From Upgraded Titanium Slag to Modern Metallurgical Slag Engineering: Technologies, Industrial Applications, and Readiness- A Critical Review
Metallurgical slags are increasingly treated as dynamic secondary resources rather than inert residues, yet the literature remains fragmented across metal recovery, mineral-phase engineering, material valorization, carbon management, and industrial implementation. This structured critical narrative review evaluates a recent core corpus of 111 publications from 2020–2026 and supplements it with selected pre-2020 foundational sources required to establish the historical Upgraded Slag (UGS) lineage in titanium metallurgy. The recent corpus was descriptively coded by primary slag family, evidence function, and implementation maturity; steelmaking/ferrous systems account for 60 studies, copper/fayalitic slags for 16, cross-cutting or mixed systems for 15, ferroalloy/Cr–Mn slags for 7, Ti-bearing slags for 4, and Ni/ferronickel slags for 3. Eighty publications primarily provide mechanistic or product-performance evidence, 13 provide system-assessment evidence, and 12 are reviews or contextual sources. The synthesis distinguishes historical titanium-slag upgrading from the broader family of modern thermal, chemical, redox, physical, hydrometallurgical, carbonation, and electrochemical interventions. It shows that credible upgrading requires more than high recovery: a controllable phase transformation must be coupled with effective separation, qualified metal or material products, a safe and useful residual matrix, and realistic integration with plant infrastructure. Representative industrial evidence is strongest in copper-slag flotation and settling, selected hot-stage steel-slag operations, and heat-recovery applications, whereas many high-value functional-material routes remain laboratory-led. A five-gate framework—value inventory, engineerability, separation and safety, integration, and system performance—is applied to representative routes to connect mechanisms with product specifications, TEA/LCA, and industrial readiness. The resulting perspective positions slag upgrading as site-specific product and process design within circular metallurgy rather than as generic residue reuse.