May 5, 1954
SEPARATING O F
RAREEARTH MIXTURESINTO PURE COMPONENTS
One conclusion derived from these equations is extremely important and accounts for the unique success of the ion-exchange method in separating rare earths. It will be noted that once the ammonia in the eluant is fixed, all variables in the equilibrium eluate and on the column are determined for each individual rare earth present. As the equilibrium constants of the complexes change from rare earth t o rare earth, the Cit" concentration in the eluate changes. This means that the rare earths must separate into bands on the resin bed, each band having a specific citrate concentration, hydrogen-ion concentration and ionic strength associated with it. As a result of this, the bands develop autosharpening boundaries, and one band rides immediately on the tail of the preceding band. If an ion of a given rare earth species gets ahead of or behind its own band for any reason, i t will be subjected to an adverse citrate concentration and will either be accelerated or retarded in its movement down the column, so that it returns to its own band. It will be seen, while in theory it is possible to separate tracer quantities of rare earth ions on ionexchange columns, that in practice, under equilibrium conditions as carried out in these experiments, i t is impossible to do so. There is always a slight tilting of the band and a small amount of channeling so that when the band front leaves the column one part of the front will have passed the boundary while the other part still remains on the resin bed. For an infinitely thin band, there will always be found a mixture of rare earths, a t least binary in composition, in the eluate. The requirement for good separation of rare earths is that the band be long compared with its diameter, so that any tilting of the band is small compared with the length of the band. If tracer quantities are to be separated under these conditions, very small diamekr col-
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umns should be used and carriers should be present to give the proper type of bands. Of course, carrier-free tracers may be separated if non-equilibrium conditions are used in other citrate ranges, but i t is our conclusion that they cannot be separated under the conditions of these partkular experiments. Under equilibrium conditions, the equations indicate that flat-topped elution curves should be obtained. However, i t is interesting that, if the bands are narrow due to only a small amount of a rare earth species being present, a bell-shaped elution curve may be observed. This results from the fact that the columns are circular in cross-section and that, if a narrow band is tilted, the concentration of a particular rare earth in the eluate is proportional to the area of the band front crossing the boundary a t a given moment. This area will vary from zero, a t the edge, to a maximum corresponding to the diameter of the column, and then decrease to zero again. The bell-shaped curve observed is really the envelope of a series of equilibrium increments weighted according to the geometry of the sorbed band. Under non-equilibrium conditions, where the bands are continually spreading out as they progress down the column, true bell-shaped curves are obtained, but the height of these curves continually diminishes as the length of the column is increased, and there is always a certain amount of overlap of one rare earth with the other. Under these conditions, equation 17 is no longer controlling. Instantaneously, a t any point in the column, the solution adjusts itself, according to the other ten equations, to the concentrations of the components on the resin a t that point, or vice versa, but the adjustment is different a t each succeeding point, and the concentrations constantly change as the band spreads out. AMES, IOWA
INSTITUTE FOR ATOMICRESEARCH AND DEPARTMENT OF CHEMISTRY, IOWA STATE COLLEGE]'
The Use of Copper as the Retaining Ion in the Elution of Rare Earths with Ammonium Ethylenediamine Tetraacetate Solutions BY F. H. SPEDDING, J. E. POWELL AND E. J. WHEELWRIGHT RECEIVED DECEMBER 19, 1953 A rapid method for separating mixtures of rare earths into the pure components is described. The method consists of eluting a band of mixed rare earths adsorbed on a cation-exchange resin through a second cation-exchange bed in the copper" state. The eluant consists of an ammonia-buffered solution of ethylenediaminetetraacetic acid. I t was found that gram quantities of pure heavy rare earths could be obtained in a few days by this method.
Introduction In a preliminary note,2 the possibility of using ethylenediamine tetraacetic acid as an eluting agent for the separation of adjacent rare earths on cation-exchange resins was discussed. This earlier publication described an experiment in which a mixture of neodymium and praseodymium was (1) Work was performed in t h e Ames L a b o r a t o r y of the Atomic Energy Commission. (2) F. H. Spedding, 1. E. Powell a n d E. J. Wheelwright, THIS JOURNAL, TO, 612 (1964).
