Online yield stress measurement for real-time process control

Abstract

The authors report a robust estimate of the shear yield stress of a slurry from inline measurement of a flowing slurry process stream. At present, slurry density is often used as a proxy in estimating the shear yield stress based on laboratory-scale measurements performed as a function of solids concentration. However, this approach leads to inaccuracies due to poor translation of laboratory data to operational situations and fluctuations in the sensitive yield stress-concentration dependence due to material and process variations. A robust inline measurement that directly relates to the in situ yield stress is therefore both a valuable and significant advance. In order to provide continual, inline rheology data for process control, the OnLine Rheometer Series 1000 (OLR) was piloted for a high clay, gold process thickener underflow in a pilot pipe loop facility. Benchtop measurements of the vane yield stress, and solids concentration were performed on samples drawn from the line as the circulating liquid was progressively diluted. The measured vane yield stress was compared with the Yield Stress Index (YSI), a native function available in the software of the OLR, over a concentration (c) ranging between 49.4 wt% < c < 64.5 wt%. Over the concentration range studied, the vane yield stress exponentially increased with concentration and ranged between 4 and 65 Pa while the YSI values ranged between 16 and 131 units showing an identical dependence on concentration. Importantly, it was found that the YSI correlated linearly with the yield stress values measured using the vane. It follows that the YSI metric can be used to estimate the yield stress within a small factor in actual operations. This finding supports the use of the YSI as an inline control variable in mineral processing and tailings management operations; in thickening and paste preparation for example.