RU2422366C1 - Method of extracting uranium from sodium fluoride sorbent - Google Patents

Abstract

FIELD: chemistry. ^ SUBSTANCE: invention can be used in processing wastes containing uranium fluorides. Sodium fluoride sorbent which contains uranium (V) is treated with water vapour at temperature 100-250C. Uranium is then extracted through fluorination at temperature not higher than 400C. ^ EFFECT: invention lowers fluorine consumption and prevents sorbent agglomeration. ^ 4 tbl, 4 ex

Description

The invention relates to the technology of uranium production and, in particular, can be used in the processing of waste containing uranium fluorides.

As a result of the gas separation production of uranium isotopes, gas mixtures are formed containing one or another amount of uranium hexafluoride. Known methods for the extraction of uranium hexafluoride from process and waste gases are sorption technologies using sodium fluoride. In the sorption-desorption cycles of uranium hexafluoride on sodium fluoride in the latter, uranium (V) gradually accumulates as a result of partial thermal decomposition of the complex salt and reduction of uranium:

Na ₂ UF ₈ → Na ₂ UF ₇ + 0,5F ₂

During long-term operation, a significant amount of pentavalent uranium from 3.5 to 10 g per 1 kg of sorbent per cycle accumulates in the sorbent, which leads to a decrease in the working capacity of the sorbent for uranium hexafluoride.

Known methods for the extraction of uranium (V) from sodium fluoride. Well-known hydrometallurgical methods for processing substances containing uranium fluorides [Gromov B.V. Introduction to the chemical technology of uranium. - M .: Atomizdat, 1978; Turaev N.S., Zherin I.I. Chemistry and technology of uranium. - M.: Publishing. House. Ore and Metals, 2006], are unacceptable due to a number of serious drawbacks: the complete loss of uranium in the form of hexafluoride, fluorine, sorbent, the formation of large volumes of circulating and mother liquors and the corresponding problems associated with the purification and disposal of these solutions, etc.

A known method of fluorination of the sorbent with elemental fluorine or chlorine trifluoride. However, even at temperatures of 500-600 ° C, complete desorption of uranium cannot be achieved. In addition, at such temperatures, the sorbent is sintering and practically loses its ability to absorb UF ₆ and requires replacement [Gromov OB, Sergeev GS, Evseev VA et al. “Analysis of existing methods for the extraction of non-desorbable uranium residue from spent sodium fluoride”. - M.: VNIIKhT OJSC, TI-2381, 2000].

The known method [Galkin N.P., Zaitsev V.A. and Seregin M.B. Capturing and processing fluorine-containing gases. - M .: Atomizdat, 1975, p.90], adopted for the prototype, the extraction of uranium from the complex salt of Na ₂ UF ₇ . The method consists in fluorinating the salt with elemental fluorine at a pressure of up to 1 atm and a temperature of 300-450 ° C. The disadvantages of this method are the sintering of sodium fluoride at temperatures above 400 ° C, and, therefore, a decrease in its sorption characteristics [Galkin NP, Zaitsev V.A. and Seregin M.B. Capturing and processing fluorine-containing gases. - M .: Atomizdat, 1975, p. 121], and the low fluorination reaction rate at a temperature of 300 ° C and below, as well as a significant fluorine consumption equal to 40 g per 1 kg of sodium fluoride. The degree of extraction of uranium from sodium fluoride does not exceed 80%. In addition, the extraction of the sintered sorbent from the sorption apparatus is a rather time-consuming operation.

The technical result of the proposed method for the extraction of uranium from sodium fluoride is to prevent sintering of the sorbent.

The technical result of the invention is achieved by the fact that in the method of extracting uranium in the form of uranium hexafluoride from a sorbent of sodium fluoride before fluorination, a sorbent containing uranium (V) is treated with water vapor at a temperature of 100-250 ° C. Fluorination is carried out at a temperature not exceeding 400 ° C.

