Diethylenetriamine pentaacetic acid (dtpa) or hydroxyethylenediamine triacetic acid (hedta) can be used as retaining agents in the separation of rare-earth (re) elements on a cation-exchange resin. The re elements are adsorbed onto a cation-exchange resin (nh4 or h cycle) used in a conventional column. The effluents from this column are passed through a 2nd cation-exchange column or columns, having the resin in the dtpa or hedta cycle. An aqueous edta solution is subsequently passed in sequence through the column containing the adsorbed re's and the column containing the resin in the dtpa or hedta cycle. The columns are heated to a temperature sufficiently high to maintain the acid form of edta, as well as the dtpa and hedta, in solution. During the edta elution operation the re elements are eluted from the 1st resin bed in the form of re-edta complexes and these complexes in turn are exchanged with dtpa or hedta agents on the 2nd column. The process is operable over a range of eluant ph values, concentrations, and temperatures. In general, the influent eluting solution ph must be such that la edta chelate does not precipitate in the column and low enough that the ph of the re sample column is approximately 2.5. The operable ph range will vary with concentration, but for practical reasons it is best to use the highest possible eluant ph since edta is more effectively used than at lower ph values. The process uses a nonmetallic retaining agent to eliminate contamination of the re product and allows for the recovery and reuse of eluant and retaining agent.
U.S. Pat. 3,615,173; Oct. 26, 1971; Chem. Abstr. 76:101,765C