A design of experiments investigation into lithium extraction by phosphoric acid leaching of spodumene

Lithium leaching from β-spodumene (LiAlSi2O6) with phosphoric acid was explored by response surface methodology. A four-factor central composite design evaluated the effects of temperature (55–95°C), leaching time (2–12 h), liquid-to-solid ratio (2–8 mL/g), and phosphoric acid concentration (2–8 M) on lithium extraction and aluminum co-extraction. The models indicated that leaching temperature and time were the only statistically significant factors that improved leaching performance within the leaching conditions. The optimal model leach conditions (95°C, 12 h, 8 mL/g, 8 M) resulted in 58% Li and 11% Al extraction. A final extrapolation experiment considered lower acid concentration for an extended period of time (95°C, 24 h, 8 mL/g, 2 M), resulting in 59% Li and 14% Al extraction. Powder X-ray diffraction analysis of the solid residues from the validation and extrapolation experiments showed that increasing Li extraction shifted the d-spacing of the characteristic peaks for the (201) and the (102) planes from β-spodumene towards those of HAlSi2O6. These shifts occurred with corresponding decreased intensity and increased full-width at half-maximum (FWHM) of the β-spodumene peaks, and vice versa for the HAlSi2O6 peaks, confirming the lithium leaching process.