By Arash Roshdieh – Principal Engineer
In the third of our series focusing on water conservation imperatives in the mining industry, we discuss practical and pragmatic solutions to tailings and water management challenges and outline the process for decision-making in this area.
Recapping on our previous articles:
Prompted by the horrendous bushfire season experienced across Australia early in 2020, we acknowledged a new norm in our outlook towards water as a resource.
While recognising water as a scarce commodity, particularly in drier climates like Australia, we strongly believe that a congenial relationship can exist between the environment, the community and the mining industry in terms of access to water resources.
We then took a first principles view of water in mining, with emphasis on tailings management, being a significant demander of water in a mining context. Dewatering was profiled as a targeted initiative to reduce water consumption. The use of both thickening and filtration was reviewed.
The final piece of the puzzle as related to water conservation in tailings management is selecting the most viable scheme, which is not typically a straightforward process; requiring knowledge, experience and capability in this area. A holistic and lateral view is also most important, ensuring the development of a solution that is not only robust but operationally flexible and adaptable to changing conditions and circumstances.
At ATC Williams, with a wide range of specialists and tremendous
experience in the area of tailings and water management, our key
objective is to clearly understand our clients’ needs, coupled with the
needs of the environment and nearby communities, to create tailor-made
solutions for any mining situation.
Various tailings storage schemes
The safe and efficient storage of mine tailings is critical; with emphasis on this need, none-so-more emphasised than in the past five years with the experience of tailings dam failures.
With safety and efficiency being two primary factors, the selection of an appropriate tailings storage scheme is very much site-and operational dependent. It may well vary significantly from site to site, even if located nearby. Nonetheless, there is a generic list of schemes that would likely suit most tailings management program:
- Paddock or Ring-Dyke impoundment (centre or perimeter deposition)
Valley impoundment, either Down Valley discharge (DVD) or Up Valley discharge (UVD)
- Central thickened discharge (CTD)
- Dry stack (Filtered tailings)
- Co-Disposal with mine waste, either with tailings in slurry form; or as a filter cake (sometimes referred to as an Integrated Waste Landform or IWL)
- In-pit disposal
- Paste backfill (to underground workings)
Criteria used to assess a suitable scheme are:
- Environmental setting/constraints
- Mine site layout and development timing
- Legislative and permitting requirements
- Closure and rehabilitation expectations/requirements
In particular cases, the current mining layout and forward schedule may provide the opportunity for tailings deposition into a completed open cut, provided that environmental conditions are suitable. In other cases, the topography may dictate a scheme option, prompting the need for a valley fill rather than a paddock style arrangement. The availability of construction materials also influences the approach adopted. In all cases, though, the style of engineering and the engineering skills required site to site will differ, which will impact on CAPEX and OPEX.
The problem of water loss
Depending on the circumstances, the selection of a tailings management scheme may well be controlled by the potential for loss of water. In this context, water loss is not just a concern from a water resource perspective, but from an environmental protection viewpoint.
Water loss from a typical tailings storage scheme occurs by any of the following:
- Evaporation from the surface of any water pond or tailings surface
- Seepage into the ground from the storage
- Interstitial water losses (i.e. water trapped and retained in the tailings mass)
An understanding of the water inventory within the system (where its located, its phase, and its quantity), is therefore important. This is referred to as the Water Balance.
Situations where the exposure of water loss potential can be reduced are as follows:
- Schemes where steeper tailings beach slopes are achievable, typically producing smaller pond areas
- Schemes with greater attention to water recovery with smaller wet beach areas (part of the deposited tailings surface that is saturated)
- Schemes where tailings are deposited at a higher solids concentration, thus reducing bleed water quantities
The permutations are endless, with the need to consider all factors in assessing scheme options to achieve an optimum solution.
Combining tailings storage and dewatering
In our May 2020 News article, we discussed available dewatering technologies. Combinations between these technologies and storage scheme options are worthy of discussions. The following table lists some typical combinations.
An additional factor is revealed as these combinations are considered; this being the method of tailings transportation, either as a slurry (experiencing non-Newtonian flow conditions) or as a “spadable” filter cake. Options for tailings transportation are:
- Slurry via pipeline or channel
- Filter cake by mobile plant or mechanical conveyor
Multi-criteria decision-making process
Due to its complexity, a Multi-Criteria (or Multi-Asset) decision-making process is typically used to identify a preferred tailings management scheme. Such a process involves workshopping the range of scheme options with key stakeholders, supported by technical and engineering analysis.
In simple terms, the process usually starts with defining key decision-making criteria, which are typically focused towards technical, environmental and financial aspects. Weighting to each criterion is then applied, followed by ranking of the schemes against the criteria. This process represents a semi-quantitative approach for scheme options assessment, particularly where the application of weighting and comparison between options is supported by technical analysis.
Of note is that this process can be used to create emphasis on any particular factor. Specific weighting can be applied, as required, to the issue of water availability and emphasis on conservation.
A case study
Typical tailings management scheme options for a mining project, and apply financial analysis to assess the preferred scheme:
A financial analysis of these options can be viewed in terms of Net Present Value (NPV), with hypothetical outcomes graphed below. Note that these NPV costs intentionally exclude the cost of water, which, as a specific commodity, will vary depending on location and situation.
While this analysis suggests that the most efficient option is Option 2 and perhaps the most expensive is Option 4, if the cost of water is added, the ranking of the options could be completely different. This is shown in the following diagram, based on water costs varying from $0.5/m3 to $5/m3, considered to be a typical range for mining projects in Australia.
Clearly, the preferred scheme option will vary depending on the cost of water. It can conclude, at least in this case, that the preferred option can be quite sensitive to water cost, as demonstrated below by comparing Options 2 and 8:
A final word and your next steps
While this financial analysis is robust and definitive, the complete answer may not lie in this detail. The use of the multi-criteria decision-making process has a role in balancing financial outcomes with other critical non-financial factors.
Our final word is that it is essential to recognise that each tailings management project is subject to different conditions. Geotechnical and rheological characterisation of your tailings is a necessary step to fully define the options that are available to your project and to identify the preferred approach.
Arash Roshdieh is a Principal Engineer in ATC Williams’ Melbourne office. Arash is a civil/water-tailings engineer, specialising in dry climate hydrology, tailings and water management and engineering.