Mine Closure Supplement Article by Professor Mark Tibbett

Philippines remediation of mine tailings: a new collaborative research initiative

By Professor Mark Tibbett, University of Reading, UK

The world is moving to a low carbon economy that requires a greater amount and variety of metals. More circular approaches to the economy can only partially meet the rising demand for metals – realistically mining will have to continue and indeed increase. The Philippines is the fifth most mineral-rich country in the world, and therefore stands to benefit from this increased demand. However, mines can negatively impact the environment and surrounding communities. Mining and mineral processing consume and contaminate water, contribute to at least 7% of global forest loss each year, are responsible for 8% of the world’s CO2 annually, and compete with local communities for land and ecosystem services. They also produce waste, including wet slurries of finely ground minerals left over from processing, known as tailings. Large mines may generate hundreds of thousands of tonnes of such waste every day.

As one of the world’s most mineral-rich countries, mineral extraction in the Philippines is a critical industry that offers significant potential benefits and returns to both the economy and local livelihoods. Whilst the global demand for minerals to support clean energy technologies is growing, mining for minerals remains restricted in the Philippines due to past environmental impacts, illegal operations, and mismanagement. To address the causes of the current inhibition of mining in the Philippines, a new program of research is essential to deliver a sustainable pathway for Philippine mineral supplies. Consequently, the Philippine Council for Industry, Energy, and Emerging Technology Research and Development and the UK’s Natural Environment Research Council brought forward a new collaborative initiative for sustainable mineral resources in the Philippines. This has led to the development of a series of new research projects (https://www. ukri.org/news/nerc-invests-in-sustainable-future-for- philippine-minerals/).

A key new research program concerns the Philippines Remediation of Mine Tailings (PROMT). This pulled together a wide range of Filipino and UK researchers. The consortium integrates complementary interdisciplinary expertise from Philippines and UK science to build an innovative strategic project to test new sustainable approaches to tailings management and remediation, and ultimately rehabilitation.

The consortium is co-led by the Centre for Sustainable Resource Extraction, University of Leicester, and the Philippine Nuclear Research Institute, together with The University of the Philippines Diliman and Los Baños, the British Geological Survey, University of Exeter, Camborne School of Mines and University of Reading. The consortium built between these institutes provides a wide-ranging and interlinked compendium of disciplines to address tailings remediation in a holistic and novel manner (Figure 1).

Figure 1. Disciplines involved in the PROMT consortium

The team of interdisciplinary Philippine and UK researchers and industrial partners is building an innovative research program addressing research to test sustainable tailings and mine waste management, remediation and rehabilitation. Our ambitions are to produce tailings with less water consumption and greater stability, and show how they can be monitored and adaptively managed in real time; and to enable the processing of modern and legacy tailings to recover more metals, whilst decontaminating them, encouraging rehabilitation and long-term stabilisation and re-use of the associated ecosystem services. The potential outcomes, impacts and benefits of this integrated research will be to:

  1. Reduce community and environmental impacts from tailings.
  2. Provide compliance with international standards.
  3. Improve social license to operate for mining
  4. Reduce long-term liabilities and risks from legacy sites.
  5. Create potential sources of revenue by secondary extraction of additional metals and land re-use.

Recent disasters caused by the failure of tailings storage facilities (TSF), including the 2019 Brumadinho event with over 200 lives lost, as well as tailings releases in the Philippines (Marcopper disaster 1996), have brought extra scrutiny to the management of operational and legacy TSF. Investors of over $13 trillion in mining assets have driven the development of new standards in tailings safety in the aftermath. However, for the Philippines, with rugged topography, high rainfall (including typhoons), and regular seismic events, TSF remain at risk of failure, and continue to discharge contaminated water downstream. These issues not only affect operational mines but also include legacy tailings at abandoned or closed mine sites that are persistent environmental hazards. Only with innovation will new sustainable standards for TSF management be achieved. The strategic PROMT project aims to deliver the fundamental science to underpin such innovation, combining interdisciplinary expertise and novel ideas integrating three science areas (Figure 2).

Figure 2. Science areas and examples of novel interdisciplinary research questions.

Science Area 1: Observing Technologies – Geophysical, Geotechnical and Remote Sensing. Here we enable minimally invasive 3D and 4D assessment and characterisation of TSF, with near real-time monitoring of processes including solvent propagation during in situ leaching, biological colonisation and root development, and tailings evolution through compaction, cementation, and pedogenesis.

Science Area 2: Novel Solvents and their Flow. This will address the key knowledge gaps to enable application of novel environmentally benign solvents to leach metals and remove contaminants from tailings by ex situ and in situ reprocessing.

Science Area 3: Ecosystem Development. Here we will assess and understand the development of ecosystems in tailings through plant- soil–biota interactions; how ecosystem development may be manipulated to reduce toxicity and improve tailings stability, water balance and end-use; and the effects of ecosystem perturbation with novel solvents used for in situ leaching.

The PROMT project aims to build an innovative research program to test sustainable tailings management, remediation and rehabilitation. The specific science being undertaken by the project includes a wide range of research based on the novel and interdisciplinary questions in each science area (Figure 2).

The project will undertake in situ geophysical tomographic monitoring of tailings facilities to understand the evolution of tailings mineralogy, structure, water flow, and stability through time and enable adaptive management.

