simultaneous determination of uranium and thorium in high grade thorium ores
TRANSCRIPT
Journal of Radioanalyti c al and Nuclear Chemistry, Articles, VoL 170, No. 1 (1993) 165-170
S I M U L T A N E O U S D E T E R M I N A T I O N OF U R A N I U M A N D T H O R I U M IN H I G H G R A D E T H O R I U M ORES
S. AHMAD, A. MANNAN, I. H. QURE~HI
Nuclear Chemlstty Division, Pakistan Institute of Nuclear Science and Technology, P. O. Nilore, lslamabad (Pakistw 0
(Received May 11, 1992)
A routine procedure has been developed for the simultaneous determination of uranium and thorium in high concentration thorium ores. INAA is used to determine the uranium and thorium concentration. However, for very low concentrations of uranium a radiochemical procedure based on the use of NPy/benzene as an extractant has to be employed. The precision and accuracy of the method has been determined by analyzing IAEA and NBL standard thorium/uranium ores.
Introduction
Uranium and thorium ore prospecting involves analysis of a large number of samples to evaluate their economic value and feasibility of chemical processing. This interest has
emphasized the need for simple and accurate techniques and the development of
standard reference materials for calibration of those techniques. Uranium and thorium
determination of uranium and thorium in low grade uranium and zirconium ores. 4-5 The
been reported from a number of laboratories, t-3 Activation analysis is one of the most widely used techniques owing to its high sensitivity and adaptability to a wide variety of materials.
In our earlier communication we have demonstrated the usefulness of INAA for determination of uranium and thorium in low grade uranium and zirconium ores. 4-5 The
present investigation extends the utility of the same technique for the quantification of
uranium in high grade thorium and REE ores. It also brings out the inadequacy of INAA in certain circumstances and the necessity of using a radiochemical neutron activation analysis when U levels in the ores are very low. For such cases, we developed a
technique using NPy/benzene as an extractant and utilized it successfully for the determination of low levels of uranium in the presence of h i g h concentration of thorium
and rare earth elements. The procedure was also applied for the determination of U and thorium in high grade thorium ores supplied by IAEA SRMs.
Elsevier Sequoia S. A., Lausanne Akaddmiai Kiad6, Budapest
s. AHMAD et al.: SIMULTANEOUS DETERMINATION OF URANIUM AND THORIUM
Experimental
Reagents
4-(5-nonyl)pyridine (NPy) was obtained from K. K. Labs. Plainview, N. Y., and was purified by vacuum distillation before use. All other reagents used in this study were of
analaR grade. Radioactive tracers were prepared locally by neutron irradiation of their appropriate spec-pure compounds.
Preparation of standard and samples
Standard solutions of uranium and thorium were prepared by dissolving accurately
weighed high purity uranium dioxide and thorium metal in appropriate acids (reagent grade), heating to dryness and finally making up to a known volume of aqueous solution. Accurately weighed aliquots of the standard solution were then transferred
onto a piece of clean filter paper (No. 589 Selecta from R. F. P., Germany) and dried at room temperature. For the INAA of uranium and thorium in the ores, the samples were
hermetically sealed in appropriate vials and irradiated along with the standard as prescribed in our earlier communication. 6 If INAA could not be used conveniently for
the determination of U, the following radiochemical procedure was employed.
Preconcentration procedure and irradiations
About 500 mg of the sample (S-14 and IAEA SRM thorium ore) was dissolved in
aqua regia and heated until the solution was clear. The solution was then evaporated and its strength adjusted to 7M HC1. To this solution 10 ml of NPy/benzene was added, and 5 minutes standing time was allowed at room temperature to ensure the completion of
reaction. The mixture was agitated for 2 minutes. Organic phase containing U was
separated and dried at room temperature in polythene vials. Four samples and two standards sandwiched between them were packed in another polythene container and irradiated for 2 to 10 minutes. All irradiations were carried out in our 5 MW swimming
pool type reactor PARR-I at a thermal neutron flux of 2.5 �9 10 J-3 n �9 cm -2- s -1.
