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Tailings: how to deal with them? That is the question!

Dear reader, after almost one year, I am happy to write to you again.
This time we will focus on mineral processing, to be more specific, on the waste generated from it.

We know that mineral processing aims to reduce the bulk of the ore by using methods to separate the valuable (desired) mineral(s) from the gangue [1]. Transforming the raw ore coming from the mine into a marketable product is something that happens inside a mineral processing plant, using many available methods. However, starting from the removal of the ore from the ground to its transformation into a concentrate, several unsellable products are generated. They are called mining waste and can be divided, basically, into waste rock and tailings..

Waste rock refers to the overburden or surrounding rocks that must be removed to allow access to the ore. For the opening and during the design of the mine, usually, hundreds of tons of waste rock are removed and stored in a waste pile. Also, tailings refer to the non-marketable product(s) generated during the beneficiation of the raw ore in the processing plant (see Figure 1).

Figure 1: Generic mineral processing route. Modified from Tecnologia de Barragens e Disposição de Rejeitos [2].

Due to the typically low percentage of valuable mineral(s) in raw ores, tailings account for the largest fraction generated in the processing plant. In addition to the solid content, tailings, also contain remnants of chemical products and water, added during the beneficiation processes. This makes tailing management more challenging than waste rock management.

How are tailings typically dealt with?

Throughout the history of mining, and even today, tailings have been commonly disposed of in dams or ponds. Due to large volumes that need to be stored, tailing dams are huge anthropic structures (the highest tailing dam today is as high as 230 m [3]) and require intense monitoring and control to maintain their physical-chemical-geomechanical stability. Tailing dams fail ten times more often than conventional dams [3]. The most common causes of failure are related to the forces of water, e.g. by internal erosion of the dam material or by overtopping of the dam. Dam integrity is thus particularly important under extreme weather conditions [3]. International data indicates that only 5% of the tailing dams created in 2018 were dewatered, producing pasty, thickened, or dried residues. In 2025, it is estimated that 13% of the existing tailings dams will undergo dewatering [4] targeting to increase their stability.

Throughout the existence of the tailing dam, the mining company is responsible, which means spending large amounts of money on something that will not generate revenue for the company and that needs to be monitored even after mining operations cease due to the exhaustion of the mineral deposit.

Why change something that has been done that way for so long?

In recent times, changes in the market (e.g. price increase or demand), advances in technology, and exhaustion of high-grade deposits highlight the possibility of reprocessing tailings. For example, one of the most efficient processes used nowadays to concentrate minerals is called flotation. Flotation as an industrial application is a relatively recent technology [5]. Also considering that old analytical techniques were not sensible to detect some minerals or even that some minerals were not well known and consequently ignored at that time, implies that in many European regions, those old mining wastes are a potential resource for several sought-after metals and semi-metals, including minerals which are nowadays used in modern high-tech applications [6].

The exhaustion of high-grade deposits can also enable the reprocessing of tailings, such as those which have been constructed in Brazil since the 1970s, especially in Minas Gerais where you can find iron ore mining waste. The tailings deposited in some of these dams are extremely rich in iron, with contents varying between 35% and 55% of iron, which represent high grades for the mining industry, surpassing the levels of iron formation deposits in exploration (around 42% of iron). Thus, these dams have become real reserves, with great potential for exploitation and high added value [7].

Speaking of minerals that were of no interest in the past, an article published in 2019 in Forbes, presents the Rio Tinto´s case, where the company started out looking for gold in the waste and found in the waste rock pile “what could turn out to be the biggest deposit of lithium in the U.S.” [8] which is an essential mineral for the production of lithium batteries (See Figure 2).

Figure 2: A satellite view of the Rio Tinto Borax mine in California. The mine is the largest borax mine in the world [8].

Who does it matter to?

Investigating the content of old waste piles and tailing dams is becoming more common in mining as the demand for minerals changes and technology improves [8]. The reprocessing of tailings is interesting from an operational standpoint, as it has one less step compared to the mining cycle: the removal of overburden material, which entails reduced costs [7]. Additionally, regarding the beneficiation phase, the reprocessed material has already been ground; thus, allowing for the removal of one of the most energy-consuming steps.

Above all the reasons already mentioned in favor of reprocessing of tailings, there is the environmental concern, fueled by ruptures of tailing dams (such as the failure of the Fundão (2015) and Brumadinho (2019) in Brazil). Environmental issues put pressure on the mining sector to modernize management practices and adopt stricter rules, to ensure the long-term stability of these structures and the protection of the environment, since it is known that in addition to valuable minerals, tailings can also contain hazardous components, such as As or Cd.

