1. Basic introduction of ion exchange resin
The full name of ion exchange resin is composed of classification name, skeleton (or gene) name, and basic name. The pore structure is divided into two types: gel type and macroporous type. Any resin with physical pore structure is called macroporous resin, and “macroporous” is added before the full name. If the classification is acidic, add “yang” before the name, and if the classification is alkaline, add “yin” before the name. Such as: macroporous strong acid styrene cation exchange resin.
Ion exchange resins can also be classified into styrene resins and acrylic resins according to the type of matrix. The type of chemically active groups in the resin determines the main properties and types of the resin. Firstly, it is divided into two categories: cationic resin and anionic resin, which can be ion exchanged with the cation and anion in the solution. Cationic resins are further divided into two categories, strong acid and weak acid, and anionic resins are further divided into two categories, strong alkaline and weak alkaline (or medium strong acid and medium strong alkaline).
2. The basic type of ion exchange resin
(1) Strong acid cationic resin
This kind of resin contains a large number of strong acid groups, such as sulfonic acid group -SO3H, which is easy to dissociate H+ in the solution, so it is strongly acidic. After the resin is dissociated, the negatively charged groups contained in the body, such as SO3-, can adsorb and bind other cations in the solution. These two reactions exchange the H+ in the resin with the cations in the solution. Strong acid resin has a strong dissociation ability, and can dissociate and produce ion exchange in acid or alkaline solutions.
After the resin has been used for a period of time, it needs to be regenerated, that is, the ion exchange reaction is carried out in the opposite direction with chemicals to restore the functional groups of the resin to its original state for reuse. The above-mentioned cationic resin is regenerated with strong acid. At this time, the resin releases the adsorbed cations and combines with H+ to restore the original composition.
(2) Weak acid cationic resin
This type of resin contains weakly acidic groups, such as carboxyl -COOH, which can dissociate H+ in water and become acidic. The remaining negatively charged groups after the resin dissociates, such as R-COO- (R is a hydrocarbon group), can adsorb and combine with other cations in the solution to produce cation exchange. The acidity of this resin is weak, and it is difficult to dissociate and perform ion exchange at low pH. It can only work in alkaline, neutral or slightly acidic solutions (such as pH 5-14). This kind of resin is also regenerated with acid (it is easier to regenerate than strong acid resin).
(3) Strongly basic anion resin
This kind of resin contains strong basic groups, such as quaternary amine group (also known as quaternary amine group)-NR3OH (R is a hydrocarbon group), which can dissociate OH- in water and become strongly basic. The positively charged groups of this resin can adsorb and combine with the anions in the solution to produce anion exchange.
This resin has strong dissociation and can work normally under different pH. It uses strong alkali (such as NaOH) for regeneration.
(4) Weakly basic anion resin
This type of resin contains weakly basic groups, such as primary amino groups (also known as primary amino groups) -NH2, secondary amino groups (secondary amino groups) -NHR, or tertiary amino groups (tertiary amino groups) -NR2, They can dissociate from OH in water and become weakly alkaline. The positively charged groups of this resin can adsorb and combine with the anions in the solution to produce anion exchange. In most cases, this resin adsorbs all other acid molecules in the solution. It can only work under neutral or acidic conditions (such as pH 1-9). It can be regenerated with Na2CO3 and NH4OH.
(5) Transformation of ionic resin
The above are the four basic types of resin. In actual use, these resins are often converted to other ionic operations to meet various needs. For example, the strong acid cationic resin is often reacted with NaCl to convert it into sodium-type resin for reuse. When working, the sodium-type resin releases Na+ and exchanges and absorbs cations such as Ca2+ and Mg2+ in the solution to remove these ions. No H+ is released during the reaction, which can avoid the pH drop of the solution and the resulting side effects (such as sucrose conversion and equipment corrosion, etc.). After the resin is used in sodium form, it can be regenerated with brine (without strong acid). Another example is the anion resin can be converted to chlorine type and used again. It releases Cl- and adsorbs and exchanges other anions during work. Its regeneration only needs to use salt water solution. Chlorine resin can also be converted into hydrogen carbonate type (HCO3-) operation. Strong acid resins and strong alkaline resins no longer have strong acidity and strong alkalinity after they are converted into sodium and chlorine types, but they still have other typical properties of these resins, such as strong dissociation and wide working pH range Wait.
