OA16467A - Method for converting UO3 and/or U3O8 into hydrated UO4. - Google Patents

Abstract

The invention relates to a method for converting UO3 and/or U3O8 into hydrated UO4 having the formula UO4, nH2O, where n is 2 or 4, including the following consecutive steps: a) preparing an aqueous suspension of a UO3 powder and/or a U3O8 powder; b) adding hydrogen peroxide H2O2 to the aqueous suspension of UO3 and/or U3O8 powder, converting the UO3 and/or U3O8 into hydrated UO4, and precipitating and crystallizing the hydrated UO4 in the suspension; c) recovering the hydrated UO4 precipitate; d) optionally washing the hydrated UO4 precipitate; e) optionally repeating step d); and f) optionally drying the precipitate, wherein the addition of H2O2 to the aqueous suspension is carried out such that the suspension contains a stoichiometric excess of H2O2 with respect to the stoichiometry of the reaction from UO3: UO3 + nH2O + H2O2 UO4, nH2O+ H2O (1) or of the reaction from U3O8: UO2.67 + 1.33 H2O2 + nH2O UO4,nH2O+ H2O (2), and the pH of the suspension is maintained in steps a) and b) at a value between 2 and 4.

Description

The invention relates to a method for converting UO₃ or U₃O₈ into hydrated UO₄ (UO₄ hydrate) .

STATE OF THE PRIOR ART

The treatment of uranium ores has the purpose of extracting uranium from the ores, of purifying it and of combining it so as to obtain a product called a concentrate, or uranate or further a « yellow cake » rich in uranium, for example comprising more than 70% by weight of uranium.

Uranium ores are first of ail crushed, and then milled, and they are then subject to an operation for putting the uranium into solution by means of a base or an acid, such as for example sodium carbonate or sulfuric acid, called etching or leaching.

After purification and concentration of the solutions from the leaching of the ore, the uranium is recovered in the form of uraniferous, uraniated, generally acid, liquors, solutions, in a sulfates medium for example.

These solutions may also be in a chloride, ammonia, nitrate or carbonate medium according to the preliminary purification-concentration step.

The uranium concentrate or « yellow cake » is obtained from these uraniferous liquors, solutions, by précipitation with précipitation reagents such as soda, ? * magnesia, ammonia, ammonium uranyl tricarbonate, and hydrogen peroxide H₂O₂, by filtration and drying.

According to the précipitation reagent used, the uranium concentrate or « yellow cake » will thus be respectively based on sodium uranate, magnésium uranate, ammonium diuranate, ammonium uranyl tricarbonate, or uranium peroxide.

The thereby prepared uranium concentrate or « yellow cake » is then transformed, notably into UF₄ and then into UFe.

Uraniferous concentrâtes such as « yellow cake », the préparation of which was described above, but also of other uraniferous concentrâtes containing uraniferous uranium trioxide or uranium octa-oxide are not able to be directly converted notably into UFeIndeed, they contain too many impurities for the subséquent isotopic séparation step (also designated as enrichment) relatively to the ASTM enrichment standards on the one hand, the presence of certain compounds may be redhibitory for the fluorination method on the other hand.

Moreover, certain oxides hâve insufficient reactivity for the reduction/hydrofluorination step leading to UF₄.

In order to find a remedy to ail these problème, the concentrâtes are purified before converting them.

A purification method is thus known in which the « yellow cake » is, first of ail, dissolved in nitric acid and the solution is then sent into a counter current liquid-liquid extraction apparatus in which the uranyl nitrate of the solution is extracted by using a mixture of TBP and of kerosine.

This method is complex and uses nitrates and volatile organic compounds which hâve to be handled.

A treatment of the used solvent and of the nitrate effluents has notably to be carried out.

The method for precipitating uranium with H2O2 is known to be decontaminating towards many impurities.

However, ail the known methods for precipitating uranium with H2O2 apply preliminary dissolution of the uranium, before reprecipitation of UO<i,4H2O by addition of oxygenated water.

However, this technique has the two following major drawbacks:

the addition of an acid and therefore of the associated anions such as sulfates, chlorides, nitrates, etc. during the dissolution step. These anions are impurities which prove to be bothersome in the subséquent conversion steps, the addition of a base and therefore of the associated cations such as sodium, potassium, ammonium, cations during précipitation of hydrated UO₄ in order to maintain the pH constant. These cations are impurities also very bothersome for the formation of UF₄ (Na, K...), or else generate gaseous effluents.

