Due to rising demand and widespread supply-chain turmoil, researchers around the world are investigating creative methods to obtain rare-earth elements (REEs) and other critical minerals and metals (CMMs). The ash from combusting coal has previously been seen as a potential source for these materials, and now a new technology developed by the National Energy Technology Laboratory (NETL; www.netl.doe.gov) enables significantly more efficient mineral recovery from coal ash than previous methodologies. “The Targeted Rare Earth Extraction (TREE) process takes advantage of how REEs and other CMMs like scandium occur in calcium-rich coal ash, which allows certain portions of the ash to be selectively dissolved. By leaching the ash in stages, or step-leaching, different materials can be targeted,” explains Thomas Tarka, director of NETL’s Minerals to Materials Supply Chain research facility. “Mild mineral acids are added to the ash to reduce pH to the optimal levels for recovering desired materials, and this pH is maintained until the targeted phase is dissolved. Then the REEs and CMMs are separated from the leachate using a traditional solvent-extraction step. The TREE process is performed at ambient temperature and pressure, reducing equipment and energy costs. Also, since the process targets only certain phases without dissolving the entirety of the ash, there is residual, undissolved ash left over from the leaching process. We are exploring how this and other process outputs, notably the leftover acidic liquor streams, can be reused in the process or repurposed as a saleable product for commercial use. If successful, we’ll have further reduced the costs and waste generated by this process,” says Tarka.
“The TREE process is designed to only target a limited number of phases in the ash — first, the alkaline portion, and then those that host the REEs and CMMs. The ability to first dissolve and separate the alkaline phase from the remainder of the leachate is important because those components can complicate downstream purification. This compares favorably to other practices currently designed to liberate REEs and CMMs from other types of coal ash, which require additional processing steps to crack a glassy matrix in the ash that encapsulates the REEs and CMMs, and generally requires the entirety of the ash to be dissolved. Therefore, chemical and processing costs are reduced compared to REE extraction processes for other types of coal ash, both in terms of acid consumption and the costs associated with cracking the REE phase,” says Tarka.
The TREE process has been demonstrated as effective not only with fly ash, but also the bottom ash produced from the combustion of coal from the expansive Powder River Basin reserve in Montana and Wyoming, and NETL is currently evaluating additional feedstocks. A TREE pilot plant has been operated at the Energy Capital Economic Development (ECED) Center in Gillette, Wyo. in partnership with the University of Wyoming Center for Economic Geology Research and other municipal and state entities. Plans for further scaleup are being considered.