Introductory Chapter: Slurry Technology – New Advances

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 “non-settling” slurries, usually of very fine particles, to slurry fluidized beds¹.

Figure 1.

Solid-liquid mixtures are not always slurries.

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 “non-settling 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.

2. Horizontal slurry ducts

Workers in the 1950s, following an influential study by Durand & Condiolis [1], identifiedregimes for the flow of slurries in horizontal ducts. Categories included a classification by size, but inter-relationships with pipe velocities, particularly those for deposition added a layer of complication in identifying the flow regime. Subsequently four regimes were proposed, based on increasing axial pipe velocity (other factors having been defined):

1. Flow with a stationary bed ($0<U\le U_{M1}$).

2. Flow with a moving bed ($U_{M1}<U\le U_{M2}$).

3. Heterogeneous flow with saltation heaps. As velocity progresses, these saltation heaps erode and move downstream with a rolling action. At the higher end of this velocity interval, all the solids are in asymmetric suspension ($U_{M2}<U\le U_{M3}$).

4. Pseudo-homogeneous mixtures in which all particles are in suspension follow in sequence from the heterogeneous pattern ($U>U_{M3}$). Ultimately all the solids are in a symmetric suspension and this can be termed a homogeneous mixture. Note that many researchers use the terms “pseudo-homogeneous” and “homogeneous” interchangeably.

It is not always straightforward to identify visually the flow regime of a particular slurry against the list of velocity categories alone. An interesting, if rather controversial, approach to this problem was proposed by Turian and Yuen [2] who proposed coefficients based on friction factor, efflux concentration, pipe diameter, and relative density of solid particles in addition to pipe velocity. These coefficients were to be used to choose the regime (0…3). A critical evaluation of the Turian and Yuen method is given by Mediema [3]. The incorporation of slurry flow variables, in addition to pipe velocity, underlines the over-simplification implied by the list, but the convenient four-fold categorization by velocity remains in common use.

Homogeneity in flowing slurries is aided by turbulence. At a small scale, the flow might be extremely complex, but particles are spread by strong mixing actions. At high velocities, concentration profiles across the duct are sensibly uniform. Conversely, non-settling fines in high concentration can be in laminar flow. When the mixture is very well mixed prior to transmission, the medium is confined to streamlines and very little mixing takes place within the duct.

When a slurry can be treated as a homogeneous or pseudo-homogeneous liquid, continuum techniques (including rheological modeling) can be applied to the flow as a whole. Strictly, these methods are not really applicable to other regimes, but that does not always prevent them from being applied.

In the discussion above, particle concentration has only briefly been mentioned as an important factor. The transportation of slurry flows in turbulent flow has usually been constrained by a 40% v/v limit. The reasons for this limit are in the fear of pipe blockage if greater concentrations are used. Interestingly, much greater concentrations are possible at the cost of improved infrastructure as authors will explore in later pages.

3. Vertical slurry ducts

The flow of slurry in a rising main brings a number of specific challenges. As already mentioned, the most obvious is the increased pressure that the pump must provide in addition to the frictional losses from the pipe wall. The weight in a rising main comes from the density of the slurry being pumped and this changes with elevation. The first task is therefore to establish the density of the slurry in the duct.

The in situ concentration, the concentration across a section of duct, yields the density of the slurry at that section. The in situ concentration ($C_r$) should not be confused with the concentration supplied to, or delivered from, the duct ($C_v)$. Holdup, the ratio of the relative velocity of the solids to the pipe velocity governs the in situ concentration, and this determines the mean density. Holdup is defined here as

$$\mathcal{H}=\frac{U-U_s}{U}=1-\frac{C_v}{C_r}$$

where $U_s$ is the velocity of the solids component, and U is the pipe velocity.

Engineers usually have knowledge of the efflux or delivered concentration required ($C_v)$ for an application, but holdup and in situ concentrations ($C_r$) are not generally known at the outset. An Artificial Neural Network (ANN) developed by Lahiri and Ghanta [4] is claimed to predict holdup with an absolute average accuracy of 2.5%. The value can be used to determine the mean in situ concentration and the appropriate density. Their definition of holdup ($\frac{C_r}{C_v}$) is algebraically linked to the definition given above, but can exaggerate the accuracy obtained, particularly for high values of holdup.

Vertical rising mains are found in many situations. They assume great importance in the mining industry both historically and in present day operations. Interesting, examples of interest to researchers in coarse particle slurry technology are deep-ocean mining and the extraction of coal-water mixtures from depth.

