Extraction of rare earth elements from their oxides using organophosphorus reagent complexes with HNO3 and H2O in supercritical CO2

Direct extraction of metals from solids with complexing agents in supercritical CO2 (SC-CO2) has recently attracted interests in separation, purification, recovery, and analysis of metals. In the present study, the static/dynamic extraction of rare earth elements (Nd, Ce) from their oxides (Nd2O3, CeO2) with organophosphorus complexes with HNO3 and H2O in SC-CO2 was investigated. The static extraction efficiency of Nd from Nd2O3 with the tri-n-butylphosphate (TBP)-HNO3 complex could reach 95% under optimized experimental conditions. However, the static extraction efficiency of Nd from Nd2O3 with the trilkyl phosphine oxide (TRPO)-HNO3 complex was less than 10%, and the static extraction efficiency of Ce from CeO2 with the TBP-HNO3 complex was less than 1%. The dynamic extraction efficiency of Nd from Nd2O3 with the TBP-HNO3 complex could reach more than 97%. Effects of parameters were evaluated on the extraction efficiency to ascertain the optimum conditions. This study illustrated the extraction process using the TBP-HNO3 complex in SC-CO2 would be feasible for separation, purification, recovery, and analysis of some rare earth elements from various oxide mixtures.