Anion Exchange Resin Separation of the Rare Earths, Yttrium, and Scandium in Nitric Acid-Methanol Mixtures JOHN P. FARIS and JOSEPH W. WARTON' Argonne National laboratory, Argonne, 111.
b The adsorption of the rare earths, yttrium, and scandium from nitric acid-methanol solutions has been studied using strongly basic anion exchange resins. Adsorption measurements were made at room temperature using the column elution technique and employing emission spectrographic analysis. Distribution coefficients were a function of both the volume percentage of alcohol and of the nitric acid concentration. The rare earths were eluted from a column in order of decreasing atomic number with little individual separation of those heavier than dysprosium. Those lighter than terbium had relatively large separation factors. Yttrium behaved like lutecium, while scandium was not adsorbed to an appreciable extent. Application of the system for macro separations has been demonstrated in the preparation of extremely pure rare earths and for fractionation of crude rare earth mixtures. The simplicity of operation is attractive for analytical separation of trace impurities.
A
NUMBER OF COMPLEXING AGENTB
have been successfully used for the chromatographic elution of the rare earths from cation resin columns. A history of the development and a thorough evaluation of the various separation systems for production of high purity materials as well as for analytical separation a t the tracer level have been given recently (16, 22-24).
Although strong base anion exchange resins do not adsorb the rare earths to any great extent from mineral acid solutions (1, 2, 16, 18), appreciable adsorption has been reported from other media. Adsorption from nitrate salt solutions has been noted by Danon (4) and tracer studies have been made by Marcus, Nelson, and Abrahamer (19, 20) in which considerable fractionation of the lanthanides occurred in inorganic nitrate solutions of low Present address, Eugene Dietzgen co.,
Chicago 14, 111,
acidity. Information for lithium chloride (14), thiocyanate (26), citrate (IS), and ethylenediaminetetraacetic acid (EDTA) systems (12, 21) has been presented. The present work centered on the anion exchange adsorption of the rare earths, yttrium, and scandium from mixtures of nitric acid and miscible organic solvents. The effects of acid concentration and solvent composition were investigated in detail using methanol as the added solvent. Because good control of rare earth separations could be achieved using gradient elution techniques, and quantitative separations could be made with simple equipment and inexpensive materials, the system offered good advantage for both micro and macro applications. In recently published studies, a considerable increase in the adsorption of many elements occurred when the aqueous component was decreased by adding miscible organic solvents. Data for anion exchange behavior of the rare earths have been included. Their low adsorption from methanolhydrochloric acid mixtures was used for a group separation from uranium by Tera and Korkisch (26). Korkisch and Tera (17) found the rare earths and thorium to be adsorbed strongly from nitric acid-methanol mixtures and thus separated from many other elements. Fritz and Pietrzyk (11) reported distribution coefficients for dysprosium, lanthanum, and scandium in mixtures of hydrochloric acid and several alcohols. Yttrium and scandium were separated from lanthanum in hydrochloric acid-ethanol mixtures by Wilkins and Smith (27), while Edge (6) found yttrium, neodymium, and lanthanum to be unresolved a t low acidities. Adsorption from n i t r a t e acetone and nitrateethanol (19) mixtures has been reported as a function of the organic solvent content. Edge (6) separated yttrium, neodymium, and lanthanum in dilute nitric acid solutionp containing ethanol and also found appreciable distribution coefficients ( 7 ) for these elements in sulfuric acidethanol media.
