Behavior of small amounts of thorium in the separation of heavy rare earth elements by ion exchange

J.inorg. nucI.Chern.,1970. Vol. 32,pp. 3091to 3099, PergamonPress. Printedin Great Britain

BEHAVIOR OF SMALL AMOUNTS OF THORIUM IN THE SEPARATION OF HEAVY RARE EARTH ELEMENTS BY ION EXCHANGE ZENZI

H A G I W A R A . I S A O T E R A S H 1 M A * and Y O S H I O K O Y A M A * Faculty of Engineering, T o h o k u University, Sendai, Japan

(Received 26January 1970) Abstract- U n d e r various experimental conditions, elutions of the heavy rare earth mixtures involving

small a m o u n t s of thorium have been carried out in order to investigate the contamination of the rare earth fractions by thorium. With the HEDTA-eluant, the degree of contamination was reduced with an increase in the elution temperature, but the ytterbium and lutetium fractions obtained in elution at 60°C were still contaminated with thorium. O n the other hand. in the elution system with E D T A , the bulk of the rare earth fractions of lutetium and ytterbium obtained in elution at ordinary temperature was slightly contaminated with thorium, while the rare earth fractions obtained at 70°C, for the most part. were free of thorium, due to complete elution of thorium ahead of lutetium. For the removal of 1 or 2 per cent thorium in the heavy rare earth mixture, the EDTA-eluant gave excellent effects, relative to the HEDTA-eluant. INTRODUCTION

IN THE ion-exchange separation of the individual heavy rare earths using such polyaminopolycarboxylic acids as ethylenediamine-N,N,N',N'-tetraacetic acid ( E D T A or H4Y) and N'-(2-hydroxyethyl)-ethylenediamine-N,N,N'-triacetic acid ( H E D T A or H:~Ch), the contamination of small amount of thorium present in the raw material comes to an important problem in order to prepare these elements of high purities. Powell and Burkholder [1] achieved the separation of ytterbium and lutetium at a high temperature with a solution of dilute E D T A (2.5 g/l, pH = 8.4) as the eluant and with hydrogen as the retaining ion for the rare earth. The concentration of E D T A , as noted above, was limited to a low value to avoid the deposit of free acid (H4Y) in the hydrogen-retaining bed. On the contrary, the HEDTA-eluant has an advantage of increasing the concentration, but the separation factors [2-3] for the two adjacent rare earths in the heavier group seem to be slightly lower in the H E D T A system than in the E D T A system. One [4-5] of the authors reported the elution mechanism of the rare earth and also found that the behavior of thorium in minute quantity in the heavy rare earth group was dependent upon the composition and concentration of the HEDTA-eluant as well as upon the elution temperature. The advanced study was necessary to investigate precisely the behavior of *Present Address: Takefu Factory, Shinetsu Chemical Industry Corp. Japan. 1. 2. 3. 4. 5.

J. E. Powell and H. Burkholder, Chemistry (UC-4), TIID-450(J (Oct. 1, 19651. J. E. Powell and F. H. Spedding, Chem. Engng. Prog. Sym. Ser. 24.55, 101 (19591. M. Noguchi, A. Yoshifuji and Z. Hagiwara, Bull, chem, Soc. 3apan 42, 2286 (1969). Z. H agiwara. J. inorg, nucl. Chem. 31, 2933 (19691. Z. Hagiwara. J. phys. Chem. 73, 31 (12 (1969). 3091

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Z. H A G I W A R A , I. T E R A S H I M A and Y. K O Y A M A

trouble-some thorium occurring in the isolation of these heavy elements on the cation exchange columns in combination with a chelating agent. Therefore, using the E D T A or HEDTA-eluant, elutions of the rare earth mixtures containing small amounts of thorium were performed under different conditions from those mentioned in the previous paper [4]. EXPERIMENTAL

