Protactinium—I analytical separations

J. Inorganic and Nuclear Chemistry. 1956, Vol. 2, pp. 46--59. Pergamon Press Ltd.. London

PROTACTINIUM--I A N A L Y T I C A L SEPARATIONS J. GOLDEN and A. G. MADDOCK Radiochemical Laboratory, Department of Chemistry, Cambridge

(Received 7 July 1955) Abstract--The conditions for the separation of protactinium from other elements by solvent extraction of a chloride complex by diisopropyl ketone have been investigated. The complex appears to contain three moleculesof the solvent. Its heat of formation is small. Evidence for the existence of a sparingly soluble salt BaPaF7 is advanced. INTRODUCTION A RECENT report on the solvent extraction of protactinium (1~ and another on its separation from iron by anioH exchange t2) lead us to record the methods of separation that have been developed in this laboratory during the last three years. These methods were devised for the estimation of the natural protactinium content of a large variety of materials, ranging from siliceous barium sulphate precipitates to simple solutions of uranyl salts. They have been applied to the analysis of more than a hundred samples. A combination of these procedures has been applied successfully to the separation of some two hundred milligrams of protactinium. The latter process will be described in a subsequent paper. Recent studies of the chemistry of protactinium emphasize the necessity of handling the element in its anionic complexes, especially when tracer concentrations are concerned, otherwise losses by adsorption and irreproducible behaviour of the element are inevitable, t3' 4) The presence of soluble, colloidal, or suspended silica accentuates these difficulties. None the less, the co-deposition of protactinium on manganese dioxide is often a useful preliminary step in the isolation of.the element from dilute solutions. This reaction is ineffective if the protactinium is appreciably complexed, and cannot be used, for instance, in fluoride or citrate solutions. The complex fluoride anion, PaF72-, is perhaps the most stable and tractable anion, t4) Hydrofluoric acid has been shown to leach protactinium from a variety of matrices, tS) and, in addition, it permits rapid isotopic equilibration of Pa 23x and Pa 233, so that the latter isotope may be added to monitor separation yields. This equilibration process may be slow in solutions of the other mineral acids, re) Three principal modes of recovery of the protactinium from these solutions have been described, in addition to the inconvenient method of displacement of the fluoride by evaporation with sulphuric acid. The protactinium may be reduced to the fluoride insoluble tetravalent state tS~ and collected, for instance, on a thorium fluoride precipitate. Alternatively, it may be co-deposited on a barium fluozirconate precipitate. ~7~ cx~ F. L. MOORE, Anal. Chem., 27, 70 (1955). c2~ K. A. KRAUS and G. E. MOORE, J. Amer. Chem. Soc., 77, 1383 (1955). Iz~ R. E. ELSON, G. MASON, D. F. PEPPARD, P. A. SELLERS, and M. H. STUDIER, dr. Amer. Chem. Soc., 37, 4974 (1951). ¢4~ A. G. MADDOCK and G. L. MILES, dr. Chem. Soc., S.248 (1949). ~5) G. BOUISSi/~RESand M. HAiSSINSKY, Bull. Soe. Chim. 18, 146 (1951). ~6) A. G. MADOOCK and G. L. MILES, dr. Chem. Soc., S.253 (1949). ~7~ W. W. MEINKE, U.S.A.E.C. Declassified Document A.E.C.D. 2750. 46

•

Protactinium--I: Analytical separations

47

Finally, it has been shown possible to extract the p r o t a c t i n i u m into solvents after preferentially c o m p l e x i n g the fluoride ion with aluminium/~) A n u m b e r o f the n e u t r a l complexes o,f p r o t a c t i n i u m can be extracted f r o m aqueo us solutions by immiscible organic solvents. The c o n d i t i o n s for the extraction o f the t h e n o y l t r i f l u o r o a c e t o n e (s) a n d the c u p f e r r o n complexes ~6~ have been investigated in detail. However, since p r o t a c t i n i u m can also be extracted from miner~d acid solutions b y a variety o f solvents, a t t e n t i o n has been focused on these simpler processes. These extraction processes a p p e a r to have been studied in some detail, but few rcport.~ have been p u b l i s h e d in the general literature, tl, 9.10.1]~ M o s t o f the d a t a refer ~, h y d r o c h l o r i c or nitric acid solutions, usually in the presence o f considerable atnotHat~ o f dissolved salts. F o u r solvents----dichlorodiethyl ether, tl]l diisopropyl carbinol and ketone/9) a n d tributyl p h o s p h a t e , (12) seem c a p a b l e o f d e v e l o p m e n t for an analytical separation. T h e first o f these solvents is only suitable with aqueous solutions o f m o d e r a t e o r high ionic strength. The last is not generally selective enough for a n a l y t i c a l purposes. The allied p r o b l e m o f the s e p a r a t i o n o f n i o b i u m a n d t a n t a l u m has been solved, tan) using solvent extraction with diisopropyl ketone, by varying the p r o p o r t i o n s o f h y d r o f l u o r i c a n d h y d r o c h l o r i c acids in the a q u e o u s phase. A similar technique seemed possible for p r o t a c t i n i u m . This ketone does n o t require a salting-out agent. I n this p a p e r the c o n d i t i o n s for the s e p a r a t i o n o f p r o t a c t i n i u m from fluoride solutions on b a r i u m fluozirconate a n d b a r i u m fluoride precipitates are described, and a detailed study o f the solvent extraction s e p a r a t i o n o f the element is reported. P r o t a c t i n i u m cart also be s e p a r a t e d f r o m other elements by cation, or better, anion exchange. T h e latter process will be the subject o f a separate c o m m u n i c a t i o n . EXPERIMENTAL

