RARE EARTH ELEMENT ABUNDANCES IN FLORIDA PHOSPHATE MINING WASTE (Invited Presentation)

Processing phosphate ore results in the large-scale production of waste products throughout the mining and processing of phosphatic sands. The mining process involves extracting ore body and isolating the concentrated phosphate grains from the host lithology in a process known as beneficiation. During beneficiation, rock waste piles are generated in the early stages. Later stages of beneficiation involve washing and flotation that concentrates the phosphate. The process generates phosphatic clays that are stored in clay settling ponds, of which cover tens of thousands of acres throughout Florida. The concentrated phosphate is processed into phosphoric acid, which ultimately generates phosphogypsum waste. Phosphogypsum is mildly radioactive and stored in large gypsumstacks, some exceeding 400 feet above land surface.

The Florida Geological Survey, through the the United States Geological Survey’s (USGS) Earth Mapping Resources Initiative, has been funded to sample phosphate mine waste throughout the Central Florida mining district at previously mined areas, clay settling areas, and phosphogypsym stacks. Previous studies have shown an association between phosphate deposits and rare earth elements (REE). Samples acquired through surficial composite grab sampling and core drilling were submitted to USGS labs for geochemical analysis. Sand and clay from abandoned mine sites display the lowest abundance of REEs, with the lowest abundances associated with increasing amounts of quartz sand. Clay from settling areas have the highest measured concentration of REEs. REE content in the clay displays a strong positive correlation with U content and a weaker correlation with increasing P₂O₅. Phosphogypsum has comparable REE concentrations to what is observed in the clay from settling areas. However, phosphogyspum has much lower U and P₂O₅ content. The clay likely represents natural compositions, while the discrepancy observed in phosphogypsum is the result of element mobilization during chemical processing. The chemostratigraphy of multiple boreholes through a gypsumstack show a steady decrease in REE content with increasing depth, and maximum concentrations at 10 – 20 ft depths. These results have implications for the targeting and feasibility of extracting REEs from phosphate mining wastes in Florida.