1、 Introduction to water treatment resin

          Water treatment resin is divided into cationic resin and anionic resin. Cationic resin is subdivided into sodium type and hydrogen type. Sodium type resin exchanges calcium and magnesium ions in water into sodium ions to soften water. Hydrogen resin is to exchange calcium and magnesium ions in water into hydrogen ions to soften water. Anion resin contains replaceable hydroxyl ions, which can replace acid ions in effluent. At the same time, anion resin and hydrogen cation resin can change water into purified water. In the water treatment industry, ion exchange is the equal charge molar reaction between ions in water and ions on ion exchange resin.

2、 Classification of water treatment resins

        Composite bed: two exchangers are used to load anion and cation exchange resins into their respective exchangers according to the design requirements. The raw water is first loaded with cation exchanger, the cations in the water such as Ca2 +, Mg2 +, K +, Na + are adsorbed by the exchanger, and the exchangeable H + on the exchanger is replaced into the water, and the corresponding inorganic acid is generated with the anions in the water; The effluent then passes through the anion exchanger, the anions in the water such as SO42 -, Cl -, HCO3 - are adsorbed by the exchanger, and the exchangeable ion Oh - on the exchanger is replaced in the water and combined with H + in the water to form H2O. For the water treated by the above anion and cation exchangers, the salt in the water is removed, which is the desalting treatment of the primary compound bed, and the effluent quality is ≤ 10us / cm.

          Mixed bed: in the same exchanger, the anion and cation exchange resins are filled according to a certain volume proportion, and the anion and cation exchange are carried out under the condition of uniform mixing, so as to remove the salt in the water, and the effluent quality is ≥ 5m Ω. Cm.

          The purpose of deionization method is to remove inorganic ions dissolved in water. Like hard water softener, it also uses the principle of ion exchange resin. In this, two resins are used - cation exchange resin and anion exchange resin. Cation exchange resin uses hydrogen ion (H +) to exchange cations; The anion exchange resin uses hydroxyl ions (OH -) to exchange anions, and hydrogen ions and hydroxyl ions combine with each other to form neutral water.

3、 Physical properties of ion exchange resin

1. Resin particle size

          Ion exchange resins are usually made into bead like small particles, and its size is also important. When the resin particles are finer, the reaction speed is higher, but the fine particles have greater resistance to the passage of liquid and need higher working pressure; In particular, the viscosity of concentrated sugar solution is high, and this effect is more significant. Therefore, the size of resin particles should be selected appropriately. If the resin particle size is less than 0.2mm (about 70 mesh), it will significantly increase the resistance of fluid passage and reduce the flow and production capacity.

          The size of resin particles is usually determined by wet sieving method. The resin is sieved after full water absorption and expansion, and its retention on the 20, 30, 40, 50 mesh screen is accumulated. 90% of the particles can pass through the corresponding screen hole diameter, which is called the "effective particle size" of the resin. The effective particle size of most general resin products is between 0.4 and 0.6mm. Whether the resin particles are uniform is expressed by the uniformity coefficient. It is used to determine the "effective particle size" of the resin, take the cumulative retention of 40% particles, and the ratio of the corresponding sieve diameter to the effective particle size. If the effective particle size of a resin (IR-120) is 0.4 ~ 0.6mm, and its retained particles on 20 mesh sieve, 30 mesh sieve and 40 mesh sieve are 18.3%, 41.1%, and 31.3% respectively, the calculated uniformity coefficient is 2.0.

2. Density of resin

          The density of resin during drying is called true density. The weight of wet resin per unit volume (including the gap between particles) is called apparent density. The density of resin is related to its crosslinking degree and the properties of exchange groups. Generally, the density of resin with high crosslinking degree is higher, the density of strong acid or strong alkaline resin is higher than that of weak acid or weak alkali, while the density of macroporous resin is lower.

3. Solubility of resin

        The ion exchange resin shall be insoluble. However, substances with low degree of polymerization and substances generated by resin decomposition will be dissolved during operation. The resin with low crosslinking degree and more active groups has a greater dissolution tendency.

4. Expansion of resin

        Ion exchange resin contains a large number of hydrophilic groups, which absorb water and expand when contacting with water. When the ions in the resin change, such as the cation resin from H + to Na +, and the anion resin from Cl - to OH -, they expand due to the increase of ion diameter and increase the volume of the resin. Generally, resins with low degree of crosslinking have a high degree of expansion. When designing the ion exchange device, the expansion degree of the resin must be considered to adapt to the resin volume change caused by ion conversion in the resin during production and operation.

5. Durability of resin

          The resin particles will change in transfer, friction, expansion and contraction during use, and there will be a small amount of loss and fragmentation after long-term use, so the resin should have high mechanical strength and wear resistance. Generally, the resin with low crosslinking degree is easy to break, but the durability of the resin mainly depends on the uniformity and strength of the crosslinking structure. For example, macroporous resin with high crosslinking degree has stable structure and can be regenerated repeatedly.

4、 Strong acid cationic resin

          This kind of resin contains a large number of strong acidic groups, such as sulfonate-so3h, which is easy to dissociate H + in the solution, so it is strong acidic. After the resin dissociates, the negative groups contained in the body, such as SO3 -, can adsorb and bind other cations in the solution. These two reactions exchange H + in the resin with cations in the solution. The strong acid resin has strong dissociation ability, It can dissociate and produce ion exchange in acidic or alkaline solution. After using the resin for a period of time, it shall be regenerated, that is, use chemicals to make the ion exchange reaction in the opposite direction, so that the functional groups of the resin can return to the original state for reuse. For example, the above cationic resin is regenerated with strong acid, at this time, the resin will release the adsorbed cations, Then combine with H + to restore the original composition