(a) Many of the solutions used contained free hydrofluoric acid and were therefore handled in polythene apparatus. A complete range of such apparatus--flasks, centrifuge tubes, measuring cylinders, etc.--was used. Even with other solutions in which the protactinium was present in a complex anion, it was found advisable to use polythene vessels. Considerable adsorption, or possibly ion exchange, fixes protactinium on the walls of most kinds of glass vessels from these solutions, as well as from the readily hydrolyzed cationic solutions. Losses due to this cause vary enormously with the kind of glass and its immediate history. It is better to use plastic vessels which do not suffer from this disadvantage. Polythene withstood the action of all the aqueous solutions as well as the organic solvents used. With aqueous solutions the vessels could be heated to 80-90 ~ when necessary without ill-effect. (b) Pa 23~was obtained from various uranium refinery residues, as will be described in a subsequent paper. (c) Pa ~a8was prepared by the neutron irradiation of samples of acid-soluble "thorium carbonate." This material, of uncertain composition, is reasonably thermally stable, but completely soluble in dilute nitric and hydrochloric acids. After irradiation the material was dissolved in 8N hydrochloric acid and extracted with diisopropyl ketone. The extract was treated with 2N hydrochloric acid when the protactinium passed back to the aqueous layer. The aqueous extract was then made 8N again and the extraction repeated. The protactinium was kept in solution in 8N hydrochloric acid c8~R. E. ELSON,U.S. National Nuclear Energy Series Div. IV, Vol. I4A, p. 125 (1954). ~g~E. K. HYDEand M. J. WOLF,National Nuclear Energy Series Div. IV, Vol. 14A, Chap. 5, Section 9.1, also Chap. 15, Section 5.2 (1954). tt0) R. C. THOMPSON,U.S.A.E.C. Declassified Documents MDDC 1770 (Jan.) MDDC 1897 (Feb.) (1948). tXl~A. G. MADDOCKand L. H. S'rEIN,J. Chem. Soc., S.258 (1949). 112)R. E. ELSON,et. aL ; D. F. PEPPARO,et al., U.S. National Nuclear Energy Series Div. IV, Vol. 14A, p 125. (~a) p. C. STEVENSONand H. G. HICKS,Anal. Chem., 25, 1517 (1953). Most of the references to the United States Atomic Energy Commission publications quoted in papers II) and (10) are not available in Cambridge.

48

J. GOLDEN and A. G. M A D D O C K

in a polythene vessel. It was observed that losses"by adsorption quickly took place on the walls of glass containers even with solutions of the chloride complex in diisopropyl ketone. Better recoveries were obtained if the irradiated carbonate was dissolved in 8N hydrochloric acid 0.6N in hydrofluoric acid. This solution was nearly saturated with aluminium chloride before the first solvent extraction. The subsequent separation followed the first procedure. Neither product contained detectable amounts ofthorium. (d) 0c-Activities were measured with a scintillation counter. The efficiency of the counter was estimated by measuring the activity of a number of thin trays prepared by evaporating and igniting aliquots of a standard solution of A.R. uranyl nitrate solution. It was 30.5 + 0-3 per cent. Samples were prepared for counting by the evaporation and ignition of aliquots of the solutions, or by transference of precipitates to the trays as slurries in a suitable solvent, and drying after careful distribution of the precipitate. Accurate aliquots of solutions were obtained using mercury calibrated micropipettes. A micrometer-controlled pipette ("Agla" micrometer syringe) was also used for small aliquots. The "trays" consisted of standard discs of aluminium, copper, nickel, or platinum foils. The choice of foil for a particular series of experiments depended on the nature of the solutions involved. The samples were not allowed to exceed 1 mg/cm~ thickness. (e) The ~/- and y-activity of the trays used for 0c-measurements were determined, using an endwindowed counter with an aluminium window of about 7 mg/cm'. Some early measurements were also made, using an annular liquid Geiger counter, but the decontamination of the counter was troublesome, so later measurements were made with an annular scintillation counter with a Nal/TI crystal about 3 cm high and 2 cm diameter. In this counter the sample holder was an annular polythene vessel. This arrangement proved most satisfactory. RESULTS