Materials used. The heavy rare earth oxides were supplied by the Shinetsu Chemical Industry Corp. and converted to chlorides. After adding proper amount of thorium nitrate to the mixed solution of rare earth chlorides, the resulting solution was charged on the ion-exchange columns with a hydrogen form of resin in order to adsorb the rare earth and thorium ions. Prior to elution, the columns were filled with either Dowex 50W-X8, 50-100 mesh or the same type of resin of X12, and then preconditioned with hydrochloric acid and citric acid. Here, the symbol X means the crosslinkage of resin. Both E D T A and H E D T A were of analytical grade, and these were buffered with ammonium hydroxide before use. The other chemicals were also the same grade as above. Elution experiments. In elutions at high temperatures, a similar experimental apparatus described in the recent papers [4, 6] was employed. The ion-exchange columns constructed of i.d. 22 mm-Pyrex glass tube jacketed with i.d. 60 mm-glass tube, were closed at the bottom with coarse sintered glass disks in order to support the resin; all retaining beds were about 100 cm long and the rare earthsorption beds were kept to proper lengths, depending on the loaded amount of the rare earth. Further. for a larger scale experiment, the jacketed acryl columns with a bed dimension of i.d. 50 × 1000 mm (HR-form basis) were used in series. Before starting elution, the rare earth and retaining beds were back-washed with hot water kept at slightly higher than the elution temperature so that dissolved gases and fine particles in the resin could be removed. Then, a series of the columns was maintained at a constant temperature by circulating hot water through the columns from the constant-temperature bath, and the elution experiment was started by charging a degassed solution of the eluant into the top of the exchange column saturated with the rare earth mixture. In elution at ordinary temperature, the ion-exchange columns consisted of i.d. 22 mm-Pyrex glass tubes were used, and the eluant was introduced to the top of the leading column without degassing. Analytical method. According to the described methods[4], the compositions of the prepared E D T A and HEDTA-eluants were determined. The rare-earth eluates issuing from the column were precipitated as oxalates, followed by ignition to the oxides for weighing. In most cases, the oxide samples were analyzed by X-ray fluorescent analysis, and the spectroscopic measurement, sometimes, was made by the Analytical Group, Rare-Earth Section, Shinetsu Chemical Industry Corp.

RESULTS

AND

DISCUSSION

Results obtained by HEDTA-eluant. Except for the elution temperature, similar elution conditions were employed for Runs H T - H R - I & II, where about 18.8 g of the L u - Y b - T m oxide mixture involved approximately 1 per cent ThO~ were loaded on the sorption bed for the rare-earth; elutions were made at 25 ° and 60°C for H T - H R - I & II, respectively, using the columns filled with D o w e x 50W-X12, 5 0 - 1 0 0 mesh, and the rare earth-sorption band with a bed dimension of i.d. 22 x 270 mm was moved down the hydrogen-retaining bed with a bed dimension of i.d. 22 x 1000 mm, employing 0.01529 M HEDTA-eluant buffered with N H 4 O H (molar ratio: NH4/Ch = 2-5) and a flow rate of 5 ml/min. As is shown in Table 2, elution at 60°C gives a sharp cut at the boundary between ytterbium and lutetium, but contamination attributed to small amounts of thorium is still observed over the entire rare earth region. It is obvious from Tables 1 and 2 that the degree of Th-contamination can be reduced to some extent with the increase in the elution temperature. 6. Z. Hagiwara and H. Oki. Ball. chem. Soc. Japan 42.3177 (1969).

Behavior of small a m o u n t s of thorium

3093

Table 1. Composition of rare earth fractions obtained in elution at 25°C (Run No. H T - H R - 1 ) Fraction No.

Lu203 (%)

YbzO3 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

88.2 84-4 79.7 74.1 67.7 61.1 54.6 49-6 42-8 37.7 33.3 29.3 26-0 22.6 19-6 16-7 14.6 13.1

I 1'2 15.2 20.0 25.5 31.9 38.5 45,0 50,1 56,9 61-9 66.4 70-4 73.7 77.1 80-1 83.0 85.1 86.6

Tm203 (%)

< 0.01 < 0.01 0.01 0.01 0.02 0.02 0-03

*Total eluate v o l u m e = 2 2 . 5 1 : fraction = 500 ml.

Volume

ThO2 (%) 0.31 0-33 0.28 0.31 0.35 0.36 0.34 0-32 0.30 0.33 0.30 0.29 0.28 0.29 0.25 0-26 0.27 0.24 of each

Table 2. Composition of rare earth fractions obtained in elution at 60°C ( H T - H R - 1 1 ) Fraction No. 2* 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Lu203 (%)

Yb~Oa (%)

99.7 99.1 96.2 92-8 84-7 73 "3 58"8 40.9 26"6 16.2 9-7 6.2 3-7 2-4 2"4

0.5 1'2 3"1 7.7 15-7 27.1 40.9 59.5 73.8 84-2 90.6 94.2 96.6 98-0 98.7

Tm20:~ (%)

< 0.01 0"01

ThO2 (%) 0.29 0.18 0'12 0.12 0.10 0.09 0"08 0.07 0-07 0'08 0.06 0"07 0'06 0-05 0-05

*Total eluate volume = 24.0 I: Volume of each fraction = 500 ml.

