Researchers at Idaho National Laboratory have developed a dimethyl ether-driven fractional crystallization process for separating rare earth elements and transition metals. The process has been successfully applied in the treatment of rare earth element-bearing permanent magnet leachates as an atom-efficient, reagent-free separation method.

An open-access paper on their work is published in Nature Communications.

Using ~5 bar pressure, the solvent was dissolved into the aqueous system to displace the contained metal salts as solid precipitates. Treatments at distinct temperatures ranging from 20–31 °C enable crystallization of either lanthanide-rich or transition metal-rich products, with single-stage solute recovery of up to 95.9% and a separation factor as high as 704. Separation factors increase with solution purity, suggesting feasibility for eco-friendly solution treatments in series and parallel to purify aqueous material streams.

Staged treatments are demonstrated as capable of further improving the separation factor and purity of crystallized products. Upon completion of a crystallization, the solvent can be recovered with high efficiency at ambient pressure. This separation process involves low energy and reagent requirements and does not contribute to waste generation.

—Stetson et al.

a Schematic depicting the experimental apparatus whereby DME gas is sparged into an aqueous solution at elevated pressure, permitting dissolution of DME into the liquid. Reaction temperature is controlled via a water bath, recirculation is carried out through a gear pump, and crystallization of metal salts occurs on the nucleation scaffold. b Photograph of the experimental apparatus during treatment of the Sm-Co leachate. c Process schematic depicting the DME-FC solid-liquid separation followed by a gas-liquid separation to recover and reuse DME with high efficiency. d Photograph of the experimental apparatus after FC of CoSO₄ from the leachate, showing the visible change in CoSO₄ concentration concurrent with crystal growth on the nucleation scaffold. Stetson et al.

The process begins with a magnet that’s no longer useful, which is cut and ground into shavings, said Caleb Stetson, lead author. The magnet shavings are then put it into a solution with lixiviants, a liquid used to selectively extract metals from the material. Once the desired metals are leached from the material into the liquid, the researchers can then apply the treatment process.

The dimethyl ether-driven process uses far less energy and pressure than traditional methods, typically conducted at hundreds of degrees Celsius. Fractional crystallization can be carried out at ambient temperatures and requires only slightly elevated pressures of around five atmospheres. In comparison, the pressure in an unopened 12-ounce can of soda is 3.5 atmospheres. The lower energy and pressure needs also save money.

Competing technologies also use added chemical “reagents” to drive precipitation and other separations, which inevitably become additional waste products with financial and environmental consequences. This is not the case with dimethyl ether-based fractional crystallization.

Resources

• Stetson, C., Prodius, D., Lee, H. et al. (2022) “Solvent-driven fractional crystallization for atom-efficient separation of metal salts from permanent magnet leachates.” Nat Commun 13, 3789 doi: 10.1038/s41467-022-31499-7