The series of Tim’s Top Tips has been expanded to now include three Tip’s on:
• How to measure thixotropy
• How to measure yield stress
• How to measure flow and viscosity curves
These tips are available as printed hard copies free on request.
These new Tips have a different theme – they provide details on how to measure each of the rheological parameters, the measurement techniques and pitfalls of each technique, a summary plus a dictionary covering the working definition for the technical terms included in the note.
Yield Stress
Background and Discussion
Yield stress has an impact on the mining industry in a variety of ways. In pumping and mixing at start-up, it must be overcome so that the impellers can turn. In pipeline transport yield stress influences the velocity profile of the material, and under some circumstances can cause the material to flow as a solid plug carried by a lubricating liquid layer at the wall where shear stresses are high. The yield stress of a material dictates whether or not a solid fraction will settle to the bottom of a tank, pipe or other container, and also whether gaseous materials can rise through it. Disposal of mining waste is an important issue and the yield stress influences the slope of beaches in disposal areas, the ability of high solids pastes to be compressed and to flow after disposal and so on.
Thixotropy
Background and Discussion
Thixotropy is a relative measurement, as such depends on the experimental conditions and technique used to measure it. There are several such techniques, the simplest being modified flow curves, or constant shear rate or shear stress measurements. A more complicated, but perhaps more intuitively understood method is the shear and recovery method. In order to attain repeatability the most appropriate technique should be selected and the sample handling and experimental procedures defined fully. These should be unchanged for all tests, so that thixotropy for different materials can be properly compared. Thixotropy is a relative measure of the extent and speed of recovery of the internal structure of a material during and after shear. It is useful because it allows an estimate of the effects of agitation, pumping etc for prolonged periods, and also the effects of ceasing the agitation etc (i.e. how quickly the structure will rebuild, and how difficult it will be to restart the process as a result).
Flow and Viscosity Curves
Background and Discussion
A flow curve is used to define the interdependency of the shear rate and shear stress for a material. To generate a flow curve either a range of shear rates or of shear stresses can be imposed on the material, and the other of the pair – the response of the material – is measured. A viscosity curve shows the viscosity of a material at each shear rate from the flow curve. Flow and viscosity curves can be presented either on logarithmic or linear axes. They describe the flow behaviours at a variety of shear rates, so that pump, pipe, impeller, etc sizing can be correctly achieved. They are also important because they define the type of flow behaviour one can expect from a material – Newtonian, pseudoplastic (shear thinning), dilatant (shear thickening) etc.
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