Study On Mercury Emission Control And Reclaim Technology From Nonferrous Metal Smelting Flue Gas

The “Minamata Convention on Mercury”, a new global legally binding treaty, was signed in October 2013 in Japan, which marked the international attention to mercury pollution up to unprecedented height. China is considered to be the world’s largest source of anthropogenic mercury emissions. In China, the mercury emission from nonferrous metal smelting industry is an important part of the total mercury emission. Therefore, the reduction of mercury emission in nonferrous metal smelting industry is directly critical to the efficiency of implementing the treaty.The measurement of mercury concentration from real nonferrous metal smelting flue gas was performed. The emission characteristics and pollution control technology were analyzed as a breakthrough point in this paper. On the basis of before analysis, with the existing mercury chloride absorption process as a research object, effect factors and mechanism on Hg0 removal were studied, and methods of improving and intensifying the mercury removal efficiency from nonferrous metal smelting flue gas were discussed. Furthermore, new high-efficiency absorption solutions, new methods and technologies for Hg0 and SO2 synchronous removal from nonferrous metal smelting flue gas were developed, and the relevant mechanisms were analyzed. The main contents and conclusions including:The first, the measurement of mercury concentration from real nonferrous metal smelting flue gas was performed. The results show that the mercury concentration in initial smelting flue gas is about tens mg/m3 which is thousand times higher than in coal-fired flue gas, and the SO2 concentration in initial smelting flue gas is about 2%-6%. Through the particulate matter collection device, wet scrubber unit and Boliden-Norzink process, the removal efficiency of total mercury reaches 90%, but the outlet Hg0 concentration is still much higher than the emission limit in “Emission standard of pollutants for lead and zinc industry(GB 25466-2010)”.The second, the research on technological parameters and mechanisms of the existing mercury chloride absorption process were conducted. The results show that increasing the HgCl2 concentration in solution is helpful for improving Hg0 removal efficiency and stabilizing the removal efficiency from the undulate flue gas. SO42- and NO3- ions don’t affect the Hg0 removal efficiency in the HgCl2 solution. The existence of SO32- causes the conversion from Hg2+ to Hg0, which weaken the ability of HgCl2 to absorb Hg0. The reduction of pH can inhibit the hydrolysis of HgCl2 in solution and promote the ionization of HgCl2 to HgCl+ or Hg2+, so that enhance Hg0 removal efficiency. When the mole ratio of Cl- to HgCl2 is 10:1, the Hg0 removal efficiency is the highest. Combined with the experimental data, Raman instrument characterization and Density Functional Theory computation, the results show that the HgCl3- is more efficient than HgCl2 and HgCl42- on mercury removal in mercury chloride absorption system, and the efficiency of HgCl42- is the lowest. In the research, it is found that the low p H, the moderate Cl- concentration and adding H2O2 in HgCl2 solution can effectively inhibit the influence of SO2 and have synergistic effects on improving the Hg0 removal efficiency. On the basis, the two new composite HgCl2 absorption solutions contain moderate HgCl2, Cl-, H2O2 and low pH for Hg0 removal in different situations of flue gas in the presence of SO2 are confirmed.The third, a new two stages integrated process for the mercury removal and sulphuric acid reclaim from nonferrous metal smelting flue gas was developed. The process contains a pre-desulfurization unit with Fe2(SO4)3 solution(Stage I) and a Hg0 removal and deep desulfurization unit with HgSO4/H2O2(Stage II). In the Stage I, 90% of SO2 can be efficiently absorbed by Fe2(SO4)3 and reclaimed sulfuric acid without losing much Hg0 from flue gas. Furthermore, the Hg2+ absorbed can be converted to the Hg0 by S(IV) and Fe(II) and re-emitted to the flue gas, so that the proportion of Hg2+ and Hg0 in the flue gas is redistributed in this stage. The conversion of Hg2+ to Hg0 is favorable for promoting the recovery of the total mercury. In Stage II, the composite absorption solution, which is constituted of 0.368 mmol/L HgSO4, 1.0% H2O2 and 0.1 mol/L H2SO4, can effectively remove 98.8% Hg0 and 99.7% SO2. Moreover, the concentrations of HgSO4 and H2O2 were adjusted with the changes of the concentrations of Hg0 and SO2 in the flue gas. In this paper, the absorption characteristics of Hg0 in HgSO4 solution are studied. The theory of mercury cluster Hg46+ is put forward, and it is verified reasonable using the Density Functional Theory computation.The fourth, in order to realize the Hg0 and SO2 removal in one purification facilities, the method of Hg0 and SO2 removal synchronously using I2+HI/H2 O solution was developed. The results show that when I2+HI/H2 O complex solution is used as absorbent for the control of Hg0 and SO2 flue gas, the concentration of I2 and HI should be 2.5 g/L and 0.1 mol/L at least, respectively. Under these conditions, the Hg0 and SO2 removal efficiencies can get higher than 99% and 95%, respectively. And the utilization rate of I2 can reach to 95%. In addition, H2SO4 has adverse impact on the absorption of SO2. So it should reduce the H2SO4 concentration as small as possible in the system.Three absorption technologies were studied in this paper. The improved mercury chloride process had better sulfur tolerance, and it was suitable for the flue gas containing high concentration of Hg0 and high concentration of SO2 which is reclaimed by the process of producing sulfuric acid after the process of Hg0 removal. The two stages integrated mercury sulfate process had higher mercury removal efficiency and better SO2 removal effects for the flue gas containing high concentration of Hg0 and medium-low concentration of SO2 which could not be made sulfuric acid by double contact double absorption process. The process of iodine cycle for Hg0 removal and acid reclaim could eliminate the influence of SO2 for mercury removal, and realize the Hg0 and SO2 removal synchronously in one purification facilities. In summary, the research above provided new ideas, new approaches and theoretical foundation for mercury and sulfur dioxide removal from nonferrous metal smelting flue gas. It is quite important and practical significance for the reduction of mercury emission.