Remining of Mine Water and Tailings – an Initial Assessment
Introduction
The Research Center of Post-Mining (FZN) at the TH Georg Agricola University (THGA), Bochum/Germany, is involved on both a scientific and a practical level with the many different challenges arising from the mining lifecycle, this applying in particular to the post-mining phase. The provision of a sustainable flow of raw materials has a key role to play in this respect and the FZN is doing its bit to ensure that mining remains both workable and viable in the years ahead. Only by combining sustainable mining practices with the circular economy will it be possible to create the basis that is needed to meet the future raw material requirements of the energy, mobility and heat transition processes (1).
There is one basic precondition for this: Both industry and consumers must be made aware of sustainable raw materials and what they mean. From an industry perspective sustainable raw materials production means bringing operations into line with the UN’s 2030 Agenda and the 17 sustainable development goals (SDG) and it also means creating transparency, communication and participation. And from the consumers’ viewpoint it requires an awareness of the importance of producing raw materials in a sustainable way.
There is no doubt that the mining industry is facing huge challenges as far as its “social license to operate” is concerned. The most effective way ahead will be to adopt a transparent strategy of risks and opportunities management and to establish reliable communications with all stakeholders, thereby enabling them to participate fully in the process. Actions aimed at monitoring the mining processes and the impact on the environment play a key role in all this and one of the areas of responsibilities identified by the FZN has been to optimise “mine water management” as one of the so-called “eternity obligations”. The findings obtained from longstanding mine-water monitoring operations in the Ruhr, Saar and Ibbenbüren coalfields have indicated that there is promising potential here for the recovery of critical raw materials.
Looking at this aspect in a broader focus it is possible to see how it interconnects with the EU Green Deal, the EU Conflict Minerals Regulation and the European Critical Raw Materials Act (2). The aims laid down in these and other legal regulations comprise the sustainable supply of raw materials, the strengthening of supply chains, the development of European capacities and the promotion of a sustainable and recycling-oriented raw materials industry. By the same token, the Federal Government has declared that it aims to bring the national mining industry into line with the 17 SDGs and to adapt it to the circular economy (3). The Federal Ministry for Economic Affairs and Climate Action (BMWK) has specifically highlighted the benefits of domestic raw materials extraction: It is more environmentally friendly, more occupationally safe and more participative when it comes to local jobs and labour market effects (4).
In this context the following paper will seek to examine the potential that mine water can offer for the national recovery of critical raw materials. Consideration will also be given to embedding the principles of the circular economy in the mining lifecycle and to efforts aimed at recovering sustainable resources from mining industry waste such as mine tailings. This issue is currently being discussed at ISO level with a view to drafting a standard.
In recent years a number of global, and in some cases concurrent, disasters (pandemic, war and economic crises) have served to expose the associated problems that exist in the form of commodity dependence on third countries and escalating raw material prices. Back in 2011, with the financial crisis still of major relevance, the European Commission drew up a list of 14 critical raw materials. This was updated to 20 in 2014 and to 27 in 2017 and was then further extended to 30 in September 2020. The current list, which dates to 2023, accounts for 34 critical materials (Figure 1) (5). Many of these commodities, which are mostly metallic in nature, are classified as “critical” because they play a fundamental role in a wide range of technologies and in some cases are also 100 % import dependent (Table 1).
Table 1 presents a selection of metal resources (listed according to atomic number), along with examples of where they are used and their respective dependence on third-country imports from outside the EU.
One of the key objectives of the European raw materials strategy is to reduce the current supply dependence on third countries and to tap into new sources of raw material production within the borders of the EU. Even though the German coal industry has now ceased production the mine water that continues to accrue can make an important contribution towards reducing these dependency levels. Investigations that have been carried out in Portugal (7), the USA (8, 9, 10), South Africa (11) and elsewhere have indicated that significant quantities of rare earth elements can accumulate in the ferrous sludges that have been created from the local mine water. The research project under review here therefore seeks to examine whether the mine water that has built up in the former German coalfields areas, and/or its precipitates, can also exhibit similar levels of enrichment and as a result can make a practical contribution towards reducing the nation’s dependence on raw material imports from third countries.
