EP0408468B1 - Process for producing uranium from oxygen-containing uranium compounds by chlorination - Google Patents

Description

TECHNICAL AREA

The invention relates to a process for obtaining uranium metal in stages, from an oxidized compound, for example UO₃, U₃O₈, using a chloride route.

STATE OF THE ART

To produce uranium metal from an oxide, in general UO₃, a process is usually used which successively involves reduction to the state of UO₂ at high temperature using hydrogen or a carrier gas d hydrogen, such as NH₃ for example, followed by fluoridation using hydrofluoric acid at high temperature or in the aqueous phase, to obtain UF₄, and metallothermic reduction, for example with Mg or Ca, to obtain on the one hand l uranium in the form of ingots, on the other hand a byproduct which is a fluoride (for example Mg or Ca) which it is necessary to decontaminate before rejection.
Although commonly used this method has some drawbacks. In particular, it requires the use of hydrofluoric acid which is both a dangerous product, therefore very difficult to handle, and expensive, with a reducing agent such as Mg or Ca which are also expensive. Furthermore, these two expensive products (fluorine and reducing agent) are found "in fine" in the form of an alkaline-earth fluoride by-product requiring an expensive wet decontamination process and itself generating liquid effluents. Furthermore, this decontamination, necessary to remove and recover the uranium contained, leaves behind some traces which limit the possibilities of possible recovery of said fluoride.

Thus the Applicant has sought to develop a process which makes it possible to avoid the use of expensive and particularly dangerous products, such as hydrofluoric acid, the production of a byproduct which is also expensive to treat and to eliminate. It also sought a process which, preferably, is continuous and insensitive to the presence of impurities in the starting oxide or better operates a purification.

Description of the invention

• (1) on fait réagir un mélange, tel quel ou aggloméré, d'une poudre dudit composé oxydé et d'un excès de poudre de carbone avec du chlore gazeux à une température supérieure à 600° C pour obtenir UCl₄ gazeux qui est filtré et condensé après épuration éventuelle par distillation,
• (2) on réduit UCl₄ à température élevée inférieure à la température de fusion de l'uranium de façon à générer de l'uranium sous forme solide et un ou des sous- produits, et
• (3) on recycle ledit sous-produit dans le procédé, éventuellement après transformation pour le mettre sous forme élémentaire recyclable.

The invention is a process for the production of uranium, from one of its oxidized compounds, which does not generate liquid or solid effluent, characterized by the sequence of the following stages:

• (1) a mixture, as it is or agglomerated, of a powder of said oxidized compound and of an excess of carbon powder is reacted with chlorine gas at a temperature above 600 ° C to obtain UCl₄ gas which is filtered and condensed after possible purification by distillation,
• (2) reducing UCl₄ at a high temperature below the melting temperature of uranium so as to generate uranium in solid form and one or more by-products, and
• (3) said by-product is recycled in the process, possibly after transformation to put it in elementary recyclable form.

• soit une électrolyse ignée, de préférence en milieu de chlorures alcalins ou alcalinoterreux fondus, pour obtenir d'une part l'uranium solide, d'autre part du chlore sous forme élémentaire qui est directement recyclé à la première étape.
• soit une réduction métallothermique à l'aide d'au moins un agent réducteur métallique, tel que Mg, Ca, Na, K, donnant d'une part l'uranium à l'état solide, d'autre part le chlore sous forme de chlorure métallique, ce sous-produit étant transformé sous forme élémentaire pour être recyclé, c'est-à-dire en ses éléments constitutifs qui sont également recyclés : le chlore à la première étape et le métal en réduction. Cesdits élements constitutifs sont en général obtenus, ou séparés, par électrolyse.

This reduction is generally:

• or an igneous electrolysis, preferably in the middle of molten alkali or alkaline earth chlorides, to obtain on the one hand solid uranium, on the other hand chlorine in elemental form which is directly recycled in the first step.
• or a metallothermic reduction using at least one metallic reducing agent, such as Mg, Ca, Na, K, giving on the one hand uranium in the solid state, on the other hand chlorine in the form of metal chloride, this by-product being transformed into elementary form to be recycled, that is to say into its constituent elements which are also recycled: chlorine at the first stage and the metal in reduction. These constituent elements are generally obtained, or separated, by electrolysis.

