Study finds a better way to separate zirconium and hafnium

Researchers at Oregon State University have published a paper in the Journal of the American Chemical Society on a new method of separating zirconium and hafnium, both of which are important for nuclear energy, and which are generally quite challenging to separate due to the elements' similarities.

According to the paper, more than 90 percent of Zr and Hf derived from zirconium silicate (zircon) is utilized by the nuclear industry, with high-purity Zr being widely used for nuclear fuel rod cladding. High-purity Hf is also used in the nuclear applications, typically for specialized control rods, such as those in nuclear submarines.

"Despite their chemical similarity, the Zr nucleus is essentially neutron-transparent, while Hf has a high cross-section for neutron capture," the paper states.

These properties are exploited in their nuclear applications, but it means that the "high-purity" part is key for these difficult-to-separate elements: For Zr that means it contains less than 0.01 percent Hf, and for Hf that means it contains less than 1 percent Zr.

Current industrial-scale Zr/Hf separation processes are energy intensive and use large quantities of methyl isobutyl ketone, a toxic organic solvent, roughly 4 percent of which ends up lost to the air as noxious pollution.

Led by OSU chemistry professor May Nyman and graduate research assistant Alex Roseborough, the process described in the JACS paper uses a water-based solution that combines natural Zr, with its few percentage points of Hf impurity; thiocyanate ligands, which bind to the Hf and Zr ions; and choline, an inexpensive, nontoxic chemical commonly used as a food additive, according to OSU.

The demonstrated process eliminates the organic solvent and reduces energy requirements, resulting in a precipitation of Hf-rich species. It also achieves a separation factor-a measure of the process's ability to separate two components in the mixture-that (under optimized laboratory conditions) is more than four times higher than the industry standard.

Nyman told Nuclear News that work would need to be done to translate the process from a bench experiment to a pilot demo to understand how it could scale up to an industrial production level.

"That would likely require a start-up company and/or industry investment to study scale-up and adaption to an automated process," she said.

Nyman said this likely would require converting it into a precipitation-based process, which can be automated, but the current industrial separation process is not precipitation-based, so existing facilities would require significant renovation if they were going to adopt this new process.

"We describe in atomic-level detail how the separation works and how precipitation-based separations can compete with solvent extraction," said Nyman. "We still have questions to answer and milestones to achieve, but these findings are really exciting and impactful, especially as society must move toward more carbon-free and high-density electricity generation, including nuclear energy."