WO1997038785A1

WO1997038785A1 - Metal fluorides separation

Info

Publication number: WO1997038785A1
Application number: PCT/GB1997/001023
Authority: WO
Inventors: Mark Fields
Original assignee: British Nuclear Fuels PLC
Current assignee: Sellafield Ltd
Priority date: 1996-04-17
Filing date: 1997-04-15
Publication date: 1997-10-23
Anticipated expiration: 1998-10-17
Also published as: GB9607901D0; ZA973140B

Abstract

A method of removing molybdenum hexafluoride from a gas containing both uranium hexafluoride and molybdenum hexafluoride. The method comprises irradiating the gas with ultraviolet light and allowing the molybdenum pentafluoride which is formed to condense from the gas.

Description

METAL FLUORIDES SEPARATION

This invention relates to the separation of metal fluoπdes and specifically to the separauon of molybdenum hexafluoπde from uranium hexafluoπde. Typically, the rnixture will be present in the form of a UF₆ gas stream in which the MoF₆ is present as an unpuπty

Uranium may be enπched by converting it to UF₆ and usmg centπfuges to separate the 235 isotope. When the uranium source matenal for preparing UF₆ is obtamed from irradiated nuclear reactor fuel, vaπous metal fluoπdes are present as impuπues. The vapour pressures of several hexafluoπdes are similar to that of UF₆ and therefore fractional disullation is not a suitable techmque for separating out these impuπues. Additionally, MoF₆ may be present as an impuπty if the UF₆ has been prepared from uranium ore.

The use of visible laser radiation has been proposed for the removal of impunties from UF₆. However, MoF₆ has no absorption bands in the visible region and accordingly cannot be separated by this techmque.

It has also been proposed (Transuranium Elements - A Half Century ( Amencan Chemical Society 1992. authors J V Beitz and C W Williams, editors L R Morss and J Fuger) to use ultraviolet radiauon to separate the fluoπdes of neptunium and plutomum from UF₆. Irradiation of NpF₆, PuF₆ and UF₆ with ultraviolet light causes the hexafluoπdes to be reduced to NpF₅, UF₅ and PuF₄ which are involaQle. Because of its reactivity, UF₅ reacts further to remove a fluorine atom from either PuF₆ or NpF₆, thereby reforming UF₆. In this way, separation takes place and the bulk of the UF₆ is recoverable. If the reacuon is earned out m the presence of carbon monoxide, which reacts with fluorine atoms to remove them from the system, then high decontaminauon of UF₆ from NpF₆ and PuF₆ is achieved, but approximately 20% of UF₆ is lost. If the reaction is earned out without carbon monoxide present, and with the peπodic removal of fluoπne gas from the system, poorer decontaminauon is achieved, but only 4% UF,, is lost.

In their proposal, the authors (Beitz and Williams) states that their method is likely to be effective in the removal of other hexafluoπdes which are more reactive than UF₆. On this basis it is postulated that AmF₆, UF₆ and RhF₆ will probably be photochemicaily removal from UF₆. is postulated that AmF₆, UF₆ and RhF₆ will probably be photochemically removal from UF₆. However, MoF₆ is shown as being less reactive than UF₆ and it is clear that the authors do not expect this hexafluoride to be removed from UF₆.

It has now been discovered that MoF₆ is in fact removable from UF₆ usmg ultraviolet radiation. Accordingly the present invention provides a method of removing MoF₆ from a gas containing both UF₆ and MoF₆, the method comprising irradiating the gas with ultraviolet light and allowmg the M0F₅ which is formed to condense from said gas.

Examples showing the effect of UV light on MoF₆ and on a mixture of MoF₆ and UF₆ will now be described.

In order to carry out the photolysis, samples were co-condensed with an excess of nitrogen onto a Csl window at 12°K to provide a satisfactory matrix. Nitrogen was used as the matrix gas. A protective layer of nitrogen was laid onto the window prior to deposition of the samples. MoF₆ and UF₆ were held in passivated nickel cans cooled to about approximately 150°K (methylcyclohexane slush) and 140°K (methy lcyciopentane slush) respectively, to achieve a suitably slow and controlled deposition rate onto the Csl sample window. Sample depositions of two to three hours gave the best results. Samples of MoF₆ -¹- UF₆ were prepared by condensing both substances onto the window at the same time, in an attempt to get an intimate rnixture of the two gases rather than layers of the condensed gases.

Photolysis was carried out through the quartz window of the vacuum shroud using a mercury lamp to provide UV radiation, the wavelength being 254nm. Each of the deposited samples was examined by infrared spectroscopy before and after irradiation with UV light. .All infrared spectra were recorded with 2cm^' resoluuon and 128 scans or 0.5cm^' resoluuon and 256 scans. The results obtained at the cryogenic temperatures used here can be directly translated to gaseous samples. The matrix-isolauon technique has been used for ease of handling the samples. In fact, the process is expected to be more efficient in the gas phase due to the much greater mobility of the species. and 621cm^"1 which are attributed to MoF₆ and UF₆ respectively. In addition a band was observed for MoF₆ at 264cm^"1.

After photolysis, the infrared spectrum of MoF₆ shows three new bands at 721cm^"1, 707cm^*1 and 678cm^"1, all of which grew in intensity with increased irradiation. The band at 737cm^"1 showed a marked decrease in intensity as irradiation of the sample proceeded. In total. the sample of MoF₆ was irradiated for 225 minutes and other new bands were not observed.

As a result of irradiation of the rnixture of MoF₆ and UF₆, new infrared bands were observed at 674cm^{" 1} , 580cm^"1 and 560cm^" . Other bands observed during photolysis of pure MoF₆ were only present as indistinct shoulders. The intensity of the 737cm^" band of MoF₆ and the 621cm^'1 band of UF₆ dropped markedly during photolysis.

