etermination of Thorium and Uranium in Solul ay Spectrometry GEORGE PlSH and AUDREY A. HUFFMAN M o u n d Laboratory, Monranto Chemical Co., Miamisburg, Ohio
The determination by chemical methods of thorium and uranium in the aqueous and organic solutionsfrom solvent extraction investigations was slow and inadequate. A rapid and accurate x-ray fluorescent spectrametriernethod hasbeendevelopedin which theaqueous and organic solutions could be analyzed directly. Sample preparation required only the addition of a suitahle internal standard added as a solution. Internal standards of strontium solutions were used for the aqueous samples and bromohenzene solutions for the organic samples. Working curves obtained by plotting intensity ratios of the element to the internal standard versus the concentration of that element gave straight lines. In all the ranges studied the effect of matrix impurities was negligible. The accuracy of the final results is as good as present chemical methods. A sample can be analyzed for both thorium and uranium in 0.5 hour or less.
Table I.
X-Ray Spectrometer Pa Working Curves
Spectrometersetting (28deg.I Aqueous fced Raffinate organic phase
35.27
x X
37.10 X X
x
33.87
x x -
40.47
-
-
X
ground 38.75 X
x
X
series (8)are used, is described. The accuracy and relhbility of the method, as well as the time required for analysis, are eonsidered. APPARATUS
T
HE extraction of thonum and uranium from raw materids . ' . . . . and their partitioning' by use of organic solvent techniques can he accomplished by countercurrent extraction methods. The establishment of the proper conditions for extraction and partitioning of thorium and uranium required numerous reliable analyses for these two elements. A number of variables are involved in the development of any solvent extraction process; therefore, numerous chemical analyses me needed for design purposes, as well as for the determination of the best operational conditions. A single experiment resulted in many aqueous and organic solutions which required immediate analysis, hut weeks were needed t o obtain suitable analyses by chemical means. Some of the analyses were unreliable, because of the presence of certain impurities which are difficult to remove. The preparation of solid samples from solutions was difficult and a t times resulted in some loss of thorium and/or uranium. Therefore x-ray flucrescent methorls (2, 3) applicable t o solids resulted in doubtful data attributed to dubious sampling techniques. X-ray fluorescent analysis of liquids (2, 7) appeared to be feasible, since the materials are in solution throughout the various processes. A rapid method for the quantitative determination of thorium s n d uranium direetlv from solutions bv use of fluorescent x-mv mtrometry, in which the characteristic spectra of the K and L
Figure 1.
Completed cell, accessory parts, loading syringe
EXPERIMENTAL
For most of the studies the x-ray apparatus was operated a t ratings up to50 kvp. and 45 ma. The raw materialsand processed sludges used in the present investigation contained up t o 60% thorium and 0 t o 2% uranium. Therefore, greater emphasis was placed on the thorium analysis. Figure 2 gives the region and x-ray spectral positions which were used, ohtahed by B rapid scan. rhe spectra shown were free of interfering element and absorption edge effects. The analyses were arranged into three classi6cations: Aqueous Feed Solutious. These include nitric acid solutions of new minerals, processed sludges, feed stock for solvent extraction development experiments, and product solutions containing both thorium and uranium. Aqueous Ratiinate Phase. These represent nitric acid solutions containing 0 to 10 mg. per ml. of thorium and the impurities which were present in the original material. Organic Phase. Organic solutions (40% trihutyl phosphate and 60y0aliphatic diluent solvent) are included. These solutions contain eaacrrlially thorium, uranium, and small amounta of impurities. Table I gives the spectra criteria and spectrometer positions needed in the determination of working curves.
