RU2542286C1 - Method of uranium tetrafluoride purification - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of purification of uranium tetrafluoride from admixtures of volatile fluorides consists in thermal processing of uranium tetrafluoride at temperature from 350 to 520°C with gaseous hydrogen fluoride, which is obtained by thermal decomposition of alkali metal bifluoride, in mixture with dried air, taken in ratio from 1:7 to 1:2, after which gaseous mixture is directed into apparatus, in which thermal thermal decomposition of alkaline metal bifluoride is realised.

EFFECT: increase of uranium tetrafluoride quality, reduction of hydrogen fluoride consumption.

2 cl, 4 ex, 2 tbl

Description

The invention relates to a technology for producing uranium compounds and, in particular, to purifying uranium tetrafluoride from compounds of carbon, phosphorus, nitrogen and other impurities.

Upon receipt of uranium tetrafluoride in various ways, it can be contaminated above acceptable standards by any impurities, most often carbon, phosphorus or nitrogen. In addition, for some special purposes, high purity uranium tetrafluoride is necessary, in particular, on the carbon content in the processing of depleted uranium hexafluoride.

A known method of purification of uranium tetrafluoride at the stage of dehydration of crystalline hydrate of uranium tetrafluoride (Gagarinsky Yu.V., Khripin L.A. Uranium tetrafluoride. - M.: Atomizdat, 1966). Uranium tetrafluoride is calcined at a temperature not lower than 550 ° C in a stream of hydrogen fluoride, while uranium tetrafluoride is purified from certain impurities by no more than 60%, but at the same time the content of uranium oxidized to oxides increases (see below). The main disadvantage of this method is the need to use significant concentrations of hydrogen fluoride (above 50 vol.%) In the gas mixture, a high temperature of 550 ° C and above. In addition, an increased concentration of hydrogen fluoride under these conditions leads to severe corrosion of the equipment.

The closest in technological essence is the method of purification of uranium tetrafluoride (patent No. 1047185, Germany), in which uranium tetrafluoride is treated with gaseous hydrogen fluoride at a pressure above atmospheric and a temperature of 550 ° C. The disadvantage of this method is the use of expensive diluent gases (neon, helium) or explosive hydrogen, a significant process temperature, the use of sophisticated equipment and operation at elevated pressure.

We found that when calcining uranium tetrafluoride in accordance with the conditions of the prototype in the atmosphere of hydrogen fluoride obtained according to GOST 14023-78, practically no purification of uranium tetrafluoride from phosphorus is observed; carbon removal is about 55%, and nitrogen up to 96%. In addition, uranium tetrafluoride can be contaminated with hydrogen compounds, silicon and phosphorus compounds present as impurities in the process fluoride.

Hydrogen fluoride formed during thermal decomposition of sodium bifluoride at a temperature of 190-230 ° C (or lithium bifluoride at 80-100 ° C) does not contain impurities of phosphorus, carbon, nitrogen and silicon, since at these temperatures sodium bifluoride (respectively, from lithium bifluoride) only hydrogen fluoride is released. Significantly higher temperatures are required to burn impurity volatile components. For example, to isolate silicon as tetrafluoride from sodium fluoride, temperatures above 550 ° C are required.

When calcining uranium tetrafluoride in an atmosphere of hydrogen fluoride obtained by thermal decomposition of alkali metal bifluorides, the degree of purification of uranium tetrafluoride from carbon, nitrogen, phosphorus and other impurities significantly increases.

The technical result of the invention consists in improving the quality of uranium tetrafluoride, reducing the consumption of a fluorinating agent, reducing the corrosivity of the latter and eliminating the processes of trapping and utilizing spent hydrogen fluoride, as well as reducing the cost of the cleaning process.

The technical result is achieved by contacting at a temperature from 350 to 520 ° C of uranium tetrafluoride and hydrogen fluoride formed during thermal decomposition of alkali metal bifluorides - lithium, sodium, in a mixture with dried air, taken in the ratio of HF: air from 1: 7 to 1: 2 . Next, the gas mixture containing hydrogen fluoride, air and impurity fluorides, is sent to clean the impurities in an apparatus in which thermal decomposition of lithium bifluoride or sodium bifluoride is carried out.

The process of purification of uranium tetrafluoride is carried out at a temperature of from 350 to 520 ° C, depending on the type of impurity. At temperatures below 350 ° C, the cleaning of almost all impurities, with the exception of nitrogen, is significantly reduced (see Tables 1 and 2): on the other hand, the processing time is significantly increased. With increasing temperature above 520 ° C, the mass fraction of uranium air oxidized by oxygen sharply increases.

