IntroductionIn particulate processing, the flow of the suspension is critically important to both the energetics and, indeed, to the ultimate success of the unit operation. There are a large number of primary and secondary operations in the minerals and manufacturing industries that fall into this category, and processes such as solids filtration, pumping, and thickening are but three relevant examples to this work. In nearly all particulate processing, the state of dispersion of the particles in the fluid and the volume concentration of particles are important, simply because these two parameters are the prime determinants of suspension viscosity. Concomitantly, the process is more often than not viscosity limited at high solids concentrations.The solids concentration is an easily understood parameter, but the role of the surface chemistry of a particle in particulate processing is less explicitly resolved. For example, in filtration and dewatering operations, the suspension needs to be flocculated to effect the process in a timely manner, however, this in turn results in the suspension exhibiting nonNewtonian flow behavior with increasing solids concentration. The transition concentration is considered classically as the gel point, and flocculated particulate suspensions in excess of this concentration typically exhibit a yield stress. This yield stress is a mechanical rigidity exhibited by the three-dimensional, space filling particulate network, with each particle exhibiting an attractive interparticle force.Yield stress plays an important role in a wide variety of industrial systems and products: the manufacture, storage, and application of paints, dyestuffs, pigments, paper coatings, printing inks, detergents, ceramic slips, carbon pastes, cosmetics, pharmaceutical formulations, and soon, and in the transportation of coal-water slurries, beneficiation of mineral ores and treatment, and disposal of fine particulate wet wastes. Indeed, in many situations pertaining to the processing of flocculated or semistabilized concentrated suspensions, yield stress turns out to be of greater relevance than viscosity. It is also of considerable theoretical interest as yield stress is dependent on the spatial disposition of particles, that is, their structural arrangement in the suspension and the nature and magnitude of the surface forces operating between them (Strenge, 1993). This is distinct from a classical suspension of noninteracting hard spheres, where model considerations negate the forces between particles and the spatial arrangement of particles is most usually geometric. Charged stabilized monodisperse particle suspensions also form crystalline yield stress fluids at a high volume fraction. Both of these examples show a mechanically rigidity as a result of a geometric particle constraints, the gel point of flocculated, ultrasmall particle suspensions may be as low as 5 vol. %.
AIChE JournalMay 1997 Vol. 43, No. 5
1171In order to predict the yield stress of a given suspension, as well as extract from the measure...
