High Concentration, Coarse Particle, Hydraulic Conveying

*Lionel Pullum*

### **Abstract**

Conventional coarse particle hydraulic conveying is performed under turbulent flow conditions, usually at concentrations of less than 40% v/v. The last three or four decades have seen the development of much higher concentration conveying, with the successful transport of suspensions of 70% v/v or more. These suspensions can be conveyed at very low velocities and generally exhibit very benign characteristics, having the capability of being stopped and restarted at will. There are generally two methods of pumping coarse materials safely at low velocities. The first method, using a Newtonian such as water, can be applied when the particle size distribution is sufficiently broad to minimize percolation and the concentration sufficiently high to prevent particle restructuring during transport. A second method uses a non-Newtonian, visco-plastic carrier fluid, normally fine particle slurry, to convey the coarser particles. This second method removes the constraints of the first method, allowing a greater range of coarse solid distributions and concentrations to be pumped. In both cases, the conveying characteristics appear similar to laminar flow. Both methods are described and analyzed in this chapter.

**Keywords:** hydraulic transport, coarse particle, high concentration, no-Newtonian, hybrid suspension

### **1. Introduction**

Except for the transport of pastes and non-Newtonian slurries, most hydraulic transport applications operate under turbulent conditions at low to moderate volumetric concentrations. This contrasts with pneumatic transport systems which operate in both dilute, or lean, phase conveying and dense phase conveying. Dilute phase is analogous to this form of hydraulic conveying, while dense phase conveys solids at much higher solids' concentrations and at lower velocities. Over the last three or four decades, advances have been made in the development of higher concentration hydraulic transport systems, which share many characteristics in common with dense phase pneumatic systems.

Conventional hydraulic transport systems are turbulent, relying on the action of turbulent eddies to suspend and convey the solids in the pipe. At high velocities, the solids'suspension is almost uniform across the pipe, while at lower velocities, a pronounced concentration profile appears, forming sliding and even stationary beds of solids on the bottom of the pipe. Such flows are unstable and solids' transport will stop when the mean velocity falls below a minimum conveying velocity, and may even block the pipe. To minimize this risk of blockage and assist in restarting, a rule of thumb is used whereby the in-line solids' concentration, *Cvi*, is generally limited to less than 40% v/v. The minimum conveying velocity is a function of the solids' concentration, size and density, and can typically range from 1 to 2 m/s for fine particles (d < 1 mm say) to 7–10 m/s for coarse particles (d > 5 mm say).

Despite these unstable characteristics, such conveying has many advantages in the main areas where it is practiced, namely inter-process transfer, dredging, tailings disposal and long distant transport of ore or concentrates. It is simple to operate, requiring only that those solids are mixed with water in a suitably agitated vessel of some form, and pumped, normally using centrifugal pumps, through the pipeline to their destination. The low concentration also affords easy separation of the solids from the conveying fluid.

By contrast, outside of the food and some process industries, high concentration conveying is rarely seen, concrete pumping being the most obvious exception. However, high concentration systems have been developed and are finding increasing use in tailings disposal, hoisting and other niche applications. Their attractiveness lies in their benign operation, as they generally do not require a minimum conveying velocity, so lines can be stopped and restarted at will. Their low velocity, typically of order 1–2 m/s, several meters per second lower than their coarse particle low concentration counterparts, results in low particle attrition and pipe wear. This mode of conveying also allows very coarse particles to be conveyed at low particle pipe diameter ratios, e.g., *D*/*d* ≤ 5. The high concentration, 40<*Cv* ≤75% v/v, also produces low specific energy consumption (SEC), producing low operating costs (OPEX) and low greenhouse gas emissions. These advantages, however, are offset against a more complex operating system and the need for more specialized equipment (CAPEX).

Two forms of high concentration conveying have been developed, one using Newtonian fluids, like water, as the carrier fluid to convey the solids, and a second using non-Newtonian carrier fluids. Although sharing many characteristics, these methods will be described separately starting with the Newtonian system.

