How to deal with "iron poisoning" of ion exchange resin?

Ion exchange resins have advantages such as good chemical stability, high exchange capacity, and high mechanical strength. Therefore, they are widely used in power plant boilers, industrial boiler water treatment, and the production of desalted water and pure water. However, during use, the resin may be contaminated by harmful impurities (such as iron compounds and organic matter), resulting in resin "poisoning". "Poisoning" incidents. If reasonable measures are not taken in time to revive it, it may cause the resin to fail or even be scrapped. Iron "poisoning" is the most common type of resin "poisoning".

What is iron "poisoning"?

When the surface of the ion exchange resin is covered with iron compounds or the exchange channels inside the resin are blocked by iron impurities, the working exchange capacity and regeneration exchange capacity of the resin are significantly reduced, but the resin structure remains unchanged. This phenomenon is called iron "poisoning" of the resin.

Analysis of the causes of iron "poisoning" of resin

There are four main reasons for iron "poisoning" of resin:

① The water source is groundwater with high iron content or surface water contaminated with iron;

② Iron compounds are produced by corrosion in the inlet pipe or inside the exchanger;

③ The regenerant contains iron impurities;

④ The water contains macromolecular organic matter.

Iron "poisoning" of cation resin generally only occurs in the softening water process using salt as a regenerant. There are two main situations: one is that when iron enters the sodium ion exchanger in the form of colloidal or suspended iron compounds, it is adsorbed by the resin and forms a layer of iron compound coating on the resin surface, preventing the ions in the water from effectively contacting the resin; the other is that iron enters the exchanger in the form of Fe2+, reacts with the resin, and Fe2+ occupies the exchange position. Because Fe2+ is easily oxidized into high-valence iron compounds, it deposits inside the resin, blocking the exchange channels.

The main reasons for iron "poisoning" of anion resin are also as follows: Firstly, the purity of the alkali used to regenerate the anion resin does not meet the specified standard, especially when the liquid alkali contains more iron compounds, it is easier to cause anion resin poisoning; Secondly, when the water contains macromolecular organic matter, it is easy to form chelates (i.e., organic iron) with iron, which can exchange with strong alkaline anion resin, gather at the exchange group position, block the exchange channels of the resin, reduce the exchange capacity and regeneration capacity, reduce the regeneration efficiency, increase the consumption of regenerant and washing water, and further lead to iron "poisoning" of the resin.

Identification methods for iron "poisoning" resin

1. Appearance color identification: For resins that have undergone iron "poisoning", the color changes significantly from transparent yellow (cation resin) or milky white (anion resin) to darker, and in severe cases, even black.

2. Experimental identification: Determining the iron content of water to determine the degree of iron "poisoning" of the resin is a more accurate method.

The method is as follows: The "poisoned" resin is washed with clean water, regenerated in 10% brine for about 30min, the brine is poured off, and then washed 2-3 times with distilled water (or deionized water). A portion of the resin is taken out and placed in a test tube or glass bottle, then 6mol/L hydrochloric acid (about twice the volume of the resin) is added, the mixture is sealed and shaken for 15min, and then the acid solution is taken out and poured into another clean test tube. Saturated potassium ferrocyanide solution is added dropwise. The depth of the Prussian blue color (from light blue to dark brown) in the test solution can be used to judge the degree of iron "poisoning" of the resin. It should be noted that some units only use the method of determining the resin exchange capacity to judge whether the resin is iron "poisoned", which is inaccurate. Because iron "poisoning" only reduces the working exchange capacity of the resin, but has almost no effect on the total exchange capacity.

Recovery methods for iron "poisoning" resin

Since the exchange capacity of iron "poisoned" resin can be restored after proper treatment, iron "poisoned" resin should be treated promptly and correctly, otherwise, it will increase the possibility of resin damage and lead to resin scrapping.

There are three main methods for the recovery of iron "poisoned" resin, which are compared as follows:

1. Hydrochloric acid recovery method

Mechanism: The selectivity order of strong acid resin for cations is: Fe3+＞Fe2+＞Ca2+＞Mg2+＞Na+＞H+. After adding 10% hydrochloric acid to the iron "poisoned" resin, the hydrochloric acid dissolves the colloidal Fe2O3•XH2O on the surface or in the gel pores of the resin into Fe3+, and the H+ in the hydrochloric acid exchanges with Fe3+, Ca2+, and Mg2+ on the resin, so that the resin gradually turns into the hydrogen form, and then it is converted into the sodium form before operation. This method is simple and easy to implement. However, in practical applications, in order to fully recover the iron "poisoned" resin, the concentration of hydrochloric acid must be increased to more than 10%, which increases the processing cost and easily damages the anti-corrosion layer of the exchanger.

2. Hydrochloric acid-salt recovery method

Mechanism: 4% hydrochloric acid and 4% brine solution are added to the "iron-poisoned" resin and fully soaked. The main function of hydrochloric acid is to dissolve Fe2O3•XH2O. Na+ in salt, together with H+ in hydrochloric acid and Fe3+, Fe2+, Ca2+, Mg2+ on the resin, exchange, so that the resin gradually turns into a mixed hydrogen-sodium type, and it can be regenerated into a sodium type before operation. This method is a commonly used method. However, it also has disadvantages such as large amount of hydrochloric acid and salt used, long time consumption, and incomplete recovery treatment.

3. Hydrochloric acid-salt-sodium sulfite recovery method

Mechanism: A mixture of 4% hydrochloric acid, 4% sodium chloride, and 0.08% sodium sulfite is added to the iron "poisoned" resin and fully soaked. The roles of hydrochloric acid and sodium chloride are the same as above. The SO32- in Na2SO3 reduces Fe3+ to Fe2+, thereby reducing the resin's binding to Fe3+, and the H+ generated by the reaction also promotes the dissolution of Fe2O3•XH2O. The reaction formula is: SO32-+2Fe3++H2O≒SO42-+Fe2++2H+. Finally, the hydrogen sodium mixed resin is converted into a sodium resin, which can then be put into use. It should be noted that the concentration of Na2SO3 should be determined by experiment and should generally not exceed 0.1%, because if the concentration of Na2SO3 is too high, SO2 gas is easily generated, and the concentration of the product SO42- increases, which will produce CaSO4 precipitation. Practice has proved that this method of treating iron "poisoned" resin consumes less restorative agent, takes less time, and the hydrochloric acid concentration in the restorative agent is low, resulting in less corrosion to the exchanger, and the restorative effect is good. It is a relatively ideal treatment method.

Preventive Measures

①Iron-containing groundwater must undergo necessary iron removal treatment before entering the exchanger. Commonly used iron removal methods include: aeration iron removal, manganese sand filtration iron removal, etc.

②When using deep well water or tap water directly as the water source, a filter should be installed before the cation bed inlet pump. When producing pure water, stainless steel pipes or other pipes without iron elements should be used for the inlet pipeline to prevent the flowing water from bringing some iron corrosion products into the exchanger.

③Strengthen the anti-corrosion work of water treatment equipment and pipelines. Regularly check the internal regeneration device and anti-corrosion layer of the exchanger, and deal with any damage promptly. Use stainless steel pipes for the salt solution transport pipeline to prevent pipe corrosion from producing iron compounds and polluting the resin.