Molecular modelling of tantalum fluoride in sulphuric acid medium: A DFT study

Abstract

During the solvent extraction (SX) experiments of tantalum (Ta) and niobium (Nb), the aqueous phase consisted of tantalum- and niobium penta-fluoride (Ta(Nb)F5), water (H2O) and sulphuric acid (H2SO4), which was contacted with an organic phase. In this study the aqueous phase used during SX was modelled by studying the interactions and resulting reactions of specifically TaF5 when it is contacted with H2O and H2SO4. Different functional and basis set combinations within density functional theory (DFT) were investigated. From previous modelling it was seen that by increasing the number of water molecules, the reaction energy decreased due to molecule stabilisation (hydrogen bonding) and subsequently a 1:1:10 metal:acid:water ratio were used.Results showed that the deprotonation of H2SO4 was exothermic, leading to the formation of HSO4−. Furthermore, from the various reactions and geometries between TaF5, H2SO4 and H2O investigated, it was observed that only four species would be available in the aqueous phase during solvent extraction, namely TaF5·H2O in water or diluted acid medium, TaF4·HSO4 in a concentrated H2SO4 medium and TaF4OH or TaF3OH·HSO4 if the aqueous phase was aged. From the results obtained a reaction mechanism that might occur during SX of TaF5 was predicted.