1. Preliminary Experiments It was observed that, even f r o m strongly acidic solutions that had previously been shown not to lose protactinium when centrifuged at high speed, c4~ losses o f Pa 238 took place in glass vessels when tracer concentrations were used. Similar behaviour was observed with solutions of the nitrate and chloride complexes in organic solvents. Nitrate solutions were avoided, since the nitrate complex seemed to be especially prone to such behaviour. It was found that such losses were greatly reduced by the use o f polythene or platinum vessels. Since evaporation of solution was occasionally necessary, and since the trays for the estimation o f Pa ~zx were to be prepared by evaporation nod ignition, the possible losses o f protactinium by volatilization in these processes were measured. Experiments with Pa 2az showed that there was less than 0.5 per cent loss on evaporation of 50-ml a m o u n t s o f concentrated hydrofluoric, nitric, sulphuric, and hydrochloric acid solutions to 1 ml. Distillation of hydrochloric acid containing Pa ~31 at tracer concentration showed that less than 0.1 per cent o f the protactinium appeared in the distillate. It has been suggested, however, that protactinium m a y volatilize from mixtures of hydrochloric and hydrofluoric acids. A similar experiment with Pa ~ at tracer concentration in the mixed acids showed the loss was less than 1 per cent. To ensure that the concentration of protactinium does not influence the loss in this system, a distillation was conducted in a polythene system at 95°C, using 25 ml of a solution containing 2-0 m g of Pa ~m. T h e ~-activity of the distillate could not be detected. The loss by volatilization on ignition of the counting trays was tested, using both Pa ~33 and Pa ~31 trays. Aliquots of the tr~tcer solution were evaporated on a n u m b e r of copper and platinum discs. The trays were heated for varying lengths of time at temperatures from 80°-360°C. In no case did the activity fall m o r e than 1 per cent.

Protactiniurn--I: Analytical separations

49

The platinum discs were then heated to 800°C for one hour without greater loss of the activity. The latter observation was important, since it permitted the volatilization of the active Po 2x° which might have interfered with the estimation of Pa 2al. Samples containing Po m° were ignited to constant activity at 800°C. Test experiments showed that tracer amounts of Pa ~aa could be extracted from aqueous chloride and nitrate solutions by diisopropyl ketone even in the absence of considerable concentrations of dissolved salts. Since the disadvantages of the nitrate solution, referred to above, quickly became apparent, further effort was diverted to chloride solutions.

2. Co-deposition on Manganese Dioxide The previously reported conditions for this separation were confirmed, CG~as was its inefficiency in the presence of the strongly complexing fluoride or citrate ions. An advantage of this process was its tolerance of siliceous material in the solution. A new method of separation of this protactinium from the precipitate was developed. Leaching with hydrofluoric acid solution gave the following results. Normality of acid Per cent Pa 2~ leached out

0.1 68

1.0 94

I0 98

3. Behaviour in Alkaline Solutions Protactinium is generally co-deposited on hydroxide precipitates, and this reaction was found convenient for the concentration of the element from large volumes of dilute aqueous solutions. However, it was found necessary to avoid the use of too great a concentration of alkali hydroxide, or co-deposition became inefficient, possibly because of peptization of the protactinium. The incomplete co-deposition from ammonium carbonate solutions, previously observed, ~4) was confirmed. Between 55 and 88 per cent of the Pa ~aa in hydrochloric acid solution at tracer concentration, and in the presence of iron and zirconium, was carried down by the addition of excess ammonium carbonate solution. But the same reagent only leached a small amount of the protactinium from a hydroxide precipitate. To this extent protactinium resembles thorium in its behaviour at comparable concentrations314~ An attempted separation of tracer amounts on a carbonate precipitate formed by the addition of strong sodium carbonate solution was also unsuccessful. At tracer concentrations true solution of the protactinium content of these precipitates needs a strongly complexing acid; indeed, only hydrofluoric acid is entirely reliable. But at higher concentrations the protactinium dissolved perfectly in concentrated hydrochloric acid.

4. Precipitation in Fluoride Solutions (a) Co-deposition on barium fluozirconate--The conditions for the co-deposition of protactinium on barium fluozirconate from hydrofluoric acid solutions were investigated, using Pa 2~ at tracer concentrations on a 10-ml scale. Selected favourable conditions were checked at a much higher concentration with Pa ~zl as tracer and with larger volumes of solution. u4~ M. BACHELET,Ann. Chim., 12, 348 (1939).

