Separation of uranium from other metals by partition chromatography

Ttita. 1966,Vol. 13.pp. 123u) 128. Per@monRen Ltd. Prtatcdin NorthernIreland

SEPARATION OF URANIUM FROM OTHER METALS BY PARTITION CHROMATOGRAPHY* JAMES S. FRITZ and DONALD H. SCHMITT Institute for Atomic Research and Department of Chemistry, Iowa State University Ames, Iowa, U.S.A. (Received 27 July 1965. Accepted 9 September 1965) Samtnar~--Uranium(VI) can be separated quantitatively from most other metal ions by partition chromatography on a silica-gel column. The column is treated with aqueous 6Mnitric acid; after sorption of the sample, uranium(VI) is selectively and rapidly eluted by methyl isobutyl ketone. In addition to the separation of macro quantities of metal ions, the method has been used successfully for the isolation of trace amounts of metal ions from uranium(VI). INTRODUCTION

A NUMBERof inorganic analytical separations are based on the batch extraction of ion-association complexes by an organic solvent. Separation by batch extraction requires that one metal or group be quantitatively extracted, while the extraction of other metals is essentially zero. However, separation by partition chromatography merely requires an appreciable difference in extractability. Although several useful separations based on reversed-phase chromatography have been developed, it is surprising that so little has been done on the selective elution of metal ion-association complexes from a column by an organic solvent. A short note1 described the successful elution of iron(II1) with di-isopropyl ether from a Hyflo Super-Cel column treated with 6.5M hydrochloric acid. Two other papers2ss described the elution of the uranyl ion from cellulose columns with nitric acid in ether as the eluent. Hara4 separated uranium(W) from several other metal ions by elution with 1% nitric acid in ethyl ether from a specially prepared silica column. Seiler and Seiler5 and also Takitani, Fukazawa and Hasegawas have separated a number of inorganic ions by thin-layer chromatography using a silica-gel coating. In the present work, uranium(V1) is separated quantitatively from other metals using a silica-gel column treated with 6M nitric acid. Uranium is eluted quickly and selectively from the column with methyl isobutyl ketone (MIBK). The metal ions remaining on the column can be readily eluted with 6iU nitric acid and determined without difficulty by standard analytical methods. EXPERIMENTAL Reagents Methyl isobutyl ketone (MZBK). Eastman Reagent grade was used. Silicagel. This was standard chromatographic grade which was sieved to 60-80 mesh and washei repeatedly with 6M hydrochloric acid until no evidence of iron was present, then washed with 95 y0 ethyl alcohol and dried in an oven at 110” for 24 hr. 6M Nitric acid. Prepared and equilibrated with MIRK by shaking for 1 min in a separatory funnel. The 6M nitric acid was equilibrated before each group of separations. * Work was performed at the Ames Laboratory of the U.S. Atomic Energy Commission, Contribution No. 1762. 123

124

J. S. FRITZ and J. H. Scmfrrr

@lM Uranium(Yr) nitrate solution. Prepared from reagent-grade UOa(N0&..65Hn0 and kept in 1% nitric acid. QlM Metal ion solution. Prepared by dissolving reagent-grade nitrate or chloride salts in water with sufficient acid present to prevent hydrolysis. O.OSMEDTA. Prepared from the reagent-grade disodium salt. It was standardised by a titration of standard zinc@) solution using Naphthyl Azoxine S (NAS) as the indicator.” Apparatus

