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X-RAY SPECTROGRAPHIC DETERMINATION OF HAFNIUM AND ZIRCONIUM CONCENTRATION IN THE SPONGE ZIRCONIUM METALv
05/02/2012 12:29
An energy dispersive X-ray fluorescence method is described for the simultaneous determination of hafnium and zirconium in the metal zirconium. The concentration of Hf and Zr in zirconium samples was calculated base on the standard calibration. With a single exposure, it is possible to cover the concentration range of Zr/Hf from 100 to 1000 times. The analytical results showed a good agreement with those obtained by ICP-MS analyses and titrimetric method.

 X-RAY SPECTROGRAPHIC DETERMINATION OF

HAFNIUM AND ZIRCONIUM CONCENTRATION IN THE SPONGE ZIRCONIUM METAL

Dinh Cong Bot, Tran Thi Ngoc Diep, Nguyen Quoc Hoan, Nguyen Van Sinh

INSTITUTE FOR TECHNOLOGY OF RADIOACTIVE AND RARE ELEMENTS

 

Abstract

An energy dispersive X-ray fluorescence method is described for the simultaneous determination of hafnium and zirconium in the metal zirconium. The concentration of Hf and Zr in zirconium samples was calculated base on the standard calibration. With a single exposure, it is possible to cover the concentration range of Zr/Hf from 100 to 1000 times. The analytical results showed a good agreement with those obtained by ICP-MS
analyses and titrimetric method.

1.     Introduction

Zirconium is one of the high commercial materials because of its resistance to corrosion, high melting point and good thermal conductivity. For these reasons, it is used in a number of industry applications such as military industry, chemical industry, electricity, electronic, metallurgy, etc. Nowadays, about 80-90% of zirconium metal production is consumed by commercial nuclear power generation. Zirconium metal exhibits a low thermal neutron capture across-section as well a resistance to corrosion which makes it ideal for cladding on fuel rod in nuclear reactor. The high thermal neutron capture across-section of hafnium makes it ideal to be used as control rod in nuclear reactor. It shares almost all of zirconium’s other chemical attributes, such as its resistance to corrosion which makes zirconium an ideal material in reactor design. Unfortunately, due to their similarity these elements specially are extremely difficult to separate which creates practical problems when preparing pure zirconium or hafnium for nuclear applications. In order for zirconium to be used in reactor, however, it is necessary for hafnium concentration to be less than 0.01%.

In Vietnam, the Center for Science and National Defense Technology has cooperated with the Institute for technology of Radioactive and Rare elements (ITRRE) in researching and manufacturing Sponge Zirconium metal with quality criteria in order to provide Zirconium metal for military industry. It also lays the foundation for nuclear grade zirconium metal production in Vietnam in the future. Determination of trace amounts of hafnium in zirconium is important in nuclear technology where Hf is an undesirable element. Hafnium and zirconium have almost identical chemical properties and therefore separation of the two elements is both difficult and expensive. So, the analysis of Hf by conventional chemical methods is troublesome owing to the difficulty of separating Hf from Zr. Instrumental methods such as Optical Emission Spectroscopy (OES), Neutron Activation Analysis (NAA), X-ray Fluorescence (XRF) etc. are quite useful for such an analysis[2]. One major disadvantage of NAA is that the technique requires access to a high-flux neutron source in the nuclear facilities. Even one error in dilution can have large effects on analytical result in the case of ICPMS. X-ray fluorescence has been employed successfully under carefully controlled conditions for analysis of mixtures of zirconium and hafnium because of its capability of giving high precision and accuracy.

 

 

2. Experimental

2.1. Instrument

- Energy Dispersive X-ray fluorescence system (ED-XRF).

+ Excited source: X-ray tube, Mo anode (Model CK-5 made in Italy). Beryllium window (an area of 20 mm, a thickness of 0.3 mm. Mo: the secondary target.

+ Si (Li) Dedector (Model SL 30165), 30 mm in diameter, Beryllium window (a thickness of 25mm), energy resolution 165 eV on the Fe line (5.9 KeV).