Certain added impurities may partly follow the uranium, in spite of repeated washings, and are bothersome for the UF₄ conversion method.

In particular, sodium and potassium form eutectics while the sulfates release corrosive H2S.<3x/ ? » précipitation from concentrâtes with H2O2,

Among the documents which describe for purposes of purification, mention may be made of document

WQ-A1-2009/013759, which describes a method for refining « yellow cake » in order to préparé uranium of nuclear quality in which précipitation is achieved in a single step in order to simultaneously remove heavy metals, boron and other rare earth metals. In this method, one begins with dissolving the « yellow cake » in nitric acid with moderate stirring in order to produce a solution of uranyl nitrate, and hydrogen peroxide is added at a predefined température and pH in order to selectively precipitate hydrated uranium peroxide.

The method of this document includes a preliminary step for dissolving the concentrate with nitric acid with ail the drawbacks of such a step as listed above.

Document FR-A-2 438 623 relates to a method for purifying hydrated uranium(VI) peroxide wherein uranium concentrate is digested in an acid aqueous solution, notably a solution of nitric acid, in the presence of a fluoride-complexing agent so as to obtain an aqueous solution of uranium, and this aqueous solution of uranium is reacted with a peroxide in order to precipitate hydrated uranium(VI) peroxide.

Again, the method of this document includes a preliminary step for dissolving the concentrate with an acid such as nitric acid with ail the drawbacks of such a step as listed above .Qa!

Document FR-A-2 429 747 relates to a method for preparing hydrated uranium(VI) peroxide from hydrated uranium tetrafluoride, in which the hydrated uranium tetrafluoride is digested in an acid solution, notably a solution of nitric acid, in the presence of an agent for precipitating fluorides, for precipitating fluoride ions and obtaining an aqueous solution of uranium, the aqueous solution of uranium is filtered and the pH is adjusted and the aqueous uranium solution is reacted with a peroxide in order to precipitate the hydrated uranium(VI) peroxide.

There again, the method of this document includes a preliminary step for dissolving the concentrate with an acid, notably with nitric acid, with ail the drawbacks of such a step as listed above.

Therefore considering the foregoing, there exists a need for a method for converting UO3 or Ü30g into hydrated UO₄ which allows préparation of an hydrated uranium peroxide which has a low impurity content, in particular a content of impurities which is sufficiently low so that this hydrated uranium peroxide may be directly converted into UF₄ and then into UFg.

More speeifically, there exists a need for such a method which allows préparation of UO₄ hydrate which totally or for a major part meets the ΑΞΤΜ C-787 standard relating to the purity of hydrated UO4 for conversion into UFe.

This method should also allow préparation of hydrated uranium peroxide having a high spécifie surface area and great reactivity with view to its conversion into

There further exists a need for such a method which is simple, reliable, safe and which includes a limited number of steps.

There also exists a need for a method which uses non-toxic reagents, not causing any harm to the environment and of low cost.

The goal of the présent invention is to provide a method for converting UO3 or U3O8 into hydrated UO₄ which meets the whole of the needs and requirements, as listed above.

The goal of the présent invention is also to provide such a method which does not hâve the drawbacks, defects, limitations and disadvantages of the methods of the prior art, such as notably illustrated by the documents mentioned above, and which solves the problems of the methods of the prior art.

SUMMARY OF THE INVENTION

This goal, and further other ones are attained according to the invention with a method for converting UO3 and/or U₃O_B into hydrated U0₄ of formula UO^nlhO wherein n is 2 or 4, comprising the following successive steps:

a) preparing an aqueous suspension of a UO3 powder and/of a U3O8 powder;

b) adding hydrogen peroxide H2O2 to the aqueous suspension of a UO3 and/or U30e powder, converting the UO3 and/or the U₃O₀ into hydrated UO₄ and precipitating, crystallizing UO₄ hydrate in the suspension;

c) recovering the hydrated UO₄ precipitate, crystals;

d) optionally, washing the recovered hydrated

UO4 precipitate, crystals;

e) optionally, repeating step d);

f) optionally drying the precipitate, the crystals;

wherein addition of H2O2 to the aqueous suspension is carried out so that the suspension contains a stoichiometric excess of H2O2 relativeiy to the stoichiometry of the reaction from UO₃:

UO₃ + H₂O₂ + nH₂O -> UO₄,nH₂O + H₂O (1) or of the reaction from U₃O₈:

UO₂.67 + 1.33 H₂O₂ + nH₂0 -> UO₄,nH₂O + H₂O (2), and the pH of the suspension is maintained in steps a) and b) at a value comprised between 2 and 3 (2 and 3 inclusive).

