Conference Agenda

Rare earth elements (REE) are a critical resource for the green energy transition, but their production is associated with significant negative environmental impacts. A shift to sustainable procedures and diversification of supply chains is crucial to ensure a stable long-term supply, and will be driven by economic and political incentives requiring traceability of REE products back to their origin. Here, we employ double spike MC-ICP-MS for combined stable (δ^146/144Nd) and radiogenic (ε^143/144Nd) Nd isotope analysis for geochemical fingerprinting of prospective Australian REE deposits, representing distinct lithologies. With light δ¹⁴⁶Nd of ca. -0.11‰ and for Allanite-bearing historical tailings and even lighter signatures (ca. -0.14‰) for REE-enriched alkaline volcanics on one hand, and isotopically heavy signatures between +0.09 and +0.11‰ δ¹⁴⁶Nd for phosphate ore and waste products, all these REE deposits are strongly fractionated from the bulk silicate Earth composition. Simultaneously obtained ε¹⁴³Nd range from strongly unradiogenic (−40) to slightly radiogenic (ca. +6), reflecting the age and geology of the deposits. This large variability allows a clear distinction between the resource types tested in combined δ¹⁴⁶Nd vs. ε¹⁴³Nd space. Signatures of REE concentrates and leachates generated by conventional hydrometallurgical processing using sulfuric acid baking and a subsequent water leach, fall within uncertainty of their initial feed materials with tailings-derived REE concentrates exhibiting δ¹⁴⁶Nd between −0.11 to −0.14‰, and leachates of the volcanics and phosphate a δ¹⁴⁶Nd of ca. −0.15‰ and ca. +0.11‰, respectively, indicating only negligible fractionation of Nd isotopes, so that the characteristic signatures are preserved for successful fingerprinting.

Flue dust from pyrometallurgical metal extraction is an important secondary raw material source for many metals, including strategic ones. However, it often remains unused when it cannot be directly returned to the process and is instead safely deposited or landfilled as hazardous waste. These materials are continuously altered by (geo)chemical processes, producing heterogeneous, sometimes toxic, and often small-volume residues.

Since processing these materials individually is often uneconomical and may create additional environmental pollution, we developed the FINEST concept. This approach is to mix different fine-grained waste streams to create a raw material optimized for efficient pyrometallurgical processing.

We experimentally implemented and confirmed this concept for processing “Theisenschlamm” mixed with fluxing and reducing agents from secondary raw material sources. While “Theisenschlamm” was already established as a raw material, the selection of the other recipe components was initially made largely by chance during screening of local raw material sources.

Systematic thermodynamic modeling of different mixture systems can replace this alchemical approach with a structured procedure. For this purpose, we developed a PostgreSQL database to collect chemical, phase, and physical parameters of fine-grained secondary raw materials, including quarry fillers, residual material from building recycling, processing residues from tailings piles, and flue dust. These materials were classified using Shiny Apps and prepared as input data for thermodynamic modeling. We will discuss the database structure and its potential applications.

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

The transformation of the cast iron industry toward climate-neutral and resource-efficient production requires not only the electrification of melting technologies, but also the development of sustainable raw material strategies. In electrified foundries, especially induction furnace-based processes, increasing demands are placed on scrap quality, process stability, and the control of impurity accumulation. At the same time, metallurgical residues and by-products such as slags, filter dusts, and sludges represent underutilized secondary iron-bearing resources with considerable potential for circular material use.

This contribution presents current investigations from 3 projects concerning the recycling of multiple iron-containing residues through the production of self-reducing agglomerates. Particular emphasis is placed on the selection of potential secondary raw material as well as the integration of renewable carbon carriers such as biochar and biomass-based binders into agglomeration concepts for sustainable metal processing. Experimental studies evaluate different agglomeration techniques, binder systems, fibre reinforcements, and reducing agents with regard to their mechanical, thermal, and metallurgical performance.

The investigated agglomerates achieved cold compressive strengths exceeding the industrially relevant threshold of 5 N/mm², while selected biochar-based reductants exhibited comparable reduction and melting behaviour to conventional coke breeze systems. Laboratory melting and induction furnace trials further demonstrate the technical feasibility of recovering metallic iron from filter dust-based agglomerates under electrically heated conditions.

The presented approach contributes to reducing landfill demand, lowering Scope 3 emissions, and decreasing dependency on primary raw materials and imported coke. Simultaneously, it supports the implementation of circular economy strategies in electrified iron foundries while maintaining metallurgical quality and process robustness.

