Suppression of Zr, Nb, Hf and Ta coprecipitation in fluoride compounds for determination in Ca-rich materials

We have evaluated recovery yields of Zr, Nb, Hf and Ta, which are called high field strength elements (HFSE), in the synthetic Ca-Al-Mg-HF solution system using two different methods: at <70°C in an ultrasonic-bath (denoted as the ultrasonic method) and at 205°C using a TFE Teflon® bomb (denoted as the bomb method). Full recovery of HFSE into the supernatant HF solution was not achieved in cases where fluorite (CaF₂) forms in the precipitate in both methods and sellaite (MgF₂) forms in the ultrasonic method. In order to suppress fluorite precipitation, an 'Al-addition method' was developed in which Al solution is added to the sample before decomposition to change the matrix solution to a composition in which no fluorite forms. Using the Al-addition method, HFSE concentrations of Ca-rich materials were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) by an isotope dilution method for Zr and Hf and by a calibration curve method for Nb and Ta. Furthermore, we found isotopic disequilibria of Zr and Hf between sample and spike without Aladdition for Ca-rich samples, and that the Al-addition method was very effective in achieving isotopic equilibria and full recovery of HFSE, resulting in high accuracy in their determination.