Advantageously, the pH of the suspension is adjusted during step a) to a value comprised between 2 and 3 by adding an acid to the suspension.

Advantageously, said acid is selected from oxalic acid, sulfuric acid and mixtures thereof.

Advantageously, the stoichiometric excess of H₂O₂ is from more than 1 to 10, preferably from 1.5 to 3, relativeiy to the stoichiometry of the reaction (1), and from more than 1.33 to 10 relativeiy to the stoichiometry of the reaction (2).

Advantageously, the hydrogen peroxide is added in the form of an aqueous solution at a concentration from 30% to 70% by weight.

Advantageously, the aqueous suspension of U0₃ and/or U₃O₈ has a uranium concentration from 10 to<Jv/

J I

500 g/L (gU/L), preferably from 100 to 200 g/L for UO₃, and from 10 to 500 g/L, preferably from 100 to 200 g/L for example 250 g/L for U₃C>8.

Advantageously, steps a) and b) may be carried out with stirring.

Advantageously, during step a) and/or step b) , complexing anions are added to the suspension.

Advantageously, said complexing anions are selected from sulfate anions, oxalate anions and mixtures thereof.

Advantageously, the duration of step b) is selected so that the conversion of UO₃ and/or of U₃Oe into hydrated UO₄ is total or substantially total, for example of more than 99%, or even 99.9%.

In an embodiment, step b) may comprise the following successive steps bl) and b2):

bl) adding hydrogen peroxide H₂O₂ to the aqueous suspension of a UO₃ and/or U₃Oe powder, preferably with stirring, and then stopping the addition;

b2) ripening the suspension, preferably with stirring.

Advantageously, the duration of said step bl) may be from 1 to 8 hours, preferably from 1 to 3 hours, and the duration of step b2) may be from 1 to 24 hours, preferably from 1 to 3 hours.

In another embodiment, the addition of hydrogen

peroxide H2O2 is	achieved	during	the whole	duration	of
step b), i	.e. step b2) is	omitted.
In	this	embodiment, the	duration of	step b)	is
generally	from	1 to 8	hours,	preferably	from 1	to

hours.

U *

Advantageously, during steps a) and/or b), the suspension is subject to the action of ultrasonîc waves.

Notably in this case, the water of the suspension may be removed by évaporation, and the precipitate, the crystals of hydrated UO4, are then recovered as a dry solid, for example with humidity less than 7% by mass, generally consisting of υο₄,2Η₂Ο, or else during step c), the precipitate, the crystals of UO4 hydrate are separated from the suspension by a solid/liquid séparation operation, for example a filtration or centrifugation operation, in the form of a humid solid, for example with a humidity from 30 to 80% by mass, generally consisting of UO4,4H₂O.

On the other hand, the évaporation will not generally allow removal of the impurities.

Advantageously, said humid solid is washed at least once with a washing liquid.

Advantageously, said washing liquid is selected from among demineralized water; acidified aqueous solutions preferably at a pH from 2 to 3, for example with sulfuric acid; solutions containing an agent complexing the impurities contained in the humid solid.

Advantageously, the washing ratio defined by the ratio of the mass of the washing liquid to the mass of the humid solid is from 1 to 30, preferably from 1 to 10.

Advantageously, the oxide UO3 and/or the oxide U₃O₈ appear in the form of a uraniferous concentrate called a « yellow cake », or the oxide UO3 and/or the oxide U3O8 stem from the drying, and then from the calcination of a uranium concentrate based for example on UO4 hydrate, ammonium diuranate, or uranium tricarbonate obtained by précipitation in a reactor, notably in a fluidized bed reactor, from an uraniferous 5 solution.

