6N Ultra-High-Purity Sulfur Distillation and Purification Process with Detailed Parameters‌

The production of 6N (≥99.9999% purity) ultra-high-purity sulfur requires multi-stage distillation, deep adsorption, and ultra-clean filtration to eliminate trace metals, organic impurities, and particulates. Below is an industrial-scale process integrating vacuum distillation, microwave-assisted purification, and precision post-treatment technologies.

‌I. Raw Material Pretreatment and Impurity Removal‌

‌1. Raw Material Selection and Pretreatment‌

• ‌Requirements‌: Initial sulfur purity ≥99.9% (3N grade), total metal impurities ≤500 ppm, organic carbon content ≤0.1%.
• ‌Microwave-Assisted Melting‌:
  Crude sulfur is processed in a microwave reactor (2.45 GHz frequency, 10–15 kW power) at 140–150°C. Microwave-induced dipole rotation ensures rapid melting while decomposing organic impurities (e.g., tar compounds). Melting time: 30–45 minutes; microwave penetration depth: 10–15 cm
• ‌Deionized Water Washing‌:
  Molten sulfur is mixed with deionized water (resistivity ≥18 MΩ·cm) at a 1:0.3 mass ratio in a stirred reactor (120°C, 2 bar pressure) for 1 hour to remove water-soluble salts (e.g., ammonium sulfate, sodium chloride). The aqueous phase is decanted and reused for 2–3 cycles until conductivity ≤5 μS/cm.

‌2. Multi-Stage Adsorption and Filtration‌

• ‌Diatomaceous Earth/Activated Carbon Adsorption‌:
  Diatomaceous earth (0.5–1%) and activated carbon (0.2–0.5%) are added to molten sulfur under nitrogen protection (130°C, 2-hour stirring) to adsorb metal complexes and residual organics
• ‌Ultra-Precision Filtration‌:
  Two-stage filtration using titanium sintered filters (0.1 μm pore size) at ≤0.5 MPa system pressure. Post-filtration particulate count: ≤10 particles/L (size >0.5 μm).

‌II. Multi-Stage Vacuum Distillation Process‌

‌1. Primary Distillation (Metal Impurity Removal)‌

• ‌Equipment‌: High-purity quartz distillation column with 316L stainless steel structured packing (≥15 theoretical plates), vacuum ≤1 kPa .
• ‌Operational Parameters‌:
• ‌Feed Temperature‌: 250–280°C (sulfur boils at 444.6°C under ambient pressure; vacuum reduces boiling point to 260–300°C).
• ‌Reflux Ratio‌: 5:1–8:1; column top temperature fluctuation ≤±0.5°C.
• ‌Product‌: Condensed sulfur purity ≥99.99% (4N grade), total metal impurities (Fe, Cu, Ni) ≤1 ppm.

‌2. Secondary Molecular Distillation (Organic Impurity Removal)‌

• ‌Equipment‌: Short-path molecular distiller with 10–20 mm evaporation-condensation gap, evaporation temperature 300–320°C, vacuum ≤0.1 Pa .
• ‌Impurity Separation‌:
  Low-boiling organics (e.g., thioethers, thiophene) are vaporized and evacuated, while high-boiling impurities (e.g., polyaromatics) remain in residues due to differences in molecular free path.
• ‌Product‌: Sulfur purity ≥99.999% (5N grade), organic carbon ≤0.001%, residue rate <0.3%.

‌3. Tertiary Zone Refining (Achieving 6N Purity)‌

• ‌Equipment‌: Horizontal zone refiner with multi-zone temperature control (±0.1°C), zone travel speed 1–3 mm/h.
• ‌Segregation‌:
  Utilizing segregation coefficients (K=Csolid/CliquidK=Csolid​/Cliquid​), 20–30 zone passes concentrate metals (As, Sb) at the ingot end. The final 10–15% of the sulfur ingot is discarded.

‌III. Post-Treatment and Ultra-Clean Forming‌

‌1. Ultra-Pure Solvent Extraction‌

• ‌Ether/Carbon Tetrachloride Extraction‌:
  Sulfur is mixed with chromatographic-grade ether (1:0.5 volume ratio) under ultrasonic assistance (40 kHz, 40°C) for 30 minutes to remove trace polar organics .
• ‌Solvent Recovery‌:
  Molecular sieve adsorption and vacuum distillation reduce solvent residues to ≤0.1 ppm.

‌2. Ultrafiltration and Ion Exchange‌

• ‌PTFE Membrane Ultrafiltration‌:
  Molten sulfur is filtered through 0.02 μm PTFE membranes at 160–180°C and ≤0.2 MPa pressure.
• ‌Ion Exchange Resins‌:
  Chelating resins (e.g., Amberlite IRC-748) remove ppb-level metal ions (Cu²⁺, Fe³⁺) at 1–2 BV/h flow rates.

‌3. Ultra-Clean Environment Forming‌

• ‌Inert Gas Atomization‌:
  In a Class 10 cleanroom, molten sulfur is atomized with nitrogen (0.8–1.2 MPa pressure) into 0.5–1 mm spherical granules (moisture <0.001%).
• ‌Vacuum Packaging‌:
  Final product is vacuum-sealed in aluminum composite film under ultra-pure argon (≥99.9999% purity) to prevent oxidation.

‌IV. Key Process Parameters‌

‌V. Quality Control and Testing‌

1. ‌Trace Impurity Analysis‌:

• ‌GD-MS (Glow Discharge Mass Spectrometry)‌: Detects metals at ≤0.01 ppb.
• ‌TOC Analyzer‌: Measures organic carbon ≤0.001 ppm .

1. ‌Particle Size Control‌:
   Laser diffraction (Mastersizer 3000) ensures D50 deviation ≤±0.05 mm.
2. ‌Surface Cleanliness‌:
   XPS (X-ray Photoelectron Spectroscopy) confirms surface oxide thickness ≤1 nm .

‌VI. Safety and Environmental Design‌

1. ‌Explosion Prevention‌:
   Infrared flame detectors and nitrogen flooding systems maintain oxygen levels <3%
2. ‌Emission Control‌:

• ‌Acid Gases‌: Two-stage NaOH scrubbing (20% + 10%) removes ≥99.9% H₂S/SO₂.
• ‌VOCs‌: Zeolite rotor + RTO (850°C) reduces non-methane hydrocarbons to ≤10 mg/m³ .

1. ‌Waste Recycling‌:
   High-temperature reduction (1200°C) recovers metals; residue sulfur content <0.1% .

‌VII. Techno-Economic Metrics‌

• ‌Energy Consumption‌: 800–1200 kWh electricity and 2–3 tons steam per ton of 6N sulfur.
• ‌Yield‌: Sulfur recovery ≥85%, residue rate <1.5%.
• ‌Cost‌: Production cost ~120,000–180,000 CNY/ton; market price 250,000–350,000 CNY/ton (semiconductor grade) .

This process produces 6N sulfur for semiconductor photoresists, III-V compound substrates, and other advanced applications. Real-time monitoring (e.g., LIBS elemental analysis) and ISO Class 1 cleanroom calibration ensure consistent quality.

Footnotes

1. Reference 2: Industrial Sulfur Purification Standards
2. Reference 3: Advanced Filtration Techniques in Chemical Engineering
3. Reference 6: High-Purity Materials Processing Handbook
4. Reference 8: Semiconductor-Grade Chemical Production Protocols
5. Reference 5: Vacuum Distillation Optimization