As the global demand for energy storage batteries surges, the recycling of spent lithium iron phosphate (LiFePO4) batteries becomes increasingly crucial to address resource depletion and environmental concerns. This study delves into the selective flocculation process, employing polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA), to enhance the flotation separation of mixed electrode materials from spent LiFePO4 batteries. The interaction mechanisms between PVP, PAA, and the electrode materials are meticulously analyzed.
Challenges in Flotation Separation
The flotation process, leveraging the hydrophilicity of cathode metal oxides and the inherent hydrophobicity of anode graphite, faces significant challenges due to the organic binder coatings on the electrode materials. These coatings diminish the wettability differences, hindering efficient separation. Additionally, the fine particle size of the cathode material during direct recycling leads to entrapment within graphite aggregates, further compromising separation efficiency.
Experimental Approach
The study employed a systematic experimental approach, including:
- Material Preparation: Discharged and dismantled 18650-type spent LiFePO4 batteries were processed to obtain mixed electrode materials.
- Selective Flocculation Tests: Utilized a沉降管 to evaluate the flocculation efficiency of PVP and PAA, individually and in combination, on the mixed electrode materials.
- Flotation Tests: Conducted flotation experiments using a laboratory flotation machine to assess the impact of PVP and PAA on the separation efficiency of LiFePO4 and graphite.
- Characterization Methods: Employed laser particle size analysis, Zeta potential measurements, and Fourier transform infrared (FTIR) spectroscopy to investigate the interaction mechanisms between PVP, PAA, and the electrode materials.
Results and Discussion
- Selective Flocculation Tests: The combined use of PVP and PAA demonstrated a significant improvement in LiFePO4 recovery, achieving a recovery rate of 85.57% with a selectivity index (S) of 0.22.
- Flotation Tests: The joint application of PVP and PAA resulted in a remarkable increase in LiFePO4 recovery rate from 71.41% to 83.59%, while maintaining a relatively high grade.
- Particle Size Distribution: PVP effectively dispersed graphite, inhibiting its spontaneous hydrophobic flocculation. PAA induced the aggregation of LiFePO4, increasing its apparent particle size (D50) from 15.01 μm to 26.17 μm.
- Entrapment Analysis: The addition of PVP and PAA significantly reduced the entrainment of LiFePO4 during flotation, as evidenced by a decrease in the entrainment index (ENT) from 0.95 to 0.76.
- Zeta Potential Measurements: PVP adsorbed onto the graphite surface through hydrogen bonding, while PAA interacted with both graphite and LiFePO4. The sequential addition of PVP and PAA ensured selective flocculation of LiFePO4.
- FTIR Spectroscopy: The FTIR analysis confirmed the interaction between PVP and graphite through hydrogen bonding and the adsorption of PAA onto LiFePO4 via hydrogen bonding between the carboxyl groups of PAA and the hydroxyl groups on the LiFePO4 surface.
Mechanism of Selective Flocculation
The selective flocculation process can be summarized into three stages:
- Dispersion Stage: PVP selectively adsorbs onto the graphite surface, inhibiting its spontaneous hydrophobic flocculation and reducing the entrainment of LiFePO4.
- Selective Flocculation Stage: PVP occupies the active sites on the graphite surface, preventing PAA from flocculating graphite. Simultaneously, PAA selectively flocculates LiFePO4 due to its inability to interact with PVP-covered graphite.
- Flotation Stage: During flotation, hydrophobic graphite is captured by air bubbles and floats to the surface, while the hydrophilic LiFePO4 flocs remain in the flotation cell.
Conclusion
This study demonstrates the effectiveness of selective flocculation using PVP and PAA in enhancing the flotation separation of LiFePO4 and graphite from spent LiFePO4 batteries. The combined use of these reagents significantly improves LiFePO4 recovery while minimizing entrainment losses. This approach offers a promising solution for the efficient and environmentally friendly recycling of spent energy storage batteries, contributing to the sustainable development of the energy storage industry.