Modes of occurrence and migration patterns of trace elements during thermal simulation of sub-bituminous coal

Investigating the geochemical behavior of trace elements in coal under high-temperature conditions not only enables the reconstruction of their geochemical characteristics during thermal evolution but also holds great significance for the prevention and control of coal-derived pollution and the resource utilization of high-value-added elements. This study takes the No. 6 coal seam of the Guanbanwusu Mine in the Jungar Coalfield as the research object. Based on thermal simulation experiments at approximately 500°C, combined with sequential chemical extraction and multiple analytical methods, the modes of occurrence, migration, and enrichment of trace elements during thermal simulation are revealed. The results show that among 48 trace elements, except for Sr, U, and As, the concentration coefficients of the remaining elements in partings are higher than those in raw coal. In raw coal, 23 elements, including Li, Ga, Sr, Zr, In, Pb, Th, U, most rare earth elements and yttrium (REY), have contents higher than the average values for Chinese coals, warranting focused investigation. As the thermal simulation temperature increases, moisture and volatile matter decrease significantly, while ash yield increases. The trace element contents in semicoke show an increasing trend, indicating that trace elements do not readily migrate with gaseous and liquid products but are more likely to remain in the semicoke. Sequential chemical extraction reveals that in raw coal, most trace elements are primarily present in the silicate fraction, followed by carbonate and organic-bonded fractions. With increasing temperature, the proportion of organic-bonded trace elements in semicoke increases, while the proportion of silicate fractions decreases, which is attributed to enhanced complexation and physical encapsulation within the organic matrix. At each thermal simulation temperature, elements with mobility above the average are classified as highly mobile, whereas those below the average are considered weakly mobile. A comparison of relative migration capacities shows that Pb, Y, Li, Zr, Pr, Th, Tm, Er, Gd, Ga, and La exhibit stronger migration ability than the average, while Lu, Dy, Ho, Yb, Nd, Tb, Ce, In, Sm, U, Sr, and Eu exhibit weaker migration ability than the average. The migration process of elements is jointly influenced by temperature, raw coal properties, thermal stability of minerals, and modes of occurrence.