Conference Agenda

Critical elements, especially rare earth elements (REEs), are essential in various applications, including clean energy, medical, communication, and defense technologies. Waste associated with coal mining operations, such as coal-based acid mine drainage (AMD), has recently enticed great interest as a potential unconventional source of REE.

We investigated the distribution, modes of occurrence, and relative extractability of REE from AMD-based materials at Tab Simco, an abandoned coal mining operation in the Illinois Basin, U.S.A. We collected aqueous and solid samples and analyzed the concentrations of REEs in: (1) size fractionated (0.01, 0.22, and 0.45 μm) AMD samples; (2) Fe(III)- and Al-bearing colloids; and (3) AMD-based sediments accumulated in a bioreactor constructed to treat the AMD.

In AMD, the REEs are partitioned between the truly dissolved fraction (<0.01 μm) and the suspended solid fraction (>0.01 μm), which carry a significant fraction of REE. The suspended colloidal and particulate fractions included detrital REE-bearing minerals (e.g., clay minerals, monazite, apatite, and zircon) from the weathering coal mining waste and newly formed Fe(III)-phases. During AMD treatment in the bioreactor, dissolved Fe(II) is effectively removed from AMD through oxidation and precipitation of Fe(III)-phases, onto which REE can adsorb or co-precipitate. Concurrently, the detrital minerals dominated by silicates are sequestered in the AMD-based sediments accumulating in the bioreactor. The overall REE content of AMD suspended fraction, with an average value of 40 mg/kg in total REE, is orders of magnitude higher than that of AMD dissolved fraction. Bulk AMD sediments accumulating in the bioreactor have total REE contents varied from ~20 mg/kg in Fe(III)-rich sediments up to 250 mg/kg in silicate-rich sediments. A consistent positive correlation occurred between REE and Al contents, indicating that detrital fraction was an essential carrier of residual REE.

The preferential enrichments of REE-bearing phases in the silicate-rich particulate fractions, which can contain up to 300 mg/kg of REEs (corresponding to 0.04% REO), make an attractive target for REE extraction. Therefore, if current technologies, which primarily target extraction of REE from AMD, can be extended to include the silicate-rich particulate fractions, it could contribute to improved economic feasibility of the REE extraction operation at abandoned coal mines.

Valuable metals are strategically important for the global economy, as they play a crucial role in industries such as renewable energy and modern technology. However, their availability is becoming increasingly limited, making it essential to develop new technologies for their efficient recovery and reintegration into the supply chain. Within this context, the Horizon Europe Resilex project introduces an innovative circular treatment process to recover metals like zinc (Zn), copper (Cu), and cobalt (Co) from acid mine drainage, while also producing sulfuric acid from mining waste.

The technology comprises seven bench-scale units, forming a circular process to treat acid wastewater and mining wastes from Tharsis mines in the Iberian Pyrite Belt. The first unit, treating 25 L/h, employs physico-chemical treatment to remove iron (Fe) and aluminium (Al) from acid wastewater. A 20-L anaerobic membrane bioreactor (AnMBR) follows, producing hydrogen sulfide (H₂S) from sulfates (SO₄) and organic waste. This H₂S is used to precipitate Zn and Cu as metal sulfides. In the next stage, Co is captured via ion-exchange resins. Simultaneously, solid mining waste undergoes thermal valorisation at high temperatures to produce sulfuric acid, which is used for resin regeneration. A ceramic nanofiltration membrane recovers sulfuric acid from the resin regeneration eluent. Finally, an evapocrystalliser extracts bieberite (CoSO₄·7H₂O) from the concentrated nanofiltration membrane stream, completing the circular process.

The system is currently in the operational and optimisation phase. During the physico-chemical treatment, acid wastewater with a pH of 2.5 and containing Fe (2 g/L), Al (30 mg/L), SO₄ (5.5 g/L), Cu (50 mg/L), Zn (260 mg/L), and Co (5.6 mg/L) is neutralised with NaOH, removing over 99% of Fe and 70% of Al at pH 4.5. Cheese whey has been identified as the most effective substrate for the H₂S production in the AnMBR, enabling 90% removal of Cu and Zn as sulfides, yielding up to 6.4 g CuS and 34.7 g ZnS recovery per 100 L of feed water. The ion-exchange resins concentrate Co up to 3 g/L and 1.8 g of bieberite can be recovered per 100 L, while the thermal valorisation unit can produce sulfuric acid up to 50% purity.

These circular economy-based treatment lines valorise acid effluents and mining waste, maximising metal and sulfate recovery. This strategy has the potential to make mining operations worldwide more sustainable, turning waste into valuable resources.

