Fractional Separation of Rare Earths by Oxalate Precipitation from Homogeneous Solution BOYD WEAVER, Stable Isotope Research and Production Division, O s k Ridge National Laboratory, O a k Ridge, Tenn.
within the first 4 hours, though they were usually complete within 24 hours. This indicates a possible increase in rate of hydrolysis after its inception. Preparation for precipitation from hot solutions was made in the same manner. The solutions were heated with stirring on a hot plate until precipitation became visible, usually a t about 80' C. The beaker was kept a t 75' to 90" C. for at least an hour and then allowed to cool to room temperature. Precipitated oxalates were filtered on KO.42 or 44 Whatman pa er, dried in porcelain crucibles, ignited a t 800" to 850' C. for at ]Peast an hour, cooled in a desiccator, and weighed. Protection from absorption of carbon dioxide is essential for the lighter rare earths. The rare earths left in solution were recovered by the conventional precipitation with oxalic acid and conversion to oxide.
A study of the fractional precipitation of oxalates from homogeneous solution has established semiquantitative relationships among most of the rare earths. By fractionation of pairs and multicomponent mixtures of these elements, separation factors have been determined for pairs throughout the series. Samarium has been found to precipitate preferentially to all of the other rare earths, with the lighter elements following in order of decreasing atomic number, while the heavier elements follow samarium in order of increasing atomic number. .411 of the rare earths precipitate preferentially to yttrium. The effect of rate of precipitation on separation has been studied. This method of separation of rare earths has been partially evaluated in comparison with some other methods involving precipitation from homogeneous solution.
Analyses. Whenever binary oxide mixtures contained praseodymium, neodymium, or samarium, analyses were made by measuring the characteristic spectral absorption of their solution (6). X Beckman hfodel DU spectrophotometer was used to measure absorption at optimum wave lengths by filtered solutions of chlorides of suitable concentration, essentially free of excess acid. Where only one element could be determined in this manner, sufficient accuracy for the purposes of this investigation was attained by regaiding the remainder as the oxide of the other element of the pair. A11 mixtures of other elements were analyzed spectrographically by a solution technique developed a t Oak Ridge Sational Laboratory ( 7 ) . d n accuracy of 3 ~ 2 %was normal for these analyses
T
HE fractional precipitation of rare earth oxalates from homo-
geneous solution by hydrolysis of methj 1 oxalate and by osalic acid from solutions containing chelating agents has been studied by hIarsh (3-5), Gordon et al. ( I ) , Vickery ( 9 , I O ) , and Gordon and Shayer ( 2 ) . The former investigations involved only some of the so-called cerium group of rare earths, while the last also included yttrium as an example of the yttrium group. In the methyl oxalate precipitation, the heavier rare earths precipitated preferentially to the lighter ones, in accordance with the findings of Sarver et al. (8) that the solubilities of the oxalates decrease R ith increasing atomic number. The Sequestrene procedure produced fractional separations in the opposite direction and gave a good separation between cerium and yttrium. The present study applies the methyl oxalate separation to almost all of the rare earths and yttrium and assigns semiquantitative numerical relationships to the different elements. The study of effects of experimental conditions on separation efficiency was limited to temperature, which influences the rate of hydrolysis of methyl oxalate and consequently the rate of precipitation. Different ratios of mixed elements and different degrees of precipitation were used in order to validate the method of evaluating the results.