50

J. GOLD~ and A. G. M~DOCK

(i) Effect of concentration of zirconium. Each solution was 2.5N in hydrofluoric acid, and the barium-zirconium concentration ratio was constant at 2.5. Zr concentration mg/ml Per cent Pa carried down

2 67

1 61

0.25 61

0.1 50

0.03 4.2

(ii) Effect of hydrofluoric acid concentration. Each solution contained lmg/ml of zirconium, and the barium-zirconium concentration ratio was constant at 2.5. HF normality Per cent Pa carried down

2.5 61

5 56

10 36

15 23

20 0

Washing with 20N hydrofluoric acid removed 94 per cent of the protactinium codeposited on a barium fluozirconate precipitate. (iii) Effect of barium-zirconium concentration ratio. Each solution contained 2 mg/ml of zirconium, and was made 2.5N in hydrofluoric acid. Ba/Zr ratio Per cent Pa carried down

0.5 0

1.5 19

2.5 67

5 82

10 96

(iv) Time allowed for complexing of the zirconium. The published directions for this separation allow the fluozirconate mixture to stand one hour before addition of barium. (7) The desirability of this delay was tested in the following experiment. Each solution contained 2 mg/ml of zirconium, the barium-zirconium concentration ratio was adjusted to 5 and the solution was 2.5N in hydrofluoric acid. Time elapsing between zirconium and barium additions (rain) 0 12 30 90 Per cent Pa carried down 82 91 94 95 The conditions of the last experiment were adopted for general use and were tested over a wide range of protactinium concentrations and solution volumes. (b) Co-deposition on barium fluoride--The enhancement of the co-deposition in the previous series of experiments by a large excess of barium suggested that the sparingly soluble barium fluoride might co-deposit protactinium as well as or better than the fluozirconate. The conditions were investigated over a wide range of protactinum concentrations and volumes of solution. (i) Effect of quantity of added barium. hydrofluoric acid.

Solutions were each made 2-5N in

Barium added, mg/ml Per cent Pa carried down

0.5 42

1.0 57

2.5 69

10 >99

It was found that the co-deposition is even more favourable in less acid solution, and in ammonium fluoride solution is unaffected by the concentration of ammonium fluoride up to 10M. A further test of the necessary concentration of barium was made, using a solution that was 5M in ammonium fluoride but to which no free hydrofluoric acid had been added. Barium added, mg/mI Per cent Pa cmried down

0.5 34

0.9 49

1.8 89

4.3 98 .

8.2 >99

Protactinium--I: Analytical separations

51

The barium fluoride precipitated in this way is easily soluble in large amounts of cold water. The barium fluoride can be preferentially dissolved, thus further concentrating the protactinium; for instance, solution of 50 per cent of a precipitate in cold water dissolved less than 0.01 per cent of the protactinium.

5. Solvent-extraction of the Protactinium (a) Choice of solvent--Most of the data to be reported refer to diisopropyl ketone. and indeed, this solvent remains the most suitable for the separation. However, during the investigation it ceased to be produced commercially, so a comparison was made with some alternative, more readily available, solvents. Equal volumes of solvent and aqueous phases were used in each experiment. The initial normality of the aqueous phase in hydrochloric acid and the percentage of Pa 2an tracer found in the organic phase are given after each solvent. Dichlorodiethyl ether, 6-0N, 13-30 per cent; amyl alcohol, 6.0N. 79 per cent: diethyl ketone, 6.0N, 87 per cent; mesityl oxide, 4.7N, 94 per cent; phorone, 6.0N 90 per cent; methylisobutyl ketone, 6.0N, 97 per cent; diisopropyl carbinol, 6.0N, 95.2 per cent; diisopropyl ketone, 6.0N, 99 per cent. The first of these solvents was only satisfactory in the presence of salting-out agents. Amyl alcohol, diethyl ketone, and mesityl oxide showed too great a mutual solubility with water. Phorone, which had to be used above its melting-point, 28 °, was too liable to condensation reactions which were catalyzed by aluminium chloride. To study the mutual miscibility and extraction of hydrochloric acid by the solvent. equal volumes of diisopropyl ketone and varying concentrations of hydrochloric acid were equilibrated by shaking together for twenty minutes in ampoules in a thermostat at 25 ° . The change in acidity was determined by titration of the aqueous phase. Initial normality of aqueous phase Final normality of aqueous phase