+ Bin/Power supply (Canberra, Model 2000).

+ Pre-amplifrier(Canberra Model 2008B).

+ Automatic amplifier (Canberra Model 2025AFFT0).

+ Canberra Multiport Multichannal Analyzer.

- GENIE 2000 spectroscopy software.

- Axil PC software.

2.2. Preparation of standards samples

            Suitable proportions of Hf and Zr standard solutions were mixed in separate clean beakers so as to yield various standard ratios of Hf and Zr in solutions followed by precipitation with    NH4OH as hydroxides. The obtained precipitate was filtered, washed, dried and finally transferred to platinum dishes and was ignited at 9000C to the oxides. These standard oxides were mixed thoroughly with equal proportions by weight of starch as the binding material and then were pressed in to pellets of 24 mm diameter.

            The sample pellet was prepared by weighing accurately about 0.1 g of sample into 30-ml platinum crucible. Then the sample was dissolved in concentrate HF and then the hydroxide mixture was precipitated by the addition of ammonium hydroxide. The hydroxide was filtered, dried and fired at 900°C in a platinum crucible for 1 hour. Sample compression process is also similar to the standard pellet preparation.

3. Results and discussion

3.1. Comparison of Mo- and Cu secondary target

      An X-ray source can excite characteristic X-rays from an element only if the source energy is greater than the absorption edge energy for the particular line group of the element. The secondary target material is excited by the primary x-rays from the x-ray tube, and then emits the secondary x-rays that are characteristic of the elemental composition of the target. For the best results, the optimized secondary target material in respect of each element of interest has to be found. In this research, Mo and Y secondary targets were used. The results are shown in the table 1.

Table 1. The XRF intensity of Zr and Hf using different secondary target

 

Element

Cu secondary target

Mo secondary target

Peak area

Peak area

Zr

5138

1.0

33480

54.3

Hf

50273

28.6

3264

1.7

 

The intensity of Zr measured in the sponge Zirconium sample using Mo as a secondary target is 6 times bigger than the same sample excited by Cu secondary target. In case of excitation with a Mo secondary target, the resulting spectra shown in Fig. 2 demonstrate the effect of Mo line to Zr fluorescence line. We can assume the peak overlap between Mo line and Zr line due to the energy of X ray emitted from the Mo secondary target (17,48 KeV) is near the one of Zr(15,77 keV). This overlap can be a significant problem for elemental quantification. In successive approximations the values of the parameters are altered until the minimum in  is reached [7]. However, in this experiment chi-square reached high value. For the above practical reasons, Cu is used as the secondary target to excite Zr. This is also the reason why Mo is chosen as the secondary target for Hf . 


3.2. Voltage and current of X-ray tube

 The X ray spectrum depends on the accelerating voltage as well as the electron beam current. The intensity of the x-ray radiation was regulated during the measurements of the current variations in the tube with each prescribed accelerating voltage. The results are shown in the table 2

 

Table 2: The dependence of the XRF intensity on the voltage and current of Xray tube

I

U    

40 kV

45 kV

50 kV

Z2

H1

Z2

H1

Z2

H1

15 mA

4414

1804

4194

2206

3936

2990

Dead time

9.67

11.40

12.77

13.35

15.27

17.62

20 mA

5518

2488

7536

2442

4590

3262

Dead time

12.15

13.82

13.05

16.74

18.39

19.69

25 mA

7657

2722

10660

3836

5266

3408

Dead time

13.47

18.96

13.63

19.10

21.09

20.73

30 mA

9374

3282

11004

3672

11004

4186

Dead time

15.12

21.57

24.61

26.15

29.76

28.32

 

Our experimental data indicated the dependence of experimental fluorescence intensities as a function of the operational voltage. When the tube voltage increases, the X-ray fluorescence also increases. However, by increasing the voltage on the X-ray tube, two effects which are unfavorable to the X-ray fluorescence intensity, are intensified. By increasing the anode voltage, the dead time of the electronic system and the intensity of the scattered radiation also increase. The dead time causes longer measuring time and high number of coincidences while scattered radiation causes a high back ground. The experimental research proves that the dead time is less than 20% to ensure the accuracy and stability of analytical result [7,11].

