Continuous hydrothermal leaching of LiCoO2 cathode materials by using citric acid

The first run of continuous hydrothermal leaching of lithium-ion battery cathode materials, LiCoO2, was performed using citric acid as the leachant at 200 °C. The flow system was specially designed and customized. Prior to the hydrothermal leaching experiments, a three-layer model was used to predict the flow state in this flow system, and a cold flow test using two kinds of flow lines was performed to determine the conditions in the preliminary experiments. Finally, the pulp density of the slurry and the flow rate were set to 10 g L−1 and 30 ml min−1, respectively, for the continuous hydrothermal leaching experiments. At 60 min after the start of slurry feeding, the leaching efficiency of Li and Co reached 81.3% and 92.7%, respectively, and can continue to increase with the extension of time. The successful run indicated that the process of hydrothermal leaching is feasible and promising to be applied in practice. Meanwhile, a problem of acid corrosion caused by the use of citric acid during this process was revealed and is expected to be resolved using inner coating materials with high acid corrosion resistance or organic acids with low or no acid corrosion as the leachant.