Alkaline leaching of spent SCR catalysts: Mechanochemical activation vs mechanochemical leaching

Recycling of vanadium (V) and tungsten (W) from spent WO 3 -V 2 O 5 /TiO 2 selective catalytic reduction (SCR) catalysts is critical for resource circularity and environmental protection. This study investigates the application of mechanochemical processing (MCP) to enhance the alkaline leaching, comparing (i) mechanochemical activation (ACT; milling before leaching) and (ii) mechanochemical leaching (MCL; simultaneous milling and leaching) against a non-activated control (NAC). In NAC, temperature dominates extraction for both V and W, with NaOH concentration and liquid-to-solid ratio as secondary factors. MCP improves extraction under most conditions, notably, MCL delivered the highest performance: at high milling intensity and small media, V recovery reached ~99 % and W ~ 66 % at ≤75 °C. Kinetic analysis showed that classical shrinking-core models fail to capture the behavior, whereas a stretched-exponential (Avrami/Weibull) law describes the rapid initial dissolution and decelerating tails, consistent with heterogeneous, overlapping pathways. Mechanochemically induced microstructural changes lowered apparent activation energies (V: ~34 to ~11 kJ/mol; W: ~67 to ~62 kJ/mol) and reduced kinetic heterogeneity, particularly in MCL. Overall, MCP enables low-temperature, energy-efficient recovery of critical metals from spent SCR catalysts and reconfigures the controlling leaching mechanisms.