Effect of organophosphorus ligands on supercritical extraction of neodymium from NdFeB magnet

Abstract

Supercritical fluid extraction is receiving growing attention for the enhanced extraction and separation of metals. A metal must be charge neutral and coordinatively ready to dissolve in non-polar supercritical carbon dioxide (sc-CO2). This can be accomplished by bonding the metal with a combination of negative and neutral ligands. Here, we investigate the effect of four organophosphorus reagents including triethyl phosphate (TEP), tri-n-butyl phosphate (TBP), tributyl phosphine oxide (TBPO), and trioctyl phosphine oxide (TOPO), on the extraction of neodymium from a neodymium-iron-boron magnet in sc-CO2. The COSMO-vac model is used to predict the solubility of these reagents in sc-CO2 showing the order of TEP > TBPO ~ TBP > TOPO. The stoichiometry of Nd-ligand complexes is determined via UV-Vis spectroscopy, showing a 1:1 Nd–TEP, 1:3 Nd–TBP, 1:3 Nd-TBPO, and 1:4 Nd–TOPO complex chemistry. Highest neodymium extractions are achieved with TEP followed by TBP, TBPO, and TOPO, respectively. This is due to the increase in coordination number which results in more hydrophobic interactions between aliphatic functionalities, leading to larger micellar assemblies with lower solubility in sc-CO2, which result in lower extraction efficiency.