EXPERIMENTAL
Materials. The anion exchange resins employed were analytical grade Dowex 1 x 4, 200 to 400 mesh, and Amberlite CG-QOO, 8% DVB, 100- to 200-mesh. Each batch was converted from the chloride form by washing with dilute nitric acid until chloride free and rinsed with distilled water and the fines were decanted. The rcsin was air dried, placed in an oven a t 100' C. for 2 to 3 hours, and then stored in an air-tight container. A weighed amount of resin was placed in a column and conditioned by washing thoroughly with the appropriate solvent mixture. Overnight soaking of the resin in the solvent beforehand eliminated formation of gas bubbles and gave better column performance. Most of the work \vas carried out with 1-gram portions of resin placed in '/,-inch 10-ml. burets having glass wool plugs inserted as a support. The average height of a resin bed was about 3 inches. All eluting mixtures were prepared by taking the required percentage volume of aqueous mixture and diluting to the mark with alcohol-for example, a 10% 7 M "01 (90% methanol) mixture was prepared by adding sufficient alcohol to 10 ml. of 7M HN03 to bring the final volume to 100 ml. Procedure. A stock solution of the rare earths in 10% "03-90% methanol was prepared. A 0.1-ml. aliquot, containing quantities of each element from 5 to 200 bg,, was diluted with 1 to 2 ml. of the eluting solution and allowed to flow slowly on the column, allowing an hour or so for the sorption step. As the elution was continued, the flow rates were increased from 5 ml. to about 30 ml. per hour. For the 1-gram resin columns, fractions of 5 ml. were collected for the first 50 ml.; fractions of 10 ml. or more were collected thereafter. All experiments -\\ere conducted a t room temperature. Analysis. The rare earths in each fraction were determined spectrographically using the copper spark method (10). Samples were evaporated to dryness, redissolved in hydrochloric acid, and brought to volume. Aliquots of 0.1 ml. were evaporated on the flat ends of copper electrode pairs and sparked under standard conditions. VOL 34, NO. 9, AUGUST 1962
1077
I0,OOO
I-
z
w
IO00
c)
LL L
W
0 0 2
0
t 3
m -
IO0
a c
'D
n
IO I
0.005 0.01
0.05 0.1 MOLARITY
0.5
1.0
HNOj
Figure 2. Variation of distribution coefficient with acid molarity in mixtures of 5% nitric acid-95% methanol. Dowex 1 X 4, 200- to 400-mesh resin I
0.01
0.05
1.0 1.5
0.5
0.1 MOLARITY
HNOJ
Figure 1. Variation of distribution coefficient with acid molarity in mixtures of 10% nitric acid-90% methanol. Dowex 1 X 4, 200- to 400-mesh resin
The spectra were photographed in appropriate wavelength regions. Elution curve maxima were approximated from visual examination and comparison of the spectra with those of previously prepared standard plates. A moderate error was introduced because of the asymmetry of some curves, particularly for the more strongly adsorbed elements. Distribution coefficients were calculated from the expression Kd = V / M , where V is the volume in ml. that has passed Table 1.
through the column a t the elution curve peak corrected for the first volume of interstitial liquid displaced and M is the mass of resin in grams. RESULTS
An adsorption dependence on the degree of cross-linking similar to that observed in lithium chloride solutions (24) was found when the two resins were compared. The results following were measured using Dowex 1 X 4
Separation Factors in Several Nitric Acid-Methanol Mixtures
(Expressed relative to gadolinium) 0.3M
80%
90% Methanol10% "01
Element La
Ce Pr
Nd
Pm Sm Eu Gd
Tb
DY Ho Er
Tm Yb
Lu Y
SC
1078
1 . OM 117 67 39 18 (6.8) 2.90 1.55 1.00 0.75 0.65 0.64 0.64 0.62 0.62 0.62 0.55 0.06
7.OM 163 93 54
1.86 1.00 0.73 0.61 0.55 0.53 0.51 0.48 0.46 0.47 0.08
ANALYTICAL CHEMISTRY
15.7M 194 110 61 27 (10) 4.11 1.95 1.00 0.68 0.46 0.38 0.31 0.27 0.24 0.23 0.29 0.02
95% Methanol- Methanol5%HNOa 20%HNO: 7.0M 15.7M 7.OM -30
A20
10.6
2.17 1.00 0.61 0.46 0.40 0.35 0.33 0.30 0.28 0.30 0.01
2.22 1.00 0.60 0.43 0.30 0.25 0.21 0.19 0.18 0.22
...