As a result of our studies, it was shown that in the treatment of sodium fluoride containing pentavalent uranium with water vapor in the temperature range from 100 to 250 ° C, the disproportionation of uranium (V) to uranium (IV) and uranium (VI) takes a relatively short time ( from seconds to fractions of a second), which depends on the diffusion rate of water molecules in the particles of sodium fluoride:

2Na ₂ UF ₇ + 2H ₂ O = Na ₂ UF ₆ + Na ₂ UO ₂ F ₄ + 4HF

Indirect thermodynamic substantiation of the processes occurring in the system can be obtained only from the fluorination reactions of individual uranium fluorides due to the practical lack of similar data on complex sodium and uranium fluorides. Fluorination of uranium fluorides in oxidation states +4 (tetrafluoride) and +6 (oxyfluoride) occurs with much greater ease than fluorination of uranium pentafluoride (see table 1) [Chizhevsky S. V., Seleznev V. P. and Yagodin G.A. “Interaction of uranium-containing compounds of alkali metals with elemental fluorine”, 5th All-Union Symposium on the Chemistry of Inorganic Fluorides. Abstracts of reports. Dnepropetrovsk, June 27-30, 1978 - M .: Nauka, 1978, p.290; Morachevsky A.G., Sladkov I.B. Physicochemical properties of molecular inorganic compounds. - L .: Chemistry, 1987].

Table 1 The likelihood of uranium fluoride fluorination reactions Reaction -ΔG ° ₂₉₈ , kcal / mol lgK _{p, 298} -ΔG ° ₆₇₃ , kcal / mol lgK _p , ₆₇₃ UF ₅ + 0.5F ₂ = UF ₆ 31,0 22.7 25.5 8.3 UF ₄ + F ₂ = UF ₆ 53.7 39,4 42.5 13.8 UO ₂ F ₂ + 2F ₂ = UF ₆ + O ₂ 120.3 88.2 128.6 41.7

The sorbent containing U ⁵⁺ was treated at 120 ° C with water vapor, the content of which in the gas was 2.3-47.0 vol.%. After that, NaF was fluorinated with fluorine at 400 ° C and a pressure of 120 mm Hg. within 0.5-4 hours. Table 2 shows the results of processing NaF containing U ⁵⁺ after hydrolysis and fluorination. The initial content of uranium (V) in the sorbent was 0.92-3.08 wt.%.

table 2 Uranium content after primary steam-air treatment No pp [H ₂ O], vol.% Processing time, min Uranium content, wt.% (After fluorination for 1 hour) 0.5 one 2 four one 2,3 one 0.34 0.29 0.24 0.22 2 2,3 2 0.34 0.21 0.17 0.15 3 2,3 5 0.25 0.20 0.17 0.14 four 7.4 one 0.24 0.20 0.17 0.13 5 7.4 2 0.24 0.19 0.17 0.14 6 7.4 5 9.22 0.17 0.16 0.14 7 19.8 one 0.22 0.19 0.17 0.14 8 19.8 2 0.21 0.18 0.16 0.14 9 19.8 5 0.19 0.17 0.14 0.13 10 47.0 one 0.19 0.17 0.14 0.13 eleven 47.0 2 0.19 0.17 0.14 0.13

The degree of uranium recovery was not more than 93%.

An insufficient degree of desorption of uranium after hydrolysis and subsequent fluorination is caused by the occurrence of opposite processes — desorption of UF ₆ and reduction of uranium. Therefore, the system again contains uranium in reduced form, and the sorbent containing at least 0.13 wt.% Uranium continues to be qualified by radiation material. To achieve the required value of the uranium content in the sorbent equal to not more than 0.1 wt.%, It was necessary to carry out a repeated hydrolysis cycle at a temperature of 220 ° C and fluorination of the sorbent at 400 ° C (see Table 3).

Table 3 Uranium content after secondary steam-air treatment No pp [H ₂ O], vol.% Processing time, min Uranium content, wt.% (After fluorination for 1 hour) 0.5 one 2 four one 7.4 one 0.15 0.12 0.09 0.09 2 7.4 2 0.11 0.08 0.08 0,07 3 7.4 four 0.10 0.09 0.08 0,07 four 19.8 one 0.10 0.09 0,07 0.06 5 19.8 2 0.10 0.08 0,07 0.06 6 19.8 four 0.09 0,07 0.05 0.05

As follows from the data in Tables 1 and 2, the total degree of uranium extraction from NaF after a double cycle of “hydrolysis-fluorination” reaches 97.3%, while the uranium content in the sorbent is at the level of not more than 0.1 wt.%.

It should be noted that after carrying out the above operations, the sodium fluoride sorbent did not reduce its sorption characteristics with respect to uranium hexafluoride.

