A new interpretation of viscosity and yield stress in dense slurries: Coal and other irregular particles

Abstract

Illinois coal was ground and wet-sieved to prepare three powder stocks whose particle-size distributions were characterized. Three suspending fluids were used (glycerin, bromonaphthalene, Aroclor), with viscosities ηs that differed by a factor of 100 and with very different chemistries, but whose densities matched that of the coal. Suspensions were prepared under vacuum, with coal volume fractions φ that ranged up to 0.46. Viscosities were measured in a cone-and-plate over a shear rate {Mathematical expression} range 10-3-102 s-1. Reduced viscosity ηr = η/ηs is correlated in the high-shear limit (η∞) with φ/φM∞, where φM∞ is the maximum packing fraction for the high-shear microstructure, to reveal the roles of size distribution and suspending fluid character. A new model that invokes the stress-dependence of φM is found to correlate ηr well under non-Newtonian conditions with simultaneous prediction of yield stress at sufficiently high φ; a critical result is that stress and not {Mathematical expression} governs the microstructure and rheology. Numerous experimental anomalies provide insight into suspension behavior. © 1985 Steinkopff.