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Degeneration And Contamination Of Ion Exchange Resin

Jul 18, 2020

During the operation of the ion exchange water treatment system, various ion exchange resins often gradually change their performance. There are two reasons: one is that the nature of the resin has changed, that is, its chemical structure is destroyed; the other is contamination by foreign impurities. The change in resin performance caused by the former situation cannot be recovered; the change in resin performance caused by the latter situation can take appropriate measures to eliminate these contaminants, so as to restore or improve the performance of the resin.


   1. Deterioration


  (1) Cationic resin


The main reason for the deterioration of   Yang resin in application is the oxidant in the water, such as free chlorine, nitrate, etc.; when the temperature is high, the resin is more severely corroded by the oxidant. If there are heavy metal ions in the water, it can play a catalytic role.


  The oxidation process of the cationic resin is not yet fully understood. It is generally believed that as a result of resin oxidation, the carbon chains between the benzene rings are broken, and carboxylic acids and ketones are formed.


  The phenomenon that occurs after the oxidation of the positive resin is: the color becomes lighter and the resin volume becomes larger, so it is fragile and the volume exchange capacity decreases, but the mass exchange capacity does not change much.


  Practice has proved that the strong acid H resin suffers the most severe damage. For example, when the feed water contains 0.5mg/L Cl2, the resin will change significantly after running for 4-6 months.


The oxidation product of the carbon chain of the sulfonic acid-based cation resin is shed from the resin and becomes a soluble substance, which also contains weak acid groups. When it enters the anion exchanger, it will first be adsorbed by the anion resin, which reduces the water quality.


   There are two commonly used methods to remove free chlorine in water: one is to use an activated carbon filter, and the other is to add sodium sulfite.


  (2) Anion resin


   Generally speaking, the chemical stability of the anion resin is worse than that of the cation resin, so its resistance to oxidants and high temperatures is also poor. However, in the desalination system, the anion is located after the cation exchanger, so the damage is relatively light; generally only the oxygen dissolved in the water damages the anion resin.


The most vulnerable part of anion resin is the nitrogen in its molecule. For example, when the strong alkaline anion resin is invaded, the quaternary nitrogen gradually changes to tertiary, secondary, and primary nitrogen, which weakens the alkalinity, and finally reduces to a non-alkaline substance. Therefore, during the oxidation deterioration of the strong base anion resin, the total amount of exchange groups and the number of strong base exchange groups gradually decrease, and the latter is larger than the former. This is because in the early stage of oxidation of the anion resin, the quaternary amine group becomes a weakly basic group capable of anion exchange in most cases.


   Type II strongly basic resin is more susceptible to oxidation than Type I.


   Increasing the water temperature during operation will speed up the oxidation of the resin.


   2. Pollution causes and treatment methods


  (1) Cationic resin


  Yang resin will be polluted by suspended solids, iron, aluminum, grease CaSO4 and other substances in the water. The cationic resin used in the desalination system is unlikely to be contaminated by iron and aluminum, because acid can be used as a regenerating agent to dissolve and remove iron and aluminum deposits. When sulfuric acid is used as a regenerant, calcium sulfate may be deposited on the resin surface.


   If the cation resin is contaminated, it can be treated with the following methods.


   1) Air scrubbing. It can be seen from the microscope that there are deposits on the resin surface, which can be removed by air scrubbing. The compressed air used should be purified.


   2) Pickling method. For those substances that cannot be removed by scrubbing, such as salts of iron, aluminum, calcium and magnesium, they can be pickled with hydrochloric acid.


   First, perform laboratory tests to determine the acid concentration, acid amount and time required to process the resin.


  Industrial equipment is pickled. In the desalination system, the original regeneration system can be used to prepare an acid solution of the required concentration. Acid immersion or low flow rate circulation of the resin, or alternate immersion and circulation.


  Yang resin pickling treatment can be supplemented by compressed air scrubbing. The cation resin contaminated by CaSO4 precipitation can be cleaned with a dilute solution of EDTA.