It can be seen that this process uses only inexpensive products (C), the other reagents being recycled, and that it does not produce solid or liquid effluent. The only gaseous effluent produced is CO / CO₂ which it is easy to filter before discharge.

Such a process thus allows significant savings in manufacturing cost: neither treatment nor rejection of solid effluent, and simplified installations due to the absence of F₂ and / or HF.

According to the invention, the starting product is any oxidized compound of uranium, pure or impure, for example an oxide such as UO₂, U₃O₈, UO₃, UO₄ or a mixture thereof, usually U₃O₈ and rather UO₃ is used, or a uranate, preferably ammonium diuranate because the presence of alkali or alkaline earth is not always desirable. The starting uranium-containing compound is preferably mixed in dry and divided form (powder, scales, granules, etc.) with carbon (coke, carbon, graphite, etc.) also in divided form. This mixture is introduced as it is, or possibly after granulation or agglomeration, in a high temperature reactor where it reacts with chlorine gas, diluted or not in an inert gas such as argon, helium, nitrogen, introduced preferably against the current when operating continuously and / or so that it percolates through the load.

mais il peut se former également UCl₅ et UCl₆.
On opère à une température supérieure à environ 600° C et de préférence entre 900 et 1100°C, pour obtenir de préférence UCl₄ et limiter la formation d'UCl₅ ou UCl₆, et à pression quelconque; cependant pour des raisons pratiques, il est plus aisé d'utiliser une pression voisine de la pression atmosphérique. De la température de réaction dépend la proportion de CO et/ou CO₂ obtenue. La réaction est complète. On préfère opérer en présence d'un excès de carbone d'au moins 5% en poids pour éviter la formation d'oxychlorures, et obtenir UCl₄ sous forme gazeuse. La quantité de Cl₂ utilisée est au moins suffisante pour consommer la totalité de l'uranium, un léger excès est favorable mais doit être limité pour éviter la formation des chlorures supérieurs UCl₅ et UCl₆.With UO₃, the reaction generally produces UCl₄ and is written: $$\text{UO₃ + 3C + 2 Cl₂ → UCl₄ + 3 CO (and / or CO₂)}$$

but it can also form UCl₅ and UCl₆.
One operates at a temperature above about 600 ° C and preferably between 900 and 1100 ° C, to preferably obtain UCl₄ and limit the formation of UCl₅ or UCl₆, and at any pressure; however for practical reasons, it is easier to use a pressure close to atmospheric pressure. The proportion of CO and / or CO₂ obtained depends on the reaction temperature. The reaction is complete. It is preferred to operate in the presence of an excess of carbon of at least 5% by weight to avoid the formation of oxychlorides, and to obtain UCl₄ in gaseous form. The amount of Cl₂ used is at least sufficient to consume all of the uranium, a slight excess is favorable but must be limited to avoid the formation of the higher chlorides UCl₅ and UCl₆.

The reaction can be conducted in a wide variety of modes. One can for example operate in the middle of molten salt, such as alkali chlorides, not reacting with the reagents used. The molten salt bath is then regularly fed with the mixture of said uranium-oxidized compound and carbon, while bubbling the chlorine. Such a process is particularly advantageous when the starting uranium-containing compound is an impure concentrate, containing in particular troublesome elements such as alkali or alkaline-earth, rare earths or others. This bath containing UCl₄ can optionally be used for electrolysis, but it is preferable to recover UCl₄ in gaseous form.

We can also operate in solid phase.
The uranium-bearing compound, alone or preferably in admixture with carbon, can then be introduced directly into a reactor containing a carbon bed which ensures the excess of carbon. All kinds of reactor or furnace can be suitable, for example belt, rotary, sliding bed furnace ..., but the most interesting is the fluid bed reactor, comprising a carbon bed fluidized by chlorine and the reaction gases, which is supplied with said mixture of uranium compound and carbon preferably in the form of powder. But more generally, the supply of the different types of reactors can also be in the form of granules, tablets, balls, etc. This type of process is especially advantageous when the uranium-containing compound contains few alkaline elements and preferably a low level of impurities.

