2022, №1

сс. 3-25

Supercritical Fluid Extraction of Cerium from Aqueous Solutions Using Tributyl Phosphate as a Ligand

2022, №1

сс. 3-25

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M.O. Kostenko, K.B. Ustinovich, M.Yu. Sinev, O.O. Parenago, V.B. Baranovskaya

Key words: extraction, supercritical fluid, cerium complexes, aqueous solutions, tributyl phosphate

The extraction of Ce(III) and Ce(IV) complexes with tributyl phosphate (TBP) from aqueous solutions of various acidity in the presence of a salting-out agent (magnesium nitrate) into supercritical (SC) fluids, CO2 and freon R23, was studied under static and dynamic conditions. It is shown that SC-CO2 is a more efficient extractant than freon R23. The efficiency of Ce(III) extraction increases with an increase in the concentration of magnesium nitrate and the amount of TBP in the system and decreases with an increase in the acidity of the aqueous solution. The dependence of the extraction efficiency on pressure is more complex: at a pressure of SC-CO2 below 10 MPa (at 40 °C), the solubility of TBP in it drops sharply. As the pressure rises above this value, the degree of transition of the Ce(III) complex into the fluid phase decreases. An equation for the extraction isotherm is obtained and its parameters are determined for conditions close to optimal for the extraction of Ce(III). Cerium in the form of Ce(IV) is more efficiently extracted by SC-CO2 even at high acidity of the aqueous solution; however, the state of Ce(IV) in the aqueous solution of its complex is unstable; the complete transition to the Ce(III) state occurs within ~14 days. Dynamic extraction using a countercurrent extraction column cannot be described in terms of the «theoretical separation steps» (or «theoretical plates») model. The reason, apparently, is the instability of the TBP solution in the SC-fluid and the formation of a suspension of small drops of liquid TBP, which is a more efficient extractant than the true solution of TBP in the fluid. At the same time, such a transition leads to a sharp increase of the efficiency of the extraction column as a mass transfer apparatus.