Iron pollution and treatment:

Iron pollution may occur in both cation and anion resins. The appearance of the contaminated resin is dark brown and can turn black in severe cases. Generally, when the iron content in every 100g of resin exceeds 150mg, it shall be treated. The existence of iron will accelerate the degradation of anion exchange resin.

During the use of cation resin, most of the iron ions brought in by raw water exist as fe2+. After they are absorbed by the resin, some of them are oxidized to fe3+. During regeneration, they cannot be completely exchanged by h+, so they are retained in the resin and cause iron pollution. When iron salt is used as coagulant, part of alum will be brought into cation bed, which will be accumulated on the surface of resin layer due to filtration. During regeneration, acid solution will dissolve alum, making it fe3+, and part of it will be absorbed by cation resin, causing iron pollution. A large amount of fe3+ in industrial hydrochloric acid will also cause certain iron pollution to the resin. Cation resins used for sodium ion exchange are more susceptible to iron contamination.

The iron content of anion resin is sometimes many times greater than that of cation resin. Iron in anion resin mainly comes from regeneration solution. Caustic soda produced by general diaphragm method contains 0.01%~0.03% Fe2O3 and 6~7mg/l NaClO3. Such caustic soda will generate ferrate (FeO4) when it contacts with iron containers and pipelines (without anti-corrosion coating) during storage and transportation. After ferrate enters the anion bed with alkali liquor, it will decompose due to the decrease of pH value. The reaction formula is as follows:

2FeO42- + 10H+ → 2Fe3+ + 2/3 O2 + 5 H2O

Fe3+ further generates Fe (OH) 3 and attaches to anion resin particles, causing iron pollution.

After the resin is contaminated by iron, it cannot be removed in the general regeneration process and must be cleaned with hydrochloric acid.

The common cleaning method is to use 10%hcl solution. Before this method, the corrosion resistance of the exchanger equipment must be checked, otherwise the hydrochloric acid with inhibitor must be used.

Enter 10%hcl solution equivalent to 0.5 times the volume of the resin bed from the top of the resin bed (considering the residual water in the resin bed and maintaining the concentration of HCl solution), drain water equivalent to the residual water in the bed from the bottom of the resin bed, stir the solution and contact the resin for 12 hours. Drain the acid, wash it from top to bottom, and then backwash it for 30 minutes to remove loose substances. After regeneration of the resin bed, it can be put into operation.

Measures to prevent iron pollution of resin include:

1. reduce the iron content in the influent of cation bed. The groundwater with high iron content should be treated by aeration and manganese sand filtration to remove iron. For surface water with high iron content or when iron salt is used as coagulant, alkaline agents, such as Ca (OH) 2 or NaOH, shall be added to increase the pH value of the water and prevent iron ions from entering the cation bed.

2. necessary anti-corrosion measures shall be taken for pipelines and storage tanks transporting raw water with high iron content to reduce the iron content of raw water.

3. the storage tank and transmission pipeline of caustic soda for anion bed regeneration shall be rubber lined for corrosion prevention to reduce the iron content of alkali regeneration solution.

4. when the iron content of the resin exceeds 150g/gr, pickling shall be carried out.