**1. Introduction**

Mixtures of particles and liquids are often considered to be *slurries*, but this is not always an accurate description. A gravel extraction might comprise large pebbles in the water, but the solid and liquid components would have distinctly independent behavior (**Figure 1**). A *slurry* has its own character quite different from carrying liquids or entrained bulk solids. The term covers a wide range of mixtures from "nonsettling" slurries, usually of very fine particles, to slurry fluidized beds1 .

Before we can study these intriguing mixtures, we must briefly discuss their *settling* propensity. The distinction between *settling* slurries and others is not precise. All particles in a liquid have a gravitational potential however minute that potential might be. A settling rate of 0.6 mm/s has been suggested as the upper limit for a "nonsettling slurry", but this is might be considered a little over-prescriptive. If a fine particle slurry can be shown to be effectively a non-settling continuum for a specific application, deposition concerns can be largely ignored and many offline and online measurements carried out. However, many industrial slurries have a settling component, and practitioners are finding new ways to face the challenge of handling them.

When studying the flow of a slurry, frictional headloss and pipe velocity are most important not least because they have a direct bearing on the requirements for a suitable pump. The orientation of the pipe run (horizontal, vertical, or graded) is another major factor. Determination of deposition patterns in horizontal pipelines is a non-trivial task while rising mains have axially symmetric velocity distributions but increased pressure drops. Lastly, the design concentration of the slurry or sludge has a pivotal influence on the choice of pumping system.

**Figure 1.** *Solid-liquid mixtures are not always slurries.*

<sup>1</sup> Slurry fluidized beds are gas–liquid–solid mixtures often given a category of their own. They have specialized industrial uses, for example in the synthesis of methane.