eluted through a cation-exchange bed in the ironIII state with a solution of ammonium ethylenediamine tetraacetate. The iron"' resin bed served as a barrier through which a solution of rare earth-ethylenediamine tetraacetate complex could not pass without decomposition, since the iron"' ion forms a more stable complex with EDTA than does any trivalent rare earth ion. In this preliminary wperiment, only short sections of resin bed were used and a moderate degree of separation was attained. Subsequent experiments with longer resin beds,
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F. H. SPEDDING, J. E. POWELL AND E. J. ~ ' H E E L W R I G H T
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using concentrated solutions of EDTA, have indi- the resin bed. The effluent solution was collected after the cated however that iron'II is not the most desirable rare earth breakthrough occurred and any copper present was eliminated by electrolysis after the solution was made retaining ion. It was found that the pH range of acidic with sulfuric acid. The rare earths present were reeluant was limited when iron was used, for if the covered by the addition of oxalic acid to the copper-free solution was too acidic, the acid HIY tended to solution. 1. The Elution of Certain Troublesome Rare Earth precipitate in the interstices of the resin bed and, if Mixtures.-In the separation of rare earths by the citrate it was too basic, hydrous ferric oxide clogged the method,*j5 there are certain groups of elements which separesin pores. When the ammonium EDTA solu- rate less readily than other members of the series. These tion encountered the rare earth on the resin bed, groups (lutetium-ytterbium, dysprosium-yttrium-terbium hydrogen ions were liberated. 9 s the concentra- and gadolinium-europium-samarium) offer a severe test any separation method which does not make use of diftion of EDTA in the eluant was increased, the in- to ferent oxidation states. creased hydrogen ion concentration resulted in the In the following three experiments the rare earths were formation of insoluble H4Y a t increasingly higher adsorbed on -40+50 mesh h'alcite H C R beds, 90 cm. pH values. On the other hand, as the pH in- long and 22 mm. in diameter. The loads consisted of creased, insoluble hydrous ferric oxide tended to I. 53 0 g. of R203 containing 53.0% LU&, 33.0% Yb203, 11.9% Tmz03, 2.1% Erz03, etc. form and this resulted in the trailing of iron into 11. 47.7 g. of Rz03 containing 45.9% Gd203, 1.7% the rare earth bands when concentrated EDTA was Euz03, 52.4% Smz03 used, even in the most favorable cases. Actually, it 111. 43.5 g. of Rz03 containing 0.1% Dyz03, 30.7% YQOI. is desirable to use concentrated EDTA in order to 65.5% Tb40r,3 794 GdQOs,etc. obtain rapid separation of larger amounts of rare earths, but the trailing tendency of the iron1" dissolved in a minimum of hydrochloric acid. The rare makes necessary a subsequent separation of iron earths were eluted through - 100+200 mesh Salcite H C R beds, 90 cm. long and 22 inm. in diameter, in copper" state, from the rare earths. with a solution of approximately 2 % EDTA at a pH of 8.a I t is obvious that some ion having a more soluble and a linear flow rate of 0.5 cm. per minute. The solution hydroxide than ironIII would be a more desirable was prepared by diluting 900 g. of diammonium dihydrogeii choice for the retaining ion in this type of elution. ethylenediamine tetraacetate and 300 ml. of concentrated hydroxide to 45 liters with distilled water. Consideration of the stability constants for the ammonium The rare earths displaced the copper11 ions from the resin EDTA complexes of the rare earths and other metal beds very nicely and except for channeling effects the copper-rare earth boundary was sharp. In each experiment ions3 showed that copper1', nickel11 and lead" should serve to retain a number of the rare earth TABLE I ions. I n solutions having an ionic strength of 0.1, O F A MIXTURE O F LUTETIUM,YTTERBIUM, the stability constants of these ions lie between the THEELUTION AND ERBIUM WITH EDTA 1HROUGH A R F D O F constants for erbium and dysprosium. However, THULIUM XALCITE HCR ix THE COPPER" STATE the relative stabilities of the EDTA complexes are not the only factors which determine the order of FracTmnOi 'ipl tion Total W t . R?Oi LUZO', q 'b no vol., 1. R. elution of metal ions from ion-exchange beds. It is also necessary to consider the effect of the relative 0.2 .. 1 0,4036 99.8 4.6 affinities of the metal ions for the resin. The 0 ,5 , . 2 99.5 1.2157 4.8 general rule for the order of affinity of ions for cat1.0 , . 99.0 2.2398 3 5.0 > M+3> hl+?> I f + . ion-exchange resins is 1 .0 99.0 2.2642 4 5.2 This means that, while copperII ion for example is 2 5 . . 97.5 2,2613 > 6 .4 complexed by EDTA more strongly than holmium 2 .5 .. 97.5 2.2755 B 5.6 and the lighter rare earths, the rare earths in gen. . 1. -5 95.5 2.3111
