The Mechanism of Extraction of Tantalum in the Tantalum-Hydrofluoric Acid-Tributyl Phosphate System

Abstract

The mechanism of extraction of Ta by TBP in the Ta-hydrofluoric acid-TBP (tributyl phosphate) system is investigated in this report. The results obtained can be summarized as follows: (1) Ta present at a low concentration of HF seemed to be essentially HT_aF₆ at a high concentration of HF, H₂T_aF₇, and at a much higher concentration of HF, H₃T_aF₈ seemed dominant. (2) One molecule of Ta compounds at various concentrations of HF combined with three molecules of TBP organic solvent, and was extracted by TBP in the form of a Ta compound-3 TBP complex. (3) when the polymerization of Ta in the aqueous phase was higher than that in the organic phase, the distribution coefficient of Ta and the percentage extraction of Ta became lower. At a higher concentration of HF the polymerization number of Ta in the aqueous phase was constant and larger than that in the organic phase, independent of the concentration of Ta in the aqueous phase, but at a lower concentration of HF the ratio of the polymerization number in the aqueous phase to that in the organic phase was larger at a higher concentration of Ta, but was smaller at a lower concentration of Ta. (4) The process of Ta-HF-TBP extraction was an exthothermic reaction, and the heat of extraction was 2.41 kcal/mol at 6.36 N HF concentration and 16 g/L Ta concentration. (5) The rate-controlling step in the extraction of Ta in the Ta-HF-TBP extraction system was considered to be the formation reaction of a Ta compound-3TBP complex. (6) In this extraction system, the concentration of Ta in the aqueous phase showed little change by the distance from the interface, but the change in the TBP phase was remarkable. (7) The activation energy for diffusion of Ta species in a 1.28 N HF solution was 3.495 kcal/mol, and that of a Ta compound-3TBP complex in the TBP phase was 5.836 kcal/mol.