Demineralization plant is employed for removal of minerals or dissolved salts from the water. Salts on dissolving dissociate into electrically charged particles called ions: for example common salt will be split into sodium ion (a positively charged ion or cation) and chloride (a negatively charged ion or an anion). If such a solution is brought into contact with a suitable ion exchange material (called resin), some ions from the solution are taken up by the resin and an equivalent number are transferred from the resin to the solution. Ion exchange is thus a reversible interchange of ions between a liquid and a solid.

A simple Demineralization Plant consists of two beds of chemically treated resin beads operating in series. The first column-cation exchanger-converts the dissolved solids in the raw water to the equivalent acids; these acids are removed as the water passes through the second column-anion exchanger. The final product from this process consists essentially of pure water. When exhausted, the cation exchange resin is regenerated with acid and the anion exchange resin with alkali.

In essence the DM plant comprises of resin vessels with charge of strong cation and anion resin; control-panel encompassing a conductivity measurement and alarms, etc; acid and caustic regeneration facility with bulk, semi-bulk regenerant chemicals storage arrangement.

The high-purity water from a demineralization plant is typically used as feed water for high pressure boilers in many industries; as wash water in computer chip manufacture and other micro-electronics manufacturing processes, as pharmaceutical process water, and any process where high-purity water is a requirement. DM water is used as process water in the manufacture of chemicals and fertilizers, food products such as soft drinks, automobiles for rinsing of components, textiles, etc.

Two-bed INDION DM plants are made in all sizes, from small portable units for laboratories to large multi-stream installations for Thermal power stations, refineries, petrochemical and steel plants.

The type of resins employed and selected depends on numerous factors: Treated water quality required – If silica removal is not required, anion exchange resin used in two-bed DM plants is usually INDION 850 weak base anion resin. If silica level of 1.0 ppm can be tolerated, then INDION N-IP strong base Type-2 resin is offered. When water free from silica is required, the anion exchanger is charged with INDION FF-IP strong base Type-1 anion resin.

Input water quality

• Presence of organic foulants – In cases where water has high level of organic foulants such as humic and fulvic acids occurring in natural surface waters, Macroporous resins such as INDION 810-Type 1 strong base resin are better suited for the application than INDION FF-IP
• Flow through plant required
• Considerations of minimization of operating costs in terms of regenerant chemical consumption: In order to reduce regenerant chemical consumption in large plants, INDION 850 resin (which is very efficient for removal of strong acids such as HCl and H2SO4 with minimal requirement of alkali for regeneration) is used in combination with INDION FF-IP strong base resin which is best suited for removal of weak acids such as carbon dioxide and silica from water

The regeneration is usually carried out in three steps. Firstly, the ion exchange column is backwashed with an upflow of water. The pressure vessel has about 75% free space above the resin bed (known as free board). This free space allows removal of any entrained solids and re-classification of the resin bed by backwashing. Backwashing also relieves bed compaction. Secondly, a predetermined amount of acid or alkali is injected into the column in a downward direction (same direction as the service flow or co-current) to displace sodium/ calcium/ magnesium in the cation exchanger and chlorides/ sulphates/ alkalinity in the anion exchanger taken up during the service cycle. Lastly, the column is rinsed to remove excess regenerant. The entire operation takes about 3 hours for a two-bed DM plant.

With counter-flow regeneration, the regenerant acid or caustic passes in the direction opposite to the flow of water during the service cycle. With counter-flow regeneration, the fresh regenerant enters at the bottom of the resin bed and passes in an upward direction (opposite to the downflow direction during service cycle or counter-current). Hence, bottom layer of the resin bed is always in highly regenerated condition. This means lower leakage or slip of ions during the service cycle producing better quality of treated water than the co-current method.

The mixed bed is a single column of INDION 225 cation exchanger and INDION FF-IP anion exchanger mixed together. Water passing through the column comes into contact with these materials and is subjected to almost infinite number of demineralizing stages. Thus demineralized water of high purity is produced.

Two-bed demineralizers & mixed bed units are regenerated with acid and alkali.
Ion exchange resins in mixed bed units must be separated before the regeneration. Bed separation is accomplished by backwashing: this carries the lighter INDION FF-IP resin to the top of the bed and the heavier INDION 225 remains at the bottom. Two completely separated layers are thus formed, into which the acid and alkali solutions and rinse water are introduced through specially designed distributors. After regeneration, the two resins are mixed with compressed air.

Normally mixed bed unit treats water from the two-bed DM plant that is already of high purity and their ionic load is low. They can consequently be operated at high flow rates, and are of small dimension.

Electrical conductivity is used to express the purity of demineralized water. Depending on the application pH and/or reactive silica in DM water may also be specified as parameters to measure the purity of DM water.

The quality of the water depends on the type of scheme used:

Cation-Anion-Polishing Mixed Bed

Our guarantees can be as follows:
(Depending upon the scheme)

1. Conductivity 0.1 micromhos/cm – 1.0 micromhos/cm at 25℃ (We guarantee conductivity of 0.1 micromhos/cm in very large systems depending upon operating pressure of boilers)
2. pH: 7 +/ - 0.2
3. Reactive silica 0.2 mg/l to 0.02 mg/l

Cation-Anion (Counter-Current Regeneration)

For standard plants our guarantees are as follows:

1. Conductivity less than 10 micromhos/cm at 25℃
2. pH: 7.5 – 8.0
3. Reactive silica less than 0.2 mg/l

Cation-Anion (Co-Current Regeneration)

With typical co-current regeneration, the outlet quality will depend on the regenerant applied, resin employed and raw water quality

1. Conductivity less than 30 μS/cm at 25℃
2. pH: 7.5 – 9
3. Silica less than 1.0 ppm

For the sizing of a demineralization plant, a detailed water analysis is required which gives all the cations & anions present & their concentration along with potential, heavy metals & organic foulants. The final water quality specification, as well as flow rate and end use of water.

The alkalinity or bicarbonates and carbonates present in raw water appear as carbonic acid or dissolved carbon dioxide at the outlet of cation exchanger. Degasser tower is used to remove unstable carbonic acids from water. It reduces ionic load in water which is to be treated in downstream anion exchangers.

All INDION DM plants are provided with conductivity indicators that have two basic elements: a conductivity cell with electrodes of special design between which demineralized water flows and a sensitive milliammeter for measuring the current passing between the electrodes. This current is proportional to conductivity of the water.