Selective membrane based solvent extraction of Hf from a (NH4)3Zr(Hf)F7 solution

Abstract

Zr and Hf, which co-exist in nature, have similar chemical properties, except for their Zr and Hf, which co-exist in nature, have similar chemical properties, except for their absorbance of neutrons which implies that Zr must contain < 100 mg.L-1 before it can be used as fuel cladding in nuclear reactors. In a newly proposed process, where Necsa uses a plasma and fluoride chemical process to produce Zr from Zircon, a (NH4)3Zr(Hf)F7 complex is formed. It was the aim of this study to investigate the solvent extraction (SX) based separation of Zr and Hf using the (NH4)3Zr(Hf)F7 complex as feed stock. The following parameters were investigated in this study: i) the acid type (HNO3, HCl, H2SO4), and concentration (0 – 8 mol.dm-3), ii) the extractant type (amine- and phosphorus-based extractants) and concentration (0 – 10 wt %), iii) the stripping compound (H2SO4 and NaCl for the amine-based extractions and H2SO4, (NH4)3CO3, CaCl2 and C2H2O4 for the phosphorus-based extractions) and concentration (0 – 2 mol.dm-3), iv) the ageing of the feed solution, v) the time to reach equilibrium during SX experiments and vi) the influence of single vs binary metal complexes on extractions. Finally the suitability of membrane based solvent extraction (MBSX) was investigated for the phosphorus-based extraction process. For the amine-based extractions, Alamine 336, Alamine 300, Alamine 308, Aliquat 336 and Uniquat 2280 were selected and evaluated. While Alamine 336 and Aliquat 336 yielded the highest extractions, the most promising combinations of solvent extraction (Zr = 60 %, Hf = 41 %) and selectivity (19 %) was obtained when extracting with 9 wt% Alamine 336 from a 0.5 mol.dm-3 H2SO4 solution. When stripping from the loaded Alamine 336, it was found that nearly 100 % of the Zr and 95 % of the Hf could be recovered when stripping with 0.01 mol.dm-3 H2SO4. On the other hand, a highly selective 46 % of the Hf and almost no Zr was stripped when using 2 mol.dm-3 NaCl confirming the suitability of both these stripping liquors depending on the required need. D2EHPA, Dio-PA and Ionquest 801 were selected as examples of the phosphorus-based extractants. For all three extractants both in HCl and H2SO4, more than 90 % Zr and Hf was extracted. Ageing of the feed solution had no effect on the extraction efficiency while the equilibrium time was found to be 30 minutes. When optimising the E/M ratio, it was found that 9 wt% D2EHPA gave the best results (82 % Hf and 48 % Zr extraction) when extracting from a 4 mol.dm-3 H2SO4 solution. When stripping from this loaded organic phase, the best results were obtained with 1.2 mol.dm-3 (NH4)2CO3, which gave a v stripping of 53 % Zr and nearly 100 % Hf, 2.0 mol.dm-3 CaCl2, which gave a stripping of 75 % Zr and < 1 % Hf and 1.2 mol.dm-3 C2H2O4, which gave a stripping of 93 % Zr and nearly 100 % Hf. When extracting with 9 wt% D2EHPA using MBSX, a 69 % Zr and 83 % Hf extraction was obtained. The membrane based stripping with CaCl2 yielded 54 % Hf and almost 0 % Zr stripping, while C2H2O4 yielded 74 % of both Zr and Hf. However, both the extraction and the stripping took considerable longer time to reach equilibrium when using MBSX. From these results it is clear that it was possible to extract Zr and Hf from the (NH4)3Zr(Hf)F7 complex. Both the amine- and phosphorus-based extractants have specific advantages. For example, the amine process is Zr-selective, while the phosphorus process is Hf-selective. When combining the extraction, scrubbing and stripping steps using Alamine 336, 53.3 % of Zr with a purity of 99.0 % was obtained. When combining the extraction, scrubbing and stripping steps using D2EHPA, a combined 97.2 % Zr yield, again with a purity of 99 % in both product streams, were obtained showing that the D2EHPA based extraction yielded significantly more of the purified Zr. When using MBSX, 4.5 % less Zr product was obtained over the two product streams while the purity