Electrical methods such as resistivity and induced polarisation using multi-electrode arrays can be used with machine-learning to image the distribution of minerals and porewater saturation in the near surface. These methods have recently been used in imaging landfills with the aim of mining them.

Large permanent arrays are now possible that can image the subsurface in real time allowing monitoring of dynamic processes, such as rainfall infiltration or leachate propagation, in a volumetric and minimally invasive manner, but have yet to be widely adopted in tailings monitoring and management.

The project will assess the use of novel environmentally- benign solvents, which exhibit low toxicity and/or enhanced selectivity for specific target metals and metalloids, compared to conventional strong mineral acids. Previous work suggests that certain types of organic acids are able to perform such functions at near-ambient geochemical conditions, due to their additional capability for metal chelation. Similarly, non-aqueous solvents, such as deep eutectic solvents (DES) are capable of leaching a wide range of minerals including chalcogenides, arsenides, and native gold and mercury with a range of potential benefits including low volatility, high target metal selectivity, and potentially low ecotoxicity. Such solvents can result in a step change in our ability to both remove environmental contaminants and recover metals, including previously discarded by-products, and render the final tailings more amenable to new use such as aggregate or soils. These solvents can also provide an alternative way to process modern ore streams to produce more environmentally benign and stable tailings.

Whilst these environmentally benign solvents may be used for ex situ leaching of mine waste we also aim to investigate whether they can be applied for in situ reprocessing of tailings by injection of solvents to remove contaminants, recover value and facilitate tailings stabilisation and CO2 sequestration. This has the crucial advantage of not requiring disruption of tailings and may even be used to strengthen them. Metal recovery from pregnant leach solutions can be via a combination of standard techniques such as electrowinning together with novel approaches such as sorption onto engineered nanomaterials.

Microbes have long been used in conventional heap leaching of low-grade ores, but to date no research has focused on understanding and harnessing geomicrobiological processes when exposed to the novel organic solvents outlined above. Microorganisms can produce and degrade organic acids, with implications for environmental metal behaviour. Indeed, the presence of residual organic substrates may even be beneficial for microbial community development in nutrient-limited tailings and support the associated provision of ecosystem services that are essential for plant colonisation. This fundamental understanding is needed to (a) predict and numerically model the behaviour and degradation of these novel solvents as a function of time and across different environmental conditions, including the critical zone and (b) inform phytoremediation strategies.

Phytoremediation: Plants are an integral component of restoring soil and ecosystem functions after mining and some species can be employed to accumulate and recover contaminant metals and to stabilise tailings for ecosystem rehabilitation. We intend to identify local plant species that have both the appropriate pH, salinity and metal tolerances to grow on treated tailings and to conduct field and glasshouse trials to facilitate their establishment on these substrates. We have already gathered preliminary data from a literature review that shows a large number of metal concentrating plants in the Philippines with nickel primarily accumulated in aboveground parts, copper accumulated in the roots, and cobalt being evenly distributed (Figure 3).

Figure 3. Filipino flora metal concentration for aboveground (green) and belowground (brown) plant parts for (a) nickel (n = 57); (b) copper (n = 83); (c) cobolt (n = 40); +/- standard error of the mean.

The facilitation of pedogenic activity in tailings materials will be required to underpin new ecosystem development and land use for the benefit of local communities.

This will require detailed materials characterisation and understanding of what materials are present from the outset of a mining project and hence what will remain in the tailings and stored overburden with a perspective on soil development and target land end-use. Understanding the interaction of plants, symbionts and the wider soil biotic communities will be essential in facilitating incipient ecosystem development.

Underpinning all these studies will be a holistic geometallurgical approach to material characterisation to understand all the materials involved including ores, tailings, and soils, and their variability.

This will use state-of-the- art material characterisation at various scales, from hyperspectral imaging of mineralogy and µ-CT of textures at the drill-core scale through to automated- SEM (MLA, QEMSCAN, etc.) quantification and mapping of mineralogy and liberation at the grain scale. These data will enable predictive modelling, such as mineral separation performance, the leaching behaviour of metals from tailings with solvents, hydraulic properties of tailings including macropores and the development of soil textures during tailings pedogenesis.

Whilst each of these approaches individually promises significant advances, we propose that it is their integration and synergies that will deliver a step change in innovation and enable us to achieve our ambitions of sustainable tailings management. Furthermore, the interfaces between approaches provide fruitful and timely science opportunities for novel research, e.g.:

  • Can novel solvents be used to recover metals from metal phytoaccumulator species? In turn, canplants and the microbiota remediate traces of residual solvents such as DES from tailings?
  • Can in situ monitoring be used to assess effectiveness of phytostabilisation or other induced stabilisation of tailings by adaptive management, to identify areas amenable for in situ reprocessing, and to monitor the reaction progress during reprocessing?

The project work has already started and we have secured access and permissions from companies, government agencies, the mining regulator and local communities for fieldwork, which commenced earlier this year.

We already have good contacts and approval from a number of mine sites (Figure 4) including porphyry Cu-Au mines at Philex Mining, Benguet and Carmen Copper, Cebu; Filminera Resources’ Masbate gold mine; and NickelAsia’s Ni-laterite deposits. Early data is currently pouring in from all three science areas and I look forward to updating you on results at a future Mine Closure conference.

Figure 4. Mines, deposit type and collaborate companies
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Professor Tibbett can be emailed at: m.tibbett@reading.ac.uk

M Tibbett University of Reading, UK

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