Slurries factors affecting viscosity

Solids concentrations can vary from a few percent to well over 50% in a typical stirred tank. Solids concentration, particle shape, and the viscosity of the suspending phase are the main factors affecting the rheology and settling characteristics of the slurry. Cubic- and spherical-shaped solids tend to form Newtonian slurries, while needle-, oblong-, and plate-shaped solids form thixotropic slurries. Such slurries exhibit yield stresses even at quite low solids concentrations. This can lead to the development of caverns, as shown in Section 9.4. Proper design can usually overcome these stagnation problems. 

The viscosity of slurries is a function of the solution and solid involved, as well as the slurry density. The viscosity can also be significantly affected by the particle size, size distribution, and particle shape. As a general rule, as particle shape varies from spheres to needles, the viscosity moves further from Newtonian behavior. A detailed discussion of factors affecting the viscosity of suspensions can be found in Sherman (1970). 

Figure 55 indicates that the level of percent solids is extremely important in determining the proper separation, the higher the concentration, the coarser the separation. The correction factor is a relative measure of slurry viscosity and is affected by such parameters as particle present and particle shape. A feed that contains a large amount of clay would tend to shift the curve to the left, resulting in a coarser separation. In contrast, the absence of fines would shift the curve to the right and result in a finer separation. 

When we consider many particles settling, the density of the fluid phase effectively becomes the bulk density of the slurry, i.e., the ratio of the total mass of fluid plus solids divided by the total volume. The viscosity of the slurry is considerably higher than that of the fluid alone because of the interference of boundary layers around interacting solid particles and the increase of form drag caused by particles. The viscosity of a slurry is often a function of the rate of shear of its previous history as it affects clustering of particles, and of the shape and roughness of the particles. Each of these factors contributes to a thicker boundary layer. 

Viscosity and Plasticity—Viscosity and plasticity are closely related. Viscosity may be defined as the force required to move a unit-area of plane surface with unit-speed relative to another parallel plane surface, from which it is separated by a layer of the liquid of unit-thickness. Other definitions have been applied to viscosity, an equivalent one being the ratio of shearing stress to rate of shear. When a mud or slurry is moved in a pipe in more or less plastic condition the viscosity is not the same for all rates of shear, as in the case of ordinary fluids. A material may be called plastic if the apparent viscosity varies with the rate of shear. The physical behavior of muds and slurries is markedly affected by viscosity. However, consistency of muds and slurries is not necessarily the same as viscosity but is dependent upon a number of factors, many of which are not yet clearly understood. The viscosity of a plastic material cannot be measured in the manner used for liquids. The usual instrument consists of a cup in which the plastic material is placed and rotated at constant speed, causing the deflection of a torsional pendulum whose bob is immersed in the liquid. The Stormer viscosimeter, for example, consists of a fixed outer cylinder and an inner cylinder which is revolved by means of a weight or weights. 

The types of sedimentation encountered in process technology will be greatly affected not only by the obvious factors—particle size, liquid viscosity, solid and solution densities—but also by the characteristics of the particles within the slurry. These properties, as well as the process requirements, will help determine both the type of equipment which will achieve the desired ends most effectively and the testing methods to be used to select the equipment. 

To obtain the absolute sound attenuation in the coal slurry, the diffraction loss, the acoustic mismatch loss, the attenuation due to the Teflon window, and the oil coupling must be calculated. Thus, it is difficult to accurately determine the absolute attenuation. In practice, one measures the relative attenuation with respect to a standard. The attenuation of ultrasonic waves in a solid suspension is attributed to three major factors, namely, scattering, viscosity, and thermal effects. Although the presence of particles affects the fluid viscosity and thermal conductivity, the primary source of attenuation may be due to particle scattering. Hence, one may define the relative attenuation of the HYGAS coal slurry by comparing the slurry attenuation with that of the carrier fluid, i.e., the toluene/benzene mixture. This can be expressed by the equation... 

The performance of cathodes is highly affected by the slurry properties and drying methods. Viscosity is one of the most important properties of slurries. It is determined by the slurry composition, including the choice of solvents, the solvent concentration, the properties of active material particles, the choice and concentration of binders, the choice and concentration of carbon blacks, and the sohds loading. It is also affected by additional factors, including mixing sequence, mixing time, and de-aeration processes. 

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