A process for recovering iodine-129

International Journal of Applied Radiation and Isotopes, 1963, Vol. 14, pp. 611-613. Pergamon Press Ltd. Printed in Northern Ireland

A Process for Recovering Iodine- 129 H . A. O ' B R I E N , JR. a n d J. M . S U L L I V A N Oak Ridge National Laboratory, Oak Ridge, Tenn.* (Received 11 March 1963) A process was developed on a laboratory scale for recovering, in 90-95 per cent yield, I 1~9 from Berl saddles impregnated with AgNO s. The iodide ion is displaced from silver iodide by sulfide ion in alkaline solution, and I ~29 is distilled from the 1 M NaOH-0.01 M Na~S solution after oxidation with hydrogen peroxide. UN PROCEDE P O U R LE R E C O U V R E M E N T DE L'IODE-129 On a ddvelopp6 un procddd £ l'dchelle du laboratoire pour le recouvrement, en rendements de 90-95 pour cent, del'p29horsdessellesBerlimpregn~esd'AgNO3. L'iond'iodureest ddplac~ de l'iodure d'argent par l'ion de sulfure en solution alcaline, et l'I a~9 est distilM de la solution 1 M NaOH-0,01 M Na2S suivant l'oxydation ~ l'eau oxygdnde. IIPOIIECC OBPATHOFO IIOdlqEHH,q I t~9 BHa pa3pa6owax~ aa6opavopHbIt~ npoRecc 06pawH0rO n o a y ~ e H ~ noaa, e BblXO~OM ~0 90--95 %, n~ ceaaOBHg Bepna, npormwaHHi,ix AgNO 3. I/Ion Ho~a ~aMeglaewcg n I/IO~HCTOM cepe6pe HOHOMcyaLdpn~a B meaoqnoM pacwBope,a I ~90TrOHAeTCAU3 1 M NaOH-0,01 M NaaS pacwBopa nocae oKHcaeimFt nepeHncmo no~opoaa. EINE M E T H O D E Z U R R U C K G E W I N N U N G VON I a29 Eine Methode zur 90-95% Ruckgewinnung vom 1129 aus mit AgNO 3 getr~nkten Berl Sfittel wurde entwickelt. Das Iod Ion wird aus silber Iodide dutch Sulfid Ion alkalischer L6sung verdr~ingt und nach Oxydierung mit Wasserstoffsuperoxid wird das 1129 aus der LOsung 1 M NaOH--0,01 M Na2S abdestilliert. INTRODUCTION DURINO t h e processing o f spent r e a c t o r fuel, I n g - - a p r o d u c t o f U 235 fission--is released in e l e m e n t a l form u p o n dissolution o f the u r a n i u m m e t a l in nitric a c i d ; it is then c a r r i e d in the offgases to a s c r u b b i n g tower c o n t a i n i n g Berl saddles i m p r e g n a t e d w i t h A g N O 3 w h e r e it is t r a p p e d as the h i g h l y insoluble A g I . Previously, the 11~9 has b e e n recovered in 50 p e r cent y i e l d from the i n t a c t Berl saddles b y r e d u c i n g the silver w i t h p o w d e r e d zinc a n d o x i d i z i n g the i o d i d e to i o d i n e w i t h nitric a c i d ~1). I t is possible to increase r e c o v e r y to n e a r l y 90 p e r

cent b y g r i n d i n g the saddles to 100 mesh, b u t the h a z a r d of h a n d l i n g the d r y r a d i o a c t i v e p o w d e r precludes the use o f this modification. T h e m e t h o d presented here p e r m i t s r e c o v e r y o f 1129 in 90-95 p e r cent yield a n d is b a s e d on the d i s p l a c e m e n t of the i o d i d e from the A g I b y sulfide ions present in a n alkaline solution. L i m i t e d previous w o r k has involved the s t u d y o f the A g I - A g 2 S e q u i l i b r i u m in a c i d m e d i a . ~2,3) A l k a l i has the a d v a n t a g e s o f r e d u c i n g the h y drolysis o f the sulfide ion a n d increasing t h e sulfide ion concentration, t h e r e b y shifting t h e equilibrium 2 A g I + S 2- -~ Ag2S + 2 I -

* Operated by Union Carbide Corporation for the U.S. Atomic Energy Commission.

