The Application of Membrane Separation Technology in the Recycling of Hydrofluoric Acid and Nitric Acid in the Special Steel Industry

In the production process of special steel, the pickling process is an indispensable link. Hydrofluoric acid (HF) and nitric acid (HNO₃), as common pickling agents, play a key role in removing metal surface oxides and improving material surface quality. However, the pickling process generates a large amount of waste liquid containing HF and HNO₃. If such waste liquid is discharged directly without treatment, it will not only cause resource waste but also lead to serious environmental pollution. As an efficient separation and purification method, membrane separation technology has shown great application potential in the recycling of HF and HNO₃ in the special steel industry.

I. Roles of Hydrofluoric Acid and Nitric Acid in Special Steel Pickling and Current Status of Waste Liquid Treatment

(1) Roles of Hydrofluoric Acid and Nitric Acid in Pickling

• Hydrofluoric Acid (HF): It has the characteristics of dissolving silicon-containing compounds and effectively dissolving metal oxides such as aluminum and chromium, making it commonly used for etching castings, stainless steel, and other workpieces.
• Nitric Acid (HNO₃): As a strong oxidizing acid, it can rapidly dissolve the oxide scale on the metal surface and has good corrosion resistance to the steel matrix at appropriate concentrations.
• The synergistic effect of the two can effectively remove impurities and oxide layers from the surface of special steel, improving product quality.

(2) Current Status of Waste Liquid Treatment

• Traditional treatment methods (such as neutralization) can reduce the acidity of waste liquid but cannot realize effective acid recovery, leading to resource waste.
• Pollutants such as metal ions and fluoride ions in the waste liquid will cause serious harm to the environment if improperly treated. Therefore, developing an efficient and environmentally friendly waste liquid treatment method to realize the recycling of HF and HNO₃ has become an urgent problem to be solved in the special steel industry.

II. Principles and Advantages of Membrane Separation Technology in the Recycling of Hydrofluoric Acid and Nitric Acid

(1) Principles of Membrane Separation Technology

Membrane separation technology is a method of separating and purifying different components in a mixture by using the selective permeability of semi-permeable membranes. Commonly used membrane separation technologies for HF and HNO₃ recycling include:

 

• Reverse Osmosis (RO) Membrane Technology: RO membranes have extremely small pores that block most solutes and allow only small molecules (such as water) to pass through, realizing effective separation of acid liquid and impurities.
• Bipolar Membrane Electrodialysis (BMED) Technology: Using the property of bipolar membranes to dissociate water into hydrogen ions (H⁺) and hydroxide ions (OH⁻) under direct current electric fields, it realizes the separation of acid anions from metal salt ions and the regeneration of acids.

(2) Advantages of Membrane Separation Technology

1. High-efficiency Separation: It can efficiently separate and purify HF and HNO₃, and the recovered acid liquid has high purity, which can be directly reused in the pickling process to improve resource utilization.
2. Environmental Protection and Energy Saving: Compared with traditional waste liquid treatment methods, it does not require adding a large number of chemical reagents, reducing secondary pollution. Meanwhile, it has low energy consumption, meeting the requirements of energy conservation and emission reduction.
3. Simple Operation: The equipment has a high degree of automation, simple operation, and low maintenance cost, enabling continuous and stable operation and improving production efficiency.

III. Specific Applications of Membrane Separation Technology in the Recycling of Hydrofluoric Acid and Nitric Acid in the Special Steel Industry

(1) Application of Reverse Osmosis Membrane Technology in Preliminary Acid Liquid Purification

• RO technology can be used for the preliminary purification of pickling waste liquid in special steel plants. Through the selective permeability of RO membranes, most water and partial small-molecule impurities in the waste liquid are removed, obtaining a high-concentration acid liquid. After further treatment, this acid liquid can be used as a supplementary liquid for the pickling process, reducing the consumption of new acid.
• Case: In a pickling waste liquid treatment project of a special steel enterprise, RO technology was used to treat the waste liquid, successfully increasing the water removal rate to over 90%, and the concentration of the recovered acid liquid reached about 80% of the original acid liquid, realizing partial recycling of the acid liquid.