## **2. Newtonian high concentration conveying**

#### **2.1 Concentration considerations**

While weight concentration, *Cw* w/w, or worse weight of solids per volume of liquid, *Cwv* w/v, are common in industry, it is the volumetric concentration, *Cv* v/v, that characterizes and controls the way a suspension of solids behaves. At high concentrations, another more useful way of describing the solids' concentration is the ratio of the volumetric concentration to the maximum packing concentration of the solids, or more usually the maximum dilated or loose poured concentration, i.e., *Cv*/*Cb* known as the reduced concentration. The term *Cb* is used here to denote this packing concentration, as this value corresponds to the in-situ concentration of a settled or sliding bed of solids within the pipe. For non-Newtonian systems, described in §3, the bed concentration will be lower than for Newtonian systems, and will be described in greater detail there. This value may be considered to indicate the pipelines obscuration, or particle's mobility.

*Cb* is a function of the particle's size distribution (PSD) and shape distribution (SSD), from which this value can be calculated (for example [1, 2]), or measured

using a procedure outlined by Leuenberger [3], as follows. A tank of initially dry solids is continuously mixed with a pitch-bladed agitator, and the power drawn by the agitator monitored while water is progressively added. An example of such a test is shown in **Figure 1**.

The typical response to the addition of water (**Figure 1b**) is to change the solids from a purely granular flow in air, to one where an increasing number of water bridges are formed. Shearing these as well as the solids requires increasingly more energy. Once all the interstices are filled with water, further addition of water simply separates the particles, reducing the mixer energy requirements. Thus, the value of 1- *Cvwater* corresponding to the peaks in the characteristic curves approximates *Cb*.

The calculated value of *Cb*, based on the coal's PSD and SSD, is approximately 89%, whereas the empirical value is around 86%. The discrepancy, although small, is because the empirical value is for a sheared, and hence a dilated bed. It is recommended that the empirical value is used, or that calculated values should be reduced by a couple of percentage points.

#### **2.2 Conveying characteristics**

As mentioned in the introduction, both dilute and high concentration conveying is commonplace in pneumatic conveying systems, and the operational or transport characteristics are displayed in what is known as a diagram of state (**Figure 2**).

At moderate to high velocities, the transport characteristics exhibit the familiar hook curves of conventional hydraulic conveying systems, moving from a fully suspended state at high velocities, to sliding beds as the velocity is decreased. After going through a minimum, the pressure gradient rises due to partial obscuration of the pipe and increased particle wall contact. For hydraulic systems, which normally use centrifugal pumps, this is an unstable region, but for pneumatic systems, which use a variety of different feeding systems, e.g., blow vessels, transport can still be obtained down to very low velocities. At these low velocities, the line is filled with solids that

#### **Figure 1.**

*(a) Broad PSD of typical run of mine (ROM) coal, (b) experimental value for two different agitator speeds and calculated (dashed line) determination of Cb for the same ROM.*

**Figure 2.** *Diagram of state for coarse particle pneumatic conveying.*

move along as a continuous plug, although this plug is often deliberately broken into smaller plugs to reduce the pressure requirements. At intermediary velocities, however, the flow is unstable, exhibiting dune flow that can block the pipe.

Wilson et al. [4] examined the behavior of dense phase hydraulic hoisting, subsequently publishing an analysis of the frictional component of such plug flow [5], and such techniques were employed in the Hansa underground mine to hoist -60 mm coal, 850 m to the surface [6]. To the author's knowledge, though, it was not until the invention of a feeding system called a Rotary Ram Slurry Pump (RRSP), described in §2.7, that dense phase horizontal coarse particle hydraulic conveying was considered at an industrial scale.

The RRSP enabled very high concentrations of coarse particles to be loaded into a pipe using a valving system that opened to the full bore of the conveying pipe. These particles could be conveyed at very low velocities with little, if any, inter-particle movement. This feature was ideal for friable materials such as coal. Transport characteristics for ROM coal conveyed in a 300 mm pipe using this technique are shown in **Figure 3**.

