Modes of occurrence of rare earth elements and yttrium in bituminous coals with different ranks from the Hedong Coalfield, northern China

Abstract

As the promising critical metal elements in coal, rare earth elements and yttrium (REY) can occur in minerals or complex with organic matter. Some differences have been observed in the modes of occurrence of REY in coals with different ranks, which raises interesting questions on their variation characteristic and mechanism during complex coalification. In this study, the modes of occurrence of REY and organic structure characteristics in bituminous coals with different ranks from the Hedong Coalfield are investigated. The sequential chemical extraction procedure shows that REY mainly remain in the residue in the low-rank bituminous coals and display an increasing total leaching rate as vitrinite reflectance increases. While in the high-rank bituminous coals, REY are dominated by HCl-leachable and residual phases. These results demonstrate that organic matter is the dominant carrier of REY in the low-rank bituminous coals. While in the high-rank bituminous coals, REY predominantly associate with phosphates and organic matter, indicating a gradual transformation from organic to mineral forms during coalification. Fourier Transform infrared (FTIR) spectroscopy experiments have found that the peak areas of most oxygenated functional groups are negatively correlated with vitrinite reflectance. In comparison to other functional groups, the content of phenolic group (Ar-OH) displays a strong positive correlation with the proportion of residual REY in the sequential chemical extraction experiment. This indicates that Ar-OH probably serves as the dominant organic binding site of REY during coalification. The loss of organic binding sites during coalification may result in the gradual decline of the proportion of organic-bound REY. Meanwhile, some REY released from these binding sites probably form authigenic minerals, leading to the higher proportion of mineral-associated forms of REY in the high-rank bituminous coals.