Knight, A, Sofrà, F, Stickland, A, Scales, P, Lester, D & Buscall, R 2017, 'Variability of shear yield stress – measurement and implications for mineral processing', in A Wu & R Jewell (eds), Proceedings of the 20th International Seminar on Paste and Thickened Tailings
, University of Science and Technology Beijing, Beijing, pp. 57-65.
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.
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