5. Operational Plan
5.3. Key Resources
The main elements of a slurry transport system are shown in Figure 4.8.
4.5.1 Slurry Preparation
Initially the slurry must be prepared by physical and/or chemical processing in order to achieve the proper slurry characteristics for effective transport. Slurry
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preparation involves slurrification, or addition of the liquid phase to the dry solids. In addition, chemical treatment for corrosion inhibition, or modification of slurry rheology by particle size reduction, are often aspects of slurry preparation.
In the grinding process, the desired particle size is small enough such that the generated slurry is homogeneous and easily transported, but not too small that the slurry is difficult to dewater. If the particle size is too coarse, a heterogeneous slurry will result necessitating higher pumping velocities and, consequently, higher energy costs. In addition, higher pipeline wear rates are associated with larger particles. Size reduction of larger bulk solids is typically accomplished by crushing or grinding, such as with jaw crushers (Chapter 12), until the particle size is about 2 mm. Further particle size reduction is then performed in rod mills or ball mills (Chapter 12). If significant size reduction of the solid is required, the cost of slurry preparation can be one of the two largest components (rivaling the pumping cost) of the total cost of slurry transport.
The typical practice is to add the liquid phase to the solids in agitated tanks, forming a slurry with a slightly higher concentration than the ultimate pipeline concentration. Final adjustments to the slurry concentration are then made by the addition of more liquid as the slurry enters the pipeline.
4.5.2 Pumps
Several types of pumps are useful for handling slurries. The selection of a pump for a specific slurry transport line is based on the discharge pressure requirement and the particle characteristics (particle size and abrasivity). The pumps that are used are either positive displacement pumps or centrifugal (rotodynamic) pumps.
For discharge pressures under approximately 45 bar, centrifugal pumps offer an economic advantage over positive displacement pumps. Due to the lower working pressure, the application of centrifugal pumps is generally restricted to shorter distances; they are typically used for in-plant transportation of slurries.
The efficiency of a centrifugal pump is low due to the robust nature of the impeller design; the impellers and casings have wide flow passages. Efficiencies of 65% are common for centrifugal pumps compared with efficiencies of 85–90%
for positive displacement pumps. The wide flow passages, however, enable the transport of very large particles, even up to 150 mm in size, in centrifugal pumps.
SLURRY
Figure 4.8 Components of a slurry conveying system
In contrast, for positive displacement pumps, the maximum particle size is typically on the order of 2 mm. In order to minimize wear, centrifugal pumps for coarse-particle slurries are lined with rubber or wear-resistant metal alloys.
The head versus flow characteristic for a centrifugal slurry pump is relatively flat. Therefore, if the flow resistance of the system increases and the flow rate drops below the critical velocity, a fixed bed of deposited solids, potentially developing into a plugged pipeline, can result. To prevent this situation from occurring, most centrifugal pumps have variable speed drives to maintain the flow rate.
For slurry transport systems requiring discharge pressures greater than 45 bar, only positive displacement or reciprocating pumps are technically feasible. These pumps fall into two main categories: plunger type and piston type. The choice of which type to employ depends upon the abrasivity of the slurry. The plunger pump and the piston pump are similar in construction in that both have a plunger or a piston that is being caused to pass back and forth in a chamber. In plunger pumps, the plunger reciprocates through packing, displacing liquid through cylinders in which there is considerable radial clearance (Figure 4.9). The two valves alternate open and closed as the plunger moves up and down.
Plunger-type pumps are always ‘single-acting’ in that only one end of the plunger is used to drive the fluid. For slurry applications, the plunger is continuously flushed with clear liquid during the suction stroke to greatly reduce internal wear.
MOTION
PACKING
PLUNGER FLUSH
LIQUID
VALVE
FLOW Figure 4.9 Vertical plunger pump
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Piston pumps may be either single- or double-acting (Figures 4.10 and 4.11). In double-acting piston pumps, both sides of the piston are used to move the fluid.
In piston pumps, as in plunger pumps, the valves (two for single-acting and four for double-acting) alternate open and closed as the piston moves back and forth.
In double-acting piston pumps, both suction and discharge are accomplished with the movement of the piston in a single direction. Since the throughput of positive displacement pumps is much lower than centrifugal pumps, these
Figure 4.10 Single-acting piston pump
Figure 4.11 Double-acting piston pump
pumps are often arranged in parallel in a slurry line for transporting solids over long distances at high volumetric flow rates. Another characteristic of positive displacement pumps is that throughput is a function of piston or plunger speed, and is relatively independent of discharge pressure. Therefore, a constant-speed pump that moves 20 m3/h at 30 bar will handle very nearly 20 m3/h at 200 bar.
4.5.3 Pipeline
The most important considerations when specifying the pipeline are that the pipe material should be able to withstand the applied pressure and that the pipe material should be wear-resistant. Erosive wear is likely to be a problem for transporting abrasive particles at higher velocities ð> 3 m=sÞ. Based on these considerations, pipe materials generally fall into the broad categories of hardened metals, elastomers (rubbers and urethanes), and ceramics.
Steel is the most widely used material; linings of rubber or plastic are often used with steel pipes when handling abrasive slurries. Cost savings can be realized when installing the pipeline if pipe sections of reduced wall thickness are used where the pipeline pressure is lower.
Rubber and urethane tend to wear better than metals. However, high tem-peratures or the presence of oils or chemicals may render this option not feasible.
Ceramics are the most wear resistant materials, but they are low in toughness and impact strength. In addition, ceramic pipelines are typically the most costly. For this reason, ceramics are often used as liners, particularly in localized areas of high wear, such as pipe bends or in centrifugal pumps.
4.5.4 Slurry De-watering
The capital and operating costs of de-watering a slurry at the discharge end of a pipeline can be the deciding factor in a slurry pipeline feasibility study. In addition, the difficulty of de-watering the slurry will often dictate whether to transport the solids in a coarse or finely ground state. In a slurry conveying systems, the common de-watering processes are:
1. particle sedimentation by gravity or assisted by a centrifugal field;
2. filtration by gravity, assisted by a centrifugal field, pressure or vacuum;
3. thermal drying.
In one slurry conveying system, all three methods of de-watering may even be used.
Particle sedimentation techniques can involve the use of a screen if the particle size is relatively large. For smaller particles, the particles in the slurry can settle naturally due to the gravity in large tanks. For continuous settling operations, a
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thickener (Chapter 3) is employed. Solids settle into the conical bottom and are directed to a central outlet using a series of revolving rakes. Clear liquid is discharged from the top of the thickener.
Hydrocyclones are also used for liquid–solid separation. Hydrocyclones are similar in design and operation to gas cyclones; the slurry is fed tangentially to the hydrocyclone under pressure. The resulting swirling action subjects particles to high centrifugal force. The overflow of the hydrocyclone will carry predomi-nantly clear liquid and the underflow will contain the remaining liquid and the solids. Some small fraction of the particles will be discharged in the overflow; this fraction will depend on the particle size range in the slurry and the cut size of the hydrocyclone.
4.6 FURTHER READING
For further reading on slurry flow, the reader is referred to the following:
Brown, N.P. and N.I. Heywood (1991), Slurry Handling Design of Solid-Liquid Systems, Elsevier Applied Science, London.
Shook, C.A. and M.C. Roco (1991), Slurry Flow: Principles and Practice, Butterworth-Heinemann, Boston.
Wilson, K.C., Addie, G.R. and R. Clift (1992),Slurry Transport Using Centrifugal Pumps, Elsevier Applied Science, London.