(598e) Hydrodynamic Cavitation-Based Leaching of Spent Cathode Material: Comparison of Orifice, Venturi and Vortex-Diode Based Cavitation Reactors

Background: The decarbonization of the transport sector and the steady increase in electrification push the demand for battery metals to unprecedented levels. The key metals for the production of lithium-ion batteries (LIB) are lithium, cobalt, nickel and manganese. The Energy Transitions Commission estimated that the potential demand of these metals is expected to spike by 2030; while the supply of lithium and nickel is forecasted to be secured by mining, the production of cobalt is expected to be sufficient to satisfy only 60% of the demand, leaving a supply gap up to 170,000 tons. Thus, the recycling of existing feedstock such as spent cathode material from end-of-life automobile batteries will be required to fulfill the deficit in demand sustainably.

Motivation: The current state of the art in recycling of spent cathode materials consists in hydrometallurgy, which uses harsh mineral acids such as H₂SO₄, and large processing times, which runs into hours for the leaching of the crushed cathode material in the acid solution. Generally, the recovery of lithium and nickel is satisfactory, while that of cobalt and manganese tends to remain low even at high acid concentrations. However, the addition of a reducing agent increases the recovery significantly, due to the change in the oxidation number of cobalt and manganese. In addition, the liquid solution is heated up to 90ºC to improve the leaching rate, which posestechnical challenges in the materials selection for the equipment. Typical process time varies largely among processes, ranging from 45 minutes to 4 hours depending on the temperature, the acid concentration and the solid concentration. Therefore, leaching is a sensitive step in the process, both in terms of processing volumes, as its efficiency is inversely proportional to the solid concentration, and in terms of residence time. The leaching rate regulates the dimension of the leaching reactors, which in turn dictates the space requirements and the energy cost of the industrial facility. An intensification of the leaching rate lowers process volume and residence time, enabling the utilization of more diluted acid solutions or weaker organic acids.

Scope of this work: Cavitation - the formation, growth, and collapse of bubbles in a liquid - is a well known tool for process intensification. It generates intense shear in a confined region of the system of interest, intensifying the leaching rate. This intensification is beneficial to reduce the processing time of leaching, thus shrink the size of process equipment, and to operate at ambient conditions. To generate cavitation, the pressure of the liquid has to be lowered to a sufficient extent to form cavities. This can be achieved by applying ultrasonic waves (acoustic cavitation) or by specific fluid-flow patterns (hydrodynamic cavitation). Despite the extensive work on the applicability of acoustic cavitation, the latter is energy intensive and has limited scalability. Moreover, the lifetime of the ultrasonic probe is short due to the erosion caused by the acid and the bubble field. Being the probe a titanium alloy, its frequent substitution in a large scale scenario is resource-demanding. Compared to acoustic cavitation, hydrodynamic cavitation is less energy intensive and has good scalability potential to large process volumes. The hydrodynamic cavitation devices are less prone to erosion compared to ultrasonic probes, and are built in stainless steel. In this work, hydrodynamic cavitation was employed to increase the leaching rate, using different types of cavitation devices.

The beneficial effect of cavitation on the leaching rate is related to the enhanced species transport through the boundary layer around the cathode particles. The solid offers heterogeneous nucleation sites for the cavitation bubbles, which mainly form and collapse on the particles’ surface. This improves the local transport properties, firstly favoring the penetration of the acid in the crevices on the surface of the particles, and secondly easing the transport of the leached species towards the bulk of the liquid phase. Due to the improved penetration of the acid in the particles, the average particle size is reduced by 35% compared to conventional processing techniques, for the same conversion. Therefore, the surface area exposed to the acid is higher. Being the leaching rate proportional to the exposed surface area, the latter is also higher.

Methods: To assess hydrodynamic cavitation-based leaching a cavitational rig was constructed with a centrifugal pump, pipe fittings and a pipeprimarily of 1’’ size, in order to operate the different hydrodynamic cavitationdevices, namely a venturi tube, an orifice and a vortex diode. The schematic of the rig operating a vortex-diode (nominal flowrate: 20 LPM, at ΔP = 250 kPag) is shown in Figure 1.

The leaching experiments were performed on NMC (Li_1.05Co_0.33Ni_0.33Mn_0.33O₂) cathode material, using diluted acetic acid as leaching agent and hydrogen peroxide as a reducing agent. All experiments are performed at ambient temperature. As a first step, the best cavitation device was identified, using different solid loadings. The venturi tube and the orifice are particularly sensitive to clogging when processing slurries, therefore the gradual increase in the solid loading is a key step to assess their performance. On the other hand, the vortex-diode appeared to be the best device for handling solid/liquid flows, therefore it was selected for pressure drop optimization. The pressure drop is a fundamental parameter to determine the intensity of hydrodynamic cavitation. In this work, the upstream pressure was varied from 400 to 100 kPa. Finally, the acid concentration and the hydrogen peroxide dose were optimized. In all the experiments, samples were taken at various process times to reconstruct the concentration profiles of the metals in the solution, measured by ICP-OES. The recovered solid was analyzed using SEM, to measure the particle size distribution and compare the morphological features of the particles before and after leaching.

Results: A typical leaching profile is plotted in Figure 2.The concentration of the leached metals in the acid solution reaches a plateau after 20 minutes of processing. To obtain the same conversion using the same chemicals concentration and solid loading, the observed leaching time using mechanical stirring (conventional method) is two hours.

Implications: Hydrodynamic cavitation is a technological platform relevant to the energy, water, and dairy sectors. This study expands the application horizon in the energy sector, by testing the applicability of hydrodynamic cavitation to the leaching of spent cathode material. The study reports the optimization of key reactor choices such as reactor type and operating pressure, in addition to operating parameters. The results from this study show a leaching efficiency of 75% in 15 minutes, by the use of acetic acid and hydrogen peroxide.