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06 Aug 2014

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Basic ion exchange processes
in water treatment

Introduction

The ion exchange technology is used for different water treatment applications:

You will find here a description of the above processes, the exchange reactions and the changes in water. Resin types are described in another page, as well as regeneration methods. See also the general introduction to ion exchange, and an overview of ion exchange column designs in other pages.

Softening

Natural water contains calcium and magnesium ions (see water analysis) which form salts that are not very soluble. These cations, together with the less common and even less soluble strontium and barium cations, are called together hardness ions. When the water evaporates even a little, these cations precipitate. This is what you see when you let water evaporate in a boiling kettle on the kitchen stove.

Hard water also forms scale in water pipes and in boilers, both domestic and industrial. It may create cloudiness in beer and soft drinks. Calcium salts deposit on the glasses in your dishwasher if the city water is hard and you have forgotten to add salt.

Strongly acidic cation exchange resins (SAC, see resin types) used in the sodium form remove these hardness cations from water. Softening units, when loaded with these cations, are then regenerated with sodium chloride (NaCl, table salt).

Reactions

Here the example of calcium:

2 R-Na + Ca++ ---> R2-Ca + 2 Na+

R represents the resin, which is initially in the sodium form. The reaction for magnesium is identical.

The above reaction is an equilibrium. It can be reversed by increasing the sodium concentration on the right side. This is done with NaCl, and the regeneration reaction is:

R2-Ca + 2 Na+ ---> 2 R-Na + Ca++

What happens to the water

Raw water
Raw water
SAC (Na)
-->
soft water
Softened water

The water salinity is unchanged, only the hardness has been replaced by sodium. A small residual hardness is still there, its value depending on regeneration conditions.

Uses

Examples for the use of softeners:

Softening the water does not reduce its salinity: it merely removes the hardness ions and replaces them with sodium, the salts of which have a much higher solubility, so they don't form scale or deposits.
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De-alkalisation

This particular process uses a weakly acidic cation resin. This resin type is capable of removing hardness from water when it also contains alkalinity. After treatment, the water contains carbon dioxide, that can be eliminated with a degasifier tower. The cation resin is very efficiently regenerated with an acid, usually hydrochloric acid.

Reactions

Here the example of calcium:

2 R-H + Ca++(HCO3)2 ---> R2-Ca + 2 H+ + 2 HCO3

and the hydrogen cations combine with the birarbonate anions to produce carbon dioxide and water:

H+ + HCO3 ---> CO2 + H2O

What happens to the water

Raw water
Raw water
WAC (H)
-->
decarbonated water
Decarbonated water
Recombination of hydrogen and bicarbonate and removal of carbon dioxide with the degasifier:
Decarbonated water
Decarbonated water
DEG
-->
Degassed water
Degassed water

The salinity has decreased. Temporary hardness is gone.

Uses

De-alkalisation is used:

De-alkalisation reduces the salinity of water, by removing hardness cations and bicarbonate anions.
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Decationisation

The removal of all cations is seldom practiced, except as a first stage of the demineralisation process, or sometimes in condensate polishing where the decationiser precedes a mixed bed unit. A strongly acidic cation exchange resin (SAC) is used in the H+ form.

Reactions

Here the example of sodium, but all cations react in the same way:

R-H + Na+ ---> R-Na + H+

The equilibrium reaction is reversed for regeneration by increasing the hydrogen concentration on the right side. This is done with a strong acid, HCl or H2SO4:

R-Na + H+ ---> R-H + Na+

What happens to the water

Raw water
Raw water
SAC (H)
-->
Decationised water
Decationised water
DEG
-->
Degassed water
Decat + degassed water

In the second step, a degasifier is used again to remove the carbon dioxide formed by combining the bicarbonate anions and the released hydrogen cation. The water salinity is reduced, and the water is now acidic. A small sodium leakage is shown.

