The Separation Factor A Criterion for Evaluation of Fractional Separation Processes BOYD WEAVER,
Stable lsotope Research and Production Division,
O a k Ridge National Laboratory, O a k Ridge, Tenn.
In reporting the results of research on fractional separation of a pair of similar elements, more use should be made of the “separation factor” as a criterion for determining the effectiveness of a single stage in a division of a mixture into two fractions. The separation factor is the ratio between two elements in one fraction divided by their ratio in the other fraction. It is independent of the original composition and of the extent of the separation process. The literature dealing with separation of the hafnium-zirconium pair and the rare earth group has been surveyed as a means of evaluating separation methods. Separation factors have been calculated and compiled from the small amount of useful data. This criterion has been applied usefully in development of a separation process by the author.
T
HE members of several groups of elements are so similar in
chemical properties that their separation from each other can be accomplished only by multistage processes. This is especially true of the zirconium-hafnium pair and the rare earth group. Chemists who work with these separations need some universally applicable criterion by which they can judge the effectiveness of a single stage in order to determine the effects of various conditions on a given process or to compare it with other processes. I n the past most workers have usually depended solely on analyses of their starting materials and their products after several stageg. This was because analyses were difficult and often consumed valuable material. Currently, advances (3, 4, 17, 2 2 ) are being made which promise to increase greatly the availability of all of these elements and extend their applications in science and industry. Accompanying these developments there have been improvements in analysis, including p-bromomandelate ( l a )method for zirconium-hafnium mixtures and quantitative spectrographic analyses of both groups (7-9, 15, 16, 20). The need for a uniform system for evaluation of results in separation processes is increasingly great. The author has found the separation factor most useful for this purpose. This term is applicable whenever a mixture of two elements has been divided into two fractions. The separation factor is then the ratio of one element to the other in one fraction divided by the corresponding ratio in the other fraction.
It does not involve an analysis of the initial material and is independent mathematically of the original ratio, the presence of other elements, the extent to which a separation process has taken place, or the units of measurement employed. The term is not new, as it has been used to evaluate liquid-liquid extraction methods for the separation of rare earths (1, 10, 17, $2). I n extractions it is simply the ratio of distribution coefficients for the two elements involved. APPLICATION
I n the course of studying a new process for the separation of rare earths (21), the author surveyed the literature concerning the
separation of these elements and extended the inquiry to zirconium and hafnium. The method most frequently used for determining changes in the composition of complex mixtures of rare earths has been the determination of average atomic weights of the fractions. The significance of such data is decreased by the presence of yttrium in most mixtures, and there is little chance of duplicating conditions sufficiently well to obtain any objective comparisons. Several investigations have dealt with the easily separable pair, lanthanum and neodymium. Usually the information given is too fragmentary to permit calculation of separation factors. It is probable that in many cases sufficient additional information was available or could have been easily obtained. However, in several cases it has been possible to obtain separation factors from the information given. Table I contains a compilation of factors derived from the data of various workers. Thus, the statement by Young et al. ( 2 5 ) that from an equimolar mixture of lanthanum and neodymium bromides reaction with ethyl benzoate gave a yield of 25% of the neodymium in a purity of 95%, is sufficient for the calculation of a separation factor of 24 for the process.
Table I.
Comparison of Rare Earth Separation Factors
Reference (3.7) (14)
(14) ilL) (11) (11 ) (1
f)
(18)
”” ((”) 29) (19) (19) (1 9 ) (19) (19)
Present Present Present Present Present Present Present Present Present Present Present
author author author author author author author author author author author
Process
Element Pair
Difference of Ionic Radii, A
Separation Factors
Ethyl benzoate Nitrate fusjon Nitrate fusion Kitrate fusion Methyl oxalate Methyl oxalate Methyl oxalate
Nd-La Xd-La Pr-La Kd-Pr Pr-La Ce-La Pr-Ce
0.10 0.10 0.08 0.02 0 08 0.07 0 01
24 25 17 1.5 4 9 2.2,2 5.2 8 1 8
Homogeneous carbonate Sulfate Oxalate Alkali carbonate N H I double nitrate Basic magnesia Basic ammonia Basic urea Basic electrolyte
Pr-La
0.08
2.0-4.5
La-Sd Nd-La Xd-La
0.10 0.10 0.10
1 .i 4.0 4.9
Methyl oxalate Mandelate Mandelate Nandelate Mandelate Mandelate Mandelate Mandelate Mandelate Mandelate Mandelate
Sm-Sd Kd-La Sm-Kd Gd-Sm Tb-Gd Dy-Tb Dy-Y Y-Tb Dy-Gd Ho-Dy Er-Ho
0.045 0 10 0.045
1.4.1.5 14 3.8-8.7 2.3-4 5 1.6-2.3 1 6-1.8 1.1-1.4 1.0-1.5 2.7-3.6 1.4 1.6
La-Sd Nd-La Nd-La Sd-La Nd-La
0.028 0 031 0 0175
....
