Cation exchange separation of scandium from rare earths in oxalic acid media

IHORG,

NUCI..

CHEM.

LETTERS

Vol.

5,

pp.

325-331,

1969.

Perg~on

Pre...

Printed

in

Great

Britain

CATION EXCHANGE SEPARATION OF SCANDIUM FROM RARE EARTHS IN OXALIC ACID MEDIA K. A. Orlandini Chemistry Division, Argonne National Laboratory, Argonne, 111.60439 (Received 24 J ~ u a ~ 1969)

Introduction One of the more difficult chemical separations to achieve, using ion exchange techniques, is that of scandium from the rare earths including yttrium. available

(1-4)

Most of the ion exchange methods

for this purpose exhibit low separation factors

(< 20) between scandium and the rare earth group.

A notable

exception in this respect is a recent cation exchange method employing an organo-phosphorus

complexing agent in a mixed

organic-aqueous system in which a separation factor of over 4000 was attained (5). The separation method presented in this paper is based on the elution of. scandium as an oxalate complex from a cation exchange column which under the same conditions strongly retains the rare earths and yttrium.

The technique features a high

separation factor (> 800) between scandium and the rare earth group and allows high column flow rates (up to i0 ml/min) with small (2 gram) resin columns. Experimental Apparatus and Reagents.

Pyrex columns of 0.8 cm I.D. and 15 cm

length fused to a 200 ml reservoir were used. controlled by including a pressure coupling on the reservoir.

Flow rate was

(ball joint inlet)

All chemicals were of reagent grade. 325

CATION EXCHANGE SEPARATION

326

Distribution Coefficients and Tracers.

Vo{. 5, No. 4

The distribution coeffi-

cients of various elements were determined by a batch technique reported earlier (6).

A number of radioactive tracers were used.

These included 46Sc, 144Ce, 140La, 169yb, 22Na, 137Cs, 45Ca, 85Sr,

44Tij 95Zr, 181Hf, 95Nb, 54Nn, 60Co, 230Th, 233pa, 241Am, and 239pu" Column Preparation and Workin$ Procedure (Example).

The

following procedure employs a 2 gram resin column and is suitable for separating up to 150 mg scandium containing rare earths and yttrium as minor constituents (up to 1% by weight). A nitric or hydrochloric acid solution of scandium (up to 150 mg) is evaporated to near dryness.

The acid moist residue is

taken up in distilled water to a volume corresponding to 1.8 ml for every milligram of scandium present.

The resulting solution

is made 0.i M in oxalic acid by addition (with stirring) of an appropriate volume of saturated (~ i M) oxalic acid solution. Upon passage of this scandium oxalate solution through the cation exchange column, the rare earths and yttrium are retained and the scandium is eluted.

For example:

2 grams of air-dried Dowex

AG50WX8, 100-200 mesh, hydrogen form, were transferred to an ion exchange column with distilled water.

The resulting resin bed,

(0.8 cm diam by 5.5 cm length) supported by a small quartz wool pad, was washed with 10-15 ml 0.i M oxalic acid in water (eluent solution).

I00 mg of scandium metal was dissolved in concentra-

ted hydrochloric acid and the solution evaporated to near dryness. The acid moist residue was taken up in 180 ml distilled water followed by addition of 20 ml saturated (~ i M) oxalic acid.

The

resulting feed solution was passed through a 2-gram cation column (prepared as above) at a flow rate of i0 ml/min.

More than 90%

Vol. 5, No. 4.

CATION EXCHANGE SEPARATION

327

of the scandium accompanied the feed fraction (200 ml volume). The remaining scandium (< 10%) was removed from the resin column by passing an additional 80 ml of 0.i M oxalic acid solution. Residual oxalic acid was rinsed from the column with 10-15 ml 0.2 M HNO 3 before the rare earths including yttrium were eluted with 5 M HNO 3 (or 5 M HCI).

Scandium was quantimtively recovered

as the oxide by evaporation of the effluent and ignition of the oxalate residue. Flow rate is not critical in the above procedure and column runs have been made at 12 ml/min.

High flow rate compensates for

the inconvenience of larger feed volumes necessary with 100-150 mg of scandium.

For less than 25 mg scandium a smaller column

(i gram) is sufficient. Modified Workin$ Procedure.

The above procedure can be modified

to separate a matrix of rare earths including yttrium from scandium occurring as a minor constituent.

The rare earths which

would otherwise precipitate as insoluble oxalates are held in solution by making the feed solution 1 M in nitric acid prior to addition of oxalic acid.

The resulting solution is passed

through a cation exchange column and the rare earths are retained while scandium is eluted by the nitric-oxalic acid mixture.

For

example - 20 mg lanthanum plus 5 mg scandium containing 46Sc, 144Ce, and 169yb tracers were dissolved in 18 ml 1 M HNO 3.

2 ml

saturated (~ 1 _~ oxalic acid were added with stirring and the resulting solution (i M HNO3, 0.i M H2C204) was passed through a 1 gram cation resin bed (0.5 cm diam. x 9 cm length) at a flow rate of approximately 1 ml/min. accompanied the feed fraction.

About 75% of the scandium Scandium remaining on the column

(~ 25%) was eluted~ at the initial flow rate, with 40 ml of a

328

CATION EXCHANGE SEPARATION

i M HNO3, 0.i M H2C204 solution.

