The effect of impeller speeds on the nanobubbles flotation efficiency of ultrafine coal particles

Ultrafine particles exhibit poor flotation behavior due to the low collision efficiencies with conventional gas bubbles. Introducing nanobubbles are expected to improve the collision probability of ultra-fine particles with bubbles. However, it remains unclear whether nanobubbles can enhance flotation at high impeller speeds during flotation process as strong turbulence may scour away the nanobubbles from solid-liquid interface of particles. This study investigates the influence and the underlying mechanism of nanobubbles on flotation performance of ultrafine coal particles under varying impeller speeds. Specifically, the surface nanobubbles (SNBs) and bulk nanobubbles (BNBs) were utilized respectively, which were produced based on the temperature difference method and hydrodynamic cavitation, and characteristized by atomic force microscopy (AFM) and nanoparticle tracking analysis (NTA), respectively. The formation of ultrafine coal particle aggregates at different impeller speeds was analyzed through laser particle size analyzer (LPSA) and high-speed camera imaging. Result showed that as the impeller speed increases, the radius of the aggregates increases, and the number and radius of aggregates in SNBs/BNBs slurries is significantly larger than that in conventional slurry at varying impeller speeds from 1200 to 2800 rpm, which is consistent with the increased flotation recoveries and flotation rates. It is demonstrated that nanobubbles remain stable even at high impeller speeds, and thus promote aggregation and thereby enhance flotation performance. The findings of this study provide theoretical support and valuable insights for ultrafine particle separation technologies.