For the optimization of tube current and voltage, the tube voltage and current were varied while measuring Zr and Hf. We observed from the result that the intensity of Zr and Hf became higher with the increase of the tube voltage and anode current. In general, the change in current has the same effect on the intensity than the change in voltage. For the same voltage of 45 kV, the XRF intensity were found to vary sharply when the current was changed from 15 mA  to 25 mA at intervals of 5 mA. At the same current of 25 Ma, a big acceleration of the intensity was observed when the voltage was varied from 40 kV to 45 kV, but the intensity became lower when the voltage reached 50 kV. To avoid the wearing of the X ray tube by applying unnecessary high voltage, a voltage of 45 kV was chosen. The current of 25 mA was also chosen for the routine analysis. Therefore, the chosen voltage current pair was 45 kV and 24 mA.

3.3 Calibration

            Five standard samples were prepared in accordance with the norm in order to construct the calibration curves of Zr and Hf.

 

Sqrt (count)

Concentration(ppm)Hinh 3_duong chuan Hf.JPG

Fig.3  Calibration curve of  Hf

                 Sqrt (count)

Hinh 4_duong chuan Zr.JPG                                                    Fig.4  Calibration curve  of Zr

                  Fig. 3 and Fig. 4 show the relationship between the XRF intensity and the given concentration of Zr and Hf. The results exhibit the calibration graphs are linear in the concentration range of 500-10000 ppm of Zr and 5-500 ppm of Hf.

 

3.4. Concentration of Zr, Hf in some samples

The analytical procedures were validated by the posterior prediction of Zr in 3 additionally prepared standards used as unknown samples. Determination of Hf and Zr in these samples was carried out. The precision of each analysis has been estimated and compared with the data achieved by ICPMS and titration method.

 Table 3 Comparison of concentrations determined by XRF, ICPMS, titration with the predicted concentrations in three unknown samples

 

 

Sample

Predicted concentration

XRF (%)

 

Bias (%)

ICP-MS (%)

Titration

Zr

Hf

Zr

Hf

Zr

Hf

Hf

Zr + Hf

ArtZr1

95

1,5

96.25 ± 1.23

1.45 ± 0.06

+1.32

-3.3

1.58

96.83

ArtZr2

97.5

1

96.87 ± 1.62

1.05 ± 0.08

- 1.05

+ 5.0

0.93

98.78

ArtZr3

98

0.5

98.56± 1.50

0.52 ± 0.09

+ 0.57

+ 4.0

0.58

98.92

 

ICP-MS, titration and XRF give consistent results for Zr and Hf analyzed by three methods. It is clear that the accuracy of the methods is similar for zirconium, hafnium contents in these samples. The error of XRF is not more than the absolute permissible error. Therefore we can use XRF to determine the concentration of Hf, Zr in the sponge zirconium sample.

Table 4. The Hf, Zr content in the sponge zirconium sample

Sample

XRF (%)

Titration (%)

Zr

Hf

Zr + Hf

RZr1

96.92 ±1,72

0.66 ± 0,12

97.00

RZr2

96.67±1,34

0.50 ± 0,08

96.50

RZr3

97.6 ±1.52

0.32 ± 0.08

97.50

RZr4

98.2 ±1.06

0.187 ±0.055

98.3

RZ5

98.5 ±1.86

0.108 ±0.073

98.7

RZ6

98.7 ±1.67

0.08 ±0.006

98.9

The results in the table 4 indicate that Zr/Hf ratio in the sample is in the range of 100-1000 that appropriate to ratio of Zr/Hf in the standard sample. There was not a much variation in the concentration of Zr and Hf in the variety of samples.

4. Conclusion

An Energy Dispersive XRF method has been developed for the simultaneous quantitative analysis of Zr and Hf concentrations in the sponge zirconium. The method is found to be inferior when being compared with ICPMS and titration method.

                                                             

References

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