International experiences and the “first-flush effect”
Tests carried out at a former gold mining site in Portugal showed (7) that an enrichment of rare earths had taken place in the sludges that were left over from a passive mine-water preparation plant. Reports indicate a concentration of up to 185 mg of neodymium and nearly 300 mg of cerium per kilogram of sludge. In overall terms, the sum total of the concentration levels of all the rare earths exceeded 700 mg/kg of sludge, whereas the original concentration level in the mine water was in the lower microgram range. Similar values were measured in the deposited mine-water sludges in the American Appalachians (8, 9, 10). The competitive concentrations of rare earths found in these sludges have persuaded the authors to underline all the advantages associated with the recovery of rare earth elements from settling ponds containing iron sludges. For one thing there are no radioactive byproducts such as uranium and thorium, as is the case with conventional reclamation processes, and for another, the extraction costs are relatively low, as the sludge has already built up or been accumulated and this can help reduce the cost of processing the mine water. Moreover, there would be no need for a new set of mine workings below ground – a fact that will also benefit the social acceptability of the project – and as well as meeting the all-important requirement of reducing the dependence on third countries such a scheme would also create new employment opportunities in regions badly hit by the process of structural change.
While the continuous replenishment of dissolved metals due to the non-stop pumping of the mine water is often seen as one of the advantages of recovering raw materials from mine-water bodies, this is not always the case when the first-flush effect is taken into account. The first-flush effect (the term was coined by P. Younger (12) in 1997 and later translated into German by C. Wolkersdorfer (13)) refers to the steady decline in the overall mineral content of the mine water over time (Figure 2).
This happens because the secondary minerals that are found below ground and formed during the mining process – particularly iron-rich sulphates (14, 15, 16) – are dissolved by the mine water and gradually leached out. Areas of deposits that are not flooded and are exposed to atmospheric oxygen also contribute to the formation of these secondary minerals and to the maintenance of dissolved metals in the mine water. As a general rule, the metal concentration levels in underground workings or deposits that are completely flooded tend to be fairly similar to the natural, geogenically determined background levels that exist in the area in question. According to empirical assessment, this process, i. e. beginning with an initially very high mineral content and then running through to the geogenic background stage, lasts for about four times the duration of the flooding phase.
Similar developments in respect of overall mine-water mineralisation are therefore to be expected at mines in Ibbenbüren and in Saarland (17). At Ibbenbüren the mine water is in future to be drained off via a newly constructed passageway that will be located above the former mine workings. As a result, the major part of the deposits will be flooded and any further formation of secondary minerals can only take place within the strata lying above the water-filled former working zone. Even after the flooding phase invasive rainwater and atmospheric oxygen can lead to the formation of secondary minerals in this area. Similar conditions are also likely to develop in Saarland at some point in the future. In the former Ruhr coalfield the mine water will in future be pumped out on a long-term basis via a number of drainage stations (18) so that, where necessary, a safe distance can be maintained to vital drinking-water reserves. This means that the genesis of secondary minerals is likely to endure over the long term, something that will contribute to the prolonged mineralisation of the mine water.
The IAW33 research project
Once the water management scheme has been put into effect it is predicted that as much as 95 M m3 of mine water will accrue each year in the Ruhr coalfield alone (19). Much of this water will have a high mineral content, this including many dissolved metals – few of which are currently being analysed using the standard tests that are routinely carried out. The concentration levels of many of these metals is often too low to be detected using standard analysis methods. A different approach is therefore required in order to be able to investigate the mine water as a source for the supply of critical raw materials. It was for this reason that the IAW33 research project (“Innovative processing technologies and their potential for the recovery of recyclable materials from mine water, precipitates and tailings from the Ruhr, Saar and Ibbenbüren areas with special focus on critical metal resources”), which was funded by the RAG-Stiftung, developed a set of reactors that were specifically adapted to the particular characteristics of mine water and were designed to trigger the precipitation of the dissolved iron by means of targeted oxygen enrichment. This operation also causes other dissolved metals to become enriched in the resulting precipitate as part of the co-precipitation process, this product comprising a mostly reddish sludge consisting of various iron oxide/iron hydroxide particles. The precipitate material that is produced in the precipitation reactors is then analysed with the help of the DBM, where ICP-MS (Inductively Coupled Plasma – Mass Spectrometry) technology is used not just to look for all the critical metallic materials as defined by the European Commission (5) but also to identify other metals of high economic value with a view to ensuring that the extraction process is as cost-effective as possible. The findings thereby obtained can then be used to evaluate the annual yield of valuable substances and/or the quantity of raw material becoming available at the existing water pumping sites.
