Showing papers by "Richard Buscall published in 2017"

Variability of shear yield stress – measurement and implications for mineral processing

Abstract: At high concentrations, mineral tailings exhibit solid-like behavior up to a critical shear stress, the shear yield stress , which must be exceeded before flow may occur. The magnitude of has a significant influence upon a wide range of important mineral processing operating parameters, including pipeline pressure gradient, thickener operation and tailings beach slope, for example. Although the concept of a critical shear stress is a simple and convenient engineering metric, in practice the transition from solid to fluid-like behavior is much more complicated, resulting in a range of apparent yield stress values that vary with the applied shear conditions. For example, in constant rate or flow start-up rheological testing, the peak stress (one measure of ) can more than double depending on the applied strain rate. In constant stress tests, there is a range of stress over which the slurry exhibits solid behavior over short timescales, but may yield and flow on much longer timescales. Other methods for yield stress determination, such as extrapolation from steady-state stress versus rate data, the onset of non-linearity in oscillatory flows or slump testing can generate an even broader range of yield stress values. Whilst it is neither practical nor desirable to incorporate such complex yielding behaviour into basic models of engineering operations, it is of paramount importance to use an appropriate measure of shear yield stress for a given application. The time and rate dependencies of the selected measurement method must match the time and rate conditions of the intended application. Failure to do so can introduce significant errors with attendant design, operation and cost implications. The present work takes recent insights into the shear rheology of minerals tailings and discusses appropriate measurement techniques for a range of processing applications, including beach slope and pipeline pressure drop prediction, as well the implications of using inappropriate measurement techniques. Rheological measurements of a model system are used to demonstrate the variability of the apparent yield stress with measurement technique. Comparisons with model values of pipeline pressure drop and beach slope are used to demonstrate the scale-up implications of using inappropriate rheological data.

Correction of wall adhesion effects in the centrifugal compression of strong colloidal gels

Abstract: Several methods for measuring the compressive strength of strong particulate gels are available, including the centrifuge method, whereby the strength as a function of volume-fraction is obtained parametrically from the dependence of equilibrium sediment height upon acceleration. The analysis used conventionally due to Buscall & White (1987) ignores the possibility that the particulate network might adhere to the walls of the centrifuge tube, even though many types of cohesive particulate gel can be expected to. The neglect of adhesion is justifiable when the ratio of the shear to compressive strength is small, which it can be for many systems away from the gel-point, but never very near it. The errors arising from neglect of adhesion are investigated theoretically and quantified by synthesising equilibrium sediment height versus acceleration data for various degrees of adhesion and then analysing them in the conventional manner. Approximate correction factors suggested by dimensionless analysis are then tested. The errors introduced by certain other approximations made routinely in order to render the data-inversion practicable are analysed too. For example, it shown that the error introduced by treating the acceleration vector as approximately one-dimensional is minuscule for typical centrifuge dimensions, whereas making this assumption renders the data inversion tractable.