Insights into critical metal element migration and enrichment in coal gasification

With the rapid development of China's semiconductor, military, and new energy industries, ensuring a stable supply and acquisition of critical metal elements, including lithium, gallium, germanium, and indium, has gained increasing attention. The migration and enrichment of critical metals during coal gasification can provide essential insights for developing metal extraction strategies from gasification slag. This work focused on the patterns of migration and enrichment of critical metal elements under varying gasification temperatures through comprehensive element distribution analysis and molecular structure characterization. Results show that the main component of gasification slag is mullite, which is composed of aluminate- and silicate-containing groups, namely, Si-O, O-Si-O, Al-O, and O-Al-O. As the gasification temperature increases from 1000 °C to 1500 °C, lithium, gallium, and germanium exist in the residual state, while the enrichment state of indium is complex. The concentrations of the residual state of each element gasified at 1500 °C are 121.2 μg g^-1, 46.93 μg g^-1, 0.12 μg g^-1, and 0.03 μg g^-1. However, lithium is mainly associated with clay minerals in coal slag. Meanwhile, gallium exhibits a relatively high bonding affinity for aluminates and silicates in the slag when the gasification temperature exceeds 1400 °C. By contrast, germanium and indium are demonstrated to have low affinity for aluminates and silicates in coal gasification slag. Germanium has more potential to combine with sulfur than aluminates and silicates, as revealed through enrichment factor analysis. Indium displays a complicated occurrence form in coal slag, demonstrating a better bonding affinity for oxygen and hydrogen in high-temperature gasification.