The effect of particle size distribution on the antithixotropic and shear thickening properties of coal–water dispersions

Abstract

The shear thickening rheology of coal–water suspensions prepared from six different naturally generated particle size distributions with mass median particle diameter d50 from 0.8 to 28 μm were investigated. Refined coal samples were thoroughly deflocculated with an anionic dispersant in water at particle concentrations from 47.7 to 56.4 vol %. The variations in flow behavior of the dispersions were observed in Couette flow as a function of shear rate in ramp tests and of time as a constant rate of shear was applied. Ramp test results indicated a decrease in the severity of shear thickening with increasing particle size distribution. Discontinuous shear thickening that occurred with the smaller particle sized samples was found to be approximated by a single function of the particle concentration reduced by the maximum packing fraction. The experimental results from constant shear rate experiments were compared in terms of the rate of antithixotropy, the extent of stress increase, and the total energy dissipated prior to shear thickening, as determined from characteristic points in the stress‐time rheograms. Significant differences in the shape of the antithixotropic curves were observed for the different particle size distributions. The magnitude of stress increase observed during the antithixotropic event was found to diminish with increasing particle size distributions which limited the detection of the antithixotropic event at the larger sizes. The extent of stress increase for the smaller particle sized samples, i.e., d50 < 3 μm, was such that the antithixotropic shear thickening appeared to dominate the flow behavior. A bimodal mixture of particles at d50 = 0.77 and 7.9 μm was also examined. An 8% increase in volumetric particle concentration was observed when compared to a monomodal suspension with equivalent d50 and average viscosity. No antithixotropic tendencies were observed with the bimodal suspension.