Effect Of Chemical Components In Red Mud On Removal Of Elemental Mercury From Coal Combustion Flue Gas

Mercury,a global pollutant,has attracted wide attention due to its long-distance transport,high toxicity,persistence and bioaccumulation.As the main important anthropogenic source of mercury pollution in China,mercury emitted from coal combustion flue gas as well as a variety of pollutants can be removed simultaneously through existing pollutant control devices,such as injecting sorbents before and after dust-removing equipment.Activated carbon injection?ACI?is a typical method,but its adaptability to low rank coal is not good.Therefore,the development of efficient and practical mineral sorbents as alternatives to activated carbon is of vital importance.Bayer red mud,a solid waste from alumina production,is rich in Fe₂O₃ and has been used for the mercury removal.However,due to the complexity of the composition of red mud,the mercury removal mechanism on red mud is unclear.Ferric oxide,alumina and silicon dioxide,the highest content in red mud,were prepared to obtain sorbent Fe/Al-SiO₂?FAS?,then loaded into a small amount of metal oxides MgO,CaO and TiO₂.By simulating the main and trace chemical components of red mud,the effects of preparation conditions,reaction temperature and flue gas components on the mercury removal efficiency were studied.The mechanism of mercury removal on the surface of sorbents was explained.The results show that the composite oxide FAS prepared by sol-gel method has large specific surface area,pore volume and a large number of mesoporous pores.Fe₂O₃,the active component,provides chemisorption oxygen and lattice oxygen for mercury oxidation;SiO₂ improves the removal of mercury from pore structure.The mercury removal of FAS samples increases firstly and then decreases with the raise of temperature in the range of 100-400 °C.Mercury is mainly in the form of HgO after reaction.It implies that chemical adsorption and catalytic oxidation are dominant reactions.The FAS composite oxides calcined at 400 °C has the best mercury removal performance at 350 °C,and the average mercury removal rate in 3 h is 94.8%.Magnesium and calcium react with some hematite in sorbent to form spinel phase of magnesite,calcite and calcareous magnesite,which reduces the cluster phenomenon of iron oxide and makes the crystal more complete.The pore volume and surface area also decreases.Ti forms amorphous titanium dioxide structure.Compared with the FAS,the mercury removal performance on M/FAS,MC/FAS and MCT/FAS decreases.XPS analysis show that the ability to provide lattice oxygen of the three sorbents decreases,total O content is also descended.Some Fe³⁺cannot play its own oxidation performance,so chemical adsorption and catalytic oxidation are limited,and physical adsorption is inhibited by the decrease of specific surface area and pore volume.M/FAS and MCT/FAS have the best mercury removal performance in the temperature of 350 °C.The average mercury removal rate on M/FAS and MCT/FAS decreases to 60.5%and 33.8%respectively.M/FAS is more sensitive to the temperature,and the mercury removal performance decreases slightly at 300-400°C than MCT/FAS.Combined with Hg-TPD and XPS,the effects of O₂,HCl,SO₂ and NO in flue gas on mercury removal of FAS,M/FAS,MC/FAS and MCT/FAS are similar.The O₂ in SFG atmosphere provides consumed lattice oxygen and chemisorbed oxygen,and enhances the mercury removal performance of FAS.The addition of HCl,NO and SO₂ to promote the oxidation of Hg⁰,forming corresponding mercury compounds.However,with the continuous addition of SO₂,the promotion effect is gradually offset by the toxic effect of SO₂ on the sorbent,following the inhibition effect on Hg⁰ by the end of reaction.MCT/FAS removes part of NO and SO₂ in flue gas;while H₂O occupies the active sites of the MCT/FAS and competes with Hg⁰ that leads to the reduction of mercury removal efficiency.