Mineralogical characteristics of fossil-free steel slags

Abstract

Steel production based on hydrogen direct reduction represents one emerging solution to reach the decarbonization targets of the steel industry. This technological transition will affect the quality and quantity of generated by-products. This study presents the first in-depth results of the mineralogical characteristics of slags originating from Hydrogen Direct Reduced Iron as a feed for pilot scale Electric Arc Furnace steel production (H₂-DRI-EAF), complemented by comparison to contemporary scrap-based EAF slags. The results highlight that each slag type has its own characteristics depending on temperature, bulk composition, oxygen partial pressure and cooling. Higher CaO content increases calcium silicate phases and decreases gehlenite content in scrap-based slags. Higher basicity in combination with high Cr₂O₃ content leads to the formation of ellinaite and chromium-containing spinel phases, and more oxidizing process conditions also result in the extensive formation of brownmillerite in scrap-based slags. On the other hand, due to the intrinsic properties of the iron ore, higher TiO₂ and V_tot contents in H₂-DRI-EAF slags lead to the formation of perovskite and calcium vanadate. The documented characteristics are important in evaluating future utilization possibilities, particularly emphasizing usage as Supplementary Cementitious Material (SCM) and valuable metal recovery but also management of potential environmental impacts. This study underscores the importance of detailed mineralogical analysis and multimethodological characterization work to overcome limitations and reduce misinterpretations based on one characterization method alone. It also forms the basis to evaluate and consider alternative slag formation and processing strategies to maximize the utilization of the steel industry’s future by-products.