Combining optical and X-ray measurements of an overflowing liquid foam

Froth flow is of central importance for mineral processing by froth flotation. In flotation plants, the recovery of solid mineral particles and liquid from the overflowing froth is monitored by optical observation and, therefore, limited to the froth’s free surface. The laboratory-scale experiment in this work investigates the flow behaviour of an aqueous foam at a horizontal overflow (Fig. 1) in combined optical and X-ray radiographic measurements. Simultaneously, the foam’s liquid fraction was determined by measuring the electrical conductivity between electrode pairs. The optical measurements, performed both through a transparent wall and at the free surface of the overflowing foam, captured light reflexions on the foam bubbles, which were analysed by adapting particle image velocimetry algorithms. While the opacity of the foam limits optical measurements to the surface-near bubbles, our approach of X-ray particle tracking velocimetry (X-PTV) sheds light on the three-dimensional foam flow. The customised tracer particles used in this work consisted of a 3D-printed tetrahedral polymer structure with a total of four small metal beads at its corners (Fig. 1). Owing to their shape and the light-weight material composite, the tracers adhered to the bubble-scale foam structure and were carried by the foam. X-ray radiography visualises the motion paths of each tracer’s metal beads, representing the local streamlines of the foam flow. Further, the X-ray radiographs map the foam’s liquid fraction distribution, thus extending the local measurement of the liquid fraction by means of the electrode pairs. X-PTV reveals comparatively high flow velocities of the three-dimensional foam flow, in particular near the overflow, whereas the optical measurements are sub-jected to wall or surface effects, yielding lower flow velocities. However, X-PTV with customised foam flow tracers comes to its limit in unstable foams at high liquid fraction and high flow velocity.