RU2456591C1 - Method of determining silicon content in uranium materials - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: inductively coupled plasma mass-spectrometric method of determining silicon content, for example, in uranium hexafluoride is realised using a dynamic reaction cell with ammonia as the reaction gas, with hydrolysis of the uranium hexafluoride sample, dilution of the obtained solution to uranium concentration of 1 g/l, plotting a calibration curve, measuring intensity of the silicon analytical signal in the analysed sample. The unknown silicon content is found from the calibration curve. The method does not require preliminary separation of silicon from the analysed sample, and to suppress undesirable spectral interference, ammonia reaction gas is used, which is released into the cell at a constant rate of 0.24 ml/min with selected optimum operating parameters of the cell quadrupole: high-frequency (RPq=0.6B) and constant voltage (RPa=OB), which enables direct, reliable detection of silicon in solutions when its content is on the level of 5 mcg/l with uranium content of 1 g/l.

EFFECT: high accuracy and rapidness of analysis.

1 ex, 4 tbl, 1 dwg

Description

The present invention relates to analytical chemistry, and in particular to methods for determining the content of elements in uranium materials, for example in uranium hexafluoride, in particular silicon.

The claimed technical solution can be used to determine the components whose content in uranium hexafluoride is subject to control.

Uranium hexafluoride as a feedstock for enrichment using the uranium-235 isotope has stringent requirements for the purity of the product, in particular for the content of silicon, an element that forms volatile fluorides. The silicon content in uranium hexafluoride according to ASTM 787-2006 [1] is limited to 100 ppm (100 · 10 ^-4 % of uranium). It should also be noted that for the analysis of high purity uranium oxide, it is necessary to determine silicon at the level of 5 ppm.

Direct spectrophotometric methods for determining silicon in uranium materials are known. Among them are the methods described in the collection of standard ASTM methods: spectrophotometric determination of silicon with ammonium molybdate in HFCs (ASTM C 761, Annual book of ASTM standards: C 761-2004. Spectrophotometric Determination of Silicon and Phosporus, section No. 45) [2] and uranyl nitrate solutions (ASTM C 799, Annual book of ASTM standards: C 799-2005. Silicon by the Molybdenum Blue (Photometric) Method, section No. 62) [3]. The disadvantages of these methods include low selectivity, high complexity and duration of the analysis.

Known is the direct express method for inductively coupled plasma mass spectrometric (ICP-MS) determination of the silicon content in uranium materials (ASTM C 1287, Annual book of ASTM standards: C 1287-2003. Standard Test Method for Determination of Impurities in Uranium Dioxide by Inductively Coupled Plasma Mass Spectrometry) [4]. It should be noted that the sensitivity of the determination of silicon in uranium materials by this method is limited by a high background level and distortion of the analytical signal due to spectral interference from polyatomic and molecular ions CO ⁺ , N ₂ ⁺ , C ₂ H ₄ ⁺ and C ₂ H ₂ N ⁺ . In order to eliminate the interfering effect of polyatomic ions, ASTM 1287 [4] used separate conditions for the determination of silicon, namely, the conditions of a “cold” plasma, which are achieved with a low power high-frequency (HF) generator (1000 W) and a high consumption of atomizing gas (1, 1 l / min). The disadvantage of this method is that, firstly, a low plasma temperature leads to a deterioration in the sensitivity of determination of silicon due to its inefficient ionization, and secondly, when analyzing samples with a high content of the matrix element (uranium) in a cold plasma, there is a noticeable increase the inhibitory effect of uranium, making it difficult to determine silicon at 5 ppm or lower.

Known adopted as a prototype method for determining silicon in uranium hexafluoride with preliminary separation (Industry instruction. OI 001.464-2009. Silicon. The method of distillation-spectrophotometric determination in HFC, inv.16 / 12101) [5], which consists in heating the analyzed samples at a temperature 105 ° C for 5-6 hours in a mixed acid (mixture of nitric, sulfuric and hydrofluoric acids) into polyacetal autoclave compartment silicon tetrafluoride as a gaseous silicon compound (SiF _4), silicon tetrafluoride absorbed alkali Coated in a thin layer on the lid of the autoclave, dissolving an alkali layer with the absorbed silicon tetrafluoride to saturated solution of boric acid, spectrophotometric determination of silicon in the resultant solution to the blue color silicomolybdic complex.

This method has the following disadvantages:

- the need for additional operations to separate silicon from the analyzed sample, which leads to an increase in the complexity and duration of the analysis;

- additional operations lead to an increase in error.

