Backfill pipeline and delivery systems
Factors affecting pipeline pressure gradients
Gilchrist (1988) shows that with increasing backfill velocity the pipeline pressure gradient increases. This work was carried out in a 40 mm NB schedule 40 pipe. Figure 1 after Steward (2007) shows that pipeline pressure gradients increase with increasing pipeline velocities for a range of pipe sizes. This trend is seen for all backfill types.
Figure 1: Velocity versus pressure gradient for cyclone classified tailings at 1.75 relative density in different pipe sizes
Backfill relative density
Figure 2 shows that at a solids relative density greater than 1.75 there is a sharp increase in pressure gradient of a 2.65 SG tailings.
Figure 2: Velocity versus pressure gradient for increasing backfill relative density in a 65NB pipe
This relative density corresponds to a mass % solids concentration of 68.8%. The degree of the increase and the shape of the relationship is dependent on particle characteristics such as size, shape, specific gravity and mineralogy. In this instance it is clear that the backfill product is transitioning from a settling backfill to one displaying slow settling characteristics. The movement of the backfill pressure gradients from passing though the origin on both the X and Y axis indicates a pressure gradient existing as the flow rate tens for zero flow this is indicative of a yield stress developing in the backfill. The backfill system is moving from a turbulent, inertial force driven system to a laminar, viscous force driven one. Backfill relative density and the associated pressure gradients define our description of backfills.
Hydraulic fills manufactured from cyclone classified tailings (CCT) are typically settling backfills from which the water drains. These backfills are typically less than 50 volume % solids and the flow rate pressure gradient relationships tend to pass through 0.
Full plant tailings (FPT) backfills that are slow settling may best be termed thickened tailings and contain a binder content. These backfills still have shrinkage and possibly some water drainage; however, they have a volume percent solids that may exceed 50%. These backfills may manifest a yield stress, typically less than 100Pa, and the flow rate pressure gradient may not pass through the origin.
Paste backfills tend to be non-settling, with solids volumetric concentration above 50%. They have a binder content and are typically non-draining. These backfills have yield stresses in excess of 200Pa. As a progression, the pressure gradients increase from the hydraulic backfill to the paste backfill.
Particle size distribution
Backfilling has changed over the years from being dominated by draining backfills to being dominated by slight and non-draining backfills.
Initially metallurgical plant tailings were cycloned to remove the minus 10 micron fraction to produce cyclone classified tailings, to aid percolation and drainage. Now the metallurgical plant tailings are dewatered and used as thickened tailings or paste backfills. The major changes in these operations, besides the backfill tailings becoming more fine and the need for more capital intensive equipment, is the need to introduce cementitious binders to take up the water remaining in the backfill and generate strength.
This move from cyclone classified tailing to full plant tailings based backfill, i.e. from a coarse material with little fines to a high fines backfill, results in increased pressure gradients. Figure 3, after Steward (2007) indicates the increase in pressure gradient from a cyclone classified tailings to full plant tailings for the same metallurgical tailings material at a relative density of 1.75, or a volume solids concentration of 45%.
Figure 3: Velocity versus pressure gradient for cyclone classified tailings and full plant tailings at 1.75RD from the same metallurgical milled tailings
Cooke, (1991) reports a decrease in pipeline pressure gradient with increasing pipe diameter and this is reflected in Figure 1 and even more clearly in Figure 4 after Steward (2007).
Figure 4: Pipe inside diameter versus pressure gradient for backfill at 1.75RD at various flow rates
Cooke (1991) discusses flow observations and the corresponding flow regimes in terms of the following three observations:
In general, a stationary bed—where a bed of stationary particles or solitary particles is observed on the pipe invert—occurs if the transport velocity is insufficient to suspend the solids and the concentration is sufficiently low to manifest no hindered settling of the particles. This is possible in cyclone classified tailings and less so in thickened tailings and seldom apparent in paste backfills, although laminar flow settling can occur in thickened and paste backfills.
