Sublimation Kinetics of Zirconium Tetrafluoride

Abstract

Abstract.An important step of a new process being developed for the beneficiation of the mineral zircon (Zr(Hf)SiO 4 ) to produce nuclear grade zirconium (Zr) metal, is the separation of the Zr from the hafnium (Hf). Zr ores typically contain between 1 and 3 % Hf, whereas the use of Zr metal in the nuclear industry requires a Hf content <100 ppm, owing to its relatively high neutron-capture cross section. The separation step is therefore key in the preparation of nuclear grade Zr, which is considered to be very difficult due to the various similarities in their chemical properties.The preparation of hafnium free zirconium relies on traditional wet separation systems, for example solvent extraction systems. In contrast to the traditional aqueous chloride systems, Necsa focusses on dry fluoride-based processes. Dry processes have the advantage of producing much less hazardous chemical waste.In the work reported here, separation is achieved by sublimation/de-sublimation in the tetrafluoride form. The tetrafluoride is prepared by fluorination of plasma dissociated zircon (PDZ or Zr(Hf)O 2 •SiO 2 ) with ammonium bifluoride (ABF). The separation involves the selective sublimation of the two tetrafluorides in an inert atmosphere under controlled conditions, and subsequent similarly selective desublimation.An accurate estimation of the sublimation rates of both the zirconium tetrafluoride (ZrF 4 ) and hafnium tetrafluoride (HfF 4 ) as a function of temperature is required since this forms the basis of the development of a sublimation model to determine whether the concept under consideration is theoretically possible. The sublimation kinetics of ZrF 4 is reported in this paper. IntroductionZirconium and hafnium in both metallic and compound states share a chemical similarity that is unequal by any other two homologous chemical elements in the periodic table [1]. This similarity results from their near identical atomic radii [2], and makes them difficult to separate. It is therefore quite a challenge to produce zirconium with hafnium levels of less than 100 ppm.The literature describes several methods used for the separation of Zr and Hf. Amongst these are fractional crystallization [3][4][5][6], solvent extraction (also liquid-liquid extraction or partitioning, [3,[7][8][9][10][11][12][13] [20,21] and sublimation [22,23].Information on sublimation purification of ZrF 4 from non-volatile impurities practically only became available from early 1970 [24], after which its use as a purification technique became more apparent [25][26][27][28][29][30][31][32].The AMI zirconium metal process developed by Necsa [33] involves the plasma dissociation of the mineral zircon to make it chemically reactive. The plasma dissociated zircon (PDZ) is fluorinated with ammonium bifluoride (ABF) to produce ZrF 4 . Hafnium and zirconium are then separated and the zirconium compounds are reduced to the metal which can then be purified. The focus here is dry fluoride-based processes which have the advantage of producing much less hazardous chem...

EKSTRAKSI Zr DAN Hf MENGGUNAKAN CAMPURAN EKTRAKTAN TBP DAN CYANEX 921

“…Beberapa metode yang digunakan untuk pemisahan Zr dan Hf, antara lain adalah kristalisasi fraksional (4) , ekstraksi pelarut (5)(6)(7)(8)(9) , pertukaran ion (10)(11)(12)(13) , distilasi ekstraktif dalam garam cair (14), kromatografi (15) dan sublimasi (16)(17)(18) . Ekstraksi cair-cair dengan memakai pelarut dapat digunakan metil iso butil keton (MIBK) maupun tri butyl fosfat (TBP).…”

EMULSION LIQUID MEMBRANE EXTRACTION OF Zr AND Hf FROM ACID NITRIC USING EXTRACTANT TOPO

“…For the separation of Zr of Hf is used in various ways, each using tools, solvents and different systems. Some of the methods used for the separation of Zr and Hf, include fractional crystallization (4), solvent extraction (5-9), ion exchange (10-13), extractive distillation in molten salts (14), chromatography (15) and sublimation (16)(17)(18). Liquid-liquid extraction using a solvent can be used methyl iso-butyl ketone (MIBK) and tri-butyl phosphate (TBP), tri-octyl phosphine oxide (TOPO).…”