Resumo

UNGERER, M.J.; VAN SITTERT, C.G.C.E.; VAN DER WESTHUIZEN, D.J.  e  KRIEG, H.M.. Molecular modelling of tantalum in an aqueous phase. J. S. Afr. Inst. Min. Metall. [online]. 2017, vol.117, n.6, pp.541-544. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/2017/v117n6a4.

The transition metals tantalum (Ta) and niobium (Nb) are of significant importance, for example in the nuclear energy sector where they are used as cladding materials, as well as in capacitors and specialized materials. For these applications a high-purity metal is needed. The separation of Ta and Nb is always a challenge since they are found together in nature and have similar chemical and physical properties, resulting in costly and laborious separation processes. A technology that has been used successfully for the separation of these metals entails solvent extraction (SX)¹. While separation was achieved in a previous SX study using a sulphuric acid (H₂SO₄) medium with the extractants diiso-octyl phosphinic acid (PA) and di-(2-ethylhexyl) phosphoric acid (D2EHPA), due to the absence of speciation data for Ta and Nb it is not clear how the separation occurred. One method that might be suitable for determining the speciation of a reaction is molecular modelling. Calculations based on the density-functional theory (DFT) are now used not only for light elements and small molecules, but also metal complexes, heavy metals, and especially metal separation in SX². In this study the aqueous phase used during SX was investigated by studying periodic systems of Ta, as a metal and in salt form, when it is in contact with H₂O and H₂SO₄. The results were used to predict the reaction mechanism occurring during SX. Results showed that (i) in a 1:1 acid-water ratio, the deprotonation of H₂SO₄ was endothermic, (ii) in a 1:5 ratio deprotonation was exothermic forming HSO₄^-, and (iii) in a 1:10 ratio double deprotonation occurred to form SO₄2- exothermically.

Palavras-chave : tantalum; niobium; solvent extraction; reaction mechanism; molecular modelling.

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