Selective two-stage redox-controlled acidic leaching and alkaline hydrothermal leaching of vanadium and tungsten from spent selective catalytic reduction catalysts with experimental and thermodynamic evaluation

Efficient recovery of critical metals from spent selective catalytic reduction (SCR) catalysts is fundamentally constrained by the contrasting dissolution behaviors of vanadium (V), tungsten (W), and titanium (Ti), which hinder selective separation in conventional hydrometallurgical systems. Here, a selectivity-driven two-stage strategy is developed by deliberately integrating a redox-controlled acidic leaching stage with an alkaline hydrothermal leaching (HTL) stage. In the first step, reductive H2SO4 leaching in the presence of Na2SO3 selectively promotes the dissolution of V, achieving 86% extraction, while W and Ti remain largely in the solid residue. The resulting W- and Ti-enriched residue is subsequently treated under alkaline hydrothermal conditions, where elevated temperature and hydroxide activity intensify W dissolution, enabling >99% recovery with minimal Ti solubilization (<5 mg/L). This coupling of reductive chemistry and hydrothermal processing provides a process-intensified pathway for the stepwise separation of V and W from a Ti-rich matrix. Thermodynamic analysis further corroborates the feasibility of the proposed reactions and elucidates the governing role of temperature and alkalinity in controlling phase stability and dissolution behavior. Overall, the proposed approach establishes a robust and highly selective route for the valorization of spent SCR catalysts, enabling efficient recovery of critical metals while preserving a Ti-enriched residue with potential for downstream utilization.