Rare Earth Elements in coal fly ash and their potential recovery

Coal fly ash is a potential resource of valuable elements, such as rare earth elements (REEs), which are retained and concentrated upon combustion. Understanding REE occurrence within fly ash is vital to developing recovery methods. Some of the highest REE contents occur in fly ash derived from U.S. Appalachian Basin coals, and coals influenced by input volcanic ash are especially enriched. Leaching studies of bulk fly ash show that, as a proportion of the total REEs present, samples from eastern U.S. coals are generally less extractable than fly ash derived from lower REEs western U.S. coals. Aluminosilicate glasses formed during combustion comprise the largest mass fraction of coal fly ash. REE-enriched domains are present locally in fly ash at the nanometer scale. Annual fly ash production, combined with coal ash already in storage, makes up a large resource for potential recovery of rare earths and associated critical elements. Further extraction technology developments are needed to overcome difficulties in REE concentration and purification to produce coal-ash-derived REE materials of saleable purity.