The band at 674cm^"1 is attributed to the formation of MoF₅ in the matrix. The bands at 580cm^"1 and 560cm^"1 are attributed to UF₅. It is apparent that both MoF₆ and UF₆ were reduced to MoF₅ and UF₅.

On warming the matrix to 26°K (1 minute) in order to anneal it, and recooling it to 12°K. sigmficant changes in the infrared spectrum were observed. The bands at 560 and 580cm^"1 completely disappeared whilst the peak at 678cm^"' decreased in intensity. The areas of the bands at 737cm^'1 and 621cm^"1 increased. At 12°K the MoF₆ and UF₆ molecules and their photolysis products are held rigidly by the nitrogen molecules which make up the matrix. On annealing, the structure becomes considerably less rigid, allowing the fluorine radicals, produced by photolysis, to move through the matrix then recombine with MoF₅ and UF₅ molecules. Continued irradiation of the sample further reduced the intensity (and area) of both the MoF₆ band at 737cm^{" 1} and the UF_ό band at 621cm^" . However, die bands attributed to UF₅ formation were not apparent in the spectrum, even with prolonged irradiation.

Comparison of the band areas of the UF₆ band (621cm^'1) and the MoF₆ band (737cm^{" 1}), before photolysis and after annealing of the matrix, shows tiiat the band area of the UF₆ band returned to 84% of its original area, whilst that of MoF₆ only returned to 50% of its original area after annealing. Thus, it can be concluded that UF₅ is a more reactive species than MoF₅, smce it more readily reforms the hexafiuoπde than MoF₅.

These results show that the important factor in the separation of MoF₀ from UF₆ is not the relative reactivities of the hexafluoπdes. Rather it is the relative ease with which the pentafluoπdes produced by photolysis react to reform the hexafluoπdes.

In a preferred process according to the present invenuon, a fluorme radical scavenger is present. Examples of such scavengers are xenon or carbon monoxide. With a scavenger present, the photolysis reaction occurs much more rapidly because fluoπne radicals are effectively removed from the system by the scavenger and regeneration of UF₆ then has to occur by removal of a fluoπne radical from MoF₆ by UF₅.

An example of a method in accordance with the present mvention will now be descnbed. by way of example only, and with reference to the accompanying drawings, in which: -

Figure 1 is a schematic diagram of apparatus for carrying out the method of the present mvention; and

Figure 2 is a schematic diagram of the reaction vessel of the apparatus m Figure 1

Referring to Figure 1 of the accompanying drawings, the reacuon vessel 1 is fed with a mixture of UF₆ and MoF₆ from storage vessel 3 and with fluorme from vessel 5 The pipework between vessels 3 and 5 and vessel 1 is equipped with suitable valves 7 to allow passage of either gas mto the reaction vessel 1.

Reaction vessel 1 is shown in more detail m Figure 2. It is made from nickei or Monel and is passivated to prevent reaction of UF₆ with the vessel. A senes of ultraviolet sources are held in the walls of the vessei at suitable positions to enable them to irradiate the whole internal volume of the vessei. These sources may be high, medium or low pressure mercury lamps, xenon lamps or any other convenient sources of UV radiation mcluding lasers operated in the UV range. They should provide sufficient energy to photodissociate MoF₆ and UF₆. Withm vessel 1 , the UF₆ and MoF₆ mixture is irradiated with UV radianon to remove MoF₆ from the vapour phase by reduction to invoiatile MoF₅. Additionally, UF₆ is reduced to UF₅ but. smce UF₅ is more reacnve than MoF₅, UF₆ is regenerated preferentially by the reaction of UF₅ with other molecules of MoF₆ and/or free fluoπne atoms.

On completion of the punfication. UF₆ is allowed to flow from the reacuon vessel through filter 11 (preventing passage of pamculate matter) mto cryogenic trap 13. Trap 13 condenses UF₆, whilst any fluorme gas produced duπng the process passes through the system to a fluorme store at 15 for recycling. The mvolatile MoF₅ remains in the reaction vessei. The UF₆ may then be revoiarilised and passed into a vessel 17 for storage.

Fluoπne (or an aitemauve fluoπnatmg agent) is then introduced into reaction vessel 1 to react with MoF₅ and regenerate MoF₆ Depending on the fluoπnating agent which is used, vessel 1 may be heated or irradiated with UV radiation to speed up the reaction to reform the MoF₆.

On compleuon of the reaction, the MoF₆ is passed through the cryogenic trap to remove any fluoπne (for recycling) and is then passed to storage vessei 19 for disposal.

In an alternative embodiment, a senes of reaction vessels 1 are operated sequentially so as further to puπfy the UF₆.

Claims

1. A method of removing MoF₆ from a gas contaimng both UF₆ and MoF₆, the method compnsmg irradiatmg the gas with ultraviolet light and allowing the MoF₅ which is formed to condense from such gas.

2. A method according to Claim 1 in which the MoF₅ is reacted with a fluoπnating agent to regenerate MoF₆.

3. A method according to Claim 2 wherem the fluoπnating agent is fluoπne.

4. A method according to any of the preceding claims wherem the irradiation with UV light is earned out in the presence of a fluoπne radical scavenger.

5. A method according to Claim 4 wherem the fluoπne radical scavenger is xenon or carbon monoxide.

6. A method according to Claim 1 and substanually descnbed herem.

7 A method of removing MoF₆ from a gas containmg both UF₆ and MoF₆ substanually as descnbed herewith with reference to the accompanying drawings.