Nanobubbles: Generation using a multiphase pump, properties and features in flotation

The importance of nanobubbles is now widely known, particularly their applications and potential in mineral processing. The actual challenge is to generate a high concentration of bubbles (number per water volume) in a sustainable manner at high flow rates. The main objective of this work was to develop a new method for generating highly-loaded nanobubbles aqueous solutions by hydrodynamic cavitation using a centrifugal multiphase pump (CMP) and a needle valve. Nanobubbles (150-200 nm) were formed, at 22 degrees C, with the pump and a recycle column, at various operating pressures and air/liquid surface tension. Nanobubbles were resistant to shearing caused by pump impellers and to high operating pressures (up to 5 bar) throughout several bubble generation cycles. The size of the nanobubbles remained constant, and their numeric concentration increased as a function of these cycles, reaching equilibrium after 29 cycles; this was dependent on pump pressure and the surface tension of the solution. The highest concentration (4 x 10(9) nanobubbles mL(-1)) was obtained at 5 bar and 49 mN m(-1) surface tension (air holdup = 6.8% and D32 of microbubbles in the range between 62 and 70 mu m). These phenomena can be explained by Henry's Law and the lower energy required for bubble formation when the interfacial tension (air/water) decreases and when the differential pressure in the cavitation zone increases. The mean diameter and concentration of these nanobubbles did not vary significantly over a period of two months, demonstrating the high stability of these concentrated nanobubbles. It is concluded that the procedure has great potential in future applications in ore flotation and wastewater treatment and reuse.