Sublimated UCl₄ obtained during the reaction is filtered at the outlet of the reactor, for example on quartz cloth or silica.
In the case where UCl₄ contains volatile impurities, one can then carry out a purification by distillation and condensation. If this purification is not necessary, UCl₄ is condensed directly, in solid (snow) or liquid form, which makes it possible to separate from Cl₂ possibly present and / or dilution and non-condensable gases such as Ar, He, N₂, CO, CO₂ .....
When UCl₄ contains higher chlorides such as UCl₅, UCl₆, it is possible to carry out a disproportionation operation consisting in demoting said higher chlorides to UCl₄. This operation consists simply in heating the mixture of chlorides either in the solid phase at a temperature of 150 to 500 ° C under reduced pressure, generally about 6 mm of mercury, or in the gas phase at a temperature at least equal to 800 ° C . Downshifting can also be done by electrolysis as will be seen below.

Then, during the second step, the reduction is carried out in order to obtain the uranium metal according to any of the variants already mentioned.

1st variant : UCl₄ electrolysis

An igneous electrolysis is carried out in the medium of molten salt preferably in a bath based on chlorides, for example alkaline and / or alkaline-earth, making it possible to recover the solid uranium at the cathode and a release of chlorine at the anode. In general, NaCl or a mixture of Na Cl + KCl is used. A bath containing exclusively fluorides, although possible, is not recommended, since it tends to stabilize the presence of oxyfluorides which are difficult to reduce without significantly increasing the oxygen content of the deposited metal.

The composition of the bath can vary within wide limits. In general, it is adjusted so that the molten bath has a low vapor pressure in UCl₄, and so that the temperature corresponds to the desired morphological structure of the uranium deposit at the cathode. Indeed, the crystalline morphology and the quality of the cathodic deposit depends to a large extent on the temperature at which it is carried out, on the chemical constitution of the bath and on its concentration of UCl₄ and / or UCl₃.
The average uranium content of the bath is very variable. In general, it is greater than approximately 2% by weight (expressed in U) to have a sufficient diffusion speed, and less than approximately 25% by weight to avoid excessive departures of UCl₄ in the vapor phase; a content of between 5 and 12% by weight is satisfactory. UCl₄ is introduced in solid, liquid or gas form.

adéquat est en général inférieur à 6, la quantité pondérale.
du fluorure alcalin introduit dans le bain étant en général comprise entre 2,5 et 5%.
La température d'électrolyse est environ de 25°C à 100°C au-dessus du point de fusion du bain choisi. En général on opère entre 650 et 850°C et de préférence entre 650 et 750°C.
La densité de courant est adaptée à la composition du bain et est en général inférieure à 0,8 A/cm2 et de préférence à 0,2 A/cm2, sinon il se forme de fines particules d'U qui peuvent tomber en fond de cuve avec les boues et présenter un danger dû à leur grande oxydabilité.
Habituellement:

• la cuve d'électrolyse est métallique et équipée d'un dispositif de chauffage, pour en faciliter l'exploitation, et d'une protection cathodique.
• l'ensemble anodique comprend au moins une anode en matière carbonée, comme le graphite, ou en métal non corrodable par le bain ou le chlore et est équipé d'un dispositif de captation de Cl₂ dégagé
• l'ensemble cathodique comprend au moins une cathode métallique, par exemple uranium ou acier, ou autre métal de façon que l'uranium déposé soit aisément détachable.

However, in order to stabilize the IV valence of uranium chloride, it is advantageous to add to the bath a limited amount of a fluoride, generally alkaline such as NaF or KF. In the absence of such an addition, the formation of UCl₃ is noted, the presence of which has an influence on the cathodic deposition.
The molar ratio $\frac{\text{F}}{\text{U}}$

adequate is generally less than 6, the amount by weight.
alkaline fluoride introduced into the bath being generally between 2.5 and 5%.
The electrolysis temperature is approximately 25 ° C to 100 ° C above the melting point of the chosen bath. In general, it is carried out between 650 and 850 ° C and preferably between 650 and 750 ° C.
The current density is adapted to the composition of the bath and is generally less than 0.8 A / cm2 and preferably less than 0.2 A / cm2, otherwise it forms fine particles of U which can fall to the bottom of tank with the sludge and present a danger due to their high oxidizability.
Habitually:

• the electrolysis tank is metallic and equipped with a heating device, to facilitate operation, and cathodic protection.
• the anode assembly comprises at least one anode made of carbonaceous material, such as graphite, or of metal which cannot be corroded by bath or chlorine and is equipped with a device for capturing released Cl₂
• the cathode assembly comprises at least one metal cathode, for example uranium or steel, or another metal so that the deposited uranium is easily detachable.

