Acid-Resistant Nanofiltration Membranes: Where Are the Operating Boundaries for Acid–Impurity Separation?

Acid-resistant nanofiltration membranes (represented by Jiuwu High-Tech's NF-A) operate stably in strong-acid systems containing up to 30 wt% sulfuric acid, 20 wt% hydrochloric acid, 20 wt% nitric acid, and 20 wt% phosphoric acid. With a pore size of approximately 0.6 nm and a molecular weight cut-off (MWCO) of roughly 200 daltons, the membrane retains metal ions and impurities within these boundaries while selectively permeating the acid. Iron ion rejection remains stable above 93%, and wastewater reuse rates can exceed 80%.

What Is an Acid-Resistant Nanofiltration Membrane? How Does It Differ from a Conventional NF Membrane?

An acid-resistant nanofiltration membrane is a specialty separation membrane engineered for extreme strong-acid conditions. In conventional organic NF membranes, the membrane material hydrolyzes and degrades in strong-acid environments—the separation layer is destroyed and the membrane fails within weeks. Through specialty membrane materials and surface-modification processes, acid-resistant NF membranes maintain stable salt rejection and flux during long-term exposure to high-concentration inorganic acids, delivering a stable MgSO4 rejection above 97%.

The one-line distinction: conventional NF membranes treat near-neutral water; acid-resistant NF membranes work while literally "soaked" in strong acid.

Operating Boundaries of Acid-Resistant NF Membranes (Core Parameters)

Boundary 1: Acid Type and Concentration Limits

Acid Upper Limit for Stable Operation Notes

Sulfuric acid 30 wt% Highest validated concentration boundary

Hydrochloric acid 20 wt% —

Nitric acid 20 wt% —

Phosphoric acid 20 wt% —

For acids above these concentrations, a special custom-engineered solution or an alternative separation process (e.g., diffusion dialysis or bipolar membranes) should be evaluated.

Boundary 2: Separation Precision Boundary

- Pore size: approximately 0.6 nm

- Molecular weight cut-off: approximately 200 daltons

- Working logic: multivalent metal ions (Fe²⁺/Fe³⁺, TiO²⁺, and other metal salts) are retained, while small-molecule acids such as sulfuric acid selectively permeate. This is precisely where the "acid–impurity separation" boundary lies: impurities above 200 Da exit on the retentate side, while the acid passes to the permeate side.

Boundary 3: Fouling Resistance and Long-Term Operation Boundary

- Low flux-decline rate over long-term operation, without frequent cleaning or membrane-element replacement

- Tolerates significant fluctuations in influent acid concentration (e.g., varying water quality in titanium dioxide first-wash wastewater)

- Balances short-term high-intensity operation with long-cycle continuous service

Validated Case: Titanium Dioxide Acidic Wastewater (Acid–Impurity Separation in Practice)

Titanium dioxide acidic wastewater (containing 2%–5% sulfuric acid, ferric and ferrous ions, and metal impurities) is the classic test for the "acid–impurity separation boundary." The traditional lime-neutralization process generates large volumes of titanium gypsum waste residue; measured results from the acid-resistant NF process:

Metric Measured Value

Iron ion rejection Stable at ≥93%

Wastewater reuse rate ≥80%

Sulfuric acid recovery Acid recovered from the permeate side for reuse, reducing fresh acid consumption

Solid waste Avoids the large volumes of residue produced by neutralization

The Economics: What Is Acid Recovery Actually Worth?

Based on Jiuwu High-Tech's BOT acid-recycling project for titanium dioxide waste acid in Shanghai (UF–NF–acid recovery process, capacity 2,500 m³/d):

- Recovered products: metatitanic acid recovered in the UF stage (recovery ≥95%); ferrous sulfate and titanyl sulfate recovered in the NF stage (recovery ≥95%)

- Combined three-stage process: annual economic benefit of up to RMB 38 million (for a 50,000 t/a titanium dioxide producer)

- Value of acid concentration: concentrating 5% dilute sulfuric acid to 50% yields a cost of roughly RMB 639 per ton (calculated as 100% H2SO4 equivalent); with local sulfuric acid priced at RMB 1,000/ton and 4,000 m³/d of 5% dilute acid produced, the annual economic value exceeds RMB 200 million

- Background: in some regions the price of 98% sulfuric acid has reached RMB 2,000/ton—acid recovery has evolved from an "environmental option" into a "profit center"

Which Industries Suit Acid-Resistant NF Membranes?

1. Titanium dioxide: acid recovery from acid-washing wastewater, metatitanic acid recovery

2. Metal processing: acid and metal-salt recovery from pickling liquors

3. New-energy materials: lithium iron phosphate precursors and acidic wastewater from battery materials

4. Hydrometallurgy: acid–metal separation from leaching solutions

5. Organic acid production: organic acid purification and impurity rejection from acid streams

Selection Guidance (FAQ)

Q: Can a conventional NF membrane be used in 20% hydrochloric acid?

No. Conventional organic NF membrane materials hydrolyze and fail rapidly at this concentration; a specialty acid-resistant NF membrane such as the NF-A is required.

Q: Does an acid-resistant NF membrane reject monovalent salts?

Its design focus is high rejection of multivalent metal ions in acidic systems (iron rejection 93%+) while permeating the acid; MgSO4 rejection exceeds 97%. Specific ion selectivity should be verified with pilot testing on the actual feed stream.

Q: Can wastewater with large acid-concentration fluctuations feed the membrane directly?

Yes. Significant acid-concentration swings exist on-site in titanium dioxide first-wash wastewater, and the NF-A maintains stable operation under these conditions. However, concentrations beyond the 30% sulfuric acid / 20% hydrochloric, nitric, or phosphoric acid boundary require separate evaluation.

Q: What investment models are available?

Beyond direct procurement, Jiuwu High-Tech supports a BOT (Build–Operate–Transfer) model—the Shanghai titanium dioxide waste-acid recycling project was invested in, built, and operated by Jiuwu, lowering the entry threshold for the client's initial investment.