Innovative methods to identify critical and/or strategic elements from unconventional domestic sources

Sourcing sufficient quantities of precursor chemicals at the appropriate purity can be a major challenge for scaling up production of advanced materials containing critical and/or strategic elements (many of which are imported). The USGS Minerals Program has been applying several innovative technologies for in-situ quantification of the amounts and forms of critical and/or strategic metals in potential domestic sources including historic ores, unconventional mineral deposits, and various types of solid and liquid waste streams. The goal is to identify previously unknown domestic resources of these elements and evaluate their potential for future development. In the past, technological barriers limited chemical analysis of rocks and wastes to a small suite of elements and/or to elements at concentrations greater than 1000 mg/kg. Rapid screening laser ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS) methods developed at USGS allow for the detection of critical/strategic elements in the $\mu$g/kg-mg/kg range. An example application is the discovery of potentially-economic concentrations of REE in 23 domestic phosphate deposits which were not previously considered as significant REE resources.

In addition to element concentration, the elemental form (also known as chemical species) is also an important consideration when evaluating the potential for development of a mineral resource. For example, many of the identified REE deposits contain highly insoluble REE-bearing phases that are inadequately dissolved by standard metallurgical processes. X-ray absorption spectroscopy (XAS) and Raman scattering with laser-stimulated photoluminescence are being used to: (1) identify the forms of gallium and REE in unconventional mineral deposits (acid-sulfate alteration zones); (2) track the changes in the forms of Ni, Cu, and PGE in ore after long-term reaction with the environment; and (3) identify the predominant forms of PGE in municipal biosolids (the end-product of sewage treatment). All of the above-mentioned techniques can be performed in bulk and spatially-resolved modes (1-10 micron, depending on the technique) to provide information at the appropriate spatial scale.

This research is funded by the US Geological Survey Mineral Resource Program. Synchrotron experiments were conducted at the Stanford Synchrotron Radiation Laboratory, a Department of Energy Office of Science User Facility.