Fractional extraction of the lanthanides as their di-alkyl orthophosphates

Abstract

The extraction of lanthanides(III) and yttrium(III) into a solution of di(2-ethyl hexyl) orthophosphoric acid (symbolized as HDEHP) in a carrier solvent from aqueous mineral acid phases has been investigated as a function of HDEHP concentration in the organic phase, mineral acid concentration in the aqueous phase, and Z, using the radioactive-tracer technique and employing americium as a normalizing element. The distribution ratio, K, defined for a given radioactive nuclide as its concentration in the organic phase divided by its concentration in the aqueous phase, has been found to have a direct third-power dependency upon the HDEHP concentration in the organic phase and an inverse third-power dependency upon the mineral acid concentration in the aqueous phase. In experiments involving gross concentrations of extracting cation, it has been shown that none of the anion associated with this cation in the initial aqueous phase reports in the equilibrated organic phase. On the basis of these data, the extracting species has been formulated as M(DEHP) 3, possibly with solvate water. Operationally, HDEHP may be considered as the high-acid analogue of thenoyl trifluoroacetone (symbolized as HTTA); and analogously the M(DEHP) 3 is tentatively considered to be a chelate complex. A plot of log K vs. Z is well represented by a straight line of positive slope corresponding to an average value of r, defined as the ratio of K Z + 1 to K Z , of 2·5. This average r of 2·5, to be compared with the value of 1·63 as the ratio of molar aqueous solubilities of the dimethyl phosphates of adjacent lanthanides as reported by Marsh, is sufficiently large to make fractionation of lanthanides by liquid-liquid partition an attractive possibility. Successful application of such a technique to a gross sample has been demonstrated. In the plot of log K vs. Z, Y falls on the straight line if given an artificial Z approximately 67·6, as it does in Marsh's plot of log molar solubility vs. Z for the lanthanide dimethyl phosphates.