Mineralogical constraints and beneficiation perspectives for ultrafine rare earth elements in flotation tailings

Rare earth element (REE)-bearing tailings are dominated by ultrafine particles, posing significant challenges for characterisation and beneficiation. In this study, a complex, low-grade flotation tailings system dominated by ultrafines (~80% for <20 μm and ~ 42% for <5 μm) was systematically examined using inductively coupled plasma-mass spectrometry (ICP-MS), mineral liberation analysis (MLA), and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) to determine the characteristics and deportment of REEs and associated gangue species. Results revealed that REEs are strongly enriched in the ultrafine fraction (<5 μm), with florencite-(La) and monazite-(Ce) identified as the dominant host minerals, while quartz and magnetite constitute the principal gangue phases. Below 10 μm, liberation could not be reliably resolved due to particle agglomeration, surface coatings, and inherent limitations of 2D automated mineralogy. These constraints highlight the need for ultrafine-specific beneficiation strategies, including dispersant-assisted pre-treatment, advanced flotation technologies (e.g. Reflux Flotation Cell and Jameson Cell), selective aggregation (floc-flotation), phase modification prior to REE pre-concentration (e.g. magnetising roasting or microwave pre-treatment), and desliming. Furthermore, a correlative multi-scale workflow integrating 3D techniques (e.g. micro-XCT and X-ray microtomography) with synchrotron-based methods such as micro-X-ray fluorescence (XRF) and X-ray absorption near-edge spectroscopy (XANES) is proposed to overcome the limitations of conventional 2D characterisation. • Systematic size-by-size mineralogical characterisation of complex REE tailings down to <10 μm. • Demonstrated ultrafine enrichment of REEs. • Identified phosphate-host control (florencite/monazite). • Explained flotation inefficiencies through mineralogical constraints. • Proposed ultrafine-focused beneficiation pathways.