Demineralization of water is the removal of essentially all inorganic salts by ion exchange. In this process, strong acid cation resin in the hydrogen form converts dissolved salts into their corresponding acids, and strong base anion resin in the hydroxide form removes these acids.
Ion exchange or deionization or demineralization process:
Principle: A reversible exchange of ions takes place between the stationary ion exchange phase and the external liquid mobile phase.
Ion exchange resins are insoluble, cross-linked, porous, high molecular weight, organic polymers and the functional groups attached to the chains are responsible for ion exchange properties.
Ion exchange resins are generally copolymers of Styrene- divinylbenzene
Ion exchange resins are of two types
- Cation exchange resins
- Anion exchange resins
Cation exchange resins (RH+):
- These are capable of exchanging H+ ions with the cations.
- It is mainly styrene divinyl benzene copolymer
- They have acidic functional groups like -SO3H, -COOH, etc
Anion exchange resin (ROH-)
- These are capable of exchanging OHions with the anions.
- It is nothing but a copolymer of styrene-divinylbenzene
- They contain basic functional groups like quarternary ammonium ions.
- On treatment with dil NaOH, they have the capability to exchange its OHions with anions in the water
Process:
The hard water is passed first through the cation exchange column.
The cation exchanger removes all the cations like Ca+2, and Mg+2 from it and an equivalent amount of H+ ions are released from the column into water.
2RH+ + Ca2+ ---->R2Ca + 2H+
2RH + + Mg2+ -----> R2Mg + 2H+
The hard water is now passed through the anion exchange column, which removes all the anions like SO4 -2, and Cl from it, and an equivalent amount of OH ions is released from the column to water.
R'OH- + Cl- ----> R'Cl + OH-
2R'OH- + SO4 2- ----> R'2SO4 + OH-
H+ and OH ions are released from the cation exchange and anion exchange columns respectively get combined to produce water molecules.
H+ + OH- ----> H2O
Water coming out from the exchanger is free from cations and anions and is known as deionized or demineralized water
What is the demineralization process of water softening?
Removal of contaminants or impurities like calcium, magnesium, silica, and iron which are prime causes for scale formation is known as water softening or demineralization. Some of the conventional methods to do so are:
- Lime Softening.
- Sodium Zeolite Softening.
- Reverse Osmosis.
- Electro dialysis.
- Ion Exchange method etc.
The treatment method employed mainly depends upon the raw water quality, demand, and purity requirements. It must be also noted that the process also depends upon the cost involved- budget.
Regeneration of exhausted bed:
When the beds are getting exhausted (capacity to exchange H+ & OH ions is lost) then they are regenerated
The exhausted cationic exchanger is regenerated by washing with acids like HCl, H2SO4, etc
R2Ca2+ + 2H+ → 2RH+ + Ca2+
The exhausted anion exchanger is regenerated by washing with bases like NaOH
R'2SO4 2- +2OH - → 2R'OH- + SO4 2-
The columns are washed with deionized water and washings are passed to the sink or drain. Now they are ready for softening process.
Demineralization of water
Advantages
- Produce water of very low hardness (2ppm )
- The process can be used to soften highly acidic or highly alkaline waters.
- Water produced by this process water can be used in high-pressure boilers
Disadvantages
- High capital costs and chemicals & equipment are costly
- If the water contains turbidity efficiency of the process decreases.