Novel Phosphate Ester Extractant for Preferential Extraction of Hafnium and its Mechanism

Abstract

Nuclear-grade zirconium and hafnium play an essential role in the nuclear industry. It is difficult to separate and purify zirconium and hafnium because of the similar properties caused by lanthanide shrinkage and the preferential extraction of hafnium is challenging. In this work, a new extractant, di(p-tert-butylphenyl)phosphate (DTBPPA), was synthesized, and its separation performance of zirconium and hafnium in the sulfuric acid system evaluated. Under identical extraction conditions, DTBPPA shows excellent preferential extraction properties for hafnium compared to current commercial extractants, such as D2EHPA and PC88A. The highest SFHf/Zr value is up to 16.3. Crystal ofhafnium extracted species was grown to study the extraction mechanism and the principle of preferential extraction of hafnium. It was found that the crystal growth of zirconium-extracted species was difficult under the same conditions. Hafnium extracted species crystal reveals a six-coordinated hafnium ion bound to six extractant molecules. The electron drawing effect of aromatic substituents results in the low electron density of O atoms in the P-O group of the new extractant, and the extractant coordination group shows the characteristics of soft bases. It matches the soft acid properties of hafnium ions. DTBPPA shows a solid ability to identify hafnium ions, thus achieving preferential separation of hafnium from zirconium and hafnium mixtures.

Keywords: Zirconium, Hafnium, Solvent extraction, Crystal of hafnium extracted species, Cation exchange mechanism