${\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right)\to {\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{I}\right)}$