RU2154028C1 - Method of determining content of perfluorocarbon compounds in uranium hexafluoride - Google Patents

Abstract

FIELD: analytical methods. SUBSTANCE: prior to be subjected to mass-spectrometric analysis, uranium hexafluoride with perfluorocarbon compounds contained therein is condensed into nickel reactor. The latter is heated to temperature allowing thermal destruction of perfluorocarbon compounds to form perfluoromethane and carbon. Simultaneously, uranium hexafluoride interacts with reactor nickel to release atomic fluorine, which, in turn, reacts with carbon to form perfluoromethane. After reactor is cooled to ambient temperature, content of perfluoromethane is measured by means of gas mass-spectrometer and measured data is used to calculate original content of perfluorocarbon compounds in uranium hexafluoride taking into account mass of fluorine added in the course of thermal destruction. EFFECT: increased accuracy and sensitivity of analysis. 2 dwg, 4 tbl

Description

The invention relates to the field of analysis of materials, and in particular to methods for determining the content of perfluorocarbon compounds (C _n F _m ) in uranium hexafluoride.

 Currently, analogues of the invention are the method of IR spectroscopy [1,2] and direct (classical) mass spectrometric method [3,4].

When determining the content of perfluorocarbons (PFCs) in uranium hexafluoride by IR spectroscopy, first hydrolysis of UF _{6 is} carried out, then PFCs are extracted using some extractant (for example, carbon tetrachloride) and then, passing infrared radiation through the obtained extract, along an absorption band of 1244 cm ^-1 determine the content of PFCs. The main disadvantages of the method are: laborious sample preparation, a large amount of uranium hexafluoride required for one analysis (~ 100 grams), and the semi-quantitative nature of the method.

When determining the content of PFCs in uranium hexafluoride by direct mass spectrometric method, the gas is directly introduced into the ionization chamber of the gas mass spectrometer directly through a needle dispenser. The determination of the PFC content in UF _{6 is} carried out by the value of the output signals CF ₃ ⁺ (m / e = 69 amu) or C ₃ F ₅ ⁺ (m / e = 131 amu) corresponding to the content of PFC . The detection limit of the method, expressed in molar fractions, is 0.01%, which is due to the maximum allowable pressure of the analyzed gas mixture in the ionization chamber of the order of 1 • 10 ^-4 mm Hg.

 For the prototype of the proposed method taken direct mass spectrometric method.

When determining the content of organofluorine compounds in uranium hexafluoride by direct mass spectrometric method, the following actions are performed:
- let the analyzed gas sample UF ₆ into the vacuum system of the mass spectrometer to the metering valve;
- open the metering valve and let gas into the ionization chamber of the mass spectrometer;
- record the mass spectrum of the sample;
- the magnitude of the output signals of the mass spectrum corresponding to the pressures of uranium hexafluoride and organofluorine compounds in the inlet system, calculate the molar fraction of organofluorine compounds.

 The task to which the claimed invention is directed is to increase the accuracy and sensitivity of the existing method.

The task is carried out as follows:
The analyzed gas mixture UF ₆ - C _n F _{m is} condensed into a nickel reactor. The reactor is a nickel pipe 12-15 cm long with an outer and inner diameter of 10 and 14 mm, respectively. The region of the reactor (~ 10 cm ³ ) is cut off from the volume of the vacuum system. The reactor is heated to a temperature of 900-950 ^o C with a gradual rise in temperature at a speed of 30-50 ^o C / min The total heating time of the reactor is 40-60 minutes. In this case, chemical reactions take place in the reactor, as a result of which uranium hexafluoride is converted into non-volatile compounds with the release of atomic fluorine:
UF ₆ ---> UF ₅ + F ^o ,
3UF ₆ + Ni ---> NiF ₃ + 3UF ₅ ,
2 • NiF ₃ ---> 2NiF ₂ + F ^o .