3.12

3.47

4.16

6.26

8.00

9.76

12.00

3.08

3.46

4.10

6 - 2 5 7.80

9.20

9.20

Below 7N the volume change of the phases on equilibration is less than 2 per cent. (b) Conditionsfor the solvent extraction by diisopropyl ketone(i) Effect of acid concentration. 1-ml volumes of hydrochloric acid at various co~Lcentrations were equilibrated with equal aliquots of a solution of the chloride complex of protactinium in diisopropyl ketone at 18° in polythene capsules. Equilibrium was found to be established in less than five minutes. The results are presented in Graph I. Curve A refers to experiments with Pa ~31 at 0.02 mg/ml, and curve B refers to experiments with Pa 2~ at tracer concentrations, the latter experiments bei~lg conducted in glass vessels. The same data are presented in a different form in Graph V. (ii) Effect of dilution of ketone with benzene. The extraction of protactinium from hydrochloric acid solutions between 3 and 8N by pure benzene was shown to be much less than 0.1 per cent. The variation of the extraction coefficient with the concentration of diisopropyl ketone in benzene was studied with an aqueous phase 6N in hydrochloric acid and with an initial concentration of Pa 2al of about 0.04 mg/ml. 2-ml aliquots of each phase were equilibrated in each experiment. The results are presented in Graph 1I.

52

J. GOLDEN and A. G. MADDOCK

/-

_/ )

•

]//'

+2"O

//

0

-I.C

o ~

J

÷3"0

J~

r

+0'8 +O.4 +O'6 LOC% NORMALUY OF HCI GRAPH I GRAPH L--Relation between the extraction coefficient for the protactinium and the con-. centration of hydrochloric acid in the aqueous phase. Curve A, Pare: 0.02 mglml. Curve B. Pare: tracer concentration (glass vessels). Curve C, Pare: 0.002mg/ml in presence of aluminium fluoride complex. +J.

+'~ ,

//IS/ A

+

< LOGm MOLARITY OF ~I.P.K. SOLUTION GRAPH II GI~ApH l|,--Relation between the extraction coefficient f o r the protactinium and the

concentration of ketone in the benzene phase. × First experiment O Second experiment • Third eFperiment Two di/sopropyl ketone purification cycles were carried out between the first and second, and second and third experiments.

Protactinium--I: Analyticalseparations

53

(iii) Effect of fluoride on the extraction. The easiest method of returning the protactinium from the solvent to true solution in an aqueous phase was found to be extraction with aqueous hydrofluoric acid. The effect of the concentration of the hydrofluoric acid on the re-extraction was studied, using a solution of the chloride complex in diisopropyl ketone, prepared by extraction from 6N hydrochloric acid solution and re-extracting the organic phase with 7.4N hydrochloric acid containing varying concentrations of hydrofluoric acid. The experiments were conducted with Pa 2al at a concentration of about 0.004 mg/ml, using 2 ml of each phase. The results are presented in Graph III.

/

I(30

/

0

oi

/

0"I 0"01 NORMALITY OF HF GRAPH TIT

O-OOI

GaAPH IlL--Re-extraction of complex by 7.4N hydrochloric acid containing varying concentrations of hydrofluoric acid.

It was found that, if a strongly fluoride complexing ion was added to these fluoride extracts, it became possible again to solvent-extract the protactinium. Aluminium chloride was found convenient for this purpose. The ratio of the concentration of aluminium to fluoride ion necessary for solvent-extraction of the fluoride-containing solution was investigated. The experiments were made with an 8N hydrochloric acid solution, 0.6N in hydrofluoric acid and containing about 0.01 mg/ml of Pa aal. 3 ml of each phase were used and the alumini~m added as anhydrous aluminium chloride. Ratio A1/F Per cent Pa z~l extracted

0.22 2.0

0.44 4-3

0.88 88

1-6 100

The elfect of the hydrochloric acid concentration on the solvent extraction from ftuoride-complexed solution was studied with the aid of a solution with an aluminium to fluoride concentration ratio of 1.72 and a fluoride concentration of 0.51N. The

54

J. GOLDEN a n d A . G . MADDOCK

protactinium concentration was about 0.002 mg/ml, and 2 ml of each phase were used. The results are presented in curve C on Graph I. (iv) Homogeneity ofprotactinium solutions. Since the fraction of the protactinium extracted from an aqueous 6N hydrochloric acid solution was greater than that previously reported, ca~some repetitive solvent extractions were performed to determine if all the protactinium in the aqueous phase showed the same solvent-extraction behaviour. In each experiment the two phases were re-equilibrated with equal volumes of the protactinium-free opposite phase, and the distribution of the protactinium was redetermined. 7.0N. hydrochloric acid: z~ / ketone phase (Pa 1 ) \

/ ketone phase . . . . . . aqueous phase E1 ketone phase . . . . . aqueous phase E3

* E, E, Es E4

= = = =

0.041 0"038 0"068 0"037

~ ketone phase fresh aq. phase E2 >-ketone phase

E

fr- sh aq-: p- ase '

:k 0'004 ~: 0"004 -4- 0"005 ± 0"004

* Percentages refer to a q u e o u s phase.