4. Low pipe velocities and non-homogeneous mixtures

At the low to medium pipe velocities found in industrial processes and pipelines, particle concentrations usually vary across a section. We recall the reluctance to pump a slurry of greater efflux concentration than 40% v/v for fear of blockages. For horizontal pipe runs, dense regions accumulate at the lower pipe wall as we have already discussed. Despite these difficulties, it is an economic proposition to reduce pipe velocity for at least two reasons:

1. Pumping power is approximately proportional to the cube of pipe velocity. If one can halve pipe velocity, energy consumed in pumping the slurry will be reduced by 87.5% in theory.

2. Pipe wear (per tonne) is approximately proportional to velocity to the power of 2.5. This implies a reduction in wear of approximately 82% for a halving of pipe velocity.

These factors have encouraged the development of swirl-inducing ducts (Figure 2). If sediment from a low to medium-concentration slurry can be swept into suspension at a low velocity, there will be savings in power consumption and pipe wear, not to mention the prevention of pipe blockages. The interested reader is encouraged to visit various contributions to the subject (e.g., Jones [5]).

Figure 2.

Swirl-inducing duct. Two-lobe, a low-loss design (illustrated). Four–lobe, for a greater swirl intensity.

High-concentration conveying offers an alternative way to exploit the benefits of low pipe velocity as explained in later pages of this volume. An example can be found in the pumping of liquid concrete. This material is a mixture of several particulate elements of widely differing sizes. Utilizing specialized pumping equipment, liquid concrete can be pumped at high concentrations and low velocity. Coarse particles cannot pack at the pipe wall, so a particle-depleted layer exists there. This is sometimes given the rather misleading term: the “lubricating layer” [6].

Broad size distribution in the material to be pumped is an important pre-requisite for pumping at high concentrations in a Newtonian fluid, such as water. There is a very inventive way around this stipulation. Some of the particulate materials can be ground to make a non-settling visco-plastic carrier, and the rest of the material can be carried within it. Essentially, at low velocity, the carrier is in laminar flow. Machines to create this carrier and to separate coarse particles add to plant costs. Dense-phase horizontal coarse particle transportation for coal slurries became available when a Rotary Ram Slurry Pump (RRSP) was invented [7]. To the incredulity of plant engineers, high-concentration slurries were not required to exceed a minimum velocity and could be stopped and restarted at will.

There are empirical methods to determine headloss in non-homogeneous slurries, but an alternative is a two-layer model invented by Wilson [8, 9]. Shook and Roco [10] presented the model “… in a form which should be easier to follow or modify.” Over the years, modifications were needed to improve stability and usability of the model, and a completely new interpretation (2LM) is now available. The essence of the model is the simplification of the non-homogeneous flow of slurry into two layers. The upper layer represents a component completely supported by hydrodynamic forces, and the lower layer is a component gaining support from pipe walls and hindered settling. This is a severe oversimplification in some eyes, but very good estimates can be obtained.

Designers rely on an estimate of the critical deposit velocity, which is the velocity at which a single particle, or perhaps a raft of several particles, will rest immobile at the base of a horizontal pipe. The roughness of the particle surface is greater than that of the pipe alone and this encourages greater accumulations of particles. These particles accumulate into dune-like saltation heaps. The curvature of streamlines around the tops of the heaps has an erosive action on the upper particles, which causes a downstream rolling action. This low-velocity downstream motion of dunes characterizes heterogeneous flow, but it can be overwhelmed, ultimately causing a pipe blockage. Hence, a fundamental design requirement is to apply a pipe velocity greater than the deposition velocity. The 2LM model estimates the pipe velocity for a stationary bed. This velocity is not identical to the critical deposit velocity because the stationary bed is not necessarily a single layer of particles but the differences in estimates are relatively unimportant.

5. Instrumentation and measurement of slurry flows

Slurries are fundamental mixtures, and this lies at the heart of problems in making useful measurements. Designers have little choice but to use historical empirical evidence often based on examples of differing flow regimes and differing circumstances. It is encouraging to note how successful these designs have been in the hands of experienced practitioners.

Size distributions of industrial slurries can often be broad, non-symmetrical, or perhaps multimodal. Similarly, distracting is the spread of property values in a particle population. This is particularly true of coal slurries in which constituents can have hydrophilic (easily wetted) or hydrophobic (resisting wetting) tendencies and a broad spread of density. The particles themselves can differ markedly in solid geometry from spherical models from which many theoretical predictions have been built.

Setting aside difficulties in accurate prediction, the situation is not entirely bleak. Great progress has been made in online measurements of slurry flows. Radiometric methods have been used to measure the physical properties of a slurry flow. The same methods, at a higher energy level, can indicate the concentration of a flow by interrogating the transmission of the radiometric beam across it. Most interesting of all is visualization technologies which can give us concentration and distribution mapping. These include Particle Image Velocimetry (PIV), Laser Doppler Anemometry (LDA), and Tomographic techniques [11]. Electrical Resistance Tomography stems from early work in the 1930s and 1940s [12], before computational methods opened it up to widespread use. Electrical Capacitance Tomography used to form computed images of pipe contents was investigated from the 1980s [13]. The techniques involve the measurement of electrical properties of the slurry from the pipe periphery. The difficult job has been to reconstruct the interior structure from the measurements obtained—“The Reconstruction Problem.”