The accepted range of the ratio in the gas mixture of air and hydrogen fluoride is caused by the fact that, at a ratio of less than 7: 1, the role of oxidation processes significantly increases due to atmospheric oxygen, and an increase in the ratio of more than 2: 1 does not lead to a noticeable effect of hydrogen fluoride on the cleaning process.

At a temperature of 190-230 ° C, sodium bifluoride decomposes (respectively, at 80-100 ° C for lithium bifluoride) into sodium fluoride and hydrogen fluoride. In the thermal decomposition apparatus — a sorption column with heating — the gas mixture is cleaned of impurities after contacting with uranium tetrafluoride, and hydrogen fluoride, without being adsorbed by sodium fluoride (lithium fluoride), is again fed to the processing of tetrafluoride. Impurities removed from uranium tetrafluoride after their hydrofluorination and conversion into volatile fluorides with a stream of hydrogen fluoride are sent for absorption by a sorbent - sodium fluoride or lithium fluoride.

In case of termination of the heat treatment of uranium tetrafluoride, the thermal decomposition apparatus is cooled to room temperature. In this case, hydrogen fluoride, which is in a closed circuit, is completely absorbed by the sorbent, forming sodium bifluoride (or lithium bifluoride).

Thus, the proposed method from the prototype differs in that the purification of uranium tetrafluoride from impurities is carried out at a lower temperature equal to 350 to 520 ° C, the value of which depends on the removed impurities, with hydrogen fluoride with diluted air to a lower concentration. Hydrogen fluoride is obtained by thermal decomposition of alkali metal bifluorides - lithium or sodium - at the corresponding decomposition temperature, and after contact with uranium tetrafluoride it is again passed through an apparatus in which thermal decomposition of bifluoride occurs.

When the mixture of dried air and hydrogen fluoride is successively passed through uranium tetrafluoride and sodium bifluoride (lithium), impurities transfer from tetrafluoride to the gas phase in the form of fluorides, and then from the gas phase to the solid phase, sodium bifluoride (lithium bifluoride). The process of separation from a gas mixture of sodium bifluoride (fluoride) at 190-230 ° C or lithium bifluoride (fluoride) at 80-100 ° C and gas circulation provides a high degree of extraction of impurities from the gas mixture on sodium or lithium fluorides due to the formation of active centers on their surfaces in the places from which the HF molecules were desorbed. At active centers, effective physical sorption of impurity molecules occurs, followed by the chemical reaction of adsorbed substances. This does not happen during the separation of impurities from the gas mixture on ordinary sodium fluoride.

However, such chemically bound impurities as sodium hexafluorosilicate (decomposition temperature> 550 ° C), sodium monofluorophosphate (≥620 ° C), sodium hexafluorophosphate (600 ° C) ("Fluorine and its compounds" / Ed. J. Simons, t.1 - M .: Foreign literature, 1953).

The maximum filling of impurities in sodium fluoride (bifluoride), after which the sorbent will not work as intended, is Si - 14.9; N - 15.4; P - 21.5 wt.%.

The use of dried air and hydrogen fluoride allowed to exclude the possibility of the following reaction:

UF ₄ + H ₂ O + 1 / 2O ₂ = UO ₂ F ₂ + 2HF (reaction possible at temperatures> 400 ° C).

In the ranges of the proposed temperatures and oxygen contents, the formation of uranium oxides by the following reaction does not occur:

2UF ₄ + O ₂ = UO ₂ F ₂ + UF ₆ (the reaction is feasible at a temperature of ≥800 ° C).

A positive effect is given by the combination of all the listed essential features.

Example 1

In order to confirm the effect of hydrogen fluoride on the cleaning process, we process uranium tetrafluoride for 60 minutes at various temperatures from 300 ° C to 550 ° C with an air pressure of 740 mm Hg.

Under these conditions, only a small purification of uranium tetrafluoride from carbon (less than 10%), titanium (16.6%) is observed. Only nitrogen and chromium compounds are burned well, and the purification of uranium tetrafluoride from phosphorus does not occur at all. In addition, the content of oxidized uranium increases to 7.7% in terms of uranyl fluoride.

