CA1096583A - Method of preparation of uranium tetrafluoride - Google Patents

Abstract

Abstract of the Disclosure In a method of preparation of uranium tetrafluoride by reaction of hydrofluoric acid with uranium oxide which is present in the form of dioxide at the time of the reaction, the temperature rise resulting from the heat generated by the reaction between the gaseous hydrofluoric acid and the uranium oxide is limited by employing at least part of the heat for evaporating and depolymerizing at least a fraction of the hydrofluoric acid which is necessary, this fraction being introduced in the liquid state within the reaction zone or in the vicinity of this latter

Description

58;3 This invention generally relates to an important improvement made in a method of preparation of uranium fluoride from the oxides of this metal which is at present ; in use on an industrial scale.
~ This method was disclosed in French patent ; No 1,107,592 filed on June 18th, 1954 and in two patents of Addition No 72,440 and No 74,259.
The objective to be accomplished in accordance with this method is the conversion to uranium tetrafluoride of higher uranium oxides such as U03 or U308 which have in turn been obtained by chemical processing of concentrates derived from uraniferous ore deposits. Vranium tetrafluoride constitutes an intermediate product which is subsequently employed in the preparation of nuclear fuels having a base of either natural uranium or enriched uranium in the form of metal, oxide or any other compound.
he method consists of two successive steps : first of all the reduction of higher oxides such as U03 or U308 to U2 and then the simultaneous reduction and fluorination of U02 in order to obtain UF4 as an end product.
The first step is carried out by means of the action produced by cracked ammonia on the higher oxide or oxides at a maximum temperature of the order of 600 to 700C.
The uranium dioxide obtained is then contacted with hydro-fluoric acid and the reaction process which takes place can be described in a general manner by the following equation :
U2 + 4 HF ~ UF4 + 2 ~2 ;SB~

