Differences in the occurrence and enrichment causes of sodium in coal from Zhundong and Hami of Xinjiang

Xinjiang has abundant coal resources, primarily consisting of high-quality power coal and feed coal for the coal chemical industry. Additionally, it is also rich in sodium elements. However, the issues of staining and slagging during the combustion process of sodium-rich coals have significantly hindered the clean development and utilization of Xinjiang coal resources. The coals from the Xiheishan Open Pit Coal Mine in the Zhundong Coalfield and the Second Mine of the F1 Nanjing Field in the Dananhu Mining Area in the Tuha Basin were used as research objects. An improved stepwise chemical extraction experiment was used, along with testing methods such as ICP-AES, SEM-EDS and ion chromatography, to analyze the distribution and characteristics of sodium in the coals from the two mining areas and the differences between them, and to clarify the enrichment mechanism and main control factors of sodium in the coals. The results show that in terms of content distribution characteristics, the coal samples from the two mining areas are both typical low-rank high-sodium coals. The lithology of the roof slate of different coal seams has resulted in the Na content of the Dananhu coal gradually decreasing with the depth of the coal seam, while the Xiheishan coal shows the opposite trend. In terms of occurrence characteristics, the sodium and moisture in the coal have a strong affinity with the organic matter. This is also evidenced by the results of stepwise chemical extraction. The sodium in Xiheishan coal is mainly water-soluble sodium (54.32%), followed by ammonium chloride-soluble sodium (30.35%). The sodium in Dananhu coal is mainly ammonium chloride-soluble sodium (41.10%), followed by water-soluble sodium (33.15%). The water-soluble sodium in the coals from the two mining areas is mainly NaHCO3 and NaCl, respectively, and the inert group is its affinity microscopic component. The differences in the characteristics of coal rock, coalification degree and the depositional system of the source will lead to differences in the characteristics of the occurrence of Na in coal. The enrichment of sodium in Xinjiang coal is the result of the superposition of multiple factors, mainly controlled by the double effects of surface water leaching (vertical recharge) and groundwater driving effect (lateral recharge). Among them, the enrichment of sodium in Dananhu coal is mainly affected by the leaching of saline-alkali soil, while Xiheishan coal is mainly affected by the sodium-containing minerals in the aquifer rocks and the roof of the coal seam. The pretreatment method for sodium removal should also be improved according to the state of occurrence of sodium. For high-sodium coal rich in sodium organic acids, a low-concentration ammonium salt solution and organic acid can be used as a solvent for sodium removal. For high-sodium coal with high water content and high inert group, flotation and ammonium acetate can be considered for pretreatment, which can achieve the removal of sodium and separation of the inert group, and increase the content of the active component, the mirabilite group, in the clean coal.