By Arash Roshdieh – Principal Engineer
In our February News article Leading Sustainable Water Conservation in the Australian Mining Industry, we discussed the challenges that mining companies are facing due to climate change conditions. In this article, we outline how the selection of a tailings management scheme may improve a mine’s water conservation situation.
At ATC Williams, achieving sustainable management outcomes for mine tailings is always a long-term focus. New norms in our environment need to influence approaches to mining and create new ways of thinking about water management issues.
Tailings behaviour and management
In a mining context, tailings are the residue from the mineral processing operation. In most cases, tailings are derived hydro-metallurgically, as a slurry. At low densities, tailings slurries are typically not so viscous (behaving more like water). However, as slurry densities increase the tailings behave as non-Newtonian fluids.
Selection of a scheme for management of a tailings slurry is a complex process to find the best option. The basic principles that apply to the selection process include:
- Achieving practicality and simplicity
- Balancing the availability (and cost) of raw process water against recycled water
- Minimising risks, primarily the trade-off between production and environmental threat
- Overall cost of the scheme (capital and operational)
How is water lost from a tailings management scheme?
Water loss from a typical mine processing circuit occurs by any of the following:
- Evaporation from the surface of any water pond, tailings surface or the metallurgical process
- Seepage into the ground from ponds and storage
- Interstitial water losses (i.e. water trapped and retained in the tailings mass)
- Water lost into the mining product, such as a concentrate
Each of these components is quantifiable, either directly or by laboratory tests. At ATC Williams’ NATA accredited laboratory, we have developed specific tailings tests that enable the estimation of water losses from tailings materials, either enroute or within a tailings storage facility.
Assessing tailings dewatering technologies
The conventional approach for disposal of a tailings slurry is discharge into a storage as an unthickened slurry (less than 30% solids by weight). Such a scheme is generally low technology, and therefore low cost, however, exposes the operation to significant water losses, particularly evaporation and seepage from the storage. Upgraded lining systems to improve containment, is a newer approach, although it has no significant effect on water losses, with long term safety and rehabilitation issues.
Tailings dewatering is a common technique used to reduce the moisture content of a tailings material. The clear advantages of a dewatered tailings are the immediate reduction in potential water loss from the storage area, the overall improved geotechnical stability, and the improvement in rehabilitation outcomes.
Two popular tailings dewatering methods are:
- Filtration (for filtered tailings disposal or “Dry Stacking”)
Both are mechanical processes applied to achieve different solids concentrations.
The common thickening types are*:
- Conventional thickener (up to 45% solids)
- High rate thickener (up to 59% solids)
- High-density thickener (up to 64% solids)
- Deep cone or paste thickener (up to 67% solids)
(* Stated solids concentrations are considered representative for a conventional ore with a specific gravity of around 2.7 t/m3 and are presented by weight which is equal to weight of solids divided by total weight)
Filtration usually employs one of the following techniques:
- Vacuum filters such as belt and disc filters, or
- Filter presses
Filtered tailings would typically achieve a solids concentration of 75 to 80%, which visually would appear to possess “spadable” soil properties. Occasionally, centrifuges and vibrating screens are also used.
Importantly, it is expected that CAPEX and OPEX will increase with increased solids density.
What does dewatering of tailings do?
Dewatering separates water from a slurry and increases the ratio of solid to liquid. The benefit of tailings dewatering is the controlled recovery of water from the process. Recovered water can be recycled directly as process water. Fundamentally, the more water recovered at this time, the lesser is the raw water demand. Added benefits include:
- The rheological properties of the tailings will be improved, which results in steeper tailings beaches, therefore smaller decant pond surface areas and less evaporation
- The quantity of bleed water from the surface of the deposited tailings will be less, hence less water will report to the decant pond. In dry climates, this reduces water return demands and also reduces potential loss of decant water to evaporation before it is recovered
- The settled density of the tailings achieved in the storage usually increases, which means:
- Less water lost or retained in the tailings
- Reduced availability of water to report as seepage
- More efficient use of the available storage capacity
- Greater tailings strength, therefore improving rehabilitation potential
Water conservation in various schemes
ATC Williams has extensive experience with tailings thickening and filtration. Application is driven by a range of critical factors, such as:
- Safety concerns (particularly liquefaction potential under earthquake loading)
- Environmental concerns (loss of water to the environment causing potential environmental harm)
- Cost and availability of raw water
- Regulatory direction
Due to the available range of technologies, a balance needs to be created between the performance expectation defined by each of these factors, measured against both CAPEX and OPEX. The available storage schemes will be addressed in a later article. However, from a performance perspective, the following shows the volume of water (in cubic metres) that would be retained within each tonne of tailings material following dewatering after the initial settling phase.
These retained water volumes can be taken as losses to the processing circuit, creating a need to make-up these volumes via an alternative raw water source. (We would add that there is potential to recover a small portion of these retained volumes, particularly at the higher end, by decant recovery from the storage area.)
As a comparison, the retained water volume in an unthickened tailings stream would be in the order of 3 m3/tonne, being three times the volume of a conventional thickening process, and more than ten times the volume from filter presses.
In simple terms, the application of tailings dewatering is very effective in achieving high levels of water conservation. However, we reiterate that this comes at a price, which will be addressed later in this series of articles.
Next steps for your site
It is essential to recognise that each tailings management project is subject to different conditions, and not all dewatering technologies may suit. Geotechnical and rheological characterisation of your tailings is a necessary step to understand the options that are available to you. Future articles will assess the options for storage of dewatered tailings, providing an understanding of the constraints that may apply to the selection of a preferred scheme, and which may suit the setting of your site.
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.