Separation of Uranium by Reversed-Phase Partition Chromatography on a KEL-F Column A. G. HAMLIN,
B. J. ROBERTS, W. LOUGHLIN,
and S. G. WALKER
United Kingdom Afomic Energy Authority, Capenhurst Works, Nr. Chesfer, Cheshire, England.
b Reversed-phase partition chromatography i s shown to b e a convenient and effective method for the quantitative recovery of microgram to gram quantities of uranium from highly impure solutions of unknown composition. The uranium i s obtained in a form suitable for determination b y a volumetric, colorimetric, or a-counting technique. Uranium i s retained b y a column of Kel-F supporting tri-n-butyl phosphate. Impurities are eluted with 5.5M nitric acid, and the uranium i s eluted with water. Thorium(lV), plutonium(lV), and cerium(lV) interfere, but interference b y cerium can b e avoided b y reduction to cerium(ll1) prior to separation. Recovery of The utanium i s better than 99%. method can be adapted to the separation of uranium and plutonium.
determination of uranium. The method has given very satisfactory routine service for the past 2 years. The technique is simple and effective and appears to be of general application provided that suitable solvent and aqueous phases are available, as is demonstrated by the separation of uranium and plutonium described below. The slowness of the chromatographic step is offset by the fact that large numbers of separations can be handled concurrently by one operator. EXPERIMENTAL
T
of uranium compounds for Atomic Energy projects gives rise to a variety of wastes and r c d u e s in which the uranium content is of considerable importance. The analysis of these materials for uranium is complicated by their varied composition and by the relatively large numbcr of elements and compounds that interfere with the accepted met'hods. Thus, either the sample must be known to be free from interfering agents or, more usually, a separation of the uranium must be undertaken. S o universally applicable method for the separation of uranium is known, perhaps the most widely applicable being that based on treatment of the sample solution with ammonia followed by resolution of the precipitate and separation of the remaining unwanted elements by the addition of sodium carbonate solution ( 6 ) . This method has been applied to the analysis of residues, but experience shows that many substances interfere and that, when the proportion of uranium is low, the recovery is unsatisfactory, being sometimes as low as SO%%,. To develop a more widely applicable method for t'he routine analysis of uranium residues, the use of selective extraction oi uranium with tri-n-butyl phosphatr OBP) W ~ Fconsidered, but the USE of multipie extraction* OE a r o u t ~ nb~a w by thc normal sepnrntory HE PROCESSING
Figure 1. column
Chromatographic
funnel technique was considered impracticable. As a n alternative, the practicability of performing these multiple extractions on a chromatographic column, using the method of reversedphase partition chromatography first proposed by Boscott ( I ) , was investigated. This development has led to the production of a simple method for the separation of uranium, which appears to be equally applicable to separations from the gram to microgram scale and to be affected only by the very few elements that partition strongly into the selective reagent, TBP. Fortunately, most of these elements either do not occur in the residues handled a t the Capenhurst Works of the Atomic Energy Authority or do not interfere in the subsequent
Apparatus and Reagents. After preliminary experiments had established t h e approximate size of chromatographic column required, t h e majority of t h e experiments were made with solutions t h a t contained 10 to 50 mg. of uranium using a chromatographic tube as shown in Figure 1. Kel-F low density 300 moulding poa-der (Minnesota Mining and Manufacturing Co.) was reduced to the required size by grinding it in a micro hammer mill and sieving i t to isolate the particles of desired range. Commercial quality T B P (100 ml.) was steam distilled with 0.5M sodium hydroxide solution (150 ml.) until the distillate was free of the odor of butyl alcohol, separated from the aqueous phase, and washed with water (100 ml.) and 1M nitric acid (3 X 50 ml.). Uranyl nitrate solution was prepared from analytical reagent quality salt and was adjusted to be 5.5M in free nitric acid. Other reagents and solutions were prepared as required from analytical reagent quality acids and salts, usually nitrates. Procedure. To 1.5 grams of Kel-F powder in a small beaker, a d d 1.5 ml. of TBP and stir t h e mixture until i t is uniform. Add a few milliliters of 5.5-W nitric acid with stirring and pour t h e slurry into t h e chromatographic tube fitted with a small pledget of cotton wool above t h e stopcock. Stir t h e slurry t o release air bubbles, allow it to settle, and compact it by running off the nitric acid. The upper surface of the column should be level with or slightly below the level of the top of the side tube so that i t is impossible for the column to run dry. A small pledget of cotton wool protects the Kel-F column from disturbance. Pour the sample solution containing up to 50 mg. of uranium(V1) in 5 to 10
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Table I.
Retention of Uranium b y Different Types of Column Packing (Sample charged, 5 ml. of 5.5M HNO, containing 50 mg. of uranium; eluent, 30 ml. of 5.5M HxO3)
Column Characteristics Eluate Support TBP ~ l Uranium ~ ~ Band , Pafticle abU't., sorbed, Length, !b%l./-Length, Cm. size, BSS grams ml. em. A b . Initial Eluted Polymer DIFFUSE 15 1-2 4.5 Uranium 1.5 Polyethylene (Poly36-52 3.0 thene) 52-72 1.5 15 1-2 4 eluted 3.0
Polytetrafluoroethylene (Teflon) Polytrifluorochloroethylene (Kel-F)
72-100 Pass 100 36-44
3.0 3.0 3.0
1.5 1.5 1.5
15 14 13
