Nanofiltration (NF) Membrane Process for Phosphonic Acid Separation and Purification
07 Aug 2025
Here is a professional English translation of the Nanofiltration (NF) Membrane Process for Phosphonic Acid Separation and Purification tailored for high-purity applications in electronics and flame retardant synthesis.
Process Fundamentals
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Feed Solution Characteristics:
- Phosphonic acid (H₃PO₃): 10–30 wt% │ pH 1.5–3.0
- Major impurities: Na⁺/K⁺ (≤5,000 ppm), SO₄²⁻ (≤1,000 ppm), Cl⁻ (≤2,000 ppm)
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Target Product:
- High-purity phosphonic acid: H₃PO₃ ≥40 wt%, total impurities ≤100 ppm
- Recovery rate ≥90%
- Core Technology: Multi-stage NF Fractional Separation & Concentration System
Process Flow Diagram
graph LR
A[Feed Tank] --> B[Pretreatment System]
B --> C[Primary NF Stage (NF1)]
C --> D[Secondary NF Stage (NF2)]
D --> E[Phosphonic Acid Concentrate]
C --> F[Salt Permeate]
D --> G[Permeate Reuse]
F --> H[Salt Recovery]I. Core Process Units
1. Pretreatment System (Membrane Protection)
| Unit | Parameters | Function |
|---|---|---|
| Precision Filtration | 0.5 μm titanium filter | Remove particulates (prevent abrasion) |
| Cooling System | Plate heat exchanger (5–10°C) | Inhibit oxidation (H₃PO₃→H₃PO₄) |
| Reducing Agent Dosing | Ascorbic acid (50–100 ppm) | Maintain reducing environment (ORP≤-50 mV) |
| pH Adjustment | H₃PO₃ (no foreign ions) | Stabilize pH at 2.0±0.2 |
Critical Control Points: Temperature≤10°C, SDI₁₅<3, zero suspended solids
2. NF Fractionation System (Dual-Stage Concentration)
| Parameter | Primary NF (NF1) | Secondary NF (NF2) |
|---|---|---|
| Membrane Type | Acid-resistant (e.g., Duracid NF) | Tight separation (e.g., NF270) |
| Rejection Mechanism | Charge repulsion (+ve surface) | Size exclusion (MWCO: 200–300 Da) |
| Operating Pressure | 2.0–2.5 MPa | 1.5–2.0 MPa |
| H₃PO₃ Rejection | ≥95% | ≥98% |
| Monovalent Ion Passage | Cl⁻≥90%, Na⁺≥85% | Cl⁻≥95%, Na⁺≥92% |
| Concentration Factor | 3–5× (feed→concentrate) | Final conc. to 40 wt% |
Separation Mechanism:
- NF1: Positively charged membrane rejects multivalent anions (SO₄²⁻) while permeating monovalent ions (Cl⁻, Na⁺).
- NF2: Small-pore membrane deeply retains H₃PO₃ (MW=82 g/mol) while removing residual salts.
3. Permeate Treatment
- Salt Recovery: NF1 permeate → Electrodialysis (ED) → Separate NaCl/Na₂SO₄.
- Permeate Reuse: NF2 permeate (TDS≤500 ppm) recycled for cleaning or formulation.
II. Equipment Specifications
| Equipment | Specifications | Qty |
|---|---|---|
| NF Membrane Elements | Acid-resistant spiral-wound, 4"×40", 7.9 m² | NF1: 6, NF2: 4 |
| High-Pressure Pump | Q=5 m³/h, H=250 m, Hastelloy C276 | 3 (VFD) |
| Pressure Vessels | FRP, pH 0–14 resistant | 10 |
| In-line Conductivity Meter | ±1% FS, acid-proof | 6 |
| Automated Valve Bank | PTFE-lined diaphragm valves | 1 set |
III. Fouling Control & Stability
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Flow Dynamics:
- Cross-flow velocity ≥1.0 m/s minimizes polarization and crystallization.
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Cleaning Protocol:
- Acid wash: 0.5% HNO₃ (remove scales, every 24h).
- Reducing wash: 1% NaHSO₃ (prevent oxidation fouling, weekly).
- Flux recovery ≥98% post-cleaning.
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Material Protection:
- Temperature ≤35°C to prevent PVDF/PES hydrolysis.
- Full-system N₂ blanketing to exclude O₂.
IV. Performance & Economics
| Metric | Performance |
|---|---|
| Phosphonic Acid Purity | ≥99.5 wt% |
| Cl⁻ Removal | ≥99.8% (2,000 ppm→≤5 ppm) |
| SO₄²⁻ Removal | ≥99.9% (1,000 ppm→≤1 ppm) |
| Specific Energy | 15–18 kWh/m³ (main pumps) |
| OPEX | 11–18 USD/m³ feed |
Cost Breakdown:
- Membrane replacement: 3.7 USD/m³ (≥2-year lifespan)
- Chemicals: 3 USD/m³ (ascorbic acid/HNO₃/NaHSO₃)
- Energy: 6 USD/m³
V. Critical Issue Mitigation
1. H₃PO₃ Oxidation Control
- Process design: Low temperature (5–10°C) + reducing agents.
- Emergency response: Auto liquid-N₂ injection when ORP>0.
2. Salt Scaling Risk
-
Concentration limits:
- NF1 concentration factor ≤5× if Ca²⁺≤50 ppm.
- Realtime LSI monitoring with auto-dilution.
3. Deep Impurity Removal
- Polishing option: NF concentrate → Continuous Electrodeionization (CEDI) → Total ions ≤10 ppm.
Advantages vs. Evaporation Crystallization
| Metric | NF Membrane Process | Evaporation Crystallization |
|---|---|---|
| H₃PO₃ Decomposition | <0.1% | 5–20% (thermal degradation) |
| Energy Consumption | 18 kWh/m³ | 150–200 kWh/m³ |
| Product Purity | ≥99.5 wt% | 95–98 wt% (salt entrapment) |
| Capital Investment | Moderate (membrane-focused) | High (evaporators/vacuum) |
Implementation Guidelines
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Pilot Testing:
- Conduct 200 L/h tests; optimize membrane selection (Toray TX40/DOW NF90 recommended).
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Material Compatibility:
- Piping/tanks: High-purity PVDF or PTFE-lined.
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Automation:
- DCS monitoring: Temperature/pH/ORP/pressure/conductivity with auto-shutdown.
Extended Applications:
- Sodium hypophosphite purification │ Flame retardant intermediates │ Semiconductor etching waste recovery
This process is field-proven in a 3,000-ton/year electronic-grade phosphonic acid plant, demonstrating <5% flux decline over 18 months. Final product compliance: SEMI C36 standard for chip manufacturing.
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