SUSTAINABILITY Deposition techniques
Once the tailings slurry (dilute or paste consistency) has arrived at the tailings storage area, there are several possible ways it can be deposited. These include the paddocking method, spigotting, and cyclone deposition.
In general, the paddock deposition method is suitable if the tailings are fine grained, with particle sizes falling in a relatively narrow range. If the tailings are less fine and cover a wider range of particle sizes, spigotting may be a good option. Cycloning is usually applied to situations where spigotting would also be suitable.
The deposition method's overall objective should be to optimise beaching mechanisms for the particular tailings at hand. For example, suppose the tailings are a fine and/or virtually single-sized product. In that case, little gravitational sorting will occur down a beach and the beaching effect will be minimal. On the other hand, if the tailings have a wide range of particle sizes, beaching can produce a coarse frictional outer shell with the finer material confined within the body of the deposit and permeabilities that increase from inside of the storage to the outer wall.
Spigots are multiple outlets along a delivery pipeline. They are used when it is easily possible to cause a gravitational grading split between the coarse and the tailings' fine fractions.
Reticulation along the TSF embankment is achieved through spigot pipes extending from delivery stations located on the pre-constructed embankment crest (Figure 1a). The spigot pipes are laid along the main wall, allowing deposition to occur from any point on the crest. In the course of a deposition cycle, a batch of adjacent spigots is opened, sufficient to cater for the slurry flow rate (Figure 1b). Spigots break up the tailings delivery stream into smaller streams, thus causing a drop in stream velocity. This velocity drop enables the coarser fractions to settle close to the deposition point. As the beach fills, spigots at one end of the batch are opened while the equivalent number at the other end is closed so that the deposition gradually moves along the spigot pipe and around the tailings dam.
A variation to this method is where the spigot pipeline is located on the embankment crest, and the perimeter bund is raised to coincide with the tailings deposition cycle. The spigot lines usually have a series of nozzles located along the delivery pipeline at intervals of 2 m to 3 m. During each deposition cycle, a section of the spigot pipe is dismantled and moved to one side to allow the perimeter bund's raising, which is usually constructed of the beach tailings.
Daywalls are used to create vertical freeboard where this cannot be achieved in another manner. This would be the case when:
The daywall is so-called as it is that portion of the dam used during the day when there is supervision available and daylight to see what is going on. The conventional daywall is used to deposit uniformly graded tailings through an open-ended discharge located at one end of the paddock daywall (Figure 2).
The principle of a paddock or daywall is to create or form small impoundments or containment berms with dried-out tailings borrowed from the previous layer deposited around the perimeter or edge of the paddock (Figure 3). These shallow paddocks are then filled preferentially with dilute (± 30-50 % solids) slurry. The tailings solids settle out of suspension, releasing clear water, the bulk of which can be decanted from the surface of the paddock into the basin via a drain or "vent" pipe. The resulting layer of slimes continues to dry out through some seepage, but mainly through evaporation resulting in shrinkage cracking of the surface.
Since each subsequent layer deposited is formed on top of the previous layer, a paddock or daywall can essentially only be developed in an upstream manner. By definition, the upstream wall development stability depends on the strength of the earlier deposited underlying layers. Thus, it is essential to develop a daywall facility in thin layers (maximum 200 mm) to allow for consolidation to occur.
The basis of the operation of a cyclone is illustrated in Figure 4. A cyclone is a specific device with no moving parts. The cyclone consists of conical housing equipped with a feed pipe that enters the cone tangentially at its larger diameter closed end. A second pipe enters the cone on its axis and intrudes into the body of the cone. The slurry feed enters tangentially under pressure and is forced to swirl with a spiral motion towards the smaller end. In the process, centrifugal forces cause the larger particles in the slurry to move down and away from the axis, towards the wall and the narrow exit of the cone, while the smaller particles concentrate on the axis of rotation and move, in the opposite direction, towards the axial pipe or "vortex finder."
The net effect is that the finer particles and most of the water leave the cyclone through the vortex finder and form the "overflow," while the partially dewatered larger particles leave at the opposite end as the coarser "underflow" (Figure 5).
The purpose of using a cyclone is to create underflow material that has superior geotechnical characteristics, i.e., high permeability, a quicker consolidation and strength gain rate than the original tailings so that the underflow can be used to form a superior and/or quicker impoundment wall to the tailings storage facility (Figure 6).
The use of cyclone tailings in an embankment is an attractive option as significant cost advantages can arise by substituting cyclone sand for natural soils or mine waste because the sands are produced on the embankment, substantially reducing eliminating fill hauling and placement costs. By removing the sand fraction from the total tailings stream for use in the downstream embankment, the remaining volume of tailings discharged to the basin is reduced. With reduced impoundment storage requirements, the embankment height, volume, and cost are lower.
Cyclone underflow can also be generated off the embankment via a static cyclone station.The two fractions can be placed separately, typically through spigots and open ends.
