The “Theisenschlamm” Challenge: Exploring the potential and limitations of valorizing a stockpiled flue dust sludge by blending it with other environmentally toxic waste streams.

"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.