Comparison of extractants for the separation of TaF5 and NbF5 in different acidic media

To optimise the solvent extraction (SX) of Ta and Nb, eight different extractants were investigated, including the cation exchangers di-iso-octyl-phosphinic acid (PA) and di-(2-ethylhexyl)-phosphoric acid (D2EHPA), the neutral solvating extractant 2-thenoyl-trifluoro-acetone (TTA), and the anion exchangers tri-n-octylamine (Alamine 336), tri-octyl-methyl ammonium chloride (Aliquat 336) and 1-octanol, 2-octanol and 3-octanol. In addition, four different acids, hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4) and perchloric acid (HClO4), were used at different concentrations to determine the best combination for extraction. Using the fluoride salts of Ta and Nb (Ta(Nb)F5), the highest extraction was obtained with PA and D2EHPA, followed by Alamine 336, Aliquat 336 and then TTA and the octanols. Similarly, irrespective of the acid used, the extraction with PA and D2EHPA increased with increasing acid concentration. While extraction of both Ta and Nb was achieved with all the acids tested, and the highest selectivity (log D = 3) for the two metals was obtained in H2SO4 using 0–2 mol/dm3 and 15 mol/dm3 of PA and D2EHPA, respectively. Non-selective precipitation occurred in the absence of acid when using Alamine 336, Aliquat 336 and TTA. The optimum extraction was achieved with an extractant to metal (E/M) mole ratio of 3:1 using PA and D2EHPA as the extractant and 10 mol/dm3 H2SO4 in the aqueous phase.