Ion-Exchange Resins

Membrane Works now offer extensive Ion-exchange (IX) resin testing services in our Sydney laboratory.

Our services include:

What are the different types of ion exchange resins and what are they used for?

Ion exchange resins are commonly used to demineralise make-up water in power stations, remove contaminants from drinking water and in processing mine-water. We will discuss the different types of resins and their applications below.

Strong acid cation resins

Strong acid cation (SAC) exchange resins are among the most widely used resins and have a strongly acidic sulphonate (SO3^–) functional group. SAC resins are regenerated with either a sodium salt solution (NaCl) for softening applications, or with an acid (H+) for demineralisation applications.

SAC resins are used extensively for softening applications, and are very effective at removal of hardness such as magnesium (Mg²⁺) or calcium (Ca²⁺). They may also be used in split-stream dealkalisation, where two SAC beds operate in parallel, the first as a softener that produces an alkaline solution, and the second as a demineraliser (in the H+ form), with the result that alkalinity is removed from the blended stream. SAC resins can be damaged by oxidants and fouled by iron or manganese, so care must be taken to avoid exposure of the resin to these materials.

Weak acid cation resins

Weak acid cation (WAC) exchange resins remove cations associated with alkalinity (carbonate and bicarbonate ions). WAC resins typically employ carboxylic acid functional groups (RCOO–), and are regenerated with sulphuric or hydrochloric acid. Similar to SAC resins, WAC resins swap cations for hydrogen ions, resulting in slightly greater acidity in the treated stream.

WAC resins are used for demineralisation and dealkalisation applications. Their high affinity for divalent cations ions (Ca²⁺ and Mg²⁺) makes them a good choice for applications demanding the removal of hardness ions even in difficult waters such as in the presence of high sodium concentrations. Additionally, WAC resins tend to have relatively high oxidation resistance and mechanical durability, making them a good choice for streams containing oxidants such as hydrogen peroxide and chlorine, among others.

Strong base anion resins

Strong base anion (SBA) exchange resins are available with a wide variety of functionalities but are broadly made up cross-linked polystyrene functionalized with quaternary amine salts.

SBA resins are typically used for demineralisation, dealkalisation and desilication, as well as removal of organics. Type 1 SBA resins can also be used for selective removal of nitrates (NO3-), sulphates (SO3-), and perchlorate (ClO4-)and for desilication where low levels of silica are required. Type 2 SBA resins are used for applications where complete removal of anions is desired but where low silica levels are not critical to operational processes.

Weak base anion (WBA) exchange resins

Weak base anion (WBA) exchange resins are the only major type of IX resins where the exchange site is not ionized. The most common functional group is R-CH₂-N(CH3)₂. These resins are not ionised in their regenerated free base form and therefore, they do not react with neutral salts but will adsorb free mineral acidity (Cl and SO₄). WBA resins have amine functional groups, and are typically regenerated with sodium hydroxide (NaOH), ammonia (NH3), or sodium carbonate (Na₂CO₃).

WBA resins are used for partial demineralisation. In larger plants, WBA resin beds may be paired with SBA units for complete demineralisation applications, and WBA resins are also effective for removal of organic carbon ahead of SBA beds. WBA is also used for acid adsorption applications involving the removal of chloride, sulphate, nitrate, and other anions associated with strong acids, though it is not effective for removal of weak acids such as silica (SiO₂) and carbon dioxide (CO₂)

Common ion exchange applications

Design engineers use various combinations of ion exchange units to produce a final water of any desired quality. The more common system designs are for water softening, dealkalisation, and demineralisation.

Water Softening

Water softening accounts for the majority of ion exchange applications. Hard waters containing calcium and magnesium ions cause scale in power plant boilers, water pipes and domestic houses. Water softening involves the exchange of hardness ions (calcium/magnesium) for sodium using a SAC resin in the sodium form. After softening the resin is regenerated by a concentrated solution of sodium chloride.

Dealkalisation (hydrogen)

Dealkalisers are used to remove both hardness and alkalinity from the raw feedwater. This is can be done using a SAC resin in either the hydrogen form or chloride form. The primary application of dealkalisation is the reduction of carbonate in boiler feedwater which would otherwise break down into carbon dioxide and cause corrosion.

In acid resins, the acid groups on the react with the bicarbonate alkalinity to form carbonic acid and free mineral acidity (FMA). Once formed, the carbonic acid readily breaks down to release free carbon dioxide which can be removed by aeration. However, the acid groups will also react with other cations and remove them to be replaced with more H+ ions.  Since the strong acid resin replaces all cations with hydrogen, the effluent from the dealkaliser is both softened and acidic.

The reactions involved can be seens as:

R-H + HCO₃^– => R^– + H₂CO₃

2R-H + Ca²⁺ => R₂-Ca + 2H⁺

Chloride SAC dealkalisers are used as an alternative to hydrogen SAC dealkalisers and are regenerated with salt brine rather than acids. The carbonate and bicarbonate alkalinity are removed by the SAC resin and replaced with chloride. Frequently, a sodium SAC softener is combined with a chloride SBA/WBA dealkaliser to produce a water that is both soft and low in alkalinity.

Water from dealkalisers can be neutralised by blending with effluent from a sodium softener and the desired alkalinity and pH can be controlled by the blending ratio. After blending, the water is passed over an aerating tower (or decarbonator) to remove the free carbon dioxide released in the neutralisation step.

Demineralisation 

Demineralization (or deionisation) is a process that removes both anions and cations to produce a high–purity water with extremely low conductivity. This is accomplished by using a SAC resin in the hydrogen form to remove cations, followed by an SBA resin in the hydroxide form to remove anions. In this way, all the mineral salts are removed, producing a water of exceptional purity.

This can be accomplished with a two-tank system or in a mixed bed where both exchange reactions occur simultaneously.

Ultra Pure Water 

Ultrapure water is required in the semiconductor industry as well as hospital and pharmaceutical manufacturing. This can be produced using a wide array of single resin or mixed bed resin systems, depending on the requirements. 

Contaminant removal

Ion exchange resins are now more frequently used to remove specific contaminants from water. This can include heavy metals (lead, cadmium, uranium) or organic materials (PFAS, pesticides), or nitrate removal from polluted waters.

Condensate polishing

The purity of steam condensate generated in power plants is critical to maintaining the operation of the power station. Single or mixed beds can be used to remove particulate matter and corrosion products that could cause harmful deposits in boilers.

Industrial Processing

Ion exchange can be used in a wide variety of industrial applications where their chemical functionality is used as a reagent or catalyst.