In the quest to develop new waste-to-resource pathways for industrial residues, this work aims to recover iron from Waelz slag (WS), the main residue of electric arc furnace dust processing in the Waelz process, using a combined hydrometallurgical and pyrometallurgical process. WS was pre-treated via alkaline leaching using a 6M NaOH solution, which was designed to extract those undesirable elements (e.g., Zn, S, and Pb) from both the perspective of metallurgical quality of the recovered iron and the yield of pyrometallurgical processing. Hydrometallurgical alkaline leaching pretreatment removed over 60 wt.% of Zn and over 45 wt.% of Pb and S while preserving over 95 wt.% of the initial Fe content. The solid residue was then subjected to carbothermic reduction at 1500 °C and the characteristics of the reduction products were compared to those achieved by the reduction products of an untreated WS. Regardless of the hydrometallurgical pre-treatment, the reduction product consisted of an interconnected matrix of metallic iron and secondary slag. Nevertheless, alkaline leaching had a beneficial effect on the amount of metallic iron in the reduction products of the pre-treated WS. They contained up to two times more the amount of iron (19.78-23.34 wt.% vs. 9.52 wt.%), with a metallization degree higher than 95 wt.%. Furthermore, the decrease in undesirable elements associated with alkaline leaching made the reduction products of the pre-treated WS more suitable to be considered as a secondary iron source for metallurgical furnaces, thereby opening new valorization routes for the WS.

Biomass fuels (comprising ~51 wt.% waste wood and ~49 wt.% household waste) combusted in commercial grate-furnace of heat-and-power plants produce critical (complex and cohesive) slags requiring multi-method characterization to understand formation temperatures, mineralogy, and geochemical evolution. Bulk fuel ashes formed at 550 °C (DIN EN ISO 21656) contain Ca-Mg phases (e.g., calcite, dolomite), silicates (talc, illite, feldspar), quartz, and low-melting K/Na salts/chlorides that initiate sintering and initial melting at 1050–1250°C.

Slags show grain-size zoning and variation from wall to combustion side. Wall‑proximal zones are coarser‑ and composed of zoned melilite, wollastonite, clinopyroxene, and sphene in equilibrium assemblages with ~ 5–8 modal % glass. Combustion‑side zones are finer-grained, with plagioclase and/or melilite microlites in ~30–40 modal % glass exhibiting honeycomb textures from degassing (950 °C at roof to 1200 °C at the grate). EPMA reveals TiO₂-enrichment (~ 2.31 wt.%) along walls, with Mg-rich åkermanite cores grading into Fe-Al-rich gehlenite rims.

Time-resolved differential high-T XRD captures the full phase-transition sequence during slag genesis, while T-X modelling (assuming: closed-system, P = 1 bar, volatiles < 1 wt. %, constrained fO₂, equilibrium crystallisation) predicts melting of the biomass-fuel at 1190–1260°C in a multi-component system. Although not representative of the entire process, titanium‑in‑melt pockets in slags along the wall at ~2.31 wt.% TiO₂ coexist with titanite, indicating titanite saturation in the melt (aTiO₂^melt = 0.3, TiO₂^titanite ≈ 30–33 wt. %) at cooling temperatures of ~870 ± 25 °C, validating modelled equilibria against dynamic conditions.

This integrated analytical framework provides novel insight into biomass slagging.

The “Siliziplants” project explores the potential of silicon-rich ashes derived from regionally produced plant biomass as a sustainable alternative to imported rice husk ash. Currently, rice husk ash is widely utilized in steel and foundry industries due to its low density, high amorphous silica content, and favorable thermal insulation properties. However, its supply relies heavily on imports from Asia and Southern Europe, resulting in significant transport-related CO₂ emissions and increasing market dependency.

This study investigates locally available plant species of Central Europe, which are known for accumulating biogenic silica as tall fescue. The project combines laboratory-scale experiments, biomass cultivation trials, and thermochemical processing to evaluate their suitability for producing high-quality, silica-rich ashes. Particular emphasis is placed on optimizing silicon uptake through targeted fertilization using converter slags, and on developing controlled combustion processes that preserve the amorphous silica as well as residual carbon—key properties for metallurgical performance.

Comprehensive chemical, mineralogical, and thermophysical analyses are conducted to assess ash composition, impurity levels, and insulation efficiency. Furthermore, laboratory-scale metallurgical tests evaluate the interaction between the produced ashes and molten iron, ensuring chemical inertness and functional performance comparable to conventional materials.

In addition to technical feasibility, the project examines ecological and economic implications, highlighting the potential for reduced greenhouse gas emissions, regional value creation, and improved resource security. Overall, Siliziplants demonstrates a novel circular approach by integrating biomass utilization, waste valorization, and sustainable material development for the metallurgical industry.