....
0.0485 0.014 0.022
Data by Marsh (14) for the nitrate fusion process have been used to calculate factors involving lanthanum, praseodymium, and neodymium. The decreasing difference along the rare earth series is illustrated by the over-all factor of 25 for the three-element range with a factor of only 1.5 between the adjoining praseodymium and neodymium. Gordon et al. (11) fractionated lanthanum-cerium and lanthanum-praseodymium mixtures by precipitation from homogeneous solution with methyl oxalate and reported the original composition, the fraction precipitated, and the lanthanum content of each
V O L U M E 26, NO. 3, M A R C H 1 9 5 4 fraction. These are sufficient data from which to calculate separation factors. The present author has extended this process to the neodymium-samarium pair. Quill and Salutsky (18), in reporting the fractionation of lanthanum and praseodymium by precipitation as carbonate from homogeneous solutions containing trichloroacetic acid, gave the weights and praseodymium contents of source material and precipitate in each step. Separation factors have been calculated from these values. Selwood (19) made a laboratory comparison of various rare earth separation techniques as applied to lanthanum and neodymium and reported the weights and neodymium contents of the original mixtures and of the two fractions. This is more information than necessary for calculation of separation factors, and these factors add much to the meaning of the results. The author has found that useful separation of rare earths can be accomplished by precipitation of their complexes 1%ith mandelic acid (21). Fractionations of synthetic mixtures of neodymium and samarium and complex mixtures of several of the heavier rare earthb and yttrium under various conditions have been evaluated by calculation of separation factors. Separation factors from all of these sources have been compiled in Table I. Since the chemical properties of these elements are, in general, functions of their ionic radii, the compilation includes the difference in radii for each pair. These values are derived from data of Bomnier ( 2 ) and Zachariasen (26). I t is interesting to note that yttrium falls between terbium and dysprosium, rather than in its usual position bet\\ een holmium and erbium. This fact can be observed by making a single precipitation, rather than through a long series.
47s Table 111. Hafnium-Zirconium Separation Factors Phosphoric Acid Process Stage
HfOz Duritv. % -.
Factor
Triethyl Phosphate Process HfOz Duritv. -70 " ' Factor
...
Yield 70 10.0 a Adjusted values are given in parentheses.
...
23.8
~
IS%, both separation fsctors fall very well into line. The adjusted values are in parentheses in the table. With slight changes in the products the separation factors for triethyl phosphate precipitation could become more consistent also. The superiority of the precipitation from homogeneous solutionappears in the factors. DISCUSSION
There may be much information in the notebooks of chemists by which separation factors could be calculated for the fractionation of similar elements by various methods. While this survey has been limited to two groups of elements, there may be much broader fields of application, such as the fractional separation of organic rompmnds. I t is hoped that a t least some of the most promising processes may be re-evaluated by this means and that future workers will use this criterion for determining the efficiency of their fractionations. LITERATURE CITED
Table 11. Hafnium-Zirconium Separation Factors Factors b y Phosphate Process
Factors b y Fluorophosphate Process
4.6 6.3
3.3
...
...
... .. .. ..
8.1
5.7 3.3 4.9
5.8 4.5 I 0.5
Appleton, D. B., and Selwood, P. W., J . Am. Chem. Soc., 63, 2029 (1941).
