DE1134975B - Process for the production of uranium tetrafluoride from oxygen-containing uranium compounds - Google Patents

Description

Process for the production of uranium tetrafluoride from oxygen-containing uranium compounds The invention relates to a process for the production of uranium tetrafluoride from oxygen-containing uranium compounds, in which the uranium compounds are initially mixed with at least one fluoride of a basic nitrogen compound in an intimate mixture at temperatures between 100 and 200 ° C , preferably between 130 and 1150'C, and the reaction products are then converted into uranium tetrafluoride in a second process stage at temperatures of more than 300'C.

It is known from Swiss patent 292 792 to react oxides of tetravalent uranium with ammonium fluoride or bifluoride at temperatures of up to 150 ° C. and then to decompose the reaction product in a second process stage at temperatures of not more than 400 ° C. , whereby uranium tetrafluoride is obtained.

One of the advantages of this method is that you can use a relatively can work at low temperatures, which are definitely lower than those Temperatures necessary for fluorination with gaseous hydrogen fluoride are. The problems of corrosion are thereby considerably simplified and the costs for the Vorrichtuncen for carrying out the procedure reduced. Furthermore is this procedure is also relatively easy to carry out; the solid reactants are mixed and subjects it to a thermal process carried out in two successive stages Treatment, while the gases evolved in the reaction by a light Purge gas flow are removed.

However, if this known process is based on uranium compounds in which the uranium has a valency higher than 4, for example oxides or oxygen-containing salts of hexavalent uranium, then a reduction of the higher valued uranium compounds is first necessary in order to benefit from the advantages of the process, which are described in can be seen essentially in the low reaction temperatures, can be exploited. Such a previous reduction of the starting material, which usually requires the use of higher temperatures and leads to a reduction in the size of the reacting surface, makes the subsequent fluorination of the reduction product more difficult. In addition, the conversion to uranium tetrafluoride then requires two different processes, which cannot always be carried out in the same device.

It is known that ammonium fluorides are oxides and other metal salts can reduce and fluorinate at the same time. Still, it was found that under the above process conditions the ammonium fluoride and bifluoride none, have a reducing effect on the compounds of hexavalent uranium. Such a one Effect only takes effect at increased temperature; it is evidently due to the ammonia caused by the dissociation of the ammonium fluoride. The reaction plays then act as if you were a mixture of ammonia and hydrogen fluoride can act on the uranium compounds; the benefits of the procedure disappear.

From British Patent Specification 704,601 it is known loading to produce uranium tetrafluoride by reaction between an oxide of hexavalent uranium and aqueous hydrofluoric acid in the presence of a reducing agent; SnCl is preferably used as the reducing agent.

In this process, the uranium is reduced and fluorinated at the same time in an aqueous medium and by two different reactants. The method also has a number of disadvantages which make its application uneconomical. The yield based on the fluorine ions used is low; the filtration of the UF4 - H, 0 precipitated in the reaction makes difficulties, and very careful drying and dehydration of the UF4 - H, 0 must be carried out so that no hydrolysis of the UF4 in U02 and 4HF occurs.

It has now been found that the indisputable advantages of the first-mentioned method using ammonium fluoride or bifluoride can also be used in the reduction and fluorination of oxygen-containing uranium compounds, which obtain uranium with a valence of 6 if instead of ammonia one substantially stronger reducing substance, namely hydrazine, is used.

The use of hydrazine salts to reduce high-grade uranium is not in itself new; US Pat. No. 2,761,758 already proposes to convert higher-value uranium, which is found in very small amounts in phosphoric acid solutions, into the tetravalent state by the action of hydrazine salts before the corresponding uranium compounds are deposited. In this case, however, the hydrazine salt is only used as a pure reducing agent.

The present invention consists essentially in the fact that in a process for the production of uranium tetrafluoride from oxygen-containing uranium compounds, in which initially in a first process step the uranium compounds with at least one fluoride of a basic nitrogen compound at temperatures between 100 and 2001 C, preferably between 130 and 150'C, and the reaction products are then converted into uranium tetrafluoride in a second process stage at temperatures of more than 300 ° C, in the first process stage oxides and / or oxygen-containing salts of hexavalent uranium, such as UO., UO, uranyl fluoride, uranyl carbonate, Uranyl oxalate and ammonium diuranate, reduced and fluorinated at the same time by reacting them with hydrazine dihydrogen fluoride (N., H4 - 2H F).