Example 2

Uranium tetrafluoride containing nitrogen 6.0 10 ^-3 wt.%, Calcined in an atmosphere of hydrogen fluoride obtained by thermal decomposition of lithium bifluoride at 80 ° C, and air taken in the ratio 1: 7, at a temperature of 550 ° C for 60 minutes The result is a product with a mass fraction of nitrogen of 1 10 ^-4 wt.%.

Example 3

Uranium tetrafluoride containing (mass fraction): carbon - 3.3 10 ^-2 %,

phosphorus - 4.2 10 ^-3 %, nitrogen - 4.9 10 ^-3 %, is treated with a gas mixture consisting of hydrogen fluoride and air in a ratio of 1: 2 at various temperatures and pressures equal to 760 mm Hg, for 30 min (see table 1).

Table 1 Purification of uranium tetrafluoride from impurities Impurity The degree of purification depending on temperature,% 300 ° C 350 ° C 400 ° C 460 ° C 500 ° C 520 ° C 540 ° C Carbon 38.1 48.5 51.5 57.6 78.8 85.3 87.8 Phosphorus 0.5 4.8 16.7 73.6 90.5 95.5 97.6 Nitrogen 56.8 63.3 32,2 93.9 95.9 96.7 96.9

Purification of uranium tetrafluoride from impurities is noticeably observed with respect to carbon at temperatures above 350-400 ° C, phosphorus - above 460 ° C, nitrogen - at 300 ° C and above.

Example 4

Uranium tetrafluoride is treated with hydrogen fluoride formed as a result of thermal decomposition of sodium bifluoride at a temperature of 190-230 ° C in a mixture with air (ratio 1: 3) for 60 minutes at various temperatures and the tetrafluoride is purified from impurities of carbon, nitrogen, phosphorus, titanium and chromium (see table 2).

table 2 Purification of Uranium Tetrafluoride Impurity The degree of purification depending on temperature,% 300 ° C 350 ° C 400 ° C 460 ° C 500 ° C 520 ° C 540 ° C Carbon 30.5 54.6 63.6 66.7 84.8 86.9 87.9 Phosphorus 0.7 9.5 33.3 76,2 95.2 97.3 97.8 Nitrogen 60.9 69,4 92.5 94.6 96.4 96.9 97.0 Titanium 0.2 4.2 62.9 96.8 95.8 96.0 96.2 Chromium 52,2 54.1 54.5 54.9 59.1 81.0 90.9

A noticeable purification of uranium tetrafluoride from carbon and nitrogen compounds begins already at a temperature of 350 ° C, phosphorus - at 460 ° C, titanium - above 400 ° C. The purification of tetrafluoride from chromium compounds is most fully observed at a temperature of 500 ° C and above.

The proposed method for purification of uranium tetrafluoride in comparison with known methods has the following advantages:

- the degree of purification of tetrafluoride from compounds of carbon, phosphorus and other impurities increases;

- the resulting product is not contaminated with impurities present in hydrogen fluoride (GOST 14022-78);

- depending on the removed impurities, the processing temperature of uranium tetrafluoride decreases;

- reduces the consumption of hydrogen fluoride per unit of production and provides for its multiple use by circulating the mixture in a closed volume;

- it is not required to develop special methods for collecting and processing spent hydrogen fluoride;

- does not require the use of relatively expensive inert diluent gases;

- simplified process maintenance.

The application of the proposed method will allow:

- improve the quality of uranium tetrafluoride by reducing the amount of impurities in it;

- reduce the consumption of hydrogen fluoride by reducing its concentration in the reaction gas mixture and reusable by circulation;

- reduce the cost of the cleaning process by using atmospheric air as a diluent gas;

- to reduce losses from corrosion of the apparatus by reducing the concentration of hydrogen fluoride in the gas mixture and reducing the temperature of the process.

Claims (2)

1. The method of purification of uranium tetrafluoride from impurities of volatile fluorides by treatment with gaseous hydrogen fluoride at elevated temperature, characterized in that the heat treatment of uranium tetrafluoride is carried out at a temperature of from 350 to 520 ° C. Hydrogen fluoride obtained by thermal decomposition of alkali metal bifluoride, in a mixture with dried air taken in a ratio of 1: 7 to 1: 2, after which the gas mixture is sent to an apparatus in which thermal decomposition of alkali metal bifluoride is carried out. 2. The method according to claim 1, characterized in that hydrogen fluoride is obtained by thermal decomposition of sodium bifluoride at a temperature of 190-230 ° C or lithium bifluoride at a temperature of 80-100 ° C.