The invention is illustrated, merely by way of example, in the accompanying drawings in which:
Fig. 1 is a schematic illustration of a prior art apparatus for preparing uranium tetra-fluoride, Fig. 2 is a schematic illustration of the apparatus for preparlng uranium tetrafluoride in accordance with the present invention, Fig. 3 represents a vertical cross-section of part of the apparatus of Fig. 2, and Fig. 4 represents a horizontal cross-section of the section of the apparatus illustrated in Fig. 3~
Fig. 1 is a schematic representation of an apparatus for successively carrying out the two steps hereinabove described in accordance with the technique which has now become standard practice as disclosed, for example, in the ' - 2A -~6583 second patent of Addition No 74,259 to French patent No l, 107,592.
In this apparatus, granules consisting substantially of one or a number of higher uranium oxides such as U03 or U38 are continuously introduced through the hopper 1 lnto a tube 2 of heat-resisting metal such as, for example, a steel containing 0.15 ~ C, 25 % Cr and 20 ~ Ni whilst the remainder consists of Fe + minor.additions and impurities. This tube is heated on the exterior by means of an electric furnace 3 which serves to bring the charge to a maximum temperature between 600 and 700C. Ammonia gas is injected at 4 into the base of the tube 2 and passes upwards within this latter in counterflow to the charge. The ammonia first plays a contributory role in cooling the uranium dioxide which has already formed and is then progressively cracked as its temperature rises in contact with the charge. The nascent hydrogen which is formed reduces the higher uranium oxides to U02. The overall heat balance of this first reaction step is negative, which explains the need to supply heat by means of the furnace 3.
The granules which pass through the throat ~, and are then discharged at the base of the tube 2 are therefore substantially constituted by uranium oxide U02. Said granules penetrate through the passage lO into the tube 5 of Monel (registered trademark of International Nickel Co in respect of a NiCu alloy containing 63-68 % Ni and small quantities of Fe, Mn, Si and C, the remainder consisting of Cu) in which they are contacted at an increasing temperature with the hydrofluoric acid introduced at 7 which circulates countercurrentwise in the gaseous state. A separating device in which recourse is had to injection of a neutral gas such as nitrogen through the tube 9 as described for example in the first and second patents of Addition to the French patent No 1,107,592 dispenses with the need for mixing between the reagent gases introduced or formed in the tube 2 and those which are introduced or formed in the tube 5, these latter being discharged through the orifice 11. Under these condi-tions, the combined reaction of reduction of U02 and form-ation of UF4 as represented by the equation written above is highly exothermic. However, in order to initiate this reaction, it may be necessary to heat the charge as this latter is admitted into the tube 5 when its temperature at the outlet of the tube 2 is too low. This heating process is performed by means of the furnace 6. Once the reaction has been initiated, the quantity of heat generated corresponds to 525 small calories per gram of injected gaseous HF. Rapid heating of the charge of U02 therefore takes place and is liable to result in a runaway condition of the reaction at a short distance beneath the admission zone. It has been obsérved that, when the temperature exceeded approximately 600C, this gave rise to rapid densification of the U02 granules and formation around each grain of a surface layer of UF4 having low permeability which subsequently prevented fluorination at the center of said granules.
In order to reduce these disadvantages, it has been found necessary to limit the diameter of the tube 5 to a value such that the heat generated can be removed through the tube wall in such a manner as to ensure that the thermal gradient does not attain an excessive value. This limiting diameter can scarcely be permitted to exceed about 250 mm.
Moreover, in order to remove the large quantlties of heat produced, it has also been found necessary to equip the furnace 6 which serves to initlate the reaction with cooling means such as the coil 12 through which a coolant fluid is passed after start-up of the r~action. This does not prevent very high heterogeneity of temperature in the tube cross-section since the axial zone i5 heated to a temperature in the vicinity of 600C whilst the temperatures in the peri-pheral zone can be too low for a complete reaction. In order to provide a partial remedy to these disadvantages, it has been necessary to provide a reaction zone of substantial height and also, as described in the first Addition to French patent No 1,107, 592, to provide an extraction worm 13 of greater length than would have been necessary for a simple extraction. This worm makes it possible to mix the granules delivered by the tube 5 and to complete their fluorination by countercurrentwise in~ection of gaseous HF through the tube 14 and heating by means of a furnace 15.
In spite of the improvements thus achieved, it was not possible to exceed a production rate of approximately 50 kg/hour of UF4, which would have made it necessary to construct a large number of similar units in order to satisfy rapidly rising demands.
The possibility of constructing reaction tubes of much larger diameter equipped with internal cooling devices had been contemplated by the present Applicant but it was both difficult and hazardous to circulate a coolant fluid within ducts which were exposed both to the mechanical actlon of the charge at a relatively hlgh temperature and especially to the highly corrosive action of hydrofluorlc acid . By reason of the potential danger of leakages, highly complex safet~ devices therefore had to be contemplated.
