V. Drozd, Y. Katsenovich, A. Medley, T. Levitskaia, P. Zhang
Florida International University,
United States
Keywords: phosphogypsum, rare earth elements, mineralogical characterization
Summary:
Phosphate mining generates vast quantities of waste materials that represent an unconventional but significant resource for rare earth element (REE) recovery. In Florida, more than 2 billion tons of phosphogypsum (PG) waste have accumulated in Polk County. The application of advanced leaching technologies offers the potential to recover approximately 11,080 tons of REEs annually, elements that are critical to the U.S. economy and technological competitiveness. This collaborative research effort, uniting DOE national laboratories, academic institutions, and industry partners, seeks to recover mixed rare earth oxides and salts from PG, thereby advancing both environmental stewardship and economic growth in Polk County. Recovery of critical elements from low-grade waste streams presents both economic and environmental challenges. The objective of this study is to recover REEs from PG produced during phosphate fertilizer processing in Polk County, Florida, using a tetraoctyl diglycolamide (TODGA)-based extraction system to produce high-purity mixed rare earth oxides (MREOs). The research focuses on characterizing solid samples to assess the mineralogy, surface morphology, and elemental composition of PG materials. These analyses aim to elucidate the mineralogical framework and elemental distribution within the waste matrix to better understand REE leaching behavior. We also assessed the distribution of radioactive Ra-226 across different particle-size fractions of PG. Mineralogical characterization of PG samples from Riverside and Bartow stacks was performed using powder X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS). Both materials exhibited dominant calcium sulfate phases like gypsum (CaSO₄·2H₂O), bassanite (CaSO₄·0.5H₂O), and anhydrite (CaSO₄), but differed markedly in their textural and mineral associations. Riverside PG was primarily composed of bassanite and gypsum, with bassanite enrichment in coarse fractions and minor quartz impurities. In contrast, Bartow PG was dominated by gypsum across all fine fractions (up to 97 wt. %), with quartz reaching 36–47 wt. % in coarser fractions (+20 to –50 mesh). Minor fluorosilicate and aluminofluoride phases (Na₂SiF₆ and Ca₄AlSiSO₄F₁₃·12H₂O) were also identified. No discrete REE phases were detected by XRD, implying they are present bellow detection limit of the method. The rare earth elements (REEs) in Bartow PG were predominantly associated with iron (Fe) and iron-phosphate (Fe-P) bearing particles. Specifically, dysprosium (Dy) and terbium (Tb) were found to be linked with Fe, while yttrium (Y) and scandium (Sc) were aligned with phosphorus (P) and fluorine (F). Additionally, larger-sized particles, particularly those in the +35 and -35+100 mesh fractions, exhibited higher concentrations of P and Fe and were associated with Y, Dy, europium (Eu), and Tb. The distribution of Ra-226 was similar in Bartlow and Riverside PG, with majority of Ra-226 concentrated in the finest fractions.