Selective metal extraction from solid matrices with TBP complex with nitric acid in supercritical carbon oxide

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Selective recovery of lanthanide elements from a mixture of metal oxides was demonstrated using tri-n-butylphosphate complex with nitric acid and water dissolved in supercritical carbon dioxide (SF-CO_2). Recovery yields were experimentally evaluated under various temperatures and pressures ; influences of temperature and pressure were studied and optimum conditions for metal extraction were determined. Higher temperature is preferable for rapid dissolution because of activation energy. Concerning with pressure, the optimum value exists because density becomes larger, but mole fraction of the complex decreases as pressure increases. The optimal pressure is very close to the critical point for the mixture of the complex, and SF-CO_2. To study mechanisms of selective dissolution and extraction of metals form their oxides, dissolution and extraction rates was correlated with the mole ratios of TBP/HNO_3/H_2O and a fact that, the higher ratios of HNO_3/TBP and, H_2O/TBP are preferable was found. ^3H-NMR measurements on TBP complex supports that TBP complex contains a solvent structure of reverse micelle like. It was also found that ultrasound application is so effective that it may enhance the extraction rates ten times larger than without its application. The solubility of TBP complex in SF-CO_2 was well predicted by a molecular simulation.