Composition of the KCl–AlCl3–ZrCl4–HfCl4 Melt as Applied to the Extractive Rectification of Zirconium and Hafnium Chlorides

Abstract—AO Chepetsk Mechanical Plant implemented the production of a high-purity zirconium sponge for the manufacture of nuclear fuel components. One of the main stages of production is the purification of zirconium from hafnium to a residual concentration of less than 0.01 wt %. Purification is carried out by the rectification separation of a mixture of zirconium and hafnium tetrachlorides in a KCl–AlCl3 melt. The experience of operation of the zirconium and hafnium tetrachloride separation installation has shown that, to determine the operational properties of the melt, it is not enough to know the content of K, Al, Zr, and Hf in it. During the operation of the installation, the KCl–AlCl3–ZrCl4–HfCl4 melt is studied by a set of the following independent methods: X-ray diffraction, reducing melting in the presence of carbon, and the determination of the residual zirconium content after sublimation of volatile components by an inert gas flow. The content of the phases ZrCl4, K2ZrCl6, and AlCl3 against the background of the matrix KAlCl4 phase is determined in frozen melt samples by X-ray diffraction. The KCl phase is found not to form. The contents of ZrCl4, AlCl3, and K2ZrCl6 in the melt at different sites of the technological scheme of the installation are analyzed. ZrCl4 and AlCl3 or K2ZrCl6 are found to exist in the melts of the unit for preparing a mixture of zirconium and hafnium tetrachlorides to be separated, a sublimation column, and an evaporator depending on the excess or deficiency of AlCl3 relative to KCl. ZrCl4 and AlCl3 are absent in the melt after the desorption column; in some cases, K2ZrCl6 is detected, and its content correlates with the Zr content determined by inductively coupled plasma atomic emission spectrometry. An amorphous (as determined by X-ray diffraction) component, which contains up to 1.5 wt % aluminum and up to 3.5 wt % zirconium, is detected in frozen melt samples. Reducing melting in the presence of carbon in frozen melts is used to determine oxygen (up to 1.8 wt %), which is part of the amorphous component, which is likely to consist of AlOCl and ZrOCl2. The residual zirconium content is analyzed in melt samples after the sublimation of volatile components by an argon flow at a temperature of 550°C at various AlCl3 contents. The experimental results are used to develop a technique to determine the ratio AlCl3/KCl, which based on the difference in the physicochemical properties of the melt components. The molar ratio Al/K calculated using the mass fractions of Al and K in the melt is shown to be higher than the molar ratio AlCl3/KCl determined by the developed technique. Based on the results obtained, we corrected the melt composition during the operation of the zirconium and hafnium chloride separation installation at AO Chepetsk Mechanical Plant.