Experimental Study On Leaching Toxicity Of Spent SCR Catalyst And Its Harmless Treatment

At present,selective catalytic reduction?SCR?technology is widely applied to remove nitrogen oxides from coal-fired power plants.However,the life of SCR catalysts is generally 3⁵ years,which will lead to a large number of spent SCR catalysts to be produced in the 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.Firstly,the total contents and distributions of heavy metals such as V,Cr,Ni,Cu,Zn and As in spent SCR catalyst were studied by microwave digestion and sequential extraction experiments.The results showed that the contents of V and Zn in the spent SCR catalyst were higher,which were 7434.70 mg/kg and 906.40 mg/kg,respectively.The order of the mobility of heavy metals in the spent SCR catalyst is:Ni>Zn>V>Cr>As>Cu.Next,the HJ/T300-2007 acetic acid buffer solution was used to test the toxic leaching of the spent SCR catalyst.The leaching characteristics of heavy metals in spent SCR catalyst,such as pH value,leaching time and liquid/solid ratio,were further investigated through the leaching characteristics test.The results show that the leaching concentration of Cr,Ni,Cu,Zn and As in the spent SCR catalyst is lower than that of the GB5085.3-2007hazardous waste certification standard.The pH of the leachate has an important influence on the leaching concentration of heavy metals in the spent SCR catalyst.Low pH conditions contribute to the migration and release of heavy metals from the spent SCR catalyst.High pH conditions help to improve the stability of Cr,Ni,Cu and Zn,but it will promote the release of Vand As.The release of heavy metals in the spent SCR catalyst is a rapid process,but the leaching concentration of V will decrease with the increase of the leaching time.With the increase of liquid-solid ratio,the leaching concentration of heavy metals in SCR catalysts showed a decreasing trend.Increasing the liquid-solid ratio only dilutes the leaching concentration of heavy metals in the spent SCR catalyst.Then,the influence of different temperature and mixing ratio on the migration and release of heavy metals was studied by the spent SCR catalyst and coal co-firing experiment.Meanwhile,the toxicity identification experiment of the burned slag was carried out.The result shows that Cu and Zn are volatile heavy metals,which are mainly distributed in the gas phase.Cr and Ni are semi-volatile heavy metals,and about 55%of them are distributed in the bottom ash.V and As are non-volatile heavy metals,which are mainly retained in the bottom ash.With the increase of mixing ratio of spent catalyst,the distribution of V,Cr,Cu and Zn in the bottom ash tends to increase,and the distribution of As in the bottom ash is reduced.At last,the XRD phase,SEM micromorphology and leaching toxicity were analyzed.The results showed that when the co-firing temperature was higher than 1300?,the bottom ash began to melt,and the leaching concentration of heavy metals was much lower than that of the national hazardous waste.It achieved the purpose of harmless treatment of waste SCR catalyst.