Continuous flow rheometry for settling slurries

Abstract

The rapid settling nature of some industrial mineral slurries can cause problems in the measurement of their rheological properties. To address this problem a flow rheometer based on the principles of helical flow was developed. The rheometer designed, is a modified Couette flow system, whereby slurries are circulated through the concentric cylinders by the addition of an axial flow. The purpose of this axial flow is to prevent particles from settling and to maintain a homogeneous suspension. However, the addition of an axial flow component to Couette flow complicates the analysis procedure for non-Newtonian fluids particularly in wide gap geometries. Thus a specific emphasis in this study was placed on developing a correct analysis procedure for helical flow that eliminated the need for rudimentary calibration procedures. Experimental measurements with different liquids, including those with Newtonian and non-Newtonian flow properties showed good agreement between data obtained from the flow rheometer and data obtained using other standard laboratory instruments. Typical differences between the results from the flow rheometer and results from other laboratory instrument varied between 1-2%, with standard deviations in the flow rheometer data of between 2-4%. The flow properties of several non-Newtonian slow settling slurries were examined using the flow rheometer and also with a specially modified tube rheometer. As with the pure liquid results good agreement was obtained between the results from the flow rheometer and those obtained with the modified tube rheometer. Several rapid settling slurries were examined using the flow rheometer, but due to the rapid settling nature of these slurries they could not be examined with any other laboratory instruments. However, internally consistent results were obtained from different tests with the flow rheometer using different values of axial flow rate. These results demonstrate that the correct data analysis method was developed for the helical flow of non-Newtonian fluids Particle migration is a phenomenon known to affect the results of both rotational and axial flow rheological equipment. Whilst the motion of particles within the helical The University of Adelaide Continuous Flow Rheometry for Settling Slurries flow geometry could not be directly observed, careful examination of the results from several experiments with slurries showed that the effects of particle migration were minimal or non-existent within the flow rheometer. It is presumed that the circulation of the fluid through the geometry minimises the residence time in the geometry, which reduces the likelihood of particle migration. The development of Taylor vortices in a Couette type geometry can cause substantial errors in any rheological measurements. The flow rheometer is based on helical flow, which is a combination of both Couette and axial flow and as such may also suffer from measurement errors if instabilities develop in the flow. A stability criterion for the helical flow of non-Newtonian fluids is therefore required to ensure measurements from the flow rheometer were obtained in the laminar flow region. The stability criterion for laminar Couette flow of a Newtonian fluid was well known, as was the effect of imposing axial flow on Newtonian Couette flow. However, the effect of the rate of acceleration of the inner cylinder and the effect of non-Newtonian fluids on the onset of Taylor vortices was unknown. An increase in the rate of acceleration of the inner cylinder was found to have a destabilising effect on Couette flow. A modified Taylor number was developed for non-Newtonian fluids using the power-law model and was experimentally validated for a range of non-Newtonian fluids. These results were then used to develop a laminar flow stability criterion for rheological measurements of non-Newtonian fluids in the flow rheometer. To test the suitability of the results from the flow rheometer for use in the design and optimisation of process units, the power requirements to turn an impeller in a small baffled mixing vessel were investigated. Good agreement was obtained in the laminar and turbulent flow regions for a variety of Newtonian and non-Newtonian fluids between measured values of impeller power and those predicted using rheological measurements from the flow rheometer. Altering the density of the solid particles in a slurry is known to affect the overall rheological properties of the slurry. However, the effects of changing the liquid density were not so clearly defined and thus several artificial slurries of PMMA (poly-methylmethacrylate) spheres in water/NaCl and water/glycerol solutions were used to investigate this phenomenon. It was found that the slurry rheology was altered by The University of Adelaide Continuous Flow Rheometry for Settling Slurries changes in the suspending liquid density, however, these changes could be entirely attributed to changes in the liquid viscosity associated with the changes in liquid density. To summarise, the work presented in this Thesis provides a fundamental approach for the absolute measurement of the rheological properties of settling slurries, under conditions that more accurately represent those found in actual mineral processing operations.