Design, construction and operation of the precipitation reactors
In order to ensure conclusive analysis results and to be able to make the best possible predictions as to quantity of precipitate that can be obtained per cubic metre of mine water it was decided that two types of reactor should be developed. The first of these was designed for weakly mineralised mine water (iron concentration < 5 mg/l) and was to be powered using a solar energy and battery storage system so that it could still operate at sites where no mains electricity was available. This unit consisted of two commercial 600 l IBC containers connected up in series so as to create a large operating volume and thereby to deliver the largest possible quantity of precipitate. The mine water was pumped from one chamber to the other and then enriched by means of atomisation using a nozzle fed with atmospheric oxygen before being delivered back to the first chamber to establish a circulatory system with a hydraulic connection. Hydraulic barriers or flow restrictors are installed within the chambers to promote the sedimentation of the precipitation particles (Figure 3). This type of reactor was used to process mine water at the Friedlicher Nachbar, Robert Müser and Heinrich pumping sites.
A different concept was used for processing highly mineralised mine water (iron concentration > 5 mg/l, saline). In this case the solar-powered pump was replaced by a commercial air compressor installed in a single IBC container from which six air outlets led out and were then suspended in the mine water (Figure 4). This system was used for processing mine water from the Walsum and Ibbenbüren sites. This second reactor type has two disadvantages compared with the system used for the weakly mineralised water in that it has a lower processing volume and requires a mains power supply. However, its great advantage is that there are no electronic or mechanical components operating within the body of mine water and therefore there is no risk of clogging or encrustation, a problem that frequently affected the first reactor type in spite of the weakly mineralised water being processed.
Initial analysis results and assessments
The precipitation products recovered at the Ruhr sites (Friedlicher Nachbar (FN), Robert Müser (RM), Heinrich (HR) and Walsum (WS)) and at Ibbenbüren (IB) yielded significant results as far as overall metal concentration levels were concerned. The analysis findings in respect of alkaline and alkaline earth metals, transition metals (excluding iron), metals and semimetals and rare earths are presented in Table 2. Here the results have been extrapolated to grams per tonne of precipitate.
It was found that the settling tanks at Friedlicher Nachbar (Ruhr) and at Ibbenbüren presented the most promising total metal concentrations as far as potential raw material extraction was concerned, even though – as expected – the proportion of iron oxide (Fe2O3) was highest in each case. The precipitation reactor at the Friedlicher Nachbar site, along with those set up at Robert Müser, Heinrich and Walsum, produced very low metal concentrations in the precipitate. However, it must be borne in mind that alkaline and alkaline earth metals, which are neither deemed “critical” nor considered to be of economic value – this includes sodium, potassium and calcium – were not analysed or taken into account as part of the test. X-ray spectroscopic analyses (XRD) indicate, however, that the precipitate obtained at these sites has a high content of calcite and aragonite, which would increase the metal concentration levels accordingly. Nonetheless, these findings show that the extraction of raw materials from these bodies of mine water will be an extremely difficult operation and at most could only yield elements associated with the alkaline and alkaline earth metals. More particularly, the alkaline earth metals magnesium, barium and strontium exhibit relatively high concentration levels at all the Ruhr sampling sites and not only in the mine water bodies themselves but also in the precipitates. The metal concentration level in the precipitates obtained from the precipitation reactor at Ibbenbüren was also found to be exceptionally high (Figure 5).
With a metal content of over 63 % this was almost twice as high as the figure indicated by the analysis results for the slurry sample taken from the settling tank. However, this is only due to the high iron concentration, as all the other metal species present much lower concentration levels. The reason for this can be found in the pH value of the mine water in the reactor, as monitored over the course of the experiment, which within a few days and weeks fell from values of over 6 to a figure of 3.2. At such a low pH value many metals remain in solution, to the effect that a phased precipitation process is now being implemented at the Ibbenbüren site. Phased precipitation means setting different pH values with the same body of water so as to take account of the different solution equilibria of the different metals at various pH values. In this way it should be possible, by setting several different pH values in the precipitation reactors, to ensure that specific individual metals can be targeted for precipitate enrichment.
Preparations are now under way to extend these investigations to the Duhamel site in Saarland.
