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

The extraction of Ce(III) and Ce(IV) complexes with tributyl phosphate (TBP) from aqueous solutions with various acidities in the presence of a salting-out agent (magnesium nitrate) into a medium of supercritical (SC) fluids, CO 2 (SC-CO 2 ), and Freon R23, is studied under static and dynamic conditions. It is shown that SC-CO 2 is a more efficient extractant than Freon R23. The efficiency of the extraction of Ce(III) increases with the increase in the concentration of magnesium nitrate and amount of TBP in the system and decreases with the increase in the acidity of the aqueous solution. The dependence of the efficiency of extraction on pressure is more complex; thus, at a pressure of SC-CO 2 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 that are close to optimal for the extraction of Ce(III). Cerium in the form of Ce(IV) is more efficiently extracted with SC-CO 2 even at high acidity of the aqueous solution; however, the Ce(IV) state is unstable in the aqueous solution of its complex; 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 model of “theoretical separation steps” or “theoretical plates”. This is apparently due to the instability of the solution of TBP in the SC fluid and formation of a suspension of small drops of liquid TBP that is a more efficient extractant than a true solution of TBP in the fluid. However, such a transition leads to a sharp increase in the efficiency of the extraction column as a mass transfer apparatus.