Chemical-free flotation enhancement via hydrodynamic cavitation-generated nanobubbles: A cleaner production approach

Froth flotation is a cornerstone separation process for recovering critical minerals but conventionally depends on multiple organic and inorganic reagents that increase operating costs and environmental burdens. This study presents a chemical-free flotation enhancement strategy using hydrodynamic cavitation (HC) to generate micro-nanobubbles (NBs) and provides new mechanistic insight into how these NBs improve flotation by directly enhancing bubble-particle attachment. A multiscale experimental framework combining mechanical flotation cells and a Hallimond tube enables decoupling of collision and attachment subprocesses and quantifies NB effects under near-unity collision probability – an approach rarely applied in HC-NB flotation studies. HC-generated NBs markedly improve flotation performance: for coal, recovery increased by 15 % with a 19 % rise in flotation rate and collection efficiency; for hydrophobized silica, recovery increased by 21 %, flotation rate and attachment efficiency increased threefold, and attachment time decreased by 22 %. Mechanistically, NBs adsorbed on particle surfaces and interfacial gas enrichment increase apparent hydrophobicity, promote particle agglomeration, and facilitate multipoint attachment. By explicitly linking HC conditions, NB characteristics, and attachment efficiency, this study advances understanding beyond prior work focused solely on overall performance. Importantly, HC-induced enhancement is fundamentally chemical-free and does not rely on additional reagents, and practical implementation does not necessarily require dedicated extra energy input because cavitation tubes can be integrated into existing feed pipelines. In cleaner-production terms, HC-generated NBs can reduce reagent demand and associated effluent loads. While full techno-economic and life-cycle assessments remain future work, these findings establish a mechanistic foundation for more sustainable, reagent-reduced mineral processing.