DEMINERALIZATION UNIT ( DI ) Demineralization means that all the salts in the water are removed. The cation resin is regenerated with acid, where­by the positive ions contained in the water are exchanged for hydrogen ions, H +, The anion resin is regenerated with caustic solution, whereby the negative ions are exchanged for hydroxide ions, OH- When an anion resin follows a cation resin, the result will be demineralized water.

The ion exchange process is carried out in two cycles:

Resin and softening, What It Is and How It Works ?

The softening media is called Zeolite.The proper name is Polystyrene Resin.
A softener tank is filled approximately 2/3 full with Zeolite beads.
Even the smallest softener will contain hundreds of thousands of Zeolite beads. Each bead is a negative in nature and can be charged or regenerated with positive-charged ions.
In a softener, the Zeolite is charged with positive "soft" Sodium ions by passing through the softener tank a rich Sodium Chloride (common salt) brine solution which contains the "soft" Sodium ion (Na+).
The plus (positive) charges of the Sodium are attracted to the negative-charged Zeolite beads and remain "stored" upon the beads ready for the softener to deliver soft water when hard water containing Calcium (Ca++) and Magnesium (Mg++) are passed through the mineral bed. When this happens, "positive" ion exchange occurs.
You should note that Sodium (Na+) has only one positive charge where Calcium (Ca++) and Magnesium (Mg++) both have two positive charges. When the hard water passes by the Zeolite beads, which have been loaded or charged up with Sodium (Na+) from the salt brine solution, the attraction of the negative Zeolite bead for the two positive charges in each ion of Calcium and Magnesium is greater than for the one positive Sodium ion.
As a result, they are attracted to the bead and the Sodium ion is pushed off, displaced, and takes the place of the Calcium and Magensium ions originally in the water supply in the ratio of two Sodium ions for each ion of Calcium and Magnesium.
In effect, the Sodium is "exchanged" for the Calcium and Magnesium in the water supply with the Calcium and Magnesium remaining on the Zeolite beads and the Sodium ions taking their place in the water flowing through and out of the softener tank which, as a result of the "exchange" process, has become "soft" water.
It can now be readily understood that a softener will continue to give soft water only as long as there are sufficient Sodium ions remaining on the Zeolite beads to provide two Sodium ions for every ion of Calcium and Magnesium in the hard water flowing through the softener mineral (storehouse) tank. When the supply of Sodium ions has been depleted, the "storehouse" must be refilled.
Restocking the mineral tank with a new supply of Sodium ions is accomplished by flowing through the mineral tank a rich solution of common salt brine (Sodium Chloride) which effects another regeneration, ion exchange process, only in reverse.
The salt brine solution is automatically prepared and stored in a separate tank normally called the brine tank.
When the rich brine solution flows through the Zeolite beads, which are now depleted of Sodium ions and saturated with the "trapped" Calcium and Magnesium ions which are no longer wanted, the "Positive Ion Exchange" process reoccurs.
In the regeneration process, the reason the Sodium with the single positive charge is attracted to the negative Zeolite bead and the Calcium and Magnesium are given up is due to the overwhelming multitude of Sodium ions present in the rich brine solution.
When so many millions of ions are close to the negative Zeolite bead, the bead cannot resist this attraction and in the process throws off the double positive Calcium and Magnesium ions as it gathers to itself the single positive-charged Sodium ions.
The enemy in this instance are the entrapped Calcium and Magnesium ions and the "overwhelming power", the rich brine solution containing the multitude of Sodium ions.
One single Zeolite bead will attract numerous positive ions, not just one per bead.
More brine with the Sodium ions is passed through the softener mineral bed than can ever be exchanged. This is to assure the reverse positive ion exchange will occur as explained. The excess brine is rinsed away to a drain facility and with it, the unwanted Calcium and Magnesium ions that were given up by the Zeolite bead in the regeneration process. At the completion of the regeneration process the mineral bed is regenerated -- the storehouse is restocked with Sodium ions -- and the softener is ready to produce soft water once again. The frequency of this regeneration process is determined, therefore, by the capacity of the softener, the hardness of the water and the water usage. In most softeners, the brine solution is transferred from the brine tank to the softener mineral tank by the vacuum created when water is directed to flow through the "Injector" located within the control valve. This is "the" most important function of the control valve. If the softener fails to draw brine from the brine tank into the mineral tank there can be no soft water.