Emsbo, et al., (2015) reported that the critical role of rare earth elements (REEs), particularly heavy REEs (HREEs), in high-tech industries has created a surge in demand that is quickly outstripping known easily mineable global supply and has triggered a worldwide scramble to discover new sources. The chemical analysis of 23 sedimentary phosphate deposits (phosphorites) in the United States demonstrates that they are significantly enriched in REEs.
Leaching experiments using dilute H2SO4 and HCl, extracted nearly 100% of their total REE content and show that the extraction of REEs from phosphorites is not subject to the many technological and environmental challenges that vex the exploitation of many identified REE deposits. Our data suggest that phosphate rock currently mined in the United States has the potential to produce a significant proportion of the world's REE demand as a byproduct. Importantly, the size and concentration of HREEs in some unmined phosphorites dwarf the world's richest REE deposits.
Secular variation in phosphate REE contents identifies geologic time periods favorable for the formation of currently unrecognized high-REE phosphates. The extraordinary endowment, combined with the ease of REE extraction, indicates that such phosphorites might be considered as a primary source of REEs with the potential to resolve the global REE (particularly for HREE) supply shortage.
The authors concluded that oceanic REE contents in phosphorites vary in a secular fashion as a result of changes in the redox conditions of the oceans. Consequently, the hypothesis that the relative abundances of REEs are a proxy for redox cycling of metals, and may indicate episodes of phosphogenesis merits additional study. Of particular significance to REE exploration, however, is the consistency of REE abundances within individual time horizons and that these may identify time periods, like the Late Mississippian, Devonian, and Ordovician, [and late Cambrian in Australia] that were favorable for the formation of phosphorites with high-REE abundances.
The authors' evaluation indicates that the concentrations and quantities of HREEs in U.S. phosphorites exceed those of all known resources. Their contained REEs are entirely hosted in francolite and, are nearly 100% recoverable, using the same process currently used to leach and produce the world's phosphate supply. The beneficiation, dissolution, and production of sedimentary phosphate is sufficiently low-cost to produce enough phosphate to fertilize the fields that feed the world's population.
In addition, the REE content in even the lowest grade of these deposits exceeds the current value of the phosphate itself. Thus, the extraordinary endowment of easily extracted REEs hosted in environmentally friendly phosphorites indicates they are a REE source with the potential to meet the global REE demand.
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