The objective of the proposed technical solution is to create such a method for the determination of silicon in uranium materials, which, excluding the disadvantages inherent in known methods, would provide a direct, reliable and rapid determination of the silicon content in uranium materials at the level of 5 ppm to uranium.

The problem is solved in that in the claimed method for determining silicon in uranium hexafluoride using a mass spectrometry method, including hydrolysis of a HFC sample, diluting a solution of hydrolyzed uranium hexafluoride to a uranium concentration of 1 g / l and establishing a calibration characteristic, measuring the intensity of the analytical silicon signal in the analyzed the sample is carried out directly, without preliminary separation of silicon from the analyzed sample, at the selected optimal parameters of the dynamic reaction ion cell into which is fed a constant flow of the reactive molecular ammonia gas. The combination of thermochemical reactions involving ammonia occurring in a cell with a certain passband provides an effective suppression of spectral interference of polyatomic ions.

The proposed method for determining the silicon content in uranium materials was implemented in the analysis of samples of production of uranium hexafluoride using a mass spectrometer equipped with an Elan DRC II dynamic dynamic cell. Hydrolysis of uranium hexafluoride samples was carried out to obtain a solution of uranyl fluoride with a uranium concentration of 200-300 g / l. Before taking measurements, the uranyl fluoride solution was diluted with deionized water to a uranium concentration of 1 g / L.

As it was established during the development of the proposed method for determining the silicon content in uranium materials, polyatomic ions interact with the molecules of ammonia gas poured into a dynamic reaction cell and are eliminated from the general ion flow. The figure shows the plot of the mass spectrum in the mass range from 25 to 30 amu obtained for silicon isotopes in standard operation without gas inlet (type A of Fig.) And at a flow rate of ammonia of 0.3 ml / min and RPq = 0.6 (type B of Fig.) In the analysis of a solution containing 1 g / l of uranium (“blank” sample). As can be seen from the figure (view B), the use of ammonia leads to a significant decrease in the background signal from polyatomic ions at a mass of 28 amu

As a result of research, the following patterns were identified.

1. The optimal reaction gas for the most effective elimination of polyatomic interference in direct ICP-MS determination of the silicon content in uranium-containing solutions is ammonia.

2. The use of ammonia as the reaction gas of the cell can improve the detection limit for the direct determination of silicon from the ²⁸ Si isotope in uranium solutions by at least 3 times in comparison with the standard measurement mode (without gas inlet).

3. The sensitivity of the determination of silicon in uranium materials can be increased tenfold when selecting the optimal flow rate of the reaction gas and the electrical parameters of the cell quadrupole — AC RF voltage (parameter RPq), DC voltage in the cell (parameter RPa), DC voltage applied to the rods of the cell quadrupole and a mass analyzer (CRO and QRO) and cell acceleration voltage (CPV) by reducing the overall background and increasing signal stability and ion transfer efficiency.

4. The optimal flow rate of the reaction gas is from 0.10 to 0.25 ml / min. It was established that at an ammonia flow rate of more than 0.25 ml / min, the analyzed element may be lost as a result of ion-molecular reactions of silicon with ammonia, and at a flow rate of less than 0.1 ml / min, the suppression of background signals from polyatomic ions decreases.

5. As calculations showed, an increase in the value of the parameter RPa (RPa> 0) slightly affects the lower limit of the passband when measured on a mass m _an = 28 amu, therefore, the value of RPa = 0 was adopted as the optimal value for this parameter.

6. The optimal value of the parameter RPq for the effective removal of interfering ions from the cell, causing the formation of new overlays, is in the range from 0.55 to 0.7 V. At high values of the rf voltage RPq> 0.7, a decrease in the intensity of the analytical silicon signal due to the occurrence of an endothermic charge transfer reaction between the ²⁸ Si ⁺ ion and NH ₃ , leading to the loss of the analyzed element. At low RPq values <0.5, the cell efficiency decreases due to the formation of new interfering ions with a mass number of 28 amu

7. The optimal value of the constant voltage was U _CRO = -2 ÷ -3 V.

8. In order to ensure sufficient bandwidth of the quadrupole mass analyzer during operation of the dynamic reaction cell, the potential applied to the quadrupole of the QRO analyzer must have a higher negative value than the potential applied to the quadrupole of the cell — CRO, namely in the range from - 5 to –6 V. Changing the QRO parameter to a smaller or larger side leads to a deterioration in the sensitivity of determining the silicon content.

