Insights into non-aqueous solvent extraction of gadolinium and neodymium from ethylene glycol solution using Cyanex 572

Abstract

Abstract The reprocessing of rare earth elements (REEs) by conventional hydrometallurgical processes remains a challenge because of the difficulties of separation. Solvometallurgy is a new non-aqueous solvent extraction approach including two immiscible organic phases which has been advanced for the extraction of rare earths. The less polar organic phase (LP) was a solution of Cyanex 572 (Cy-572) as an acidic extractant dissolved in kerosene. Whereas, the more polar organic phase (MP) was ethylene glycol with a limited amount of water dissolved in metal nitrate salts and lithium nitrate as a salting-out agent. The extraction of Gd(III) and Nd(III) from the ethylene glycol solution was compared with extraction from an aqueous solution. The solvent extraction equilibrium was estimated by slope analysis method, and the composition of the Gd(III) extracted species in the less polar phase was proposed to be Gd(OH) L 2 . 2 HL ¯ . The non-aqueous system performs better separation of rare earths than the corresponding conventional extraction system. Subsequently, 0.1 mol/L HCl is the best stripping agent for both metal ions after two steps from the less polar phase, while a solution composed of 10% HF + 0.05 mol/L HCl is superior for nearly complete separation between the two elements. Eventually, the separation of binary mixtures of Gd(III) and Nd(III) is possible by this new non-aqueous solvent extraction system. A complete flow sheet process, including extraction, scrubbing and stripping steps for the separation of Gd(III) and Nd(III) was proposed.