CRITICAL MINERAL COMMODITIES IN PHOSPHATE MINING WASTE

The Florida Geological Survey (FGS) began a 2-year project funded through the the Earth Mapping Resources Initiative (Earth-MRI) of the United States Geological Survey (USGS) with a focus on the economic abundance of critical mineral resources in wastes associated with phosphate mining in Florida. Previous studies have shown an association between phosphate deposits and rare earth elements (REE), but the project’s interest in critical minerals goes well beyond REE and includes a list of 53 critical mineral commodities.

The mining process involves extracting the ore body and then 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. The overburden and waste rock piles are often used as backfill in the reclaimed mined areas. Later stages of beneficiation involve washing and flotation, a process that concentrates the phosphate. The process generates phosphate “slime” and froth waste that are stored in clay settling ponds, which cover tens of thousands of acres throughout Florida. The concentrated phosphate is processed into phosphoric acid by way of sulfuric acid, which ultimately generates phosphogypsum waste. This material is variably acidic and slightly radioactive and, therefore, stockpiled permanently on site. The phosphogypsum is stored in large stacks, referred to as gypsum stacks, some exceeding 400 feet above land surface.

Samples from the phosphogypsum stacks, reclaimed mined areas, and clay settling ponds have been collected and submitted to USGS labs for geochemical analyses. These analyses are used to characterize the various mine waste products and determine if they are reservoirs for critical minerals like REE. Mining waste in most of the study area is not easily accessible at the surface, thus samples were dominantly recovered through drilling sediment core via an AMS power probe direct push drilling and roto-sonic drilling. To date, over 1000 feet of sediment core has been collected from previously mined areas and clay settling ponds, as well as 300 feet of core collected from a gypsum stack. Here we present XRF and ICP-OES geochemical results for samples obtained from several waste sites throughout the central Florida phosphate mining district.