The treatment resin is divided into cation resin and anion resin, and the cation resin is subdivided into sodium type and hydrogen type. The sodium type resin exchanges calcium and magnesium ions in water into sodium ions to soften the water.
Hydrogen type 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 water. At the same time, anion resin and hydrogen type cation resin can change water into pure water.
In the water treatment industry, ion exchange is the reaction of equal charge molar quantity between ions in water and ions on ion exchange resin.
Physical properties: 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 resin is usually made into beads, and its size is also very important. The reaction speed is higher when the resin particles are smaller, but the resistance of the fine particles to the liquid is larger, which requires higher working pressure.
Especially when the viscosity of concentrated sugar is high, the 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), the resistance of fluid passing through will be increased obviously, and the flow rate and production capacity will be reduced.
The particle size of resin is usually determined by wet sieving method. The resin is sieved after full water absorption and expansion, and its retention on 20, 30, 40, 50 mesh sieve net is accumulated. 90% of the particles can pass through the corresponding sieve diameter, which is called the “effective particle size” of the resin. The effective particle size of most common resin products is between 0.4 and 0.6 mm.
The uniformity of resin particles is expressed by the uniformity coefficient. It is to determine the “effective particle size” of resin, take the cumulative retention of 40% particles, the corresponding ratio of sieve diameter and effective particle size.
If the effective particle size of a resin (IR-120) is 0.4-0.6 mm, the retained particles on 20 mesh sieve, 30 mesh sieve and 40 mesh sieve are 18.3%, 41.1% and 31.3% respectively, the uniformity coefficient is 2.0.
Density of resin
The density of the resin when it is dry is called true density. The weight of wet resin per unit volume (even the space between particles) is called apparent density. The density of the resin is related to its crosslinking degree and the properties of the exchange group. In general, the density of resin with high degree of cross-linking is higher than that of resin with strong acid or strong base, while that of macroporous resin is lower.
Solubility of resin
The ion exchange resin should be insoluble. However, the substances with low degree of polymerization and the substances generated from the decomposition of the resin will be dissolved during the operation. The resin with lower crosslinking degree and more active groups has a greater tendency to dissolve.
Expansion of resin
Ion exchange resin contains a large number of hydrophilic groups, which swells when contacting with water. When the ion in the resin changes, such as the cation resin changes from H + to Na +, and the anion resin changes from Cl – to OH -, it will expand due to the increase of ion diameter and increase the volume of the resin. Generally, the resin with low crosslinking degree has higher expansion degree.
In the design of ion exchange equipment, the expansion degree of resin must be considered to adapt to the volume change of resin caused by ion conversion in the resin during production and operation.
Durability of resin
The resin particles have the changes of transfer, friction, expansion and shrinkage when used, and there will be a small amount of loss and breakage 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 withstand repeated regeneration.
This kind of resin contains a large number of strong acid groups, such as sulfonate SO3H, which is easy to dissociate h+ in the solution, so it is strong acid. After the resin dissociation, the negative groups in the body, such as so3-, can absorb other cations in the binding solution.
These two reactions make the H + in the resin exchange with the cations in the solution. The strong acid resin has strong dissociation ability and can dissociate and exchange ions in both acidic and alkaline solutions.
After a period of use, the resin should be regenerated, that is, the ion exchange reaction should be carried out in the opposite direction with chemicals to make the functional groups of the resin return to the original state for reuse. For example, the cation resin is regenerated with strong acid, at this time, the resin releases the adsorbed cation, and then combines with H + to restore the original composition.