Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

In mineral processing, froth flotation is based on recovering valuable mineral particles by means of the overflowing froth. Industrial-scale froth flotations cells are typically equipped with optical measurement systems, which monitor bubble sizes and flow velocities at the froth surface. However, the velocity profile of the overflowing froth underneath its free surface is not accessible by optical observation. The present study combines X-ray radiography and particle tracking velocimetry in a laboratory-scale experiment, aiming to measure local flow velocities within an opticially opaque foam at a horizontal overflow. For this purpose, we prepared custom-tailored tracer particles: light- weight tetrahedra with an edge length of 4 mm were 3D-printed from a polymer material, and metal beads of 0.5 mm in diameter glued at each corner of a tetrahedron served as radiopaque features. In parallel to the velocity measurements by means of X-ray particle tracking, we determined the liquid fraction of the overflowing foam by electric conductivity measurements using electrode pairs. The experiment was performed with aqeuous foams of two different surfactant concentrations, but similar bubble size range and superficial gas velocity, yielding around 10 % liquid fraction near the overflow. Employing the particles as tools for flow tracing in X-ray image sequences, we identified an unexpected velocity maximum underneath the free surface of the overflowing foam. In a sequel, we will compare the X-ray radiographic measurements with optical measurements of the foam flow velocity through a transparent wall and at the free surface.