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Demineralisation

For many applications, all ions in the water must be removed. In particular, when water is heated to produce steam, any impurity can precipitate and cause damage. As there are cations and anions in the water, we must use two different types of resins: a cation exchanger and an anion exchanger. This combined arrangement produces pure water, as presented in the general introduction. Demineralisation is also called deionisation. The cation resin is used in the hydrogen form (H+) and the anion resin in the hydroxyl form (OH), so that the cation resin must be regenerated with an acid and the anion resin with an alkali.

A degasifier is used to remove the carbon dioxide created after cation exchange when the water contains a significant concentration of bicarbonate.

The cation resin is usually located before the anion resin: otherwise if the water contains any hardness, it would precipitate in the alkaline environment created by the OH form anion resin as Ca(OH)2 or CaCO3, which have low solubility.

Layout SAC – (DEG) – SBA
Let us first consider a simple demineralisation system comprising a strong acid cation exchange resin in the H+ form, a degasifier (optional) and a strong base anion exchange resin in the OH form. The first step is decationisation as shown above:

RSAC-H + Na+ <---> RSAC-Na + H+

With calcium insead of sodium (also valid for magnesium and other divalent cations):

2 RSAC-H + Ca++ <---> (RSAC)2-Ca + 2 H+

In the second step, all anions are removed with the strong base resin:

RSBA-OH + Cl <---> RSBA-Cl + OH

The weak acids created after cation exchange, which are carbonic acid and silicic acid (H2CO3 and H2SiO3) are removed in the same way:

RSBA-OH + HCO3 <---> RSBA-HCO3 + OH

And finally, the H+ ions created in the first step react with the OH ions of the second step to produce new molecules of water. This reaction is irreversible:

H+ + OH ---> H2O

What happens to the water

1: Cation exchange removing all cations (as in decationisation) followed by degassing:
Raw water
Raw water
SAC (H)
-->
Decationised water
Decationised water
DEG
-->
Degassed water
Decat + degassed water
2: Anion exchange removing all anions (strong and weak acids):
Degassed water
Decat + degassed water
SBA (OH)
-->
Demin water
Demineralised water

Demineralised water is completely free of ions, except a few residual traces of sodium and silica, because the SAC and SBA resins have their lowest selectivity for these. With a simple demineralisation line regenerated in reverse flow, the treated water has a conductivity of only about 1 µS/cm, and a silica residual between 5 and 50 µg/L depending on the silica concentration in the feed and on regeneration conditions.
Note that the pH value should not be used as a process control, as it is impossible to measure the pH of a water with less than say 5 µS/cm conductivity.

Regeneration

The SAC resin is regenerated with a strong acid, HCl or H2SO4:

R-Na + H+ <---> R-H + Na+

And the SBA resin is regenerated with a strong alkali, NaOH in 99 % of the cases:

RSBA-Cl + OH <---> RSBA-OH + Cl

Layout WAC/SAC – DEG – WBA/SBA
Because weakly acidic and weakly basic resins offer a high operating capacity and are very easy to regenerate, they are used in combination with strongly acidic and strongly basic resins in large plants. The first step with the WAC resin is dealkalisation (removal of bicarbonate hardness), and the second step with the SAC removes all the remaining cations. A WAC resin is used when both hardness and alkalinity are present in large relative concentrations in the feed water.

WBA resins remove only the strong acids after cation exchange. They are not capable of removing the weak acids such as SiO2 and CO2. In the regenerated, free base form, they are not dissociated, so no free OH ions are available for neutral anion exchange. On the other hand, their basicity is enough to adsorb the strong acids created after cation exchange:

RWBA + H+Cl ---> RWBA.HCl

In the last step, a SBA resin is thus required to remove the weak acids, as shown in the preceding section:

RSBA-OH + HCO3 <---> RSBA-HCO3 + OH

What happens to the water

1 & 2: Cation exchange beginning with the removal of temporary hardness (WAC, as in dealkalisation) followed by the removal of all remaining cations (SAC):
Raw water
Raw water
WAC (H)
-->
Decarbonated water
Decarbonated water
SAC (H)
-->
Decationised water
Decationised water
3 & 4: Anion exchange begining after degassing with the removal of strong acids (WBA) followed by the removal of weak acids (SBA):
Degassed water
Decat + degassed water
WBA (FB)
-->
Partial demin water
Partially demineralised
SBA (OH)
-->
Demin water
Demineralised water

A full demineralisation line is shown below, with a cation exchange column (WAC/SAC), a degasifier, an anion exchange column (WBA/SBA), and a polishing mixed bed unit. The use of a weakly acidic resin and the degasifier column are conditioned by the presence of hardness and alkalinity in the feed water, as explained in the previous sections.