In order to solve all these problems in an extremely 3L(~C~6583 simple manner, the present Applicant conceived the unexpected idea of resortlng to the use of hydrofluoric acid itself in order to control the reaction. In the chemical lndustry, this acld is mostly employed in the form of aqueous solutions.
When it is necessary to employ said acid in the anhydrous state as in the present instance, it iS found preferable to distribute the acid in the form of gas from a storage tank in which it is present in the liquld stateO Said storage tank is heated so as to obtain the desired pressure of gaseous HF
above the liquid and throughout the distribution system~ This does not give rise to any particular difficulties since hydrofluoric acid boils at about 20~C at atmospheric pressure.
In order to achieve the object in view, the present Applicant has decided to employ hydrofluoric acid in the liquid state and not in the gaseous state, directly at the reaction locationO To this end, profitable use has been made of a very characteristic property of hydrofluoric acid, namely its polymerized state at low temperature.
In fact, when hydrofluoric acld is brought to the boil, the vapor which is fo:rmed has the formula : (HF)6 ;
this vapor progressively depolymerizes as a function of the temperature and is completely depolymerized at about 100C.
The evaporatlon of HF in the vicinity of 20C
absorbs approximately 97 small calories per gram. Its de-polymerization between 20 and 100C absorbs approximately 333 small calories per gram. It is therefore apparent that, in order to pass from the liquid state to the depolymerized gaseous state, lt is therefore necessary to supply the hydro-fluofic acld with ~30 small calories per gram.
This ~Lgure is comparable with the 525 small calories per gram of HF released by the overall fluorination reaction of U02 by means of gaseous HF. It is therefore apparent that, if the gaseous HF is replaced by liquid HF, the reaction balance will then be only very slightly exo-thermic ; lt will then be possible to control the temperature of this latter much more easily and to establish the desired temperature profile within the reaction tube in order to ensure that the reaction takes place under optimum conditions of reaction rate and efficiencyO Provided that it proves feasible in practice to carry out evaporation and depolymeriz-ation of the HF at the actual center of the reaction zone,enhanced heat transfer wlll be achie~ed and temperature gradients will be reduced ; it will then be possible to in-crease to a very substantial extent the flow cross-sections of the reaction tubes and consequently to provide units having considerably enhanced productivity compared with those constructed in accordance with the conventional technique.
One mode of execution of the invention is described ; in the following e~ample which is not given in any limiting sense, reference being made to the accompanying drawings, in which : Fig. 2 shows a furnace which has been modified for the practical application of the method according to the invention in which the portion corresponding to the first step of the reaction, that is, to the production of U02 by reduction of the higher uranium oxides has been omitted from the drawings sînce it has not been modified to any appreciable extent. Figs. 3 and 4 show respectively in elevation and in plan the details of the device which permits the application of the method according to the invention.
There is shown in the top portion of the figure the lower end of the tube 16 in which the reduction of the higher uranlum oxides such as U03 and U308 is carried out, ~9~i83 then the connecting section 17 through which pass the granules resulting from the reduction and having a composi-tion which corresponds substantially to uranium dioxide.
There are provided within said connecting section conventional means for separating the gases injected into the lower portion of the tube 16 from those issuing from the tube 18, such means being described by way of e~ample in the two Additions to French patent No 1,107,592. The reduction-fluorination tube 18 is ]oined to the flange of the connecting section in a conventional manner. Within said tube, the granules supplied through the tube 16 first pass through the connecting section and are then distributed annularly between the internal wall of the tube 18 having an external diameter of approximately 400 mm and the external wall of a concentric tube 19 having an internal diameter of approximately 150 mm. Said tube 19 is open at the lower end and closed at the top by a leak-tight cap 20 whlch is preferably conical.
The tubes are of material such as Monel, this metal being capable of affording resistance to temperatures of up to 600C and to the corroslve atmosphere constituted by liquid or gaseous HF mixed with water vapor. The tubes 18 and l9 are joined together by means of three substanti~lly radial connectlons 21 which are angularly spaced at approximately 120V and constituted by sheet elements of Monel, said elements ; being welded to the two tubes over a distance of approximately 500 mm in the vertical direction. A cylindrical reservoir 22 approximately 800 mm in length and lO0 mm in diameter is secured concentrically withln the lnterior of the tube l9, also by means of three radial connections consisting of Monel sheet elements 23 spaced at angular intervals of 120, ~ald element~ belng welded to the reservoir 22 and to the tube ~6583 19. Finally, the pipe 24 serves to supply the reservoir 22 with