It is desirable to have a diaphragm between the anode and the cathode to prevent the recombination of the elements and facilitate the collection of chlorine. It must be sufficiently porous (10 to 60% of vacuum preferably 20 to 40%) and is made of a material resistant to temperature and to corrosion of the bath. It is preferable to use a conductive material, for example a metal, or better a graphite material which can be cathodically polarized to avoid any migration of U towards the anode and re-formation of chloride. A metal deposit can be made there tending to clog it; the metal deposited is then redissolved by depolarization. The polarization of the diaphragm leads to different concentrations in the anode (anolyte) and cathode (catholyte) compartments.

The metal deposited on the cathode must be sufficiently adherent not to fall to the bottom of the tank and be unrecoverable, but not too much either to be easily recoverable.
The crystal form of the deposit and its characteristics are a function, as has already been said, of a certain number of factors such as the nature of the bath, its composition, its concentration, its temperature, the current density, etc.

The interpolar distance between electrodes is variable and depends largely on the form in which the metal is deposited; it is advantageous to fix the electrolysis conditions so as to avoid strong growths of said metal, therefore to deposit it in a fairly collected form, while preventing it from being too compact to then facilitate harvesting. Usually the interpolar distance is between 50 and 200 mm.

Once the cathode is sufficiently charged with a deposit of uranium contaminated by bath inclusions, it is washed and it is harvested either by mechanical means, such as scraping, machining, etc., leading to a metal. in divided form which is washed with acidulated water and / or melted to eliminate the said inclusions, either by physical means such as fusion ..., leading to a purified ingot surmounted by a layer of slag coming from the bath inclusions .
The chlorine obtained at the anode is recycled in the previous step, after a possible addition of fresh Cl₂ intended to compensate for the losses.

• . à entourer à distance l'anode, plongeant dans le bain, d'un panier ajouré, également immergé, formant cathode, par exemple en treillis métallique; il peut être constitué de deux cylindres coaxiaux verticaux, délimitant un espace annulaire vertical, solidaires d'un fond
• . à disposer à l'extérieur dudit panier au moins une cathode complémentaire immergée
• . à appliquer à ladite cathode complémentaire une tension qui la rend polarisée cathodiquement par rapport au panier
• . à alimenter le bain électrolytique en introduisant lesdits chlorures ou chlorures d'uranium dans le panier, de préférence dans l'espace annulaire.

On observe alors dans le panier, formant cathode, le dépôt d'uranium brut, la réduction des chlorures supérieurs en UCl₄, tandis que se dépose sur la ou les cathodes complémentaires l'uranium raffiné.An improvement in this electrolysis is particularly interesting; it allows both to deposit uranium metal, to carry out its electrorefining, to downgrade the higher chlorides to UCl₄ and to do without diaphragm between anode and cathode.
It consists :

• . surrounding the anode, immersed in the bath, with a perforated basket, also submerged, forming a cathode, for example in a wire mesh; it may consist of two vertical coaxial cylinders, delimiting a vertical annular space, integral with a bottom
• . to have at least one submerged complementary cathode outside said basket
• . applying a voltage to said complementary cathode which makes it cathodically polarized with respect to the basket
• . feeding the electrolytic bath by introducing said chlorides or uranium chlorides into the basket, preferably in the annular space.

One then observes in the basket, forming cathode, the deposit of crude uranium, the reduction of higher chlorides in UCl₄, while the refined uranium is deposited on the complementary cathode (s).

2nd variant : metallothermic reduction of UCl₄.

M représente un métal fusible et capable de réduire UCl₄ à des températures inférieures à environ 1100°C, au besoin avec apport d'énergie extérieur. On utilise de préférence Mg, Ca, mais aussi Na, K ou un de leur mélange.
Cette étape du procédé selon l'invention, consiste à faire réagir le métal réducteur liquide contenu dans un réacteur ou creuset fermé généralement en acier ordinaire ou inox, avec UCl₄ introduit régulièrement, généralement sous forme liquide ou gazeuse, à une température et dans des conditions telles que UCl₄ réagisse à l'état gazeux avec le réducteur, que le chlorure résultant soit liquide et que l'uranium produit reste solide.Metallothermic reduction methods for obtaining uranium metal are well known, in particular the reduction of UF d'U by Mg or Ca where the reaction products go through a molten state.
Given the thermal balances, such a process is not possible in the context of the reduction of UCl₄.
It is therefore preferable to operate in the following manner using the reaction: $$\text{UCl₄ + 4M → U + 4M Cl}$$

