Researchers from the Idaho National Laboratory and Ames National Laboratory, Ames, Iowa, USA, have published a paper in Nature Communications detailing the use of a solvent-driven fractional crystallisation process for separating Rare Earth Elements (REEs) and transition metals.
The process begins with a ground magnet, the shavings of which are put into a solution with lixiviants (a liquid used to selectively extract metals from the material). Once the desired metals are leached into the liquid, the researchers then applied the treatment process.
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 were reported to have enabled the crystallisation 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 crystallised products. Upon completion of crystallisation, the solvent can be recovered with high efficiency at ambient pressure.
The dimethyl ether-driven process uses far less energy and pressure than traditional methods, typically conducted at hundreds of degrees Celsius. Fractional crystallisation can be carried out at ambient temperatures and requires only slightly elevated pressures of around five atmospheres (for comparison, the pressure in an unopened 355 ml can of pop is 3.5 atmospheres).
‘Solvent-driven fractional crystallization for atom-efficient separation of metal salts from permanent magnet leachates’ is available here, in full.