Stepwise heat treatment for fluorine removal on selective leachability of Li from lepidolite using HF/H2SO4 as lixiviant

•Sulfuric acid with fluorine additives can enhance the Li extraction from lepidolite.•Stepwise heat treatment was proposed at 120 °C and 200 °C for F removal.•Retention rate of Li, Al and F is 94.26%, 75.24% and 0.68%, respectively. Considering that 2%-8% F contained in lepidolite, an improved sulfuric acid method with introducing fluorine additives has been proposed to enhance its lithium extraction. Herein, an acid mixture of hydrofluoric acid and sulfuric acid (HF/H2SO4) was employed to theoretically investigate the dissolution mechanism due to that the HF molecules are the real reactive components of the fluorine additives. A stepwise heat treatment for fluorine removal has been proposed based on the different boiling point of HF-H2O and H2SO4-H2O subsystems of the acid leaching solution. Effects of heating temperature (200 °C-350 °C), heating time (6 h, 12 h), initial ratio of ore/H2SO4 (1:1.5–1:3.5 g/mL) and different heating style (one step or stepwise) on fluorine removal and lithium recovery have been carried out. Based on the elemental and composition analysis of liquid and corresponding solid phase, an optimal condition for fluorine removal was established: about 0.68% fluorine remained in leaching slurry after the stepwise heat treatment at 120 °C for 3 h and 200 °C for 6 h with initial ratio ore/HF/H2SO4 of 1:2:2 g/mL. The retention of Li was remained at a relatively high level as 94.26%. The resulted siliceous residues and undissolved quartz indicates that selective leaching of Li was achieved during this stepwise heat treatment, which is beneficial for subsequent purification and lithium production.