Migration, transformation, and enrichment of strategic metal elements including Li, Ga, Ge, In, and Re during the coal pyrolysis process

The migration, transformation, and enrichment of strategic metal elements in coal in the process of coal processing and utilization are significant for their effective extraction and application. In this work, the migration, transformation, and enrichment of Li, Ga, Ge, In, and Re during coal pyrolysis were investigated by six-step sequential chemical extraction. The results showed that Li mainly exists in a residual state in raw coal and pyrolytic products under 500 ℃; however, the content of sulfide state Li increases largely when the pyrolysis temperature exceeds 500 ℃. Ga mainly enriches in residual form, and the content of Ga in the residual state rises with the pyrolysis temperature increasing. Ge mainly presents in the form of carbonate-binding state, sulfide state, and residual state. As the temperature rises to higher than 500 ℃, Ge combines with the carbon and oxygen compounds produced by pyrolysis to form a carbonate-binding state, resulting in a significant increase in its content. The content of Ge in the residual state also increases significantly. The ratio of carbonate-binding state and organic state In gradually decreases with the increase of pyrolysis temperature, while that of residual state In gradually increases. Due to the low content of Re, the reliability of the test results is poor. However, Re may exist in sulfide, organic, and residual states. With the pyrolysis temperature increasing, Li, Ga, Ge, In, and Re in pyrolysis products are more likely to combine with aluminosilicate than those in raw coal.