Radioassay technique
All the irradiated samples and standards were counted with a coaxial 30 cm 3 Ge(Li)
detector (Canberra Inc.) coupled to a charge sensitive preamplifier (Canberra Model 9700) and a spectroscopic amplifier (Ortec Model 451), both incorporating a zero pole cancellation. A Nuclear data 4410 computerized multichannel analyzer with an 8I( memory was used for pulse height analyses. The system had a resolution (FWHM) of
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S. AHMAD et al.: SIMULTANEOUS DETERMINATION OF URANIUM AND THORIUM
2.0 keV for the 1332.4 keV ,/-ray of 6~ and a peak to compton ratio of 40:1. The
system was calibrated with a standard source prior to every set of experiment. The spectroscopy of the irradiated samples was performed by placing them at an appropriate distance from the detector depending on the activity of the sample. A counting time of 10 minutes - 1 hour was found sufficient. The areas under the desired full energy peaks were calculated by subtracting the background form the integral area of the peak.
Results and discussion
In case of uranium determination by a non-destructive method, the samples (IAEA SRMs, S-15 and S-16) after irradiation showed a fairly high matrix activity due to 233pa, 24Na, 56Mn and REE (those are--4-8%). This interference can be minimized by reducing the irradiation time and increasing the cooling time. Since 239Np is produced according to the nuclear reaction,
Z'nU(n , ,/)239U �9 239Np (2.35 days)
a reduction of 2 minutes in the irradiation time will not decrease the production of 239Np by the same factor as that for the short-lived nuclides mentioned. The `/-ray spectra of
samples namely, S-15 and S-16, recorded at successive time intervals showed that after 3 days cooling, the 239Np ,/-my photopeaks were clear. 4,5 The relatively prominent peaks
of 239Np at 228.2 and 277.6 keV are the most suitable for uranium determination. The
determination of thorium was made via the 300.1, 311.9, 340.5, 398.5 and 415.8 keV full energy peaks of 233pa. The details of peak clarity and interferences have already been discussed elsewhere. 6 The amounts of uranium and thorium in samples were
determined by comparing the relevant photopeaks of samples with those of standards. Uranium and thorium concentrations in filter paper used in the preparation of
standards were also measured by the prescribed method and were found to be (5.2 • 0.4) and (11.5 • 0.65) ppb, respectively. The contribution made by these concentrations was calculated to be less than 0.61% and was hence considered negligibly small.
However, the procedure described could not be applied successfully to uranium determination in the IAEA-S-14 sample. In this case the 239Np peaks could not be detected because of a very high background generated by the ,/-rays of thorium and rare
earth elements (0.06 and 3.0%, respectively). In order to see the extent of their interferences in the respective 239Np (U) photopeaks, the contribution factors were estimated by selecting a locally available monazite sample (1 5 -5 %) . The concentration of the interfering elements are given in Table 1. Some of the possible interferences and their contributions are also shown in Table 1. Contribution factor (f)
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S. AHMAD et al.: SIMULTANEOUS DETERMINATION OF URANIUM AND THORIUM
Table 1 Uranium y-ray energies and possible interferences
Gamma-ray, Possible interferences keV
(239Np) Element Value, Radio- Energy, Contribution p,g/g* nuclide keV factor (f)**
103.7 Sm 6 153 Sm 103.0 62.8 Gd 250 153 Gd 103.2 0.01
106.1 Lu 1.0 # 177m Lu 105.3 0.25 Ta 33 t83 Ta 107.9 2.24
117.7 Yb 30 169yb 118,6 0.05 120.7 Lu 1.0 # 177m Ln 121,0 1.19
Eu 55 lSaEu 121,7 18.73 209.75 Lu 1.0 # 177Lu 208.3 34.73 228.2 W 27 188 W 227.0 0.003
Lu 1.0 # 177m Lu 228,4 1.59 Ta 33 182Ta 229.0 0.08
277.6 Ba 107 133m Ba 276.9 0.01 Hg 32 203 Hg 279.0 0.21
Se 32 75 Se 279.5 0.02 Ta 33 183 Ta 313.2 24.93
334.2 Sn 86 125 Sn 332.0 0.001
*Estimated concentration of REE and other elements in a local monazite sample, # value expressed in %.