EVALUATION OF RESULTS
The cnonventional method of evaluation of the efficiency of separation processes has been to calculate an enrichment ratio, m-hich is simply the ratio of the abundance of a given element in one of the fractions divided by its abundance in the starting material. This term, however, is mathematically dependent on the original composition and the mass distribution between the two fractions. A much more useful criterion, which is independent of these conditions, is the separation factor ( 1 1 ) . This criterion may be used to determine whether the efficiency of the separation process is chemically dependent on the original composition or the extent to which the process has taken place, or to determine the optimum conditions for a given process. I n the present study, mixtures of samarium and neodymium of various
EXPERIVEhTAL
Materials Used. Several of the elements were on hand as oxides with purities of 98 to 99.9% through purchases from such commercial sources as Research Chemicals, Inc., and Lindsay Chemical Co. or through purification by workers a t Oak Ridge National Laboratory by means of various kinds of fractional precipitation and crystallization, ion exchange chromatography, and liquidliquid extraction. Various mixtures of two or more of the heavier elements, some of them containing yttrium as well, were also available. Binary mixtures n w e made from equal weights of two oxides. Ceric oxide was dissolved in hydriodic acid and converted to the chloride by treatment with aqua regia. The other oxides were dissolved in hydrochloric acid. Most of the excess hydrochloric acid was removed by evaporation to incipient crystallization. The only other reagent required for the precipitation was commercial methyl oxalate. Procedure. For most of the data of this paper, a total of 1.000 gram of oxide mixture was prepared as chloride in 100 ml. of water. T o this was added 100 ml. of water containing 0.500 gram of methyl oxalate, enough to precipitate approximately half of a mixture having the atomic weight of samarium but free of yttrium. Most of the experiments were performed a t the prevailing room temperature, 20" to 35" C. Under these conditions the hydrolysis of methyl oxalate is so slow that it is not necessary t o add it slowly. In fact no precipitates were ever observed
Table I.
Original
Fractionation of Samarium-Neodymium Mixtures (Variation in material composition) Ratios of SmnOs to lid203 Precipitate Filtrate 0.366 0.913 1.37 1.34 1. 9 4 5.06
0,250 0.667 1 .oo 1.00 1.50 4.00
Table 11.
0.233 0.549 0.828 0,829 1.21 3.05
Separation Factors 1.57 1.66 1.66 1.62 1.60 1.66
Fractionation of Samarium-Neodymium Mixtures (Variation in degree of precipitation)
D~~~~~of p p t n , ,
5%
16.8 36.3 55.7 75.1
479
Ratios of SmrOa to xdz03 Precipitate Filtrate 1.66 1.01 0.89 1.48 0.78 1.33 0.67 1.22
Separation Factors 1.64 1.66 1.70 1.83
480
ANALYTICAL CHEMISTRY Table 111.
Rare Earth Fractionations at Different Temperatures
Element Pairs Sm-Pu’d Nd-Pr Sm-Gd Gd-Dy
Separation Factors Hot Coel 1.38 1.65 1.32 1.37 1.27 1.66 1.13 2.09
compositions were fractionated by the addition of equal amounts of methyl oxalate. The results are given in Table I. There are no significant variations which can be correlated with original compositions. Likewise several mixtures of equal amounts of samarium and neodymium oxides were fractionated with various amounts of methyl oxalate. These results are given in Table 11. There appears to be a slight trend for the separation to improve as precipitation is increased. OPTIMUM PROCEDURE
Some of the variable conditions which might affect the efficiency of separations in homogeneous solution are temperature, pH, and nature and concentration of anions present. Since temperature determines the rate of hydrolysis of methyl oxalate, it also determines the rate of precipitation of the rare earth oxalates and could have a predominant influence on the separation efficiency. Most of the experiments in this study were performed at room temperature in order to obtain the maximum practical separation. However, some rapid precipitations were made a t temperatures near the boiling point as a means of comparison. In Table I11 are separation factors for a few pairs of elements under the t x o sets of conditions. It is evident that separations at room temperature are much more efficient. APPLICATION OF PROCEDURE
The procedure described was applied to synthetic mixtures of pairs of elements and to mixtures derived from various separation procedures. These mixtures cover the whole range of rare earths and yttrium, omitting only the very rare elements europium, thulium, and lutetium. Table IV Summarizes the results of these experiments. In addition, practical application of the technique was made in the purification of samarium and yttrium by repeated schematic fractionations. Some of these data were obtained under circumstances somewhat different from those described above, and the separation factors may have been influenced to some extent by some condition not investigated, such as concentration. However, there is no doubt about the order of preference of precipitation of the elements.
Data from the present work indicate clearly that samarium oxalate is precipitated preferentially to the oxalates of all of the other rare earths, while yttrium falls behind all of them. In schematic fractionation of mixtures containing all of the rare earths and yttrium, samarium will become concentrated to the greatest extent in the head fraction and yttrium in the tail section. The lightest members of the cerium group will be found along with some of the yttrium group. In this unique case, yttrium itself is definitely not a typical example of the yttrium group and should be readily separable from dysprosium and holmium, which it closely resembles in most separation processes. Table IV.