Conventional 1 x 15 cm, coarse frit, chromatographic columns with Teflon stopcocks were used. A Nuclear-Chicago scintillation counter model DS5 with a sodium iodide crystal was the detector; a Nucl~-Chi~go recording spectrometer, model 1820, isolated the y-emission of seFe; and a decade scaler counted the pulses received from the spectrometer. Procedure Column packing. A sufficient amount of 60-80 mesh silica gel for four columns was siurried with 6M nitric acid equilibrated with MIBK. The slurry was added to the colurmis with gentle tapping of the columns to ensure uniform packing. The excess 6M nitric acid above the gel was drained off. Separation procedure. The samples, each of which contained 3066 pmole of uranium(X) plus another metal ion, were evaporated to a volume of l-2 ml. This evaporation usually made the sample 2-3M in nitric acid; if not, additional nitric acid was added to bring it to 3M. Before adding the sample, 2-5 ml of dry silica gel were added to the top of each column to ensure a better sorption. The samples were transferred to each column using equilibrated MIBK to wash them onto the columns; and the elution was continued with the equilibrated MIBK at a flow rate of @S-1*1ml/mm. The complete elution of ur~ium~ was qualitatively tested with potassium he~yanof~at~ as a spot test and was found to take about 35 ml. The other metal ion was stripped with 50 ml of 6M nitric acid, except for molybdenum(W), thorium(W), zirconium(IV) and titanium(IV). The molybdenum(VI) and thorium(IV) required 8~100 ml of 6M nitric acid to strip them quantitatively from the cohunn, while zlrconium(IV) and ti~~~~~ were stripped with 60 ml of 25M sulphuric acid and with 50 ml of 4M sulphuric acid plus @6% of hydrogen peroxide, respectively, Analysis of column eluates. The uranium(W) in the MIBK fraction was analysed by first adding 50 ml of water to each sample and evaporating off the MIBK. The sample was then evaporated to a few ~iiiii~~ to remove excess nitric acid and d&ted to 200 ml with water. The sample was passed through a 1 x 3.5 cm column containing the cationexchange resin Dowex 50 x 8 to remove the nitrate. Uramum(VI) was eluted from the column with 60 ml of 3M hydrochloric acid and analysed by the lead reductor method with cerium(IV).* The other metal ions were analysed by either direct or back-titrations with 0,OSMEDTA except for molybdenum(VI) which was determined gravimetrically with S-hydroxyqumoline. The JO pg of copper(H) were analysed by a spectrophotometric method using dieth idithlocarbamate. Trace amounts of “Fe were determined with the scintillation counter. Sma r1 amounts of dysp~sium~ were determmed by flame photome~.

RESULTS

AND

DISCUSSION

Excellent separations of uraniu~~1) from other metal ions are obtained provided the proper technique is employed. It is necessary to equilibrate the aqueous 6M nitric acid, used to treat the dry silica gel, with the MIBK used to elute the uranium. If this is not done, cracks and fissures develop in the column as the elution proceeds. Addition of a little dry silica gel to the top of the column and a small sample volume are required to prevent the sample band from moving too far down the column before elution with MIBK. A typical elution curve for uranium is shown in Fig. 1. From the curve, the theoretical plate height was calculated from the relation rr = 16fvIJ~)~ a and found to be 1.48 cm. The actual retention volume (VR) was also compared with the theoretical retention volume using the relation V, = V, + (l/D)V,.lO The actual Vn was 12-4 ml, while the calculated Vn was 10.6 ml. Results for quantitative separations are summarised in Table I. In general, the results for separation and analysis of the metals separated from uranium fall within normal analytical error. The relative standard deviation for 68 individual analyses

125

Separation of uranium

I

I

I

I2

16

20

I 0

4

6

EFFLUENT,

I

\

24

26

ml

Fm. l.-Blution curve for uranium(VI) from a 60-80 mesh silica-gel column using a 6M nitric acid-MIBK system [column: 1 x 16.6crn (including dry gel); flow rate: 0+8 ml/mm; sample solution volume: 1 ml; uranium(W): 306.6 pmole].

TABLE &-SmmTION RESULT

OF URANIUM(VI)

IS THE AVERAGE

Metal ion

OF FOUR

(306.6qOL~)

iNDIVIDUAL

FROM

SEPARATIONS

OTHER AND

METAL

Taken,

Found,

Difference,

pmole

pmole

pmole

3w9

305.9

CaO Ce(II1) COUI) CUOI) Er(II1) WIII) LaWI) M&II) Mo(V1) NdO NiO IWID Th(IV) Ti(IV) Zr(Iv)

126.1 307.6 534.6 299.6 327.3 301.9 311.1 240.0 322.4 308.4 198.6 301-8 306.3 296.7 95.5 348.4

125.7 307-g 535.4 299.3 327.6 302.4 311.4 239.9 323.7 309.4 198.7 300.4 3060 297.0 96.3 348.2

+1*0 -0.4 +0*3 +0x8 -0.3 +0*3 +05 +0*3 -0.2 -t-l*3 +1*0 -l-o*1 -1.4 -0.3 +0*3 +o*s -0.2

WVI)

306.6

3065

-0.1

AWI)

