Method-dependent variation of yield stress in a thickened gold tailings explained using a structure based viscosity model

Mine tailings may be sufficiently dewatered prior to deposition such that they exhibit a yield stress and therefore they will form gently sloped deposits, which result in a number of advantages from an engineering perspective. Predicting the slope and the shape of these deposits at the field scale remains challenging, and is probably the key technical unknown holding back more general adoption of thickened tailings disposal in the mining industry. Methods for estimating the slope are very sensitive to rheological data, in particular the yield stress. This paper presents data from rheometry on a gold tailings that presents yield stress values in the range of 18–125 Pa. A rheometer with a vane fixture was used in a number of techniques, including controlled increments in strain rate to generate a flow curve, stress relaxation, stress growth and creep techniques. A controlled stress technique was used to simulate the stress history that the material would experience in the field as the tailings slow to a rest. The measured yield stress varied substantially (18–125 Pa) depending on the measurement method. This result is explained using a structure based viscosity model, modified from work created by others working on clays. Ageing and shear rate appear to be significant factors that influence the rheology, though the mechanism for ageing may be partly due to gravity driven particle settling, as opposed to or in addition to the buildup of a network structure