Summary and outlook
There is a distinct possibility that precipitates obtained from mine water bodies at a number of sites in the former coalfield areas could be used for extracting raw materials to provide a sustainable supply of resources and to reduce the national dependence on third-country suppliers. The results obtained from the settling tanks at Friedlicher Nachbar and Ibbenbüren show high metal concentrations along with a replenishment of dissolved metals that is likely to remain secure in the medium to long term. Direct extraction from precipitates where the highly mineralised mine water exhibits high metal concentration levels, as is the case at Ibbenbüren, is also possible if the phased precipitation process that is currently being carried out is able to match, or even exceed, the results achieved in the settling tanks. A promising approach for extracting individual metals from precipitates has recently been published by Jovičević-Klug et al. (22). In this case the products are rapidly heated up by an electric arc in a reducing hydrogen atmosphere and then partially smelted, thereby producing pure elemental iron that can be used directly for further processing into steel products. The elimination of the main constituent, namely iron, causes the remaining metals to become enriched to the extent that post-processing is likely to be economically viable. In a subsequent stage of the project these findings are to be applied to mine-water precipitates and the results evaluated.
Extracting materials from precipitates with low metal concentrations, as have been identified at all the Ruhr sites (Friedlicher Nachbar, Robert Müser, Heinrich and Walsum), can however only be successful if restricted to the alkaline earth metals magnesium, strontium and barium. Nevertheless, the processing technologies required for such an undertaking, and the investment that will be needed to complete the task, still constitute a major obstacle.
In overall terms it can be said that individual sites have a good potential for the extraction of metals from mine water and as these materials can be regarded as a new resource as far as “remining” is concerned they can and indeed should be used to this end. It is now important to realise that material that for tens and indeed hundreds of years has been seen merely as a waste product should be recognised as offering new possibilities for a sustainable and reliable source of raw material supply and that this should be viewed as an opportunity for eco-friendly and environmentally responsible raw materials production. Using existing and/or accruing resources of this kind does not require the building of new mines or the accessing of new deposits. At the same time, the quantities of sludge from mine-water processing plants that have to be sent to landfill will be dramatically reduced – and as well as potentially generating profits from the marketing of recyclable material this will also mean lower costs for sludge disposal and a much reduced land usage due to the lower demand for large settling tanks.
Acknowledgements
We wish to express our thanks to the RAG-Stiftung for supporting this research project and to the RAG Aktiengesellschaft for providing the relevant data and for their collaborative co-operation. Thanks are also due to Michael Bode of the DBM in Bochum who kindly provided ICP-MS analysis services.
References / Quellenverzeichnis
(1) Goerke-Mallet, P.; Melchers, C. (2022): The Mine Life Cycle and the United Nations 2030 Agenda – A Sustainability Analysis. In: Mining Report Glückauf 158, Heft 1, S. 59 – 71. https://doi.org/10.48771/a8cx-dr76.
(2) Europäische Kommission (2024): Europäisches Gesetz zu kritischen Rohstoffen. Online verfügbar unter https://commission.europa.eu/strategy-and-policy/priorities-2019-2024/european-green-deal/green-deal-industrial-plan/european-critical-raw-materials-act_de (Stand 28.06.2024).
(3) Goerke-Mallet, P.; Melchers, C.; Rudolph, T. (2022): Bergbau und Nachhaltigkeit – ein Zielkonflikt? In: bergbau 73 (6), S. 248 – 254. Online verfügbar unter: https://doi.org/10.48771/eern-2r82.
(4) Bundesministerium für Wirtschaft und Energie (BMWi) (2021): Rohstoffe. Bergbau, Recycling, Ressourceneffizienz – wichtig für Wohlstand und Arbeitsplätze. https://www.bmwi.de/Redaktion/DE/Publikationen/Industrie/rohstoffe-bergbau-recycling-ressourceneffizienz.pdf (Stand 28.06.2024).
(5) European Commission (2023): Critical raw materials. Online verfügbar unter: https://single-market-economy.ec.europa.eu/sectors/raw-materials/areas-specific-interest/critical-raw-materials_en (Stand: 27.06.2024).
(6) Europäische Kommission (2020): Widerstandsfähigkeit der EU bei kritischen Rohstoffen: Einen Pfad hin zu größerer Sicherheit und Nachhaltigkeit abstecken. COM(2020) 474 final. Online verfügbar unter: https://ec.europa.eu/docsroom/documents/42849 (Stand: 3.11.2022).