M represents a fusible metal capable of reducing UCl₄ at temperatures below about 1100 ° C, if necessary with an external energy supply. Preferably, Mg, Ca, but also Na, K or one of their mixture is used.
This step of the process according to the invention consists in reacting the liquid reducing metal contained in a closed reactor or crucible generally made of ordinary steel or stainless steel, with UCl₄ introduced regularly, generally in liquid or gaseous form, at a temperature and under conditions such that UCl₄ reacts in the gaseous state with the reducing agent, that the resulting chloride is liquid and that the uranium produced remains solid.

This is usually carried out between about 600 ° C and 1100 ° C and preferably between about 800 ° and 1000 ° C, under a reducing or inert atmosphere (H₂, He, Ar ...), in a reactor generally made of steel which can be heated from the outside, possibly in several zones regulated at different temperatures.
First, a charge of reducing metal is introduced into the crucible in solid or liquid form, it is closed with a cover, the air is purged by evacuation and / or sweeping with a reducing or neutral gas, it is heated so as to bring the enclosure to the chosen reaction temperature, and bring or maintain the reducing metal in liquid form. UCl₄ is then introduced, for example in gaseous form, which reacts with the molten reducing agent, U collects at the bottom of the crucible and / or along the walls in more or less agglomerated solid form; the liquid chloride of the reducing metal and the unreacted liquid reducing metal float above the uranium in two successive layers classified in order of their density; usually the layer of reducing agent is above and the liquid salt in contact with uranium.
It is advantageous to regularly withdraw said liquid chloride, to increase the processing capacity of the crucible.
At the end of the reaction, therefore, a more or less compact mass of uranium is obtained which is contaminated by inclusions of reducing metal and of salt (chloride) formed. The reducing metal not consumed, therefore the excess to be expected, can reach 20% to 30% relative to the stoichiometry of the UCl₄ used.
To purify the U obtained from these inclusions, it is possible either to heat the crucible under vacuum to distil the reducing metal, or to wash the uranium-bearing mass with acidulated water, after having extracted it from the reactor and possibly crushed, to remove the inclusions from the salt formed. It is also possible to effect a fusion, a decantation and a pouring of the uranium, previously extracted from the crucible, either before or preferably after distillation of the excess reducing agent.
This fusion is carried out according to techniques known to those skilled in the art: for example induction furnace with electronic bombardment, crucible graphite coated with a refractory inert towards uranium, cold crucible ..., and the casting can be done in the form of ingot, wire, ribbon etc ... using all known methods of one skilled in the art.

The reducing metal chloride by-product is preferably subjected to electrolysis to recover the chlorine and the reducing metal, respectively recycled in the first and in the second stage, according to methods well known to those skilled in the art.
The process according to the invention therefore avoids the production of annoying by-products or effluents to be treated and eliminated, is economical and makes it possible to obtain a metal of a purity at least sufficient to be used in particular in an enrichment process isotopic by laser; If one starts from an oxidized compound of nuclear pure uranium, such as that obtained in conventional conversion processes, the quality obtained according to the invention is as follows:
C <50 ppm
O <200 ppm
Σ Fe and transition metals <250 ppm
Cl <20 ppm
expressed by weight with respect to U
the contents of the other impurities being lower than those present in the starting product.
If one starts from an impure compound, one obtains a quality identical to the previous one with regard to C, O, Cl, Fe and also with regard to the other impurities provided that the first step is carried out in a molten medium, a distillation of UCl₄ as described, and possibly electroraffinage, for example by means of the basket device.
It is obviously possible to improve the quality of the U metal obtained by carrying out purifications by any process known to those skilled in the art.
One can for example practice an electroraffinage by soluble anode with a bath of the type of those described in the first variant. If the reduction is carried out by electrolysis (first variant), it is possible to add to it a simultaneous electroraffinage by introducing into the bath at least one complementary electrode, cathodically polarized with respect to the main cathode where the crude uranium is deposited.

