Application of an in-line flow visualization technique based on ultrasonics for paste and thickened tailings

Abstract

Ultrasonic velocity profiling (UVP) is a technique that can measure an instantaneous one-dimensional velocity profile in a fluid containing particles across the ultrasonic beam axis or measurement line. A methodology for in-line rheometry combining the UVP technique with pressure difference (PD) measurements, commonly known as UVP+PD, has been developed and improved at the Swedish Institute for Food and Biotechnology and the Cape Peninsula University of Technology (Wiklund, (1); Wiklund et al., (2); Kotzć et al., (3)). The UVP+PD methodology allows measurements that are not possible with common rheometers such as radial velocity profiles and yield stress directly in-line and under true dynamic process conditions. Furthermore, it has advantages over commercially available process rheometers and off-line instruments in being noninvasive, applicable to opaque and concentrated suspensions and having small sensor dimensions. It has been evaluated for several potential industrial applications including, for example, paper pulp, foods, transient flows and model mineral suspensions. Similarly, the UVP technique can be applied to open channel flow by combining flow depth measurements in order to obtain rheological properties in-line. Industrial fluids, such as thickened paste etc., commonly found in tailings transportation exhibit wide particle size distributions, large particle sizes and very high viscosities. These industrial fluids cause strong attenuation of the ultrasound energy, which can significantly distort velocity profiles measured with the UVP technique or even make it impossible to conduct flow measurements at all with optical techniques. Initial results obtained in concentrated cement pastes and grouts as well as bentonite showed that UVP is a feasible and promising technique for flow visualisation and rheological characterisation in complex fluids. The results obtained showed significant potential for application in more viscous fluids and larger pipe diameters using new transducers and acoustic coupling technologies.