Study On The Migration Behavior Of Heavy Metals During The Synthesis Process Of Coal Fly Ash-based Cu-SSZ-13

A large amount of fly ash(CFA)waste was produced from China’s coal industry produces a large amount of fly ash(CFA)waste every year,and the abandoned CFA can cause serious environmental pollution and resource waste.The pollution control and utilization of fly ash is always an issues for national attention.The synthesis of molecular sieves using CFA as raw material can not only realize the high value-added utilization of CFA,but also develop the lowcost preparation technology of molecular sieve.CFA contains Hg,As,Se,Pb,Cd and other heavy metal elements and these harmful heavy metal elements may affect the performance and application of molecular sieves and other products.If heavy metal elements in fly ash products are released during the application process,it will cause potential safety hazards and environmental hazards.Therefore,it is very necessary to clarify the migration law and occurrence form of heavy metal elements in the process of CFA utilization,especially in the preparation of molecular sieves,for the development of high value-added chemicals such as CFA based molecular sieves.In this paper,the contents of Hg,As,Se,Pb,Cd and Cr in raw materials and products were measured by atomic fluorescence spectrometry(AFS)and inductively coupled plasma mass spectrometry(ICP-MS),respectively.It was investigated of the distribution of heavy metal elements in the products and their migration in solid and liquid phases during the preparation of SSZ-13 molecular sieve by two-step hydrothermal method using fly ash as raw material.The stability of heavy metal elements in molecular sieves during denitrification was also studied.The specific experimental results are as follows:(1)SSZ-13 molecular sieve was synthesized by two-step hydrothermal method using the pulverized coal furnace fly ash and the circulating fluidized bed fly ash as raw materials.The analysis results showed that more Pb,Cr and Cd elements were retained in SSZ-13 molecular sieve.The vast majority of Hg,As and Se elements migrate to the filtrate during hydrothermal reaction.The migration of heavy metal elements in the synthesis process of molecular sieve using fly ash was mainly affected by the nature of the elements,and the type of fly ash and the synthesis method of SSZ-13 molecular sieve had little effect on it.(2)The element speciation of Hg,As,Se,Pb,Cr and Cd elements in PC1 #-CFA and corresponding solid products were analyzed by the improved BCR continuous extraction method.The results showed that Pb,Cr and Cd elements mainly exist in the form of residual state in CFA,analcime and SSZ-13 molecular sieve,and their properties are stable and difficult to migrate.Most of Hg,As and Se elements exist in the form of transferable state in the fly ash and analcime.In addition,the content change of residual heavy metal elements in SSZ-13 before and after NH3-SCR denitrification was also investigated.Fly ash based Cu-SSZ-13 has excellent denitrification performance but poor hydrothermal stability.The contents of Pb,Cd and Cr elements in Cu-SSZ-13 had little change before and after denitrification and hydrothermal aging,indicating that these heavy metal elements were stable in denitrification.(3)The removal of heavy metals from fly ash by wet magnetic separation and hydrochloric acid leaching was investigated.The results showed that Fe,Pb,Cd and Cr elements in fly ash are enriched in magnetic components after magnetic separation.The contents of Fe,Pb,Cd and Cr elements in non-magnetic components decreased and the content of Al and Si increased.Acid leaching can effectively remove Hg,As and Se,but excessive temperature will increase the loss of Al.The combined removal process of magnetic separation and acid leaching can greatly reduce the contents of Hg,As and Se,but the removal of Pb,Cd and Cr is limited.