Elemental Geochemistry of Float-Sink Products from Late Paleozoic Coals with Different Ranks: Implications for Clean Utilization and Resource Potential

Coal rank exerts a fundamental control on the distribution of elements during density-based separation, yet this influence remains poorly understood. The primary objective of this study is to elucidate how coal rank governs the enrichment and partitioning of major, trace, and rare earth elements (REY) in float–sink products, and to assess the implications for clean coal utilization and critical metal recovery. To achieve this, three Late Paleozoic bituminous coals of different ranks from Shanxi Province, China, were subjected to density fractionation (1.3–1.8 g/cm3) combined with proximate and ultimate analyses, X-ray fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), and coal petrography. The results show that coal rank fundamentally governs element distribution and enrichment patterns. With increasing rank, the dominant inorganic minerals shift from clay minerals to carbonates, leading to pronounced differentiation in elemental affinities. In medium- to high-rank bituminous coals, chalcophile elements (e.g., As, Mo, Tl) associated with sulfides are significantly enriched in high-density fractions, whereas in high-rank bituminous coals, carbonate-related elements (e.g., Sr, Ca, Mg) show marked enrichment. Rare earth elements are primarily hosted in clay and phosphate minerals. Light rare earth elements dominate in medium- to high-rank coals, while middle rare earth elements increase in high-rank coals due to carbonate influence. Density-based separation effectively concentrates hazardous elements (e.g., As, Pb, Cd) in high-density tailings, demonstrating substantial potential for mitigating environmental risks. Meanwhile, critical metals such as lithium (Li), strontium (Sr), and REY are enriched in medium- to high-density products, with Li hosted in clay minerals and Sr strongly enriched in carbonate-rich high-rank coal (up to 1525 μg/g), indicating recoverable resources from coal processing wastes.