decreased with 3 % compared to the traditional SX when using D2EHPA. Keywords: Solvent extraction, membrane based solvent extraction, amine-based extractants, phosphorus-based extractants, zirconium, ha absorbance of neutrons which implies that Zr must contain < 100 mg.L-1 before it can be used as fuel cladding in nuclear reactors. In a newly proposed process, where Necsa uses a plasma and fluoride chemical process to produce Zr from Zircon, a (NH4)3Zr(Hf)F7 complex is formed. It was the aim of this study to investigate the solvent extraction (SX) based separation of Zr and Hf using the (NH4)3Zr(Hf)F7 complex as feed stock. The following parameters were investigated in this study: i) the acid type (HNO3, HCl, H2SO4), and concentration (0 – 8 mol.dm-3), ii) the extractant type (amine- and phosphorus-based extractants) and concentration (0 – 10 wt %), iii) the stripping compound (H2SO4 and NaCl for the amine-based extractions and H2SO4, (NH4)3CO3, CaCl2 and C2H2O4 for the phosphorus-based extractions) and concentration (0 – 2 mol.dm-3), iv) the ageing of the feed solution, v) the time to reach equilibrium during SX experiments and vi) the influence of single vs binary metal complexes on extractions. Finally the suitability of membrane based solvent extraction (MBSX) was investigated for the hosphorus-based extraction process. For the amine-based extractions, Alamine 336, Alamine 300, Alamine 308, Aliquat 336 and Uniquat 2280 were selected and evaluated. While Alamine 336 and Aliquat 336 yielded the highest extractions, the most promising combinations of solvent extraction (Zr = 60 %, Hf = 41 %) and selectivity (19 %) was obtained when extracting with 9 wt% Alamine 336 from a 0.5 mol.dm-3 H2SO4 solution. When stripping from the loaded Alamine 336, it was found that nearly 100 % of the Zr and 95 % of the Hf could be recovered when stripping with 0.01 mol.dm-3 H2SO4. On the other hand, a highly selective 46 % of the Hf and almost no Zr was stripped when using 2 mol.dm-3 NaCl confirming the suitability of both these stripping liquors depending on the required need. D2EHPA, Dio-PA and Ionquest 801 were selected as examples of the phosphorus-based extractants. For all three extractants both in HCl and H2SO4, more than 90 % Zr and Hf was extracted. Ageing of the feed solution had no effect on the extraction efficiency while the equilibrium time was found to be 30 minutes. When optimising the E/M ratio, it was found that 9 wt% D2EHPA gave the best results (82 % Hf and 48 % Zr extraction) when extracting from a 4 mol.dm-3 H2SO4 solution. When stripping from this loaded organic phase, the best results were obtained with 1.2 mol.dm-3 (NH4)2CO3, which gave a stripping of 53 % Zr and nearly 100 % Hf, 2.0 mol.dm-3 CaCl2, which gave a stripping of 75 % Zr and < 1 % Hf and 1.2 mol.dm-3 C2H2O4, which gave a stripping of 93 % Zr and nearly 100 % Hf. When extracting with 9 wt% D2EHPA using MBSX, a 69 % Zr and 83 % Hf extraction was obtained. The membrane based stripping with CaCl2 yielded 54 % Hf and almost 0 % Zr stripping, while C2H2O4 yielded 74 % of both Zr and Hf. However, both the extraction and the stripping took considerable longer time to reach equilibrium when using MBSX. From these results it is clear that it was possible to extract Zr and Hf from the (NH4)3Zr(Hf)F7 complex. Both the amine- and phosphorus-based extractants have specific advantages. For example, the amine process is Zr-selective, while the phosphorus process is Hf-selective. When combining the extraction, scrubbing and stripping steps using Alamine 336, 53.3 % of Zr with a purity of 99.0 % was obtained. When combining the extraction, scrubbing and stripping steps using D2EHPA, a combined 97.2 % Zr yield, again with a purity of 99 % in both product streams, were obtained showing that the D2EHPA based extraction yielded significantly more of the purified Zr. When using MBSX, 4.5 % less Zr product was obtained over the two product streams while the purity decreased with 3 % compared to the traditional SX when using D2EHPA.