Process And Mechanism Of Extraction And Separation Of Zirconium And Hafnium With TOPO-N1923 Synergistic Extraction System

Zirconium and hafnium are important strategic resources,and their nuclear properties are different.In nature,zirconium and hafnium tend to co-exist.Generally,hafnium accounts for 1 to 3%of the total zirconium and hafnium.However,the content of hafnium in nuclear-grade zirconium must be no more than 100 ppm.The content of zirconium in nuclear-grade hafnium must be no more than 2%.Therefore,the high-efficiency separation of zirconium and hafnium is the key to the production of nuclear-grade zirconium and hafnium.Solvent extraction separation technology is the main way for industrial separation of zirconium and hafnium.At present,more than 2/3 of nuclear grade zirconium in the world is extracted and separated by MIBK-HSCN system.However,this system has the shortcomings of MIBK’s high toxicity,easy loss,poor operating environment,and serious water pollution.Previous studies have shown that neutral phosphorous extractant can extract hafnium with less content from zirconium and hafnium solution preferentially.However,the concentration of thiocyanic acid used in this system is as high as 3 mol/L,and emulsification is easy to occur during the extraction process,and the phase separation effect is not good.Synergistic extraction system can not only enhance the stability of the extraction compound in the organic phase,but also eliminate the third phase and emulsification,and increase the extraction reaction rate.Therefore,the secondary carbon primary amine N1923 was selected as a co-extraction reagent,which forms a synergistic extraction system with the neutral phosphorus-containing extractant TOPO to selectively extract hafnium with less content in the zirconium-hafnium feed liquid in the HSCN medium.The system further improves the separation efficiency of zirconium and hafnium.At the same time,with the aid of the Lewis alkalinity of N1923,the dissolution stability of HSCN in the organic phase is improved,and problems such as difficulty in emulsification and phase separation caused by the decomposition of HSCN are avoided.First of all,orthogonal experiment and single factor experiment are used to study the process for the selective separation of hafnium from the high-concentration zirconium-hafnium feed liquid(C_Zr+Hf=110 g/L)in the TOPO-N1923 synergistic extraction system.The factors such as extractant concentration,feed liquid thiocyanic acid concentration,pre-saturated thiocyanic acid concentration,water phase acidity,salting-out agent type and concentration,extraction time,and comparison on the performance of selective extraction of hafnium are explored.The results show that the optimized process parameters for separation of zirconium and hafnium by co-extraction system are:the TOPO concentration in the organic phase is 10 vol%,the N1923concentration is 6 vol%,the thiocyanic acid concentration in the feed solution is 2mol/L,the pre-saturated thiocyanic acid concentration is 2 mol/L,the acidity of the feed solution is 2 mol/L,the salting-out agent ammonium sulfate concentration is 0.5 mol/L,the extraction time is 12 minutes,the phase volume ratio is O/A=2/1.Under optimized process parameters,the extraction rate of hafnium by TOPO-N1923 synergistic extraction system can reach 86.3%,the extraction rate of zirconium is 23.1%,and the separation coefficient of hafnium and zirconium can reach 20.9,which is much higher than that of traditional MIBK method（the separation coefficient is about 10）.Under the optimal process parameters,the concentration of thiocyanate in aqueous phase was only 2/3 of that in MIBK method,the organic phase and the water phase are separated clearly,and the difficulty of emulsification and phase separation is inhibited.At the same time,the acidity of the feed solution and the concentration of the salting-out agent ammonium sulfate are significantly reduced,which can further reduce the environmental pollution problem.Secondly,the washing and stripping performances of sulfuric acid,hydrochloric acid,ammonium carbonate and ammonium bicarbonate solution on the synergistic extraction system loaded organic phase of zirconium and hafnium were studied and compared.The results show that sulfuric acid is suitable as a detergent before loading organic reverse extraction.When the sulfuric acid concentration is 1.25 mol/L,the elution rate of zirconium is 45.9%,and the elution rate of hafnium is only 3%.In the concentration range of 0～3 mol/L,the elution effect of hydrochloric acid solution on zirconium and hafnium is not good,not exceeding 25%.Compared with acidic solution,ammonium carbonate has stronger stripping ability.When its concentration reaches 2mol/L,the stripping rate of zirconium and hafnium is the highest,reaching 90.7%and88.6%,respectively.Finally,the slope analysis method,Fourier infrared spectroscopy（FT-IR）,nuclear magnetic resonance spectroscopy（NMR）and other modern instrumental analysis methods were used to study the mechanism of selective extraction of hafnium with TOPO-N1923 system in HSCN medium.The effects of TOPO concentration,N1923concentration,H⁺concentration,SCN^-concentration,SO₄^2-concentration on the ratio of hafnium and zirconium were investigated.The composition of the raffinate of hafnium in the loaded organic phase is determined as4Hf O²⁺·2H⁺·4SCN^-·3SO₄^2-·4TOPO·RNH₂,the composition of the raffinate of zirconium is determined as 2Zr O²⁺·SCN^-·SO₄^2-·Cl^-·TOPO.The results of infrared spectroscopy showed that DIBK does not participate in the formation of extractive compounds,only serves as a diluent.The two new absorption peaks of S-C≡N-at 2025cm^-1 and 2072 cm^-1 in the pre-saturated organic phase shifted to 2012 cm^-1 and 2060cm^-1 in the infrared spectrum of the loaded organic phase,respectively,and the intensity of the peaks becomes higher,indicating that S-C≡N-is coordinated with zirconium and hafnium ions.The peak at 611 cm^-1 is the characteristic peak of SO₄^2-,indicating the presence of SO₄^2-in the extraction compound.In addition,in the infrared spectrum of the pre-saturated organic phase,new characteristic peaks of NH₃⁺appeared at 1518 cm^-1 and 1618 cm^-1.The infrared spectrum of the loaded organic phase shifted to 1508 cm^-1 and 1607 cm^-1 respectively.And the proton peak of NH₃⁺appearing at 7.68 ppm in the NMR ¹H spectrum of the pre-saturated organic phase shifted to 7.82 ppm in the NMR ¹H spectrum of the loaded organic phase,indicating that N1923 combines H⁺protonation to generate NH₃⁺in the pre-saturation process,and participates in the coordination reaction in the form of NH₃⁺.At the same time,the P=O peak in the loaded organic phase in the ³¹P spectrum of NMR has shifted from 58.74 ppm in the pre-saturated organic phase to 77.09 ppm,indicating that the P=O of TOPO is coordinated with zirconium and hafnium ions.