How Ion Exchange Works

Ion exchangers are materials that can exchange one ion for another, hold it temporarily, and then release it to a regenerant solution. In a typical demineralizer, this is accomplished in the following manner: The influent water is passed through a hydrogen cation-exchange resin which converts the influent salt (e.g., sodium sulfate) to the corresponding acid (e.g., sulfuric acid) by exchanging an equivalent number of hydrogen (H +) ions for the metallic cations (Ca+\ Mg+2, Na+). These acids are then removed by passing the effluent through an alkali regenerated anion-exchange resin which replaces the anions in solution (CI", S04", N03") with an equivalent number of hydroxide ions. The hydrogen ions and hydroxide ions neutralize each other to form an equivalent amount of pure water. During regeneration, the reverse reaction takes place. The cation resin is regenerated with either sulfuric or hydrochloric acid and the anion resin is regenerated with sodium hydroxide. Figure 1 illustrates a basic scheme for ion exchange demineralization.

that we have, in effect, a number of multiple demineralizers in series. Higher-quality water is obtained from a mixed-bed unit than from a two-bed system, (see Figure 2 for an example). Operation of cation and anion exchanges is shown in Figure 3 (for fundamental processes) and Figure 4 (operation modes for both cation/anion exchanges).

Scdlum cyco inflsient

v (Source)

Parallel ofwration of

2nd alternative

Hydrogen cycle influent C«++MH++| COj" N«+ K* I MCOf M +

Mixed efi

Removed c,'h

CaL'tin wct-jr^e

Kegencrated

Regenerated HX

Cation

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