3094

z. HAG1WARA, 1. TERASHIMA and Y. KOYAMA

As one of the elution experiments at 60°C, employing 0.06 M H E D T A and a flow of 10 ml/min, a mixture of about 160 g of the E r - T m - Y b - L u oxide containing about 10 per cent ThO2, which had been loaded on four columns [bed dimension of each column: i.d. 22 x910 mm(HR-form basis)], was eluted down ten columns [bed dimension of each column: i.d. 22 x 910 mm(HR-form basis)]. In this run, Dowex 50W-X8, 50-100 mesh was used and the elution distance of the formed rare earth band was about 2.5 band lengths. The result is graphically represented in Fig. 1, in which all of the rare earth fractions collected are contaminated by thorium.

y

J°°It-,i 50 1

og;

"

'

'

50'

"

' '5'5

Eluate volume,

I

60

i

i

i

65

Fig. 1. Elution curves of the heavy rare earth containing thorium with HEDTA-eluant at 60°C. -O-O- Total metal concn.; -A-A- Er; -A-A- Tm; - ~ - ~ - Yb; - O - I - Lu; -x-x- Th.

In another elution experiment at 60°C, about one Kg of the T m - Y b - L u oxide mixture was adsorbed on the i.d. 5 cm-columns, and eluted down the hydrogenretaining beds at a flow rate of 30 ml/min using 0.016 M HEDTA-eluant. The elution distance was corresponded to three band lengths. The compositions of the major rare earth eluates obtained are tabulated in Table 3 where the maximum contamination by the presence of thorium seems to appear in the boundary region of lutetium and ytterbium. From the experimental results obtained in the H E D T A - s y s t e m , it may be concluded that the contamination due to the presence of small amount of thorium is developed through thulium to lutetium, depending on the elution condition, and this phenomenon will not be reduced to an insignificant degree by employing such an elution system. Results obtained by EDTA-eluant. In Run 5 at 25°C, three columns [bed dimension of each column: 22 x 1000 mm (HR-form basis)] were saturated by passing an excess of a mixed solution of erbium, thulium, ytterbium, and lutetium containing thorium (1 to 2 per cent as ThO2), and the formed rare earth-sorption bands were connected in series, and displaced with 0.015 M EDTA-eluant through the mixed beds composed of hydrogen and ferric iron ions. In this case,

Behavior of small amounts of thorium

3095

Table 3. Composition of rare earth fractions obtained in elution at 60°C ( H T - H R - I I i )

Fraction

Fraction

No,

volume

1" 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

3 3 3 3 3 10 10 10 10 10 10 10 10 10 10 10 10 10 10 I0 10 10 10 10 10 20 20 4O 30 15 30 20 25 10 10 10 10 10 10 10

Lu20:~ (%)

Yb20:~ (%)

Tm20:~ (%)

ThO2 (%)

98.2

1-35

98.6

1.55

98.5

1.49

98-7 98.4 98-5 86.9 19.6 < 0.01 none

0.08 11.1 76.6 95.8 95-4

1-34 1-63 1.44 2-06 3.83 4.22 4.67

96.2

3-80

97-9

2.12

98.7 98.3 98-2 98. I 98-2

1.32 1.69 1.80 1.89 1-73

98-0

1,91

97.7

2.29

97.6

2-32

98-9

1-07

> 99.9

0.02

> > > >

99.9 99.9 99.9 99.9 92.5

7.07 28.6

0.02 < 0.01 < 0.01 < 0.01 0-02

*Total eluate volume = 1348 1.

the hydrogen-retaining bed cannot be e m p l o y e d at ordinary temperature, due to the limited solubility of E D T A itself. Here, D o w e x 5 0 W - X 8 , 5 0 - 1 0 0 mesh was used, and the elution distance of the rare earth band w a s approximately 2.7 band lengths at a flow rate of 4 ml/min. As Table 4 s h o w s , the contamination based on thorium tends to gradually increase toward the Lu-side.

3096

Z. H A G I W A R A , I. T E R A S H I M A and Y. K O Y A M A Table 4. C o m p o s i t i o n of rare earth fractions obtained in elution at 35°C (Run No. 5) Fraction No.