The method according to the invention may be defined as a method for direct conversion without preliminary dissolution of U3O8 and/or of UO3 by addition of H2O2 to an aqueous suspension of a U3O8 powder and/or of a UO3 powder.

The method according to the invention includes a sequence of spécifie steps which has never been described in the prior art.

The method according to the invention is fundamentally distinguished from the method of the prior art in that no preliminary dissolution of U₃O₈ and/or UO3 is carried out before achieving their conversion by adding hydrogen peroxide.

The conversion of U3O0 and/or U0₃ into hydrated UO₄ (UO4 hydrate) is thus, in the method according to the invention, achieved in a dispersion and not in a solution.

The method according to the invention, which does not include any preliminary step for dissolving the uranium, does not hâve ail the drawbacks due to this preliminary dissolution step. In particular, the method according to the invention thus avoids the formation of many impurities which may prove to be extremely bothersome in the subséquent steps for conversion of UO4 hydrate, for example into UF^./^J

J »

Ail the comparable methods of the prior art include such a dissolution step, and there does not exist any indication in the prior art which would hâve lead the man skilled in the art to suppressing this dissolution step.

The method according to the invention is further defined by the fact that the suspension contains a stoichiometric excess of H2O2 relatively to the reactions (1) and (2), which gives the possibility of obtaining total or quasi-total conversion.

The method according to the invention is further characterized in that the pH of the suspension

is maintained in steps a)	and	b) at a spécifie	value
comprised between 2 and 3.
The sélection of	this	very narrow PH	range
gives the possibility	of	avoiding risks	of

redissolution of UO₄ hydrate at too acid pH's, generally less than 2, on the one hand, and of avoiding the risks of formation of compounds other than Ü0₄ hydrate at more basic pHs, generally greater than 3, as well as the précipitation of impurities following the uranium, on the other hand.

The method according to the invention does not hâve the drawbacks of the method of the prior art and provides a solution to the problème of the prior art.

Thus, the method according to the invention allows préparation of uranium peroxide or of uranium peroxide hydrate which has a low content of impurities, in particular a sufficiently low impurity content so that this uranium peroxide or this uranium peroxideoJ

J ί hydrate may be directly converted into UF₄ and then into

UF₆.

The method according to the invention notably allows préparation of UO₄ hydrate which totally or for a major part meets the ASTM C-787 standard relating to the purity of UO₄ hydrate for conversion into UF₆.

The method according to the invention further allows préparation of a uranium peroxide which has high reactivity for rapid conversion into UF₄.

Indeed, the method according to the invention gives the possibility of obtaining a UO₄ hydrate having a high spécifie surface area, which may range up to 30 m²/g.

The invention will now be described in a detailed way in the detailed description which follows notably in connection with preferred embodiments. This description is given as an illustration and not as a limitation, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

-	Fig. 1 illustrâtes	the	DRX	spectrum of	the
purified	uranium powder in	the	form	of UO4.2H2O	as
obtained	in Example 1.
-	Fig. 2 illustrâtes	the	DRX	spectrum of	the
purified	uranium powder in	the	form	of UO4.2H2O	as
obtained	in Example 2.
-	Fig. 3 illustrâtes	the	DRX	spectrum of	the
purified	uranium powder in	the	form	of UO4.2H2O	as
obtained	in Example 4.
-	Fig. 4 is a photograph	taken with	a
scanning	électron microscope (SEM)	of nanometric <2*/

needles of (JO4.2H2O with a size of 200 nm, as obtained in Example 1.

The scale indicated in Fig. 4 represents 200 nm.

Fig. 5 is a photograph taken with a scanning électron microscope of needles of UO4.2H2O with a size from 1 pm to 2 pm, as obtained in Example 2.

The scale indicated in Fig. 5 represents 200 nm.

Fig. 6 is a photograph taken with a scanning électron microscope of agglomérâtes of UO4.2H2O with a size from 100 nm to 200 nm, as obtained in Example 6.

The scale indicated in Fig. 6 represents 200 nm.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the first step of the method according to the invention, an aqueous suspension of a uranium trioxide UO3 powder and/or of a uranium octa-oxide U₃Og powder is prepared.

