Research on the combination model of water-soluble elements in coal from eastern Xinjiang

The high content of water-soluble sodium, chlorine and sulfate ions in Xinjiang’s high-alkali coal seriously affects the clean and efficient utilization of Xinjiang’s coal. The combination mode of water-soluble elements in high-alkali coal and the influence on coal quality has become a basic problem to be solved in the scientific understanding and evaluation of Xinjiang’s coal. We collected coal seam and commercial coal samples from Dajing, Xiheishan, Santanghu, Naomaohu and Shaerhu mining areas in eastern Xinjiang, as well as surface soil overlying on the coal seams, roof and gangue samples in different mining areas, including surface salt rock samples from some mining areas. The content of water-soluble ions in relevant samples was determined by the leaching experiment. Based on the method of coal ash composition, proportion of water-soluble ions, water-soluble ions ratio parameter and Piper trilinear diagram, the difference of the combined characteristics of water-soluble elements in coal in different mining areas were analyzed. The possible causes and influencing factors of water-soluble elements in coal were discussed by comparing the variations of water-soluble ions in coal seams, other rock layers of coal-bearing strata and surface soil. The result shows that there are significant differences in the content and combination modes of water-soluble elements in coal from different mining areas in eastern Xinjiang, and the degree of difference in content can be about 3 times. In terms of combination modes, there are Cl-Na type and Cl·HCO₃-Na type, as well as other types of combinations such as SO₄-Na·Ca type and SO₄·HCO₃-Na type. The ratio parameters of various water-soluble ions show that there have been ion-exchange between water-soluble ions in the coal. The formation of high-alkali coal in Xinjiang is a modern geological processes currently still in development. There has been a distinct possibility that water-soluble ions in the salt rock and soil on the surface were carried gradually into the coal seams along with the groundwater infiltration, which leads to the formation of high-alkali coal, and the process is relatively complex. Affected by ion-exchange and precipitation, the combination forms and occurrence states of alkali metals, alkali earth metals, chlorine, sulfur and other elements in coal become more complex, but the water-soluble state is still the main form.