How Do Polishing Ion Exchange Resins Work?

What Is Polishing Resin?

Polishing resin, also known as mixed bed resin, is a specialized type of ion exchange resin designed to achieve the highest possible water purity. Polishing mixed-bed resin is composed of anion and cation exchange resins mixed in a certain proportion. Its primary function is to remove trace ionic and organic impurities from water or other solutions, resulting in ultra-pure water suitable for critical industrial processes and laboratory applications. The resistivity of production water treated with polishing resin can reach more than 16 megohms, or even more than 18 megohms. Polishing resin can be further divided into electronic grade polishing resin, nuclear grade polishing resin, mixed bed polishing resin and other levels.

Principles of Polishing Ion Exchange Resin

The working principle of polishing resin involves the selective exchange of ions in solution with ions on the resin. The H⁺ ions of the cation resin undergo a displacement reaction with cations such as Ca²⁺, Mg²⁺, and Na⁺ in the water, and the OH^- of the anion resin undergoes a displacement reaction with anions such as sulfate and chlorine in the water. The H⁺ displaced by the cation resin combines with the OH^- ions displaced by the anion resin to form H₂O.

Mixed Bed Polishing Process

In polishing mixed-bed equipment, anion and cation exchange resins are filled in an ion exchange device in the same exchanger in a certain proportion. The uniformly mixed resin layer cation resin and anion resin are closely staggered, and each pair of cation resin and anion resin particles is similar to a set of compound beds. Therefore, the mixed bed can be regarded as an ion exchange equipment operating in series with countless sets of compound beds.

Polishing mixed bed equipment is generally used at the end of the reverse osmosis water treatment system or the end of the ultrapure water system, that is, it is used behind the reverse osmosis membrane or the electric desalination system EDI to produce ultrapure water with resistivity of more than 18 megaohms.

It is worth noting that as the use time increases, the exchange capacity of the resin will gradually decrease, and the static electricity between the anode and anion resin will also weaken, eventually leading to delamination after the resin fails. Due to the non-synchronization of ion exchange, the mixed bed resin water output will be unqualified, and the periodic water production volume will also be greatly affected.

Applications of Polishing Ion Exchange Resin

Polishing resin finds widespread application in industries requiring ultra-pure water, such as pharmaceutical manufacturing, power generation, electronics fabrication, and semiconductor production. It is also indispensable in laboratory settings where high-purity water is a necessity for analytical instruments and research experiments. The polishing of ion exchange resin finds extensive applications in various industries and processes, including:

• Condensate Polishing

Maintaining appropriate water quality through the removal of impurities from condensate water, which is crucial for the efficient operation of steam turbines and boilers.

• Brine Polishing

Recycling of brine and the production of high-purity brine in chlor-alkali processes through cost-effective polishing.

• Vacuum Deaeration

Removal of O₂, CO₂, and N₂ from water to produce water with ultra-low levels of dissolved gases, a critical requirement in various high-purity applications.