Separation of zirconium and hafnium via solvent extraction

Abstract

Zirconium metal (Zr) is highly desirable as a cladding material for nuclear fuel rods in nuclear power plants, because of its very low nuclear absorption cross-section for thermal neutrons, however to use this Zr metal it has to be purified (<100 ppm) from the contained (1 – 3% wt) hafnium metal (Hf), occurs in zircon ore (ZrSiO4) in nature. Because of the extensive beach deposits, rich in zircon minerals, located along the South African coasts, there is a great opportunity for zircon beneficiation in South Africa to convert the country's mineral output into high added value products rather than selling the basic mineral to countries like China. Due to the significantly similar chemical properties of these two elements, the purification of the Zr metal is a complicated process. The separation of Zr and Hf, as currently practiced, is mostly conducted through solvent extraction in which the aqueous chloride solution of metal species is contacted with an immiscible organic phase containing a reagent that selectively removes one of the two metals from the aqueous phase. The conventional multi-stage industrial approach, in production since the 1950s, presented several technological disadvantages and environmental problems that were considered acceptable when these processes were developed, but have become a serious problem as legislation became more stringent. Thus, the main objective for this study is to develop an innovative, environmentally friendly and cost-efficient solvent-extraction process that makes use of Hf-containing Zr compounds, produced by NECSA (Pty) Ltd from zircon ore by means of plasma technology, for the separation of Zr and Hf in order to produce nuclear-grade Zr metal. Results obtained from this study show that the extraction of Zr and Hf from chloride-based compounds (Zr(Hf)CI4) proceeds via an anion-exchange mechanism through the extraction with amine extractants (Alamine 336 and Aliquat 336), while the Zr species could be successfully recovered from the organic solutions. However, the extraction from fluoride-based compounds (K2Zr(Hf)F6) was unsuccessful for both Zr and Hf species. The new proposed process for Zr and Hf separation from chloride-based Zr compounds seems to be an improvement from the conventional separation processes.