(2) Application of Bipolar Membrane Electrodialysis Technology in Acid Anion Recovery and Acid Regeneration

• BMED technology is a key technology for recycling HF and HNO₃ in the special steel industry. It dissociates water into H⁺ and OH⁻ through bipolar membranes. Under the DC electric field, H⁺ combines with acid anions to regenerate acids, while metal salt ions migrate to the other electrode to form metal hydroxide precipitates, realizing effective separation of acid anions from metal salt ions.
• Case: A special steel enterprise adopted BMED technology to treat pickling waste liquid, successfully realizing the regeneration and recycling of HNO₃ and HF, saving millions of yuan in acid procurement costs annually, while reducing waste liquid discharge and environmental pollution risks. The acid recovery rate can reach over 90%.

(3) Coupling Application of Membrane Separation Technology with Other Technologies

• To improve the efficiency and quality of HF and HNO₃ recycling, membrane separation technology is often coupled with other technologies:
  • Combined with diffusion dialysis technology: Diffusion dialysis first recovers most free acids from waste liquid, and then membrane separation technology further treats the dialyzed residual liquid to improve acid recovery.
  • Combined with evaporation crystallization, ion exchange, and other technologies: Realize comprehensive recovery and utilization of multiple components in waste liquid.

IV. Challenges and Solutions in the Application of Membrane Separation Technology

(1) Membrane Fouling Problem

• Impact: Membrane fouling leads to decreased membrane flux, reduced separation efficiency, increased frequency of membrane cleaning and replacement, and higher operation and maintenance costs.
• Solutions:
  1. Optimize Pretreatment Process: Pretreat pickling waste liquid before membrane separation to remove suspended solids, colloids, macromolecular organic matter, and other impurities, reducing the pollution load on membrane modules. For example, using a double-layer filtration system can effectively reduce the solid mass fraction in waste liquid.
  2. Develop New Anti-fouling Membrane Materials: Invest in R&D of new membrane materials with high anti-fouling performance and long service life to improve membrane stability and durability. For example, membranes with special surface properties can reduce the adsorption and deposition of impurities.
  3. Optimize Membrane Cleaning Process: Select appropriate cleaning agents and methods based on the type and degree of membrane fouling, and regularly clean membrane modules to restore flux and separation performance. Combining chemical and physical cleaning methods is effective.

(2) Technical Cost Problem

• Challenge: The equipment investment and operation costs of membrane separation technology are relatively high, limiting its promotion in some small and medium-sized special steel enterprises.
• Solutions:
  1. Optimize Process Design: Reduce equipment investment and operation costs by optimizing process flows and equipment selection. For example, reasonably selecting membrane module types and specifications to improve membrane utilization and treatment efficiency; optimizing BMED operation parameters to reduce energy consumption.
  2. Strengthen Localization of Equipment: Increase R&D efforts for localization of key equipment and technologies to reduce procurement costs, and improve equipment reliability and stability to reduce maintenance and replacement costs.
  3. Policy Support and Subsidies: Governments can introduce policies to provide financial support and tax incentives for special steel enterprises adopting membrane separation technology for HF and HNO₃ recycling, encouraging the application of advanced waste liquid treatment technologies.

V. Conclusion and Prospect

• Conclusion: Membrane separation technology has important application value in the recycling of HF and HNO₃ in the special steel industry. Through technologies such as RO and BMED, it can efficiently recover and purify HF and HNO₃ from pickling waste liquid, improve resource utilization, and reduce environmental pollution risks. Although challenges like membrane fouling and technical costs exist, they can be effectively addressed by optimizing pretreatment, developing new anti-fouling membranes, and improving process design.
• Prospect: With the continuous development and innovation of membrane separation technology, its application in HF and HNO₃ recycling for special steel will become broader, promising to provide strong support for the sustainable development of the special steel industry.