From the Ashes: Coal Ash May Offer Rich Source of Rare Earth Elements

For most of history, the world got along fine without the rare earth elements. We knew they existed, we knew they weren’t really all that rare, and we really didn’t have much use for them — until we discovered just how useful they are and made ourselves absolutely dependent on them, to the point where not having them would literally grind the world to a halt.

This dependency has spurred a search for caches of rare earth elements in the strangest of places, from muddy sediments on the sea floor to asteroids. But there’s one potential source that’s much closer to home: coal ash waste. According to a study from the University of Texas Austin, the 5 gigatonnes of coal ash produced in the United States between 1950 and 2021 might contain as much as $8.4 billion worth of REEYSc — that’s the 16 lanthanide rare earth elements plus yttrium and scandium, transition metals that aren’t strictly rare earths but are geologically associated with them and useful in many of the same ways.

The study finds that about 70% of this coal ash largesse could still be accessible in the landfills and ponds in which it was dumped after being used for electrical generation or other industrial processes; the remainder is locked away in materials like asphalt and concrete, where it was used as a filler. The concentration of REEYSc in ash waste depends on where the coal was mined and ranges from 264 mg/kg for Powder River coal to 431 mg/kg for coal from the Appalachian Basin. Oddly, they find that recovery rates are inversely proportional to the richness of the ash.

The study doesn’t discuss any specific methods for recovery of REEYSc from coal ash at the industrial scale, but it does reference an earlier paper that mentions possible methods we’ve seen before in our Mining and Refining series, including physical beneficiation, which separates the desired minerals from the waste material using properties such as shape, size, or density, and hydrometallurgical methods such as acid leaching or ion exchange. The paper also doesn’t mention how these elements accumulated in the coal ash in the first place, although we assume that Carboniferous-period plants bioaccumulated the minerals before they died and started turning into coal.

Of course, this is just preliminary research, and no attempt has yet been made to commercialize rare earth extraction from coal ash. There are probably serious technical and regulatory hurdles, not least of which would be valid concerns for the environmental impacts of disturbing long-ignored ash piles. On the other hand, the study mentions “mine-mouth” power plants, where mines and generating plants were colocated as possibly the ideal place to exploit since ash was used to backfill the mine works right on the same site.