Is this something surreal or has it already been put into practice?

The recovery of raw materials is explored in a case study [6] of the old Penouta mine in Spain. Tailings deposited in abandoned old dumps and ponds are reprocessed, thus recovering tantalum and niobium (an Sn-Ta-Nb concentrate), which are Critical Raw Materials for the EU; quartz, mica, feldspar, and kaolin, which are valuable industrial minerals; and residual extractive waste, which is used for the final rehabilitation of the Penouta site. These actions also contribute to the site’s rehabilitation, minimizing the environmental impacts of the formerly abandoned mine.

Another good example comes from Goslar, in Germany, where the Rammelsberg Mines existed, already considered one of the largest mining deposits in the world. Exhausted in 1988, they stayed in operation for over 1,000 years [9]. Currently, the Technical University of Clausthal, more precisely the Institute of Mineral and Waste Processing, Waste Disposal and Geomechanics (IFAD), along with other partners, have started a project to reprocess the content of the old Rammelsberg tailing dam. In a nutshell, the project intends to recover metallic minerals, and use the residual extractive waste, mostly alumino-silicates, as raw material for the cement industry.

Link to video: https://video.tu-clausthal.de/film/mine-tailings.html

Why not just “take the cherry on top of the cake”?

According to Prof. Daniel Goldmann, the head of IFAD, reprocessing of tailings is “a complex, multi-disciplinary and multi-criteria challenge”. Furthermore, Blengini and co-authors states that the reprocessing of waste rock/tailings to recover only what is most interesting and ignore the remaining minerals may be neither economical nor sustainable. Instead, the recovery of residual extractive waste (mostly waste rock matrix) is a key aspect leading to a more sustainable circular economy model, decreasing the residue and recovering (as much as possible) all minerals contained in the waste rock/tailings [6].

What does SULTAN have to do with it?

Absolutely everything! SULTAN is based on the above principles that aim to achieve a more sustainable circular economy model, not only focusing on the recovery of valuable minerals but also working to immobilize hazardous metals and valorize mineral residues. We see mining waste (tailings and waste rock) not as trash but as a potential source of wealth and we are working hard to present feasible solutions to deal with hundreds of tons of mining waste laying around Europe.

If you have not yet seen our didactic Poster (https://etn-sultan.eu/2020/03/23/sultan-poster/), please, take a few minutes to do that. You will not regret it.

If I caught your attention until here, maybe you will be interested in reading more about this topic in the following references:

[1] Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities, 2004. Available at http://www.geology.cz/rroum/stazeni/2004_BAT_REFERENCE_DOCUMENT.pdf

[2] Tecnologia de Barragens e Disposição de Rejeitos. Available at http://www.itv.org/linha-de-pesquisa/tecnologia-de-barragens-e-disposicao-de-rejeitos/

[3] Introducing TAILSAFE. Available at http://www.tailsafe.com/introducing_tailsafe.html

[4] Barragens de rejeitos de mineração são um desafio mundial. Available at https://www.metso.com/br/blog/agregados/barragens-de-rejeitos-de-mineracao-sao-um-desafio-mundial/

[5] Advances in Flotation Technology “ (1999): B.K. Parekh and J.D. Miller (Editors). Society of Mining Metallurgy and Exploration Inc.(SME)

[6] Blengini, G.A.; Mathieux, F.; Mancini, L.; Nyberg, M.; Viegas, H.M. (Editors), 2019. Recovery of critical and other raw materials from mining waste and landfills. State of play on existing practices. Available at https://ec.europa.eu/jrc/en/publication/recovery-critical-and-other-raw-materials-mining-waste-and-landfills

[7] Sousa, G. M; Gomes, R. C., 2018. Lavra de Barragens de Rejeito: Uma alternativa para manejo e desativação de estruturas.  XIX Congresso Brasileiro de Mecânica dos Solos e Engenharia Geotécnica, Geotecnia e Desenvolvimento Urbano.

[8] Old Mines. New Tricks. Hidden Value In A Pile Of Californian Waste Rock. Available at https://www.forbes.com/sites/timtreadgold/2019/10/22/old-mines-new-tricks-hidden-value-in-a-pile-of-californian-waste-rock/#1beae38c221c

[9] Mine tailings. Raw material source and environment risk. Available at https://video.tu-clausthal.de/film/mine-tailings.html