Separation of Rare-Earth Elements by Synergistic Mixtures of Methyl(tri-n-Octyl)ammonium Nitrate and an Ammonium Di(2-Ethylhexyl)phosphoric Acid Salt

The separation of medium-heavy group of rare-earth elements along the Gd/Tb line by liquid–liquid extraction from low-acid nitrate solutions was studied. A synergistic mixture of methyl(tri-n-octyl)ammonium nitrate (TOMAN) and an ammonium di(2-ethylhexyl)phosphoric acid salt (Amm-HD2EHP) was used as an extractant. The synergistic extraction area was determined using the isomolar series method. Gel formation was observed in extraction with the mixed extractants and with an individual ammonium salt of di(2-ethylhexyl)phosphoric acid (Amm-HD2EHP) at salt concentrations higher than 0.25 mol/L. In the area of system’s stability, the highest separation factor along the Gd/Tb line was higher than 3.8. n-Octanol (OC) and tri-n-butyl phosphate (TBP) were used as modifiers to prevent gel formation.