Pure water technology - performance and regeneration of resin for ion exchange reaction

First, the exchange ability

The hydrogen-type cation exchange resin can dissociate hydrogen ions (H ⁺ ) in water. When metal ions or other cations are encountered, mutual exchange occurs, but the exchanged resin is no longer a hydrogen-type resin. For example, when the concentration of cations such as calcium ions and magnesium ions in water is relatively large, hydrogen ions in the sulfonic acid type cation exchange resin can be exchanged with calcium and magnesium ions to form "calcium type" or "magnesium type". Cation exchange resin, as follows: 2R-SO ₃ H + Ca ²⁺ → (R-SO ₃ ) ₂ Ca + 2H ⁺ (calcium type strongly acidic cation exchange resin) 2R-SO ₃ H + Mg ²⁺ → (R -SO ₃ ) ₂ Mg + 2H ⁺ (magnesium type strongly acidic cation exchange resin) The exchange capacity of the hydrogen type cation exchange resin is closely related to the valence of the exchanged cation. At normal temperature, in a low-concentration aqueous solution, the exchange capacity increases as the valence of the ions increases, that is, the higher the valence, the greater the tendency of the cations to be exchanged. Further, if the valences are the same, the tendency of the cations having a larger ionic radius to be exchanged is also greater. If the cations frequently appear in tap water as a reference, the order of exchange capacity of the hydrogen-type cation exchange resin can be expressed as follows: Strong acidity: Fe ³⁺ >Fe ²⁺ >Mn ²⁺ >Ca ²⁺ >Mg ²⁺ >K ⁺ >NH ⁴⁺ >Na ⁺ >H ⁺ weakly acidic:H ⁺ >Fe ³⁺ >Fe ²⁺ >Mn ²⁺ >Ca ²⁺ >Mg ²⁺ >K ⁺ >NH ⁴⁺ >Na ⁺ The order shows that the precursors of the strongly acidic and weakly acidic cation exchange resins have the same order of cation exchange capacity. The only difference is that the exchange capacity of H+ is different, the affinity of strong acid to hydrogen ions is the weakest, and the weak acid is hydrogen. The affinity of ions is the strongest, and this property may deeply affect their function and function in the water tank. Although the hydrogen type weakly acidic cation exchange resin has the strongest affinity for hydrogen ions, the hydrogen ion (H ⁺ ) and the hydroxide ion (OH ^- ) combine to form water (H ₂ O) with stronger affinity, so it is alkaline. In the water quality, H ⁺ in the weakly acidic cation exchange resin is rapidly consumed by OH ^- and OH ^- mainly from the hydrolysis reaction of KH hardness (HCO ^{₃ -} ): HCO ₃ ^- + H ₂ O ←→ H ₂ CO ₃ +OH ^- The "active position" left by H ⁺ is replaced by other cations such as Fe ³⁺ >Fe ²⁺ >Mn ²⁺ >Ca ²⁺ >Mg ²⁺ etc., which continue until HCO ₃ ^{- is} completely eliminated. So far (KH=0). Therefore, the main action range of the weakly acidic cation exchange resin is the water quality of pH=5-14. Since HCO ₃ ^- is an anion with a temporary hardness, when HCO ₃ ^{- is} completely eliminated, its "equivalent cation", such as calcium, magnesium, etc., is also completely replaced at the same time, so that all the "quivalent cations" of temporary hardness can be eliminated. . The hydrogen-type strongly acidic cation exchange resin has the weakest affinity for hydrogen ions (H ⁺ ), so that it has exchange capacity at any pH, so that the GH hardness (temporary hardness and permanent hardness) can be completely removed.

