Selenium, as an important semiconductor material and industrial raw material, has its performance directly affected by its purity. During the vacuum distillation purification process, oxygen impurities are one of the main factors influencing selenium purity. This article provides a detailed discussion of various methods and techniques for reducing oxygen content during selenium purification via vacuum distillation.

I. Reducing Oxygen Content in the Raw Material Pretreatment Stage

1. Preliminary Purification of Raw Materials

Raw selenium typically contains various impurities, including oxides. Before entering the vacuum distillation system, chemical cleaning methods should be employed to remove surface oxides. Commonly used cleaning solutions include:

• Dilute hydrochloric acid solution (5-10% concentration): Effectively dissolves oxides such as SeO₂
• Ethanol or acetone: Used to remove organic contaminants
• Deionized water: Multiple rinses to remove residual acid

After cleaning, drying should be performed under an inert gas (e.g., Ar or N₂) atmosphere to prevent re-oxidation.

2. Pre-Reduction Treatment of Raw Materials

Reduction treatment of the raw material before vacuum distillation can significantly reduce oxygen content:

• Hydrogen reduction: Introduce high-purity hydrogen (purity ≥99.999%) at 200-300°C to reduce SeO₂ to elemental selenium
• Carbothermal reduction: Mix selenium raw material with high-purity carbon powder and heat to 400-500°C under vacuum or inert atmosphere, inducing the reaction C + SeO₂ → Se + CO₂
• Sulfide reduction: Gases such as H₂S can reduce selenium oxides at relatively low temperatures

II. Design and Operational Optimization of the Vacuum Distillation System

1. Selection and Configuration of the Vacuum System

A high-vacuum environment is critical for reducing oxygen content:

• Use a diffusion pump + mechanical pump combination, with an ultimate vacuum reaching at least 10⁻⁴ Pa
• The system should be equipped with a cold trap to prevent back-diffusion of oil vapor
• All connections should use metal seals to avoid outgassing from rubber seals
• The system should undergo sufficient bake-out degassing (200-250°C, 12-24 hours)

2. Precise Control of Distillation Temperature and Pressure

Optimal process parameter combinations:

• Distillation temperature: Controlled within the range of 220-280°C (below selenium’s boiling point of 685°C)
• System pressure: Maintained between 1-10 Pa
• Heating rate: 5-10°C/min to avoid violent evaporation and entrainment
• Condensation zone temperature: Maintained at 50-80°C to ensure complete selenium condensation

3. Multi-Stage Distillation Technology

Multi-stage distillation can progressively reduce oxygen content:

• First stage: Rough distillation to remove most volatile impurities
• Second stage: Precise temperature control to collect the main fraction
• Third stage: Low-temperature, slow distillation to obtain high-purity product
  Different condensation temperatures can be used between stages for fractional condensation

III. Auxiliary Process Measures

1. Inert Gas Protection Technology

Although operating under vacuum, the appropriate introduction of high-purity inert gas helps reduce oxygen content:

• After evacuating the system, fill with high-purity argon (purity ≥99.9995%) to 1000 Pa
• Use dynamic gas flow protection, continuously introducing a small amount of argon (10-20 sccm)
• Install high-efficiency gas purifiers at gas inlets to remove residual oxygen and moisture

2. Addition of Oxygen Scavengers

Adding appropriate oxygen scavengers to the raw material can effectively reduce oxygen content:

• Magnesium metal: Strong affinity for oxygen, forming MgO
• Aluminum powder: Can simultaneously remove oxygen and sulfur
• Rare earth metals: Such as Y, La, etc., with excellent oxygen removal effects
  The amount of oxygen scavenger is typically 0.1-0.5 wt% of the raw material; excess amounts may affect selenium purity

3. Molten Filtration Technology

Filtering molten selenium before distillation:

• Use quartz or ceramic filters with pore sizes of 1-5 μm
• Control filtration temperature at 220-250°C
• Can remove solid oxide particles
• Filters should be pre-degassed under high vacuum

IV. Post-Treatment and Storage

1. Product Collection and Handling

• The condenser collector should be designed as a detachable structure for easy material retrieval in an inert environment
• Collected selenium ingots should be packaged in an argon glove box
• Surface etching may be performed if necessary to remove potential oxide layers

2. Storage Condition Control

• Storage environment should be kept dry (dew point ≤-60°C)
• Use double-layer sealed packaging filled with high-purity inert gas
• Recommended storage temperature below 20°C
• Avoid light exposure to prevent photocatalytic oxidation reactions

V. Quality Control and Testing

1. Online Monitoring Technology

• Install residual gas analyzers (RGA) to monitor oxygen partial pressure in real-time
• Use oxygen sensors to control oxygen content in protective gases
• Employ infrared spectroscopy to identify characteristic absorption peaks of Se-O bonds

2. Finished Product Analysis

• Use inert gas fusion-infrared absorption method to determine oxygen content
• Secondary ion mass spectrometry (SIMS) to analyze oxygen distribution
• X-ray photoelectron spectroscopy (XPS) to detect surface chemical states

Through the comprehensive measures described above, oxygen content can be controlled below 1 ppm during vacuum distillation purification of selenium, meeting the requirements for high-purity selenium applications. In actual production, process parameters should be optimized based on equipment conditions and product requirements, and a strict quality control system should be established.