Optimization of Roasting Parameters for Recovery of Vanadium and Tungsten from Spent SCR Catalyst with Composite Roasting

Abstract

Every year, large amounts of selective catalytic reduction (SCR) catalysts with losing catalytic activity and failing to be regenerated need to be regenerated, which will result in acute pollution. Recycling valuable metals from spent SCR catalysts can not only solve environmental problems, but also save resources. The process of sodium roasting and water leaching is able to effectively extract vanadium (V) and tungsten (W) from spent SCR catalysts. To improve the efficiencies of V and W, different sodium additives were first investigated in the roasting process. The results revealed that the process of NaCl-NaOH composite roasting and water leaching showed superior leaching efficiencies of V and W, which can reach 91.39% and 98.26%, respectively, and simultaneously, it can be found that adding low melting point NaOH promoted mass transfer as compared with the melting points of different sodium additives. Next, a single-factor experiment was conducted to investigate different roasting conditions, such as roasting temperature, roasting time, mass ratio of sodium additive and catalyst, and mass ratio of NaCl and NaOH, on the leaching efficiencies of V and W. Then, a three-level and four-factor orthogonal experiment and a weight matrix analysis were used to optimize the roasting parameters. The results showed that roasting temperature had the most significant effect on the leaching efficiencies of V and W, and the optimal roasting conditions were as follows: the roasting temperature was 750 °C, the roasting time was 2.5 h, the mass ratio of sodium additive and catalyst was 2.5, and the mass ratio of NaCl and NaOH was 1.5. Under the optimal roasting conditions, the leaching efficiencies of V and W were 93.25% and 99.17%, respectively. The results of XRD analysis inferred that VO2 coming from the decomposition of VOSO4 in spent SCR catalysts may first oxidize into V2O5 and then react with sodium additives to produce NaVO3. The formation of titanium-vanadium oxide ((Ti0.5V0.5)2O3) was a part reason of hindering the leaching of vanadium. With the increase of roasting temperature, TiO2 converted into Na2Ti3O7, which indicated that the main structure of the catalyst was destroyed, and simultaneously, more characteristic peaks of sodium metavanadate and sodium tungstate appeared, thus enhancing the leaching of V and W. Finally, it can be seen that the process of NaCl-NaOH roasting and water leaching remained higher leaching efficiencies of V and W and lower roasting temperature by comparing with leaching efficiencies of V and W in different processes of recycling SCR catalyst. The process of NaCl-NaOH composite roasting and water leaching provided a strategy with a highly efficient and clean route to leach V and W from spent SCR catalyst. The orthogonal experiment and weight matrix analysis in our study can be used as a reference to optimize the reaction conditions of a multiple indexes experiment.