Vanadium Distribution and Competing Slag Associations in Reductive Smelting of Spent NiMoV Catalysts: Oxygen Potential, Aluminosilicate Interactions, and Metal Recovery Constraints—A Critical Review

Spent Ni–Mo–V hydroprocessing catalysts are alumina-rich secondary resources whose reduction smelting produces valuable metal alloys and complex oxide slags. This critical narrative review addresses where vanadium resides when it is not recovered in the bulk alloy. It contrasts direct experimental evidence with thermodynamic and mechanistic analogs, explicitly separating vanadium dissolved in liquid slag, retained in crystalline oxides, entrained as metallic droplets, and transferred to dust or deposits. Under different operating configurations, reported V recoveries range from 25.1% in reduction–sulfurization smelting to 89.7% in reduction smelting of mixed catalyst/ore feeds; an independent optimized kilogram-scale catalyst-smelting study reported 97.9%. These process recoveries are not equilibrium partition coefficients and cannot be transferred across feed and slag systems. We formalize phase-specific vanadium inventories, recovery, and mass-balance closure, and assess the coupled roles of oxygen potential, carbon and sulfur activities, aluminosilicate chemistry, crystallization, liquidus margin, and droplet settling. The review presents a diagnosis-first framework for distinguishing chemical retention from physical separation losses and identifies the experimental data needed for process optimization and scale-up.