Combining optical and X-ray measurements of an overflowing foam

The flow behaviour of liquid foam is of central importance in froth flotation for mineral processing. Flotation separates valuable mineral particles from gangue material based on the surface wettability. To this end, the solids are finely ground and suspended in an aqueous solution with flotation reagents. In aerated flotations cells, gas bubbles selectively attach to the hydrophobic mineral particles, rise to the surface, and form a froth. To recover the valuables, they are transported out of the flotation cell with the froth. In flotation plants, the recovery of solid and liquid is monitored by optical observation of the overflowing froth. However, this monitoring is limited to the free surface of the particle-laden froth. Aiming for detailed insights into the flow behaviour underneath the surface-near foam bubbles, the laboratory-scale experiment in this work investigates the velocity field of an overflowing foam in combined optical and X-ray measurements. For this purpose, foam was generated continuously, moved similar to a plug-flow in a vertical channel with rectangular cross-section, and flowed off over a one-sided horizontal weir into the open surrounding. The imaging measurements focused on the foam flow in the region of interest around the weir. Simultaneously, the liquid fraction of the foam was monitored by measuring its electric conductivity between electrode pairs mounted near the weir. We used aqueous foams of two different surfactant concentrations but similar bubble size range and superficial gas velocity, yielding around 10 % liquid fraction. The optical measurements carried out through the transparent side wall of the flow channel as well as at the free surface of the overflowing foam. They captured light reflections on the foam bubbles were analysed by an adapted particle image velocimetry algorithm. While the opacity of the foam limits optical measurements to the wall- or surface-near foam bubbles, our approach of X-ray particle tracking velocimetry with custom-tailored tracer particles sheds light on the velocity field in a truly three-dimensional measurement volume. We prepared tracers consisting of small 3D-printed polymer tetrahedra with tiny metal beads glued to the tetrahedral tips. Owing to their shape and the light-weight material composite, these tracers adhered to the bubble-scale foam structure and, therefore, were carried with the foam flow very well. X-ray radiography visualised the motion paths of each tracer’s metal beads, representing the local streamlines of the foam flow. Besides, the X-ray images mapped the liquid fraction distribution in the entire field of view, i.e. also directly at the weir, thus extending the local measurement of the liquid fraction by means of the electrode pairs. The tracer-based X-ray measurements revealed the velocity profile increasing in vertical direction above the weir, whereas the optical flow measurements were subjected to wall and surface effects, resulting in lower velocities. Combining all measurement results, we identified an unexpected velocity maximum underneath the free surface of the overflowing foam.