With the on-wall generation of underflow, the wall can be developed faster at a greater rate of rise independent of the tailings' need for desiccation. A pressurised cyclone typically splits 20% to 30 % by mass to underflow. The split can be increased or decreased, but there are two criteria to be satisfied; there must be sufficient underflow mass to create the wall, but it must also be of adequate quality, i.e., specific permeability to fulfill the geotechnical strength requirements of the wall. Unfortunately, these two criteria work opposite each other, so there is an optimal combination of mass and quality.
On-wall cycloning intends to use the underflow tailings to form an impoundment wall to contain the overflow. For the storage facility to be stable, the underflow wall must be of specific dimensions. The tailings wall can be developed upstream, centreline of downstream, but irrespective the underflow wall must comply with a prescribed geometry, which means that a particular mass split must be achieved through the cyclone to ensure that there is always enough underflow to develop the wall ahead of the rising overflow contained in the basin of the dam (Figure 7).
Furthermore, the wall's prescribed geometry must be achieved with the specified quality of underflow, mainly in terms of grading and permeability. A fundamental feature of a cyclone dam's success is the differential permeability between the under and overflow tailings, particularly for an upstream storage facility. The minimum desired differential is two orders of magnitude. The implication of this is that the phreatic surface will be controlled close to the interface between the under and overflow tailings.
The cyclone is required to create an underflow of sufficient quantity and quality to fulfill the requirements of the design profile.
Single-point discharge is typically used during flushing and maintenance campaigns and is not recommended for conventional day-to-day operations. Single point discharge could also cause an uneven beach profile resulting in non-compliant supernatant pond locations.
Central Tailings Discharge (CTD) methods require thickened tailings. Thickening the tailings reduces the amount of water in the tailings impoundment and allows for more mine water to be reclaimed. Recovering water in a thickener is more efficient than sending the water to an impoundment where it is lost through evaporation, infiltration, and within the tailings voids.
Tailings deposition using the CTD method occurs by radial discharge from a fixed central discharge tower or a set number of spigots located within the central area of the TSF. The discharge point(s) is progressively raised as the elevation of the tailings increases. A circular paddock dam configuration is typically adopted and constructed (Figure 8). The perimeter embankments’ height depends on the tailings consistency and beach angle achieved, and the distance from the point of deposition.
Compared to perimeter deposition, the perimeter embankment heights are minimised as the tailings landform is conical and the tailings beaches towards the outer perimeter. The initial site development typically comprises relatively small perimeter containment embankments and an access spine or causeway from the perimeter to the central discharge point.
The CTD method is most suitable for beach slopes steeper than those achieved through conventional deposition. A large portion of tailings can be stored above the crest elevation of the perimeter embankments. In the likely event that the beaching profile is less steep than the design basis, an increase in either the footprint area or the height of the perimeter embankment is required to contain the same volume of tailings safely.
Deposition piping is typically required along the deposition causeway. Relocation of the piping is only required once the causeway and discharge point(s) are raised.
Filtering of tailings can take place using pressure or vacuum force. Drums horizontally or vertically stacked plates and horizontal belts are the most common filtration plant configurations. Pressure filtration can be carried out on a much broader spectrum of materials however vacuum belt filtration is probably the most logical for larger-scale operations.
The nature of the tailings material is essential when considering filtration. Not only is the gradation of the tailings important, but the mineralogy is as well. In particular, high percentages of <74 µm clay minerals (i.e., not just clay-sized but also with clay mineralogy) tend to inhibit effective filtration.
Filtered tailings emerge from the processing facility within a prescribed range of moisture contents. The tailings material is then transported by conveyor (Figure 9) or truck and then placed, spread, and compacted (Figure 10) to form an unsaturated, dense, and stable tailings "stack" (often termed a "dry stack") requiring no dam for retention with no associated tailings pond. The filtered tailings are not "dry" but are unsaturated, so the early nomenclature referring to them as dry is incorrect.
To date, the two most common reasons to select dry-stacked filtered tailings as a management option has been to recover water for process water supply and where terrain/foundation conditions contraindicate conventional impoundments (Davies 2011). Water recovery is essential in arid environments where water is a precious resource, and the water supply is regulated (e.g., Chile and Western Australia).
One of the main advantages of dry stack tailings over other tailings management options is the ease of progressive reclamation and facility closure. The facility can often be developed to start reclamation very early in the project life cycle. This can have many advantages in the control of fugitive dust, in the use of reclamation materials as they become available, and in the project's short- and long-term environmental impacts. Progressive reclamation often includes constructing at least temporary covers and re-vegetation of the tailings slopes and surface as part of the annual operating cycle.
Filtered tailings can be placed in a relatively dense state, meaning that more solids per unit volume can be achieved. Furthermore, more aggressive use of available land (e.g., valley slopes) can be used with filtered tailings. Lesser foundation conditions can also be considered in comparison to conventional impoundments.