The catalyst for the reactions are the nickel walls of the reactor. Heating the reactor with uranium hexafluoride at the above temperature for 40-60 minutes leads to the complete conversion of HFCs to non-volatile compounds. As the heating time decreases, some uranium remains in the form of UF ₆ .

C + 4F^o ---> CF₄.Under the influence of high temperature, thermal destruction (pyrolysis) of molecules of organofluorine compounds to perfluoromethane and carbon occurs. The carbon formed reacts with active fluorine atoms; the process ends with the formation of perfluoromethane:
C _n F _m + (4 • nm) • F ^o ---> n • CF ₄ ,

C + 4F ^o ---> CF ₄ .

The optimal temperature for thermal destruction of organofluorine compounds is 900-950 ^o C. At this temperature, the PFCs are completely converted to perfluoromethane, which is confirmed by the absence in the mass spectrum of analytical lines with mass numbers 119 (C ₂ F ₅ ⁺ ) and 131 (C ₃ F ₅ ⁺ ). Carrying out pyrolysis at a lower temperature leads to an increase in the fraction of “heavy” FUs, and the determination of the average molecular weight of FUs resulting from pyrolysis will be less accurate. In FIG. Figure 1 shows the dependence of the ratio of the output signals of ions C ₃ F ₅ ⁺ (m / e = 131 amu) and CF ₃ ⁺ (m / e = 69 amu), measured after pyrolysis, on the temperature of thermal decomposition . In the spectrum of heavy FUs before pyrolysis this ratio is 30%. The dependence of the ratio (I ₁₃₁ / I ₆₉ ) • 100 on the temperature of thermal degradation is shown in FIG. 1.

As a result of thermal degradation from one “heavy” molecule of organofluorine compounds C _n F _m , capable of easily adsorbing on the inner surface of the vacuum system of the mass spectrometer, n molecules of a highly volatile and easily analyzed compound — CF _{4 — are formed} , which leads to a significant increase in the sensitivity of the method.

An increase in the heating rate (more than 50 ^o C / min) can lead to the formation of a plug of non-volatile uranium compounds in the center of the reactor and its failure.

 After termination of thermal degradation, the reactor is cooled to room temperature (300K) and the total pressure of the gaseous pyrolysis products is measured.

 Using a gas mass spectrometer, the amount of perfluoromethane in the mixture is determined and the mass concentration of FU in HFCs is calculated taking into account the mass of fluorine added during thermal decomposition.

 The mass concentration of FU in uranium hexafluoride after their thermal destruction is calculated in the following sequence.

The mass of uranium hexafluoride condensed into the reactor is determined by the formula:
m _HFC = 0.0188 • V • ΔP, gram (1)
where V is the volume of the vacuum system from which the condensation UF ₆ , liter;
ΔP - pressure change UF ₆ in the volume V, mm Hg

 Expression (1) is obtained from the Clapeyron equation for an ideal gas at a temperature of 300 K.

где I₆₉ - величина выходного сигнала, соответствующая содержанию CF₄ в продуктах пиролиза;
K₆₉ - коэффициент относительной чувствительности CF₄;
I_i - величина выходного сигнала, соответствующая содержанию i-компонента в продуктах пиролиза;
K_i - коэффициент относительной чувствительности i-го компонента.After the thermal degradation of the FU is completed, the gas mixture containing the pyrolysis products is introduced into the ion source of the mass spectrometer and the molar fraction of perfluoromethane (CF ₄ ) is determined:

where I ₆₉ is the value of the output signal corresponding to the content of CF ₄ in the pyrolysis products;
K ₆₉ - coefficient of relative sensitivity CF ₄ ;
I _i is the value of the output signal corresponding to the content of the i-component in the pyrolysis products;
K _i is the coefficient of relative sensitivity of the i-th component.

 When using formula (2), the entire mass spectrum of thermal degradation products is prescribed and all compounds present in the mixture are identified.

