Recycling of spent SCR catalysts by near-complete tungsten and vanadium extraction via mechanochemical alkali conversion and water leaching

Abstract Spent WO 3 –V 2 O 5 /TiO 2 selective catalytic reduction (SCR) catalysts are an important secondary source of tungsten (W) and vanadium (V), but under moderate direct alkaline-leaching conditions, V is often extracted more readily than W because supported W-bearing species may remain incompletely converted. In this work, mechanochemical alkali conversion (MCC) followed by water leaching was developed to improve extraction of both metals, particularly W. Spent SCR catalysts were milled with NaOH in a high-energy ball mill and subsequently leached with water at room temperature. A 3 3 factorial design with ANOVA revealed that NaOH/SCR mass ratio and milling intensity were highly significant for both metals ( P < 0.001), whereas milling media size had only a minor effect within the explored range. Linear regression models described the data well (R 2 ≈ 0.81 for V and 0.95 for W). Extraction ranged from 86.9 to 100% for V and 39.4–99.6% for W, confirming the stronger sensitivity of W to process severity. Under optimized conditions (NaOH/SCR = 1.6, 1000 rpm, 2 mm media), near-quantitative extraction of both metals (≈ 99–100%) was achieved by water leaching at room temperature. The aqueous step reached a plateau within 5 min, and reducing the liquid-to-solid ratio from 100 to 10 mL/g did not decrease extraction, indicating that overall performance was governed mainly by the prior solid-state conversion. Additional analysis of milling time and cumulative energy input showed that V conversion responded rapidly, whereas W conversion remained more dependent on milling severity and impact intensity.