The method according to the invention may be applied with ail kinds of uranium trioxide UO₃ and/or

uranium octa-oxide U3O0, regardless of their origin and
of the shape in which	they appear.
These oxides	may for example appear	as
concentrâtes called «	yellow cake ».
These oxides	may also stem from drying,	and

then from calcination of a uranium concentrate based for example on U0₄ hydrate, ammonium diuranate, or uranium tricarbonate by précipitation in a reactor, notably in a fluidized bed reactor, from an uraniferous solution.

A method for preparing a UO₃ powder powder by drying and then by calcination of or a U₃O_a a uranium concentrate based on hydrated UO_fl, on ammonium diuranate or on uranium tricarbonate obtained beforehand by précipitation in a fluidized bed, is described in document WO-A1-2010/051855 to the description of which reference may be made.

The

UO₃ or

U₃O₈ powders obtained in this document because of the préparation of the uranium concentrate in a fluidized bed, hâve particularly advantageous properties.

The oxides in the form of concentrâtes called « yellow cake » or the oxides stemming from drying and calcination of a uranium concentrate obtained preferably by précipitation in a fluidized bed generally appear as powders and may be directly used in the method according to the invention, and be suspended in water.

However, it may be advantageous to carry out preliminary milling of the oxide powders in order to obtain a particularly fine grain size, for example of the order of one micrometer.

The suspended powders generally contain impurities and the method according to the invention notably has the purpose of reducing the content of these impurities in the obtained hydrated uranium peroxide (uranium peroxide hydrate).

Preferably, with the method according to the invention, it is sought to obtain an hydrated uranium oj t

peroxide for which the impurity contents are compatible with its transformation into UFg and for which the impurity contents meet the ASTM C-787 standard.

The UjOe powder may contain one or more of the following impurities, for example in the following contents, expressed in ppm/U:

As:102

Ca:1383

Si:2312

Zr:316

SO₄ :29205

Mo:1109

Na:20

The UO₃ powder may contain one or more of the following impurities, for example in the following contents expressed in ppm/U:

Na:404

Ca:407

Mo:9

V:5

W:2

Cr:30

The reactor used for applying the method according to the invention and notably for carrying out steps a) and b) is generally a perfectly stirred reactor generally provided with a propeller stirrer, for example a three-bladed propeller.

The reactor may further be provided with counter-blades or baffles.£7

The volume of the reactor may be easily selected by the man skilled in the art according to the suspension volume which is desirably prepared.

The reactor may further be provided with sensors and devices for measuring the values of parameters such as the pH and the température of the suspension.

The suspension is generally prepared by introducing a known amount of powder of oxide(s) into the reactor.

The intended amount of demineralized water is then added to this known amount of oxide, in order to obtain a suspension having the desired concentration.

It is quite obvious that it is also possible to begin by introducing the demineralized water into the reactor, and then adding the oxide powder to the demineralized water.

	The concentration of oxide(s)	of the suspension
is	generally from	10	to 500 gU/L, preferably	from 100
to	200 gU/L	•
	The	pH of	the	demineralized	water is	adjusted
to	a value	from 2	to	3 by adding an	acid or	a mixture
of	acids.

This(these) acid(s) may be any minerai or organic acid.

As this will be seen later on, an acid for which the anion further has a complexing action which improves kinetics of the reaction, is preferred.

The preferred acids are sulfuric acid, oxalic acid and mixtures thereof.(fj

Other acids may moreover be used for adjusting the pH but sulfuric acid has the advantage of not introducing bothersome éléments towards the nuclear purity of UFe since no ASTM spécifications dealing with sulfur exist.

Moreover, the conversion rate into hydrated UO₄ is limited by the formation of a reaction intermediate (uranyl ion UO₂ ²⁺) but may be accelerated by using at least one complexing anion like the sulfate anion or the oxalate anion or further the citrate anion, and/or, as this will be seen later on, by applying ultrasonic waves.

It is therefore possible to add a compound providing, this complexing anion during step a) and/or step b) of the method according to the invention. In the case of sulfur, the optimum ratio S/U is 0.125.

Sulfuric acid will preferably be used as a compound providing the complexing anion for suspending the uranium oxide in order to obtain fast conversion kinetics.

The powder and the demineralized water having been introduced into the reactor, stirring is started in order to suspend the powder(s) in the demineralized water.

The stirring speed is adjusted so as to allow effective suspension of the powder.

