Modes of occurrence of rare earth elements in Neogene lignite from the Puyang mine, Yunnan

Despite extensive research, the modes of occurrence of rare earth elements and yttrium (REY) in lignite remain poorly understood. Two key questions persist: ①Do REY primarily associate with organic matter or mineral matter in lignite? ②Lignite contains a large amount of oxygen-containing functional groups which are easily disturbed by acid leaching, does the high proportion of REY extracted by Hcl during sequential chemical extraction originate from minerals as carbonates, phosphates, mono-sulfides, or from organic matter? The present study investigates the modes of occurrence of REY in Neogene lignite from the Puyang mine using a multi-faceted approach. We combined conventional sequential chemical extraction with float-sink separation and Fourier transform infrared spectroscopy (FTIR) analysis. Results show that the M2 and M3 coals are classified as very low-medium ash, medium-high volatile, and mediumsulfur coals. Mineral analysis revealed quartz, calcite, kaolinite, and pyrite as the dominant minerals. Major element oxides in the M2 and M3 coals are dominated by SiO2 and Al2O3. Based on Seredin-Dai's classification, the REY distribution patterns in these coals exhibit L-type and N-type characteristics, with a slight enrichment in the M2 coal. Sequential chemical extraction results revealed that HCl leached the majority of REY, followed by HNO3. Approximately 10% of REY was leached by HF, with a small proportion remaining in the residue. Float-sink separation experiments demonstrated that REY preferentially concentrates in low-density components. FTIR analysis showed an increase in oxygencontaining functional groups, such as phenol hydroxyl and hydroxyl groups, in the analyzed samples after HCl leaching. This increase may be caused by the migration of REY, which bind to the phenol hydroxyl and hydroxyl sites during HCl treatment. Collectively, these findings indicate that REY in the M2 and M3 coals mainly occur in the organic matter, with a minor fraction associated with silicate minerals. Carbonate, phosphate, mono-sulfide, and bisulfide minerals may also host a certain portion of REY. The unusually high proportion of REY extracted by HCl during the sequential chemical extraction procedure mainly originates from the organic matter. © 2024 Geological Society of China. All rights reserved.