A Comparative Study of Bulk and Surface Nanobubbles in Enhancing Coal Flotation

Nanobubbles have been widely recognized for enhancing fine particle flotation, yet the distinct roles of bulk nanobubbles (BNBs) and surface nanobubbles (SNBs) remain poorly understood. This study systematically compares the effects of BNBs and SNBs on coal flotation using a unified hydrodynamic cavitation platform—BNBs generated in clean water and SNBs generated directly on particle surfaces by cavitating the coal slurry. Flotation tests, kinetics analysis, contact angle measurements, and agglomeration observations were conducted across varying reagent dosages and pulp concentrations. The results show that BNBs have negligible influence on flotation performance across all tested conditions, whereas SNBs significantly improve combustible recovery, particularly at low collector and frother dosages, where combustible recovery increased from 71.52% to 82.03% at a low collector dosage and from 45.03% to 61.71% at a low frother dosage. SNBs also enhanced flotation kinetics, increasing the rate constant from 1.83 to 2.10 min−1, and improved surface hydrophobicity and fine particle agglomeration, which likely account for the observed enhancement in flotation performance. Notably, although BNBs showed measurable increases in contact angle on model substrates, their inability to effectively anchor onto coal particles under real flotation conditions makes them practically ineffective. This study provides critical insights into the likely dominant role of SNBs in nanobubble-assisted flotation and offers guidance for process optimization in fine coal beneficiation.