6. Pipe wear

An unavoidable consequence of flows containing particles is wear. Enormous costs to industry are attributable to wear, but costs can be reduced by careful design. Since 2017, many advances have been made in the study of erosion-corrosion in experimental and in-situ monitoring of wear, condition monitoring, the monitoring of wall erosion and computational modeling to name but three.

Designers of slurry systems will often reach for answers from Computational Fluid Dynamics (CFD). There is certainly a promising future for CFD erosion modeling, but that advance is not yet with us. Despite improved particle impact modeling, dynamically deforming geometry and improved particle tracking, the accuracy of erosion predictions is poor. There is hope of improvement from two comparatively new approaches explored by authors in this volume. The Discrete Phase Model (DPM) plots the path of a group of dynamically similar particles through a continuous liquid, while the Discrete Element Model (DEM) tracks individual particles and solves the equations of motion as they interact with each other. In theory, the accuracy of the DPM approach can therefore be improved by a coupling with DEM. A CFD-DPM-DEM combination has been used to model air-particle flows, but no attempt at extending the technique to solid-liquid flows has been reported to date. There is great potential for this combination but a disincentive to its development is the very large computational cost that would be incurred. One of the most significant improvements in the application of CFD is the Moving-Deforming-Mesh (MDM) approach. The computational mesh is updated at regular intervals based on local erosion rate and the geometry of the wall. In this way erosive deformations are integrated into the flow field and secondary erosion patterns can be predicted.

Very little research has been carried out on the influence of pipe layout. on the life of vulnerable components. Pipe geometry upstream of a vulnerable pipe section can have a significant effect on erosion rates. For example, the swirl pipe design described above can show significant wear reductions in adjacent downstream pipe components. Unsurprisingly internal surface finish (particularly in pipe elbows) has a disproportionate effect on the erosion pattern suffered in service. Comparatively minor melioration in this area could be influential in increasing component lifetimes.

7. Sludges, Slurries and Wastewater

The problem of movement, transportation, and handleability of slurries and sludges is a major preoccupation for many industries. Their causes are many and various. The character of the particulates, particularly clays, and their size distribution have an important influence. Moisture has a complex relationship with handleability: too much and a slurry must be contained, too little and a sludge can be sticky and difficult to move. The study of flow problems from hoppers has a long history [14] and since the 1950s it has gathered pace as a subject of interest to industry and academia. The Jenike Cell [15, 16, 17] was devised to measure the shear forces in particulate materials.

Industrial slurries are generally thickened to a sludge of water content of about 25% v/v. The applied stress at which a sludge distorts or moves is termed the yield stress. A shear vane tester (Figure 3) is a widely-used and readily-available device for measurements of shear stress in cohesive sludges [18]. The vanes are distributed in a cruciform shape around a spindle which can be rotated. Immersed in a sludge the spindle is rotated at between 5 and 12 degrees per minute until the sludge fails and the vane rotates in equilibrium with the RSS or “Residual Shear Stress” [19]. The insertion of the vanes, and the rotation of them, has been suspected of changing the structure of a sludge and causing inaccuracies in measurement [20, 21, 22]. The controversy stems from the observation of viscous deformation before the yield point [23, 24]. This “wall slip” phenomenon is fairly important in geo-structures (harbor walls for example) and is not shown by vane tests. Wall slip is explored in later pages of this volume.

Figure 3.

Vane Tester.

Despite controversies over certain critical measurements, the vane tester continues to be the measurement of choice for the shear strength and handleability of sludges.

Dewatering in municipal wastewater treatment is an important field of study. Some reasons for sludge dewatering are as follows.

1. Reclamation of water.

2. The reduction of volume of municipal sludge. The cost of transportation is a large recurring cost for municipal budgets.

3. Reduction of harmful leachate emissions.

4. Creation of a friable product which can be used in agriculture.

5. Production of a calorific product for energy production. Moisture lowers the calorific value of a fuel by virtue of the energy required to evaporate the liquid component.

6. Limiting of damage to the natural environment, including the control of discharges into inland waterways.

Dewatering can be carried out in many ways, for example, in warm countries lagoons can allow the sun to evaporate the water content. Periodically, industrial machines can then be employed to carry off the dried sediment. Large plate presses can be employed to reduce the moisture content of sludge to cakes of concentrations of about 20% v/v. Plate press installations are costly to build and equip, but the running costs are relatively low. An alternative to plate presses are belt filter presses which are less expensive to install, but with relatively high running costs due to the additives which are needed [25].