Lithium extraction from the mineral zinnwaldite: Part II: Lithium carbonate recovery by direct carbonation of sintered zinnwaldite concentrate

Based on decomposition studies of zinnwaldite in Part I, a process for lithium carbonate production by supercritical carbon dioxide (sc-CO2) treatment was developed and optimised to high lithium yields. Sintered and ground zinnwaldite concentrate was subjected to CO2/water treatment at elevated pressure (100 bar) and moderate heating (230 °C). Mobilisation rates of over 70% were attained. The filtered lithium bicarbonate containing solution was concentrated by electrodialysis up to a lithium content of 8.5 g/L. Precipitation of lithium carbonate was achieved by CO2 elimination at 90-95 °C. A high selectivity of CO2 treatment for alkali metals, specifically for lithium, enables lithium carbonate product purities of ≥99.0% without additional rinsing.