This study focuses on the characterization and valorization of incinerated sewage sludge ash (ISSA) as a secondary raw material, with particular focus on its mineralogical properties and environmental behavior. Produced as a by-product of municipal wastewater treatment, ISSA presents complex potential both as a source of valuable elements such as phosphorus and selected critical raw materials and as a functional component in industrial material systems. A systematic mineralogical, chemical, and morphological analyses were performed on ISSA samples collected from a Polish wastewater treatment plant. Emphasis was placed on identifying stable and reactive mineral phases, evaluating phosphorus speciation, and assessing the material’s suitability for further processing and application. In parallel, leaching experiments were conducted to ensure the material’s safety in use. The results support the use of ISSA in material production pathways, especially in construction-related applications. Among them, the use of ISSA in cementitious materials was explored in line with circular economy principles, repurposing waste, reducing landfill dependency, and improving resource efficiency. In this context, the potential for passive CO₂ uptake through mineral carbonation was also examined, indicating a measurable contribution to carbon management in sustainable construction materials

Due to its high refractive index, TiO₂ is widely used in coatings and paints. Globally, two major industrial processes are used to produce pure TiO₂ from primary titanium ores. For rutile, the chloride process is applied by mixing finely ground ore with coke and chlorine at temperatures up to 1200 °C. In this process, titanium is reduced and converted to volatile TiCl₄, enabling the removal of impurities and production of highly pure TiO₂. However, the process also generates significant residues, mainly consisting of unreacted materials as TiO2, coke and, SiO₂ polymorphs.

To recover the TiO₂ and coke from the stream, floating by water as dense media is used to produce a concentrate of acceptable Ti grade, though large amounts of impure middlings are also generated. These fine-grained middlings are commonly used as low-value filler in concrete applications (approx. €1/t).

In this study, we present a detailed quantitative mineralogical analysis of the middlings fraction using automated mineralogy (AM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). The data show a correlation between TiO₂ content, density, and particle size. Laboratory air jet sieving allowed the separation of a 63–125 µm fraction containing TiO₂ at ore-grade levels. This simple but effective method demonstrates that low-value middlings can be upgraded to a high-value Ti resource (approx. €400/t).

This study highlights how combining mineralogical characterization with basic processing techniques can significantly improve the economic value of industrial residues.

Established pyrometallurgical processes enable the recovery of many valuable metals from lithium-ion-batteries (LIB) (e.g. Ni, Cu, Co) as an alloy or a metal sulfide. However, lithium dissolves into the slag, from which it cannot currently be recovered economically. One recently discussed approach is called “Engineering of Artificial Minerals” where the lithium is collected in a specifically designed phase.^[1] A recent study demonstrated β-eucryptite as a lithium collector phase with high potential.^[2]

To achieve an economically feasible process, the collector phase requires optimized characteristics: 1) High lithium content; 2.) Low amounts of process impeding impurities; 3.) A suitable microstructure for liberation and sorting. The sampled crystalline slag was investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to investigate these parameters.

The investigated lithium collector phase, β-eucryptite identified in slag from an industrial LIB recycler, shows a lithium content of 5.54 ± 0.19 mass %, close to the stoichiometric content of 5.51 mass %. Furthermore, it shows only minor amounts of cationic impurities, mostly iron and calcium. These impurities could be easily removed while coprocessing β-eucryptite with spodumene in a primary lithium converter.^[3] In addition to grain size distributions, for the quantitative description of the microstructure the implementation of the envelope parameters was tested. A more detailed quantification of the grain shape than conventionally used descriptors was demonstrated.

[1] Hampel, et al., ACS omega, 2024.

[2] Gantz, et al., Adv Energy and Sustain Res, 2024.

[3] Nandihalli, et al., Sustainability, 2024.

The scarcity of raw materials, especially rare earths, lithium, tantalum and vanadium, is a significant risk factor in today's society. Geological reserves of these elements are not sufficient to meet future demand. By recovering these elements from material waste streams, an independent second source of raw materials can be created. Such a second source can be created by capturing the critical elements in a phase with good separation properties through the production of engineered artificial minerals (so-called EnAMs). This approach can be used for the recovery of critical elements from slags resulting from pyrometallurgical processing.

In addition to industrial slags (e.g. Umicore), synthetically produced slags were also studied to investigate the crystallization behavior of Li, Ta and V and to identify potential EnAMs. Specific modifications (chemistry, heat supply, atmosphere) have been used to study how critical elements are incorporated into artificial phases during slag solidification. The example of Li shows that different EnAMs (LiAlO₂ and LiMnO₂) can act as potential sources of recovery. Preliminary investigations have shown that the formation of LiAlO₂ can be inhibited by other phases. For instance, due to the increasing use of lithium-manganese rich cathode material, additionally manganese enters the system, resulting in the preferred formation of spinels (MgAl₂O₄) instead of LiAlO₂. As aluminum can largely be eliminated via mechanical processing, in this study, LiMnO₂ was investigated as a potential main EnAM for the recovery of lithium. Since manganese is highly redox-sensitive, the investigation of speciation changes in the high temperature range of slag is important.

"Theisenschlamm" is a geochemically complex industrial residue resulting from the historical smelting of Kupferschiefer ores in the Mansfeld region (Germany). Produced as flue dust during copper extraction and later fed into lead smelting, this fine-grained slurry was stockpiled after the closure of the lead smelter in the 1970s. By 1990, following the termination of regional mining and metallurgical operations, 220,000 tons were safely stored in a engineered tailings facility.

The material is enriched in Pb, Zn, and Cu, and contains critical trace metals such as Re, Ag, and Ge. It consists of sulfide, sulfate, and silicate phases embedded in a carbon-rich matrix with harmful organic compounds, including biphenyls and dibenzofurans. These characteristics make “Theisenschlamm” both a valuable secondary resource and a complex environmental legacy.

Previous recovery efforts failed due to technical and economic limitations. As part of the FINEST project, a novel recycling concept has been developed, based on detailed geochemical and mineralogical insights. The approach integrates plasma furnace-based pyrometallurgy with waste stream management. Central is the co-treatment of "Theisenschlamm" with precipitates from local wastewater treatment. The process yields a metal-rich concentrate and a geochemically stable, non-classified slag suitable for reuse or disposal without long-term monitoring.

This contribution presents the geochemical basis for material selection, mass balance constraints, and the behavior of key elements during processing, offering insights into the possibilities and boundaries of resource recovery from complex legacy wastes.

The project FINEST is funded by the Investment and Networking Fund of the Helmholtz Association under grant agreement no. KA2-HSC-10.