Stirring is continued during the whole duration of the conversion into hydrated UO₄ in order to complété crystallization of the initial uranium.

It is then possible to begin with adding oxygenated water into the suspension .CJ

The addition of oxygenated water may be accomplished by means of any adéquate device giving the possibility of controlling the flow rate of oxygenated water introduced into the reactor.

The addition of oxygenated water is also preferably achieved with stirring.

Hydrogen peroxide is generally added in the form of an aqueous solution at a concentration of 30% to 70% by weight.

The total amount of added oxygenated water is, according to the invention, such that the stoichiometric excess of H₂O₂ relatively to the initial uranium is from more than 1 to 10, preferably from 1.5 to 3, relatively to the stoichiometry of the following reaction (1), and from more than 1.33 to 10 relatively to the stoichiometry of the following reaction (2):

UO₃ + H₂O₂ + nH₂O -> UO₄,nH₂O + H₂O (1) UO₂.67 + 1.33 H₂O_z + nH₂O -> UO₄,nH₂O + H₂O (2)

The reaction between the oxides and the oxygenated water is exothermic and an increase in the température of the bath for example by about 10°C is observed.

In an embodiment, the following successive steps bl) and b2) are carried out:

bl) adding, as described above, hydrogen peroxide H₂O₂ to the aqueous suspension of a UO3 and/or U3O0 powder, preferably with stirring, and then stopping the addition;

b2) ripening the suspension, preferably with stirring.

During step bl) it may be estimated that some conversion into hydrated uranium peroxide occurs, but this conversion is not total.

Step bl) may be described as a nucléation, crystallization, formation of uranium peroxide hydrate crystallites, step.

During step b2), the conversion is continued until the conversion of UO₃ and/or U₃O₈ into UO₄ hydrate is total or substantially total, for example of more than 90% or even 99.9%.

Step b2) may be described as a step for ripening, growing the crystallites obtained during step bl) .

The duration of said step bl) may be from 1 to 8 hours, preferably from 1 to 3 hours, and the duration of the step b2) may be from 1 to 24 hours, preferably from 1 to 3 hours.

The total duration of steps bl) and b2) is such that the conversion into hydrated uranium peroxide is total or substantially total.

In another embodiment, the ripening step is not carried out at the end of step bl) and step b2) is omitted.

It should be noted that during the reaction of oxygenated water with the oxides, the pH varies but remains globally stable, constant, at the value to which it had been adjusted before adding the oxygenated water by adding an acid, which means that it is generally not necessary to add further acid during step b) in order to control the pH to the intended value.

In fact, it may be estimated that some régulation of the pH is induced by the addition of H₂O₂ to UO3 and to UaOa·

At the end of the reaction, the conversion being total or substantially total, the pH is generally stabilized at a value for example from 1.6 to 2.

At the end of step b) , the conversion into hydrated uranium peroxide being total or substantially total, the precipitate, the crystals of UO4 hydrate, generally as UO₄ tetrahydrate, UO_4/4H₂O, or optionally as UO4 dihydrate, UO4,2H₂O, are recovered, notably in the case when the suspension was subject to the action of ultrasonic waves.

In a first alternative of this recovery step c) it is possible to recover, collect, the UO₄ hydrate precipitate, crystals, by removing the water from the suspension by evaporating the latter, and the UO₄ hydrate precipitate, crystals, are then recovered as a dry solid, generally with an humidity of less than 7% by mass, generally directly in the reactor without it being necessary to apply a liquid/solid séparation operation such as a filtration. The recovered UO4 hydrate crystals in this alternative are generally UO₄.2H₂O crystals.

This first alternative is in particular applied, as described in detail below, in the case when the suspension is subject to the action of ultrasonic waves.

In a second alternative of this recovery step c), it is possible to recover, collect, the precipitate, the hydrated UO₄ crystals, by sépara ting<2-J < V them from the suspension with a liquid-solid séparation operation, in the form of a humid solid, for example with a humidity from 30% to 80% by mass, also called a cake.

The recovered U0₄ hydrate crystals in this second alternative are generally crystals of UO4,4H₂O.

This liquid-solid séparation operation may be an operation for filtering the suspension.

This filtration operation may be achieved in vacuo or by action of a centrifugal force.

The collected humid solid may then be washed with a washing liquid.

