Environmentally friendly approach to recover vanadium and tungsten from spent SCR catalyst leach liquors using...

This research paper deals with an environmentally friendly approach for the treatment of spent selective catalytic reduction (SCR) catalyst. To recover vanadium (V) and tungsten (W) from spent SCR catalyst, leach liquors from hydrometallurgical processing were utilized to develop a proper methodology for extraction and possible separation of vanadium and tungsten from each other. This study investigated the solvent extraction (also called liquid-liquid extraction) of vanadium and tungsten utilizing the alkaline roasted leached solution containing approximately ∼7 g L^-1 of tungsten and ∼0.7 g L^-1 of vanadium. The commercial extractant, N-methyl-N,N,N-tri-octyl-ammonium chloride [R₃NCH₃]⁺Cl^- (commercial name Aliquat 336), was dissolved in Exxsol™ D80 (diluent) system and adopted in this research. Solvent extraction studies were performed to determine the following experimental parameters: equilibrium pH, extractant concentration, diluent influence, chloride ion concentration, temperature, and stripping reagent concentration, which were systematically scanned to ascertain the optimum conditions for quantitative extraction of both title metals. An anion exchange mechanism was proposed using the quaternary ammonium chloride solvent reagent after slope analysis. Excess supplement of chloride proved to have adverse effects, further supporting the extraction mechanism. Thermodynamics results show positive values for enthalpy (ΔH) for vanadium and tungsten, favoring the endothermic nature of the extraction reaction towards the uptake of either metal. McCabe-Thiele plots for extraction were constructed, suggesting 2 and 3 stages for vanadium and tungsten extraction, respectively, at the aqueous (A) to organic (O) phase ratio of 7 : 1, ensuring more than 99.9% and 7-fold enrichment of both title metals. The stripping trend follows the order: (NaOH + NaCl) > (NaOH + NaNO₃) > NaOH > NaNO₃ > NaCl. Stripping isotherm followed by stripping counter-current (CCS) study was carried out for quantitative stripping of the metals.