'5b.1.5270 1.00 0.84 0.80 0.78 0.78 0.78 0.77 0.75 0.67 0.23
Ieobutyrate Cation R&n (3) 34.10 16.70 10.45 6.60 4.10 2.25 1.40 1.00 0.49 0.26 0.16 0.13 0.10 0.075 0.055 0.25
resin because the lower cross-linkage gave better resolution of the elution curves and permitted more accurate estimation of a maximum. Somewhat larger separation factors m d up to 50% higher distribution coefficients were found using the 8% DVB Amberlite exchanger. Distribution coefficients of the rare earths increased with acid concentration in a mixture as shown in Figures 1 and 2. The heavier were eluted first with little resolution from lutecium to dysprosium. Best fracticnation of this group occurred in higher molarity nitric acid, where evidence for adsorp tion maxima was observed. From terbium to samarium, a gradual increase in slope of the adsorption curves led to somewhat larger separation factors as a solution became more acid. The task of collecting large volumes of effluent for elution of the lighter rare earths was prohibitive at the higher adsorptions, although smaller columns containing 0.1-gram resin were employed for some determinations. The broken lines are logical extensions of the curves and have been ascertained as qualitatively correct from batchwise experiments using irradiated rare earths and radiochemical measurements of the nonadsorbed activity. The adsorption function of yttrium was slightly lower than that of lutecium except a t high acidity. Scandium for the most part was not appreciably adsorbed and behaved differently from the rare earths. Maximum adsorption occurred in solutions of 0.7M to 1.OM nitric acid (Figure 1).
In Figure 3 the distribution coefficient relationship with percentage of water in the solvent mixture is illustrated a t a constant acid molarity. Fractionation of the rare earths was greatest in the least aqueous solutions and separation factors rapidly decreased as the aqueous percentage was increased. The behavior of yttrium was nearly that observed for lutecium. Scandium adsorption was at a maximum in mixtures of about 80% alcohol. Separation factors in several selected mixtures are given in Table I. Those of Choppin and Silva (3) for cation exchange separation with a-hydroxy isobutyric acid are included for comparison. The adsorption of promethium, while not studied in detail, was between that of samarium and neodymium. The ratios given were predicted from adsorption us. atomic number graphs. APPLICATION AND DISCUSSION
Ideally, a chromatographic sepsration completely resolves the components of a mixture. While quantitstive isolation of an element was not always possible in the present system, the results given in Figures 1 to 3 permitted selection of an optimum mixture for a given rare earth separa-
1000
tion. Because the aqueous component had the greatest effect on adsorptions, a proportion was chosen to give the most satisfactory separation factor. The rate of movement down the column was then controlled by adjusting the acidity. To illustrate application of the adsorption data, a synthetic mixture of rare earths was prepared and carried through an arbitrary separation procedure. About 1 mg. each of yttrium, gadolinium, samarium, neodymium, and lanthanum and 0.1 mg. of europium were dissolved in nitric acid, reduced to a small volume, and diluted with approximately nine parts of methanol. The sample was adsorbed on a 1-gram resin column and then eluted, collecting the effluent in fractions. An aliquot of each effluent fraction was taken and the spectra were photographed, Figure 4. A similar procedure was used to prepare high purity terbium for use as an emission source for study of the arc spectrum. Fifty mg. was passed through a 1-cm., 3-g. resin column in 5% 10M nitric acid; only the portion collected a t the peak elution was converted to the oxide for use. Macro quantities of extremely pure neodymium and praseodymium have also been similarly prepared from commercial 99.9% oxide. Purity of the final product was verified by neutron activation analysis. Promethium was separated from a
