Mining value from waste: Scandium and rare earth elements selective recovery from coal fly ash leach solutions

Global demand for rare earth elements (REEs) has been on the rise due to recent rapid technological advancements. Even though most REEs are mainly recovered from rich ores and mineral deposits, recovery of REEs from secondary sources has attracted attention. However, low metal concentration, complex matrices and high levels of other impurities in mine wastes and tailings have remained the toughest challenges to overcome. The presence of low amount of valuable metals such as Sc requires utilization of very selective separation techniques such as ion exchange. A series of ion exchange resins (chelating, solvent impregnated and strongly acidic) were studied in the recovery of Sc from synthetic and actual pregnant leach solutions obtained from acid-leaching of coal fly ash. To the best of our knowledge, this paper exhibits the first attempt in selective recovery (from other REEs as well as major Fe and Al contaminations) of Sc in coal fly ash using ion exchange separation technique. With maximum capacity of ∼24 mg Sc/g resin, Sc was adsorbed quickly on phosphorus containing Lewatit® VP OC 1026 and TP 272 resins even in the presence of high levels of potentially interfering Fe3+ and Al3+ ions. Full elution of the adsorbed REEs from VP OC 1026 was achieved with 5 M H2SO4, with only minor Sc removal. While elution of Sc from these chelating resins has been reported to be challenging, Sc could be eluted almost completely with 2 M NH4F. Recovery of REEs with 5 M H2SO4 from TP 272 resulted in co-elution of only 7% of the adsorbed Sc while the rest could be eluted efficiently with 6 M HNO3.