In gaseous products of pyrolysis are always present: carbon dioxide (CO ₂ ); carbon monoxide (CO); hydrogen fluoride (HF); silicon tetrafluoride (SiF ₄ ) and perfluoromethane (CF ₄ ). Sometimes, small amounts of boron trifluoride (BF ₃ ), oxygen (O ₂ ), and uranium hexafluoride (UF ₆ ) are present. The main components of the mixture are CO and CO ₂ released from the nickel reactor as a result of its heating. In FIG. Figure 2 shows the characteristic mass spectrum of a mixture of gases obtained by pyrolysis of 0.42 grams of uranium hexafluoride, with a molar fraction of PFCs of 5 • 10 ^-4 %.

где C_CF4 ^пр - молярная концентрация CF₄, определенная по формуле (2);
P $\genfrac{}{}{0ex}{}{\text{пр}}{\text{Σ}}$ - суммарное давление продуктов пиролиза в объеме V₁, мм рт.ст.The partial pressure of perfluoromethane in the mixture is determined:

where C _CF4 ^etc. - molar concentration of CF _4, defined by the formula (2);
P $\genfrac{}{}{0ex}{}{\text{etc}}{\text{Σ}}$ - the total pressure of the pyrolysis products in a volume of V ₁ , mm Hg

где I₆₉ - величина выходного сигнала, соответствующая содержанию CF₄ в продуктах пиролиза;
K₆₉ - коэффициент относительной чувствительности CF₄;
I_j - величина выходного сигнала, соответствующая содержанию j-компонента в калибровочной смеси;
K_j - коэффициент относительной чувствительности j-го компонента;
C_j ^кс - молярная доля j-компонента в калибровочной смеси;
P $\genfrac{}{}{0ex}{}{\text{кс}}{\text{Σ}}$ - суммарное давление калибровочной смеси перед игольчатым натекателем, мм рт.ст.The partial pressure of CF ₄ in the mixture can be determined using a calibration mixture. For this, sequential introduction of thermal degradation products and a calibration mixture into the mass spectrometer is carried out. The calculation is made according to the formula:

where I ₆₉ is the value of the output signal corresponding to the content of CF ₄ in the pyrolysis products;
K ₆₉ - coefficient of relative sensitivity CF ₄ ;
I _j is the value of the output signal corresponding to the content of the j-component in the calibration mixture;
K _j - coefficient of relative sensitivity of the j-th component;
C _j ^ks is the molar fraction of the j-component in the calibration mixture;
P $\genfrac{}{}{0ex}{}{\text{cop}}{\text{Σ}}$ - the total pressure of the calibration mixture in front of the needle leak, mm Hg

где I₆₉ ^пр - величина выходного сигнала, соответствующая содержанию CF₄ в продуктах пиролиза;
I₆₉ ^кс - величина выходного сигнала, соответствующая содержанию CF₄ в калибровочной смеси;
C_CF4 ^kc - молярная концентрация CF₄ в KC, % молярных;
P $\genfrac{}{}{0ex}{}{\text{кс}}{\text{Σ}}$ - суммарное давление калибровочной смеси перед игольчатым натекателем, мм рт.ст.If perfluoromethane is present in the calibration mixture, expression (4) is simplified:

where I ₆₉ ^CR - the value of the output signal corresponding to the content of CF ₄ in the pyrolysis products;
I ₆₉ ^ks is the value of the output signal corresponding to the content of CF ₄ in the calibration mixture;
C _CF4 ^kc — molar concentration of CF ₄ in KC,% molar;
P $\genfrac{}{}{0ex}{}{\text{cop}}{\text{Σ}}$ - the total pressure of the calibration mixture in front of the needle leak, mm Hg

The mass of perfluoromethane resulting from thermal degradation is determined:
m _CF4 = 0.0047 • P _CF4 • V ₁ , (6)
where P _CF4 is the partial pressure of perfluoromethane, determined by the formulas (3-5);
V ₁ is the calibrated volume of the vacuum system in which the pressure of the pyrolysis products is P $\genfrac{}{}{0ex}{}{\text{etc}}{\text{Σ}}$ .
Expression (6) is obtained from the Clapeyron equation for an ideal gas.

