GEOCHEMICAL CHARACTERIZATION OF RARE EARTH ELEMENTS IN COAL COMBUSTION BYPRODUCTS UTILIZING SYNCHROTRON TECHNOLOGY

Coal combustion by-products (CCBs) are being examined as a potential domestic resource for rare earth element (REE) recovery. In particular, fly ash is being explored due to its relatively abundant REE content, low-to-no-cost availability as a waste product, fine particle size, and enrichment with critical REEs. In order to develop an effective protocol for REE recovery, this study characterized speciation and distribution of REE in CCBs collected from various states.

Initial geochemical based extraction results demonstrate that heavy rare earth elements (HREE) are likely bound in different forms than light rare earth elements (LREE). BCR 4-step sequential extractions showed 5-15% REEs in ash samples were associated with reducible fractions (e.g., Fe and Mn oxides, phosphates) and 70-90% of REEs in these samples were still non-extractable (e.g., associated with glass phase, and/or aluminosilicate matrix); while 95-99% REEs in coal reject samples were non-extractable. Further sequential extractions are in progress to better understand the type of phases where the REEs are extracted in different types of CCBs.

Analyses via synchrotron X-ray Fluorescence (XRF) at the Stanford Synchrotron Light Source (SSRL) were utilized to find areas of REE enrichment in CCB samples, then micro X-ray Absorption Near Edge Structure (XANES) was used at the Ce L_III absorption edge to study REE binding environment. Ce was studied due to its concentration in the samples, different oxidation states (3+ and 4+), and as a proxy for other LREEs. Ce XANES spectra were collected at small hot spots within the samples and compared against a variety of standards. XRF mapping and Ce XANES results suggest the presence of REE phosphates, oxides, and sulfates in the fly ash and bottom ash samples analyzed. These analyses suggest, the fly ash sample consists primarily of a Ce (III) sulfate phase; while the bottom ash analyzed contained both Ce (IV) oxide and Ce (III) phosphate. The XRF mapping also confirmed separate trends between LREE and HREE.