Steam-assisted Defluorination of Lepidolite: Kinetic Analysis, Phase Reconstruction and Microstructural Evolution

Lepidolite is an important hard-rock lithium resource commonly associated with valuable rare alkali metals such as Rb and Cs, but its stable layered aluminosilicate structure and structural fluorine limit direct extraction. In this study, steam-assisted defluorination of lepidolite concentrate from Yichun, Jiangxi Province, was investigated by varying roasting temperature, roasting time, steam flow rate, and particle size. Thermodynamic calculations, kinetic fitting, XRD, SEM–EDS, and BET analyses were used to evaluate the defluorination behavior and structural changes. The results showed that particle size and roasting temperature were the dominant factors. Under the optimized conditions of −74 + 38 μm, 880 °C, 25 min, and 1.5 g/min steam flow, the defluorination efficiency reached 96.13%. Thermodynamic calculations based on LiF and AlF₃ as simplified models of Li-F and Al-F bonds indicated that direct hydrolytic defluorination was unfavorable. However, when SiO₂ participated in the reaction, aluminosilicate reconstruction substantially reduced the Gibbs free energy, thereby facilitating fluorine release. Kinetic fitting showed that the process followed the A3 random nucleation model, with an apparent activation energy of 257.73 ± 40.13 kJ/mol, indicating that new-phase nucleation, growth, and structural reconstruction jointly controlled defluorination. Structural characterization further confirmed that steam roasting destroyed the characteristic lepidolite structure, promoted F migration and removal, and induced aluminosilicate reconstruction, while excessive temperature caused pore shrinkage and structural densification. These findings provide a basis for improving the subsequent extraction of Li, Rb, and Cs from lepidolite.