Ultrasonically-enhanced leaching: correlating acoustic spectra and solid particle properties in NMC cathode...

Dissolutions are entailed in a wide variety of industrial processes, and acoustic cavitation is being researched as a powerful tool to intensify these processes. In particular, when applied to the leaching of battery cathode material in hydrometallurgical recycling, it paves the way to the utilization of organic acids in place of mineral acids, lowering the hazardousness and cost of recycling. In this framework, mapping the cavitation activity and understanding the relation between acoustic cavitation and the action of the acid is fundamental for the design and control of the intensified process. This work used the leaching of NMC (lithium nickel manganese cobalt oxide) cathode particles as a model system. Leaching reduces the average particles size from 10 μm to 1.5 μm, reflecting the 85 % reduction in residual mass over 10 min. The cavitation activity was measured by means of a hydrophone in different conditions, to isolate and assess the effect of acoustic cavitation and leachant separately. The acoustic spectra enabled the quantification of the cavitation activity. In particular, the cavitation intensity decreased by 3 orders of magnitude in the leaching system, in a specific time window of the process, to rise again towards the end of the experimental time. This effect was not observed in non-leaching systems, suggesting an interaction between the acid and the particles is the cause of such phenomenon. The characterization of the solid through scanning electron microscopy (SEM) and surface area quantification by gas adsorption helped to narrow down on the cause of such attenuation, i.e. the cushioning effect related to oxygen release during the reaction. Along with this, the acoustic spectra coupled with the characterization of the solid particles highlighted a dependance of the cavitation activity on the total surface area of the solid. These findings enable deeper understanding of the effect of the presence of particles in a cavitating system, leading to better control and design of leaching reactors.