When carrying out the process, the oxygen-containing uranium compound to be reduced and fluorinated is mixed in powder form with the hydrazine dihydrogen fluoride alone or optionally with the addition of ammonium bifluoride, and this mixture is heated immediately or after it has solidified by tabletting, extrusion or some similar measure, in a first step Step up to a temperature between preferably 130 and 150 ° C in order to obtain a reaction product of the formula N.H "UF., Or NH4UF, and then the treatment in a second step is continued until this product is heated to a temperature of more than 300 ° C, preferably between 300 and 500 ° C, is converted into uranium tetrafluoride for about 1 to 3 hours.

The oxygen-containing uranium compound used as the starting material can be U 03, U3 0, ammonium diuranate, uranyl fluoride, carbonate or oxalate. In order to keep away oxygen, it is expedient to have the heated reaction vessel flushed through by flowing inert gas, for example nitrogen, during the reaction.

The duration of treatment depends on the type of raw material, his crystalline state and the dimensions or the arrangement of its grain size. It It always proves to be advantageous to fine-tune the reactants before they are mixed to cut or grind. The gases that evolve become better known per se Way removed by a flowing inert gas (e.g. nitrogen). There the hydrazine decomposition products are all gaseous, uranium tetrafluoride is obtained in a state of great purity.

The amount of hydrazine to be used is variable, it is directed depending on the temperature and the reaction environment.

Just as in the case of the reaction of the uranium compounds with ammonium bifluoride the reaction does not lead directly to the tetrafluoride, but initially to a complex salt NH4UF decomposed under the same conditions. However, it has been found that it is sufficient to use about 0.7 to 1 mol of hydrazine dihydrogen fluoride, preferably 0.75 mol, based on atomic uranium , for the conversion of 1 mol of a salt of uranium with the valency 6.

If the starting substance does not contain any fluorine ions, the amount of hydrogen fluoride introduced into the reaction mixture by the hydrazine salt is insufficient to ensure complete conversion of the uranium compound into UF4. Instead of using an excess of hydrazine dihydrogen fluoride - compared to the amount required for the reduction - it is then more economical to additionally add hydrogen fluoride in the form of ammonium bifluoride.

The complex fluoride NH4 UF initially obtained has properties that make it valuable (great stability during storage, the possibility of obtaining a very reactive UF4 through decomposition); It is therefore advantageous to measure the quantities of the reactants so that this salt and not the tetra-fluoride is formed immediately. On the basis of the examples given below, it can be established that the conversion of the fluorine ions is practically 100% in all cases examined. In other words, it is sufficient to introduce 5 fluorine ions into the reaction for each uranium atom, the total number of moles of N, H4 - 2 HF and N H4 F - HF being 2.5 . The corresponding amount of ammonium is always greater than that required to form the complex NH4UF ". This salt is obtained very pure and it is not necessary to wash it to remove traces of the starting material or the uranyl fluoride formed in between the quantities of which are negligibly small.

The recovered complex fluorine salt is thermally according to the known equation NH4UF "- decomposes> UF4 + NH4F This decomposition can be in a vacuum or an inert atmosphere, such as nitrogen, are performed, the decomposition proceeds the more rapidly, the higher the temperature... is selected; above 400 '. C, however, the forming tetrafluoride very sensitive If totally keeps however oxygen and water vapor to traces of oxygen or, if appropriate, water vapor present, the temperature can without any risk over 500' to increase C addition.

All oxygen-containing compounds of hexavalent uranium are for carrying out the new Process usable. For example the following compounds may be mentioned: UO, and its hydrates, U.O., ammonium diuranate, anhydrous and water-containing uranyl fluoride, uranyl carbonates, uranyinitrate (anhydrous or containing water), uranyl oxalate, etc.