9. The optimal value of the accelerating voltage CPV is in the range from -18 to -19 V. Changing the QRO parameter to a smaller or larger side also leads to a deterioration in the sensitivity of determining the silicon content.

The research results are presented in tables 1, 2, 3. Thermodynamic and kinetic characteristics of the ion-molecular reactions of polyatomic ions with a mass of 28 amu with ammonia are shown in table 1.

Table 1 Polyatomic ion Ion-molecular reaction products Reaction enthalpy,
ΔH _r , kcal / mol The reaction rate constant, k _r , cm ³ / molecule · s [23] N ₂ ⁺ N ₂ + NH ₃ ⁺ -124.99 2.02 · 10 ^-9 N ₂ H ^{+ +} NH ₂ There is no data 1.80 · 10 ^-9 CO ⁺ CO + NH ₃ ⁺ -78.4 2.04 · 10 ^-9 HCO ⁺ + NH ₂ -41.18 There is no data C ₂ H ₄ ⁺ NH ₃ ⁺ + C ₂ H ₄ -7.84 1.80 · 10 ^-9 NH ₄ ⁺ + CHCH ₂ -26.94 2.06 · 10 ^-9 C ₂ H ₂ N ⁺ NH ₃ ⁺ + C ₂ H ₂ N -17.06 There is no data

According to table 1, the ion-molecular reactions of polyatomic ions with ammonia are exothermic (ΔН _r <0), thermodynamically allowed. The most effective reactions are charge transfer reactions.

The results of measurements of analytical silicon signals in a certified mixture and a “blank” sample, depending on the selected parameters of the cell, are shown in table 2.

table 2 Ammonia consumption, ml / min Signal / Background Ratio The magnitude of the RF voltage applied to the rods of the cell quadrupole (RPq), V Signal / Background Ratio 0 5 0.1 0.5 0.1 eighteen 0.2 1.8 0.2 fifteen 0.3 2.6 0.3 13 0.4 3 0.4 12 0.5 3.8 0.5 eleven 0.6 4.3 0.6 10 0.7 4.4 0.7 9 0.8 3.4 0.8 8 0.9 2

As follows from table 2, the best ratio of the analytical signal and the background is achieved when the flow rate of ammonia is from 0.1 to 0.25 ml / min and the parameter RPq value is from 0.55 to 0.7 V. Under these conditions, an effective decrease in the background signal, which indicates the chemical resolution of interfering ions and silicon, and the high reactivity of ammonia with respect to polyatomic and molecular ions: N ₂ ⁺ , CO ⁺ , C ₂ H ₄ ⁺ , C ₂ H ₂ N ⁺

The optimal parameters of the measurement program of the analytical signal of silicon are shown in table 3.

Table 3 Mass Spectrometer Operation Parameter Value Cell operation parameter Value Power of a high-frequency generator, W 1400 The flow rate of the reaction gas, Cell Gas Flow, ml / min 0.1 ÷ 0.24 Spray Gas Consumption, Nebulizer Gas Flow, L / min 0.83 ÷ 0.9 DC voltage applied to the rods of the quadrupole mass analyzer, QRO, V -5 ÷ -6 Consumption of cooling gas Plasma Gas, l / min fifteen DC voltage applied to the rods of the cell quadrupole, CRO, V -2 ÷ -1 Auxiliary Gas flow rate, l / min 1.4 Accelerating cell voltage, CPV, V -19 ÷ -18 Lens Voltage, V 5.4 ÷ 5.5 High-frequency cell voltage, RPq, V 0.55 ÷ 0.7 Detector Operation Mode Dual mode DC voltage in the cell, RPa, V 0

To determine the dependence of the analytical signal intensity on the silicon content, certified mixtures of AC ₁ , AC ₂ , AC ₃ were prepared with a mass fraction of silicon of 5, 50, 100 μg / g of uranium, respectively, which are solutions of a silicon ion in a solution of uranyl fluoride obtained by hydrolysis of uranium hexafluoride high purity (not containing silicon) with a mass concentration of uranium of 200-300 g / l.

To establish the calibration dependence, a solution of uranyl fluoride (silicon-free) with a mass concentration of uranium of 1 g / l as a solution of a “blank” sample, as well as certified mixtures with a silicon content of 5, 50, and 100 μg / g of uranium, were introduced into the discharge plasma ascending. The ion count rates of the ²⁸ Si isotope were measured.

