DFT modelling of tantalum pentafluoride extraction with phosphorus-based extractants – a molecular dynamics study

Abstract

Solvent extraction (SX) is a powerful method for both the separation and extraction of metals, resulting in high purity metals. The transition metals tantalum (Ta) and niobium (Nb) are of considerable significance, for example in the nuclear energy sector, where Nb is used as a cladding material and Ta in high-temperature, oxidation-resistant alloys, capacitors, and specialised materials. For these applications, the metals are needed in high purity. In recent studies, various Ta species that could be present during SX have been identified, including TaF5, TaF5·H2O, TaF4OH, TaF4·HSO4, and TaF3OH·HSO4.

In an attempt to understand how extraction occurs, molecular dynamics simulations have been utilised, whereby each species was simulated in a 3D periodic box. The stoichiometry of each system was determined from previous experimental (SX) conditions, and each species was investigated in 4 m and 10 m H2SO4. Simulations started at a perfectly mixed point, and the total simulation time was between 2 and 10 ns. Small-scale system results showed that TaF5·H2O formed at low H2SO4 concentrations and could be extracted with D2EHPA in both 4 m and 10 m H2SO4. If the solution was left to age, TaF4OH could form, which could not be extracted with D2EHPA at either concentration. In H2SO4-based media, TaF4·HSO4 will form, which could be extracted with D2EHPA from both 4 m and 10 m H2SO4. Further ageing of this solution resulted in the formation of TaF3OH·HSO4, which could not be extracted at either H2SO4 concentration. Furthermore, it was seen that in the 4 m H2SO4 system, the aqueous phase tends to form a droplet within an organic bulk solution, and when the H2SO4 concentration was increased, both phases showed droplet properties with break-away between them