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 used to exchange calcium and magnesium ions in water into hydrogen ions to soften water Anionic resin contains replaceable hydroxyl ions, which can replace acid ions in effluent. At the same time, anionic resin and hydrogen cationic 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.
The particle size and related physical properties of ion exchange resin have a great influence on its work and performance.
Resin particle size
Ion exchange resins are usually made into bead shaped 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, which has a more significant effect. 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 measured 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 ~ 0.6mm. Whether the resin particles are uniform is expressed by the uniformity coefficient. It is to measure 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.
Density of resin
The density of resin when dry 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.
Solubility of resin
The ion exchange resin shall be insoluble. However, substances with low degree of polymerization mixed in the process of resin synthesis 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.
Expansion degree 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 the ion conversion in the resin during production and operation.
Durability of resin
The resin particles have changes such as transfer, friction, expansion and contraction during use, and there will be a small amount of loss and fragmentation after long-term use. Therefore, 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. The resin with high repeated crosslinking structure and high crosslinking resistance.
This kind of resin contains a large number of strong acidic groups, such as sulfonate-so3h, which is easy to dissociate H + in solution, so it is strongly acidic After the resin dissociates, the negative groups contained in the body, such as SO3 -, can adsorb other cations in the binding solution These two reactions exchange H + in the resin with cations in the solution Strong acid resin has strong dissociation ability and can dissociate and produce ion exchange in acidic or alkaline solution After the resin is used for a period of time, regeneration treatment shall be carried out, that is, the ion exchange reaction shall be carried out in the opposite direction with chemicals to restore the functional groups of the resin to the original state for reuse For example, the above cationic resin is regenerated with strong acid. At this time, the resin releases the adsorbed cation and combines with H + to restore the original composition.
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