Voh 5, No. 4

Because the affinity of the

cation resin for rare earths is considerably lower in nitric oxalic acid solutions than in the pure oxalic acid solution (c.f. Table I), high flow rates were avoided to prevent rare earth breakthrough. TABLE 1 Distribution Coefficients of Sc, Yb, and Ce on Dowex 50 at Various Nitric-Oxalic Acid Concentrations H2C204

~HNO 3

46Sc

144Ce

169yb

7.4

65390

6196

0.i

--

0.i

0.5

10.3

4222

883

0.i

i*

10.6

663

250

0.i

2

9

87

49

--

i

343

519

272

1"Yttrium distribution in this media is 380.

Solubility of Rare Earth and Scandium Oxalates.

Scandium at

0.5 mg/ml concentration in both oxalic acid and nitric-oxalic acid feed mixtures remains in solution for about 20 hours.

Since

complete separation of up to 150 mg scandium requires less than 2 hours, the eventual precipitation of scandium is not considered a practical interference. Freshly precipitated rare earth oxalates are readily dissolved by nitric acid of at least 3 molar strength.

Moreover,

the formation of aqueous insoluble rare earth oxalates is suppressed in the presence of dilute nitric acid.

For example:

1 M HNO 3 solutions containing up to 1 mg/ml lanthanum were free of visible precipitation for about 15 hours after being made 0.I M in oxalic acid (c.f.~ Modified Working Procedure).

Vol. 5, No. 4

CATION EXCHANGE SEPARATION

329

Results and Discussion Table i indicates

the effect of nitric and oxalic acids on

the cation exchange behaviour members

of scandium and representative

of the rare earth group cerium and ytterbium.

exchange

separation

factor between

The cation

scandium and ytterbium

in

0.i M oxalic acid is 837 and nearly 9000 for the scandium-cerium pair.

Several

scandium

separation

experiments

containing up to I%(W)

in oxalic acid media with

rare earths and yttrium revealed

that more than 99.99% of the rare earth group including yttrium was removed small

from the scandium matrix in a single pass through a

(2 gram)

Dowex 50 column.

Furthermore,

less than 0.004% of

the scandium remained on the column after elution with oxalic acid according

to the working procedure.

For scandium samples

containing more than impurity levels of rare earths total)

mixtures

example,

of nitric and oxalic acids are employed.

using the modified procedure

Experimental), consisting

(> i mg

given above

scandium was cleanly separated

of i0 mg scandium,

For this separation

a small

For

(c.f.,

from a mixture

5 mg lanthanun, and i mg ytterbium. (i gram)

Dowex 50 column and a i

HNO3, 0.i M H2C204 media were employed. lanthanum and ytterbium accompanied

Less than 0.05% of the

the scandium of which less

than 0.01% remained with the rare earths on the column. Table 2 indicates of elements insoluble

the cation exchange behaviour of a number

in 0.i M oxalic acid.

oxalates

in oxalic acid solution

Zn), many can be separated precipitation

that form

(e.g., Ca, Co, Cu,

from scandium and rare earths by

of the latter elements with ammonium hydroxide

prior to column separations oxalate

Of the elements

soluble elements

in oxalic acid media.

Certain

such as uranium are eluted from Dowex 50

330

CATION rXCHAHGE SEPARATION

Voh 5, No. 4

more readily than scandium and are likewise separable from rare earths.

This behaviour was verified in the case of a uranium \

matrix containing radioactive rare earth tracers. It is expected that the methods given above will be applicable to analytical and purification problems. This work was done under the auspices of the U. S. Atomic Energy Con~nission. TABLE 2 Distribution

Coefficients of Various Elements in 0.i M Oxalic Acid on Dowex 50

Element Sc

Kd 7.4

Element

Kd

Element

Kd

Ti (IV)

< i

AI

~103**

Na

i00

Zr

< 1

Ga

~i02.*

Cs

473

Hf

< 1

In

i00

Mg

>103**

Nb(V)

< i

Fe(lll)

Ca*

4375

Mn (II)

5250

TI(1)

Sr*

900

Co (II)

4000

Sn (IV)

Ba*

~103

60

Pb(ll)

Ni(ll)*

Ce(lll)* 65390

Ag*

Yb*

6196

Zn(ll)*

Y*

~104

Cd(ll)*

La*

>104

Hg (II)

500 3300 104 < I

Th (IV)*

< i 70 < I 276 22

Pa(V)

< i

U(VI)

< i

Am(Ill)

>104

Pu (IV)

2.6

Observed to form insoluble species in 0.i M oxalic acid when present in milligram amounts.

**Spectrographic determination.

~ol. 5 ~qo. 4

CATION E~(CHANGr" SEPARATION

331

References i.

R. KURODA, Y. NAKAGOMI and K. ISHIDA, J. Chromatog. 22, 143 (1966).

2.

F. W. E. STRELOW and C. J. C. BOTHMA, Anal. Chem. 36, 1217 (1964).

3.

R. KURODA and I. HIKAWA, J. Chromatog. 25, 408 (1966).

4.

H. HAMAGUCHI, A. OHUCHI, T. SHIMIZU, N. ONUMA and R. KURODA, Anal. Chem. 36, 2304 (1964).

5.

K. A. ORLANDINI and J. KORKISCH, Separation Science 3 (1968)

6.

K. A. ORLANDINI and J. KORKISCH, USAEC Rept. ANL-7415 (1968)