1875
1876
ANALYTICAL CHEMISTRY
Known aqueous standard solutions were prepared from high purity thorium metal, thorium nitrate, pure uranium oxide (&Os),mixed phosphates, various rare earths, and various heavy metals. The latter three groups of materials were included in the preparations t o determine the effect of the matrix upon the working curves. Solutions were made by adding thorium, uranium, and matrix materials in predetermined, weighed amounts t o 10or 100-ml. volumetric flasks, dissolving in a small quantity of nitric acid, and diluting t o the mark. The solutions in the 10-ml. volumetric flasks were used directly, while those in the 100-ml. volumetric flasks were divided into aliquot portions for further dilution. The solutions for the internal standard were prepared from reagent grade, anhydrous strontium nitrate dissolved in dilute nitric acid: 300.0 mg. per ml. for aqueous feed samples, and 36.0 mg. per ml. for raffinate samples. In the preparation of the organic phase standards, nitric acid solutions containing known amounts of thorium and of uranium were extracted with 40% tributyl phosphate in 60% aliphatic diluent. All the organic phase extracts were carefully analyzed chemically and optically before use. As with the aqueous standards, some were used directly, while others were divided into aliquot portions and further diluted with the mixed solvent. The organic internal standard was prepared t o contain approximately 80 mg. per ml. of bromobenzene in the mixed organic solvent, and then it was carefully analyzed chemically for bromine. All subsequent standards were adjusted to correspond to the bromine content of the initial standard. The analytical sample was prepared by mixing 1.00 ml. of the appropriate internal standard with 10.00 ml. of sample solutions both being a t the same temperature. Each solution was transferred as needed to the liquid sample holder which was placed in the fluorescent x-ray unit. Each sample was set manually a t the positions indicated in Table I for the sample type and counted.
BrKa (INTERNAL STANDARD)
ORGANIC PHASE SOLUTION
Figure 2. X-ray fluorescent spectra for thorium and uranium determination
An average of three sets of readings \vas obtained for every sample counted. The scaler measured the elapsed time for a predetermined number of counts; hence, all data were reduced to counts per second. For any element counted a background correction was made :
Z(A) = A
- B and Z(S)
=
S
-R
where
Z(A)
= intensity for element sought
and
I(#) A B S
= = = =
[A]= D X R where [ A ] = concentration of A in milligrams per milliliter R = intensity ratio Z(A)/Z(S) and
D
intensity for internal standard counting rate in counts per second for element sought counting rate in counds per second for background counting rate in counts per second for internal standard
Then the intensity ratios, Z(A)/I(,”), versus concentrationq of A in milligrams per milliliter for the same sample series were plotted t o give the working curves in Figures 3 t o 5 for both thorium and uranium, These curves represent single examples of several curves made using varying amounts of different matrix blends.
= A[A]/AR DISCUSSION
The raw minerals and sludges used in the studies varied widely in composition. One sludge used in the studies contained 45% thorium oxide, 15% rare earth oxides, 4.4% silicon dioxide, 7.0% titanium dioxide, 4.3% ferric oxide, 1.4% zirconium dioxide, 5.3% phosphorus pentoxide, and 1.0% uranium oxide (U80~). The effect of the above elements on the aqueous feed curves (Figure 3) was examined. Solutions were made in which only thorium and uranium were used, and other solutions were prepared in which matrix blends were added in varying quantities.
10 I50 700. 0125
600’
ILa
jdk
AQUEOUS FEED SOLUTION
As the curves were closely linear, the concentrations of thorium and uranium were calculated more conveniently:
500.
.o I00
400.
,0075
3 00-
u,d
.0050
-0025 0 0
M O
0
IO
Figure 3.
1000
1500
Th(MG/ML) 20 30 UiMG I M L 1
2000
2500
40
50
Curves for thorium and uranium aqueous feed solutions
For each point plotted to give the curve in Figure 3 the total solid content was about 340 mg. per ml. B comparison of the data available for thorium indicated that the average values were reproducible with a precision within 3~0.5% and an accuracy within 1%in the range between 10 and 240 mg. per ml. of thorium. Above 240 mg. per ml. of thorium, the graph deviated from the 1% linear portion of the curve. As it was undesirable for plant use to correct for dead time losses in the counting tube, counting rates were adjusted to stay within the 1% linearity limit. For the range of 1 to 10 mg. per ml. of thorium the accuracy dropped to about 5%. The uranium analysis gave the same result in the 1- to 5-mg. per ml. range as the thorium analysis in the 1- to 10-mg. per ml. range. The raffinate analysis resolved itself into one in which a small amount of thorium was present in 97 to 100% matrix. The total solids were unusually high. A curve plotted from data using only t w u m in 2 to 4M nitric acid reproduced the high matrix curve remarkably well; an accuracy of about 1% of the amount present was observed in the concentration range of 1 t o 10 mg. per ml. Thorium and uranium in the organic phase curves in Figure 5 were almost free of any impurities. Checks between prepared known samples agreed to better than 1% of the amount present. The thorium and uranium curves are linear to 1% throughout the concentrations studied. From the above analyses it appeared that the effect of the impurities of the absorption of x-rays and on the emission fluorescent spectra was unimportant for the materials studied. This would
V O L U M E 27, NO. 12, D E C E M B E R 1 9 5 5
1877 be purified readily, the use of a different internal standard with new spectral positions might remedy the disturbing effect. Some of the solutions containing a high solid concentration salted out after the internal standard was added. In such cases the samples were diluted 1 to 1 to solve this difficulty. No significant error was introduced by dilution. Volumetric flasks are t o be preferred over pipets, as thoriumbearing solutions increase considerably in viscosity as the amount of that element increases. Good volumetric practice is imperative for best results. Each sample preparation required only a few minutes. Using the average of three sets of readings for each element, the determination of thorium and uranium required altogether 0.5 hour or less. With some sacrifice in accuracy, single readings on each element corrected for a predetermined average background value would permit analysis of more than 60 samples for uranium and for thorium in an 8-hour working period.