mixture of rare earths in a single column run in 20% 2M to 5M nitric acid. The isolation of neodymium from a crude rare earth niixture recovered from monazite required two passes through a resin column. A 1- to 2gram sample of the raw material was dissolved in excess nitric acid, evaporated to a volume of about 20 ml., and methanol added. The covered solution was allowed to stand overnight to allow sufficient time for cerium to be reduced. Progress of this reaction can be followed by observing the loss of bright red ceric ion color. The sample was then adsorbed on a 1.7-cm., 20gram resin column and eluted with a 20% 2M nitric acid mixture. The visible neodymium band collected contained appreciable percentages of the other light rare earths and some thorium. After a second pass through a column, neodymium was recovered in which no impurities could be detected spectrographically. Also, many of the nonadsorbed impurities (bo), such as phosphorous, iron, and magnesium, were removed in the first 100-ml. fraction and could be discarded. The heavier rare earths and yttrium were eluted in the next several fractions, suggesting possible use of the system to concentrate those less abundant for subsequent cation exchange separation. For analytical use, an effluent collected from similar separational procedures was concentrated and examined
L
I
5
IO
PER CENT
I5 20
30 40
7M HN03
Figure 3. Variation of distribution coefficient with proportion of 7M nitric acid. Dowex 1 X 4, 200- to 400-mesh resin
Figure 4. Spectrograms (4075 A. to 4 1 3 5 A.) of effluent fraction samples showing separation of a rare earth mixture. At 50 mi. of effluent the increment was changed from 10 to 20 ml. The change in composition of the eluting mixture i s also indicated at the left. The resin bed was 6.3 mm. in diameter X 75 mm. long, Dowex 1 X 4, 200- to 400-mesh VOL 34, NO. 9, AUGUST 1962
1079
spectrographically. Methods similar to that described for the analysis of thorium (8, 9) could be applied readily for determination of impurities in the parts per million range. I n the samples previously cited, at least one other rare earth was found to be present as a trace impurity in the commercial starting material. In preliminary experiments, the rare earths were adsorbed by an anion exchange resin from mixtures of nitric acid and a number of miscible organic solvents. Distribution coefficients increased when the added solvent was a higher alcohol, acetone, cellosolve (2ethoxyethanol) , dioxane, tetrahydrofuran, or an alcohol-ester mixture. Methanol was chosen primarily because the solubility of rare earth nitrates was extremely good. Nitric acid solutions of up to 175 mg. per ml. of rare earth oxides did not precipitate when methanol of any proportion was added. Only one of the other solvents tested, Cellosolve, was satisfactory in this regard.
ACKNOWLEDGMENT
The authors appreciate the counsel and encouragement given by several members of the laboratory. We are
indebted to John Hines for radiochemical analyses of the rare earth activities and for isolation of the promethium and to Morris Wahlgren for neutron activation analyses of the neodymium. LITERATURE CITED
(1) Buchanan, R. F., Faris, J. P., Conf.
on Use of Radioisoto es in Ph . Sci. and Industry, Rept. dCC/173, 6openhagen, Denmark, 1960. (2) Bunney, L. R., Ballou, N. E., Pascual, J., Foti, S., ANAL.CHEM.31,324 (1959). (3) Choppin, G. R., Silva, R. J., J.
Inorg. Nucl. Chem. 3, 153 (1956). (4) Danon, J., Ibid., 7, 422 (1958). (5) Edge, R. A., J. Chromatog. 5, 539 (1961). (6) Ibid., p. 526. (7) Ibid., 6, 452 (1961). (8) Faris, J. P., .4ppl. Specfr. 12, 157 (1958).
(9) Faris, J. P., Buchanan, R. F., Nucl. Reactor Tech. TID-7606, p. 185, 4th
Conference, Gatlinburg, Tenn., October 1960, Oak Ridge National Laboratory. (10) Fred, M., Nachtrieb, N. H., Tomkins, F. s., J . Opt. SOC.Am. 37, 279 I 1 947). \ - - - . I .
(11) Fritz, J. S., Pietrzyk, D. J., Talanta 8 , 143 (1961). (12) Higgins, C. E., Baldwin, W. H., U. S. Atomic Energy Comm., Oak Ridge Nat. Lab. Rept. ORNL 894 (19.51
\ - - - - I .
).
