Application of Nitric Acid as a Regenerator in Cationized Flotation Beds

Abstract: Hydrochloric acid is commonly used as a regenerator for cation exchange resins, with nitric acid being less common. This study used 3%-5% dilute nitric acid solution (magnesium tailings water) instead of 30% industrial hydrochloric acid as the regenerator for cation exchange resin regeneration. The regenerator mass fraction was controlled at 2.4%, the flow rate at 6 m/h, and the regeneration time at 3 h. The effluent quality, operating cycle, and cycle water production of the regenerated cation exchange bed were not significantly different from those of the cation exchange bed regenerated with hydrochloric acid.

Keywords: Cation exchange resin; Floating bed ion exchanger; Regenerant; Demineralized water

CLC Number: TQ028.3 Document Code: B Article Number: 1009-2455(2004)05-0026-02

1. Regeneration of Cation Exchange Resin

Floating bed ion exchangers are a new type of bed developed in recent years based on fixed bed ion exchange water treatment equipment. This type of bed greatly improves the utilization rate of the exchange resin and the equipment output, improves the effluent quality, and reduces operating costs. Our company's thermal power plant put into use this new type of ion exchanger, anion and cation floating bed ion exchanger, in a 450t/h demineralized water system in 2000. The cation floating bed ion exchanger is filled with H-type strong acid cation exchange resin. When the cation floating bed becomes ineffective, it is regenerated using a 30% hydrochloric acid solution diluted as a regenerant.

After cation exchange resins become ineffective, they can be regenerated using H+ ions in a strongly acidic solution. H+ ions replace Ca2+ and Mg2+ ions to restore the resin's exchange performance. Theoretically, H+ ions in nitric acid solution can also replace Ca2+ and Mg2+ ions and can be used as a regenerator for cation exchange resins. However, in my country, the regenerator used for cation exchange resins is generally 30% industrial hydrochloric acid, rather than nitric acid. There are two main reasons for this: firstly, from an economic perspective, the cost of using nitric acid is higher than that of hydrochloric acid; secondly, nitric acid has strong oxidizing properties, making cation exchange resins susceptible to oxidation. This oxidation leads to chain breakage or damage to exchange groups in the resin structure, significantly reducing the resin's exchange capacity, darkening its color, and making it more prone to breakage. Ultimately, this results in a decrease in the working exchange capacity of the cation exchange resin, a decrease in water production, and unstable effluent quality. Therefore, most domestic manufacturers use 30% industrial hydrochloric acid as a regenerator and discourage the use of nitric acid.

Currently, only a few manufacturers in China use nitric acid as a regenerator, and our company is one of them. In 2002, our company invested in a set of hydrochloric acid production equipment (using magnesium nitrate to produce 98% concentrated nitric acid from 50%-60% dilute nitric acid). During the production process, 3%-5% dilute nitric acid wastewater (without impurities) is produced as a byproduct, commonly known as magnesium tailings. If this wastewater is discharged externally, it requires the addition of an appropriate amount of alkali solution for neutralization. However, if this wastewater is used as a regenerator for cation exchange resins, it can not only save 1.8 million yuan annually in the cost of purchasing hydrochloric acid, but also eliminate the need for external discharge, reducing environmental pollution, and save more than 200,000 yuan in costs associated with neutralization with alkali solution during discharge.

When using nitric acid as a regenerator, the literature indicates that the concentration and flow rate of the nitric acid solution must be strictly controlled during regeneration. Therefore, in 2002, our company collaborated with Northeast Electric Power University and Xuzhou Water Treatment Technology Research Institute to conduct experimental research on the concentration control of nitric acid as a regenerator. Numerous experimental results show that controlling the mass fraction of nitric acid at 2%-2.5% and the flow rate at 5-8 m/h can minimize adverse effects caused by oxidation of the cation exchange resin and ensure that the exchange capacity of the ion exchange resin remains unaffected.

2. Regeneration Method Using Nitric Acid as a Regenerant

Based on the chemical properties of cation exchange resins, when using nitric acid as a regenerant, to prevent damage to the resin due to its strong oxidizing properties, the mass fraction should generally be controlled at 2%-2.5%, and the flow rate at 5-8 m/h. Simultaneously, to ensure the operating cycle of the cation floating bed ion exchanger and the quality of the effluent, the mass fraction of the regenerant should be controlled at 2.4%-2.5%. The working exchange capacity of the cation exchange resin in our floating bed is approximately 800 mol/m³, the cation exchange resin loading is 21.5 t, and the wet apparent density is 0.8 g/mL.

Since the amount of regenerant used during regeneration should be 2-3 times the theoretically calculated amount, the actual acid usage is: 1.524 × 2 = 3.048 t.

The regeneration flow rate during floating bed regeneration is 6 m/h. The cation exchanger is φ3000mm with a flow area of approximately 7 m², resulting in a flow rate of 6 × 7 = 42 m³/h. With a regenerant mass fraction of 2.4%, the regeneration time is approximately 3 hours.

3. Equipment Modification

The original regenerant pipeline used stainless steel, which is resistant to nitric acid corrosion, so it was not replaced. To better control the regenerant flow rate, a DN100 float flowmeter with a range of 0-60 t/h was added after the acid injector.

The original φ3000 cation exchange floating bed had a double-layer δ=3mm rubber lining. Because rubber is prone to aging and cracking under the strong oxidizing effect of nitric acid, losing its anti-corrosion function, a patented technology from the Xuzhou Water Treatment Technology Research Institute was adopted to perform special anti-oxidation treatment on the inner walls of the four cation exchange floating beds, applying an anti-nitric acid coating.

4. Effect Comparison

After more than a month of regeneration and commissioning, the regenerated cation exchange floating bed performed well. Table 1 compares the regeneration effects and regeneration agent costs (under the same influent water quality) using hydrochloric acid and nitric acid as regenerators.

Table 1: Comparison of Regeneration Effects and Costs of Nitric Acid and Hydrochloric Acid

Our plant has used nitric acid as a regenerator for over a year. The resin breakage rate has not changed significantly compared to before, with a slight increase. The annual resin replenishment has increased by about 5 tons compared to before. Each ton of cation exchange resin costs 5000-6000 yuan, resulting in an annual increase of 30,000 yuan in resin replenishment costs. However, compared to the annual savings in regeneration costs, this still represents a significant economic benefit.

5. Conclusion

Compared with hydrochloric acid-regenerated cation exchange beds, the regeneration effects of nitric acid-regenerated cation exchange beds are similar, with a difference of about 3 hours in operating cycle and about 400 tons in cycle water production. Nitric acid regeneration shows better operating results. Using nitric acid as a regenerator is feasible. When using nitric acid as a regenerator, the mass fraction of the regenerator must be strictly controlled between 2% and 2.5%. If it exceeds 2.5%, the cation exchange resin is easily oxidized, resulting in a short operating cycle of the cation exchange bed and unstable effluent quality.

Author Biography: Li Jing'an (1969—), male, from Huainan, Anhui, engineer, Bachelor's degree, Kunming University of Science and Technology, Thermal Engineering, Class of 1988, Tel: (0554) 2692896