Role of NO in Hg^0 oxidation over a commercial selective catalytic reduction catalyst V_2O_5–WO_3/TiO_2

Experiments were conducted in a fixed-bed reactor that contained a commercial catalyst,V2O5-WO3/TiO2,to investigate mercury oxidation in the presence of NO and O2.Mercury oxidation was improved by NO,and the efficiency was increased by simultaneously adding NO and O2.With NO and O2 pretreatment at 350°C,the catalyst exhibited higher catalytic activity for Hg^0 oxidation,whereas NO pretreatment did not exert a noticeable effect.Decreasing the reaction temperature boosted the performance of the catalyst treated with NO and O2.Although NO promoted Hg^0 oxidation at the very beginning,excessive NO counteracted this effect.The results show that NO plays different roles in Hg^0oxidation; NO in the gaseous phase may directly react with the adsorbed Hg^0,but excessive NO hinders Hg^0 adsorption.The adsorbed NO was converted into active nitrogen species（e.g.,NO2） with oxygen,which facilitated the adsorption and oxidation of Hg^0.Hg^0 was oxidized by NO mainly by the Eley-Rideal mechanism.The Hg^0 temperature-programmed desorption experiment showed that weakly adsorbed mercury species were converted to strongly bound ones in the presence of NO and O2.

In-situ investigation of melting characteristics of waste selective catalytic reduction catalysts during harmless melting treatment

Selective catalytic reduction(SCR) catalyst waste is a hazardous solid waste that seriously threatens the environment and public health.In this study,a thermal melting technology is proposed for the treatment of waste SCR catalysts.The melting characteristics and mineral phase transformation of waste SCR catalysts blended with three different groups of additives were explored by heating stage microscopy,thermogravimetric analysis/differential scanning calorimetry(TG/DSC) analysis,thermodynamic simulation,and X-ray diffraction(XRD) analysis;heavy metal leaching toxicity was tested by inductively coupled plasma-atomic emission spectrometry(I CP-AES) analysis.The results indicated that the melting point of waste SCR catalysts can be effectively reduced with proper additives.The additive formula of 39.00% Fe2 O3(in weight),6.50% CaO,3.30% SiO2,and 1.20% Al2 O3 achieves the optimal fluxing behavior,significantly decreasing the initial melting temperature from 1223℃ to1169℃.Furthermore,the whole heating process of waste SCR catalysts can be divided into three stages:the solid reaction stage,the sintering stage,and the primary melting stage.The leaching concentrations of V,As,Pb,and Se are significantly reduced,from 10.64,1.054,0.195,and 0.347 mg/L to 0.178,0.025,0.048,and 0.003 mg/L,respectively,much lower than the standard limits after melting treatment,showing the strong immobilization capacity of optimal additives for heavy metals in waste SCR catalysts.The results demonstrate the feasibility of harmless melting treatments for waste SCR catalysts with relatively low energy consumption,providing theoretical support for a novel method of disposing of hazardous waste SCR catalysts.

Comprehensive recovery of W,V,and Ti from spent selective reduction catalysts

In this study,spent WO_(3)/V_(2)O_(5)-TiO_(2) catalysts used for selective catalytic reduction were treated by a hydrometallurgical process to comprehensively recover valuable metallic elements,such as W,V,and Ti.Al and Si impurities were preferentially removed by selective micro wave-assisted alkali leaching.W and V were leached by enhanced high-pressure leaching with efficiencies estimated at 95% and 81%.The leaching of W and V followed the nuclear shrinkage model controlled by the combination of product layer diffusion and interfacial chemical reaction.A synergistic extraction was applied to separate W and V using an extractant mixture of di-(2-ethylhexyl)phosphoric acid P204 and the primary amine N1923.The extraction efficiencies of V and W reached 86.5% and 6.3%,respectively,with a separation coefficient(V/W) of 95.30.The product was precipitated after extraction to yield ammonium paratung state(APT) and NH_(4)VO_(3).The TiO_(2)catalyst carrier residue meets commercial specifications for reuse.This comprehensive recovery process with the characteristics of high-pressure leaching and synergistic extraction realizes the resourceful utilization of the spent catalysts.

A promising method to recover spent V_(2)O_(5)-WO_(3)/TiO_(2) catalyst: treatment by vanadium-titanium magnetite sintering process

A large number of spent selective catalytic reduction(SCR)denitration catalysts are produced after the ultra-low emission transformation of coal-fired power plants in China.According to the China’s“Directory of National Hazardous Wastes(Version 2021)”,these spent vanadium-tungsten-titanium catalysts are classified as“HW50”hazardous waste,and their disposal and utilization processes have been strictly controlled.Thus,an effective and low-cost technique was developed to treat and utilize these spent SCR catalysts by the vanadium-titanium magnetite sintering process.Effects of adding spent SCR catalysts on the sintering production process and product quality indexes of sinter were studied.The results showed that adding spent SCR catalysts can improve the sintering granulation and green feed permeability,thereby increasing the productivity and flame front speed.When the addition proportion of spent SCR catalysts is less than 1 wt.%,the performance indexes of the finished sinter are basically equal to those of the finished sinter without adding spent SCR catalysts.Further increasing the proportion of spent SCR catalysts to 2.0 wt.%results in a decrease in product quality indexes,which could be attributed to the increase in perovskite content in the finished sinter.