Analysis of HNO₃ and HF Membrane Separation and Recovery Technology in Polysilicon Production

Polysilicon, a key raw material for semiconductor and solar photovoltaic industries, involves extensive use of HNO₃ and HF in etching and cleaning processes during production. Wastewater containing these acids, if discharged directly, not only causes resource waste but also severe environmental pollution. Therefore, developing efficient and environmentally friendly HNO₃ and HF recovery technologies is of significant practical importance. Membrane separation technology, as a novel separation method, has shown broad application prospects in the recovery of HNO₃ and HF in polysilicon production.

1. Principles of Membrane Separation and Recovery Technology

Membrane separation technology utilizes special semipermeable membranes to selectively separate, concentrate, and purify solutes or solvents in solutions. In the recovery of HNO₃ and HF in polysilicon production, the technology primarily relies on the selective permeability of membranes to different ions to achieve acid separation. For example, ion exchange membranes can selectively allow ions such as H⁺, NO₃⁻, and F⁻ to pass through based on their charge properties and sizes, while blocking other impurity ions.

2. Process Flow of Membrane Separation and Recovery

2.1 Pretreatment

Polysilicon production wastewater typically contains large amounts of suspended solids, metal ions, and other impurities, which can foul and clog membranes, affecting their service life and separation efficiency. Therefore, wastewater must undergo pretreatment before membrane separation and recovery. Pretreatment methods include filtration, precipitation, and pH adjustment. Filtration removes suspended solids, precipitation eliminates partial metal ions, and pH adjustment ensures the acid concentration in the wastewater is within an optimal range for subsequent membrane separation and recovery.

2.2 Membrane Separation

The pretreated wastewater enters the membrane separation system for separation. Depending on the separation objectives and requirements, different types of membrane separation technologies can be selected, such as dialysis, electrodialysis, and reverse osmosis.

 

• Dialysis: Driven by a concentration gradient, dialysis allows ions like H⁺, NO₃⁻, and F⁻ in the wastewater to pass through ion exchange membranes to the recovery side, while other impurity ions are retained on the original solution side. Dialysis does not require an external electric field and has low energy consumption, but its separation efficiency is relatively low.
• Electrodialysis: Under the action of a direct current electric field, electrodialysis uses anion and cation exchange membranes to enable the directional migration of anions and cations (e.g., H⁺, NO₃⁻, F⁻) in the wastewater to the recovery side, achieving acid recovery. Electrodialysis features high separation efficiency and high-purity recovered acids but has relatively high energy consumption.
• Reverse Osmosis (RO): RO uses pressure to drive water molecules to reverse osmose, concentrating acids in the wastewater on the recovery side while allowing water molecules to pass through the membrane to the freshwater side. RO effectively removes dissolved solids, organic matter, and other impurities from the wastewater, yielding high-purity recovered acids, but requires high operating pressure.

2.3 Post-treatment

The recovered acids obtained after membrane separation may contain trace impurities and require further post-treatment. Post-treatment methods include evaporation crystallization and ion exchange. Evaporation crystallization removes water from the recovered acids to obtain high-purity nitric acid and hydrofluoric acid crystals, while ion exchange eliminates trace impurity ions to improve acid purity.

3. Advantages of Membrane Separation and Recovery Technology

3.1 High-Efficiency Recovery

Membrane separation technology can achieve efficient separation based on the property differences between HNO₃, HF, and other substances in wastewater, resulting in high recovery rates. For example, using dialysis to recover HNO₃ and HF from polysilicon production wastewater can achieve a recovery rate of over 80%.

3.2 Energy Saving and Environmental Protection

Compared with traditional acid recovery methods, membrane separation technology does not require high-temperature or high-pressure conditions, resulting in low energy consumption. Additionally, the membrane separation process produces no secondary pollution, meeting environmental protection requirements.

3.3 Simple Operation

Membrane separation equipment has a simple structure, is easy to operate, and can be easily automated. Operators only need to follow set parameters to recover HNO₃ and HF.

3.4 Resource Recycling

Recovered HNO₃ and HF through membrane separation technology can be reused in polysilicon production, achieving resource recycling and reducing production costs.

4. Challenges Facing Membrane Separation and Recovery Technology

4.1 Membrane Fouling

Polysilicon production wastewater contains numerous impurities that easily deposit on the membrane surface, leading to membrane fouling. Membrane fouling reduces membrane flux and separation efficiency and increases membrane cleaning and replacement costs.

4.2 Membrane Material Performance

Current membrane materials used for HNO₃ and HF recovery still have shortcomings, such as the need to further improve acid resistance and selective permeability. Developing high-performance membrane materials is critical to enhancing the efficiency of membrane separation and recovery technology.

4.3 Process Optimization

Optimizing the membrane separation and recovery process is essential for improving recovery efficiency and reducing costs. Currently, there are still areas for improvement in the process, such as pretreatment processes and membrane separation operation parameters.

5. Practical Application Case

A polysilicon production enterprise adopted dialysis to recover HNO₃ and HF from production wastewater. The enterprise first pretreated the wastewater to remove suspended solids and partial metal ions, then introduced the pretreated wastewater into dialysis equipment for separation. After a period of operation, the enterprise successfully recovered a large amount of HNO₃ and HF, with a recovery rate exceeding 85%. The recovered acids, after post-treatment, achieved the purity required for production and were reused in polysilicon production. This not only reduced production costs but also decreased wastewater discharge, yielding significant economic and environmental benefits.

 

Membrane separation technology holds significant application value in the recovery of HNO₃ and HF in polysilicon production. While it offers advantages such as high-efficiency recovery, energy saving and environmental protection, simple operation, and resource recycling, it also faces challenges like membrane fouling, membrane material performance, and process optimization. In the future, further research and development of membrane separation technology are needed to improve membrane performance and stability, optimize recovery processes, and reduce costs. Additionally, promoting the application of membrane separation and recovery technology will facilitate the green and sustainable development of the polysilicon industry.

 

With continuous technological advancements and increasingly stringent environmental requirements, membrane separation technology will have a broader application 前景 in the recovery of HNO₃ and HF in polysilicon production. It is believed that in the near future, membrane separation technology will become the mainstream method for acid recovery in polysilicon production, making greater contributions to the development of the polysilicon industry.