This study presents a systematic investigation into the alkaline leaching of spent aluminum electrolyte (SAE) for efficient recovery of fluorine components. Thermodynamic analysis reveals that the dissolution of cryolite (Na3AlF6) in NaOH is a spontaneous endothermic process (Δr<InlineEquation ID="IEq1"> <EquationSource Format="TEX">${G}_{m}^{\theta }$</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mi>G</mi> <mrow> <mi>m</mi> </mrow> <mi>θ</mi> </msubsup> </math></EquationSource> </InlineEquation>< 0, Δr<InlineEquation ID="IEq2"> <EquationSource Format="TEX">${H}_{m}^{\theta }$</EquationSource> <EquationSource Format="MATHML"><math> <msubsup> <mi>H</mi> <mrow> <mi>m</mi> </mrow> <mi>θ</mi> </msubsup> </math></EquationSource> </InlineEquation> > 0) when the temperature exceeds 30 °C. Single-factor experiments optimize the leaching parameters as 70 °C, 1.2 mol/L NaOH, liquid-to-solid ratio 24:1, and 30 min, achieving 88.1% fluorine leaching efficiency and 81.4% total mass leaching rate. Kinetic studies indicate that the Avrami equation can well fit the alkaline leaching kinetics of spent aluminum electrolyte. On the basis of the available macroscopic kinetic data and characterization results including X-ray diffraction (XRD) and Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), the leaching process exhibits an apparent mixed-control mechanism jointly dominated by product layer diffusion and interfacial chemical reactions, with a corresponding apparent activation energy of 36.72 kJ/mol. Integrating thermodynamic/kinetic analyses with separate XRD and Fourier transform infrared spectroscopy (FT-IR) characterizations, the leaching mechanism is elucidated: cryolite in the electrolyte transforms into Al(OH)3 and soluble NaAl(OH)4 via a two-step reaction in alkaline media, while α-Al2O3 and CaF2 enrich in the residue. This work provides a viable alkaline leaching route and technical support for the leaching of spent aluminum electrolyte. Graphical Abstract

Green and Efficient Separation of Fluorine from Spent Aluminum Electrolyte by Sodium Hydroxide Solution: Leaching Behavior and Mechanisms

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发表日期：

2026年7月29日

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Enle Xu,
Renguo Qin,
Tianxiang Hao,
Zhenyong Miao,
Shan Zhao,
Honglei Wang,
Zhihao Song

摘要

This study presents a systematic investigation into the alkaline leaching of spent aluminum electrolyte (SAE) for efficient recovery of fluorine components. Thermodynamic analysis reveals that the dissolution of cryolite (Na₃AlF₆) in NaOH is a spontaneous endothermic process (Δ_r ${G}_{m}^{\theta }$ $G_{m}^{θ}$ < 0, Δ_r ${H}_{m}^{\theta }$ $H_{m}^{θ}$ > 0) when the temperature exceeds 30 °C. Single-factor experiments optimize the leaching parameters as 70 °C, 1.2 mol/L NaOH, liquid-to-solid ratio 24:1, and 30 min, achieving 88.1% fluorine leaching efficiency and 81.4% total mass leaching rate. Kinetic studies indicate that the Avrami equation can well fit the alkaline leaching kinetics of spent aluminum electrolyte. On the basis of the available macroscopic kinetic data and characterization results including X-ray diffraction (XRD) and Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS), the leaching process exhibits an apparent mixed-control mechanism jointly dominated by product layer diffusion and interfacial chemical reactions, with a corresponding apparent activation energy of 36.72 kJ/mol. Integrating thermodynamic/kinetic analyses with separate XRD and Fourier transform infrared spectroscopy (FT-IR) characterizations, the leaching mechanism is elucidated: cryolite in the electrolyte transforms into Al(OH)₃ and soluble NaAl(OH)₄ via a two-step reaction in alkaline media, while α-Al₂O₃ and CaF₂ enrich in the residue. This work provides a viable alkaline leaching route and technical support for the leaching of spent aluminum electrolyte.

Graphical Abstract