где s - коэффициент термической деструкции;
M_CF4 - молекулярный вес перфторметана, г/моль;
M_CnFm - средний молекулярный вес ФУ, присутствующих в гексафториде урана, г/моль.As a result of thermal destruction, the mass of fluorocarbons due to the attached fluorine increases. The factor s (coefficient of thermal destruction), which establishes the magnitude of the change in mass FU, is determined from the ratio:

where s is the coefficient of thermal destruction;
M _CF4 — molecular weight of perfluoromethane, g / mol;
M _CnFm — average molecular weight of FU present in uranium hexafluoride, g / mol.

где I_i - величина выходного сигнала, соответствующая молекулярному иону с массой M_i.If the average molecular weight of fluorocarbons is not known, then it is determined using a mass spectrometer with "soft" negative ionization, in which resonance electron capture by a molecule occurs with the formation of a molecular ion. So, if the mass spectrum consists of k molecular ions, then the average molecular weight of the FU is determined by the formula:

where I _i is the value of the output signal corresponding to a molecular ion with mass M _i .

где m_CF4 - масса CF₄, определенная по формуле (6);
m_ГФУ - масса гексафторида урана, определенная по формуле (1).Determine the mass fraction of FU in HFCs:

where m _CF4 is the mass of CF ₄ determined by the formula (6);
m _HFC - mass of uranium hexafluoride determined by the formula (1).

 S is the coefficient of thermal destruction.

 For various perfluorocarbons, most often present in uranium hexafluoride and having a molecular weight of from 500 to 800 amu, the coefficient of thermal destruction is 1.8-2.2. Therefore, to calculate the mass content of PFCs after their pyrolysis, an average value of 2.0 can be assigned to the destruction coefficient. Moreover, the calculation error will not exceed 15% relative.

Example 1. The determination of the mass fraction of Freon-350 (C ₇ F ₁₄ ) in uranium hexafluoride using thermal destruction of PFCs (table 1-3). For this, three UF ₆ - C ₇ F ₁₄ mixtures with a mass fraction of Freon-350 were used: 0.02%, 0.005% and 0.0005%. These mixtures were prepared on the basis of pure substances.

В таблице 4 для сравнения приведены результаты определений содержания фреона-350 в гексафториде урана прямым масс- спектрометрическим методом и методом термической деструкции.The coefficient of thermal destruction for freon-350 is:

Table 4 for comparison shows the results of determining the content of Freon-350 in uranium hexafluoride by direct mass spectrometric method and thermal destruction method.

Literature
1. A. Smith, Applied IR Spectroscopy, Trans. From English -M.: World, 1982, 328 p .;
2. "Determination of trace quantities of volatile fluoride in uranium hexafluoride using an infrared spectrophotometer." Analytical Chemistry, vol. 44, no. 9, august 1972;
3. J. Beynon, Mass spectrometry and its application in organic chemistry. Per. from English -M .: Mir, 1964, 702 p .;
4. I. L. Agafonov, G.G. Ninth, Mass Spectrometric Analysis of High Purity Gases and Vapors, Moscow: Nauka, 1980.

Claims (1)

A method for determining the content of perfluorocarbon compounds in uranium hexafluoride, including the inlet of the analyzed gas mixture into the ionization chamber of the mass spectrometer and recording the mass spectrum of the mixture, characterized in that the uranium hexafluoride with the organofluorine compounds present in it is condensed into a nickel reactor, which is heated at a rate 30 - 50 ^o C / min up to 900 - 950 ^o C, maintained at this temperature for 20 - 30 min, cooled to room temperature and then determined by the classical mass spectrometric method the amount of perfluoromethane formed as a result of thermal destruction of perfluorocarbons is added, and the initial mass content of perfluorocarbons is calculated taking into account the mass of fluorine attached in the process of thermal decomposition.

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