Bi(III)

IONS (EACH

DElXRMrNArrONS)

126

J. S. FRITZ and D. H. m

was 0.29 ‘A and the average recovery was 100.05 %. The separation of uranium from other metals was complete, as evidenced by quantitative recovery of uranium(W) from all samples in which uranium was determined. The behaviour of many elements not studied in this research can be predicted from distribution ratios for extraction of uranium from aqueous nitric acid into MIBK. Maeck et al.ll have made a comprehensive study of the extraction of metal ions into MIBK from aqueous nitric acid. They add quaternary ammonium salts to increase the extraction of metals. Their work indicates that the following additional elements are not appreciably extracted from 5M nitric acid and should, therefore, be separated from uranium(V1) on a silica-gel column: alkali metals, strontium(II), barium(II), scandium(III), yttrium(III), vanadium(V), chromium(IIl), chromium(VI), manganese(II), silver(I), zinc(II), cadmium(II), gallium(III), indium(III), arsenic(III), antimony(II1) and protoactinium(V). Their data indicate that the following elements may accompany uranium(V1) in our separation: technetium(VII), gold(III), mercury(II), polonium(IV), neptunium(IV), plutonium(IV) and some of the platinum metals. It is interesting that thorium(W) and zirconium(IV) have a great preference for the stationary phase and are efficiently separated from uranium(VI), because both elements interfere in the reversed-phase chromatographic separation using tributyl phosphate.12*r3 The explanation seems to be that silica gel has ion-exchange properties that are sufficient to retain quadrivalent metals like thorium(IV), titanium(IV) and zirconium(IV), even from, strongly acidic solutions.14*15 These ion-exchange properties make the removal of quadrivalent metals from the column more difficult following the uranium elution. Thorium requires more than the usual volume of 6M nitric acid for elution; zirconium and titanium require the complexing action of sulphuric acid and hydrogen peroxide (peroxide not needed for zirconium) for complete elution. The effect of several common anions on the separation was investigated (Table II). Complexing anions, such as fluoride, phosphate and sulphate, slow down or prevent the elution of uranium(V1) if present in too high a concentration. However, moderate concentrations of these ions and high concentrations of chloride or perchlorate in the sample cause no interference. TABLE II.-EFPEC~

OF ANIONS ON THE ELUTIONOF URANUJM(VI)(306.6 ~MOLE) SAMPLE VOLUME OF 2 ML WHICH WAS AL.90 3&f IN NITRIC ACID)

Anion

Max. allowable concentration, M

Elution volume required, ml

ClClOaFPoIsSO*B-

>3.0 >1.5 0.25 1.00 2.25

35 (no increase) 35 35 81 72

(IN A

Larger-scale separations were carried out using a 3.1 x 20 cm column packed with 80-120 mesh silica gel. After treatment of the silica gel with 6M nitric acid, the interstitial nitric acid was displaced from the column with equilibrated MIBK. The sample was then added in aqueous nitric acid and the uranium eluted with MIBK.

Separation of uranium

127

By this method 6.2 g of uranium(W) were successfully separated from O-7g of cobalt. The maximum sample volume used with this column was 20 ml. Silica gel is low in cost and might well be utilised for many types of laboratory or large-scale separation, such as the one shown by Hultgren and Haeffner.ls The method described above is inherently more simple than counter-current, pulse columns and other larger-scale separation methods based on liquid-liquid partition. Separation of trace amounts of several metal ions from uranium(V1) was successfully carried out (Table III). Regular 1 x 15 cm columns were used ; the samples TABLE

