The recycling of interconnects from solid oxide electrolyzer (SOEL) stacks is essential for closing material loops in green hydrogen systems. Since it is mostly made of high-quality stainless steel, remelting is the most practical recovery route, but it inevitably generates slag, where strategic elements like chromium (Cr) are retained. This study investigates the mineralogical and grain characteristics of slag from SOEL interconnect remelting, with an emphasis on Cr distribution and its recovery potential. A correlative approach was applied using X-ray diffraction (XRD), scanning electron microscopy-based mineral liberation Analysis (MLA), and X-ray computed tomography (XCT). Cr was primarily found in magnesiochromite Mg(Al,Cr)₂O₄ (~54 wt.% Cr), constituting only ~5 wt.% of the slag, while lower concentrations were also detected in monticellite and åkermanite. XCT revealed the macroscopic heterogeneity of the slag system, with metallic inclusions and pores concentrated near the metal–slag interface, indicating density-driven settling. Cr-rich spinels were fine-grained (x_50,2 ≈ 55 µm), irregular in shape, and partially intergrown, presenting challenges for mechanical liberation and physical recovery. These features, combined with their compositional selectivity, suggest that Cr-rich spinels are promising candidates for future Engineered Artificial Mineral (EnAM) strategies aimed at enhancing selective recovery from slag.

Steelmaking slag represents one of the largest industrial by-products, with estimated annual generation exceeding 500 million tons worldwide. These slag materials contain valuable alloying elements such as chromium, nickel, and vanadium that remain in the slag. Chromium, in particular, has a dual role: it is essential for corrosion resistance of steels but creates environmental and resource recovery challenges when transferred into the slag. Conventional utilization of these slag types in cement and construction applications is restricted by stringent Cr(VI) leaching limits. This article reviews current understanding of chromium distribution, speciation, and stabilization mechanisms in slag derived from various steelmaking processes. The paper critically examines chromium recovery strategies, particularly the potential of engineered artificial minerals (EnAM) concept to immobilize or concentrate chromium in physically recoverable phases. Comparative analysis of chemical, mineralogical, and process parameters is presented to identify leverage points for improved slag recycling and resource efficiency. The review concludes with recommendations for improved chromium management and identifies research priorities for more sustainable steel production. Graphical Abstract

The engineered artificial mineral (EnAM) concept aims to concentrate target elements in tailored mineral phases with defined composition, grain size, and shape through controlled slag chemistry and crystallisation. This study evaluates the transfer of LiAlO₂ formation as a Li-bearing EnAM from a 5 kg laboratory-scale slag to a 125 kg pilot-scale slag. The pilot-scale slag was produced in a 300 L top-blown rotary converter and subjected to controlled cooling. A multiscale characterisation workflow combining micro-X-ray fluorescence (µXRF), X-ray computed tomography (XCT), SEM-EDX and electron backscatter diffraction (EBSD) was used to investigate EnAM distribution, morphology, intergrowth and grain structure. LiAlO₂ was formed at pilot scale, with a phase fraction of 7.5–10.5 wt. % compared with 11.2–14.4 wt. % at laboratory scale. µXRF revealed substantially larger EnAM clusters at pilot scale, whereas EBSD showed that these clusters consist of finer, more irregular LiAlO₂ grains. Nevertheless, both scales exhibited a comparable size-shape progression from smaller, compact clusters toward larger, less compact structures. XCT and SEM-EDX further revealed intergrowth of LiAlO₂ with Mn-Al spinel. The results demonstrate the feasibility of transferring EnAM formation to pilot scale while showing that scale-up modifies EnAM yield, clustering, intergrowth, and individual grain properties.