Recovery of metals from acidic pit lakes in copper-rich deposits has the potential to generate a circular economy, improve water quality, provide a domestic supply of critical materials, and demonstrate corporate investment in Environmental-Social-Governance goals. The European Union lists several constituents found in acidic pit lakes as Critical Raw Materials (CRM; e.g., antimony, cobalt, manganese, vanadium) and Strategic Raw Materials (SRM; e.g., copper, nickel). Additional metals (e.g., zinc) can have high market value. Metal recovery from pit lakes in the Iberian Pyrite Belt (IPB), Spain, has been previously investigated but not advanced. Recent investigations into zinc and copper recovery from two acidic pit lakes in the United States plus resource supply chain risks in Europe may renew interests in metal recovery from IPB pit lakes.

A screening-level economic evaluation required dissolved concentrations of metals, lake volume, and market values of metals. Surface water concentrations (2008) from thirteen, acidic (pH<5), IPB pit lakes were assumed to represent average water column concentrations. The surface of each lake was assumed to be circular allowing the volume of each lake to be estimated using the geometry of a cone. These assumptions have several limitations (e.g., concentrations at depth are typically higher than surface concentrations, particularly in meromictic lakes) that can be refined after screening. Where concentration data are missing, a novel, artificial intelligence (AI) program, called the “Pit Lake Decision Support Tool” (PLDST), can help identify additional lakes for consideration.

Three pit lakes (Corta Atalaya, San Telmo, and Aznalcollar) were found to store between € 2.2 and €10.6 million in combined cobalt, copper, nickel and zinc (Table 1). Further refinement will combine volume vs depth measurements with recent concentration vs depth data to revise values as a function of depth. Often pit lakes are connected to large “mine aquifers,” or underground networks of flooded mine tunnels storing additional metal-rich water, which will be included in revised resource assessments. The PLDST will then be used to estimate the cost and return-on-investment associated with various metal recovery methods including sulfide precipitation, electrowinning, solvent extraction-electrowinning, and copper cementation.

This study raises awareness within mining companies and government agencies of the potential value of CRM stored in European pit lakes. The screening method is simple and identifies high-value pit lakes for advanced resource investigation. The PLDST can identify new potential resources, rapidly estimate costs of different metal recovery methods, and project returns to inform a business case.

Rising prices as a consequence of running short natural resources, continuously increasing demand and a strong dependence upon third party countries led the European Commission to the publication of a list with 34 critical elements, most of them being metals.

With this background, the research project IAW33 was established at the THGA Bochum in 2022, that is founded by the RAG foundation for three years. The aim of the project is the first screening and evaluation of mine water sludges of all active pumping sites in the former coal areas of the Ruhr and the Saar region and the former mine in Ibbenbueren (NRW) for their production potential of critical metals. The sludges were collected with the help of precipitation reactors for low and highly mineralized mine waters. Besides that, two available settlement lagoons were also sampled and analyzed via ICP-MS for all critical metallic and several further economically valuable elements (55 in total).

The results show, that the sludges of some active pumping sites in the Ruhr area exhibit increased concentrations of critical alkaline earth elements, such as strontium, barium and magnesium. Nevertheless, the former mining site in Ibbenbueren (northern part of NRW) shows the most promising results with respect to critical and economically valuable metals. Three samples were taken at one of the four settlement lagoons. After drying, these samples showed just total metal concentrations of nearly 20%, but high rates of nickel, titan, cobalt and rare earth elements. Relating to 1.000 kg of dried sludge, several hundred grams of these metals are contained in it. That means that with a total area of 236.000 m² of all settlement sites, about 32 tons of cobalt, 47 tons of nickel, 38 tons of titan and 10 tons of rare earth elements are present within every meter depth of sludge. Recently ongoing experiments (October 2024) shall enhance the concentrations of these elements with the help of a graded precipitation with the adjustment of different pH levels.

As a conclusion may be drawn, that the former mining site in Ibbenbueren shows the most promising option for a re-mining of the deposited sludges. The recovery of these elements may help to reduce the costs of the mine water treatment at this site and may also reduce the need of large settlement areas. Furthermore it is possible that it contributes on a small scale to the reduction of dependencies upon third party countries.

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.