liquid HF obtained from an external storage. When the furnace is in operation, the reaction between the annularly distributed charge which moves downwards and the gaseous hydrofluoric acid which passes upwards in countercurrent flow produces a strong evolution of heat of-the order of 525 small caLories per gram. This results in heating of the charge to a temperature within the range of 500 to 600C. A large propor~ion of the heat thus produced is transmitted by radia-tion and conduction to the internal tube 19 and to the liquid HF reservoir 22. This heat gives rise successively to the evaporation of the HF, then to depolymerization of the vapors produced which pass downwards within the tube 19 to the lower end of this latter, then pass upwards within the space formed between the tubes 1~ and 19 and react on the downwardly moving charge of granules. At the base of the tube 18, an extraction system comprisin~ a worm 25 with heating furnace 26 and countercurrent injection of gaseous HF through the pipe 27 serves to mix the granules and complete the reaction as described in the first Addition to French patent No 1,107,592.
As shown in the foregolng, the heat balance of the reaction is only slightly positive when making use of liquid HF. In consequence, if the proport~on of HF introduced in the liquid state is too large, it becomes necessary not only to heat-insulate the tube 18 in order to limit thermal losses but also to heat the tube externally by a conventional means such as an electric furnace 28 o~ xelatively low power. It is possible to maintain the temperature of the reaction zone in a different manner. The procedure accordingly consists in varying the ratio between the quantities of HF introduced in the liquid state through the pipe 24 and the quantities _g_ ~65~33 introduced in the gaseous state through the pipe 27. This regulation can be carried out by means which are known to those versed in the art, for example by continuously measuring the temperatures within the annular space between the tubes I8 and 19 and correcting deviations from reference values by producing action on a system for controlling the distribution of the flo~ of HF between the pipes 24 and 27. It should be noted that, if the distributing unit supplies HF in the liquid state, a heating system for vaporizing and depolymeriz-ing the HF must be placed on the duct which connects said unit to the pipe 27. When the temperatures within the reaction zone become too low, it is possible in this mannar to reduce the supply of liquid HF at 24 and to increase the supply of gaseous HF at 27, with the result that the overall heat balance of the reactions which take place within the tube 18 is made highly positive ; this in turn produces an increase in the temperatures measured within the an~ular space. Conversely, if the temperatures within the reaction zone become too high, the regulating device produces action in the opposite dir~ction by increasing the flow of liquid HF at 24 and reducing the flow of gaseous HF at 27.
By means of this method of regulation, it is possible to obtain good stability of operation and a high degree of regularity in the quality of the product. The furnace thus described has a production capacity which can attain 400 kg/hour of UF4, that is, substantially eight times the capacity of a conventional furnace 250 mm in diameter.
The example described in the foregoing constitutes only one particular mode of application of the invention.
In regard to the introduction of liquid hydro-fluoric acid for the purpose of cooling the reaction zone, i83 it ~s possible without thereby departing from the scope of the lnvention to introduce said liquid acid dir~ctly in contact with the charge at one or a number of points. In this case, the internal concentric tube can be dispensed with and the hydrofluoric acid can be caused to flow onto the charge by means of suitably distributad sprinkler devices, steps being taken to ensure that the jet or jets of liquid hydrofluoric acid are preferably directed towards the axial zone, that is, the zone which is brought to the highest temperature.
It is also possible to employ an lnternal concentric tube filled with a packing such as Raschig rings over which the liquid hydro1uoric acld is caused to run.
Many alternative arrangements may also be devised without thereby departing from the general purview of the ; invention.

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Claims (2)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A method of preparation of uranium tetrafluoride by reaction of hydrofluoric acid with uranium oxide wherein uranium oxide substantially in the form of dioxide and gaseous hydrofluoric acid are continuously contacted into a reaction zone, which comprises introducing one part of the hydrofluoric acid required for the reaction, in the gaseous state, into the reaction zone, and introducing the other part of the quan-tity of hydrofluoric acid required for the reaction, in the liquid state, in the vicinity of said zone so that said liquid HF is evaporated and depolymerized by using at least part of the heat generated by the reaction in order to cool the reaction zone, the thus obtained gaseous HF being then contacted with uranium dioxide in the reaction zone ; and regulating the ratio between the quantities of hydrofluoric acid introduced in the liquid state and the hydrofluoric acid introduced in the gaseous state in order to control the temperature of the reaction zone.

2. A method according to claim 1, wherein the liquid hydrofluoric acid is introduced into a reservoir communicating with said reaction zone and adapted to be heated by radiation and / or conduction by means of the heat generated in the reaction zone,