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Demineralization, also known as Deionization, is the process of removing dissolved minerals from water to make it suitable for industrial or municipal uses. For many laboratory and industrial applications, high-purity water, which is essentially free from ionic contaminants, is required.
The two most common types of Deionization are :-
1) Two-bed deionization
2) Mixed-bed deionization
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Two-bed Deionization :-
The two-bed deionizer consists of two vessels - one containing a cation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form. Water flows through the cation column, whereupon all the cations are exchanged for hydrogen ions.
To keep the water electrically balanced, for every monovalent cation, e.g. Na+, one hydrogen ion is exchanged and for every divalent cation, e.g. Ca2+, or Mg2+, two hydrogen ions are exchanged. The same principle applies when considering anion-exchange.
The desalinized water then flows through the anion column. This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).
Mixed-bed Deionization :-
In mixed-bed deionizers the cation exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The thorough mixture of cation-exchangers and anion-exchangers in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants. As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant.
The vessel can be in the form of a large stainless steel or reinforced fiberglass column containing many hundreds of liters of resin, or a small disposable/redeemable cartridge which, when exhausted, can either be thrown away or sent back to the original supplier for regeneration.
Demineralization :-
Demineralization, also known as Deionization, is the process of removing dissolved minerals from water to make it suitable for industrial or municipal uses. Deionization units also remove all other positive metallic ions in the process and replace them with hydrogen ions instead of sodium ions.
As the metallic ions in the water affix themselves to the exchange material, the latter releases its hydrogen ions on a chemically equivalent basis. A sodium ion (Na+) displaces one hydrogen ion (H+) from the exchanger; a calcium ion (Ca++) displaces two hydrogen ions; a ferric ion (Fe+++) displaces three hydrogen ions, etc. (Recall that home softeners also release two sodium ions for every calcium or magnesium ion they attract.) This exchange of the hydrogen ions for metallic ions on an equivalent basis is chemical necessity that permits the exchange material to maintain a balance of electrical charges. Now because of the relatively high concentration of hydrogen ions, the solution is very acid.
At this point the Deionization process is just half complete. While the positive metallic ions have been removed, the water now contains positive hydrogen ions, and the anions originally in the raw water.
The partially treated water now flows through a second unit; this time an anion exchange material normally consists of replaceable hydroxyl anions and fixed irreplaceable cations.
Now the negative ions in solution (the anions) are absorbed into the anion exchange material. Released in their place are hydroxyl anions.
All that emerges from such a two-unit system is ion-free water. It still contains the positive hydrogen ions released in the initial exchange plus the negative hydroxyl ions released in the second exchange.
What has become of these two ions? Through the magic of chemistry they have combined (positive to negative) to produce water molecules, which are in no way different from the water in which they were produced.
The result of this two-stage ion exchange process is water that is mineral-free. Deionized water has a wide range of uses in industry. Chemical production, pharmaceuticals, electroplating, television tube production, consumer products such as liquid detergents & leather goods processing are among the many diversified applications for Deionized water.
The quality of water flowing out of a primary system, whatever its composition is determined by the ion leakage from the Cation exchanger. This ion leakage varies, depending upon the properties of the raw water and the rate of regeneration. The quality of the demineralized water obtained is not sufficient for certain uses such as that of feed water for very high-pressure boilers and various applications in the chemical, nuclear or electronics industries. Therefore, it has to be further treated in a system known as a polishing plant.
Two-bed and Mixed-Bed Demineralization :-
Resin-based demineralization systems are typically designed in two-bed or mixed-bed configurations. Two-bed deionizers have separate tanks of cation and anion resins. In mixed-bed deionizers, the anion and cation resins are blended into a single tank or vessel. Generally, mixed-bed systems will produce higher quality water with a lower total capacity than two-bed systems. They are often used as post treatment for a reverse osmosis (RO) or electro Deionization (EDI) system as a final polishing step. Many industrial high purity water systems that do not incorporate membrane treatment use a three-step process consisting of a cation exchange step, an anion exchange step, and a mixed bed step for polishing the water to high purity.
Usually a mixed bed-polishing unit contains strong acid Cation (SAC) resin and strong base Anion (SBA) resins combined to a ratio of approximately 40% Cation resin to 60% Anion resin. (All resin volumes are dependant upon the incoming water analysis and the required quantity of treated water).
Please note: The Cation resin must be a strong acid Cation type (SAC) and the Anion resin must be a strong base Anion resin (SBA).
After a service cycle, each resin is regenerated as listed :-
(SAC): Strong Acid Cation resin is washed with approx. 30% hydrochloric acid.
(SBA): Strong Base Anion is washed with approx. 26% caustic.
Advantages :-
� The water obtained is of very high purity and its quality remains constant throughout the cycle.
� The pH is almost neutral.
� Rinse water requirements are very low.
Applications :-
� Battery top up
� Laboratory and research institutions
� Pharmaceuticals
� Chemicals
� Electroplating and mirror silvering
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