III.-SEPARATIONS

OF URANIUM(M) OTHER

(306.6

,UMOLE) FROM

TRACE

AMOUNTS

OF

METALS

Metal ion

Taken, I%

Found, N

Difference, w

WII) Fe@I) DYO

50.0 0.11 21.6

50.9 0.12 , 18.5

+0*9 +0.01 -3.1

Parts of metal ion per million parts of u(vI) 675 1.6 293

contained 306.6,~ mole of uranium(W) and the sample volume was 2-O ml. The concentrations of trace metal in the uranium(W) ranged from 1.6 to 675 ppm. Other samples containing iron, copper, lead, bismuth, zinc and neodymium in concentrations ranging from 13.6 to 5730 parts of metal ion per million parts of uranium(W) present were separated and roughly estimated by emission spectrography. The recovery appeared satisfactory, although the analyses were not very accurate. One difficulty in separating trace elements from uranium is caused by the impurities introduced by silica gel. The major impurities introduced are calcium, magnesium, aluminium, sodium, boron and titanium. It may or may not be possible readily to remove these impurities. Uranium(X) can be eluted from a silica-gel column with tributyl phosphate (TBP), as well as with MIBK. However, TBP does not volatilise readily, and backextraction of the eluted uranium from TBP into water is often incomplete because of the partial conversion of TBP to mono- or di-butyl phosphoric acid. Methods in the literature have employed aluminium nitrate or some other salting-out agent for batch extraction of uranium into MIBK. However, a salting-out agent often interferes with the analysis of elements separated from uranium and such an agent is not needed in the column separation proposed here. The silica-gel column should be useful for analytical separations involving other ion-association complexes. From the batch distribution coefficients in Table IV it appears that mercury(I1) should be able to be separated from many metal ions by the TABLE

N.-BATCH IONS IN A

DISTRIBWITON COEFFICIENTS FOR METAL

6M

NITRIC

ACID-MIBK SYSTEM

Metal ion

Batch distribution coefficients

HgOI) Th(IW UVI)

1.42 0.023 1.10

128

J. S. Farrz

and D. H. SCHMITT

system described above. We have also obtained sharp quantitative separations of iron(III) from cobalt(U) and from other metal ions using a silica-gel column pretreated with hydrochloric acid (1 + 1). The iron(II1) is eluted with equilibrated MIBK, then the other metal ions are removed from the column with hydrochloric acid (1 + 1). Zusammenfassnng-UrancvI) kann durch Verteihmgschromatographie an einer Silicagelsiiule von den meisten anderen Metallionen quantitativ abgetrennt werden. Die S&de wird mit 6M w%lriger Salpetersibrre behandelt; nach Sorption der Probe wird Uran(VI) selektiv und schnell mit Methylisobutylketon eluiert. Zutitzlich zur Abtrennung von Makromengen Metallionen wurde die Methode erfolgreich zur Isolierung von Metallspuren aus Uran(VI) benutzt. R&stun&&r peut Separer quantitativement l’uranium(V1) de la plupart des autres ions metalliques par chromatographie de partage sur colonne de silica-gel. La colonne est train% par l’acide nitrique aqueux 6M puis, apr&s adsorption de l’echantillon, l’uranium(V1) est selective ment et rapidement Bue par la methyl-isobutyl-cetone. En plus de la separation de macro quantitds d’ions metalliques, la m6thode a et6 appliquQ avec succ& a l’isolement de traces d’ions m6talliques dans l’uranium(V1). REFERENCES 1 F. Glue&auf, Discussions Far&y SOL, 1949,7,238. * F. H. Burstall and R. A. Wells, Analyst, 1951,76, 388. I N. F. Kember, ibid., 1952, 77, 78. ’ T. Ham, Nippon Kagaku Zasshi, 1957,78,337. 6 H. Seiler and M. Seiler, Helv. Chim. Acfa, 1961,44,939. 0 S. Takitani, M. Fukazawa and H. Hasegawa, Bunseki Kagaku, 1963,12,1156. 7 J. S. Fritz, J. E. Abbink and M. A. Payne, Anal. Chem., 1961,33,1381. o C. W. Sill and H. El. Peterson, ibid., 1952,24, 1175. a F. ,Glueckauf, Trans. Faraday Sot., 1955,51,34. I0 J. J. van Deemter, F. J. Zuiderweg and A. Klinkenberg, Chem. Eng. Sci., 1956,5,271. l1 W. J. Maeck, G. L. Booman, M. C. Elliot and J. E. Rein, Anal. Chem., 1958,30,1902. r’ A. G. Hamlin, B. J. Roberts, W. Loughlin and S. G. Wells, ibid., 1961, 33, 1547. I* T. J. Hayes and A. G. Hamlin, Analyst, 1962, 87,770. l* S. Ahrland. I. Grenthe and B. Noren, Acta Chem. Scamf., 1960,14, 1059. IS I&m, ibid.,.p. 1077. l@A. Hultgren and E. Haeffner, Proc. Zndlnter. Conf. Peaceful Uses At. Energy, Geneva, 1958,17,324.