  3) Non-ionic surfactant cleaning method. For organic substances such as lubricating oil, lipids and proteins in the water, the process characteristics of the resin are seriously affected, and the abnormal phenomenon of resin agglomeration and reduction of resin density occurs. The main feature of the contaminated resin is that the color of the resin turns black, which is very similar to Fe contamination. But if you put a small amount in the water and shake it, there will be a "rainbow" phenomenon. For this type of pollution, the resin can be converted to sodium type first, then the non-isolated surfactant is added to the backwash water, and then it can be removed by passing it through the exchanger for backwashing.


  (2) Anion resin


   Strong alkaline anion resin is often susceptible to contamination by impurities such as organic matter, colloidal silicon, iron compounds, etc., which reduces the exchange capacity.


  1) Organic pollution


   For strongly alkaline anion resins, the biggest pollution source is organic matter. Organic matter is mainly humic acid and fulvic acid produced by the decomposition of plants and animals, but there are many types.


  The characteristic of organic pollution is that the exchange capacity is decreased, the washing time after regeneration is longer, the resin color often becomes darker, and the quality of the desalination system deteriorates and the pH decreases.


The reason why gel-type strong alkaline anion resin is easily polluted by humic acid or fulvic acid is that its polymer skeleton belongs to styrene series and is hydrophobic, while humic acid and fulvic acid are also hydrophobic. It is difficult to desorb. In addition, the weak acid group on the organic molecule-COOH again acts as a cation exchange resin.


  R¢COOH + NaOH ® R¢COONa +H2O (during regeneration)


  R¢COONa + H2O ® R¢COOH +NaOH (when making water)


   In order to prevent the strong alkaline resin from being contaminated by organic matter, filtration and activated carbon adsorption treatment should be adopted reasonably.


  The degree of organic contamination of the anion resin can be judged by the following simple method. Put the 50mm resin in a conical flask, shake it with pure water 3-4 times to remove surface contamination, then add 10% saline, shake vigorously for 5-10 minutes, observe the color of the brine, and judge the degree of contamination according to the color of the solution.


   Resin pollution degree judgment


   color pollution degree color pollution degree


   clear and bright


   pale grass yellow


   amber amber color is not polluted


   light pollution


   moderately polluted brown


  Dark brown or black heavily polluted


   Serious pollution


   Generally, the resuscitation treatment is carried out when the resin is moderately contaminated.


There are many recovery methods, the most commonly used is the alkaline salt water treatment method. The specific method: soak in a mixture of 10% NaCl and 1% NaOH with more than twice the volume of the resin for 16-18 hours, and then rinse with water to a pH of 7-8 . If the treatment liquid is heated to 40~50℃, the effect will be improved, but type II resin can only be used at 40℃.


  Industrial NaCl and NaOH will produce precipitation after mixing, it is best to remove the precipitate.


  Some use sodium hypochlorite-containing sodium hydroxide solution to treat severely polluted resin, the effect is better, but this method has an oxidizing effect on the resin, which is not easy to use.


   2) Contamination of colloidal silicon


   Colloid usually does not contaminate the strong alkaline anion resin, but when the regeneration conditions are improper, such as the amount of regenerant is small, the temperature of the regeneration solution and the regeneration flow rate are too low, there is a possibility that the strong alkaline anion resin is contaminated by colloidal silicic acid. There is no unified view on the process of colloidal silicic acid pollution. When the alkaline anion resin is in a state of failure, after a long period of non-use, a large amount of alkali is required for regeneration, which is to remove colloidal silicic acid.


  3) Iron pollution


  Resins in operation are often contaminated by heavy metal ions and their oxides, among which iron compounds are the most commonly encountered. When the anion resin is regenerated, it is caused by more iron compounds in the lye. Iron and macromolecular organic compounds form complexes and enter the resin network, which will also cause contamination of the anion resin.


  The color of the anion resin becomes black after being contaminated by iron, its performance is reduced, the regeneration efficiency is reduced, and the amount of regenerant and cleaning water consumption increase. The anion resin contaminated by iron is generally treated by the same pickling method as the cation resin.


   If the anion resin is contaminated by both organic matter and iron ions and their oxides, the iron ions and their oxides should be removed first, and then the organic matter.


  Using ultrasonic to clean contaminated anion and cation exchange resin is a new technology applied in recent years.

Source:https://www.up-resin.com/

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