Extraction of Rare Earth Elements from Fly Ash Using NaOH Hydrothermal and Ultrasound Pretreatment

Extraction of valuable and critical trace elements from coal and coal byproducts such as fly ash has been studied for over 100 years. Recently the volatility of rare earth element (REE) prices, and the closing of the only rare earth mine in the U.S., has renewed interest in trace element and REE recovery from abundant domestic coal and fly ash. Acid leaching, using 2M HCl to extract REE from a fly ash was studied with less than 10% total REE extraction. However, when a sodium hydroxide hydrothermal pretreatment was employed, a dramatic increase in REE extraction was observed. The sodium hydroxide pretreatment greatly increased the REE extraction to approximately 90% in the HCl acid leach. Sonication experiments using NaOH solution in combination with hydrothermal pretreatment were also examined and will be discussed.