Although it is possible to use the uranium compounds with hydrazine dihydrogen fluoride to be treated in the gaseous state or with a gaseous mixture of the hydrazine or hydrogen fluoride, however, it is preferable to react with a mixture of solid substances.

In this case, the hydrazine dihydrogen fluoride to be used is preferably obtained by neutralizing a solution of hydrazine hydrate by means of a solution containing hydrofluoric acid in a stoichiometric ratio. When these solutions are concentrated (pure hydrazine hydrate and 60% acid) the heat development is quite considerable and effective cooling must be provided to avoid any decomposition. A substantial part of the salt crystallizes spontaneously. When using dilute solutions, the precipitation of the salt must be stopped by adding alcohol. After drying at a temperature of less than 100 ° C, the fluoride is completely stable. It is not very hygroscopic and only attacks glass very slowly.

Ammonium bifluoride is a commercial product (ammonium hydrogen fluoride), which is optionally purified by recrystallization.

Some examples of the implementation of the process according to the invention for the production of uranium tetrafluoride are described below. EXAMPLE 1 100 g of finely powdered anhydrous UOI were mixed very carefully with 19 g of hydrazine dihydrogen fluoride N ", H4-2HF and 35 g of finely ground ammonium bifluoride. The fluorination reaction started immediately and caused an increase in temperature. Uranyl fluoride U O # F was formed. The mixture was placed in a bowl made of nickel - more conveniently made of polytetrafluoroethylene or any metal coated with polytetrafluoroethylene - which was placed in an electrically heated tubular furnace through which a weak stream of nitrogen flowed air that had penetrated the oven while the tray was being placed was removed, heating was started and the temperature was increased in about 45 minutes to 1301 ° C. This temperature was maintained for one hour, during which the evolution of water and ammonia vapors could be observed After cooling down in d He showed a nitrogen atmosphere, the reaction mixture in the form of a - 'reen, porous and very brittle or breakable union mass. In an analysis no connection of uranium with the valence 6 could be found; the reduction was therefore complete. After decomposition in a nitrogen atmosphere at a temperature of 400 ° C. , uranium tetrafluoride was obtained. His analysis showed an average of 99.7 1 / o U F4 and 0.2 "/ o UO, F ?.

The small amount of U02F2 that is still in the process product found is due to the presence of traces of oxygen in the for the discharge of the Ammonium fluoride used nitrogen.

In order to show that the ammonium biluoride does not have a reducing effect on the hexavalent uranium under the same conditions as above, the reaction was carried out with a mixture of 100 g of UO and 55 g of ammonium bifluoride. After cooling, a yellow mass is found, which is an addition compound of U Q, F, and N H4 F. After decomposition at 4W1 C , a salt is obtained which consists entirely of uranyl fluoride U 02 F and does not contain any trace of uranium with a value of 4. So there was no reduction. Example 2 A mixture of 100 g of U 03 and 65 g of hydrazine dihydrogen fluoride is reacted under the same conditions as in Example 1. Exactly the same results as in Example 1 are obtained, and the end product is an almost pure UF4. Only the consumption of 1-lydrazine dihydrogen fluoride is significantly higher.

Claims (2)

PATENT CLAIMS: 1. Process for the production of uranium tetrafluoride from oxygen-containing uranium compounds, in which, in a first process stage, the uranium compounds are reacted with at least one fluoride of a basic nitrogen compound at temperatures between 100 and 200 ° C, preferably between 130 and 150 ° C, and the reaction products are then converted into a second process stage at temperatures of more than 300 'C are converted into uranium tetrailuoride, characterized in that in the first process stage oxides and / or oxygen-containing salts of hexavalent uranium, such as U O: p U3 0., uranyl fluoride, uranyl carbonate, uranyl oxalate and Ammonium diuranate, reduced and fluorinated at the same time by reacting it with hydrazine dihydrogen fluoride. 2. The method according to claim 1, characterized in that ammonium bifluoride is added to the powdery mixture of the uranium compound and the hydrazine dihydrogen fluoride and the reaction is preferably carried out after tabletting or extrusion of the mixture. Considered publications: Swiss Patent No. 292792; British Patent No. 704,601; U.S. Patent No. 2,761,758.