1. Ion exchange capacity of ion exchange resin
The performance of ion exchange resin for ion exchange reaction is reflected in its “ion exchange capacity”, that is, the number of milligram equivalents of ions that can be exchanged per gram of dry resin or per milliliter of wet resin, meq/g (dry) or meq/mL (Wet); when the ion is monovalent, the number of milliequivalents is the number of milligrams (for divalent or multivalent ions, the former is the latter multiplied by the valence of the ion). It also has three expressions: “total exchange capacity”, “working exchange capacity” and “regeneration exchange capacity”.
- The total exchange capacity means the total amount of chemical groups that can undergo ion exchange reaction per unit quantity (weight or volume) of the resin.
- The working exchange capacity means the ion exchange capacity of the resin under certain conditions. It is related to the resin type and total exchange capacity, as well as specific working conditions such as the composition of the solution, flow rate, temperature and other factors.
- Regenerative exchange capacity means the exchange capacity of the regenerated resin obtained under a certain amount of regeneration, indicating the degree of regeneration of the original chemical groups in the resin. Generally, the regeneration exchange capacity is 50-90% of the total exchange capacity (generally controlled 70-80%), and the working exchange capacity is 30-90% of the regeneration exchange capacity (for recycled resin). The latter ratio is also called Utilization rate of resin.
In actual use, the exchange capacity of ion exchange resins includes adsorption capacity, but the proportion of the latter varies depending on the resin structure. It is still not possible to calculate separately. In the specific design, it needs to be corrected based on empirical data and reviewed in actual operation.
The ion resin exchange capacity is generally measured with inorganic ions. These ions are small in size and can diffuse freely into the resin body and react with all the exchange groups inside it. In practical applications, the solution often contains high molecular weight organic substances, which are large in size and difficult to enter the micropores of the resin, so the actual exchange capacity will be lower than the value measured with inorganic ions. This situation is related to the type of resin, the size of the pore structure and the material being processed.
2. Adsorption selectivity of ion exchange resin
ion exchange resins have different affinities for different ions in the solution and are selective in their adsorption. There are general rules for the strength of various ions subjected to resin exchange adsorption, but different resins may be slightly different. The main rules are as follows:
(1) Adsorption of cations
High-valent ions are usually preferentially adsorbed, while low-valent ions are weakly adsorbed. Among ions of the same valence, ions with larger diameters are strongly adsorbed. Some cations are adsorbed in the following order:
Fe3+> Al3+> Pb2+> Ca2+> Mg2+> K+> Na+> H+
(2) Adsorption of anions
The general order of the adsorption of inorganic acid radicals by strong basic anion resin is:
SO42－> NO3－> Cl－> HCO3－> OH－
The general sequence of weakly basic anion resin adsorption of anions is as follows:
OH－> Citrate 3－> SO42－> Tartrate 2－> Oxalate 2－> PO43－ >NO2－> Cl－ >Acetate－> HCO3－
(3) Adsorption of colored matter
Sugar liquid decolorization often uses strong alkaline anion resin, which has strong adsorption of pseudomelanin (reaction product of reducing sugar and amino acid) and alkaline decomposition products of reducing sugar, while the adsorption of carabohydrate pigment is weak. This is believed to be due to the fact that the first two are usually negatively charged, while the charge of caramel is weak.
Generally, resins with a high degree of crosslinking are more selective for ions, and resins with macroporous structures are less selective than gel-type resins. This selectivity is greater in dilute solutions and less in concentrated solutions.
3. the physical properties of ion exchange resin
The particle size and related physical properties of ion exchange resin have a great influence on its work and performance.
(1) Resin particle size
Ion exchange resins are usually made into small beads in the form of beads, and its size is also important. The resin particles are finer, the reaction speed is higher, but the resistance of the fine particles to the passage of the liquid is larger, and a higher working pressure is required; especially the high viscosity of the concentrated sugar liquid, this effect is more significant. Therefore, the size of the resin particles should be selected appropriately. If the resin particle size is below 0.2mm (approximately 70 mesh), it will significantly increase the resistance of the fluid to pass, reducing the flow rate and production capacity.
The resin particle size is usually measured by the wet sieving method. The resin is sieved after fully absorbing water and swelled, and the remaining amount on the 20, 30, 40, 50… mesh screen is accumulated, and 90% of the particles can pass through it. The diameter of the mesh 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.
