Hydrometallurgical processing of spent selective catalytic reduction (SCR) catalyst for recovery of tungsten

This paper deals with the recovery of tungsten as synthetic CaWO4 from spent V2O5-WO3/TiO2 (selective catalytic reduction, SCR) catalyst. The newly developed process included the systematic experimental steps (roasting, decomposition (using HCl solution), leaching (using NaOH), and precipitation) to recover tungsten as CaWO4. The results revealed that the effect of roasting of spent SCR catalyst was significant for the extraction of tungsten. Furthermore, CaO addition in the roasting process promoted chemical equilibrium for the formation of CaWO4, and extraction efficiency of tungsten was increased simultaneously. Decomposition process revealed that 4 mol.L-1 of HCl solution was the best condition with considerable degree of removal (92.5%) of vanadium along with negligible loss of tungsten. The most appropriate concentration of NaOH (aq) to dissolve tungstic acid (H2WO4) from the decomposed sample was 1 mol L-1 (> 99% tungsten yield) in accordance with minimized concentration of foreign metals (Si, Al, Fe). The maximum amount of tungsten ( > 99%) was obtained from the sodium tungstate solution as synthetic CaWO4 using CaCl2 (molar ratio (CaCl2/WO3): 1). The best experimental conditions for the precipitation were under following conditions: temperature, 50 degrees C; equilibrium pH, 8-10; molar ratio (CaCl2/WO3), "1". The content of CaWO4 in the final product was 96.1 wt%. The overall yield of tungsten from the feedstock using the proposed process was 96.4%.