This article presents the theoretical basis for the yield plastic rheological model for shear-thinning mineral suspensions. The standard structural model is modified to allow for the hypothesis that the average interparticle bond strength increases as the structure (aggregates) breaks down. The resulting model is algebraically simple and contains the Newtonian, Bingham, and Casson models as special cases. If minor stresses are considered, the model will also describe suspensions with a low shear rate viscosity plateau (i.e., Carreau-type suspensions). The yield plastic model accurately describes the flow behavior of most shear-thinning suspensions and is consistent with the observed behavior in rheometers and pipelines.
IntroductionSolid-liquid suspensions (slurry) made of fine particles will often exhibit nonNewtonian flow behavior (rheology), including a yield stress (i.e., they will not flow appreciably below a critical shear stress value) and an apparent viscosity that decreases with increasing shear rate (i.e., shear-thinning). These slurries may be described as ''yield plastic'' fluids and are currently modeled using the Bingham (1916), Casson (1959), or Herschel and Bulkley (1926) models, as shown in Equations (1), (2), and (3), respectively:where s o is the yield stress (or zero shear rate stress), l 1 is the plastic viscosity (or infinite shear rate viscosity), _ c c is the shear rate, K is the consistency coefficient, and n is the flow index. While these rheological models have been used for decades, there are significant problems. The measured parameters (s o , l 1 , K, n) are usually applicable over only a limited shear rate range. If the parameters for a single model are determined for a single sample over different shear rate ranges, it is often found that the values differ (e.g., Hallbom and Klein 2004). This means that the parameters for a model intended to describe the shear stress versus shear rate relationship are variable functions of the shear rate. In effect, the parameters are ''variable
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