Study On The In-situ Modification Of Red Mud Catalysts For NH₃-SCR And Recovering Iron From Red Mud By Using Desulfurized Ash As An Additive

Red mud,the main by-product of alumina industry,is difficult to be directly recycled due to its complex composition and high alkalinity.At present,stacking and landfilling are the main disposal metods for red mud,which brings a heavy burden to the environment.Red mud is a rich secondary resource because it contains a large number of minerals composed of Fe,Ti,Al,Si,Na and Ca,and generally,Fe is the most abundant element.Therefore,it is extremely urgent to use red mud in a wide range of applications.In this paper,based on the composition characteristics of red mud,two effective approaches for utilization of red mud were developed.In the first section,Fe,Ti,Al and Si in red mud were taken as the fundamental components of catalyst for selective catalytic reduction of NO_x with NH₃（NH₃-SCR）.A technology including acid leaching,alkali precipitation and calcination processes was adopted to modify the compostion and microstructure of red mud.The influence of acid concentration and calcination temperature on the catalytic activity of red mud catalyst was investigated,and the SO₂resistance mechanism was studied.In the second section,using a solid waste desulfurized ash as additive,the recovering iron from red mud via roasting reduction was studied,and impact of the roasting reduction conditions on the iron recovery was investigated.Ⅰ.The red mud catalyst was prepared via the acid leaching,alkali precipitation and calcination processes.When the concentration of hydrochloric acid is 3.1 mol/L and the calcination temperature is 500 ^oC,the obtained HRM3.1 catalyst shows the best low-tempreture activity,with the NO conversion above 90%at 240-360℃as well as the excellent SO₂resistance.The Fe,Ti,Al,Na and Ca can be leached out by hydrochloric acid,and the Fe,Ti and Al can be further precipited by ammonia.After calcination,the red mud catalysts consisted of Fe-Ti-Al-Si oxides can be obtained.With the acid concentration increasing,more Fe and Ti are leached out,meanwhile,Ti can be incorporated into Fe₂O₃ phase and the crystallite size of Fe₂O₃ becomes smaller.As a result,the HRM3.1 catalyst shows strongest reducibility,largest amount of surface chemisorbed oxygen（O_α）and medium surface acidity.In-situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）results show that more NO can be adsorbed and oxidized over the HRM3.1 catalyst to generate more nitrate species,even though in the presence of SO₂,alleviating the suppression of SO₂ on the SCR reaction via Langmuir-Hinshelwood（L-H）mechanism,and thus the SO₂ resistance is enhanced.Moreover,the influence of leaching acid type on the catalytic activity of red mud catalysts were further investigated.The results showed that the NO conversion of red mud catalyst at 240 ^oC is in the sequence of leaching by hydrochloric acid>sulphuric acid>phosphoric acid>nitric acid,when the acid concentration is 3.1 mol/L and the calcination temperature is 500 ^oC,due to the highest leaching ratio of Fe and Ti achieved by hydrochloric acid.Although a higher leaching ratio of Fe and Ti can be reached by phosphoric acid,the possible poisoning effect of P on the red mud catalyst suppresses the low-temperture activity.Ⅱ.The influence of roasting temperature,roasting time,carbon content,sodium sulfate content and sodium carbonate content on the iron recovery from red mud is investigated by using orthogonal method.The optimal iron recovery rate of 88.96%and iron grade of 49.85%can be achieved when the calcination temperature is 1000℃,the roasting time is 180 min,carbon content is 10%,sodium sulfate content is 9%and sodium carbonate content is 6%.The XRD results show that the Fe₂O₃ in red mud can be completely transformed to magnetic elemental Fe under above optimal conditions,effectively promoting the iron recovery.Based on the orthogonal experimental results,iron recovery from red mud using desulphurization ash composed of sodium sulfate and sodium carbonate as additive was studied.When the temperature is 1000℃,the roasting time is 180 min and the content of desulfurized ash is 15%,the best iron recovery result can be achieved,that is,50.83%of iron grade and 90.08% of recovery rate.