3 00'
Th(MG/ML)
Figure 4. Curve for thorium in aqueous phase raffinates
I
CONCLUSIO3
.O 300
.o 200
2 00.
.o
100
.o 0 0
Figure 5.
100 10
300
400
500
Th(MG/ML) 20 30 U(MG /ML )
40
50
200
The x-ray fluorescent method using solutions directly is employed routinely t o provide development data on materials which are difficult or time-consuming to analyze chemically. It is not limited to thorium and uranium and is being used successfully for a number of elements above atomic number 22. Usually the data are reported to &O.Ol% or other equivalent units, although the sensitivity of detection is about 0.004% for a number of elements. For uranium and thorium the limit of detection is better than 1 count per second above the corresponding background a t 0.004%.
Curves for thorium and uranium in organic phase solutions
not be true a t very low concentrations or in a very precise analysis. The homogeneity of the solutions tends to minimize the errors normally present when solid samples are used. Further, use of ratios of the corrected intensities of the element sought to that of the internal standard compensated to a large extent for gross absorption effects. Some examples of analyses of feed materials, aqueous phases, and organic phases by the x-ray fluorescent method are given in Table 11. The x-ray data are based upon single samples on each of which an average of three counts was determined. For the chemical and optical analysis two or more samples were used and the results expressed as the average. Approximately 90% of the samples showed agreement between comparative methods to 1% or better. About 10% of the samples showed differences to 5%, and isolated samples gave considerably higher errors. Further examination of the samples, which gave high deviations, indicated a high phosphate and rare earth content. Known samples were prepared containing weighed quantities of thorium, uranium, and a matrix with a high phosphate and rare earth content. Analysis for thorium and uranium by the x-ray fluorescent method v,-as accurate to 1% or better, but the chemical methods still gave errors to 5%. Further work is in progress critically to evaluate these results. Khen determining low concentrations of elements accurately, the presence of impurities in the internal standard may contribute to small errors in the final result. The curve passed through the origin of the graph only if all conflicting impurities are absent and all background effects are fully accounted for. Usually a family of straight-line curves of approximately the same slope are obtained when interfering impurities are present. To correct for this effect it is necessary to determine the intensity ratio a t zero concentration of the element in question which then is subtracted from the samples analyzed. If the reagents cannot
Table 11. Thorium and Uranium Analysis h-0. 1 2 3 4
5 6
7
8 9 10 11 12 13 14
Sample No. Feed 6-A/O .\IUR-l MUR-2 MUR-3 HBU-1 ll-A/O-Raff-l1-27 7-.4/0-RaiT-6-17 11-A/O 3-PP-Feed 3-PP-2-0-A 8-P- A- 1E-73 6-P-8-1E 8-P-OK Liq f e e d 18-.1/0
X-Ray Fluorescence, Mg./Ml. Th U 154.00 4.62 56.90 1.78 71.00 2.53 89.90 2.42 48.18 0.25 0.22 .. 13.2 182.24 5:74 2 ; ; . 93 5.89 1.22 J J .02 0.07 65.21 142.74 0 06 53.62 0.00 199.05 3.99
Chemical and/or Optical, Rlg./hll. Th U 154 ... 56 1.7 69 2.5 92 2.5 50