3. the composition of the ion exchange resin matrix
The matrix of ion exchange resin (ionresin), the main manufacturing raw materials are styrene and acrylic acid (ester), which respectively react with the crosslinking agent divinylbenzene to form a long molecular main chain and crosslinking A polymer with a chain network backbone structure. Styrene resin is used first, acrylic resin is used later.
Both types of resins have good adsorption properties, but they have different characteristics. Acrylic resin can exchange and adsorb most ionic pigments, has a large decolorization capacity, and the adsorbate is easily eluted, which is convenient for regeneration. It can be used as the main decolorization resin in sugar factories. Styrene resins are good at adsorbing aromatic substances and polyphenol pigments in sugar juice (including negatively charged or uncharged); however, it is difficult to elute during regeneration. Therefore, the sugar solution is coarsely decolorized with acrylic resin, and then finely decolorized with styrene resin, which can give full play to the advantages of both.
The degree of crosslinking 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, resins with a high degree of crosslinking polymerize relatively tightly, are firm and durable, have higher density, have fewer internal voids, and are more selective for ions; while resins with a low degree of crosslinking have larger pores and stronger decolorization ability , The reaction speed is faster, but the swelling property is greater when working, the mechanical strength is slightly lower, and it is relatively brittle and fragile. The cross-linking degree of ionic resins used in industrial applications is generally not less than 4%; the cross-linking degree of resins used for decolorization is generally not higher than 8%; the cross-linking degree of resins used solely to absorb inorganic ions can be higher.
In addition to the above two series of styrene and acrylic, ion exchange resins can also be made by polymerizing other organic monomers. Such as phenolic (FP), epoxy (EPA), vinyl pyridine (VP), urea-formaldehyde (UA), etc.
4. the physical structure of ion exchange resin
Ionic resins are often divided into two types: gel type and macroporous type.
The polymer skeleton of the gel-type resin has no pores inside when dry. It swells when it absorbs water and forms very fine pores between the macromolecular chains, usually called micro-pores. The average pore diameter of the wet resin is 2 to 4 nm (2×10-6 to 4×10-6 mm).
This type of resin is more suitable for adsorbing inorganic ions, and their diameter is small, generally 0.3 to 0.6 nm. This type of resin cannot adsorb macromolecular organic substances, because the latter is larger in size, such as protein molecules with a diameter of 5-20 nm, and cannot enter the microscopic pores of this type of resin.
The macroporous resin is made by adding a porogen during the polymerization reaction to form a porous sponge-like structure with a large number of permanent micropores inside, and then introducing exchange groups. It has both micro-pores and macro-pores. The pore size of the wetting resin can reach 100-500nm, and its size and quantity can be controlled during manufacture. The surface area of the pores can be increased to more than 1000m2/g. This not only provides good contact conditions for ion exchange, shortens the distance of ion diffusion, but also adds many chain link active centers, which are generated by van de Waals force between molecules. Molecular adsorption can absorb various non-ionic substances like activated carbon and expand its function. Some macroporous resins without exchange functional groups can also adsorb and separate a variety of substances, such as phenols in chemical plant wastewater.
Macroporous resin has many and large pores, large surface area, many active centers, fast ion diffusion and ion exchange rate, which is about ten times faster than gel resin. It has fast action and high efficiency when used, and the required processing time is shortened. Macroporous resin also has many advantages: resistance to swelling, resistance to cracking, oxidation resistance, abrasion resistance, heat resistance and temperature change resistance, and easier adsorption and exchange of organic macromolecular substances, so it has strong anti-pollution power and is more Easy to regenerate.