Study On Migration Characteristics Of Vanadium,Arsenic Heavy Metals From Spent SCR Catalyst And Its Harmless Treatment

Among the flue gas treatments for NOx emission,selective catalytic reduction（SCR）of NOx with NH₃ is the most effective post-combustion control technology and has been widely used in coal-fired power plants.As the core of SCR system,the SCR catalyst will not be able to meet the SCR performance requirements,and eventually face disposal after a long period of operation.The Vanadium and other heavy metals contained in spent catalysts can have strong toxity.If these spent catalyst are not properly disposed,it will impose a heavy burden in the environment,seriously affecting the environment and human life.The safe landfill and recycling are the main method to cope with spent SCR catalyst,which have the problems such as land occupation,high disposal cost,and long-term secondary pollution.Therefore,it is meaningful to conduct further researches for the harmless treatment and resource utilization of spent SCR catalysts.In this paper,the typical honeycomb high-arsenic spent SCR catalyst sample was selected and tested for the toxicity.The results show that the leaching concentration of As in the spent catalyst reached 30 times of the limit of the national hazardous waste identification standard,and the leaching concentration of V was also as high as 17.6mg/L.Through the continuous extraction procedure,it was found that the movable state ratio of V in spent catalyst reached to 49%,which had a certain release potential,and As was mainly in the form of a residual state.The mobility of V is greater than As.The effects of pH,leaching time,liquid-solid ratio（L/S）and sample size on the leaching characteristics of V and As in the spent catalyst were also investigated.The results show that pH has an important effect on the leaching of heavy metals.Low pH conditions contributes to the migration and release of V in the spent catalyst.High pH conditions promotes the release of As.The leaching amount of V and As in the spent catalyst in the early stage reached the maximum quickly,and it remained basically unchanged in the later stage.With the increase of liquid-solid ratio（L/S）,the leaching concentration of V and As increased first and then decreased.When the liquid-solid ratio is between 20 and 30,the leaching concentration of V and As is high.As the particle size decreased,the leaching concentration of V and As increased.The finer the particle size of the sample,the larger the contact area with the leachant,which was favorable for leaching.However,after the particle size less than 100 mesh,the further reduction is limited to increase the extent of leaching.The combustion characteristics of spent SCR catalyst with two types of coal were investigated on a high temperature tubular furnace.The experimental results show that,the ignition characteristics and burnout characteristics of pulverized coal can be improved when the blending ratio of spent catalyst is controlled within 5%.The main component of the spent catalyst was TiO₂,which catalyzed the coal to burn during the co-firing process.However,when the blending ratio reached 10%,the pulverized coal ignition characteristics and burnout characteristics were adversely affected,which was related to the incorporation of a large amount of spent catalyst to cover a part of the carbon surface mesopores in the coal,hindering its diffusion combustion.At the same time,the migration of V and As in bottom ash and flue gas during the blending process were also investigated.In the range of 1100℃¹500℃,with the increase of the blending temperature,the ratio of V in the bottom ash showed a downward trend,and the ratio of As gradually increased.With the increase of the spent catalyst blending ratio,the ratio of V and As in the bottom ash showed a downward trend at the same blending temperature.In order to analyze the safety of the blended product,the leaching toxicity tests of the bottom ash were carried out.The results show that when the blending ratio of spent catalyst was less than 5%,the leaching concentration of V and As in the bottom ash was lower than the limit of national standard.The leaching concentrations of V and As were significantly reduced when the blending temperature exceeds 1300℃.The higher the blending temperature,the more the bottom ash agglomerates.As the blending ratio increased,especially reached 10%,the content of silicon,aluminum and calcium oxides in the samples relatively decreased,resulting in the weakening of melting solidification.Therefore,controlling a suitable blending temperature and blending ratio contributes to the harmless disposal of the spent catalyst.The experimental study on the preparation of concrete by adding some spent catalysts in cement to achieve harmless treatment was also carried out.The effects of spent catalyst on the performance of concrete and the safety of concrete samples were investigated.The results show that the incorporation of the spent catalyst can affect the slump and impermeability of concrete.When the amount of spent catalyst reaches to 10%,the work performance of concrete deteriorates drastically.At this time,a certain proportion of fly ash can be mixed to improve the overall working performance and ensure the mixing ratio of concrete.The safety of concrete was further evaluated by the toxicity leaching experiment.And the leaching concentration of V and As in the concrete samples all meet the requirements of the standard.The results show that the solidification rate of heavy metal in each group of concrete samples was higher than 96.5%.The hydration occurred during the preparation of concrete has a good inhibitory effect on leaching of V and As on the spent catalyst.The main reason is that the hydration of cement increases the number of C-S-H gels.The incorporation of spent catalyst makes the microstructure of the concrete more compact,exerts a filling effect,and inhibits the leaching of heavy metals.The method of preparing concrete by using a certain proportion of spent catalyst in cement has a good application prospect for realizing harmless disposal and utilization of waste catalyst.