Iridium Extraction and Recovery from Primary Ores and Secondary Materials: Process Chemistry, Metallurgical Routes, Selectivity, and Industrial Scale-Up—A Critical Review

Iridium is recovered mainly through platinum-group-metal production and specialized secondary-processing circuits, yet its chemical resistance, heterogeneous occurrence, and difficult separation from associated noble metals complicate process selection. This critical review compares primary concentrates, refinery intermediates, industrial catalyst scrap, and end-of-life proton-exchange-membrane electrolyzer components across mineralogical characterization, thermal activation, dissolution chemistry, selective refining, and industrial integration. A structured bibliography covering 1960–2026 contrasts process mechanisms and evidence quality, without interpreting isolated laboratory dissolution as industrial recovery. The synthesis identifies mineral deportment, feed-specific oxidation requirements, chloride-complex speciation, separation from rhodium and ruthenium, and complete-circuit material accounting as decisive variables. Thermochemical and electrochemical strategies offer different trade-offs in selectivity, corrosion, safety, energy intensity, and reagent recirculation. Product specifications, secondary-residue handling, and functional recovery yields must be considered jointly, not inferred from leachate concentration alone. The review proposes a feedstock-first decision framework and reporting criteria to compare routes and identify where independent scale-up evidence remains insufficient.