EXAMPLE 1

This example illustrates the implementation of the invention according to the first variant, that is to say that after having transformed UO₃ into UCl₄, the metal is then obtained by electrolysis.
- first step: obtaining UCl₄
The operation is carried out in a vertical pilot reactor made of silica glass with a diameter of 50 mm, a height of 800 mm, its outlet is equipped with a silica cloth filter, followed by a condenser by quenching on a wall cooled with water.

At the bottom of the reactor there is a foot of carbon powder of 200 cm3; the sort of nuclear nucleus pure uranium oxide is introduced, at a rate of 600 g / h, with the carbon in approximately stoichiometric quantity, in the form of a mixture of powders. The chlorine gas flow rate is 335 g / h.
The temperature in the reaction zone is 980-1000 ° C, and the pressure slightly higher by a few millimeters of mercury at atmospheric pressure, filtration is carried out at 800 ° C.

UCl₄ is obtained at the rate of 789 g / h containing less than 2.5% by weight of UCl₄ and UCl₆.
The residual gases, CO₂, CO, Cl in excess are removed.
- second step obtaining U metal by igneous electrolysis.
. We operate in a stainless steel cell with a diameter of 800 mm, with a graphite anode with a diameter of 50 mm, a diaphragm in composite fabric nickel carbon material with 30% porosity, a steel cathode, an interpolar space of 150 mm.

= 5 ± 1The bath is an equimolecular mixture NaCl-KCl; it has a height of 600 mm for a volume of approximately 300 l and a concentration of element U of 10 + 2% by weight. NaF is added in an amount such that the molar ratio $\frac{\text{F}}{\text{U}}$

= 5 ± 1

The bath temperature is 725-750 ° C and the cathode current density of 0.18 A / cm2.
After having checked the U content, the electrolysis is carried out at 200 A and UCl₄ is added continuously at the rate of 400 gU / h.

After 20 h, after having stopped the electrolysis, the cathode was extracted and mechanically recovered the deposit of U soiled with bath inclusions.

The deposit is washed with acidulated water and then with pure water, and 8 kg of a metallic uranium powder are thus recovered, of which:
7.2 kg grain size> 0.85 mm
0.8 kg grain size <0.85 mm

This latter fraction was collected and then compacted to serve as a soluble anode in an electrorefining operation.

The FARADAY cathodic yield is approximately 90%.

The quality of the particle size fraction> 0.85 mm is as follows:
C <10 ppm
WHERE 120 to 170 ppm
Fe <20 ppm
Cr <10 ppm
Ni <10 ppm
other metals <150 ppm
Cl <20 ppm

EXAMPLE 2

This example illustrates the implementation of the invention according to the second variant, that is to say that after the transformation of UO₃ into UCl₄ the latter is reduced by metallothermy.
- first step: obtaining UCl₄
It is carried out in an identical manner to that of Example 1.
- second step:
the operation is carried out in a pilot reactor made up of an AISI 304 steel tube with a diameter of 150 mm and a useful height of 250 mm, supplied by a powder UCl₄ distributor.
This reactor can be put under vacuum for the purification operation, it is placed in a thermostatically controlled enclosure.

2.265 kg of Mg are introduced therein into an ingot and the enclosure is brought to 840-860 ° C.
After melting Mg, approximately 16 kg of powdered UCl₄ are introduced regularly for 1 hour 30 minutes. The Mg Cl₂ formed is siphone at regular intervals.
After consumption of all UCl₄, the reactor is connected to a water-cooled wall condenser, is placed under vacuum (10⁻² to 10⁻³ mm of mercury) then heated to 930-950 ° C to distill and condense, by cryopumping, the excess Mg and the remaining MgCl₂ contained in the porous bread of solid U formed during the reduction. After 5 h, practically all of Mg (i.e. 225 g) and MgCl₂ (i.e. 400 g) were recovered.

After cooling the reactor, a healthy metal U-bar weighing 9.1 kg is extracted therefrom after peeling.

