Practical Solvent Extraction of Inert Platinum Group Metals from High Level Radioactive Wastes

Abstract

Several platinum group metals (PGMs) like Ru(III), Rh(III) and Pd(II) occur in spent nuclear fuels after power generation, and may cause various problems in vitrification of high-level radioactive wastes (HLW). Solvent extraction is a promising technique, while some of PGMs like Ru(III) and Rh(III) have issues in terms of low recovery efficiencies in a realistic operating time due to their extremely low chemical reactivity in coordination chemistry. In this study, we have demonstrated that the thermal-assisted technique can accelerate the solvent extraction of these inert PGMs in the aspect of kinetics. For instance, the extraction of Ru(III) and Rh(III) in HNO₃(aq)/1-octanol system can rapidly reach an equilibrium within 2 hours at 365 K, while their extraction always takes more than one day to reach an equilibrium at room temperature. In the aspect of thermodynamics, the extraction of Ru(III) and Rh(III) was promoted to 95% and 90% from 29% and 5%, respectively, employing a phase transfer catalyst (PTC) like bis(trifluoromethylsulfonyl)amide (Tf₂N⁻). This solvent extraction still has several concerns about the difficulty of temperature control in actual extraction contactors and safety risks arising from heating organic solvents. To solve these problems, we further established and demonstrated a realistic PGMs extraction method named as complexation-distribution separated extraction process.