Rare earth element extraction from pretreated bastnäsite in supercritical carbon dioxide

Rare earth elements are a critical component in many clean energy technologies. Extraction of individual rare earth elements from natural ores or recycled material is challenging due to the very similar chemical properties across the lanthanide series. Supercritical carbon dioxide has emerged in recent years as a possible extraction medium for rare earth elements, due to its tunability and selectivity as a solvent. In this study, rare earth elements were recovered from bastnäsite concentrate using supercritical carbon dioxide extraction with nitric acid/tributyl phosphate adducts. Two bastnäsite pretreatment methods were used to render the rare earth elements amenable to recovery: 1) dry roasting of the source material at 730 °C for 3 h, and 2) decomposition with 50% sodium hydroxide solution at 150 °C for 4 h. These pretreated powder samples were extracted in supercritical carbon dioxide at 34 MPa and 65 °C, with kinetic samples obtained at 15–30 min intervals. A range of tributyl phosphate/nitric acid adduct compositions (from 2 mol/L H+ to 6 mol/L H+) were used in order to determine the effect of adduct composition on recovery rate. The results showed the fastest extraction with an adduct containing approximately 4 mol/L H+. Adducts with higher acidity showed reduced extraction of cerium, praseodymium, and neodymium. This could be due to the formation of aqueous droplets which dissolve rare earth elements and create an equilibrium limitation, or due to competition between the rare earth nitrates and nitric acid for coordination with tributyl phosphate. Extraction with various adduct concentrations in supercritical CO2 showed the expected increase in reaction rate with increased adduct concentration. For the 4 mol/L H+ adduct at 5.0 mol% adduct concentration, roasted bastnäsite recoveries were 72% for La, 96% for Ce, 88% for Pr, and 90% for Nd after 120 min. For 4 mol/L H+ adduct at 5.1 mol% adduct concentration, NaOH digested bastnäsite recoveries were 93% for La, 100% for Ce, 99% for Pr, and 101% for Nd after 90 min. Though further research is needed, these results are a key step in demonstrating applicability of supercritical extraction to rare earth element ores.