Effect of Gas Oversaturation Degree on Flotation Separation Performance of Electrode Materials from Spent Lithium-Ion Batteries

The electrode materials from spent lithium-ion batteries consist of graphite and lithium cobalt oxides (LCO), which cannot be efficiently separated by the conventional flotation technique due to the fine size distributions of graphite and LCO. In this work, nanobubbles were introduced to the flotation system of electrode materials. Nanobubbles were produced with the method of temperature difference. Different degrees of gas oversaturation in the water/slurry were achieved by raising the temperature of cold water (stored at 4 °C for at least 72 h) to target values of 20 °C, 25 °C, and 30 °C. It was found that the height and lateral distance of nanobubbles increased with the degree of gas oversaturation of water. In addition, the larger graphite agglomerations were observed to form in the presence of nanobubbles. The D50 (chord length) of graphite agglomerations increased by 8 μm, 11 μm, and 21 μm, respectively, compared with the D50 of graphite in natural water. More graphite agglomerations adhered to a captive bubble with the aid of nanobubbles than in the case of no nanobubbles, which was indicated by increased wrapping angles of graphite (agglomerations) adhering to a captive bubble. Furthermore, the maximum adhesion force between a captive bubble and substrate increases to 220, 270, and 300 μN as cold water temperature increases to 20, 25, and 30 °C, respectively. The frost of nanobubbles on a graphite surface and the resulting graphite agglomerations through the bridging effect of nanobubbles are thought to be responsible for the improved flotation performance of electrode materials. The present results indicate that the flotation performance of fine minerals can be regulated by regulating the gas oversaturation degree of the slurry.