Second, the exchange capacity

The performance of the ion exchange resin for ion exchange reaction is mainly manifested by "exchange capacity". The exchange capacity refers to the number of milliequivalents of ions that can be exchanged per gram of dry resin, in mmol/g. When the ion is monovalent (such as K ⁺ ), its milliequivalent number is its milligram number, and for divalent (such as Ca ²⁺ ) or more valent ions (such as Fe ³⁺ ), the milliequivalent is The number of milligrams of the molecule is multiplied by its ion valence. The exchange capacity is divided into three types of representations: "total exchange capacity", "operation exchange capacity", and "regeneration capacity". "Total exchange capacity" means the total amount of chemical groups per gram of dry resin that can be ion exchanged, and is a theoretical measure. "Operation exchange capacity" means the ion exchange capacity per gram of dry resin under certain conditions, which is an operational measurement. It is related to the resin type, total exchange capacity, and specific operating conditions (such as contact time, temperature) and other factors. To show operational efficiency. "Regeneration capacity" means the exchange capacity of the recycled resin obtained per gram of dry resin under a certain regenerative dosage condition, and can be used to show the resin regeneration efficiency. Due to the different structure of the resin (mainly the number of active groups), the exchange capacity of the strongly acidic and weakly acidic cation exchange resins is also different. In general, the number of weakly acidic active groups is usually more than that of strong acidity, so the total exchange capacity is about 7.0 ~ 10.5 mmol / g higher. In contrast, the strong acidity is only about 3.2 ~ 4.5 mmol / g, but in practice In the application, the weakly acidic operation exchange capacity is not necessarily higher than the strong acidity. For example, when the pH value is lower than 5, the weakly acidic operation exchange capacity is zero, and there is no exchange effect at all. At a pH of 6.5, the exchange capacity of the two is similar; but in an alkaline solution, the weak acidity is much higher than the strong acidity. In terms of regenerative capacity, weak acidity is usually higher than strong acidity, so the service life of weak acidity will be longer.

Third, regeneration

The relative concentration of ions has a great influence on the exchange properties of the resin. When the concentration of hydrogen ions in the aqueous solution is relatively large, calcium ions or magnesium ions in the calcium- or magnesium-type cation exchange resin can be exchanged with hydrogen ions to become a hydrogen-type cation exchange resin. In other words, the exchange reaction can also be carried out in the opposite direction. Since the ion exchange process is reversible, when the exchange resin exchanges a certain amount of ions, it can be replaced by a relatively high concentration of hydrogen ions, so that one of them is recycled again. This effect is called regeneration. The reaction formula is as follows: (R-SO ₃ ) ₂ Ca + 2H ⁺ → 2R-SO ₃ H + Ca ²⁺ (R-COO) ₂ Ca + 2H ⁺ → 2R-COOH + Ca ^{2+ in} the hydrogen type resin After the hydrogen ions are exchanged by other hardness ions, these resins do not soften the water. The state at this time is called the "saturated" state. The main purpose of the regeneration operation is to wash out the exchanged cations with the "regenerant" of the resin that has reached the "saturated" state, and then return the resin to the original exchange capacity, or the desired capacity level, or the original Resin type, etc. Dilute sulfuric acid or dilute hydrochloric acid can be used as a regenerant for both strongly acidic and weakly acidic cation exchange resins, but it is generally considered that dilute sulfuric acid is used as a regenerant. Because the resin adsorbs organic matter, dilute sulfuric acid can resolve organic matter more than dilute hydrochloric acid. Therefore, dilute sulfuric acid is often used as a regenerant in the general process. However, in actual application, it may be difficult to obtain sulfuric acid, so hydrochloric acid is often used as a regenerant.