5* 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45

Lu203 (%)

Yb2Oa (%)

42.4 37.7

51.2 59.7

28.9 25 -3 20.7 16.8 14.6 13"2 9-7 7.8 6-3 5.4 4.0 2.8 2.6 1.9 1.4 1.4 0.7 0.6

70.4 75.0 79.2 83.1 86-0 87.3 91.3 93.0 94.4 94.6 96.7 98.0 98.3 98.9 99.0 99.0 98.7 98.1

Tm203 (%)

none none < 0.01 0.01 0-08 0.08 0.14 0.34 0.35 1.22 0.85

Er203 (%)

none < 0-01 0.11

ThOz (%) 6.5 2"5 1.0 0-7 0.4 0.2 0.13 0.11 0.09 0.07 0.06 0.05 0-03 0.02 < 0.01 < 0.01 < 0.01 trace none none

*Total eluate v o l u m e = 139.25 1: V o l u m e of each fraction = 1.001.

In Run 3 at the 70°C-elution, a similar heavy rare earth sample with Th to that used in Run 5 was eluted down the hydrogen-retaining bed using a flow rate of 5 ml/min (i.d. 22 mm-column) and 0.00935 M E D T A buffered solution. After movement of the rare earth band (3 band lengths), the rare-earth fractions were collected and analyzed, as shown in Table 5. A comparison of Tables 4 and 5 shows that the separation efficiency of the individual rare earths is increased in elution at 70°C and the greater part of the heavy rare earth fractions is not hampered by small amount of thorium. However, in Runs 3 and 5 where dilute solutions of E D T A were employed as the eluant, slight precipitation was still observed in the column as the separation was progressed; this might be due to either E D T A or protonated rare earth E D T A chelates, whose deposition on the resin gives trouble for steady-state elution. When used a mixed bed with iron and hydrogen (Run 5), a fairly part of the rare earth fractions coming from the frontal part of the rare earth band was contaminated with trace amounts of iron, but the removal could be performed easily in the precipitation of rare earth by oxalic acid. Discussion. The separation factor (a) in ion-exchange is defined as the ratio of the total concentrations of the separating ions in the resin phase, divided by this ratio in the aqueous phase. Thus, the separation factor for the rare earth (Ln) and Th can be expressed in the form

Behavior of small amounts of thorium

3097

Table 5. Elution result obtained at 70°C with EDTA eluant (Run No. 3) Fraction No. 1* 2 3 5 6 7 8 9 10 11 12 13 14 15 - 21 22 23

Lu203 (%)

Yb2Os (%)

Tm2Os (%)

Er203 (%)

96,4 88.0 59,1 1,79 0,57 0.24 0.09 0.04 < 0.01 <0-01 < 0.01 none none none

3-9 12.5 41.3 98.7 99.3 99.8 > 99.9 > 99.9 > 99.9 >99.9 > 99.9 > 99-9 > 99.9 > 99.9 > 99.9 > 99.9

< 0.01 0.03

none none

*Total eluate volume=94.0 1: Volume of each fraction = 2-00 1. Thorium was not detected through all of the fractions given in this table.

Th = [T-hT] [LnT] Ot~Ln [Thr] [Lnr]

(1)

w h e r e t h e b a r r e d s y m b o l s r e f e r to t h e r e s i n p h a s e a n d T ' s r e p r e s e n t t h e total a m o u n t s o f t h e i n d i c a t e d s p e c i e s . I n e l u t i o n o f r a r e e a r t h a n d t h o r i u m with HEDTA (H3Ch) s o l u t i o n , b u f f e r e d w i t h N H 4 O H , t h e f o l l o w i n g s p e c i e s a r e c o n s i d e r e d to b e m a i n c o n s t i t u e n t s in t h e a q u e o u s p h a s e o f t h e r a r e e a r t h b a n d i n v o l v i n g T h : H 3 C h , H 2 C h - , H C h 2-, C h a-, H +, N H 4 +, L n C h a n d T h C h +. In g e n e r a l , as t h e a d s o r b a b i l i t y o f p o l y v a l e n t ion on t h e r e s i n is m u c h g r e a t e r t h a n t h a t o f t h e m o n o v a l e n t ion. t h e a m o u n t o f T h C h + o n t h e r e s i n m i g h t b e negligible. as c o m p a r e d to T h 4+ o n t h e resin. A s s u m i n g o n l y t h e p r e s e n c e o f T h 4+ a n d L n 3+ Th on the resin, o~t. n c a n b e w r i t t e n in the f o r m [ T h 4+] ( [ L n 3+ ] + [ L n C h ] ) C~Tn h = [L--~a+] ( [ T h 4+] + [ T h C h + ] )" Introducing we have

