Depending on their functionality the ion exchange resins can be classified as cation and anion exchangers. Resins that have both cation and anion functional groups are called polyampholytes. The polymeric matrix of the resin beads can be produced in the form of either gel or macroporous structure characterised by the different pore size and specific surface area. Synthetic resins are mainly based on styrenic and acrylic polymers.

     Cation ion exchange resins

The cation exchangers are functionalised with acidic groups and able to exchange cations (positively charged ions such as Cu²⁺, Ni²⁺, Co²⁺, Al³⁺, Fe³⁺, Fe²⁺ and other metal ions) with solutions. Depending on the ionic dissociation of the functional groups the cation resins can classified as strong, medium or weak cation exchangers.
Strong and weak acid cation exchangers are able to exchange their mobile cations to external cations in alkaline, neutral and acidic environments. Weak acid cation exchange hydrogen ions for other cations only in an alkaline medium. Strongly dissociated acidic groups include mainly sulphonic and phosphoric functional groups. Weakly dissociated acid groups are represented by carboxyl, phenolic, hydroxylic groups.
The major application of the cation exchange resins is to remove hard ions such as calcium and magnesium. These processes include softening and demineralisation. The cation exchangers can be used in either hydrogen (H⁺) or sodium (Na⁺) forms. If the cation resins are in the hydrogen form the ion exchange reaction on cation exchangers is described by the following equation:
2R — H + Ca ²⁺ <=> R — Ca — R + 2H⁺
Due to increasing concentration of hydrogen ions in the purified solution it becomes acidic. Therefore, the pH correction of the product solution is required.

In case of sodium (Na⁺) form of cation exchange resins the softening reactions can be presented by the following equation:
r>2R — Na + Ca²⁺<=> R — Ca — R + 2Na⁺
The sodium cation exchange does not change the pH level and the solution remains almost neutral.

The formulas of typical strong acid and weak acid cation exchangers are shown in picture above.

Cation exchangers are resins of high volume production. Their major application is the preparation of industrial water for power generation and softening of drinking water. For more detailed information on the resins applications please refer to our IX applications page.

     Anion ion exchange resins

Due to the presence of basic functional groups the anion exchange resins are able to exchange anions (negatively charged ions) such as Cl^-, SO₄^2-, NO₃^-, PO₄^3–, U₂O₇^2-, VO₄^3-, CrO₄^2- and other oxianions with solutions .

Depending on the degree of ionic dissociation of the functional groups the anion exchangers can classified as strong, medium or weak base anion resins. The formulas of typical strong acid and weak base anion exchangers are shown in picture on the left.

The anion exchangers are used in two main areas. The major area is the removal of acidic anions (clhorides, sulphates, nitrates, phosphates) in water preparation and purification for industrial and domestic use. Another large area is hydrometallurgical recovery of metals by extracting their oxianions from leach solutions. In particular, they are used in the production of gold, uranium, tungsten, vanadium and other metals. Also they are used for pH adjustment, decolourisation and removal of organic compounds in food and beverages industry, beer and wine industry, pharmaceutics.

Please refer to our IX applications page for more details on the applications of anion exchange resins.

      Ampholyte and chelating ion exchange resins

The ampholytic exchangers have both types of functionalities - acidic and basic. They are able to exchange both cations and anions at the same time. Particular types of the ampholytic IX materials are chelating resins, which have high selectivity to complexing metals such as copper, nickel, cobalt. An example of ampholytic resin and its chelation with divalent metal is shown in picture on the left.

At this moment chelating resins take a niche market in selective removal of divalent metals, gold and platinum, boron, mercury, arsenic and other elements. However, this emerging market could grow fast due to the development of new hydrometallurgical methods in mining industry. In particular, continuous ion exchange technology provides a number of technological, economic and environmental benefits for the mining company and may significantly increase the use of chelating resins.

     Characteristics of IX resins

Synthetic IX materials are known as ion exchange resins (functionalised macroporous or gel polymeric beads). In comparison with the natural IX materials, they have higher exchange capacity, superior stability of mechanical and exchange properties, better selectivity to the target ions and simpler regeneration processes.

Exchange capacity

One of the main characteristics of ion exchange resins is their exhcange capacity, which is expressed as amount of sorbed ions in relation to the volume or mass of the resin. Exchange capacity depends on the process conditions and influences both technical and economic parameters of the IX plant.

Selectivity

Selectivity. of the IX materials is their ability to preferentially sorb some target ions. Even similar IX materials may have different selectivity, especially under different operational conditions. Due to higher selectivity the resins enable an effective separation of valuable metals from impurities.
Each ion exchange resin is characterised by a selectivity sequence. For example, the following selectivity sequence is typical for a cation resin with sulphonic groups:
H < Na < K < Cs < Mg < Cu < Ca < Sr < Ce < Ba
For a strong base anion exchange resin the selectivity sequence is typically looks as follows:
OH < F < H₂PO₄ < HCO₃ < Cl < NO₃ < HSO₄
In both selectivity sequences the resin selectivity increases from left to right. Each element on the right may displace the element on the left from the resin.

Particle size

The major granulometric parameters of the IX resins are particle size range, effective size and uniformity coefficient. Traditionally the resins are produced with the particle sizes ranging within 0.3 – 1.2 mm and the uniformity coefficient of 1.4 -1.7. Monodisperse IX resins have the uniformity coefficient of 1.1. Typical particle size distribution of various types of the IX materials is shown in pircture. Monodisperse IX materials are used in cyclic processes in water preparation, while the resin in pulp technology requires polydisperse resins.


Mechanical strength

Mechanical strength of the IX materials is not an issue for many applications due to either low attrition rates (water industry) or low volume production (pharmaceutical). However, with the increase in the IX resins utilisation in hydrometallurgy, their mechanical durability has become an important characteristic due to cost impact. Thus, the replacement cost of the resin with an annual attrition rate of 25-50% is estimated as AU$400-700K for a typical uranium plant. Therefore the proper testing and selection of the mechanical properties of the IX materials is becoming critical with new markets being emerged.