Spread is better: An investigation of the mini-slump test

In the rapidly evolving application of surface deposition of high density, thickened tailings (paste), a key design parameter is the yield stress. A method widely used in industry to obtain quick and easy measurements of the yield stress is the slump test. This paper investigates current techniques for interpreting the cylindrical slump (or mini-slump) test. The lifting process of the cylindrical mould was taken into account in numerical simulations using a computational fluid dynamics (CFD) approach. Simulations with different mould lifting velocities were carried out to understand the influence of mould lifting velocity. Therefore, the influence of plastic viscosity and yield stress on mini-slump test results was studied using a mould lifting velocity of 0.01 m/s, which is representative of rates used in laboratory tests. The predicted slump and spread from mini-slump test simulations for three different scenarios (vlifting = 0.002 m/s, vlifting = 0.01 m/s, and without mould lifting process, i.e. instantaneous disappearance of the mould) were compared to those from laboratory experiments on kaolin. The rheological properties of the kaolin were measured using a vane viscometer and the data used directly in the modelling study. The results suggest that the lifting speed of the mould has a significant influence on the mini-slump test result, which must therefore be taken into account in both numerical simulations and laboratory tests. It was found that the variation of mould lifting velocity had a greater influence on slump than spread, indicating that spread is a more appropriate measurement for determining the yield stress in a mini-slump test. This was particularly true for relatively low yield stresses (e.g. 60 Pa or less), which are values typical of most thickened tailings deposits currently operating internationally.