The transport characteristics do not exhibit the typical hooked curves of lower density conveying, although transport below the lowest velocities shown were problematic. Suspensions with concentrations below 65% were also unstable for this particular material. Readings from an isolated electrode probe (see e.g. [7]), inserted into the top of the pipe, registered periodic high activity at these lower concentrations, indicating possible dune formation. A two-layer model, at concentrations close to plug flow, was used to model this flow with satisfactory agreement as shown. However, the adoption of a normal two-layer model is found to substantially overpredict the transport pressure gradients shown, and modification to such a model is needed before the agreement shown here is obtained.

### **2.3 Two-layer modeling modifications for coarse particles**

Only a simple two-layer model (e.g. [8]) will be used in this section for illustrative purposes, and the reader is referred to the chapter on Pipeline Modeling for a more complete description and a more nuanced approach. The early layered models were developed to model the behavior of small sand particle slurries, which become fluidized during transport. This is evident by observing slurry issuing from holes in the upper invert of the conveying pipe, where the slurry behaves as a fluidized jet [9].

*High Concentration, Coarse Particle, Hydraulic Conveying DOI: http://dx.doi.org/10.5772/intechopen.107230*

#### **Figure 3.**

*Typical high concentration, coarse particle, transport characteristics. Computed curves of constant delivered concentrations and partial deposition locus using a two-layer model. The bed depth* Ybed *is also shown.*

#### **Figure 4.**

*(a) Schematic of pipe cross section giving the normal force distribution for fine and coarse particles, (b) wear pattern in a 300 mm pipe used for conveying -32 mm solids, showing unworn original oxide coating in pipe upper invert.*

Such behavior is not observed in coarse particle flow. This difference has ramifications for the modeling of the resisting force of the bed in a two-layer model. Consider the schematic of the pipe cross section shown in **Figure 4a**.

If the solids are fine and fluidized, the bed presents a hydrostatic pressure at the pipe wall (**Figure 4a**, left hand side), which is normal to the pipe wall. If the density of the fluid and solids are *ρ<sup>f</sup>* and *ρs*, respectively, then the normal force of the bed on the pipe wall is given by

$$N = 2\text{Cb}\left(\rho\_s - \rho\_f\right) \text{g}R^2 \int\_0^\beta \cos\left(\beta\right) - \cos\left(\theta\right)d\theta \tag{1}$$

Where *β* is the half bed angle shown in **Figure 4a** and which on integrating and rearranging becomes

$$N = 2\text{Cb}\left(\rho\_s - \rho\_f\right) \text{g}\frac{D^2}{4} \left(\sin\left(\beta\right) - \beta\cos\left(\beta\right)\right) \tag{2}$$

Conversely, for unfluidized coarse particles the normal force is simply

$$N = \text{Cb}\left(\rho\_s - \rho\_f\right) \text{g}\left(\frac{\pi D^2}{4}\right) (\mathbf{1} - A) \tag{3}$$

where *A* is the fractional cross-sectional area above the bed and (1-*A*) is the area of the bed 1ð Þ¼ � *A* ð Þ *β* � sin ð Þ *β* cosð Þ *β =π* ¼ *Cv=Cb*.

