During the period from 1 October 2019 to 31 March 2023, a collaborative effort was carried out under the German Federal Ministry of Education and Research’s “r+Impuls – Innovative Resource Efficiency” programme. The consortium, comprising the Institute for Building Material Research (FEhS), Hermann Rauen GmbH & Co., DK Recycling und Roheisen GmbH, and several graduate students and engineers, set out to develop an industrial‑scale conditioning plant for electric arc furnace slag (EOS). The objective was to transform the by‑product of steelmaking into a construction material that satisfies the forthcoming stringent environmental regulations embodied in the nationwide Building Material Regulation (EBV).
In 2021 Germany produced approximately 40.2 million tonnes of raw steel, of which about 30 % came from electric arc furnaces. Each tonne of steel generates roughly 150 kg of slag, resulting in a total EOS production of 1.8 million tonnes. Under the EBV many of these slag streams would otherwise be classified as non‑sellable and destined for landfill. The project therefore aimed to condition EOS so that its leaching behaviour matches that of natural aggregate, thereby meeting environmental thresholds while preserving primary raw‑material and landfill capacities.
The conditioning concept relies on injecting a conditioning agent into the liquid slag during ladle tapping into a dedicated container. By controlling the chemical and mineral composition of the slag, the process yields a coarse metallic fraction and an oxidic fraction. Magnetic separation isolates the metallic part, while the oxidic fraction is further processed to recover a highly iron‑rich sub‑fraction, termed separation iron, which is returned to steel production. The remaining oxidic material is marketed as a construction aggregate.
A prototype conditioning plant was operated to validate the concept. Conditioning trials demonstrated that the leaching behaviour of the treated slag closely resembles that of natural aggregates, with reduced concentrations of potentially harmful elements. Mechanical testing of the conditioned aggregate revealed improved compressive strengths compared with untreated slag, although specific numerical values were not disclosed in the report. The project also involved the design, construction, and commissioning of adapted crushing aggregates and the full‑scale operational conditioning plant. Economic and ecological assessments indicated that the conditioning process can be integrated into existing steel production lines with acceptable capital and operating costs, while significantly reducing the environmental footprint of slag disposal.
The consortium’s roles were clearly delineated: FEhS provided the scientific and technical expertise in material characterization and process design; Hermann Rauen supplied the industrial infrastructure and slag feed; DK Recycling und Roheisen contributed recycling and iron recovery expertise; and the graduate students and engineers carried out experimental work, data analysis, and plant construction. The project’s outcomes include a proven conditioning technology, a functional industrial plant, and a set of performance data that support the feasibility of using conditioned EOS as a construction material under future regulatory regimes.