Aqueous acid and alkaline extraction of rare earth elements from coal combustion ash

The recovery of rare earth elements (REEs) from coal combustion fly ash has recently gained attention as a possible beneficial reuse application that could provide an alternative, low-grade source of REEs in an unstable global supply market. To economically recover REEs, an efficient process needs to be developed to remove them from the ash. This study investigated different methods of leaching REEs from fly ash and other coal combustion ashes. Aqueous acid and alkaline leaching were employed on multiple types of coal fly ash samples (representing coals from three major U.S. coal basin sources) with variable leaching parameters that include extractant type (HCl, NaOH), extractant concentration, leachate-to-ash ratio, and addition of CaO during the leaching process. Acid leaching of high calcium-containing fly ashes such as samples derived from Powder River Basin coals had the highest recoveries of REEs (near 100% of total REE contents), while leaching efficiencies were much lower (<40%) for low calcium fly ashes from Appalachian and Illinois Basin coals. If the fly ashes were subjected to aqueous NaOH (to decompose alumunosilicates) and followed with dilute acid to maintain solubility of metal cations, recoveries of the REEs was up to 85% for Appalachian ashes. Overall, these results demonstrate that REEs in the Powder River Basin-based coal fly ashes are more easily leached into acidic solution than the other ashes tested. While the Appalachian Basin-based coal ashes contained the highest total REEs concentrations out of all regional coal ash sources, these ashes required more complex leaching methods that first decompose the aluminosilicate glassy matrix of the ash (e.g. alkaline digestion) prior to acid leaching.