Effect of sample manipulation on the Couette rheometry of copper concentrates

Abstract

Abstract The measurement of slurry rheology is a key factor in ore concentrate pipeline design and operation. In addition to the known effects of the input data, the manipulation and the particular conditions of the experimental apparatus may significantly affect the measured flow curves. In the present work, an experimental study of the influence of some of these factors in copper concentrate rheology was performed. In particular, the impact of sedimentation, hydrodynamic segregation and inertial effects is assessed through wide-gap Couette rheometry experiments using an automated, variable-speed sample-loading mechanism. Two types of experiments were performed by controlling the sample loading speed: the generation of an angular velocity ramp and individual torque measurements with constant angular velocities after an impulsive start. Two concentrations, on the order of those corresponding to typical turbulent transport of ore concentrates, were tested. The flow curves obtained from the first set of experiments show slight differences for loading times between 1 and 10 s. However, differences close to 100% were observed when the results obtained with loading times in the order of 1 s were compared with the results obtained with loading times of approximately 1 min. The individual torque measurements, which were represented by the maximum value within the corresponding time series, show some previously documented features of hydrodynamic segregation in monodisperse suspensions; however, consistent effects were obtained with the different sample loading times and angular velocities tested. Using scaling arguments, a set of five dimensionless groups including gravity, momentum, particle concentration diffusion and flow inertia, assessed by means of the sample loading time and the angular rotation of the inner cylinder, is proposed. Fitting such dimensionless numbers to the relative difference between the measured torques and those that should be obtained under purely homogeneous and inertialess conditions revealed a decreasing dependence of the sample loading time and increasing effects of hydrodynamic segregation and angular velocity. In particular, the present findings suggest that hydrodynamic segregation has a significant influence on the effective measurement of torque; this finding is consistent with previous results obtained with neutrally buoyant monodisperse suspensions. The experimental data suggest that the dimensionless torque may be interpreted as the result of a nonlinear interaction between the sample loading and the transient start of the rotation of the inner cylinder.