Rheological aspects of dense lignite-water suspensions; time dependence, preshear and solids loading effects

Abstract

An investigation of the rheological properties of dense lig- nite-water suspensions is reported here in order to evaluate the parameters that affect the rheology of these suspensions. Different types of particle size distributions were achieved via wet grinding with par- ticle sizes down to 1 lm. Polyelec- trolytes and surfactants were used to improve the stability and rheology of suspensions. The achieved solid volume fraction, /, with acceptable rheological behavior was 0.45, while the ratio of ///m was close to 0.85; /m is the maximum solid volume fraction. Various types of flow tests were performed on lignite-water suspensions with various preshear times and stress levels. Typical shear rates varied in the range between 10 -2 to 10 2 s -1 . The low shear behavior was found to be quite different from the high shear behavior for the most concentrated suspensions, with a plateau value at low shear rates indicating the appearance of a yield stress. These plateau values, howev- er, depend on shear history, which is responsible for the development of different structures in the sample. The viscosity curves corresponding to the ascending and descending parts of the flow curve were found to be different; these flow curve parts can be described either by well established models or by modified ones. The non-Newtonian time dependent behavior of the lignite- water suspensions is attributed to the high value of the ratio ///m, the polydispersity of the particle size distribution, and the non-spherical shape of the lignite particles.