Using the software of the mass spectrometer using the multiple regression method, the dependence of the intensity of the analytical signal (imp./s) on the mass fraction of silicon (μg / g of uranium) in the certified mixture was obtained, which is described by the equation: I = a · C, where

C is the silicon content in the calibration sample, μg / g of uranium;

I is the intensity of the analytical signal, imp./s;

and - the calculated coefficient equal to 5.048.

Based on the obtained data, a calibration graph was constructed, according to which the silicon content in the analyzed samples was determined.

An example of determining the silicon content in uranium materials, for example in uranium hexafluoride, using inductively coupled plasma mass spectrometry and a dynamic reaction cell

A sample of uranium hexafluoride is hydrolyzed with deionized water.

Using a pipette of variable capacity 100-1000 mm ^3, a necessary aliquot of the uranyl fluoride solution obtained by hydrolysis is measured in a polypropylene flask with a capacity of 50 cm ³ , calculated under the condition that the concentration of uranium in the solution is 1 g / l, adjusted to the mark with deionized water and mix. For each sample, two parallel definitions are performed.

Sample solutions prepared for measurement are introduced into the discharge plasma. The intensity of the analytical signal of the silicon ion in solutions of samples of hydrolyzed uranium hexafluoride is measured in accordance with the measurement parameters on an Elan DRC II mass spectrometer shown in Table 3.

Mass fraction of silicon in μg / g of uranium in the solutions of samples of hydrolyzed uranium hexafluoride is determined using the established calibration characteristics.

The values of the characteristics of the errors in determining the mass fraction of silicon to uranium in uranium hexafluoride by the present method and the prototype method are shown in table 4. Table 4 - Characteristics of the relative measurement errors of the proposed method in comparison with the prototype method with a confidence probability of P = 0.95.

Table 4 Method for determination of silicon in uranium hexafluoride Detection limit, ppm The range of determined mass fractions, ppm

Convergence characteristic

Characteristic of non-excluded systematic error, ± and Confidence limits of the total error for n = 2, ± d Analysis duration Prototype method 3 5 to 10 incl. 0.12 0.38 0.42 The time required to perform measurements taking into account the time of temperature control and cooling of the autoclaves is 7.5 hours Over 10. to 20 incl. 0.10 0.19 0.24 Over 20 to 40 incl. 0,07 0.11 0.15 Over 40 to 100 incl. 0.05 0.08 0.11 The inventive method 2 From 5.0 to 100.0 incl. 0.11 0.12 0.20 The total flushing time of the sample injection system is at least 5 minutes, measurements are taken 45 minutes

The proposed method has successfully passed tests that confirmed the high accuracy and rapidity of determining the mass fraction of silicon in uranium hexafluoride. The method allows you to abandon the stage of preliminary separation of silicon in the analysis of the sample and the associated additional laborious operations that affect the accuracy and reproducibility of the analysis.

Information sources

1. Annual book of ASTM standards: 787-2006. Standard Specification for Uranium Hexafluoride for Enrichment.

2. Annual book of ASTM standards: C 761-2004. Spectrophotometric Determination of Silicon and Phosporus.

3. Annual book of ASTM standards: C 799-2005. Silicon by the Molybdenum Blue (Photometric) Method.

4. Annual book of ASTM standards: C 1287-2003. Standard Test Method for Determination of Impurities in Uranium Dioxide by Inductively Coupled Plasma Mass Spectrometry.

5. Industry instruction. OI 001.464-2009. Silicon. The method of distillation-spectrophotometric determination in HFCs, inv.16 / 12101.

Claims (1)

The method of determining the silicon content in uranium materials by measuring the intensity of the analytical signal in the test sample, including hydrolysis of the sample, for example uranium hexafluoride, and diluting the sample to a uranium concentration of 1 g / l, characterized in that the determination of silicon in uranium hexafluoride is carried out directly without prior separation silicon from the analyzed sample using a mass spectrometer with inductively coupled plasma equipped with a dynamic reaction cell, with the selected optimal pa in terms of operation of a dynamic reaction cell, namely, at a flow rate of ammonia reaction gas of 0.1 to 0.25 ml / min, an alternating HF voltage RPq = 0.55 ÷ 0.7 V, a constant voltage RPa = 0 V, accelerating voltage CPV = -19 ÷ -18 V, voltage on the rods of the quadrupole of the cell CRO = -2 ÷ -1 V, constant voltage applied to the rods of the quadrupole of the mass analyzer QRO = -5 ÷ 6 V.

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