Research On Modes Of Occurrence Of Mecury In Coal Gangues And Their Thermal Transformation Behaviors

Coal gangue, as a solid residue of coal mining, has become one of the largest industrial emission wastes in China. The great quantity of coal gangue not only occupies a large amount of land resources, but also cause serious environment problem. Coal gangue has been extensively utilized as a raw material for power plants. However, the utilization of coal gangue in power plants has become a new anthropogenic discharge source of mercury(Hg) attributing to the high Hg content in coal gangue. To develop effective Hg control technology, it is of great importance to study the modes of occurrence of Hg, and the Hg release behavior of coal gangue. By different sequential extraction procedures(SEP) and the temperature-programmed decomposition(TPD) atomic fluorescence spectroscopy(TPD-AFS), the quantitative analysis and the release temperature range of different modes of occurrence of Hg in coal gangues were obtained. The release behaviors of Hg in coal gangue were studied by controlling the thermal treatment conditions(heating rate, residence time and atmosphere). In addition, the release behaviors and transformation behaviors of different modes of occurrence of Hg during coal gangue pyrolysis were also investigated. The main progress and the conclusions obtained are the following:1. Four coal gangues(ED, GD, PL, TX) from Shanxi province were studied by SEP and TPD-AFS, and the conclusions are as follows:(1) It is demonstrated that SEP coupled with TPD-AFS technique is an effective approach in studying the modes of occurrence of Hg and their thermal stability in coal gangues.(2) In the four coal gangues, the sulfide-bound Hg is the dominant form of Hg, and the level of sulfide-bound Hg is about 51%-78%; The silicate-bound Hg is the second abundant form, accounting for 10%-22%; The iron-manganese oxide-bound Hg is undetected in coal gangue ED and TX, and accounts for 6-11% in coal gangue GD and PL; The content of organic-bound Hg is about 1.5% averagely; The content of ion-exchangeable and carbonatebound Hg is low(<1%).(3) In the four coal gangues, the sulfide-bound Hg releases at 400-600 °C; The iron-manganese oxide-bound Hg releases at 200-400 °C and 400-600 °C; The organic-bound Hg releases in the temperature range of 200-400 °C; The carbonate-bound Hg releases in the temperature range of 250-300 °C; The silicate-bound Hg releases above 1200 °C.2. The Hg release behaviors of two coal gangues(ED, GD) during thermal treatment under different heating rates(10 oC/min, 20 oC/min, 40 oC/min), residence time(5 min, 10 min, 20 min, 30 min, 40 min, 50 min, 60 min) and atmospheres(N2, air) were studied, and the conclusions are as follows:(1) The effect of heating rates on Hg release: The specified temperature ranges for the release of different modes of occurrence of Hg are scarcely affected by the heating rate. At 200 oC, the heating rate has no evident effect on Hg release ratio for two coal gangues. At 400 oC and 600 oC the higher heating rate could promote the Hg release to some extent during thermal treatment of coal gangue.(2) The effect of residence time on Hg release: The Hg release ratio of coal gangues increases with the extension of residence time, and approximately reaches to a maximum value at the specified residence time. At 200 oC and 400 oC, the optimal residence time is 30 min; while the optimal residence time is 10 min at 600 oC for both coal gangues.(3) The effect of atmospheres on Hg release: The substance in coal gangues are oxidized and decompose more easily under air atmosphere, resulting in the easily Hg releasing at lower temperatures. Moreover, the oxidizing environment has a positive effect on Hg release, and the positive effect on Hg release is mainly at temperature < 600 oC, while the positive effect can be neglected at temperature above 600 oC.3. The Hg release behaviors of coal gangue ED treated with HNO3, HCl and HF during thermal treatment were studied, and the transformation behaviors of different modes of occurrence of Hg in coal gangues(ED, GD) during thermal treatment were also studied. The conclusions are as follows:(1) The sulfide-bound Hg in coal gangue could be leached out by HNO3; the carbonate bound-Hg, sulfate bound Hg and oxidation Hg in coal gangue could be leached out by HCl; the Hg releasing at 200-600 oC could be leached out by HF.(2) During thermal treatment, the ion-exchangeable and carbonate- bound Hg in coal gangue release as the temperature increases in coal gangue; The silicate-bound Hg is stable in coal gangue relatively, and temperature has no obvious effect on the silicate-bound Hg in coal gangue.