Said washing liquid may be demineralized water or an aqueous solution, acidified, preferably at a pH from 2 to 3, for example with sulfuric acid.

An aqueous solution preferably with a pH from 2 to 3, of a complexing anion towards the impurities contained in the humid solid such as those already mentioned above, may also be used as a washing liquid.

Sulfuric acid has the advantage of playing both an acidifying and complexing rôle by means of its sulfate anions.

The washing operation may be repeated from 1 to 10 times depending on the desired impurity content of the uranium peroxide.

Advantageously, the washing ratio defined by the ratio of the mass of the washing liquid (on the totality of the washings) to the mass of the humid solid is from 1 to 30, preferably from 1 to 10, in order to limit the volumes of water requîred for washing.&J

The suspension may further be subject to the action of ultrasonic waves.

The applied ultrasonic waves may hâve a single frequency, but a combination of ultrasonic waves with different frequencies may be used, for example a combination of high frequency ultrasonic waves, with a frequency for example of 2.4 MHz, and of low frequency ultrasonic waves with a frequency for example of 35 kHz.

For example, it is possible to place the reactor containing the suspension into an ultrasonic tank or else position one or several ultrasonic probes in the reactor.

Generally the suspension is subject to the action of ultrasonic waves while adding oxygenated water to the suspension. But it is also possible to apply ultrasonic waves during the step for preparing the solution.

The conversion is then much faster than in the case when the reaction is carried out without subjecting the suspension to the action of ultrasonic waves, and the duration of step b) is then only from 1 to 2 hours, instead of for example 24 hours, in order to obtain total or substantially total conversion.

The action of the ultrasonic waves should not be mistaken for mechanical stirring.

Generally, when ultrasonic waves are used, the suspension is not stirred in another way.

It should be noted that the action of ultrasonic waves causes an increase in the température of suspension, which causes overconsumption of H2O2.ÎJV

When ultrasonic waves are used, it may therefore be necessary to cool the suspension in order to avoid dégradation of the oxygenated water which generally occurs at a température above 50°C.

Because of the heating of the suspension caused by the ultrasonic waves, the water of the suspension may be removed by évaporation, and the precipitate (the hydrated UO₄ crystals) is then recovered directly in the reactor as a quasi-dry solid, for example with a humidity of less than 7% by mass and without any washing.

The use of ultrasonic waves therefore gives the possibility of significantly accelerating conversion kinetics and notably reduces the amount of water in the obtained hydrated uranium peroxide. Consequently, the hydrated uranium peroxide may be recovered without it being necessary to pass through a liquid-solid séparation step, such as a step for filtering the suspension.

As this séparation step is suppressed, the method is therefore simplified and shortened.

The method according to the invention may optionally comprise a step for drying the recovered UO₄ hydrate crystals.

This drying step is generally carried out at a température from 60°C to 100°C for a period of 1 to 24 hours.

During this step, the recovered hydrated uranium peroxide is transformed into UO₄,2H₂O if this is UO₄,4H₂O.<3J

The obtained hydrated uranium peroxide has high reactivity for fast conversion into UF₄.

For example, a conversion of at least 90% of the uranium into UF₄ is achieved in 800 seconds.

Indeed, the method according to the invention gives the possibility of obtaining a hydrated UO4 having a high spécifie surface area which may range up to 30 m²/g.

It should be noted that the conversion rate has an influence on the morphology of the uranium peroxide hydrate obtained by the method according to the invention, which generally appears in the form of nanometric needles with a length from 300 to 500 nm and a diameter from 50 to 100 nm.

Indeed, the slower the conversion, and the longer and the finer are the needles and the more the uranium peroxide or the uranium peroxide hydrate hâve a high spécifie surface area.

The needles prepared by the method according to the invention hâve acicularity expressed by the length/diameter ratio, generally from 3 to 10.

The contents of impurities in the uranium peroxide obtained by the method according to the invention, notably because the method according to the invention does not comprise any preliminary dissolution step that may bring additional impurities, are very low.

In the following Table 1 are given the initial contents of impurities in the oxide and the final contents of impurities in the purified uranium peroxide

l.

obtained at the end of the method according to the invention.

