Filtered Tailings: Addressing Oxidation, ML-ARD, Surface Runoff, and Geotechnical Stability

Tailings management remains a significant focus within the mine closure industry, with the integration of responsible mining practices becoming imperative to minimize environmental impacts and ensure long-term sustainability.

Okane has contributed numerous technical papers, presentations, and Research and Development projects on tailings management strategies. We have also shared technical insights on the future of tailings management, including strategies for dust management at tailings storage facilities (TSFs), exploring the circular concept of using tailings as concrete aggregates for construction materials, and focusing on tailings mitigation through filtering and co-disposal approaches in a featured article for Mining Magazine.

In this month’s Conversations on Closure, we explore the application of filtered tailings as an alternative method for tailings management. We will delve into their geochemical characteristics to address potential challenges, such as ML-ARD and geotechnical risks, and consider opportunities for integrating filtered tailings into mine closure planning.

What are Filtered Tailings? Understanding its Properties and Benefits

Filtered tailings (also referred to as non-aqueous tailings) are the unsaturated byproduct of reprocessed tailings using various filter methods, such as mesh, vacuum disk, belt, and pressure (Lara et. al., 2013). Filtered tailings are generally placed with a lower degree of saturation (Sako & Pabst, 2023) or gravimetric water content in the range of 10-20% compared to conventional tailings. As such, filtered tailings have a higher solids content (~78%) compared to conventional tailings (Seabridge Gold Incorporation, 2016).

Capital and operating cost considerations are common constraints for filtered tailings management. These costs are evaluated against the potential water and storage benefits. In some instances, filtered tailings management strategies are selected at specific mine sites due to operational limitations such as land availability, suitable materials for construction, or access to fresh water supply (Lara et al., 2013). Water recovery is especially important for mine operations located in dry, arid regions like Chile, Western Australia, the southwestern United States, and Mexico, where water supply can be highly regulated and scarce (Davies, 2011).

Filtered tailings deposition offers several benefits over conventional TSFs (lower solids content storage options like slurry tailings ponds), including increased opportunities for water recovery, mechanical placement or use of conventional mining equipment (as opposed to hydraulic placement, such as pipes and pumps), progressive reclamation, and a smaller environmental footprint (Sako & Pabst, 2023). Moreover, compared to conventional TSFs, considering filtered TSFs may also eliminate the need to build dams, thereby reducing the risks associated with dams (Sako & Pabst, 2023).

Environmental Challenges with Filtered Tailings: Oxidation, ML-ARD, Surface Runoff, and Geotechnical Stability

Decisions related to filtered tailings management can also be influenced by environmental challenges, which must be considered by mining operations. These challenges include evaluating the risk of metal leaching and acid rock drainage (ML-ARD; also known as acid and metalliferous drainage, or AMD), namely, seepage and its resulting adverse impact on surface water and runoff quality.

Oxidation and Metal Leaching and Acid Rock Drainage (ML-ARD)

Tailings frequently contain sulfide minerals that can oxidize when exposed to atmospheric conditions. Reactive tailings material placed in (or evolving to) oxic conditions can result in increased tailings pore-water acidity, total dissolved solids, and metal concentrations.

Filtered tailings have a lower degree of saturation than conventionally deposited tailings; therefore, the probability increases for sulfide minerals to oxidize, leading to a potential increase in ML-ARD risk. Sako & Pabst (2023) consider that the most critical factors influencing ML-ARD risk generation with filtered tailings are the degree of saturation and mineralogy. The potential mass loading from sulfide mineral oxidation is a function of these two factors. In addition, the time frame during which the tailings are exposed to the atmosphere, along with the amount of surface area exposed, strongly influences the probability of ML-ARD loading into the environment.

In some cases, geochemical processes resulting from oxidation may impact the geotechnical stability of filtered tailings (Northwest Territories Geological Survey, 2022; Lara et al., 2013). A study on long-term stability of filtered tailings at the Cantung Mine in Northwest Territories, Canada, reported that oxidation, leading to acid generation, can weaken certain minerals and potentially precipitate secondary minerals that may vary the strength properties and alter the amount of pore space within the filtered TSF (Northwest Territories Geological Survey, 2022). Therefore, a thorough understanding of the geochemical and geotechnical changes over time is required.

Surface Runoff

In addition to ML-ARD or seepage from a TSF, the probability of adverse impacts to surface water runoff from the landform can increase when depositing filtered tailings. Caldwell et al. (2015) and Sako & Pabst (2023) state that risks of contaminant transport in runoff water can increase due to the lack of water management infrastructure, leading to potential environmental impacts.

