7N Tellurium Crystal Growth and Purification

 7N Tellurium Crystal Growth and Purification 

‌I. Raw Material Pretreatment and Preliminary Purification‌

1. ‌Raw Material Selection and Crushing‌

• ‌Material Requirements‌: Use tellurium ore or anode slime (Te content ≥5%), preferably copper smelting anode slime (containing Cu₂Te, Cu₂Se) as raw material .
• ‌Pretreatment Process‌:
• Coarse crushing to particle size ≤5mm, followed by ball milling to ≤200 mesh ;
• Magnetic separation (magnetic field intensity ≥0.8T) to remove Fe, Ni, and other magnetic impurities ;
• Froth flotation (pH=8-9, xanthate collectors) to separate SiO₂, CuO, and other non-magnetic impurities .
• ‌Precautions‌: Avoid introducing moisture during wet pretreatment (requires drying before roasting); control ambient humidity ≤30% .

1. ‌Pyrometallurgical Roasting and Oxidation‌

• ‌Process Parameters‌:
• Oxidation roasting temperature: 350–600°C (staged control: low temperature for desulfurization, high temperature for oxidation) ;
• Roasting time: 6–8 hours, with O₂ flow rate of 5–10 L/min ;
• Reagent: Concentrated sulfuric acid (98% H₂SO₄), mass ratio Te₂SO₄ = 1:1.5 .
• ‌Chemical Reaction‌:
  Cu2Te+2O2+2H2SO4→2CuSO4+TeO2+2H2OCu2​Te+2O2​+2H2​SO4​→2CuSO4​+TeO2​+2H2​O
• ‌Precautions‌: Control temperature ≤600°C to prevent TeO₂ volatilization (boiling point 387°C); treat exhaust gas with NaOH scrubbers .

‌II. Electrorefining and Vacuum Distillation‌

1. ‌Electrorefining‌

• ‌Electrolyte System‌:
• Electrolyte composition: H₂SO₄ (80–120g/L), TeO₂ (40–60g/L), additive (gelatin 0.1–0.3g/L) ;
• Temperature control: 30–40°C, circulation flow rate 1.5–2 m³/h .
• ‌Process Parameters‌:
• Current density: 100–150 A/m², cell voltage 0.2–0.4V ;
• Electrode spacing: 80–120mm, cathode deposition thickness 2–3mm/8h ;
• Impurity removal efficiency: Cu ≤5ppm, Pb ≤1ppm .
• ‌Precautions‌: Regularly filter electrolyte (accuracy ≤1μm); mechanically polish anode surfaces to prevent passivation .

1. ‌Vacuum Distillation‌

• ‌Process Parameters‌:
• Vacuum level: ≤1×10⁻²Pa, distillation temperature 600–650°C ;
• Condenser zone temperature: 200–250°C, Te vapor condensation efficiency ≥95% ;
• Distillation time: 8–12h, single-batch capacity ≤50kg .
• ‌Impurity Distribution‌: Low-boiling impurities (Se, S) accumulate at the condenser front; high-boiling impurities (Pb, Ag) remain in residues .
• ‌Precautions‌: Pre-pump vacuum system to ≤5×10⁻³Pa before heating to prevent Te oxidation .

‌III. Crystal Growth (Directional Crystallization)‌

1. ‌Equipment Configuration‌

• ‌Crystal Growth Furnace Models‌: TDR-70A/B (30kg capacity) or TRDL-800 (60kg capacity) ;
• Crucible material: High-purity graphite (ash content ≤5ppm), dimensions Φ300×400mm ;
• Heating method: Graphite resistance heating, maximum temperature 1200°C .

1. ‌Process Parameters‌

• ‌Melt Control‌:
• Melting temperature: 500–520°C, melt pool depth 80–120mm ;
• Protective gas: Ar (purity ≥99.999%), flow rate 10–15 L/min .
• ‌Crystallization Parameters‌:
• Pulling rate: 1–3mm/h, crystal rotation speed 8–12rpm ;
• Temperature gradient: Axial 30–50°C/cm, radial ≤10°C/cm ;
• Cooling method: Water-cooled copper base (water temperature 20–25°C), top radiative cooling .

1. ‌Impurity Control‌

• ‌Segregation Effect‌: Impurities like Fe, Ni (segregation coefficient <0.1) accumulate at grain boundaries ;
• ‌Remelting Cycles‌: 3–5 cycles, final total impurities ≤0.1ppm .

1. ‌Precautions‌:

• Cover melt surface with graphite plates to suppress Te volatilization (loss rate ≤0.5%) ;
• Monitor crystal diameter in real time using laser gauges (accuracy ±0.1mm) ;
• Avoid temperature fluctuations >±2°C to prevent dislocation density increase (target ≤10³/cm²) .

‌IV. Quality Inspection and Key Metrics‌

‌V. Environmental and Safety Protocols‌

1. ‌Exhaust Gas Treatment‌:

• Roasting exhaust: Neutralize SO₂ and SeO₂ with NaOH scrubbers (pH≥10) ;
• Vacuum distillation exhaust: Condense and recover Te vapor; residual gases adsorbed via activated carbon .

1. ‌Slag Recycling‌:

• Anode slime (containing Ag, Au): Recover via hydrometallurgy (H₂SO₄-HCl system) ;
• Electrolysis residues (containing Pb, Cu): Return to copper smelting systems .

1. ‌Safety Measures‌:

• Operators must wear gas masks (Te vapor is toxic); maintain negative pressure ventilation (air exchange rate ≥10 cycles/h) .

‌Process Optimization Guidelines‌

1. ‌Raw Material Adaptation‌: Adjust roasting temperature and acid ratio dynamically based on anode slime sources (e.g., copper vs. lead smelting) ;
2. ‌Crystal Pulling Rate Matching‌: Adjust pulling speed according to melt convection (Reynolds number Re≥2000) to suppress constitutional supercooling ;
3. ‌Energy Efficiency‌: Use dual-temperature zone heating (main zone 500°C, sub-zone 400°C) to reduce graphite resistance power consumption by 30% .