On the effect of cavitation on particles in leaching processes: implications to battery recycling

Abstract

Along with the transition to cleaner energy production methods, closing the processing loop on batteries is becoming one of the significant issues to tackle in this decade. The most promising recycling technique consists in the leaching of crushed cathode material, i.e. the dissolution of the solid battery material in an acid solution, to recover valuable metals from spent batteries. To lower process time and to use green organic solvents, ultrasonically enhanced leaching is a valid alternative to conventional processing. The mechanism of action of ultrasound during leaching is still unclear, and yet to be directly observed on solid particles. Therefore, this work aims to shed light on the underlying phenomena in the ultrasonically enhanced leaching process, by directly observing leached material. In particular, the focus is placed on the combined effect of ultrasound and acetic acid on NMC particles. Residual material from conventional and ultrasonically enhanced leaching was analyzed with inductively coupled plasma - optical emission spectrometry (ICP-OES), dynamic light scattering (DLS) and scanning electron microscopy (SEM). Conventional and ultrasonically enhanced leaching techniques were thus compared in terms of leaching efficiency, particle size distribution and morphological changes, demonstrating the beneficial effect of ultrasonic cavitation on mass transfer. Additionally, the NMC particles were exposed to ultrasound in water, to confirm that standalone ultrasonic cavitation does not lead to particles breakage. The understanding of the effect of ultrasound enables their targeted application in leaching processes and allows a deeper understanding of ultrasound in heterogeneous systems.