Kinetic studies on cobalt and lithium leaching from spent lithium-ion battery using tannic-acetic acid as lixiviants

Kinetic leaching on recovery of cobalt (Co) and lithium (Li) from spent lithium-ion battery (LIB) cathode using tannic acid-acetic acid as leaching agent was carried out to discern the leaching mechanism in terms of acid concentration and temperature. Three kinetic models, i.e. shrinking core model (SCM) chemical reaction and product layer diffusion and shrinking particle model (SPM) film diffusion was employed to perceive the possible mechanism of Co and Li dissolution. Fitting experimental data using the models revealed that increasing acetic acid concentration caused the dissolution mechanism to follow SCM product layer diffusion. This was probably due to the molar ratio of acetic acid to Co was not sufficient to control the passivation of the Co(OH)₂ product layer. In the case of tannic acid, increasing concentration obeyed SPM film diffusion model. Due to excess concentration of tannic acid in the liquid phase, dissolution was more influenced by the reduction rate of Co by tannic acid on the solid-liquid interface rather than mass transfer through the boundary layer (insoluble layer). Dissolution of Co and Li at elevating temperature obeyed SCM product layer diffusion, indicating higher temperature encourages the reduction rate to produce Co(OH)₂ passivation layer. Still, at the same time, increasing temperature boosted ion movement and encouraged the diffusion through product layer. Activation energy (E_a) calculation yielded 5.53 kJ/mol and 26.8 kJ/mol for Co at 30-60 °C and 60-80 °C, respectively, while Li was 21.51 kJ/mol. Relatively low Ea (< 40 kJ/mol) indicated the physical interaction was more prominent than chemical reaction control. The plot between rate constant and reactant concentration showed that the effect of tannic acid and acetic acid concentration was significant for Co dissolution rate (order 0.927 and 0.638 for tannic acid and acetic acid concentration). For Li, the reaction order was 0.451 for tannic acid and 0.021 for acetic acid concentration, implying less influence of reactant concentration on Li dissolution. Although Li dissolution was less affected by reactant conditions, the recovery of Li and Co was almost identical in each experiment, which suggested the Li dissolution was controlled by Co dissolution through product layer passivation.