**f is based on 10 minutes irradiation and 3 days cooling of 500 mg of the monazite sample, where uranium, concentration is 30 gg/g.
was estimated by taking the absolute activity ratio of the interfering radionuclide (i) to the corresponding activity generated from 239Np (U) photopeak. The photopeak count rate (P) can be given as:
P = [(w N A a/M) o �9 h ~1(1 - e4"t)][eXt~] (1)
where a and M represent isotopic abundance and atomic mass of w grams of an atom. The notations NA, O, qS, t, and t c represent Avogadro's number, thermal neutron absorption cross section, neutron flux, irradiation time and decay time, respectively. The terms h, v I and ~. denote the intensity, efficiency and decay constant of the photopeak used. Under similar irradiation conditions, the factor, f, can be estimated as:
[(W a / M ) h tJ (1 - e-kt)] i f = Pi /Pu = [(w a/M) h o (1 - e-Xt)]u " [e-t' (x~ (2)
It was not possible to determine uranium by controlling irradiation and cooling time.
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S. AHMAD et al.: SIMULTANEOUS DETERMINATION OF URANIUM AND THORIUM
Therefore, the application of a suitable radiochemical procedure became imperative.
The post-irradiation chemical separation is not recommended to avoid unnecessary
exposure of the personnel. Therefore, we decided to separate U from the ore matrix
before irradiation. For this purpose use was made of a radiochemical technique described earlier, 7 in which NPy/benzene was used as an extractant for uranium
Table 2 Concentrations of thorium and uranium in IAEA SRMs
(Thorium ores)
Our values IAEA values
Thorium Concentration expressed in %
S-14 0.065-+ 0.004 0.061 S-15 0.349 -+ 0.021 0.365 S-16 1.588 -+ 0.047 1.680
Uranium Concentration expressed in ppm
S-14 28.5 - 1.57 29 S-15 86.8 + 2.91 85 S-16 448 -+ 8.12 445
Table 3 Uranium and thorium concentrations in ores
of various nature
Sample Uranium, % Thorium, %
Monazite 0.36 5.12 Monazite 0.10 1.78 Monazite 0.008 0.437 Carbonatite 0.251 0.031 Carbonatite (138) 0.037 0.003 Carbonatite (136) 0.022 0.001 Lepedolite 0.0028 -
separation from thorium and REE and subsequently determination was made with
INAA as described earlier. To detemline the precision and accuracy of the method by
IAEA, standard ore samples were also analyzed. The results are presented in Table 2
which indicate an overall precision of 5--6%. This procedure was then applied to different ores and minerals for their uranium and thorium contents and the results are given in Table 3.
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S. AHMAD et al.: SIMULTANEOUS DETERMINATION OF URANIUM AND THORIUM
Conclusion
The method developed for the simultaneous determination of uranium and thorium is relatively simple and accurate and requires a minimum of sample handling. The technique can be effectively used on routine basis for the analysis of large samples of uranium and thorium ores (containing high levels of rare earth elements) encountered in ore prospecting and geochemical studies. The radiochemical procedure described can be
used advantageously for the analysis of samples with low contents of uranium as
compared to thorium and rare earth elements.
We are indebted to Mr. A. K. RANA for his assistance in canying out the experimental work. We are also grateful to the Reactor Operaton Group for arranging the irradiation. Reference materials were supplied by the Analytical Quality Cona'ol Services Division of IAEA.
References
1. L R. HATAWAY, G. W. JAMES, Anal. Chem., 47 (1975) 2035. 2. S. AMIEL, Anal. Chem., 34 (1962) 1683. 3. A. V. MURAL[, P. P. PARKEH, M. SANKAR DAS, Anal. Chim. Acta, 50 (1970) 71. 4. M. S. CHAUDHARY, S. AHMAD, L H. QURESHI, J. RadioanaL Chem., 42 (1978) 427. 5. M. S. CHALIDHRY, S. AHMAD, L H. QURESHI, J. Radioaeal. Chem., 57 (1980) 137. 6. S. AHMAD, M. S. CHALIDHRY, I. I-L QURESHI, J. Radioanal. Chem., 67 (1981) 119. 7. M. EJAZ, Separ. Sci. Teeh., 13 (1978) 843.
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