Bommer, H., 2 . a?torg. 7 ~ .allgem. Chem., 241, 273-80 (1939). Business Week, KO 1083, 46-51 (May 31, 1952). Chem. Eng. News, 30, 1200-01 (1952); 31, 1415 (1953). DeBoer, J. H., 2 anorg. u. allgem. Chem.. 165, 16-20 (1927). DeBoer, J. H., and Koets, P., Ibid., 165, 21-30 (1927). Fassel. V. -4.. J . Ont. SOC.Amer.. 39. 187-93 (1949). Fassel, V. A:, Cook, H. D., Krota,'L. C., ahd Kehres, P. W., Spectrochim. Acta, 5 , 201-9 (1952).
Fassel, V. A,, and Wilhelm, H. A., J . Opt. SOC.Amer., 30, 51826 (1948).
Fisher, W., Dieta, W., and Jiiberman, O., Saturwissenschaften, The fractional separation of hafnium from zirconium has been given much attention, but little is known of the single stage efficiency of any of the methods applied. I n only a very few cases can separation factors be calculated from the information published. DeBoer ( 5 )reported the quantity and hafnium content of the original material and one fraction in the first stage of a separation by phosphate precipitation. He also gave the hafnium content of each fraction in the seventh stage. DeBoer and Koets (6) investigated fractionation by complex fluorophosphates and gave the hafnium content of each fraction throughout an eight-stage fractionation. These values and the corresponding separation factors are s h o w in Table 11. Larsen, Fernelius, and Quill ( 1 3 ) made repeated 55% precipitations with phosphoric acid and reported the hafnium content of each precipitate. Willard and Freund (93, 64)in a study of precipitation from homogeneous solution by triethyl phosphate gave the number of moles of each oxide in the original material and the soluble fraction for each of five stages. Data from these two investigations are in Table 111. It is observed that the separation factors for the first two stages with phosphoric acid appear out of line with the remaining data. With no discredit to these workers in a difficult field, it may be pointed out that the analysis of zirconium-hafnium mixtures has always been extremely difficult and errors of 1 or 2% in the hafnium content of a mixture are not unusual. Presuming to estimate the hafnium content of the first fraction a t 20% rather than
25, 348 (1937).
Gordon, L., Brandt, R. A., Quill, L. L., and Salutsky, M. L., AN.4L. CHEM., 23, 1811-12 (1952). Hahn, R. B., Ibid., 2 3 , 1259-61 (1951). Larsen, E. hI., Fernelius, W. C., and Quill, L. L., IND.EXQ. CHEM.,AKAL.ED., 15, 512-15 (1943). Marsh, J. C., J . Chem. Soc., 1 9 4 6 , 9 . 1RIortimore, D. IT.,and Koble, L. A , ANAL. CHEM.,2 5 , 296-8 (1953).
Norris, J. A., and Pepper, C. E., Ibid., 24, 1399-403 (1952). Peppard, D. F., Faris, J. P., Gray, P. R., and htason, G. W., J . Phvs. Chem., 57,294-301 (1953).
Quill, L. L , and Salutsky, 11. L., ANAL. CHEM.,24, 1453-5 (1952).
Selwood. P. W., J . Am. Chem. S O C . 55. . 4900 (1933). Smith, D. D., and Spitzer, E. J., U. S. Atomic Energy Commission, AECD 2924 (1950). Wearer, B.. A s ~ L .CHEM..26, 476 11954). Weaver, B., Kappelmann, F. .4., and Topp, 4 . C., J . Am. Chem. Soc., 75, 3943 (1953). Willard, H. H., ANAL.CHEM.,22, 1372-4 (1950). Willard, H. H., and Freund, H., IND.ESG. CHEM.,ANAL.ED., \
,
1 8 . 1 9 5 (1946). R.~ C., Arch, A,, and Shyne, W.V., J . Am. Chem. Soc., 6 3 , 957-8 (1941). Zachariasen, W., Norsk Geol. T i d s s k r . , 9, 310-16 (1927). Young,
RECEIVED for review M a y 4, 1953. Accepted June 24, 1953. Presented in part before the Division of Analytical Chemistry a t the 123rd Meeting of the h E R I C A N C ~ E h r i c . 4 SOCIETY, ~ Los Angeles, Calif., 1953. Based on work performed for the Atomic Energy Commission b y Carbide a n d Carbon Chemicals Co., a division of Union Carbide and Carbon Corp., a t Oak Ridge National Laboratory.
