Synergistic H3PO4 complexation and H3PO3 reduction strategy for efficient leaching and separation of tungsten and vanadium from spent SCR catalysts

The significant accumulation of spent selective catalytic reduction catalysts has sparked environmental concerns and presented recycling opportunities for valuable metals. This study introduces a novel H3PO4-H3PO3 pressure leaching-solvent extraction process for the selective recovery of tungsten (W) and vanadium (V) from spent catalysts. Under optimized conditions (8 mol·L-1 H3PO4 + 2 mol·L-1 H3PO3, 160°C), 98.0% of W and 90.3% of V were dissolved in the leachate, whereas 99.5% of Ti remained in the residue. The leaching mechanisms were elucidated as follows: (i) mixed H3PO4-H3PO3 acids break down bulk TiO2 into Ti(HPO4)2·H2O nanoplates, exposing embedded W and V; (ii) H3PO4 selectively complexes with W to form soluble phosphotungstic acid, dissolving W while creating speciation differences with V; (iii) H3PO3 reduces V(V) to V(III), enhancing its dissolution and preventing precipitation as VOPO4. Subsequent stepwise solvent extraction using 2-octanol (at pH -0.5) and primary amine (at pH 1.0) achieved extraction efficiencies of 92.7% for W and 95.6% for V, with a separation coefficient of 197.9 (W/V). The novel process features low-temperature operations and consistent phosphorus-bearing acidic conditions throughout the leaching-extraction stages, offering advantages such as higher recoveries of valuable metals, lower energy consumption, and reduced neutralization consumption of acid-base reagents.