Aqua Regia in Metal Processing: A Critical Review of Reaction Mechanisms, Selectivity, and Industrial Applicability

Aqua regia remains one of the most chemically powerful liquid-phase systems for dissolving gold and platinum-group metals, yet its industrial value is frequently overstated when dissolution yield is considered separately from selectivity, off-gas management, corrosion, and downstream purification. This critical review integrates fundamental chemistry, thermodynamics, kinetics, feedstock-specific performance, separations, safety, environmental burden, and process integration for metal processing from 1980 through September 2026. The central argument is that aqua regia should be treated not as a universal leachant but as a high-intensity reaction module whose usefulness depends on how effectively oxidation couples to chloride complexation and how quickly the resulting mixed-metal liquor can be converted into selective product streams. Evidence from electronic waste, high-grade gold scrap, spent catalysts, and complex primary materials shows that high precious-metal dissolution can coexist with excessive base-metal dissolution, reagent consumption, and difficult liquor purification. Temperature and redox potential accelerate dissolution, but they also intensify gas evolution and co-dissolution, creating an operating-window problem rather than a simple maximization problem. Downstream precipitation, solvent extraction, ion exchange, sorption, cementation, and electrowinning can recover value, although each transfers selectivity requirements to a later unit operation. Closed gas handling, acid and oxidant recovery, corrosion-resistant materials, staged pre-leaching, and hybrid flowsheets therefore determine whether aqua regia is technically and environmentally defensible. The review proposes a decision framework that limits aqua regia to high-value, low-mass fractions or final refining duties unless integrated evidence demonstrates favorable whole-flowsheet performance.