Optimization of Lithium Extraction from Lepidolite by Roasting Using Sodium and Calcium Sulfates

The lithium extraction from a lepidolite concentrate using roasting, followed by water leaching, was studied. Several alternative additives were initially tested. The use of sodium and calcium sulfates as additives was evaluated in more detail. The influence of some process variables, namely the roasting time, roasting temperature and the additive/concentrate mass ratio, was studied applying a design of experiments. The lithium extraction was modelled and the fitted and validated model was used to optimize the process response. The increase in the additive/concentrate mass ratio, roasting time and temperature seems to result in solid state reactions and transformations that lead to phase, morphological and particle size distribution modifications, which were assessed by XRPD, SEM, and particle size analyses. In this process, lithium sodium sulfate formation constitutes a crucial step enabling the Li water leaching. High lithium extractions were estimated for several combinations of factors. At 850°C, lithium extractions over 90% are obtained when the roasting time is above 1.90 hour and the additive/concentrate mass ratios are over 0.77. An increase in the temperature to 875°C also leads to lithium extractions over 90% for a roasting time of 1 hour and an additive/concentrate mass ratio of 0.60.