to the right. 611

612

H. A. O'Brien, Jr. and J. M. Sullivan

Since the solubility product constant of A g I is 10-17 but that of Ag2S only 10-51, iodide ion is essentially totally released. The caustic also increases the porosity of the ceramic saddles and exposes additional silver iodide trapped near the surface. The procedure was developed on a laboratory scale but has more recently been adapted to a production scale. PROCEDURE

Removal of iodinefrom Berl saddles The saddles containing the AgI, in 50 g lots, are washed twice with 200 ml portions of boiling water to remove as much silver nitrate as possible and then are soaked 24 hr in 250 ml of a solution 1 M in N a O H and 0.01 M in Na2S. The supernatant liquid is filtered to remove the precipitated silver sulfide. These concentrations of N a O H and Na2S were selected for the leaching solution after preliminary tests showed that increasing the sulfide above this value had little effect on the iodine extraction (Table 1). However, decreasing the N a O H below this value decreased the sulfide ion concentration and resulted in less efficient iodine extraction. TABLE 1. Effect of sulfide and hydroxyl ion concentrations on removal of I az9 from Berl saddles NaOH concentration (M) 1.0

0.1 0.01

Na~S concentration (M)

1129 removed (%)

0.1 0.01 0.001 0.000 0.01 0.01

95 98 68 17

88 82

Recovery of iodinefrom alkaline sulfide solution The filtrate from the above step is transferred to a 1 1. distillation flask supplied with a Snyder reflux column (Fig. 1) and the p H is adjusted to 1.0 with 3 N H2SO 4. (More concentrated acid should not be used because of the danger of oxidizing some iodide to iodine at this point and distilling it off in the preliminary hydrogen sulfide removal step. In the test work no iodine

FIG. 1. Reflux system for removal of H2S. was ever detected in the vapor when the pH was kept at 1.) The elemental sulfur that forms does not hinder the subsequent steps. This solution is refluxed until the evolution of HzS ceases, as shown by a test with lead acetate paper at the top of the condenser--usually 20-30 min. To reduce any iodate still present, approximately 5 ml of 0.2 M N a H S O 3 is added through the addition funnel. The complete distillation apparatus is now assembled (Fig. 2), and 10 ml of 0.2 M N a H S O a is added to the scrubber. The iodide is oxidized to iodine with 20 ml of 10% H 2 0 2 containing 5 drops of 3% ammonium molybdate as catalyst(4)--again added through the addition funnel--and the solution is boiled until visible evolution of iodine ceases. The liquid is then cooled and the oxidation and distillation steps are repeated until the absence of iodine in the solution is shown by the absence of any brown tinge.

Sampling and analysis Material balances are obtained by determining the 1129 in the unused saddles, in t h e

613

A processfor recovering iodine-129

laminated fl-mounting card. A 250 ~t aliquot of the scrubber solution is evaporated to dryness on a 1-in. diameter glass disc which is mounted on a similar card. The height of the 39 keV 1129 7-peak of each sample is determined with a Nuclear Data 256-channel analyzer. The resolution of this instrument is 7 per cent as measured by the F W H M of the 0.663 MeV Cs 137 y-peak. CONCLUSION The procedure described is suitable for efficient recovery of 1129 and has been adapted to large-scale operations. For example, a 50 1. stainless-steel tank with suitable filter has been installed at the processing site. The saddles, after being placed in a coarse mesh bag, are immersed in the caustic-sulfide solution and allowed to remain there for 24 hr. This batch is then removed and replaced with another, and the sulfide solution drawn off and processed as required.

REFERENCES I. RUSSELL H. T. Recovery and Half-life Determination

FIG. 2. Apparatus for recovery and distillation of iodine. saddles that have been processed, and in the scrubber solution. Three saddles of each type, randomly selected, are ground to a fine powder, and about 0.1 g of each powder is weighed and mounted on a standard 3.75 × 2.5 in. one-hole

of I 1~9, ORNL-2293. Oak Ridge National Laboratory, Oak Ridge (1957). 2. NOYESA. A. and FREED E. A. J. Amer. chem. Soc. 42, 476 (1920). 3. KARENMAN I. M. J. Gem Chem. (USSR) 16~ 165 (1946). 4. KOLTHOFF I. M. and SANDELLE. B. Textbook of Quantitative Inorganic Analysis p. 33. Macmillan, New York (1952).