C

20 ppm

O

150 à 200 ppm

Fe

20 à 30 ppm

Cr

20 ppm

Ni

10 à 20 ppm

autres métaux : < 150 ppm

Cl

< 20 ppm

Mg

< 10 ppm

Its analysis carried out on several samples gave the following results:

VS

20 ppm

O

150 to 200 ppm

Fe

20 to 30 ppm

Cr

20 ppm

Or

10 to 20 ppm

other metals: <150 ppm

Cl

<20 ppm

Mg

<10 ppm

Claims (26)

1. A method of producing uranium from one of its oxide compounds without creating any liquid or solid effluent, characterised by the sequence of the following stages:

   (1) reacting a mixture, as such or agglomerated, of a powder of said oxide compound and an excess of carbon powder with chlorine gas at a temperature of above 600°C, to obtain UCl₄ gas which is filtered and condensed after possibly being purified by distillation.

   (2) reducing UCl₄ at a high temperature below the melting temperature of uranium, so as to produce uranium in solid form and one of its by products, and

   (3) recycling the by-product in the process, possibly after converting it to an elementary form in which it can be recycled.
2. A method according to Claim 1, characterised in that the oxidised compound is selected from oxides or uranates.
3. A method according to Claim 2. characterised in that the oxidised compound is UO₃.
4. A method according to any of Claims 1 to 3, characterised in that in the first stage there is at least 5% by weight of excess carbon.
5. A method according to any of Claims 1 to 4, characterised in that the UCl₄ formed in the first stage also contains high chlorides such as UCl₅ and UCl₆.
6. A method according to any of Claims 1 to 5, characterised in that the temperature is from 900 to 1100°C in the first stage.
7. A method according to any of Claims 1 to 6, characterised in that the first stage reaction takes place in a medium of molten chlorides.
8. A method according to any of Claims 1 to 6, characterised in that the first stage reaction takes place in solid phase. in a fluidised carbon bed fed with said mixture of powders and with chlorine passing through it.
9. A method according to any of Claims 1 to 8, characterised in that the second stage reduction is carried out through igneous electrolysis, to obtain solid uranium at the cathode and liberation of the chlorine at the anode.
10. A method according to Claim 9, characterised in that the electrolysis takes place in a bath of molten chloride from a KC1-NaC1 mixture.
11. A method according to Claim 9 or 10, characterised in that the U content of the bath is from 2 to 25% and preferably from 5 to 12% by weight.
12. A method according to any of Claims 9 to 11, characterised in that the molten bath contains a fluoride, in a quantity such that the molar ratio F:U is less than 6.
13. A method according to any of Claims 9 to 12, characterised in that the electrolysis temperature is about 25°C to 100°C higher than the melting temperature of the fusion bath, and generally from 650°C to 850°C.
14. A method according to any of Claims 9 to 13, characterised in that electrolysis is carried out with a diaphragm which is arranged between the anode and cathode and is preferably conductive.
15. A method according to Claim 14, characterised in that the diaphragm is of graphite containing material and is polarised.
16. A method according to any of Claims 9 to 15, characterised in that the reduction of the high chlorides and electro-refining of the uranium deposited on the cathode are carried out, by means of a said cathode, which, in this case, comprises an openwork basket surrounding the anode, and by means of at least one complementary cathode which is polarised cathodically relative to said cathode.
17. A method according to any of Claims 9 to 16, characterised in that the uranium deposited is recovered by mechanical means such as scraping or machining, or physical means such as fusion.
18. A method according to any of Claims 9 to 17, characterised in that the chlorine recovered at the anode is recycled to the first stage.
19. A method according to any of Claims 1 to 8, characterised in that the second stage reduction is carried out by metallothermy, using a metallic reducing agent to give solid uranium and a chloride of said agent.
20. A method according to Claim 19, characterised in that the reducing agent is Mg, Ca, Na or K or a mixture of one of them.
21. A method according to Claim 19 or 20, characterised in that the reaction takes place between the liquid reducing agent and UCl₄ gas, in a closed steel or stainless steel reactor, the temperature usually being from 600 to 1100°C.
22. A method according to any of Claims 19 to 21, characterised in that there is an excess of reducing agent.
23. A method according to any of Claims 19 to 22, characterised in that the solid uranium obtained is purified by distillation under vacuum to eliminate inclusions of reducing metal, then by washing to eliminate inclusions of the chloride formed.
24. A method according to any of Claims 19 to 21, characterised in that the chloride formed is electrolyted to regenerate the chlorine and reducing agent.
25. A method according to Claim 24, characterised in that the chlorine is recycled in the first stage and the reducing agent in the second.
26. A method according to any of Claims 19 to 25, characterised in that the uranium obtained is subjected to fusion, decantation and casting.