Arsenic distillation and purification process

The arsenic distillation and purification process is a method that uses the difference in the volatility of arsenic and its compounds to separate and purify, especially suitable for removing sulfur, selenium, tellurium and other impurities in arsenic. Here are the key steps and considerations:

1. Raw material pretreatment

• Sources of crude arsenic: usually as a by-product of smelting of arsenic-containing minerals (e.g. arsenite, realgar) or recycled arsenic-containing waste.
• Oxidative roasting (optional): If the raw material is arsenic sulfide (e.g. As₂S₃), it needs to be roasted first to convert to volatile As₂O₃

As2S3+9O2→As2O3+3SO2As2​S3​+9O2​→As2 ​O3​+3SO2​

2. Distillation unit

• Equipment: Quartz or ceramic still (corrosion resistant, high temperature resistant), equipped with condenser tube and receiving bottle.
• Inert protection: Nitrogen or carbon dioxide is introduced to prevent arsenic oxidation or explosion risk (arsenic vapor is flammable).

3. Distillation process

• Temperature control:
  • Arsenic sublimation: As₂O₃ sublimation at 500-600 °C (pure arsenic sublimation at about 615 °C).
  • Impurity separation: low-boiling impurities such as sulfur and selenium are preferentially volatilized and can be separated by segmented condensation.
• Condensation collection: Arsenic vapor condenses into high-purity As₂O₃ or elemental arsenic in the condensation zone (100-200°C).

4. Post-processing

• Reduction (if elemental arsenic is required): Reduction of As₂O₃ with carbon or hydrogen

As2O3+3H2→2As+3H2OAs2​O3​+3H2​→2As+3H2 ​O

• Vacuum distillation: further purification of elemental arsenic to remove residual volatile impurities.

5. Precautions

• Toxicity protection: The whole process is closed operation, equipped with arsenic leakage detection and emergency treatment equipment.
• Tail gas treatment: After condensation, the tail gas needs to be absorbed by lye solution (such as NaOH) or activated carbon adsorption to avoid As₂O₃ emissions.
• Arsenic metal storage: stored in an inert atmosphere to prevent oxidation or deliquescent.

6. Purity Enhancement

• Multi-stage distillation: Repeated distillation can improve the purity to more than 99.99%.
• Zone melting (optional): Zone refining of elemental arsenic to further reduce metal impurities.

Fields of application

High-purity arsenic is used in semiconductor materials (e.g. GaAs crystals), alloy additives, or in the manufacture of specialty glasses. Processes need to comply with strict environmental regulations to ensure safety and compliant waste disposal.