Experimental Study On Leaching Toxicity Of Heavy Metals In Spent SCR Catalyst And Its Cement Solidification/stabilization Product

Currently,selective catalytic reduction(SCR)catalyst is widely applied in coal-fired power plant for the purpose of NOx removal in China,which will definitely result in large quantity of spent SCR catalyst in the coming future.Spent V2O5-WO3/TiO2 catalyst has already been classified as hazardous waste due to its leaching toxicity of heavy metals.In prior to landfill,detoxification of heavy metals contained in spent V2O5-WO3/TiO2 catalyst must be proceeded.Heavy metals(V、Cr、Ni、Cu、Zn)content and their leaching toxicity were firstly analyzed in the dissertation.Then,the influence of liquid-solid ratio(L/S),leaching time and leaching solvents’ initial p H value on leaching behavior of heavy metals was investigated.Exprimental results show that: V,as one of the major component of V2O5-WO3/TiO2 catalyst,exhibits the highest content and leaching concentration among five havy metals,which are 7584.32mg/kg and 13.7984mg/L respectively.Leaching concentration of each heavy metal goes up initially and later flattens as leaching time increases.Increasing liquid-solid ratio leads to lower leaching concentraion.Leaching content,however,rises adversely as liquid-solid ratio increases.Strong acidic condition can enhance leaching of heavy metals.Cr、Ni、Cu and Zn demostrate extreme high leaching concentration under strong acid condition and relative low leaching concentration under strong alkaline condition.However,strong acidic and alkaline environments are both beneficial to the leaching of V thanks to the amphotericity of V2O5 in spent SCR catalyst.Cement solidification/stabilization(S/S)process was utilized to perform harmless treatment of spent V2O5-WO3/TiO2 catalyst and the influence of cement mixing ratio,curing time and stabilizing agent was studied meanwhile.Exprimental results show that: leaching concentration of each heavy metal declines with the increase of cement mixing ratio and 30% cement is determied as the optimal ratio considering the balance between economy and solidification effect.Longer curing time causes lower leaching concentration of heavy metals in cement solidifying matrix.Adding Na2 S and DDTCNa during cement solidification process will of course further reduce leaching concentration of each heavy metal.The chemical composition,phase constitution,specific surface area and microstructure of cement solidification product were characterized by X-ray fluorescence(XRF),X-Ray Diffraction(XRD),BET and scanning electron microscope(SEM)techniques.Stabilization mechanism of cement was discussed based on those characterisation results.Moreover,the influence of cement mixing ratio,curing time and stabilizing agent on the Stabilization mechanism of cement regarding to heavy metals was studied.Results show that: calcium silicate hydrate(C-S-H)gel,the major product of cement hydration,can stabilize or confine heavy matal in cement solidifying matrix through adsorption,physical encapsulation and ion exchange,which plays the most significant role in solidification of heavy metals.Increasing curing time and cement mixting ratio could reinforce solidification effect since they increase C-S-H gel concent in essence.Adding Na2 S will generate metal sulfides.Addictive of DDTCNa,however,can form chelation with heavy metal ions to stabilize heavy metals.