Effects of roasting additives and leaching parameters on the extraction of rare earth elements from coal fly ash

Coal fly ash is a promising alternative source for rare earth elements (REE), which are critical materials in many technologies. REEs are entrained in the aluminosilicate glass of the fly ash particles, hindering their ability to solubilize during acid leaching. The purpose of this research was to test the effectiveness of roasting techniques to improve extraction of REEs from fly ash, determine key parameters controlling REE extraction, and understand trade-offs in reagent use. Representative coal ash samples from major U.S. coal basins (Appalachian, Illinois, and Powder River basins) were roasted using a variety of chemical additives (Na2O2, NaOH, CaO, Na2CO3, CaSO4, (NH4)2SO4). Further experiments investigated the effects of additive:ash ratio, roasting temperature, and leachate pH on REE extraction. We found that NaOH roasting often recovered >90% of total REE content, equivalent to the USGS-recommended method (Na2O2 sintering). Other additives tested recovered 70% REE extraction are a 1:1 NaOH-ash ratio and leaching with 1–2 mol/L HNO3. [Display omitted] •Roasting coal fly ash with NaOH or Na2O2 improved the leachable rare earth elements to 80–90% of the total.•Roasting with CaO, Na2CO3, CaSO4, or (NH4)2SO4 leached 90% rare earth recovery.