Whether the resin particles are uniform is expressed by the uniformity coefficient. It is the ratio of the corresponding sieve hole diameter to the effective particle size by taking the cumulative retained amount of 40% particles on the coordinate chart of the “effective particle size” of the resin. For example, the effective particle size of a resin (IR-120) is 0.4-0.6mm, and the particles remaining on the 20-mesh sieve, 30-mesh sieve and 40-mesh sieve are 18.3%, 41.1%, and 31.3% respectively, then the calculation is The uniformity coefficient is 2.0.
(2) Density of resin
“The density of the resin when it is dry is called the true density.” The weight of the wet resin per unit volume (including the gap between particles) is called the apparent density. The density of the resin is related to its degree of crosslinking and the nature of the exchange group. Generally, a resin with a high degree of crosslinking has a higher density, a strong acid or a strong basic resin has a higher density than a weak acid or a weak basic resin, and a macroporous resin has a lower density. For example, the true density of the styrene-based gel-type strong acid cation resin is 1.26g/mL, and the apparent density is 0.85g/mL; and the true density of the acrylic gel-type weak acid cation resin is 1.19g/mL, the apparent density is 0.75g/mL.
(3) Solubility of resin
The ion exchange resin should be an insoluble substance. However, the substances with a low degree of polymerization contained in the resin synthesis process and the substances generated by the decomposition of the resin will dissolve out during operation. Resins with a lower degree of crosslinking and more active groups have a greater tendency to dissolve.
ion exchange resin contains a large number of hydrophilic groups, which will swell when it comes in contact with water. When the ions in the resin change, for example, the cationic resin changes from H+ to Na+, and the anionic resin changes from Cl- to OH-, both will expand due to the increase of ion diameter, increasing the volume of the resin. Generally, a resin with a low degree of crosslinking has a larger degree of expansion. When designing the ion exchange device, the swelling degree of the resin must be considered to adapt to the change of the resin volume caused by the ion conversion in the resin during production operation.
The resin particles have changes such as transfer, friction, expansion and contraction during use, and there will be a small amount of loss and breakage after long-term use, so the resin must have higher mechanical strength and wear resistance. Generally, resins with a low degree of cross-linking are easier to break, but the durability of the resin is more mainly determined by the uniformity of the cross-linked structure and its strength. Such as macroporous resin, with a higher degree of cross-linking, the structure is stable, and can withstand repeated regeneration.
4. Application fields of ion exchange resin:
The demand for ion exchange resin in the water treatment field is very large, accounting for about 90% of the output of ion exchange resin, which is used to remove various anions and cations in water. At present, the largest consumption of ion exchange resin is used in pure water treatment in thermal power plants, followed by atomic energy, semiconductors, and electronics industries.
(2) Food industry
Ion exchange resins can be used in sugar, monosodium glutamate, wine refining, biological products and other industrial equipment. For example, the manufacture of high fructose syrup is to extract starch from corn, and then undergo hydrolysis to produce glucose and fructose, and then undergo ion exchange treatment to produce high fructose syrup. The consumption of ion exchange resin in the food industry is second only to water treatment.
(3) Pharmaceutical industry
The ion exchange resin in the pharmaceutical industry plays an important role in the development of a new generation of antibiotics and the quality improvement of the original antibiotics. The successful development of streptomycin is a prominent example. In recent years, research has also been done on the commission of Chinese medicine.
(4) Synthetic chemistry and petrochemical industry
In organic synthesis, acids and bases are commonly used as catalysts for esterification, hydrolysis, transesterification, hydration and other reactions. Using ion exchange resin instead of inorganic acid and alkali can also carry out the above reaction, and has more advantages. For example, the resin can be used repeatedly, the product is easy to separate, the reactor will not be corroded, the environment will not be polluted, and the reaction can be easily controlled.
The preparation of methyl tert-butyl ether (MTBE) uses macroporous ion exchange resin as a catalyst, which is formed by the reaction of isobutylene and methanol, instead of the original tetraethyl lead, which can cause serious environmental pollution.
(5) Environmental protection
Ion exchange resins have been used in many environmental protection issues that are of great concern. At present, many aqueous or non-aqueous solutions contain toxic ionic or non-ionic substances, which can be recycled with resin. Such as removing metal ions in electroplating waste liquid, and recovering useful substances in film production waste liquid.
(6) Hydrometallurgy and others
Ion exchange resin can separate, enrich, purify uranium and extract rare earth elements and precious metals from depleted uranium ore.