The difference between Eqs. (2) and (3) is then a multiplier which accounts for the effect on the resisting force of particle fluidization. Thus, combining Eq. (2), Eq. (3), the bed fractional area and a degree of fluidization 0 < *f* < 1, where a value of zero = the unfluidized state, 1 = the fully fluidized state, this multiplier can be written as

$$F = \mathbf{1} + f \left[ 2 \frac{\sin \left( \beta \right) - \beta \cos \left( \beta \right)}{\beta - \sin \left( \beta \right) \cos \left( \beta \right)} - \mathbf{1} \right] \tag{4}$$

and used to describe the bed's normal force for an arbitrarily fluidized bed by

$$N = \text{Cb}\left(\rho\_s - \rho\_f\right) \text{g}\left(\frac{\pi D^2}{4}\right) (\mathbf{1} - A)F \tag{5}$$

#### **2.4 Comparison with conventional conveying**

Data reported by the Saskatchewan Research Council (SRC) for coarse coal in water [10] show typical results in a similarly sized pipe for -50 mm solids, *d*<sup>50</sup> = 3 mm, *Cv* = 0.457 v/v, *D* = 234 mm, to be *V* = 2.5 m/s, d*P*/d*x* = 0.64 kPa/m. The conveying velocity is higher than that shown in **Figure 3**, and this, combined with the lower concentration means that particle attrition would be considerable. The pressure gradient, however, is comparable to that of the 70% curve in **Figure 3**. A comparison of the operating costs can be achieved by comparing the *SEC* given by

$$\text{LECC} = \frac{\text{suspension pumping power}}{\text{solds mass flow x distance}} = \frac{dp/\_{\text{dx}}}{3.6 \text{C} \nu \nu\_{\text{/} \text{f}}} \text{ kWh/} \text{(tome km)}\tag{6}$$

where the pressure gradient, d*p*/d*x*, is in kPa/m.

Comparing the SRC data with the higher pressure gradient 75% v/v data, the *SEC* for the conventionally conveyed coal is 0.27 kWhr/(tonne km), compared with 0.23 kWhr/(tonne km) for the high concentration conveying. Furthermore, the high concentration system consumes 45% less water. A more detailed comparison of high concentration and non-Newtonian systems, described in §3, with conventional conveying is given in [11].

#### **2.5 Restart capabilities and practical considerations**

#### *2.5.1 Restart*

As mentioned above, transport at lower concentrations proved problematic through dune formation partially blocking the pipe, resulting in unsteady flow. Experience gained with the RRSP suggested that an operational concentration range should be limited to 0.75 < *Cv*/*Cb* < 0.9, and that the materials required a broad size distribution such that *Cb* > 0.75 to avoid excessive bed percolation of the low viscosity water carrier.

When operating within these constraints, restart of fully loaded pipelines several kilometers long proved to be immediate once the pumps were activated. The explanation for this is not entirely clear but is expected to relate to the low deposition velocities. The deposition locus for the material in **Figure 5** expressed as a function of *Cv*/*Cb* is shown here (**Figure 6**).

At the operational limits for this material the corresponding deposition velocities are approximately 0.6 and 0.2 m/s. Such suspensions started with ease, and it is expected that once the pumps were started, increasing the flow to the normal conveying velocity of a 1 m/s was sufficiently rapid so that reorganization of any particles

#### **Figure 5.**

*Normal force fluidization modifier for* f *= 0, unfluidized coarse particles,* f *= 0.5, partially fluidized intermediary particles,* f *= 1 for fully fluidized fine particles.*

#### **Figure 6.**

*Deposition locus as a function of* Cv/Cb *for the data shown in Figure 3. The concentration limits for successful operation are shown shaded.*

above the bed to form a plugging dune did not occur. This is despite the high velocity fluid flowing over the beds, which occupy 75 to 90% of the pipes CSA.

### *2.5.2 Practical considerations*

Production of such suspensions is comparatively easy as the mobility of these suspensions is very high, requiring only gentle agitation in suitable mixing tanks to ensure homogenization. More difficult is maintaining the high concentrations within the required limits, requiring conveyors etc. to be sheltered from rain, and sufficient surge capacity to be installed for constant operation. This last point is less of a problem, as the ability to reduce the transport velocity to very low values or stopping, provides a very large turn down ratio. Maintaining the correct feed to the pump also requires some care, as fluid can be preferentially drawn from the tank diluting the flow into the pump's inlet ducting, producing unstable operation, details of which are described in [12].
