Recovery of Rare Earth Elements from Coal Combustion Ash: Survey, Extraction, and Speciation

Abstract

This research explores the beneficial reuse of coal combustion fly ash as a source of rare earth elements (REE). We characterized fly ashes of varied geological origin, tested several extraction methods and parameters, and investigated REE location and speciation in fly ash. Total REE content in a broad sample of U.S. fly ashes were quantified using HF/HNO3 digestion, Na2O2 sintering, and HNO3 digestion. If was found that Appalachian Basin coal ashes had significantly higher total REE content than Illinois Basin or Powder River Basin ashes. However, Powder River Basin ashes had higher HNO3-extractable REE content. Sinter-based extraction methods were tested for REE recovery from fly ash. Optimal sintering conditions were found to be a 1:1 NaOH-ash ratio and 1-2 M HNO3 leaching solution. Bulk and microscale Y speciation in fly ash were compared using sequential selective extractions and x-ray absorption spectroscopy. Bulk speciation suggested Y entrained in the aluminosilicate glass phase while microscale speciation resembled trace yttrium minerals.