title: Repurposing legacy iron and steel slag: a resource potential
Creators: name: Hilderman, Robin Xiaofang
Publisher: name: University of Glasgow
Year: 2024

description: The reuse of legacy iron and steel wastes presents opportunities for resource recovery and atmospheric carbon dioxide (CO2) storage. Limited documentation of these anthropogenically derived rocks and the natural environmental processes altering slag deposits resulting in ecotoxic metal leaching pose challenges to repurposing. Increasing interest in integrating Circular Economic (CE) approaches into waste management requires a comprehensive understanding of the materials and developing innovative methods to resolve characterisation complexities. In this study, the opportunities for legacy slag are explored using the two Carnforth Iron Work deposits in Warton, South Lancashire, England that are located on the Morecambe Bay coast and ~ 435 meters (m) inland. The mineralogy and trace metal content are quantified and identified using a data-driven unsupervised learning approach from the inland deposit (A), and the lithification mechanisms and mineralogic features are identified from the coastal deposit (B).

From the inland setting deposit A, 23 sections spanning five stratigraphic horizons were analysed for microstructural compositional variation using scanning electron microscopyenergy dispersive X-ray spectrometry spectrometry (SEM-EDS). These large and highlydimensional datasets were quantified using a semi-automated approach leveraging nonnegative matrix factorisation (NMF) and Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN), the regions were segmented into mineral phases and the trace metal constitutes were quantified. The variable mineralogy and metal content between horizons reflects contemporaneous iron and steel production and possibly early iron pig production. While this approach is not fully automated, the automated dimensional reduction and clustering is a turnkey operation for processing numerous large datasets. Applying the approach on specific phases (iterative phase classification (IPC)) drove the analysis that confirmed and revealed both the microstructures and trace Ba, Fe, and Mn from the model outliers and more statistically representative phases.

At the coastal deposit B, X-Ray Diffraction (XRD) and EDS analysis of the slag and thermogravimetric analysis (TGA) of the cream-coloured material covering the slag shows lithification on the top surface and seaward side above the mean high-water mark (MHWM) is the result of carbonate mineralisation. This is driven by water and leached calcium from weathering slag minerals (i.e., gehlenite, åkermanite, pseudowollastonite) reacting with ingassed and hydroxylated atmospheric CO2, forming calcite with slightly to strongly depleted δ13C values (-6.4 ‰ to -22.7 ‰) following partial dissolved inorganic carbonate (DIC) equilibrium. Calcium-silicate-hydrate (CSH), a component in cement, precipitated and was responsible for lithifying the deposit where more frequent and abundant seawater washing prevents subsequent slag mineral dissolution and carbonate precipitation.

The trace Ba, Fe, and Mn quantified at deposit A and the carbonate and CSH precipitation identified at deposit B are evidence that legacy iron and steel slag deposits can be sources of critical raw materials (CRM) and are prone to lithification in coastal settings. The iterative approach utilising machine learning provides a tool for locating hidden and nonmajority component regions, enhancing microstructural analysis. This lithification can draw down atmospheric CO2 and the CSH precipitation could help to slow the release of toxic metals, reducing the environmental contamination risk of repurposing legacy slag for ‘hard’ protection and coastal defence. Considering how the environment affects industrial waste deposit evolution can elucidate the most appropriate and safest slag repurposing option between CRM recovery, atmospheric CO2 storage, and coastal defence.