1. Ion exchange resin

Ion exchange resin is a solid polymer compound with functional groups (composed of active ions and fixed ions), a network structure, and is insoluble in acids, alkalis, and organic solvents. It is usually a spherical particle. Its polymer active groups are generally polyacids or polybases. According to the properties of the active groups, ion exchange resins can be divided into cation exchange resins and anion exchange resins.

Among them, the exchange resin with acidic active groups that can exchange with cations in water is called cation exchange resin; the exchange resin with alkaline active groups that can exchange with anions in water is called anion exchange resin.

According to the strength of H+ or OH- ionization on the active group, it can be divided into strong acid cation exchange resin and strong base anion exchange resin, weak acid cation exchange resin and weak base anion exchange resin.

2. Transformation of ion resin

These are the four basic types of resins. In practical use, these resins are often converted into other ion type operation to meet various needs. For example, the strong acid cation resin is often reacted with NaCl and converted into sodium resin for reuse. When the resin is working, Na + is released and adsorbed with Ca2 +, Mg2 + and other cations in the solution to remove these ions. No H + is released during the reaction, which can avoid the pH drop of solution and the side effects (such as sucrose conversion and equipment corrosion, etc.). The resin can be regenerated with brine (without strong acid) after being used in sodium type operation. If the anion resin can be converted into chlorine type for reuse, Cl - will be released during operation and other anions will be adsorbed and exchanged, and only the aqueous salt solution will be used for its regeneration. Chlorine type resin can also be converted into hydrogen carbonate type (HCO3 -) for operation. Strong acid resin and strong alkaline resin will not have strong acid and strong alkaline after they are converted into sodium type and chlorine type, but they still have other typical properties of these resins, such as strong dissociation and wide working pH range.

3. Composition of ion exchange resin matrix

The matrix of ion exchange resin is mainly made of styrene and acrylic acid. They react with the crosslinker diethylbenzene to form a polymer with a long main chain and a cross-linked network framework. Styrene resin is used first, acrylic resin is used later.

The adsorption properties of these two kinds of resins are very good, but they have different characteristics. Acrylic resin can exchange and absorb most of the ionic pigments with large decolorizing capacity, and the adsorbate is easy to elute and easy to regenerate. It can be used as the main decolorizing resin in sugar factory. Styrene resin is good at adsorbing aromatic substances and polyphenol pigments (including negatively charged or uncharged) in sugar juice, but it is difficult to elute in regeneration. Therefore, the advantages of both can be brought into full play by using acrylic resin to decolorize the sugar solution first and then styrene resin to decolorize it.

The crosslinking degree of the resin, that is, the percentage of divinylbenzene used in the polymerization of the resin matrix, has a great influence on the properties of the resin. Generally, the resin with high cross-linking degree polymerizes closely, is firm and durable, has high density, less internal space and strong selectivity for ions; while the resin with low cross-linking degree has large pores, strong decolorization ability and fast reaction speed, but it has large expansion, slightly low mechanical strength, and is brittle and fragile in operation. The cross-linking degree of the ion resin for industrial application is generally no less than 4%; the cross-linking degree of the resin for decolorization is generally no more than 8%; the cross-linking degree of the resin solely used for adsorption of inorganic ions can be higher.

In addition to the two series of styrene system and acrylic acid system, the ion exchange resin can also be polymerized by other organic monomers. For example, phenolic (FP), epoxy (EPA), vinylpyridine (VP), urea formaldehyde (UA), etc.