2.80N hydrochloric acid:

ketone phase E a--~-~s p--~ase 1

* E, = Ez = E3 = E4 =

66.2 66.7 65.3 67-1

4d: d: 4-

7 ketone phase / fresh aq. phase E3 ÷ ketone phase fresh aq. phase E2 ~ x , t fresh ketone phase aq. phase E4

0.6 0.6 0-6 0.6

* Percentages refer to solvent phase.

* EI = E.. = E3= E4 =

3.20N hydrochloric acid:

85"0 4- 0'8 85"I = 0'8 87.910.8 84"8 4- 0'8

ketone phase l__~Ex ketone phase E2 fresh ketone phase E3 fresh ketone phase E4 aqueous phase fresh aq. p h a s e - - 4 aqueous phase aqueous phase

1

s

In other experiments using solutions that probably contained hydrolyzed protactinium species, particularly for instance those obtained after inadequate solution of hydroxide precipitates, a part of the protactinium could not be extracted into solvent, and variable extractions between 60 and 90 per cent were obtained, even in 6N hydrochloric acid. Evidence was also obtained that colloidal silica was particularly effective in preventing the extraction of the protactinium. This was confirmed in some experiments in which the variation in the percentage extraction with the amount of added sodium silicate

Protactinium--l: Analytical separations

55

was investigated. The aqueous phase was 6N hydrochloric acid and contained almost 0.004 mg/ml of Pa ~31. 0.5 ml of each phase was used. Sodium silicate added, mg/ml Per cent Pa extracted

20 22

2.1 56

1.5 60

0.8 64

0-2 68-5

In the first four experiments the precipitated silica was visible. (v) Effect of temperature. This influence of the temperature on the distribution of protactinium between a 2.90N hydrochloric acid solution and diisopropyl ketone was investigated, using 1 ml of each phase, and equilibrating the phases for ten minutes at the different temperatures. Since the effect proved to be very small, several measurements were made at each temperature. Only the mean and the standard deviation are recorded. Per cent extraction Temperature

72.1 q- 0.6 0.7 °

72.1 q- 0.7 13.0 °

72-3 = 0.6 18.3°

70.6 ± 0.7 40 °

During these experiments it was observed that the ketone solutions were especially unstable at elevated temperatures, and the protactinium adsorbs even on the polythene vessels. A comparison was made between the losses from the ketone solutions at 18°C and at 40°C.

18°C Time Count on aliquot of solution Time Count on aliquot of solution

0

15 min

150 min

3420

3470

3440

260 min 2880

24 hr 2641

0

10min

40°C 35min

50min

65rain

1750

1582

1333

1390

1190

For this reason the extractions at 40°C referred to above were calculated from the aqueous-phase content and the activity balance. All other experiments at room temperature were completed before adsorption occurred. (c) Separation fi'om other elements--The data above suggested the possibility of a cyclic solvent extraction process which should give a good separation from most elements. Various combinations of steps can be envisaged, and some of the factors involved are examined in the following typical process. The protactinium solution was made 7N in hydrochloric acid and 0.5N in hydrofluoric acid and extracted with solvent. The excellent separation of iron and polonium from the protactinium at this stage can be judged from Graph IV. The data displayed in this graph were obtained in control extractions using Po 21° and macroscopic concentrations of iron, the latter element being estimated gravimetrically. The initial concentration of iron in the aqueous phase was about 5 mg/ml. Naturally elements with insoluble fluorides, such a s thorium, can be separated very efficiently at this stage. The data of STEVENSON and HICKStlz) show that more than 90 per cent of the tantalum would be extracted under these conditions, and less niobium. A rough estimate of the niobium gave 30 per cent extraction. About 50 per cent of stannic tin was extracted. Similar gravimetric experiments showed that ,-~7 per cent of Zr 4+ and less tfian 2 per cent of UO2 ~+, Ti 4+, V5+. or Mn 2+ were extracted.

