From Spent NiMo Catalysts to Marketable Metal Products: A Critical Review of Recovery Selectivity, Process Integration, Mass Balances, and Industrial Feasibility

Spent nickel–molybdenum (NiMo) hydroprocessing catalysts combine recoverable strategic metals with complex alumina supports, deposited carbon, sulfur, residual hydrocarbons, and feed-dependent contaminants. This critical review evaluates pathways from spent catalyst handling to saleable molybdenum and nickel compounds, emphasizing process-wide selectivity rather than isolated leaching yields. The literature was screened by feed identity, phase transformation, separation performance, completeness of material balances, product specification, and evidence scale. Thermal conditioning, alkaline conversion followed by molybdate leaching, direct acidic and oxidative leaching, biohydrometallurgy, selective solvent extraction, precipitation, and integrated flowsheets are compared. A normalized accounting framework distinguishes extraction, purification yield, overall metal recovery, chemical consumption, effluent load, and support disposition. Alkaline-first processing offers a compelling basis for separating molybdenum from nickel when feed phases and reagent demand permit, whereas direct hydrometallurgy can reduce thermal complexity at the cost of solution purification. Reported laboratory recoveries cannot by themselves establish industrial feasibility or product marketability. The review proposes explicit acceptance gates covering compositional variability, phase-resolved deportment, sulfur and aluminum balances, recycle chemistry, saleable product quality, and pilot-scale operability. These gates provide a technically auditable basis for route selection and subsequent engineering validation.