Novel phosphate ester extractant for preferential extraction of hafnium over zirconium 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 was 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 SF_Hf/Zr value is up to 16.3. A crystal of the hafnium 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 crystals reveal a six-coordinated hafnium ion bound to six extractant molecules. The electron withdrawing effect of the aromatic substituents results in a low electron density of O atoms in the P–O group of the new extractant, and the extractant coordination group shows the characteristics of a soft base. This 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.