KLnCh= [LnCh]/[LnS+][ChS-]

and

(2)

KTIaCh = [ThCh+]/[Th4+][Ch:~- l,

Th = [ Th4+] [ Ln3+] (1 + KLnch[ChS-]) O/Ln [ t - n 3+ ] [ T h "+ ] (1 + Krhch [ C h s- ] )

(3)

w h e r e K ' s d e n o t e t h e s t a b i l i t y c o n s t a n t s o f t h e i n d i c a t e d m e t a l c h e l a t e s . In the

3098

Z. HAGIWARA, I. TERASHIMA and Y. KOYAMA

present ion-exchange systems, however, the presence of Ln 3+ and Th 4+ can be neglected as compared to LnCh and ThCh +. Thus, the approximation of Equation (3) becomes = KLnCh a~nh --- K ~

(4)

gThCh

where Ke expresses the distribution coefficient of Th and Ln in the absence of chelating agent. On the other hand, in the ion-exchange system of the rare earth containing Th in the presence of EDTA, main species in the aqueous phase of the rare-earth band are as follows: NH4 + H +, H4Y, H3Y-, H~Y 2-, HY ~-, y4-, Ln3+, LnY-, HLnY, Th 4+ and ThY. Considering only the presence of Th 4+ and Ln 3+ on the resin, the separation factor (a') in the EDTA-system takes the form ~,Th

tx. Ln ~-'~-

[Th 4+] [Ln3+] { 1 + KLnY[Y 4-] (1 + KHLnV[H +] )} [Ln 3+] [Th 4+] {1 -f- KwhY[Y'-] }

(5)

From the relationships, [Th 4+] ~ [ThY] and [Ln 3+] ¢ [LnY-] in the aqueous phase of the system, the approximation becomes [Th 4+] [Ln 3+] KLnY( 1 + KHLnV[H +] ) a"~nh-- [ Ln3+] [Th 4+] KThV

(6)

AS compared to the presence of LnY-, the amount of the protonated chelate can be neglected. Therefore, Equation (6) can be approximated in the form tTh ~___ r KLnY OL Ln /¢~~d"~-'--'- • KThY

(7)

Relation (4) and (7) will explain semi-quantitatively the behavior of thorium in the ion-exchange process involving a chelating agent. The a or a' value for Th and Ln will depend greatly on the mole fraction of the species in both phases as well as on the ionic strength of solution, but the effect of these variables on the separation factors between the two adjacent rare earth will be reduced insignificantly, due to chemical similarity of these pairs. The stability constant of LuY-, which is the most stable chelate in the lanthanide group, is equal to 1019"9, while the value of Krhv is 1023"2. Inasmuch as the EDTA-chelating agent is involved in the system, the a'~nh value will be expected to become so small that thorium tends to be eluted ahead of lutetium. Eiution at an elevated temperature results an improvement in kinetic parameters related to H.E.T.P. (height equivalent to a theoretical plate); diffusion rate of thorium species in both phases will become speedy and the contamination of rare earth by thorium will be suppressed, as may be seen in the detailed data at 70°C (Table 5). Further a lower crosslinked resin will also bring a favorable effect on diffusion of Th-species inside the resin. In the HEDTA-system, as the difference in the stability constants [7-8] of the heavy rare earth and thorium chelates is not high, the separation factors for the 7. F. H. S p e d d i n g , J. E. P o w e l l a n d E. J. W h e e l w r i g h t , J. Am. chem. Soc. 78, 34 (1956). 8. T. M o e l l e r a n d R. F e r r u s , J . inorg, nucL Chem. 2 0 . 2 6 1 (1961).

Behavior of small amounts of thorium

3099

heavy rare earth and thorium will not show significant values to permit the isolation of each other. Due to the above reason, the degree of contamination by thorium is not so improved by raising only the elution temperature, as seen in Fig. 1. T h e described elution systems can be considered to divide into the following categories: The main rare earth in macro-quantity is possible to separate with a type of displacement chromatography, while trace amount of thorium in the rare earth sample is progressed down the column with a type of elution chromatography. Therefore, trace amounts of thorium get more and more diffuse and the contaminated area of the heavy rare earth gets larger in the real ion-exchange separation.