Dynamic migration mechanisms valuable elements in low-rank coal during hydrous thermal simulation: A case study of the Hedong coalfield, Ordos Basin, China

Valuable trace elements in coal can contribute critical geological insights into the formation and evolution processes of coal-bearing basins and the development characteristics of strata. The migration and enrichment mechanism are a key scientific issue in solving the mineralization of coal-bearing metal minerals. The migration and occurrence patterns of valuable trace elements during thermal evolution are extremely complex, so it is particularly crucial to comprehend the migration mechanism of trace elements in coal seams. Taking low-rank bituminous coal in Hedong coalfield as the research object, hydrous thermal simulation experiments with a large water volume and a large sample volume (1.5 mL water: 1 mL coal) were conducted in the range of 250℃ ∼ 550℃ (interval of 50℃) to systematically explore the migration and enrichment behavior of valuable trace elements under varying temperatures. The results show the following: 1) The content of Al in clay minerals changes more with the increase in temperature than that of Si in quartz. Ca and P, associated with organic matter, readily migrate into gaseous or liquid products, and Mg and Fe coexist in sulfate minerals. 2) At 250℃ ∼ 400℃, temperature is the dominant factor, where Ga, V, Hf, Zr, Th, and REY combined with organic matter are released with volatile matter and migrate into gaseous and water products. At 450℃ ∼ 550℃, temperature and water jointly influence the dissolution and migration of trace elements in certain minerals. 3) The enrichment and volatilization rates of Li in aluminosilicate minerals are different from those of other valuable metal elements associated with organic matter. 4) Trace element content prediction models based on the random forest algorithm effectively estimate the contents of Li, V, Sr, Th, Zr, and REY, although the accuracy of the predictive model is limited by the quality of the dataset. Hydrous thermal simulation experiments revealed the occurrence and migration mechanism of valuable trace elements, providing a theoretical basis for the exploration and efficient development of key metal resources in coal-bearing minerals, which has important practical value for improving the comprehensive utilization of coal resources.