56

J. GOLDEN a n d A. G. MADDOCK

Only 0.4 per cent of the protactinium is extracted by each sweep with an equal volume of solvent from this aqueous phase, so that the extraction can be repeated two or three times without much loss of protactinium. The fluoride in the aqueous phase was then complexed with aluminium chloride and the solution re-extracted. The increase salt concentration increases the extraction of solne elements, but even when the solution

~ 8c

F

/ 2C

4 6 NORMALITY OF HCI GRAPH

~2,_--.-'d 8

GRAPE[ IV.--Percgntage extraction as a function o f the normality of hydrochloric acid in the aqueous phase. Hydrofluoric acid concentration constant at 0.SN. C) Fe; [] Poat°; 0 Pa las.

was saturated with aluminium chloride, less than 1 per cent of the Th 4÷, AIa+, Ti~-, V5+, Mn 2+, and Ba~+ in the aqueous phase was extracted. However, the extraction of zirconium increased about threefold, and that of uranyl rose to 30 per cent. The protactinium passes almost entirely into the solvent phase, as shown in Graph I. The solvent phase containing the protactinium was then washed with 8N hydrochloric IOC

~

8c

~

4C

0,-

r

/f

.

7

J

8.°2c !

// 4

NORMALITY OF HCI GRAPH

8

GRAPH V.--Percentage extraction as a function o f the normality o f the fluoride-free acid phases. [] PaZ81; C) Fe; (} PaPa; • Zr.

acid. The distribution of protactinium, iron, and zirconium at various concentrations of hydrochloric acid were inyestigated. The iron and zirconium were estimated gravimetically. The results are shown in Graph V. V~-, Mn 2+, AI3+, Mg z+, and Cr a+ are retained less than 2 per cent by the solvent; titanium (Ti~ ) was about 3 per cent and tin (Sn~-) about 65 per cent retained. More surprising, about 17 per cent of the cadmium in the aqueous phase passed into the

Protactinium--I : Analytical separations

57

solvent from 6N acid, and 24 per cent from 8N acid. This hydrochloric acid wash therefore enhances the separation from uranyl and zirconium. The protactinium was finally extracted from the solvent phase by 0.5N hydrofluoric acid 8N in hydrochloric acid, and the purification cycle was repeated. The yield of the single cycle without repetition of any stage was better than 98 per cent. (d) Alternative methods of recovery from the solvent--Besides hydrofluoric acid, various alternative aqueous reagents which re-extracted the protactinium from the solvent were examined. An ammonium fluoride solution containing 100 mg/ml of the salt extracted more than 98 per cent of the protactinium when one-thirtieth of the volume of the solvent was used. 100-volume hydrogen peroxide was nearly as effective. An ammonium thiocyanate solution (100 mg/ml) removed the protactinium, while leaving most of the iron in the solvent.

6. Destruction of the Fluoride Complex Although protactinium can be separated from its fluoride complex by the extraction process above, it was sometimes desirable to find a more direct process. Although the complex was found to be stable at pH ,~9-0, it broke down in caustic soda solution and the protactinium could be completely separated on a hydroxide precipitate. It was observed that, provided this precipitate was not allowed to age, it could be dissolved in hydrochloric acid to give a solution displaying the normal extraction coefficient with diisopropyl ketone and therefore, presumably, free of colloidal material. Repeated evaporation of the fluoride complex with strong hydrochloric acid was also found to drive off the fluoride and produce a solution from which the protactinium could be completely extracted by the solvent.

7. Examples of the Separation followed with Pa~aa Tracer (a) A heavy-metal sulphate precipitate--This siliceous material consisted mainly of the sulphates of lead, barium, and calcium. Two methods of opening up were used. In the first, 1-gm samples were treated with 10 ml of 60 per cent oleum, and a known amount of Pa 2aa tracer added to the mixture. The lead, barium, and calcium sulphates were dissolved, and the siliceous residue, containing the protactinium, was separated and dissolved in 5 ml of 40 per cent hydrofluoric acid. Alternatively it was found that the original material could be attacked directly with 40 per cent hydrofluoric acid containing the Pa 23a tracer, leaving the heavy-metal sulphates as residue and dissolving both Pa 2ax and Pa 2az. Either solution was diluted to 100 ml and an excess of barium chloride solution added. The barium fluoride precipitate was washed until no obvious decrease in bulk took place. The remaining precipitate consisted presumably of fluosilicates and was dissolved in 1M aluminium nitrate solution, 6N in nitric acid. The protactinium was then can ied down on a manganese dioxide precipitate, sufficient manganous chloride and potassium permanganate solutions to produce ,~10 mg precipitate per ml of original solution being added, and the mixture digested at 100°C for half an hour. The resulting precipitate was separated, washed, and dissolved in 20 ml 7N hydrochloric acid with a trace of sodium nitrite. Alternatively the precipitate was leached with 10 ml of 1N hydrofluoric acid. In some analyses the manganese dioxide stage was omitted entirely, and the fluosilicate precipitate dissolved in 7N hydrochloric acid also 1M in aluminium chloride. The hydrofluoric acid leach from the manganese dioxide was also made 7N in hydrochloric acid and 1M in aluminium

