Direct, Acid-Free Uranium Purification from Solid Analytical Waste by Supercritical Carbon Dioxide Extraction with Incinerable <i>N</i>,<i>N</i>-Dialkyl Amide Adducts

Sustainable method development for uranium recovery from secondary lean sources (scraps, waste matrices) is a major thrust area for achieving net zero by the nuclear industry. Herein we demonstrated a simple, two-step crude solid (analytical waste, U content ∼24%) to pure solid (ammonium diuranate, near-stoichiometric U content ∼74%) transformation aided by direct, acid-free supercritical carbon dioxide (SC CO2) extraction followed by uranium precipitation from the compact extract. Augmenting the green quotient, this is the first systematic study with incinerable N,N-dialkyl amide adducts, providing insight into relevant physical properties (density and viscosity variation with temperature) as well as exploring the relation between their composition (nitric acid and water content) and uranium extraction-purification. Temperature and pressure affected the SC CO2 extraction efficiency by influencing the solubility of metal-amide complexes. In comparison to adducts of other amides and tributyl phosphate (TBP), N,N-dibutyl octanamide (DBOA) yielded the highest uranium extraction (>88%) and purification with the highest separation factors with respect to metal ion impurities, which were further enhanced with a higher matrix amount. Spectral examination (IR, UV–vis) revealed the nature of the uranium-amide complex in the SC CO2 extract. Ammonium diuranate, obtained from direct precipitation from the concentrated SC CO2 extract, could be heated to 773 K for uranium oxide production. A comparative study of the present scheme with the multistep, chemical-intensive conventional route (overall ∼86% U extraction efficiency, SFFe ∼ 19) highlighted highly efficient uranium extraction-purification (overall ∼88% U extraction efficiency, SFFe ∼ 46) by employing the green amide adduct with minimization of process steps (single-step dissolution-complexation).