PURIFICATION OF PROMETHIUM BY LIQUID-LIQUID EXTRACTION

A process was developed for separating promethium from raixed fisBion product rare earths by continuous multistage conntercurrent extraction with 100% tri-nbutylphosphste from nitric acid of 12 N or higher concentration. Distribution coefficients at 12 N acidity for aecdamium. promethium. and samarium are 0.43. 0.82, and 1.55, respectively. Single-stage separation factors of 1.9 between successive elements can be maintained throughout the system to give separations dependent only on the number of stages. Extracted values can be recovered from the organic solution by stripping with a smaller volume of dilute nitric acid. A flowsheet for purification of promethium includes one cycle for separation of promethium from neodymum and lighter elements and a secondycle for removal of samarium and heavier elements. Each cycle consists of a series of countercurrent partitioning stages. followed by stripping stages and an evaporator. With 20 stages in the first cycle and 34 stages in the second, a 90% yield of promethium with a purity of 83% can be obtained from a typical mixture of fission product rare earths, assuming essentially perfect mechanical efficiency. An increase to 34 stages in the first cycle would permit a 93% yield of 99% promethium. (auth