De-Mineral Plant (DM Plant) : It is a process of removing ionic mineral contaminants from the water by the ion-exchange process. After treatment of water in the DM Plant the quality of purity water is high. The salts and minerals present in the water dissociates into the constituent ions. The positive charged ions present in the water is known as cations and the negative charged ions is known as anions. These resins get attached with the opposite charge resins present in the vessels. The ions are loosely bounded with the resins. They are attached with electrostatic force of attraction.
Purpose: The purpose of using DM Plant:
- Removing of ionic mineral contaminants
- Reduction of conductivity
- Control the pH
Principle: Water is made to pass through the cation resin vessel and then in anion resin vessel. In cation resin vessel, cation present in water gets exchange with H+ ions and in anion resin vessel, anion present in water gets exchange with OH– ions.
Type of ion exchange resins used in DM Plant:
- Strong Acid cation Exchange Resin: These resins are mainly used in industrial water treatment purpose. They are made up of styrene and divinylbenzene. Type of resins mostly used INDION 220 Na and INDION 225 Na. It exchange H+ ions with cation of total dissolved solid in water.
- Strong Base Anion Exchange Resin: There are two types of Resin i.e. Type 1 and type 2 resin. Type 1 resin have high basicity which gives best quality of water whereas Type 2 resin have less basicity therefore it requires less chemical during regeneration. It exchange OH– ions with anion of total dissolved solid in water.
Components of DM Plant:
- Cation exchanger vessel
- Anion exchanger vessel
- Mixed bed filter
Arrangement: The Demineralization water feed pump is attached with Cation exchanger vessel. Chemical tank containing Dil. HCL joined with Cation exchanger vessel through the pipe at the top of the tank. The cation exchange vessel is further attached with the Anion exchanger vessel with the help of pipe. The Anion exchanger vessel is also attached with Chemical Tank containing Dil. NaOH.
Working: The water containing dissolved solids is introduced in the Cation exchanger vessel. Here all the cation ions such as Ca2+, Mg2+, K+, Na+ etc, get exchanged with H+ ions. Following is the reaction that takes place:
R-H + Na+ ⇌ R-Na + H+
After this stage almost all the cations are removed. Then in next step all the anions such as SO42-, NO3–, Cl– etc, are exchanged with OH– ions. Following is the reaction that takes place:
R-OH + Cl– ⇌ R-Cl + OH–
H+ ions formed in the Cation exchanger vessel and OH– ions formed in the anion exchanger vessel both react with each other and forms water, which is further drawn out from another outlet.
H+ + OH– ⇌ H2O
After this water enter mixed bed filter. In mixed bed filter the remaining ions are removed. It uses mixed ions regenerative system. In this anion and cations resins are mixed. The anion and cations resins used in mixed bed filter is different from the one used in the Cation and anion exchanger vessel. It creates a strong driving force as hydrogen ions on cation resins attracts hydroxyl ion resins as they are very close to each other. The conductivity goes below 1 µS/cm. it takes less time for regeneration of ions. After that the water is collected in DM water tank.
Regeneration: Regeneration is power restoring method for the inactive resins. The inactive resin goes through chemical process to regenerate their ions.
In Cation exchanger vessel HCl is introduced from chemical dosing tank. The chemical reacts with the resins. Following is the reaction that takes place:
R2Na + HCl ⇌ R-H + NaCl2
In anion exchanger vessel HCl is introduced from chemical dosing tank. The chemical reacts with the resins. Following is the reaction that takes place:
R-Cl + NaOH ⇌ R-OH + NaCl