Recovery of tungsten from spent selective catalytic reduction catalysts by pressure leaching

The recovery and purification of vanadium (V) and tungsten (W) from honeycomb-type spent selective catalytic reduction (SCR) catalyst was investigated using an autoclave through a pressure leaching process. Spent SCR catalyst mainly consists of TiO2 and other oxides (7.73% WO3, 1.23% V2O5, etc.). The reaction temperature, NaOH concentration, time, additive concentration, and liquid–solid (L/S) ratio were varied during the leaching process. The optimal reaction conditions were identified for recovery of V and W. The addition of NaOH to Na2CO3 improved the amount of V and W recovered because of the enhancing effect of NaOH in Na2CO3. As the concentration of CaCl2 was increased during the precipitation process in order to separate the recovered V and W, the precipitation percentages of V and W increased, respectively. However, the use of Ca(OH)2 as the additive reduced the precipitation percentage of W. Therefore, despite full precipitation of V (98.6%), only 7.73% of W was precipitated when 3 equivalents of Ca(OH)2 was reacted with spent SCR catalyst for 30 min. The remaining W in the leaching solution was reacted with NH4OH to form ammonium tungstate, which was converted to ammonium paratungstate through evaporation. Consequently, V and W could be recovered and separated successfully through the process in this study.