The “Fifty Cent” rheometer-effect of slip, speed of lift and stability on measurement of yield stress

Abstract

Most researchers in the field of engineering accept the yield stress of a fluid as a useful engineering parameter. With the need for pumping and stacking mineral paste at higher concentration the focus has shifted towards its measurement. Since Pashias and Boger in 1996 first published the use of the 50 cent rheometer for measuring the yield stress on site, a number of authors have continued with this work. The slump cone originally developed to measure the workability of fresh concrete was the first of such measuring instruments. All the subsequent cones and cylinders include the cylindrical 50 cent rheometer have all been hand lifted. The Flow Process Research Centre at the Cape Peninsula University of Technology has developed a mechanical slump meter were the speed of lift can be controlled and lateral movements eliminated. Cones and cylinders made of stainless steel and PVC were fitted to the slump machine. In addition the 50c rheometer was also used. Tests have been conducted using Laponite, kaolin and the mineral tailing with different concentrations to ascertain whether the speed of lift and slip has an influence on the value of the yield stress measured. The results from each test set are compared with each other, the standard vane test, and with three published predictive approaches. Form the results obtained no consistent trends can be identified and it would appear that hand lifting can produce adequate results.