Enhancing Tellurium Recovery Efficiency from Waste Materials Using Microwave-Assisted Leaching Technology

Abstract

Microwave-assisted leaching technology significantly improves the recovery efficiency of tellurium (Te) from waste materials by selective heating and reduction of reaction activation energy. Typical applications demonstrate that: (1) at 800 W microwave power, the leaching rate of copper telluride waste increases from 75% by conventional methods to 98%, with processing time reduced by 60% (from 4 hours to 1.5 hours); (2) dynamic temperature control with stepwise heating between 80°C and 120°C suppresses H₂TeO₃ decomposition, limiting tellurium loss to less than 2%; (3) coupling with ionic liquid [BMIM]HSO₄ enables selective dissolution, reducing the leaching rates of impurity metals (Cu, Pb) to below 5%. This technology decreases acid consumption by 40%, and waste residue toxicity (TCLP leachate Te < 0.1 mg/L) meets environmental standards. It is suitable for complex materials such as photovoltaic waste and metallurgical slags.

1. Technical Challenges in Tellurium Recovery and Advantages of Microwave Technology

1.1 Characteristics of Tellurium Resources and Recovery Difficulties

• Material complexity: Major sources include copper anode slime (Te 0.5–5%), photovoltaic waste (CdTe thin films containing 2–8% Te), and lead-zinc smelting slags (Te 0.1–0.3%), accompanied by heavy metals such as Cu, Pb, and As.
• Tellurium commonly exists as TeO₂, TeS₂, or alloy phases. Conventional acid leaching requires strong oxidizing conditions (HNO₃ concentration >6 mol/L), incurring high reagent costs and producing NOx emissions.

1.2 Efficiency Bottlenecks

• Conventional leaching kinetics are limited (apparent activation energy >50 kJ/mol), requiring high temperatures (>90°C) and long reaction times (4–6 hours), yielding Te recovery rates of only 70–85%.

1.3 Microwave Field Mechanisms

• Selective heating: Tellurium compounds (dielectric loss factor tanδ = 0.1–0.3) absorb microwaves more readily than SiO₂ (tanδ = 0.01), generating localized superheating that promotes phase dissociation of target materials.
• Non-thermal effects: Microwave polarization lowers the Te–O bond dissociation energy from 218 kJ/mol to 190 kJ/mol, accelerating interfacial reactions.

2. Design and Optimization of Microwave-Enhanced Leaching Process

2.1 Process Innovations

• Two-stage leaching:
  
  Stage 1 (microwave pretreatment): 300 W irradiation for 10 minutes breaks down surface oxidation layers on crushed material (converting TeS₂ to TeO₂), increasing specific surface area by 50%;
  
  Stage 2 (dynamic leaching): Leaching with 2 mol/L H₂SO₄ and 0.5 mol/L H₂O₂ under intermittent 800 W microwave irradiation (on/off ratio 3:1), with temperature controlled below 120°C.
• Medium regulation: Addition of 0.1 mol/L Na₂SiO₃ inhibits silica gel formation, increasing solid-liquid separation rate by a factor of three.

2.2 Key Parameter Effects

• Microwave power and duration: Increasing power from 400 W to 800 W raises Te leaching from 82% to 98%; however, powers above 1000 W cause localized overheating (>150°C), increasing TeO₂ volatilization losses to 5%. Optimal irradiation time is 1.5 hours, shortening process duration by 62.5% compared to conventional 4 hours.
• Oxidant synergy: At 0.5 mol/L H₂O₂, Te⁴⁺ oxidation reaches 99%, avoiding excessive oxidant decomposition (decomposition rate >30% when concentration exceeds 1 mol/L).
• pH dynamic control: Initial pH of 1.5 promotes TeO₂ dissolution; during mid-reaction, pH automatically adjusts to 0.8 (via sulfuric acid self-ionization) to prevent H₂TeO₃ formation, which increases above 15% yield when pH exceeds 1.2.

3. Impurity Suppression and Selectivity Enhancement

3.1 Ionic Liquid Synergistic Extraction

• Functional design: The ionic liquid [BMIM]HSO₄ exhibits a distribution ratio for Te⁴⁺ of 10³, while that for Cu²⁺ and Pb²⁺ is below 10, yielding a selectivity coefficient above 100.
• In situ separation: Direct addition of 5% ionic liquid to the leachate with 10 minutes stirring achieves Te extraction rates above 95%, with impurity metal retention above 90%.

3.2 Microwave-Membrane Electrolysis Coupling

• Continuous purification: Leachate filtered through a ceramic membrane (0.1 μm pore size) undergoes pulse electrolysis (current density 200 A/m², duty cycle 1:1), increasing Te purity from 90% to 99.9%.
• Energy consumption comparison: Combined electrical energy consumption is 8 kWh/kg Te, a 47% reduction compared to conventional extraction-electrolysis processes (15 kWh/kg).

4. Industrial Applications and Environmental Benefits

4.1 Typical Cases

• Photovoltaic waste treatment: Germany’s SolarWorld plant employs microwave-H₂O₂ process to treat 5,000 tons of CdTe waste annually, achieving 97.5% Te recovery and reducing acid consumption by 45% (H₂SO₄ usage lowered from 3.2 tons to 1.8 tons per ton of Te).
• Copper anode slime tellurium extraction: Jiangxi Copper’s pilot data show post-microwave leach residue Te content reduced from 1.2% to 0.08%, with arsenic stabilization exceeding 99.5%, meeting GB 5085.3-2007 standards.

4.2 Environmental Benefit Quantification

• Pollutant reduction: NOx emissions are eliminated completely by replacing HNO₃ leaching; waste residue toxicity tests (TCLP) show Te leachate below 0.1 mg/L, well under the 1 mg/L regulatory limit.
• Carbon footprint reduction: Life cycle assessment (LCA) indicates carbon emissions per kilogram of recycled Te drop from 28 kg CO₂-eq (conventional process) to 12 kg CO₂-eq, a 57% decrease.

5. Technical Challenges and Future Directions

5.1 Engineering Scale-Up Bottlenecks

• Microwave uniformity control: Large reactors (>1 m³) require multi-mode cavity designs and electromagnetic field simulations to avoid localized hotspots with temperature differentials exceeding 30°C.
• Continuous production: Development of spiral conveyor microwave reactors is needed to precisely control material residence time within ±5 seconds.

5.2 New Material Development

• Microwave-sensitive catalysts: Fe₃O₄@SiO₂ supported catalysts enhance TeS₂ oxidation rates by 30% and allow magnetic separation for recovery.
• Corrosion-resistant reactors: Silicon carbide ceramic linings withstand H₂SO₄-H₂O₂ environments with annual corrosion rates below 0.1 mm, extending equipment lifespan to 5 years.

5.3 Intelligent Upgrades

• Online monitoring system: Optical fiber sensors embedded inside microwave cavities enable real-time monitoring of temperature and dielectric properties (accuracy ±2°C), dynamically adjusting power output.
• Digital twin modeling: COMSOL-based multi-physics coupling of electromagnetic, thermal, and fluid fields optimizes reactor design, improving energy efficiency by 15%.

Conclusion

Microwave-assisted leaching technology, through selective heating, oxidant synergy, and ionic liquid separation, increases tellurium recovery efficiency to over 98%, while reducing acid consumption and pollutant emissions. Industrial application requires overcoming challenges in large reactor design, corrosion-resistant materials development, and integration of intelligent controls. Future integration with continuous production and green electricity supply is expected to reduce tellurium recovery costs by 40%, promoting sustainable resource recycling in photovoltaic, electronics, and related industries.