Managing runoff water is crucial to ensure stability and integrity of the filtered tailings placement (Caldwell et al., 2015; Cacciuttolo & Pérez, 2022). In some cases, this management may include specific water collection or retention ponds downstream of the storage facility (Cacciuttolo & Atencio, 2023). Implementation of fit-for-purpose surface water collection and/or management ponds is often required for filtered tailings landforms to manage potential risks of metal release to the environment. The water management infrastructure for a filtered TSF may not require the same level of containment that is inherently required with conventional TSFs. If fit-for-purpose surface water collection is not considered in the design of filtered TSFs, this difference can result in metals being released to the environment at rates higher than expected, especially in rainy regions with high precipitation (Lara et al., 2013).

Geotechnical Stability

Filtered tailings with a lower degree of saturation are generally more geotechnically stable compared to conventional tailings, which are saturated, require containment, and are often managed with water cover systems. Despite this, filtered tailings are generally non-plastic granular material placed with minimal compaction, which can become susceptible to liquefaction if the degree of saturation within the pile increases over time due to rising pore pressure from saturation and contractive behaviour from compaction (Marques et al., 2020). Liquefaction may lead to displacement of the mass during seismic events (Condon & Lear, 2006).

Responsible Closure Planning with Filtered Tailings

Through a combination of advanced technologies, stakeholder engagement, and regulatory compliance, an integrated closure planning approach to managing filtered tailings can prioritize environmental risk and stability risk management. Integrating cover systems and landform designs for filtered TSFs is an effective closure solution to manage post-closure risks of ML-ARD and impacts on water quality.

Engineered cover systems are a preventive approach to address ML-ARD generation and can demonstrate resilient performance (Sako & Pabst, 2023). Research also suggests that thicker layers within a cover system may be more efficient in retaining water (keeping the degree of saturation over 85%) to control oxygen diffusion within filtered tailings (Sako & Pabst, 2023). By extension, the management of oxygen diffusion is crucial in limiting acidity generation (Sako & Pabst, 2023). A comprehensive closure cover system also helps mitigate risks of runoff erosion while also providing the site with an appropriate ground material and surface for reclamation works (Cacciuttolo & Pérez, 2022).

Another method to manage acidity generation during filtered tailings deposition is the placement of a layer, or lift, of tailings with higher buffering capacity (mixture of filtered tailings with higher carbonate content) (Sako & Pabst, 2023; Northwest Territories Geological Survey, 2022). Study simulations showed that this layer containing additional buffering capacity could remain close to neutral pH due to the alkaline pore water produced (Sako & Pabst, 2023; Condon & Lear, 2006). Sako & Pabst (2023) consider that this would effectively shield the underlying reactive material from oxidation processes that contribute to acidity generation until the site undergoes reclamation.

Ensuring an adequate degree of saturation in filtered tailings will help minimize oxygen exposure and meet structural stability (Shaw & Ayres, 2014; Lara et al., 2013). Densification, typically through compaction of filtered tailings, restricts oxidation because it increases a material’s capacity to retain moisture and also reduces seepage rates, as densification can reduce in-situ hydraulic conductivity (MEND Report, 2017). There is also evidence from modelling studies suggesting that oxygen consumption in the top layer in combination with limited oxygen transport through moist tailings can inhibit further oxidation in the underlying layers (MEND Report, 2010). Once sulfide minerals are depleted in the top layer, it acts as a diffusion barrier to oxygen, with a small flux of ongoing oxidation into the rest of the tailings stack occurring at a slow rate over centuries (MEND Report, 2010).

To mitigate runoff risks in filtered tailings, water management ponds are essential for handling seepage water and rainwater, especially amidst the uncertainties of climate change (Cacciuttolo & Atencio, 2023). To prevent environmental contamination, water in contact with filtered mine tailings can be collected and transported to the metallurgical process plant for reuse (Cacciuttolo & Atencio, 2023). Contact water can be discharged into the environment, but only after physical-chemical treatment and testing to comply with industrial regulations and local discharge permits (Cacciuttolo & Atencio, 2023).

Okane’s Approach

Evaluating tailing storage alternatives, including filtered tailings, is part of a comprehensive tailings management approach. For some sites, a filtered tailings strategy offers advantages, including reducing risks to geotechnical stability and enhancing opportunities for water stewardship, management of lifetime costs, and acceptance by surrounding communities (Cacciuttolo & Atencio, 2023).

Despite typically higher initial investment and operating costs as compared to conventional tailings management, long-term benefits of filtered tailings can also include opportunities to create landforms, which can be operated and managed through to closure as landforms rather than as dams. Not only does this present an opportunity to increase tailings storage volume for a given landform footprint, but this also often results in simpler closure and reclamation planning and execution, occurring over a shorter time frame and with reduced financial assurance.

