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Abstract
Direct leaching with NaOH can be an economically acceptable method for vanadium (V) and tungsten (W) recovery from spent selective catalytic reduction (SCR) catalysts. However, different chemical-physical characteristics of catalysts would affect the V and W leaching. In this paper, the V and W leaching behavior of various sources of SCR catalysts with a local difference (yellow and gray color) were systematically investigated with alkali leaching solution under ambient pressure. Different leaching efficiencies from yellow and gray color areas were correlated with oxidation states and species of V and W on catalyst surfaces, as characterized by X-ray photoelectron spectroscopy (XPS), Raman, Fourier transform infrared spectroscopy (FTIR), and other analytic methods. For the V leaching efficiency, the samples from a gray area of catalysts (40.0–51.0%) were lower than that from the yellow area (66.8–69.8%). The higher molar ratio of V3+ and a lower molar ratio of V5+, and the lower total V content on the surface of the samples from the gray area could be the main reasons for the lower V leaching efficiency. As for the W leaching efficiency, the samples from the gray area (44.6–57.3%) were slightly higher than that from the yellow area (38.0–52.6%) of catalysts. The less total W content of surface species and stronger interaction among V–W–Ti of yellow area samples resulted in the lower leaching efficiency. These differential leaching efficiencies needed to be taken into consideration for recovering V and W from spent SCR catalysts.
Highlights
The selective catalytic reduction (SCR) technology is the most efficient and industrially widespread technology for reducing NOx emission from stationary sources [1,2]
Because the commercial SCR catalyst is produced by different manufacturers, using various formulation and processes, and they are serviced under different operating conditions
V atoms lose electrons and form three different valences state (V5+, V4+, and V3+) in the SCR catalyst, and vanadium oxides exist in the forms of monomeric, polymeric, or crystalline [13,14,15]
Summary
The selective catalytic reduction (SCR) technology is the most efficient and industrially widespread technology for reducing NOx emission from stationary sources [1,2]. As a mature catalyst with a high NOx conversion rate, a wide operating temperature window, and excellent stability, a considerable amount of V2O5-WO3/TiO2 catalysts have been installed in the SCR system. Final disposal, such as landfill or recovery for other uses, is unavoidable after catalyst operating life [3,4]. This work focused on the different leaching behaviors of V and W between the yellow and gray color areas in various sources of SCR catalysts with local differences. The reasons why the yellow and the gray area samples from the same one catalyst show different leaching behaviors were discussed based on the characterizations
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