Ultrasound imaging of liquid fraction in foam

Flotation is an important process in liquid-solid and solid-solid separation, whereby desired solids in suspensions are recovered by their attachment to gas bubbles. Monitoring the process is essential for increased grade of quality and reduced water consumption. However, in situ measurements of the froth’s phases (liquid, air, particles) or volume flow, which is easily integrable in industrial process are not available. In this paper, we propose an instrumentation with ultrasound transducers of center frequency of 135 kHz and electrodes to measure the liquid fraction distribution in foam and froth with high spatio-temporal resolution of 6.65 mm (axial), 13 mm (lateral) and 1 Hz. The measurement system was calibrated in homogeneous, steady foam by an integral conductivity measurement. A backscatter model was applied to reduce the effect of ultrasound shadowing and enhance quantitative liquid fraction estimates. The measurement was validated by neutron radiography. An averaged absolute and relative measurement uncertainty of 0.17 ·10 −2 and 35.1 % for 1 s and 0.08 ·10 −2 and 17.2 % for 100 s measurement was achieved respectively. As an additional validation six cases of inhomogeneous and dynamic liquid fraction distributions were investigated qualitatively. We were able to determine the liquid fraction in froth in a spatially and temporally resolved manner with a penetration depth of 9.2 cm. This investigation distinguished between two of the three froth phases within the analyzed range. However, only processes with low liquid fraction (≤ 0.83 ·10 −2 ) can be monitored. This work be considered a a first step towards in situ monitoring of froth flotation processes.