Regeneration of Ion Exchange Resin

　　After the ion exchange resin has been used for a period of time, the adsorbed impurities are close to saturation. It is necessary to regenerate the resin and remove the adsorbed ions and other impurities with chemical reagents to restore its original composition and properties. In practice, in order to reduce the cost of regeneration, the dosage of regenerant should be properly controlled so that the properties of resin can be restored to the most economical and reasonable level of regeneration. Usually, the degree of recovery of properties can be controlled to 70-80%. If a higher regeneration level is to be achieved, the regeneration dose will be increased substantially and the utilization rate of regenerant will be decreased.

　　The regeneration of resin should select appropriate regeneration agent and working conditions according to the type and characteristics of resin and the economy of operation.

　　The regeneration characteristics of resin are closely related to its type and structure. The regeneration of strong acidic and strong alkaline resins is difficult, and the regeneration dose is much higher than the theoretical value, while the weak acidic or weak alkaline resins are easy to regenerate, and the regeneration dose used is only slightly more than the theoretical value. In addition, resins with large pore size and low crosslinking degree are easy to regenerate, while gel type and high crosslinking resins have longer regeneration time.

　　The type of regenerant should be selected according to the ion type of resin, and the acid, alkali or salt with lower price should be selected appropriately. For example, sodium-type strong acidic cation resin can be regenerated with 10% NaCl solution, the dosage is twice its exchange capacity (117g/l NaCl resin); hydrogen-type strong acidic resin is regenerated with strong acid, and calcium sulfate precipitate should be prevented from reacting with sulfuric acid when sulfuric acid is used. Therefore, 1-2% dilute sulfuric acid should be used for regeneration.

　　Chlorinated alkaline resins are mainly regenerated by NaCl solution, but adding a small amount of alkali helps to dissolve and wash out the pigments and organic matter adsorbed by the resins. Therefore, alkali salts containing 10% NaCl + 0.2% NaOH are usually used for regeneration. The conventional dosage is 150-200 g NaCl per liter of resin and 3-4 g NaOH. OH strong alkali anion resin was regenerated with 4% NaOH solution. The chemical reaction during resin regeneration is the reverse reaction of exchange adsorption of resin. According to the principle of chemical reaction equilibrium, increasing the concentration of one substance in chemical reaction can promote the reaction to the other side. Therefore, increasing the concentration of regeneration solution can accelerate the regeneration reaction and reach a higher regeneration level.

　　In order to accelerate the regeneration chemical reaction, the regeneration liquid is usually heated to 70-80 C. The flow rate through the resin is generally 1-2 BV/h. In order to give full play to the efficiency of regenerant, the method of first fast and then slow can also be adopted. The regeneration time is about one hour. Then rinse the resin with soft water downstream for about one hour (water volume is about 4BV). After the rinse water is discharged, the rinse is backwashed with water until the rinse is colorless and turbid.

　　After regeneration and backwashing, some resins have to adjust their pH values. Because the regeneration solution often contains alkali, even after the resin regeneration after washing, it is often alkaline. Some decolorizing resins (especially weak alkaline resins) should work under slightly acidic conditions. At this time, dilute hydrochloric acid can be poured into the resin, so that the pH value of the resin can be reduced to about 6, and then washed with water, back wash once each.

　　After using resin for a long time, some impurities (especially macromolecular organic colloidal substances) adsorbed by the resin are not easy to be eluted by conventional regeneration treatment, and gradually accumulate, which pollutes the resin and reduces its efficiency. At this point, it should be handled in a special way. For example, cationic resins contaminated by nitrogen-containing amphoteric compounds can be treated with 4% NaOH solution to dissolve and discharged; anionic resins contaminated by organic matter can increase the concentration of NaOH in alkali salt solution to 0.5-1.0% in order to dissolve organic matter.

　　Special regeneration treatment

　　Resins with serious contamination can be treated repeatedly with acid or alkaline salt solution, such as dissolving organic matter with 10% NaCl + 1% NaOH alkali salt solution, dissolving inorganic matter with 4% HCl or 10% NaOH and 1% HCl respectively, and then with 10% NaCl + 1% NaOH treatment, at about 70 ~C.

　　If the effect of the above treatment does not meet the requirements, it can be treated by oxidation method. That is, after washing the resin with water, the sodium hypochlorite solution with 0.5% concentration is introduced, the flow rate is controlled by 2-4 BV/h, and the flow rate is 10-20 BV. Then the resin is washed with water and treated with brine. It should be noted that oxidation treatment may oxidize the bonds of macromolecules in the resin structure, resulting in the degradation of the resin, increased expansion and easy fragmentation, so it is not suitable for common use. Usually 50 cycles are used before one oxidation treatment. Because of the strong oxidation resistance of chlorine resin, the resin is treated with brine before oxidation treatment, which can avoid the change of pH value in the process of treatment and make the oxidation more stable.