Effect of ultrasonication on the flotation of fine graphite particles: Nanobubbles or not?

It has been reported that nanobubbles can be produced by ultrasonication. However, it remains unclear whether part of the contribution of ultrasonication on flotation performance can be attributed to the generation of nanobubbles. In this work, we systematically studied this point of ultrasonication by combining a series of techniques including flotation testing, AFM (atomic force microscope) measurement, and settling testing. AFM imaging showed that no surface nanobubbles were found at the HOPG-water interface before and after subjection to ultrasonication. Further, surface nanobubbles were generated with solution exchange before ultrasonciation and then they were subjected to ultrasonication. It was found that ultrasonication did not destroy the pre-existing surface nanobubbles at the HOPG (highly oriented pyrolytic graphite)-water interface. Settling tests and flotation tests indicate that ultrasonication has a negligible influence on the interaction between graphite particles and thus flotation performance. Nanobubbles were not one of the outcomes for ultrasonication.