Study On Absorption And Purification Of Heavy Metals From The Smelter Flue Gas

Heavy metal is a kind of persistence,bioaccumulation and toxic pollutant,which has a great negative impact on human health and ecological environment.Mercury is one of the most toxic heavy metals.The Minamata Convention on Mercury,a new legally binding global treaty that aims to reduce mercury emission,was signed on Oct.10,2013.It means that the control of mercury emissions is becoming more urgent all over the world.Mercury in the flue gas primarily exists in three forms:elemental(Hg0),oxidized,and particulate-bound mercury.The oxidized and particulate-bound mercury can be effectively removed by wet flue gas desulfurization(FGD)facilities.The forms of arsenic in the flue gas are arsenic sulfide,arsenic trisulfide and arsenic trichloride etc,which are attached on the atmospheric particulates.Arsine gas was directly emitted into the atmosphere to cause pollution.The forms of lead which causes air pollution are mainly gaseous.Waste incineration was the mainly reason which causes the air pollution of cadmium.Sulfide method is mainly used for removing heavy metals from waste water,but the study on removal of heavy metals from the flue gas is less.This paper focuses on the study of removal of mercury(Hg),arsenic(As),lead(Pb)and cadmium(Cd)in simulated flue gas by(NH4)2S solution and activated carbon adsorption device.The method is based on the precipitation reactions between S2-and Hg,As,Pb and Cd in simulated flue gas to generate HgS,As2S3,PbS,and CdS precipitation.It is also beneficial to recycle the several heavy metals and realize the economic cycle.The basic research and theoretical basis can be provided to the engineering application of removal of toxic metals from simulated flue gas.In this experiment,a tubular resistance furnace was used as a reactor to simulate the several heavy metals in smelter flue gases by heating these compounds.The flue gas containing Hg,As,Pb and Cd was first treated with ammonium sulfide,and then the tail gas was put into an activated carbon adsorption device The experiment was conducted to investigate the effects of varying the concentration of ammonium sulfide,the pH value of ammonium sulfide,the temperature of ammonium sulfide,the presence of SO2 and the the best conditions of modified activated carbon which was used to absorb the mercury which has not been removed by ammonium sulfide.The prepared adsorption products and precipitate were characterized by Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),scanning electron microscopy(SEM)and testing surface area(BET).Considering removal efficiency and economic benefits,the results showed that the optimal concentration,temperature and pH value of ammonium sulfide solution were 0.8 mol/L,20℃ and 10,respectively.Under the optimum conditions,the concentration of Hg,As,Pb and Cd in the treated flue gas could be reduced to 0.42mg/m3、0.45mg/m3、0.38mg/m3、0.47mg/m3,respectively.And the removal efficiencies of Hg,As,Pb and Cd from simulated flue gas could reach 98.2%,96.3%,99.1%,95.5%respectively.The results of FTIR,XRD,XPS and SEM showed that the removal mechanism of Hg and Cd from simulated flue gas was mainly chemical deposition and the removal mechanism of As and Pb from flue gas was mainly physical adsorption.All in all,the theories and results prove that ammonium sulfide is an efficient and low-cost adsorbent for Hg,As,Pb and Cd from simulated flue gas.In addition,the existence of SO2 had an inhibitory effect on removal of Hg,As,Pb and Cd from simulated flue gas.At last,the results showed that 300℃-CuCl2-AC was the best modified activated carbon which was used to absorb the mercury in the flue gas.,and the outlet concentration of mercury was down to zero.Its adsorption capacity could reach 4.55mg/g.