Leaching Characteristic And Dissolution Mechanism Of Environmentally Sensitive Trace Elements In Low-sulfur Coal Gangue

Coal gangue is the solid waste discharged from coal mining and processing. As primary pollution source of mining area, trace elements in coal gangue are easily released under natural weathering and leaching and cause environment hazards when coal gangue is stockpiling and recycling. Based on sufficiently investigated on domestic and overseas literatures from the related fields, low-sulfur coal gangue samples collected from Linhuan coal mining area in Huaibei were systematically analyzed and studied by means of chemical experiment, instrumental analysis and dynamic leaching. The concentrations, modes of occurrence and leaching characteristics and mechanism of trace elements in low-sulfur coal gangue were also researched. The results showed as follows:(1) Ash content of low-sulfur coal gangue ranged from 78.07% to 89.54%, indicating high content of inorganic minerals and low content of organic matters and calorific value. Total sulphur content of samples ranged from 0.13% to 0.46%, and ferric sulphur content was highest in various forms of sulphur, followed by organic sulphur sulfate sulphur. Essential minerals of samples were quartz, kaolinite, illite, dickite, nacrite, chlorite and pyrite, of which quartz and kaolinite contents are the highest.(2) Ba content was highest in samples, whereas Cd content was lowest. Although the contents of Cd, Co, Cu, Hg and Ni were related to weathering time of samples, Ba, Cr, Mn, Pb, V and Zn contents were had no difference with weathering time of samples. Ba, Co, Cr, Hg, Mn, Ni, Pb and V were enriched and Cu and Zn depleted in low-sulfur coal gangue compared to trace elements in Huaibei coals, in which enrichment degree of Mn was highest and was 9.7 times more than it in Huaibei coals. Low-sulfur coal gangue was enriched in Hg, Pb and V compared with the Clark values, which the EF values of Hg, Pb and V were 4.37,2.37 and 1.33 respectively.(3) Many trace elements had similar modes of occurrence, in which residual form was highest and exchangeable form was lowest, indicating a large portion of trace elements were stable. Trace elements in low-sulfur coal gangue were mainly hosted in sulfide minerals, only several trace elements showed different occurrence characteristic. A number of trace elements were closely linked to sulphur, and low sulphur content of low-sulfur coal gangue influenced the occurrence characteristic of some trace elements.(4) Leaching rates of all the trace elements were increased with the decrease of particle size of samples and influent pH and the increased flow velocity of influent. In four groups, maximum leaching amount of Mn and V were highest and Hg was lowest, which was identical with the feature of their contents. Leaching rates of Mn, Ni and V were highest in four groups, which were in accordance with their occurrence characteristic in low-sulfur coal gangue, indicating the leaching behaviors of trace elements were bound up with their contents and modes of occurrence in low-sulfur coal gangue.(5) The contents of Ni were greater than ERL and less than ERM in 14% of samples, but all trace elements were less than ERM in 100% of samples, indicating it is less possibility of biotoxicity effects caused by trace elements in low-sulfur coal gangue. RAC risk assessment showed that environmental risk of trace elements was in order of Mn>Zn>Ni>Pb>Cd>Cu>Ba>V>Cr>Co, meanwhile, Mn, Ni and Zn posed a medium ecological risk and active forms of Mn were likely to damage the environment. Maximum leaching amount of Cr, Mn and Ni riched or exceeded groundwater environment quality standards by twice or more, also leaching rates of Mn and Ni are highest level, indicating the pollution caused by Mn and Ni in low-sulfur coal gangue deserved more attention.