Study On The Removal Of Hg~0 In Acid Production Tail Gas By Walnut Shell-based Activated Carbon Catalyst

Lead-zinc smelting is one of the pillar industries in Yunnan Province,and a variety of pollutants such as SO₂,NO_x,and heavy metal mercury are generated during the production process.In most lead-zinc smelters,high-concentration SO₂ is converted into sulfuric acid through the contact method,and NO_x is converted into N₂ by the NH₃-SCR method.Hg is usually associated with sulfide ore and will be released in the form of gaseous mercury（Hg⁰）during the smelting process.Although part of the gaseous mercury will be converted into oxidation state(Hg²⁺)or particulate state（Hg^p）and enter the dirty acid slag,there is still a large amount of Hg⁰ that will be discharged into the air along with the sulphuric acid exhaust gas.Because Hg⁰ is extremely diffusible,it easily enters the human body through the respiratory system and skin,causing damage to the central nervous system,even brain damage,and death.In addition,a large number of walnut shells are discarded every year,and walnut shells have excellent pore structure,mechanical strength and specific surface area.Activated carbon is prepared by using waste and cheap walnut shells as raw materials,which can realize waste treatment.Therefore,this paper uses the discarded walnut shells in Baoshan City,Yunnan Province as raw materials to prepare the catalyst carrier（WSC）.The Fe/WSC catalyst is prepared by the Fe loading modification method to remove Hg from the flue gas of lead-zinc smelting acid.By investigating its catalytic oxidation performance,X lattice ray diffraction（XRD）,H₂ temperature programmed reduction（H₂-TPR）,CO₂temperature programmed desorption（CO₂-TPD）,N₂ physical adsorption（BET）,X-ray photoelectron energy,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy（Raman）,thermogravimetric-differential thermal analysis（TG-DTA）,Fourier transform infrared spectroscopy（FTIR）,scanning electron microscopy（SEM）,ransmission Electron Microscope（TEM）,and other means to analyze the structure of the catalyst,the existence form of Fe in the active center,and the oxygen-containing functional groups on the surface.In-depth study of the structure-activity relationship of Fe/WSC catalysts for Hg⁰ removal,and clarified the reaction mechanism of Fe-based catalysts for Hg⁰ removal,providing theoretical basis and technical guidance for the removal of mercury in industrial production.First,the preparation conditions of walnut shell-based activated carbon were optimized.The effects of catalysts prepared at different carbonization temperatures on the removal of Hg⁰ were compared,and it was found that 700°C makes the support rich in structural defects,which is conducive to the dispersion of active components.And through the screening of active components,it is determined that FeCl₃ is the precursor and the loading is 5%,Fe/WSC catalyst has the strongest catalytic oxidation activity at the reaction temperature of 50-400°C.Secondly,on the premise of the catalyst prepared under the above optimal conditions,the influence of the catalyst on the removal of Hg⁰ before and after KOH activation was further discussed.The results show that the activity of Fe-KOH/WSC is equivalent to Fe/WSC in temperature programming,but in durability experiments at different temperatures,the activity of Fe-KOH/WSC is significantly better than Fe/WSC catalyst.The characterization results show that KOH modification is beneficial to remove impurities on the surface of the carrier,and at the same time,new pores and micropores are formed,which helps to disperse the active components.Then,on the basis of the above research,the effect of particle size on the removal of Hg⁰ was further studied,and two catalysts with different particle sizes were prepared.Studies have shown that the highly dispersed catalyst（Fe-KOH-1/WSC）has a wider reaction temperature window,while the removal of Hg⁰ by the large-particle catalyst（Fe-KOH-2/WSC）first decreases with the increases of the reaction temperature.Through characterization and analysis,it is found that this is mainly due to the agglomeration of large-particle catalysts at higher calcination temperatures,which affects the removal of Hg⁰.Finally,through durability experiments,it is revealed that the Fe particle size affects the mechanism of Fe-KOH-1,2/WSC catalysts to remove Hg⁰.Studies have shown that the chemisorption oxygen,C-Cl2p and Fe₃O₄ on the catalyst surface participate in the removal of Hg⁰,and the final products are Hg O,Hg Cl and Hg-Fe₃O₄chelate.