Oxidation And Degradation Of Resins

The chemical stability of a resin can be expressed by its ability to withstand the action of an oxidizing agent. After the oxidation of cation resin, the chain of the skeleton is mainly broken, while anion resin is mainly degraded by quaternary amine groups.

1. Oxidation of cation resin:

When cation resin is oxidized, the skeleton is broken, and low molecular sulfonic acid compounds and carboxylic acid groups are formed. The reaction is:

The oxidant encountered by cation resin is mainly the oxygen generated by the reaction of free chlorine and water, and the reaction is as follows:

In the past, the free chlorine in raw water mainly came from the disinfection of domestic water. In recent years, due to the increase of organic matter content and bacteria in natural water, chlorine should also be added before coagulation and clarification to achieve sterilization and reduce COD. Therefore, attention must be paid to the damage of free chlorine to cation resin. In the regeneration process, if the use of poor quality industrial hydrochloric acid or by-product hydrochloric acid, which contains oxidants will also cause damage to the cation resin. It is generally required that the free chlorine content should be less than 0.1mg/L in the raw water of the chemical demineralization equipment.

2. Methods to prevent cation resin from being oxidized:

(1) Activated carbon filtration. The common method to prevent the oxidation of cation resin is through activated carbon filtration. The principle of activated carbon removal of free chlorine is not simply adsorption, but a chemical reaction on the surface. When the chlorine adsorbed on the surface of activated carbon reaches a certain concentration, the following reactions will occur:

Where: C* -- activated carbon;

CO* - an oxide formed on the surface of activated carbon.

If there is sufficient chlorine to participate in the reaction, CO* can be changed into CO or CO2 escape, leaving activated carbon can continue to adsorb free chlorine. For this reason, in order to remove free chlorine, a higher filtration flow rate (about 50m/h) can be used. At the same time, activated carbon has a high adsorption capacity when it absorbs free chlorine (about 6.5mg of Cl2 per gram of activated carbon).

Removal of free chlorine from water with activated carbon can be calculated using the following empirical formula:

Where: CO -- the content of influent free chlorine, mg/L;

C -- Free chlorine content in effluent, mg/L;

L -- the height of activated carbon, m;

V -- filtration flow rate, m/h.

Considering the slow reaction rate of HOCl, the above formula is revised as follows:

The raw materials used to make activated carbon generally have no effect on dechlorination efficiency.

The presence of colloids or high concentrations of organic matter in water will seriously shorten the life of activated carbon as a dechlorination agent.

When the activated carbon filter is only used to remove free chlorine, the amount of Cl2 leakage ≥0.1mg/L can be used as the end point. The life of activated carbon is very long, for example: under the condition of activated carbon layer 0.76m and filtration rate 6.1m/h, the water with free chlorine content of 2mg/L is dechlorinated, and its service life is about 6 years.

(2) The selection of high crosslinking degree of positive resin. With the increase of the crosslinking degree of resin, its antioxidant properties were enhanced.

After the cation resin is oxidized, the skeleton is loosened due to the broken chain, the volume expands and the water content increases. Because of the high crosslinking degree, the large porous positive resin has better antioxidant properties. However, with the increase of resin crosslinking degree, its exchange capacity decreases and its price increases, so it is rarely used in practice.

3. the degradation of strong alkali anion resin:

After the strong alkali anion resin is oxidized, the main performance is the gradual degradation of quaternary amine groups, and the chain breaking of the skeleton will not occur. The degradation of strong alkali negative resin is mainly the sequential decomposition of quaternary amine groups into tertiary, secondary, primary amine, and even non-alkaline substances. In chemical desalting process, it is mainly manifested as the reduction of neutral salt decomposition capacity, especially silicon exchange capacity. The oxidizing agent encountered in the operation of strong alkali negative resin is mainly dissolved oxygen in water, and the oxidizing agent encountered in the regeneration process is mainly ClO3- and FeO42- contained in the alkali.

The reaction of oxidation of quaternary amine groups is shown as follows:

The antioxidant property of strong alkali type I negative resin is better than that of strong alkali type II. In the long-term use of strong alkali negative resin, its exchange capacity will gradually decrease.

4. Methods to prevent the degradation of strong alkali anion resin

(1) Use a vacuum degasser to reduce the oxygen content in the water in the anion bed.

(2) Do a good job of anti-corrosion work of lye storage and transportation equipment to reduce the iron content of recycled liquid.

(3) Soda ash manufactured by diaphragm method can reduce the content of NaClO3 in lye (which can be reduced to 6-7mg/L).

(4) Control the temperature of the regeneration liquid: Type I negative resin shall not be higher than 40℃; Type II negative resin shall not be higher than 35℃.

(5) The resin should be stored in chlorine at low temperatures.