The selective extraction of Hf from Zr(Hf)Cl4(aq) using organophosphorous derivatives

Abstract

While traditional solvent extraction (SX) separations of Zr and Hf focus on the preferential extraction of Zr (majority solute) from Hf (minority solute), the aim of this study was to ascertain whether the selective extraction of Hf from Zr is feasible and to determine the most important variables related to the extraction and separation process. The selective extraction was studied using SX and membrane based solvent extraction (MBSX). Two acidic feed solutions (HNO3 and H2SO4) were explored to determine which favours selective Hf extraction, for which 9 extractants were optimised for both acid and extractant concentrations. The aqueous feed consisted of 1g/L ZrCl4 + 3 wt. % HfCl4, dissolved in the corresponding acidic solution and aged 24 ± 2 hours prior to extraction. The solvent was prepared by diluting the extractant in cyclohexane with 5 % v/v 1-octanol added as modifier. The SX experiments were done using an organic to aqueous volume ratio of 1:1 by contacting 25 ml of each phase in polypropylene containers at 270 rpm in a mechanical agitator. The MBSX experiments were conducted by contacting 600 ml each of the solvent and feed (1:1) within a 2.5x8” Liqui-Cel™ Extra-Flow membrane. The experiments were run for 120 minutes at 24 ± 1°C. A preliminary study was conducted in HNO3 to ascertain whether this acidic medium can replace the more corrosive sulfate medium (indicated by literature) by means of the addition of salting-out agents (1.5 M NaCl, NaNO3, Na2SO4 or NaClO4.H2O) to the acidic feed solutions. In HNO3, Zr was preferentially extracted while Hf extraction increased relative to Zr at lower acidities. The addition of nitrates and perchlorates resulted in the preferential repression of Hf extraction with chloride increasing Zr and Hf extraction slightly. Sulfate anions repressed Zr extraction and were therefore found to be suitable for selective Hf extraction. Hence further studies were conducted in H2SO4, where it was found that 6 out of the 9 extractants (D2EHPA, Ionquest 801, Cyanex 272/302/301 and 923) exhibited selective Hf extraction. Cyanex 301GN exhibited the highest Hf selectivity in SX (Δ𝐻𝑓−𝑍𝑟 = 23.9 %) at 0.5 M H2SO4 with 47.4 % Hf and 23.5 % Zr extraction. The optimised SX systems were tested in a MBSX system to compare extraction performance. D2EHPA, Ionquest 801 and Cyanex 923 exhibited the highest extraction percentages (%Hf/%Zr: 99.4/95.7, 96.6/89.9 and 94.1/87.7 respectively) with Cyanex 923 showing the least deviation from the SX data. For D2EHPA and Ionquest 801 higher extractions were obtained in MBSX (%Hf/%Zr: 99.4/95.7 and 96.6/89.9 respectively) compared to SX (%Hf/%Zr: 87.2/70.6 and 58.4/43 respectively). Cyanex 272 displayed slow mass transfer kinetics and did not reach the SX determined equilibrium in 120 min. Cyanex 302 and 301GN exhibited enhanced extraction and selectivity compared to SX with Cyanex 301GN displaying the best performance (Δ𝐻𝑓−𝑍𝑟 = 30.3 % with 88.6 % Hf and 58.3 % Zr extraction). These results indicate that it is possible to selectively extract Hf from Zr in low concentration acidic sulfate solutions (0.35-0.5 M) even at feed concentrations of ≥ 1g/L Zr(Hf)Cl4 and high Zr:Hf ratios (97:3). Extraction and separation may be further increased by exploring the influential variables further through more detailed testing thereof in smaller variable change increments.