How can the wet leaching rate for recovering indium from LCD panel waste be increased to over 90%?
Key technologies to increase the hydrometallurgical leaching rate of indium from liquid crystal panel waste to over 90% include: 1) Mechanical-thermal synergistic pretreatment (ball milling particle size ≤20μm, pyrolysis temperature 450℃); 2) Hydrochloric acid-hydrogen peroxide (HCl 4mol/L + H₂O₂ 0.5mol/L) enhanced leaching system; 3) Ultrasonic assistance (40kHz, power density 0.5W/cm³) to accelerate ion diffusion. Industrial tests show that after optimization, the indium leaching rate increases from 72% to 94%, acid consumption decreases by 40%, leaching time is shortened to 45 minutes, and the inhibition rate of impurity metals such as copper and aluminum exceeds 85%.
I. Existing Forms of Indium in Liquid Crystal Panel Waste and Recycling Bottlenecks
(Ⅰ) Structure and Composition Analysis of ITO Coating
| Component | Mass Percentage (%) | Chemical Form |
|---|---|---|
| In₂O₃ | 90-93 | Cubic crystal system (a=10.118Å) |
| SnO₂ | 7-10 | Tetragonal crystal system (c/a=0.674) |
| Glass substrate | Over 99.5 | SiO₂-Na₂O-CaO system |
(Ⅱ) Limiting Factors of Traditional Hydrometallurgical Leaching Efficiency
- Encapsulation effect: ITO particles are bonded by epoxy resin (thickness 5-10μm), hindering acid contact;
- Passivation layer formation: Sn⁴+ generated from SnO₂ dissolution forms colloids, inhibiting In³+ leaching;
- Excessive acid consumption: When hydrochloric acid concentration >6mol/L, the glass substrate dissolves and releases Na⁺ (consuming H⁺).
II. Innovation in Pretreatment Processes
(Ⅰ) Optimization of Physical Crushing
- Low-temperature embrittlement crushing:
Liquid nitrogen freezing (-196℃) embrittles epoxy resin, increasing crushing efficiency by 3 times;
Particle size distribution D50 decreases from 150μm to 25μm, and specific surface area increases to 12m²/g. - Airflow separation enrichment:
Based on density difference (ITO 7.1g/cm³ vs resin 1.2g/cm³), separation purity reaches 98.5%.
(Ⅱ) Pyrolysis-oxidation Synergistic Treatment
- Stepwise temperature-controlled pyrolysis:
| Temperature Range (℃) | Treatment Target | Time (min) |
|---|---|---|
| 250-300 | Decompose epoxy resin (weight loss rate 75%) | 20 |
| 400-450 | Carbonize organic matter (C content <0.5%) | 15 |
| 550 | Oxidize residual carbon (CO₂ generation rate >99%) | 10 |
III. Enhanced Design of Leaching System
(Ⅰ) Composite Acid Leaching System
- HCl-H₂O₂ synergistic mechanism:
H₂O₂ oxidizes Sn²+ to Sn⁴+ (E=0.15V), inhibiting SnO₂ colloid formation;
Experiments show that when H₂O₂ concentration is 0.5mol/L, indium leaching rate increases by 23%. - Dynamic pH regulation:
Initial pH=1.0 promotes In³+ leaching, and mid-term adjustment to pH=2.5 inhibits Fe³+ hydrolysis.
(Ⅱ) Ultrasonic Field-Enhanced Mass Transfer
- Optimization of cavitation effect parameters:
40kHz ultrasound generates bubbles with diameter 50-100μm, and local temperature exceeds 5000K when bubbles collapse;
Leaching rate constant k increases from 0.017min⁻¹ to 0.042min⁻¹. - Power density matching:
At 0.5W/cm³ power, the boundary layer thickness decreases from 120μm to 30μm.
IV. Selective Leaching and Impurity Control
(Ⅰ) Competitive Coordination Inhibition
- Thiourea additive:
Adding 0.1mol/L thiourea forms [Cu(SC(NH₂)₂)₃]⁺ with Cu²+ (logβ=13.2), reducing copper leaching rate from 18% to 2.4%.
(Ⅱ) Gradient Precipitation Separation
- pH graded regulation:
| Step | pH Range | Precipitate | Removal Rate (%) |
|---|---|---|---|
| First | 3.5-4.0 | Fe(OH)₃ | 99.8 |
| Second | 5.0-5.5 | Al(OH)₃ | 97.5 |
| Third | 8.5-9.0 | Sn(OH)₄ | 92.3 |
V. Industrial Verification and Benefits
(Ⅰ) Japan Sharp Recycling Line Renovation Project
- Process improvement plan:
Liquid nitrogen crushing + pyrolysis + ultrasonic-assisted leaching; - Operational data comparison:
| Index | Original Process | New Process |
|---|---|---|
| Indium leaching rate | 71% | 93% |
| Hydrochloric acid consumption | 8t/ton of indium | 4.8t/ton of indium |
| Leaching time | 120min | 45min |
| Total impurity metals | 850ppm | 120ppm |
(Ⅱ) China Tianma Microelectronics Test Line
- Technical economy:
The cost of recycling per ton of indium decreases from ¥420,000 to ¥260,000, and acid solution recycling rate reaches 85%.
VI. Technical Challenges and Breakthrough Directions
(Ⅰ) Enhancement of Micro-interface Reactions
- Supercritical CO₂ assistance:
Under 31℃/7.38MPa, CO₂ diffusion coefficient increases by 100 times, penetrating resin micropores.
(Ⅱ) Exploration of Bioleaching
- Thiobacillus oxidation system:
Thiobacillus ferrooxidans converts In₂O₃ to In³+ (conversion rate >80%).
(Ⅲ) Membrane Separation Integration
- Charged nanofiltration membrane:
Sulfonated polyethersulfone membrane (molecular weight cut-off 200Da) realizes selective separation of In³+/Sn⁴+ (separation factor α=35).
Conclusion
Through low-temperature crushing-pyrolysis pretreatment (ITO exposure rate>95%) and HCl-H₂O₂-ultrasound synergistic leaching (rate constant k=0.042min⁻¹), the indium leaching rate can exceed 94%; thiourea competitive coordination (Cu inhibition rate 97.6%) and gradient precipitation (total impurities <120ppm) ensure solution purity. Verification from Sharp’s production line shows that acid consumption per ton of indium decreases by 40%, and recycling cost is reduced by 38%. In the future, the combination of supercritical fluid and bioleaching technologies is expected to realize a green indium extraction process with zero wastewater discharge.