Okane advocates for responsible land and water stewardship, focusing on preserving returning land use options and water conservation. When helping our clients evaluate alternative tailings management options, we work with our clients to quantify the risks and opportunities of filtered tailings storage. Our team of environmental geochemists quantifies the actual mass of the filtered tailings that will oxidize. We use coupled tailings-atmosphere modelling to assess the depth of oxygen ingress and the resulting rate of advancement of an oxidation front in the tailings as a function of:

• site-specific climate conditions
• exposure time of tailings material to atmospheric conditions
• the exposed surface area
• the degree of saturation, and
• the reactivity of the tailings, all as a function of operational and closure time frames.

In many instances, acidity generation (and ML-ARD risk) is strongly correlated to these five facets, leading to oxidation of tailings being a ‘self-limiting’ condition as oxygen diffusion becomes more challenging to occur at depth. We then use these results to evaluate the influence of thickness and placement timing of filtered tailings stacking rates, which supports making informed water flow and quality modelling predictions.

Okane’s team of geotechnical and mining engineers complements this geochemical analysis by integrating the geochemical considerations with geotechnical assessments and operational parameters to develop tailings material placement and management plans. Our designs consider geochemical, geotechnical, and environmental stability to optimize mine operations and waste management practices for a mine site.

References

Cacciuttolo, C. & Atencio, E. (2023). Dry stacking of filtered tailings for large-scale production rates over 100,000 metric tons per day: Envisioning the sustainable future of mine tailings storage facilities. Minerals 2023, 13(11), 1445; https://doi.org/10.3390/min13111445

Cacciuttolo, C. & Pérez, G. (2022). Practical experience of filtered tailings technology in Chile and Peru: An environmentally friendly solution. Minerals 2022, 12(7), 889; https://doi.org/10.3390/min12070889

Caldwell, J. & Crystal, C. (2015). Filter-pressed tailings facility design, construction, and operating guidelines [C]. http://dx.doi.org/10.14288/1.0320844

Condon, P. D., & Lear, K. G. (2006). Geochemical and geotechnical characteristics of filter-pressed tailings at the Greens Creek Mine, Admiralty Island, Alaska. Journal of the American Society of Mining and Reclamation, 2006, 350-364. https://doi.org/10.21000/JASMR06020350

Davies, M. (2011). Filtered dry stacked tailings: the fundamentals [C]. http://dx.doi.org/10.14288/1.0107683

Lara, J., Pornillos, E., & Muñoz, H. (2013). Geotechnical-geochemical and operational considerations for the application of dry stacking tailings deposits – state-of-the-art. In R Jewell, AB Fourie, J Caldwell & J Pimenta (eds), Paste 2013: Proceedings of the 16th International Seminar on Paste and Thickened Tailings, Australian Centre for Geomechanics, Perth, pp. 249-260, https://doi.org/10.36487/ACG_rep/1363_19_Munoz

Marques, A., Oliveira, S., Paes, B., Paes, I. & Coelho, A. (2020). Evaluation of the liquefaction susceptibility of filtered iron ore tailings from the iron quadrangle (Brazil). In H Quelopana (ed.), Paste 2020: 23rd International Conference on Paste, Thickened and Filtered Tailings, Gecamin Publications, Santiago, https://doi.org/10.36487/ACG_repo/2052_79

Mine Environmental Neutral Drainage (MEND). (2010). Evaluation of the water quality benefits from encapsulation of acid-generating tailings by acid-consuming tailings: Prepared by EcoMetrix Incorporated. MEND.

Mine Environmental Neutral Drainage (MEND). (2017). Study of tailings management technologies: Prepared by Klohn Crippen Berger. MEND.

Northwest Territories Geological Survey [Abstract]. (2022, January 24). Analysis of Long-term Stability of Filtered Tailings at the Cantung Mine, Northwest Territories, Canada [Video]. YouTube. https://www.youtube.com/watch?v=4AFxyJxO3ZE

Sako, C. & Pabst, T. (2023). Comparative geochemical evaluation of codisposal approaches for reactive filtered tailings deposition. Cleaner Waste Systems, 5. https://doi.org/10.1016/j.clwas.2023.100094

Sako, C. & Pabst, T. (2023). Experimental and numerical evaluation of the critical degree of saturation and critical exposure time of acid generating filtered tailings. Applied Geochemistry, 155; https://doi.org/10.1016/j.apgeochem.2023.105726

Seabridge Gold Incorporation. (2016). Best available technology (BAT) study for tailing management at the KSM project. https://ksmproject.com/wp-content/uploads/2022/09/KSM_BAT_Summary_reduced_June2016.pdf

Shaw, S. & Ayres, B. (2014, December 2-4). Geochemistry, seepage, and closure considerations for dry stack tailings facilities [Conference presentation]. NATCL Public Hearing Presentation. https://registry.mvlwb.ca/Documents/MV2014D0012/MV2002L2-0019%20MV2014D0012%20-%20NATCL%20-%20DSTSF%20Public%20Hearing%20Presentation%20Dec%202-4%202014%20Part%203%20of%203-%20Nov27-14.pdf