Sequential Leaching and Mineralogical Controls of Rare Earth Elements and Yttrium Occurrence in Bituminous Coal from Upper Silesian Coal Basin (Poland)

In this study, the occurrence and leachability of rare earth elements and yttrium (REY) in medium-rank coal—meta-bituminous B coal from the southwestern part of the Upper Silesian Coal Basin in Poland—were investigated. The coal samples contained variable amounts of siderite, dolomite, calcite, kaolinite, illite, quartz, apatite, and pyrite in their mineral composition. A five-step sequential chemical leaching procedure was used, including deionized water, 3% HCl, 5% HNO₃, 10% HNO₃ with microwave assistance, and concentrated HCl–HF also with microwave assistance. The highest concentrations of ∑REY were observed in seam 404/1. Light REY (LREY) dominated the REY composition (>75%), while heavy REY (HREY) accounted for less than 10%. The chondrite-normalised REY patterns and total REY content indicate a clastic origin of REY-bearing minerals. The most efficient leaching occurred in stages IV and V. The solutions from stages I–III preferentially mobilised critical REY, while those from stages IV–V reflected the REY distribution in the coal. Based on the C_outl index, both coal and leachates from the later stages are classified as prospective REY resources. However, absolute REY concentrations should be considered when interpreting C_outl values. The positive correlation between apatite and kaolinite contents and ∑REE concentrations suggests their role in REY enrichment.