(7) Prudencio, M. I.; Valente, T.; Marques, R.; Braga, M. A. S.; Pamplona, J. (2015): Geochemistry of rare earth elements in a passive treatment system built for acid mine drainage remediation. In: Chemosphere 138 (2015), pp 691 – 700.
(8) Ziemkiewicz, P.; He, T.; Noble, A.; Liu, X. (2016): Recovery of Rare Earth Elements (REEs) from Coal Mine Drainage.
(9) Ziemkiewicz, P. (2019a): Recovery of Rare Earth Elements from Acid Mine Drainage. Written Testimony of Paul F. Ziemkiewicz to the U.S. Senate Committee on Energy and Natural Resources, 14th May 2019.
(10) Ziemkiewicz, P. (2019b): Recovery of Rare Earth Elements (REEs) from Coal Mine Drainage. Presentation NETL REE Review Pittsburgh PA, 9th April 2019.
(11) Dube, G.; Vadapalli, V. R. K.; Malatji, M.; Mothupi, T. (2020): Recovery of Rare Earth Elements (REE) from Acid Mine Drainage (AMD) passive treatment systems – A review. Vortrag SAIMM Konferenz, 12. November 2020.
(12) Younger, P. L. (1997): The longevity of minewater pollution: a basis for decision-making. In: Science of The Total Environment 194 – 195, pp 457 – 466. DOI: 10.1016/S0048-9697(96)05383-1.
(13) Wolkersdorfer, C. (2020): Grubenwasserreinigung – Beschreibung und Bewertung von Verfahren. 114 Abb., 29 Tab.; Heidelberg (Springer).
(14) Bayless, E. R.; Olyphant, G. A. (1993): Acid-generating salts and their relationship to the chemistry of groundwater and storm runoff at an abandoned mine site in southwestern Indiana, U.S.A. In: Journal of Contaminant Hydrology, 12, pp 313 – 328.
(15) Alpers, C. N.; Blowes, D. W.; Nordstrom, D. K.; Jambor, J. L. (1994): Secondary Minerals and Acid Mine-water Chemistry. The Environmental Geochemistry of Sulfide Mine-Wastes. Mineralogical Association of Canada – Short Course Handbook, Vol. 22, Waterloo (Ontario).
(16) Cravotta III, C. A. (2007): Dissolved metals and associated constituents in abandoned coal-mine discharges, Pennsylvania, USA. Part 1: Constituent quantities and correlations. In: Applied Geochemistry 23 (2008), pp 166 – 202.
(17) RAG AG (2014b): Konzept zur langfristigen Optimierung der Grubenwasserhaltung der RAG Aktiengesellschaft für das Saarland gemäß § 4 Erblastenvertrag zur Bewältigung der Ewigkeitslasten des Steinkohlenbergbaus der RAG AG im Rahmen der sozialverträglichen Beendigung des subventionierten Steinkohlenbergbaus in Deutschland vom 14.08.2007. 17 S., Herne.
(18) RAG AG (2016): Aufgaben für die Ewigkeit. Grubenwasserhaltung, Poldermaßnahmen und Grundwassermanagement im Ruhrgebiet. 34 S., Herne.
(19) Drobniewski, M. (2019): Grubenwasserkonzept der RAG Aktiengesellschaft. Vortrag im Rahmen der Tagung „Nachbergbauzeit“ am 7.3.2019 an der Technischen Hochschule Georg Agricola in Bochum.
(20) BGR (2022): Preismonitor September 2022. Deutsche Rohstoffagentur (DERA) der Bundesanstalt für Geowissenschaften und Rohstoffe. Online verfügbar unter: https://www.deutsche-rohstoffagentur.de/DERA/DE/Aktuelles/Monitore/2022/09-22/2022-09-preismonitor.pdf?__blob=publicationFile&v=2
(21) Wei, X.; Zhang, S.; Shimko, J.; Dengler II, R. W. (2019): Mine drainage: Treatment technologies and rare earth elements. In: Water Environment Research, 91, pp 1061 – 1068.
(22) Jovičević-Klug, M.; Souza Filho, I. R.; Springer, H.; Adam, C.; Raabe, D. (2024): Green steel from red mud through climate-neutral hydrogen plasma reduction. In: Nature, Vol. 625, pp 703 – 717.
