Application of Ion Exchange Resins in Water Treatment Systems

1. Definition of Ion Exchange Resin​​

Ion exchange resins are insoluble polymeric compounds featuring functional groups and a network structure, typically appearing as spherical beads. The full name of an ion exchange resin is composed of its classification name, matrix (skeleton) name, and basic name. Presently, ion exchange resins are widely used in numerous fields including water treatment, chemical industry, metallurgy, food, leather making, and ultrapure pharmaceutical production.

​​2. Classification of Ion Exchange Resins by Water Quality​​

Ion exchange resins can be classified based on their matrix type into styrene-based resins and acrylic-based resins. The type of chemically active functional groups on the resin determines its primary properties and category. Primarily, they are divided into two major groups: cation exchange resins and anion exchange resins, which can exchange cations and anions in a solution, respectively. Cation resins are further subdivided into strong acid cation (SAC) and weak acid cation (WAC) types. Similarly, anion resins are subdivided into strong base anion (SBA) and weak base anion (WBA) types.

​​3. Application of Ion Exchange Resins in the Water Treatment Industry​​

The water treatment industry is one of the earliest application fields for ion exchange resins and accounts for a significant demand, representing approximately 90% of total ion exchange resin production. In water purification, these resins are used for water softening, desalination, and the production of softened water, pure water, and ultrapure water. In wastewater treatment, they are primarily utilized to reduce the concentration of heavy metal ions through an exchange reaction between the resin's exchangeable ions and the heavy metal ions in the wastewater, thereby achieving advanced purification.

​​3.1 Application of Ion Exchange Resins in Water Softening​​

a. Composition and Function of Softening Filters:​​

Fully automatic water softeners mainly consist of three key components: the resin tank, the automatic multi-port control valve, and the brine tank. The resin tank is filled with ion exchange resin responsible for adsorbing calcium (Ca²⁺) and magnesium (Mg²⁺) ions from the water. The multi-port control valve acts as the controller for the softening equipment, managing the automatic cycles of operation, regeneration, rinsing, backwashing, and brine tank refilling. The brine tank stores salt (NaCl) used during the resin regeneration process.

b. Working Principle of Softening Resins:​​

Softening resins primarily remove hardness ions from water via ion exchange. These hardness ions, mainly calcium (Ca²⁺) and magnesium (Mg²⁺), are the primary contributors to water hardness. The softening resin contains abundant sodium ions (Na⁺). As water passes through the resin bed, the sodium ions on the resin exchange with the calcium and magnesium ions in the water, thereby removing the hardness ions.

c. Applications of Softening Filters:​​

Applications include boiler feed water, makeup water for air conditioning systems, heat exchangers, papermaking, printing and dyeing, textiles, water for petrochemical processes, biopharmaceuticals, electronics, pretreatment for pure water systems, and industrial zero liquid discharge (ZLD) systems.

​​3.2 Application of Ion Exchange Resins in Pure and Ultrapure Water Production​​

a. Definition of Cation/Anion Exchange Vessels (CAB):​​

Cation/Anion exchange vessels, also referred to as ion exchangers or demineralizers, are water treatment equipment that utilizes the ion exchange capability of resins to remove ions from water. Their operation is based on the ion exchange characteristic of the resins, where ions on the resin are exchanged with corresponding ions in the water, achieving ion removal.

b. Working Principle of Cation/Anion Exchange Vessels:​​

A typical two-bed demineralization system consists of a cation exchange vessel (acid cation exchanger) and an anion exchange vessel (base anion exchanger) connected in series. Raw water first passes through the cation unit. Here, cation exchange resin adsorbs cationic impurities from the water, simultaneously releasing hydrogen ions (H⁺) into the water, making it acidic. Subsequently, the water passes through a decarbonator (or degasifier) to remove the carbon dioxide (CO₂) liberated as a gas, ensuring efficient exchange in the anion unit under acidic conditions. Finally, the water passes through the anion unit, where anion exchange resin removes anionic impurities.

c. Application Areas of Cation/Anion Exchange Vessels:​​

In industries such as chemical, power, and metallurgy, they are commonly used for softening boiler feed water to prevent scale formation and corrosion issues. In agricultural irrigation, using water treated by these systems can help reduce soil salinization. They also play a significant role in the pretreatment stages for seawater desalination, providing favorable conditions for subsequent advanced treatment processes.

​​3.3 Application of Polished Mixed Beds in Pure and Ultrapure Water Production​​

a. Definition of Polished Mixed Beds:​​

Polishing resin is typically used at the final stage (polishing stage) of ultrapure water treatment systems to ensure the effluent quality meets the required standards, often achieving a resistivity of 18 MΩ·cm or higher. It is a type of ion exchange resin, specifically a mixture of strong acid cation exchange resin in the hydrogen (H⁺) form and strong base anion exchange resin in the hydroxide (OH⁻) form.

b. Working Principle of Polishing Resins:​​

A polished mixed bed contains both cation and anion exchange resins thoroughly mixed within the same vessel. In this mixed bed, the cation and anion resins are intimately mixed, allowing the cation and anion exchange reactions to proceed almost simultaneously. The hydrogen ions (H⁺) produced by the H-type cation exchange and the hydroxide ions (OH⁻) produced by the OH-type anion exchange cannot accumulate and immediately combine to form weakly dissociated water molecules. This essentially eliminates the effect of counter-ions, allowing the ion exchange reaction to proceed very thoroughly, resulting in high-purity effluent. When the ions within the polishing resin are exhausted, the resin is regenerated using acid and alkali solutions to effectively restore the hydrogen and hydroxide ions, renewing the resin's working capacity.

c. Application Areas:​​

​​Electronics Industry: Production of high-purity water required for semiconductors and other electronic components.

​​Power Plants: Extensive use in pure water treatment systems for thermal power plants.

​​Synthetic Chemistry and Petrochemical Industry: Ion exchange resins can act as catalysts, replacing inorganic acids and bases in reactions such as esterification, hydrolysis, and hydration.

​​Pharmaceutical Industry:Ion exchange resins play an important role in developing new-generation antibiotics and improving the quality of existing antibiotics.