58

J. GOLDEN a n d A. G. MADDOCK

chloride. The hydrochloric-acid solution from either of these three variations was then extracted with an equal volume of diisopropyl ketone, and the 0t-activity of the extract measured. The at-activity Of this preparation was considerably reduced by ignition, since an appreciable fraction of the polonium follows the protactinium. However, separation from most of the polonium can be effected by re-extraction from the solvent into 7-8N hydrochloric acid, 0.5N in hydrofluoric acid. In the more extensive solvent cycles performed on the uranyl solution, complete separation from polonium was achieved. The yield calculated on Pa~'r~-recoveryin this separation varied from 80-85 per cent, depending on the variation employed. (b) A uranyl solution--The solution was made 8N in hydrochloric acid and 0.5N in hydrofluoric acid and the tracer Pa ~ added. Direct extraction with diisopropyl ketone removed many impurities, e.g., iron and polonium. Extraction after adding aluminium chloride to complex the fluoride present brought the protactinium into the solvent layer. Washing this layer with 8N hydrochloric acid removed those elements whose extraction had been enhanced by the presence of aluminium chloride, e.g., uranium and zirconium. Finally the protactinium was back-extracted with 6-8N hydrochloric acid, 0.5N in hydrofluoric acid. The next solvent cycle avoided the use of aluminium chloride, repeated ammonia precipitation and hydrochloric acid solution, or continued evaporation with hydrochloric acid, being used to remove fluoride. In either case the efficiency of a cycle was 98 per cent or more, without repetition of any step. At the most two of these cycles were found to be sufficient to achieve radiochemical purity. DISCUSSION

No evidence of volatility of protactinium from aqueous solutions has been found. The residues obtained after evaporation of aqueous solutions do not lose protactinium on ignition below 800°, and thus separation from polonium can easily be effected. The results obtained in the barium fluoride co-deposition experiments suggest that a compound, presumably BaPaF v is very sparingly soluble. This has been confirmed by the preparation of a macroscopic amount of this compound. Its analysis and other properties will be described subsequently. SCHUMB, EVANS,and HASTINGS, (1~ observation on the separation of barium sulphate from hydrofluoric acid solution containing sulphate ion when the protactinium content of the mixture remained in solution, suggests that the following salts are increasingly insoluble. BaFz, BaPaFT, BaSO~. A disappointing feature of this investigation was the discovery that the ketonic solution of the protactinium chloride complex is not indefinitely stable, and slowly loses activity by adsorption on the walls even in polythene vessels. Various ketonic solvents can be used for this separation, but diisopropyl ketone seems to have the best combination of properties. The slope of Graph II showing the relation between the logarithm of the apparent coefficient of extraction and the molarity of ketone in benzene solution is almost exactly three. As has been shown by McKAY and HEALY(16) (see also PEPPARO et al.tX7)), the linear relation with the integral slope at low ketone concentration indicates that the protactinium is present in the solvent 116) W. C. SCHUMB,R. D. EVANS, and J. L. HASTINGS,.Jr. Amer. Chem. Soc., 61, 3453 (1939). ¢t6~ H. A. C. MCKAY and T. V. HEAL',', Rec. Trat,. chim., to be published. ~lTj D. F. PI~I'I'ARD,J. P, FARIS, P. R. GRAY; G. W. MASOn d. Phys. Chem., 57, 294 (1953).

Protactinium--I: Analytical separations

59

as a complex containing three moles of ketone, for instance PaCIs.3(CaHT~)2CO. The linearity of Graph III, which also shows a slope of three, should facilitate identification of the fluoride complex or complexes, and a more detailed study of this subject is in progress. The re-equilibration experiments show that the extracting complex is formed, reversibly, by all the protactinium in the solution. The absence of a measurable temperature coefficient means that the heat of formation of the complex cannot be greater than ~ l kilocal. Only two examples of the combination of these processes into an analytical procedure have been given, because the most appropriate combination depends on the other components of the mixture. With small quantities of material it is often best to reverse the separation cycle, extracting from 6-8N hydrochloric acid and re-extracting the solvent with 6-8N hydrochloric acid 0.5N in hydrofluoric acid. This solution is then evaporated repeatedly with concentrated hydrochloric acid and a further cycle carried out. This avoids the use of aluminium salts for complexing the fluoride. ACKNOWLEDGEMENTS This paper is published by permission of the Admiralty, one of us (J. G.) being with the Services Electronics Research Laboratory, Royal Naval Scientific Service. The generous assistance of U.K. Atomic Energy Authority personnel in connection with materials is also acknowledged. The authors are indebted to Miss B. J. COLGRAVE for assistance with the preparation and counting of samples.