Experimental Study On Catalytic Performance Of Rare Earth Tailings For CO Reduction And Denitrification

China's energy is dominated by coal resources in China's energy structure.The main method of coal utilization is direct combustion,which is often accompanied large amounts of nitrogen oxides?NO_X?.NO_X not only pollutes the atmospheric environment but also endangers human health.At present,the most commonly used technology for controlling NO_X is NH₃selective catalytic reduction denitrification?NH₃-SCR?.Recent studies have found that NH₃-SCR technology is prone to produce corrosive substances such as sulfuric acid during the denitrification process,and the presence of ammonia escape will cause secondary pollution to the environment.Therefore,NH₃-SCR is bound to be replaced by other denitrification technologies in the future.Reburning denitrification refers to the injection of fuel into the furnace reburning zone to form a reducing atmosphere,reducing NO_X to N₂.The denitrification technology has the advantages of small change to boiler and low operating cost.But its denitrification rate is too low to satisfy NO_X emission standard.In this paper,the reburning technology is improved,and rare earth tailings containing catalytic active components are used as catalysts to be injected into the reburning zone of circulating fluidized bed boiler to promote the transformation of NO_X.Therefore,the background of this paper is the reburning denitrification of circulating fluidized bed boiler,using rare earth tailings as denitrification catalyst,and CO as reducing agent for denitrification experiment.Rare earth tailings are the remaining minerals after the iron and rare earth separation in Bayan Obo.Therefore,it has the characteristics of"abundant,poor,fine and mixed".In this study,the rare earth tailings were subjected to strong magnetic separation with1.2T.The minerals selected by strong magnetic separation were called magnetic separation concentrates,and the remaining minerals were called magnetic separation tailings.Firstly,process mineralogy was conducted on rare earth tailings,magnetic separation tailings and magnetic separation concentrates.The results showed that the main mineral species did not change after the strong magnetic separation,but the content of some minerals changed accordingly.Among them,the content of hematite and magnetite in the magnetic separation tailings decreased,the content of iron-containing silicate minerals increased,and the content of bastnaesite increased.The hematite particle size in the small particle size range of<15.77?m had the largest proportion of magnetic separation tailings,so it had the highest dispersion.The intergrowth rate of hematite and rare earth minerals in the magnetic separation tailings increased from 6.94%to 9.03%.The interlacing of hematite and rare earth minerals was beneficial to joint-synergistic denitrification between minerals.In the magnetic separation tailings after strong magnetic separation,the iron content of hematite and magnetite was reduced from81.38%to 50.71%,and the iron content of silicate minerals and carbonate minerals were increased to 5.61%to 15.61%.It showed that the iron element in the magnetic separation tailings not only existed in the form of mineral phase,but also existed in the carbonate minerals and silicate minerals in a highly dispersed form.Secondly,the denitrification performance and reaction process of rare earth tailings were studied.The results showed that rare earth tailings,as catalytic CO reduction denitrification catalyst,have the characteristics of high efficiency,high sulfur resistance and high stability.The denitrification increased with the increase of reaction temperature.The denitrification rate first increased and then decreased slowly with the increase of CO volume fraction.The presence of CO₂ and SO₂ had little influence on catalytic denitrification,while the presence of O₂ will seriously affected the denitrification rate.The denitrification rate decreased first and then stabilized with the increase of reaction time.When the reaction temperature was 800?,the CO volume fraction was 2 000×10^-6,and there was no other gas composition,the maximum denitrification rate was 98.7%.The TG-MS experiment of raw rare earth tailings was carried out in the atmosphere of N₂.When the rare earth tailings were subjected to the program heating roasting at 290⁹30?,CO₂ and SO₂ would be generated.The reason why was that the cerium fluorite and carbonate minerals would decompose into CO₂ by heat under the condition of high temperature,while the pyrite and sulfate minerals would decompose into SO₂ by heat under the condition of high temperature.The rare earth tailings obtained after roasting were denitrified again on the TG-MS.It was found that the quality of the rare earth tailings did not change during the whole denitrification process,and the emission of NO_X could be reduced at the same time.Therefore,rare earth tailings could be used as denitrification catalyst in CO reduction denitrification process.During the catalytic CO reduction denitrification reaction of rare earth tailings,adsorbed oxygen would participate in the denitrification reaction.After the denitrification reaction,the proportion of Fe²⁺and Ce³⁺in rare earth tailings would increase,which would accelerate the conversion rate between lattice oxygen and adsorbed oxygen,release more oxygen vacancies,and promoted the catalytic reaction.Finally,the denitrification performance and mineral characterization of the rare earth tailings,magnetic separation tailings and magnetic separation concentrates were compared and analyzed.The results showed that when the reaction temperature was 500⁹00?,the denitrification performance of the magnetic separation tailings was higher than that of the rare earth tailings and the magnetic separation concentrate,and when the reaction temperature was500?,the catalytic denitrification rate of the magnetic separation tailings was 82.2%,much higher than magnetic separation tailings and magnetic separation concentrate.When the volume fraction of CO,CO₂,SO₂ and O₂ were changed,the denitrification rate of magnetic separation tailings and magnetic separation concentrates were similar to that of the rare earth tailings.The oxidation-reduction ability and NO absorption and desorption ability of magnetic separation tailings were better than the rare earth tailings and magnetic separation concentrates.