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Cause analysis and treatment
In order to accurately find the reasons, we conducted a full analysis of Fe3+ according to the process shown in Figure 1, and initially determined the following reasons:
Inappropriate Design and Material Selection
The raw water, after treatment in the cation exchange bed, becomes soft water. It then passes through a decarbonation tower to remove carbon dioxide. A review of the preliminary design indicates that the grate plates (i.e., the load-bearing components at the lower end of the packing) in the decarbonation tower are made of carbon steel. However, soft water has a pH between 2.8 and 3.0, which is highly corrosive to carbon steel. Therefore, Fe3+ analysis was performed on the influent and effluent of the decarbonation tower. The results are shown in Table 1.
Table 1 shows that the Fe3+ content in the system increased significantly after the soft water passed through the decarbonation tower. This indicates severe iron leakage in the decarbonation tower, leading to iron contamination of the anion exchange resin. Consequently, the anion exchange resin not only fails to adsorb Fe3+ from the water but may also continuously release it, increasing the operating pressure of the mixed bed. We also understand that the initial Fe3+ compliance rate was maintained at 70%-80% likely because the anion exchange resin adsorbed a large amount of iron ions. Therefore, we believe that corrosion of the iron components in the decarbonation tower is the main reason for the excessive Fe3+ levels in the pure water.
Replacing the grate plates inside the decarbonization tower with stainless steel eliminated the iron leakage problem in the decarbonization tower.
Table 1. Fe³⁺ Analysis at the Inlet and Outlet of the DecarbonizationTower
Water sample number | Imported ρ(Fe³⁺ ) /(mg·L⁻¹) | Outlet ρ(Fe³⁺ ) /(mg·L⁻¹) |
1 | 0.06 | 0.21 |
2 | 0.05 | 0.16 |
3 | 0.10 | 0.23 |
Poor Construction Quality
Statistical analysis of leaks and maintenance records in the inlet and outlet pipes of the cation and anion beds revealed leaks in the outlet pipes of two cation beds and the acid inlet pipes of four cation beds. Leaks were also found in the elbows of the inlet pipes of all four anion beds. Spark testing of the outlet pipe of the shut-down mixed bed revealed extensive damage to the rubber lining. This indicated that iron leakage had been occurring in the rubber-lined pipes for a considerable period before the leaks, eventually leading to perforation.
Based on these findings, we concluded that the large-scale damage to the rubber-lined pipes within such a short timeframe was due to construction quality issues. Further investigation revealed that most leaks occurred at the connecting flanges of the elbows. The replaced elbows showed that the rubber lining at the flanges was already damaged during installation.
Replacing a section of the inlet water pipe and two elbows of the four anion beds, the outlet water pipe elbows of the two cation beds, the outlet pipe of the mixed bed, and the pure water tank with stainless steel, and replacing all the rubber-lined acid regeneration pipes with fiberglass pipes, resolved the iron leakage problem in the rubber-lined pipes.
Substandard Regeneration Alkali
According to the original design, our company has been using diaphragm alkali produced by a nearby manufacturer. However, this alkali contains a high amount of impurities and may have excessive Fe3+ content. Therefore, samples were taken for analysis. The analysis results showed that the iron content was between 0.0012% and 0.0015%, while the standard is ≤0.0005%.
In addition, experiments were conducted using an exchange column to regenerate the mixed-bed resin with both diaphragm alkali and ion-exchange membrane alkali. The test results are shown in Table 2.
Water sample number | Diaphragm alkaline regeneration of ρ( Fe³⁺ ) | Alkali regeneration of ion-exchange membrane ρ( Fe³⁺ ) | ||
Water ingress | Out of water | Water ingress | Out of water | |
1 | 0.10 | 0.051 | 0.10 | 0.039 |
2 | 0.09 | 0.042 | 0.09 | 0.033 |
3 | 0.07 | 0.039 | 0.07 | 0.028 |
Table 2 shows that the Fe3+ content in the effluent from the mixed-bed resin regenerated with ion-exchange membrane alkali is lower than that in the effluent from the mixed-bed resin regenerated with diaphragm alkali. Replacing the regeneration alkali with ion-exchange membrane alkali significantly improves the quality of the mixed-bed effluent, with a marked decrease in Fe3+ content, while also controlling iron fouling of the anion resin.
Treatment of Anion Resin
To address the anion resin contamination problem, a chemical treatment method was adopted: first, the resin was soaked in a 5%-10% Na₂S₂O₂ solution for 24 hours, then soaked in a 10% hydrochloric acid solution for 12 hours, and finally, regeneration was performed. Through this treatment, the anion resin color lightened, and iron contamination was alleviated.
Implementation Results
After implementing the above measures, iron leakage in the system has been basically controlled. Simultaneously, the quality of the mixed bed effluent has continuously improved, and the pass rate of the Fe³⁺ index in pure water has continued to rise, as shown in Table 3.
Time | ρ(Fe³⁺ ) /(μg· L⁻¹) | Pass rate/% | ||
maximum | Minimum | average | ||
2001-07 | 50.63 | 18.5 | 35.6 | 37.5 |
2001-08 | 45.34 | 17.6 | 32.5 | 50 |
2001-09 | 38.52 | 15.2 | 27.6 | 80 |
2001-10 | 30.25 | 16.32 | 23.16 | 90 |
Conclusion
Through a series of overhauls, process improvements, and technical upgrades, the factors causing the high Fe³⁺ content in the pure water have been largely eliminated. Currently, the pass rate for this indicator has reached over 90%, the system is operating stably, and there have been no further iron leaks, providing a reliable guarantee for the quality of our company's products.