(13) Huffman, E. H., Oswalt, R. L., J. Am. Chem. SOC.72, 3323 (1950). (14) Hulet, E. K., Gutmacher, R. G.,
Coops, M. S., J . Inorg. Nucl. Chem. 17, 350 (1961). (15) Ichikawa, F., Bull. Chem. SOC.Japan 34, 183 (1961). (16) James, D. B., Powell, J. E., Spedding, F. H., J. Inorg. Nucl. Chem. 19, 133 (1961). (17) Korkisch, J., Tera, F., ANAL. CHEW 33, 1265 (1961). (18) Kraus, K. A., Nehon, F., Am. SOC. Testzng Mater., Spec. Tech. Publ. No. 195, p. 27, 1958. (19) Marcua, Y., Abrahamer, I., Israel Atomic Energy Comm. Rept. IA-608 (1961). (20) Marcus, Y., Nelson, F., J . Phys. Chem. 63, 77 (1959). (21) Minczewski, J., Dybcsynski, R., J . Chromatog. 7, 98 (1962). (22) Powell, J. E., “The Rare Earths,” p. 55, F. H. S edding and A. H. Daane, eds., Wiley, ew York, 1961. (23) “Rare Earth Elements,” English transl., p. 97, OTS 60-21172, originally Dublished bv Acad. of Sci.. U.S.S.R.. 1959. (24) Stevenson, P. C., Nervik, W. E., “The Radiochemistry of the Rare Earths, Scandium, Yttrium, and Actinium,” Natl. Acad. Sci.-Natl. Research Council, Nuclear Science Series NAS-NS-3020 (1961). (25) Suds, J. P., Jr., Choppin, G. R., J . Inorg. Nucl. Chem. 4, 62 (1957). (26) Tera, F., Xorkisch, J., Anal. Chim. Acta 25, 222 (1961). (27) Wilkins, D. H., Smith, G. E., Talanta 8, 138 (1961).
k
RECEIVEDfor review March 7, 1962. Accepted May 31, 1962. Based on work performed under the auspices of the U. S. Atomic Energy Commission.
Cation Exchange Separation of Vanadium from Metal Ions JAMES S. FRITZ and JANET E. ABBINK Institute of Atomic Research and Department of Chemistry, Iowa State University, Arnes, Iowa
b Vanadium(1V) or (V) can be separated from other metal ions by elution from a cation exchange column with dilute acid containing 1% or less hydrogen peroxide. Vanadium is quantitatively removed as a vanadium(V)hydrogen peroxide complex; the other metal ions are eluted later with stronger acids. Separations of vanadium(lV) or (V) from 25 metal ions are reported. Varying ratios of vanadium(V) to iron(ll1) up to 1 : 100 are separated.
A
T THE outset, an attempt was made
to separate vanadium(V) from various metal cations, using a cation exchange column in the hydrogen form. Vanadium(V) exists either as anionic VOs- or as a cationic form such w, V02+, which can be eluted readily from a cation column with dilute acid. Experiments showed that while the
1080
0
ANALYTICAL CHEMISTRY
bulk of vanadium(V) is eluted readily from a cation exchange column, some vanadium is apparently reduced by the column to VO+s, which is not eluted. Vanadium, in the form of its hydrogen peroxide complex (4), passes readily through a hydrogen-form cation exchange column. Furthermore, any vanadium(1V) is oxidized by hydrogen peroxide to the vanadium(V)-hydrogen peroxide complex and is also eluted from the column. Alimarin and Medvedeva (1) developed an ion exchange method in which molybdenum is eluted from a cation exchange column by a dilute solution of hydrogen peroxide. They noted that vanadium and tungsten are eluted along with molybdenum. Ryabchikov and Bukhtiarov (S) found that titanium and tungsten form complexes with hydrogen peroxide at p H 5; a cation exchange column retains the titanium complex but the tungsten
complex passes quantitatively through the column. The purpose of the present work is to study the ion exchange separation of vanadium from other metal ions by elution of the vanadium from a cation exchange column with an acidic solution of hydrogen peroxide. Although molybdenum and tungsten interfere in this separation, vanadium can be separated from titanium, iron, and many other metal ions. EXPERIMENTAL
Ion Exchange Resin. Dowex 50WX8, 100- to aOO-mesh, is used in the hydrogen form. One pound of resin is purified by placing it in a large column and backwaahing with distilled water to remove the fine particles. Then it is washed with 3 liters of 10% ammonium citrate followed by 3 liters of 3M HCl. It is washed with water until the eluent gives a negative chloride test with AgNOs.