Asymmetry, or a velocity profile, is again associated with low solids concentration or coarse backfills such as cyclone classified tailings or low concentration full plant tailings systems. This can also be termed heterogeneous flow. If the
This is typical of high solids concentration thickened and paste backfills. A case can exist where typically heterogeneous backfill can develop a pseudo-homogeneous behaviour if the transport velocity is sufficiently high.
In general, asymmetry of the flow of backfill in the pipeline decreases with increasing mixture-relative density and increases with increasing pipe diameter.
Backfill velocity is also inextricably linked to the effect of solids concentration. At low solids concentrations high velocities must be maintained to achieve asymmetric and symmetric flow regimes, whereas with an increasing solids concentration the velocity can be allowed to decrease. This reaches a maximum in slow and non-settling backfills where low velocities can be achieved with little or no negative effects. The method of transport in non-settling systems at high concentrations is commonly called Plug flow, described by a central core of material that flows through a highly sheared annulus as shown in Figure 5.
Figure 5: Plug flow in non-settling backfill
Figure 6 shows the interaction between flow regime and pressure gradient.
Figure 6: Interaction between pressure gradients, flow regime and backfill types
Tests on the pipe pressure gradients for CCT show that no changes are experienced with binder additions between 3 and 9% as shown in Figure 7.
Figure 7: Flow rate versus pressure gradient for cyclone classified tailings at 1.80RD with different binder additions in a 65NB pipe
This is not the case for FPT, where increased addition of binder has reduced the pipeline pressure gradients as shown in Figure 8.
Figure 8: Flow rate versus pressure gradient for cyclone classified tailings at 1.75RD with different binder additions in a 65NB pipe
The effect of binder on the pressure gradients in FPT has been researched by Paterson and Lazarus (1989). They carried out research using a FPT from Cooke 3 shaft of Randfontein Estates Gold Mine in South Africa.
Paterson and Lazarus found that a 5 % by wet mass addition of a binder resulted in increased pressure gradients for backfill relative densities greater than 1.64. This is due to the increase in fine particles to the backfill system.
Figure 9 is a graph after Cooke and Spearing (1993) that shows the increase in yield stress, apparent viscosity and laminar to turbulent transition when binder is added to full plant tailings.
Figure 9: The effect of binder addition on pipeline pressure gradients
From these results the following observations can be made:
- Velocity effectsIncreasing the backfill velocity increases the pipeline pressure gradient. Increased velocities results in increased pipe wear.
- Density effectsIncreasing the backfill relative density increases the pipeline pressure gradients, this decreases pipe wear rates.
- Particle size effectsThe particle size distribution is determined by factors other than the transport considerations, i.e. the milling process undertaken to facilitate mineral recovery. The particle size distribution can be modified; however, this is carried out in order to achieve the desirable placement properties as a backfill such as a cyclone classified tailings.
- Pipe bore effectsIncreasing the pipe bore decreases the pipeline pressure gradient.
Summary of parameter effects
The above discussion shows that the design of a successful backfill distribution system is a balance of the above parameters. In summary, the following transport and material parameters should be considered when designing a backfill distribution system:
- Low velocities
This reduces pipe wear rates; however, low velocities also result in decreased pipeline pressure gradients and tendency to greater asymmetry in the flow regime.
- High relative densities
The benefits of high relative densities are low wear rates, increased pressure gradients, more stable flow regimes and lower settling rates.
- Large pipe diameters
As with high relative densities, pipe diameter is a balance that depends on the required flow rate and the static head, against the total pipeline frictional losses. Larger pipe diameters result in decreased specific wear rates; however, on the negative side, the flow asymmetry is increased and the pipeline pressure gradients are decreased.
- Addition of cement binder
This has a beneficial effect in decreasing pipe wear rates and increasing pressure gradients in the case of FPT. Furthermore, the performance of cemented fill as the support medium underground is exemplary.
In short, the design of a backfill distribution system requires the most effective balance of velocity, relative density and pipe diameter.
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