In-pit tailings storage facilities – easy win or long-term liability?
An in-pit tailings facility may be a low-cost tailings option for an open cut mine; but an assessment that doesn’t capture the cost of remediating residual voids risks underestimating the actual cost to the operation.
When considering tailings storage options for an open cut mine, most sites don’t have to look too far to find a void that would provide a cost-efficient solution. The loss of future earnings due to resource sterilisation will often mean that mine planners are reluctant to move to in-pit tailings storage. But, where a suitable void can be identified, a tailings storage facility (TSF) can be developed that will require limited construction works, is typically low risk and exceptionally low cost in terms of dollars to dry tonnes of storage.
From a closure perspective, in addition to providing low-cost storage, in-pit tailings can go some way to addressing the issue of residual voids in post-closure landforms – though there are environmental factors to consider. Many mine sites will have regulatory conditions in relation to post-mining landforms that will require residual voids to be minimised, and occasionally eliminated, as part of closure rehabilitation. However, experience and modelling have shown that rehabilitation landforms constructed over legacy in-pit tailings deposits can have significant post-rehabilitation settlement periods, in the order of tens of years. This can result in the formation of surface depressions that impact post-closure land use to the extent that it would no longer be acceptable for relinquishment. It is not uncommon for regulators to request details of projected settlement behaviour for rehabilitated landforms over legacy in-pit tailings deposits to ensure that adequate provision has been made to ensure a satisfactory post-closure landform in the longer term.
In its simplest form, an in-pit tailings operation can be a slurry pipe discharging over the pit wall nearest the process plant; if there is surplus water on site, this could include co-storage and evaporation of supernatant water. But typically, there will be a single point of discharge depositing tailings sub-aqueously and a return water pump will be included in the operation.
The behaviour of the in-pit tailings after deposition is dependent on a range of factors, including the material characteristics, the slurry density, the rate of placement, the deposition method, and the impact of any flocculants used in the process. As a rule of thumb, tailings that are more clayey in nature, deposited sub-aqueously and deposited rapidly (in terms of placed thickness of the deposit) will have less consolidation during deposition than more sandy tailings. The consequence of this is greater long-term consolidation and therefore a greater requirement for overfilling to provide an acceptable free-draining landform for relinquishment.
Where the deposit is very thick (a deep pit) and placement is rapid, total settlement after closure can be projected to tens of metres and the costs associated with fill placement (overfilling) to create a surface geometry that can be shown to be free draining as a long-term post-closure landform can be considerable. When this is poorly defined it presents an increase in closure cost and schedule risk.
So, what can be done to better manage this risk?
Know the magnitude of the problem
- Understand the tailings materials in terms of settling characteristics and consolidation at very low pressures (ie derived by using specialist slurry consolidometer laboratory equipment).
- Understand the variability in the tailings deposit – different streams will potentially have tailings with differing settlement and consolidation characteristics.
- Consider the site hydrology to determine drainage path lengths for consolidation processes.
- Review long-term production schedules and model the consolidation of the deposit. This can be done using iterative analysis procedures to understand the state of consolidation (and hence densities) of the various levels of the deposit at the end of deposition and estimate the time for an equilibrium condition to occur.
- Use the model to project future consolidation behaviour, including the rates of settlement and the magnitude of final settlement (residual void).
Review opportunities for operational improvements
Although changes to operations do not always have a significant impact on the long-term settlement behaviour, best practice measures like maintaining a minimum pond and maximum beach to encourage surface consolidation by solar drying may help, as may moving the deposition point around the pit to improve beaching. However, potential gains will often be a function of the geometry of the void and rate of deposition and may not always be attainable. It is also worth noting that this doesn’t align with the minimum input ethos that is part of the attractiveness of an in-pit scheme in the first place.
A more plausible operational improvement may be achieved by staging deposition such that tailings are placed to an elevated level and allowed to settle before depositing again into the void resulting from the settlement, thus, filling some of the volume that may otherwise be required to be backfilled as part of rehabilitation earthworks. In cost terms this is an attractive opportunity; however, the timeframes involved in achieving sufficient settlement to enable subsequent tailings to be placed can be problematic to schedule. It must also be considered that if a tailings deposit is relatively fresh at closure, access for cover construction is likely to be an issue.
Flocculants can be used to improve the density of deposited material and a cost-benefit analysis may support this. However, variability of tailings characteristics may result in variable efficiency, and ongoing monitoring and amendment may be required to realise any savings.
Weigh the risks and benefits
An in-pit tailings facility is likely to be the lowest cost tailings storage option for an open cut operation; however, any assessment of cost to the operation that doesn’t accurately capture the associated cost of remediating residual voids risks underestimating the actual cost.
In terms of evaluating potential long-term settlement issues, while methods for assessing the magnitude of settlement are readily available, the testing methods required to give more reliable inputs (such as the slurry consolidometer) are not commonly available. The geometry of the void is often key to settlement outcomes, with thicker deposits having greater long-term settlement issues, but there is rarely a choice of available voids, which limits opportunities to adjust on the basis of geometry.
Similarly, while operational measures may result in improvements, the overall impact may not be significant. Scheduling and further deposition campaigns are a good option where possible, but where settlements will continue for many years after relinquishment there is still likely to be a requirement for considerable fill placement to meet long-term landform geometry requirements.
Understanding this cost should be a key input to the assessment of overall cost of any proposed in-pit TSF rather than just the deposition cost. It may also promote a review of further processes and operational choices that may prove to be a worthwhile investment when considered through the lens of whole-of-life cost.