Amberpack demin line
A demineralisation line (click to enlarge)

Regeneration

Regeneration is done in thoroughfare, which means that the regenerant first goes through the strong resin, which requires an excess of regenerant, and the regenerant not consumed by the strong resin is usually sufficient to regenerate the weak resin without additional dosage.

The cation resins are regenerated with a strong acid, preferably HCl, because H2SO4 can precipitate calcium.
The anion resins are regenerated with caustic soda.

Demin line regeneration
Regeneration of the demineralisation line (click to enlarge)

The quality obtained is the same as in the simple SAC-SBA layout, but because the weak resins are practicallly regenerated "free of charge", the regenerant consumption is considerably lower. Additionally, the weak resins have a higher operating capacity than the strong resins, so the total volume of ion exchange resins is reduced.

Uses

Examples of demineralisation:

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Mixed bed polishing

MB
Mixed bed unit in service
and in regeneration
The last traces of salinity and silica can be removed on a resin bed where highly regenerated strong acid cation and strong base anion resins are mixed.

Mixed bed units deliver an excellent treated water quality, but are complcated to regenerate, as the resins must first be separated by backwashing before regeneration. Additionally, they require large amounts of chemicals, and the hydraulic conditions for regeneration are not optimal. Therefore, mixed beds are usually only used to treat pre-demineralised water, when the service run is long.

 

What happens to the water

Practically nothing is left:

Demin water
Demineralised water
SAC (H) + SBA (OH)
-->
Demineralised water
Nothing is left

Mixed bed polishing produces a water with less than 0.1 µS/cm conductivity. With sophisticated design and appropriate resins, the conductivity of pure water (0.055 µS/cm) can be achieved. Residual silica values can be as low as 1 µg/L.
The pH value should not be used as a process control, as pH meters are unable to operate at 1 µS/cm conductivity or below.

Uses

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Nitrate removal

Nitrate can be removed selectively from drinking water using strong base anion resins in the chloride cycle, i.e. regenerated with a NaCl brine. The reaction is:

RSBA-Cl + NO3 <---> RSBA-NO3 + Cl

What happens to the water

Raw water
Raw water
SBA (Cl)
<-->
Decationised water
Denitrated water

Conventional SBA resins can be used, but they also remove sulphate from water. See the selectivity table. Depending on the resin type, some (selective resins) or all (non-selective) sulphate is removed. Bicarbonate is only removed partially at the beginning of the service run.

Uses

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Selective removal of various other contaminants

Selective removal of metals and other contaminants is mainly used for drinking water and for waste. Many of these applications require special resins: chelating resin making stable metal complexes, for instance.

Examples

In many of these applications, a residual concentration in the µg/L range is possible.

Some contaminants are difficult to remove with ion exchange, due to a poor selectivity of the resins. Examples: As, F, Li. See the periodic system of the elements with some ion exchange data. See also the page about resin types (selective resins) and a separate page about ion exchange processes for drinking water.

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Other information

Abbreviations
Resin types are usually abbreviated in these pages:

See a table with a complete list of abbreviations and units.

Water
See details about the water analysis as required for the above processes.
A special page is available about drinking water applications.

Ion exchange columns
Various column types are described in a separate page. Degasifiers as well, and basic principles of plant design.

Regeneration
See details about regeneration processes, quantities and concentrations of regenerants.

Ion exchange reactions
A full page describes reaction equilibrium and chemical reactions of these resins.


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