Impact of mechanochemical activation on the extraction of rare earth elements from brown coals

This study aimed at an investigation of the concentration, distribution, and extraction of rare earth elements (REEs) from various coal types, focusing on the effects of mechanochemical processing and the role of humic acids. The study shows significant variation in REE concentrations across the coals, with Vanchin coal having the highest concentration and Azeysky, Itatsky, and Spetsugli coals containing much lower levels. X-ray diffraction analysis indicated that Vanchin coal has a higher degree of metamorphism compared to Azeysky and Itatsky coals. Before processing, a higher proportion of REEs in Azeysky coal were associated with humic acids, but mechanochemical processing significantly increased this association in Azeysky coal to 93%, while it decreased in Vanchin and Spetsugli coals. Furthermore, the fraction of REEs bound to fulvo-humic acids increased after treatment, particularly in Spetsugli coal following the addition of external humic acids. Mechanochemical treatment of Vanchin coal without reagents allowed for a 1.4-fold increase in the share of REEs associated with the fulvo-humic acids fraction (FHA), reaching 14%. Mechanochemical processing of Spetsugli with a 10% addition of humic acids, which initially did not contain REEs, allowed for a twofold increase in the content of REEs associated with the FHA fraction, reaching 16%. These findings suggest that mechanochemical processing is a promising method for enhancing REE recovery, especially in humic acid-rich, low-rank coals. However, the efficiency of this approach depends on the degree of coal metamorphism, the humic acid content, and the use of additives during treatment.