The growing demand for critical raw materials (CRMs) for a climate-neutral economy has intensified challenges in waste generation and management. Traditional mine waste characterization, focused on environmental risk assessment within a linear economy framework ("take-make-dispose"), contrasts with the circular economy approach ("make-use-return"), which aims to minimize waste and recover valuable materials. However, unlocking the potential of mine waste requires robust, multi-scale characterization techniques. In this study, we review multi-scale characterization protocols to assess mine waste streams in support of a circular economy. In particular, we highlight the use of hyperspectral imaging as a key monitoring technique, enabling the semi-quantitative assessment of hydrogeochemical parameters and metals in mine water, identification of mineral associations in waste rock, and mapping of metal concentrations in tailings. Although challenges remain regarding sensor sensitivity, cost, and large-scale integration, addressing limitations and improving on standardization of such advanced monitoring tools can enhance mine waste management by improving environmental risk monitoring and enabling resource recovery.

Mining activities generate substantial volumes of mine effluents with high concentrations of ammonia nitrogen (NH₃-N), posing severe environmental challenges. Ammonia is toxic to aquatic organisms and contributes to eutrophication, negatively affecting water quality. As regulatory agencies impose increasingly stringent limits on ammonia concentrations, effective treatment of mining effluents has become critical. This study addresses the urgent need for advanced treatment solutions to manage ammonia nitrogen and ensure compliance with environmental standards.

This research investigates the application of ozone microbubbles, an advanced oxidation process (AOP), for treating ammonia nitrogen in mining wastewater. Ozone microbubbles enhance oxidation rates by increasing the gas-liquid interface area, thereby improving contact between ozone and contaminants. A series of experiments were conducted, including precipitation methods like Friedel's salt, to evaluate treatment efficacy and reduce chloride concentrations in effluents post-ozonation. The study involved testing both synthetic and real effluents from Canadian mines to assess the effectiveness of ozone microbubbles under varying salinity conditions typical of mining environments.

Results indicated that ozone microbubbles achieved over 99% removal efficiency for ammonia nitrogen across all tested conditions, with treatment times ranging from 60 to 150 min. Notably, the presence of thiocyanates (SCN⁻) majorly increased treatment time, as they oxidize more readily than NH₃-N. Additionally, chloride concentrations were reduced by 30% after ozonation using the Friedel's salt precipitation method. Salinity, including chlorides and sulfates, had minimal influence on the removal efficiency of ammonia nitrogen, representing a positive outcome despite the elevated salinity levels. Furthermore, no chlorinated byproducts were detected during ozonation with saline effluents, likely due to operational conditions, as chlorates form at elevated temperatures, while tests were conducted at approximately 20 °C.

These findings highlight the potential of ozone microbubble technology as a viable solution for treating ammonia nitrogen in mining wastewater, particularly in cold climates where conventional methods often limited performance efficiency. The relevant reduction of chloride levels enhances the overall treatment process, aligning with regulatory standards and promoting sustainable water management practices in the mining industry. Continued optimization and further research into the integration of ozone microbubble technology could advance more effective and environmentally friendly mining operations.

AMD are a major environmental issue, but they can also contain valuable metals. A typical treatment of AMD involves neutralization and metal precipitation through liming, generating sludge primarily composed of metal oxyhydroxides and gypsum. Depending on the composition of the AMD, this sludge may contain substantial concentrations of valuable metals. This is an example of how human activity can create artificial accumulation of metals, forming over time an “anthropogenic ore deposit”.

In this study, we focus on the recovery of Cu, Zn, and the treatment of Cd from sludge produced during AMD liming, while addressing the challenges of residual contaminants. The sludge used originated from a former mining site in France. The 75,000 m³ of sludge accumulated over 150 years of AMD treatment contains around 1.3% Cu and 5.1% Zn.

This process begins with selective acid leaching, achieving dissolution of over 80% of Zn and Cu, and more than 98% of Cd. Key parameters, such as acid concentration and solid-to-liquid ratio, were optimized to maximize Cu, Zn and Cd dissolution while minimizing the concentration of unwanted elements in solution like Fe and Al. After leaching, selective precipitation recovered 97% of Cu as copper sulfide and 84% of Zn as zinc hydroxide, with some Zn co-precipitating with Cu and minor amounts remaining in the residues.

A preliminary economic assessment, based on metal yields and current market prices, indicated that the process is economically viable, with positive margins after accounting for reagent costs. However, unresolved challenges remain, including the lack of valorization options for the remaining sludge matrix—primarily gypsum and iron oxides—and incomplete depollution, as Zn concentrations in the treated residue (>0.5%) still exceed regulatory limits. Moreover, regulatory hurdles related to waste management and environmental compliance must be addressed for large-scale implementation. This process offers a promising solution for metal recovery from AMD sludge. The positive financial margins generated could contribute to the rehabilitation of the contaminated site, potentially financing techniques such as surface covering or phytostabilization. Addressing both environmental and economic challenges could thus transform a long-term liability into a sustainable opportunity.