Non-settling slurry

At high particle concentrations, slurries are often non-Newtonian. For non-Newtonian fluids, the relationship between the shear stress and shear rate, which describes the rheology of the slurry, is not linear and/or a certain minimum stress is required before flow begins. The power-law, Bingham plastic and Herschel-Bulkley models are various models used to describe the flow behaviour of slurries in which these other types of relationships between the shear stress and shear rate exist. Although less common, some slurries also display time-dependent flow behaviour. In these cases, the shear stress can decrease with time when the shear rate is maintained constant (thixotropic fluid) or can increase with time when the shear rate is maintained constant (rheopectic fluid). Milk is an example of a non-settling slurry which behaves as a thixotropic liquid. 

What type of pump would be used in pipeline transport for an abrasive, non-settling slurry requiring pressures up to 60 bar ... 

Heywood, N. I. 1991. Rheological characterisation of non-settling slurries. In Slurry Handling, Edited by N. P. Brown and N. I. Heywood. New York Elsevier Applied Sciences. 

If the particles in a slurry are very fine, they may form a truly non-settling slurry in which their immersed weight is balanced by Brownian motion which arises from the thermal energy of the fluid molecules. Clay suspensions, consisting of particles of diameter finer than 1-2 pm are of this type. 

A slurry is a liquid containing solid particles in suspension. Slurries can be divided into two classes settling and non-settling. 

Nonsettling slurries are formed with fine particles or plastics or fibers. Although their essentially homogeneous nature would appear to make their flow behavior simpler than that of settling slurries, they often possess non-Newtonian characteristics which complicate their flow patterns. In Newtonian flow, the shear stress is proportional to the shear strain,... 

Liquid-solid mixtures that do not settle out rapidly are usually treated as non-Newtonian fluids. This will usually be the case if the solid particle size is less than about 200 microns (0.2 mm). Larger particle sizes will form settling slurries and require a critical velocity to maintain the solids in suspension. Correlations for critical velocity and pressure drop are given in Perry and Green (1997). 

Shah (22, 23) investigated the rheological behavior of moderately concentrated (up to 35 vol%) suspensions prepared with non-Newtonian carrier fluids. Experimental data on the Poiseuille slit flow of suspensions of sand in HPG solutions were presented. Flow data were gathered by pumping various non-Newtonian slurries into an 8-ft-tall vertical apparatus, with care taken to avoid settling difficulties. 

Spray drying is a unit operation involving the conversion of a pumpable liquid feed to a dry particulate solid by means of contact with a hot drying medium. Solutions, emulsions, non-settling suspensions, and slurries can be spray dried. It must be remembered that to be suitable for spray drying, the feed must be in pumpable condition or should be made so by dilution. 

Settling, however, may not be feasible or practical for nanosized particles unless strong agglomeration is present. Settling is also influenced by other factors such as viscosity and structure formation. The turbidity measurements are most reliable when used on non-settling and translucent slurries. 

Figure 4.1 Examples of hydraulic characteristics for settling and non-settUng slurries...

Experimental closed tank results for sands with average particle sizes of 0.18 and 0.5 mm in pumps with impeller diameters of 0.35 and 0.6 m, respectively, were reported by Herbich [5]. Slurry densities were up to about 1400 kg/m. It was found that the NPSHR-values (expressed in m of slurry) were similar to the water values, independent of the slurry density. Similar results were also reported by Herbich [51 and Ladouani et al. [6] for non-settling clay-silt slurries with densities of up to 1300 kg/ m in pumps with impeller diametres less than 0.275 m. Ladouani et al. [6] used an open-tank loop arrangement. Detailed inspection of their data indicates that the independence of the slurry density on NPSHR was limited to flow rates smaller than about 70% of the best efficiency point (BEP). With larger flow rates, NPSHR increased with increasing slurry densities, giving values from 1 to about 2 times the water values in the BEP-region. 

Many older systems pond the natural byproduct (and coal ash). Usually, the slurried solids are pumped to the pond at a solids concentration less than 50%. The slurry either is the thickener underflow or comes directly from the scrubber, with the pond acting as the thickener/clarifier. Qarified liquor is usually returned to the scrubber (Jones, 1977). The two major environmental considerations with ponding are the potential water pollution problem associated with the soluble material and the land-degradation potential of non-settling or physically unstable solids. If the disposal site is to be reclaimed when the pond is retired, the retention of water in the byproduct is a serious concern. 

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