Fractionatioii of F-arious Rare Earth Pairs
Element Pairs Sin-Nd Nil-Pr Pr-Ce Nd-La Ce-La Sm-Gd Gd-Tb
Separation Factoi1.6 1. 4 1.4 2.8 t o 5 1 . 5 t o 7.5
1. 5 1.6 -~~ ~~~
Table V. Reference
Element Pairs Tb-Dy Dy-Ho Ho-Er I:r-Yb Yb-P Dy-P ~
Separation Factors 1 3 1’ 1.4 1 3 1 6
3 to 5
-
-
Comparison of Separation !Methods Method Seqiiesti ene Methyl oxalate Methyl ovalate 3fethyl oxalate lfandelate l f e t h ,I oxalate LIandelate Ifandelate .\fethyl ouilate
Pair . . . .. Sm-Sd
Sm-Sd Sin-Gd
Grl-Sm Dy-Y Dy-Y
Separation Factoi 1.4 to 1 . 6 1.3 to 1 . 3 L4 1.5 t o 1.8 3.8 t o 8.7 1.5 to 1.7 2.3 to 4.5 1.1 t o 1.4 3 to 5
Even under the optimum circumstances of slow precipitation at room temperature it does not offer great advantages over other methods, except possibly for the purification of yttrium. The Increased efficiency is attained only by taking as much time as is required for mandelic acid precipitations or the Sequestrene method of Gordon and Shaver. -4thorough evaluation requires much more data on other methods. Since the Sequestrene method has been found to reverse the oxalate preference among the lighter earths, the whole series should be studied. ACKNOWLEDGMENT
The author is especially grateful to J. A. Norris, C. E. Pepper, and J. B. Mundzak, whose spectrographic analyses made it possible to obtain much of the data of this paper. The study was initiated as a result of the lectures and publications of H. H. Willard ( I S ) on the subject of precipitation from homogeneous solution.
COMPARISON WITH OTHER SEPARATION PROCEDURES
-4s a means of evaluating this procedure in comparison with other fractionations by precipitation from homogeneous solution, some of the separation factors obtained in this work have been listed in Table V along with results from previously published investigations and this author’s results with mandelic acid. DISCUSSION O F RESULTS
The fractional precipitation of rare earth oxalates is a preferential process. Gordon and Shaver ( 2 ) assumed that the preference increased with decreasing solubility of the oxalates. Sarver et al. (8) measured the solubilities of oxalates from lanthanum through gadolinium and observed that in general there was a decrease in solubility as the atomic weight increased. When they found that under some conditions gadolinium oxalate was more soluble than samarium oxalate, they attributed this phenomenon to a specific abnormality of gadolinium. I t appears now that they may have been on the verge of discovering a reversal of the trend of solubilities.
LITERATURE CITED
(1) Gordon, L., Brandt, R . A , , Quill, L. L., and Salutsky, AI. L., ANAL.CHEM.,23,1811 (1951). (2) Gordon, L., and Shaver, K. J., Ibid., 2 5 , 7 8 4 (1953). (3) Marsh, J. K., J . Chem. SOC.,1950, 1819. (4) I b i d . , 1951, 1461. (5) Ibid., p, 3057. (A) Moeller, T., and Brantley, J. C., &&N.~L.CHEY.,22, 433 (1950). (7) Norris, J. A., and Pepper, C. E., Ibid., 24, 1399 (1952). (8) Sarver, L. A , , Paul, H. JI.,and Brinton, P., J . Am. Chem. SOC., 4 9 , 943 (1927). (9) Vickery, R . C., J . Cheni. SOC.,1951, 1817. (10) Ibid., 1952, 1895. ( 1 1 ) Weaver, B., AXIL. CIHEX., 26, 474-5 (1954). (12) Ibid., PP. 476-8. (13) Willard, H. H., I h i d . , 22, 13T2 (1950). RECEIVEDfor review November 16. 1953. Accepted December 1. 1953. Presented before the Division of Analytical Chemistry a t t h e 124th Meeting of t h e AYERICA~V CHEMICAL S o c i E w . Chicago, Ill., 1953. This paper is based on work performed f o r the Atomic Energy Commission b y Carbide and Carbon Chemicals Co.. a division of Union Carbide and Carbon Corp., at the Oak Ridge National Laboratory.
