| Zirconium and hafnium are the key materials in the nuclear energy industry.Their nuclear properties are different and their purity requirements are very high.In nature and zirconium chemicals,hafnium content usually accounts for only 1-3 wt%of the total amount of zirconium and hafnium.However,affected by"lanthanide shrinkage",the physical and chemical properties of the two are similar.It is very difficult to separate and purify them.The industrial production of nuclear grade zirconium and nuclear grade hafnium is mainly based on traditional solvent extraction methods such as MIBK,TBP and N235.Among them,MIBK method is the priority extraction process of hafnium with less content developed in the United States.More than 2/3 of nuclear grade zirconium and hafnium was separated by MIBK method in the word today.But MIBK method requires the using of toxic thiocyanic acid or thiocyanate,which is easily decomposed into hydrogen sulfide and hydrocyanic acid in the separation process,causing serious environmental pollution.Moreover,MIBK is easy to volatilize,water solubility is large,extractant loss is serious,and wastewater treatment is difficult.The extractant of TBP method is easy to emulsify,which affects the continuous production,and requires the use of high concentrations of hydrochloric acid and nitric acid,which is easy to cause equipment corrosion.N235 method has low separation coefficient and small extraction capacity,which is not conducive to industrial production.At present,some new extractants and extraction systems have been developed for the extraction and separation of zirconium and hafnium,but most of the separation methods are preferentially extracting zirconium with a large content,and the extraction efficiency is low.Although there are also a few separation processes that give priority to hafnium extraction,the concentration of zirconium and hafnium in the raw materials treated during extraction is relatively low,which cannot meet the requirements of industrial scale.The nuclear grade zirconium and hafnium needed by our nuclear power are mainly imported from abroad and its core separation technology is still blocked.In order to ensure development of nuclear power and nuclear submarines in our country,it is urgent to realize the localization of nuclear grade zirconium and hafnium.Therefore,it is of great practical significance to develop green zirconium extraction and separation technology to preferentially extract hafnium with independent intellectual property rights.In this paper,the technological conditions for the selective extraction and separation of zirconium and hafnium by a new phosphonic acid extractor Cyanex572in a nitric acid medium were studied.The single factor variable method was used to explore the influence of factors such as the type of diluent,the acidity of the aqueous phase,the concentration of zirconium and hafnium ions,the extraction time,the concentration of the extractant,and the type and concentration of salting-out agent on the extraction process.The optimized process parameters for the extraction and separation of zirconium and hafnium were obtained as follows:the acidity of the aqueous phase liquid is 2.6 mol/L,the concentration of zirconium and hafnium ions is115 g/L,the extraction time is 15 minutes,the concentration of Cyanex572 is 10 vol%,and the concentration of the salting-out agent ammonium sulfate is 0.8 mol/L.Under this condition,toxic thiocyanate was not needed for Cyanex572 to preferentially extract hafnium.The extraction rate EHfof hafnium was 77%,the extraction rate EZrof zirconium was 17%,and the separation coefficientβwas 15.7,which is better than MIBK method.This study also found that DIBK was a better diluent,and the two-phase separation was rapid and clear in the extraction process without obvious emulsifying phenomenon.The mechanism of extraction and separation of zirconium and hafnium by Cyanex572 in nitric acid was also studied.The slope method was used to investigate the effects of zirconium and hafnium ion concentration,nitric acid concentration,and extractant concentration on the separation factor.The composition of hafnium-containing extracted into the loaded organic phase was obtained as[(Hf O2+)2·（NO3-）3]·HA2,and the reaction equation of extracting hafnium in nitric acid medium was deduced.Furthermore,analytic methods such as UV-Vis,FT-IR,Laser Confocal Raman,and Nuclear Magnetic Resonance Spectroscopy（NMR）spectroscopy were employed to analyze and clarify the extracted compounds in the organic phase before and after extraction.It was found that DIBK was only used as diluent and did not participate in the composition of the compounds.FT-IR analysis of the loaded organic phase showed that NO3-characteristic absorption peaks appeared at1554 cm-1 and 1074 cm-1 respectively,while Raman analysis of the loaded organic phase showed that NO3-characteristic absorption peaks also appeared at 1020 cm-1and 709 cm-1,thus proved that nitrate participates in the extraction reaction.Meanwhile,FT-IR analysis showed that the peak of the loaded organic phase P-O-H was red shifted,and the characteristic peak belonging to proton in P-O-H in 1H NMR spectrum of the loaded organic phase disappeared.This phenomenon proved that H+on P-O-H in Cyanex572 was exchanged with hafnium ions,and the extraction of organic relative hafnium belonged to ion exchange mechanism.At the same time,31P NMR analysis results also showed that Cyanex572 was coordinated with hafnium by oxygen on O=P-R.Therefore,the molecular structure of hafnium extracted complex in the organic phase was obtained.This research could provide theoretical guidance and technical support for developing of green zirconium hafnium solvent extraction separation technology that preferentially extracts hafnium. |
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