Treatment of a complex salt containing pentavalent uranium at temperatures below 100 ° C leads to a significant decrease in the rate of disproportionation reaction and the possible condensation of water in the pores of the sorbent, which can lead to an explosion during subsequent fluorination. When processing at temperatures above 250 ° C, the process of pyrohydrolysis of UF ₄ to UO ₂ will be observed and, as a result of this, the flow characteristics of the fluorinating reagent will increase.

Example 1

The uranium content in the powdered sodium fluoride, which worked for 2.5 years in the sorption-desorption cycles of uranium hexafluoride in the process of processing gas mixtures containing chlorine trifluoride, was 20.9 wt.%. All uranium was in a pentavalent state.

A portion of salt weighing 15.0 g was placed in a reactor, heated to 120 ° C and treated with water vapor at a flow rate of 1 ml / cm ² · s for three minutes. After that, the reactor was evacuated and salt was fluorinated at 380 ° C with elemental fluorine for two hours. After chemical analysis, the uranium content in the sorbent was equal to 0.29 wt.%.

The operation of hydrolysis and fluorination of the sorbent at 220 ° C and the same other conditions were repeated. The uranium content in the sorbent became equal to 0.09 wt.%.

After fluorination, the sorbent was treated with a gas containing UF ₆ . A sodium fluoride capacity of 4.4 g UF ₆ / g NaF was achieved.

Example 2

20 kg of sodium fluoride, which worked 45 sorption-desorption cycles containing 42.4 wt.% Uranium (V), was treated for 5 min with water vapor at a temperature of 230 ± 20 ° C. Then, the salt was fluorinated for one hour at a temperature of 380 ° C and a fluorine pressure of 300 ± 20 mm Hg. The mass of condensed UF ₆ at a temperature of -60 ° C was 12.5 kg (yield 99.6%). The residual uranium content in the sorbent turned out to be 0.21 wt.%. During reprocessing, the uranium content decreased to 0.11 wt.%.

Example 3

The sorbent of example 2, but without hydrolysis. The residual uranium content in the sorbent was 7.8 wt.% After fluorination with elemental fluorine at a pressure of 740 mm Hg.

Example 4

Table 4 presents the conditions and results of the extraction of the non-desorbable uranium residue from the granular sodium fluoride used at the sorption plant at the Isotope Separation Plant of Siberian Chemical Combine OJSC.

The need for a double vapor-air treatment of spent NaF is primarily due to the fact that during the hydrolysis of bound uranium pentafluoride, clogging of the pores of the sorbent with hydrolysis products that prevent the penetration of water vapor into the granules takes place.

Table 4 The results of the extraction of non-desorbed uranium residue Parameter Numerical value The diameter of the sorbent granules 2-6 mm Uranium content: U _total 2.54 wt.% U ⁵⁺ 2.52 wt.% U ⁴⁺ 1.26 wt.% U ⁶⁺ 1.28 wt.% Ratio U _total / U ⁴⁺ 2.02 Gas flow rate 1.43 nl / min Primary steam air treatment Water vapor content in the air 7.4 vol.% Processing time 2 minutes Steam Processing Temperature 180 ± 10 ° C Fluoridation Fluorination Temperature 380 ± 10 ° C Fluorine pressure 120 mmHg Fluoridation Duration 2 h U _total content after desorption 0.18 wt.% Secondary steam treatment (the conditions are the same) U _total content after desorption 0.08 wt.%

Compared with the prototype method, this technical solution has the following advantages:

- sintering of the sorbent does not occur;

- the sorbent can be used unlimited time without replacement;

- significantly reduces the consumption of energy and material resources due to the use of lower temperatures, a significant reduction in the duration of the process;

- the consumption of elemental fluorine is sharply reduced.

When introducing the invention into production, the following economic factors can be achieved:

- there is no need to build sites for unloading and regeneration of spent sorbent, uranium recovery;

- sharply reduced need for sodium fluoride;

- reduced energy and material costs for the regeneration of the sorbent;

- manual labor is excluded when replacing the sorbent.

When applying this invention in industry, it will not be necessary to develop special equipment and apparatuses.

Claims (1)

A method of extracting uranium in the form of uranium hexafluoride from a sodium fluoride sorbent by fluorination at elevated temperature, characterized in that before fluorination, the sorbent containing uranium (V) is treated with water vapor at a temperature of 100-250 ° C, and fluorination is carried out at a temperature of no higher than 400 ° FROM.