Fourth, the main factors affecting the characteristics of regeneration

The regeneration characteristics of the hydrogen type resin are closely related to its type and structure. The regeneration of the strong acid hydrogen type resin is difficult, and the dose of the regenerated acid solution required is much higher than the theoretical value, and a long contact time is required. In contrast, the regeneration of the weakly acidic hydrogen-type resin is relatively easy, and the dose of the regenerated acid solution required is only higher than the theoretical value, and long contact time is not required. It is generally believed that when the amount of sulfuric acid or hydrochloric acid is twice the total exchange capacity, the contact time of each regenerated resin with the regenerated acid solution is: strong acidity of about 30 to 60 minutes; weak acidity of about 30 to 45 minutes. Further, the regeneration characteristics of the hydrogen type resin are also related to their "degree of crosslinking". The degree of crosslinking is the mass percentage of the crosslinking agent (such as styrene) contained in the quantitative resin. A resin having a low degree of crosslinking is generally characterized by a low polymerization density, a large internal void, a large mesh, and good swelling property against water, but a weak choice for ions, a fast exchange reaction rate, and easy regeneration, so each The secondary regenerated resin has a short contact time with the regenerating acid solution. Conversely, a resin having a high degree of crosslinking requires a longer period of time during which the regenerated acid solution is in contact with the resin. The "crosslinking degree" of the strongly acidic or weakly acidic hydrogen type resin can be controlled at the time of manufacture. Since the mesh of the hydrogen type resin not only provides good ion exchange conditions, but also acts like activated carbon, it can produce molecular adsorption, and it can also adsorb various organic substances, so it is easily contaminated by organic substances, which affects the operation efficiency and also makes Its regeneration operation is difficult. If the resin adsorbs organic matter, especially macromolecular organic matter, during use, the regeneration contact time must be longer, and it is usually necessary to raise the temperature (70~80 °C) to remove most of the organic matter, so as to prevent its performance from decreasing too fast, and at the same time at high temperature. Under the operation, the regeneration reaction time can also be accelerated, so that the immersion contact time can be shortened. The regenerant used in this aspect is preferably sulfuric acid, on the grounds that sulfuric acid is quite stable upon heating, and hydrochloric acid may produce toxic hydrogen chloride gas.

V. Relationship between regeneration liquid concentration and regeneration efficiency

The chemical reaction of resin regeneration is the reverse reaction of its original exchange. According to the equilibrium principle of chemical reaction, the concentration of the reactant can be increased to promote the reaction to the other side. Therefore, increasing the concentration of the acid solution can accelerate the regeneration reaction rate and further improve the regeneration efficiency. However, this does not mean that the higher the acid concentration, the better. If the amount of acid required for the exchange resin is not evaluated by experiments, the problem of "too much" will occur. When the concentration of the regenerated acid solution is insufficient, the regeneration rate of the resin is lowered, which will affect the subsequent hard water softening function. On the contrary, if too much acid is used, the acid is wasted on weekdays, which increases the cost of regeneration and is not cost-effective. In order to let consumers know the dosage of regenerative acid, some manufacturers with better service will provide the most suitable concentration for reference. Yes, if the hydrogen ion concentration in the acid solution exceeds 1 mol/l, the regeneration reaction rate may be limited by the diffusion of the mesh. Therefore, the resin with a small mesh size should not be regenerated with a high concentration of acid. Otherwise, It also causes a waste of acid. In addition, although sulfuric acid is a good regenerant, it is necessary to prevent the calcium ions absorbed by the resin from reacting with sulfuric acid, and to form a calcium sulfate precipitate in the resin. To avoid this problem, in the first operation, First, pour in 1 ~ 2% sulfuric acid and elute once. In the second operation, use higher concentration sulfuric acid. Finally, if you intend to complete the regeneration operation using only "one-time operation regeneration", it is possible to increase the operating concentration of the acid solution to increase the regeneration efficiency. Although this mode of operation is the most convenient, the regeneration efficiency will not be as good as diluting the same dose of acid solution, and soaking it in two or more times. However, to perform multiple operations, it is also necessary to consider adding more regeneration efficiency, and the value is not worth the effort to deal with.

The important properties of two hydrogen-type cation exchange resins are summarized as follows: Generally, strong acid resins can be operated in all pH ranges, but their exchange capacity is small, and they must be regenerated frequently. In addition, due to poor regeneration efficiency, the required regenerant The cost is higher, but all hardness ions can be removed, or the pH can be adjusted. The weakly acidic resin has a high exchange capacity, a high regeneration efficiency, requires less regenerant, but can only operate in a limited pH range, and can only remove temporary hardness ions.