Impurity	Initial content in the oxide (ppm/U)	Final content in the purified ÜO4 hydrate(ppm/U)	Decontamination factor(initial content/final content)
Mo	1394-1414	39-40	35.0
W	141-171	19-22	8.0
S	4669-4715	250-1400	3-18.8

TABLE 1

The contents of impurities in the final peroxide are less than those of peroxides obtained with the methods of the prior art and for most of them are compilant with the ASTM C-787 standard.

EXAMPLES:

The following examples describe the results obtained by applying the method according to the invention on several types of uraniferous concentrâtes for which the mining origin, the chemical composition and the calcination température are different.

These compounds will therefore be noted in the continuation of the text in the form of « concentrate 1 », « concentrate 2 » etc.

In the examples 1 to 5 which follow, focus is laid on the conversion of U₃Oa.ûV

Example 1 : Tests on the Concentrate 1.

In this example, précipitation of uranium peroxide with 30% hydrogen peroxide is accomplished from Concentrate 1.

The targeted concentration in the reactor is 100 g/L.

The initial content of sulfates in the mined oxide is 24,824 ppm/U.

The reactor used for this précipitation is a perfectly stirred reactor « MSU 700 » with a useful volume of 700 mL provided with 4 counter-blades and a

three-bladed	propeller	stirrer, the	diameter	of these
blades being	50 mm.
The	speed of	rotation of	the three-bladed
propeller is	adjusted	to 600 rpm	in order	to allow
efficient	suspension	of the	uranium	powder.

Précipitation of the uranium is carried out at room température.

The characteristics of the tank of the reactor as well as of the stirring device are indicated in Table 2 below: GJ

Tank
Taps on the lid	4
Tank volume	0.7 L
Tank inner diameter	80 mm
Tank height	135 mm
Counter-blades inside the tank	yes (4)
Volume of liquid in the tank (when operating)	0.5 L
Stirring
Stirring type	Rod with 3f in propellers
Blade height	15mm
Blade diameter	50mm
Blade height relativeiy to the bottom of the tank	0.05 mm
Blade diameter/tank diameter ratio	0.6
Speed of rotation of the three-bladed propeller	600 rpm (turbulent flow)

TABLE 2

Various sensors and measurement devices notably for the pH and for the température give the possibility of tracking the précipitation reaction.

After adding a known amount of the milled oxide powder into the reactor, the uranium is suspended by stirring in demineralized water, the pH of which is adjusted to pH 3 with sulfuric acid.

The oxygenated water supply is then started by means of a metering syringe pump allowing control of 15 the flow rate of reagent introduced into the reactor.

The reaction is exothermic as shown by a 10°C increase in the température of the bath, and the pH is stabilized at 1.6 at the end of the reaction.

After having achieved introduction of oxygenated water into the reactor for a duration of 3h 30mins corresponding to a molar ratio Η2θ2/ϋ=3, the oxygenated water supply is stopped and the obtained homogeneous suspension of yellow hydrated U0₄ is left with stirring for ripening during 3h 30mins.

After stopping the stirring, the uranium suspension is filtered on a Buchner (filter: 0=142mm; porosity = 0.4 5 pm) and then washed with water acidified to pH 3 with sulfuric acid. The washing ratio or « wash ratio » is 1.6.

After filtering the uranium suspension, a humid cake is obtained. The humidity content of this cake is 63%.

Analysis of the chemical composition of the filtration mother liquors shows that the residual uranium content in the filtrate is very low, i.e. of the order of 1 mg/L.

The obtained cake is then dried in the oven at 90°C for 24 hours and the dry residue is analyzed.

Analyses of the obtained solid were carried out by X-ray diffraction (XRD) (see Fig. 1) and by scanning électron microscopy (SEM) (see Fig. 4).

The XRD analyses (Fig. 1) show that the dry residue actually consists of UO₄ hydrate in the dihydrate form UO₄,2H₂O (récognition of the characteristic peaks of the defined compound)

The SEM photographs (Fig. 4) show that ÜO₄.2H₂O is in the form of nanometric needles with a length of

200 nm for example.

The impurity contents measured in the purified

UO₄ hydrates are given in the foliowing Table 3:

	Concentrate 1 (ppm/U)	Purified UO4 hydrate (ppm/U)	Decontamination factor [ ]init / [ ]final
As	87	10	8.7
Ca	1176	152	7.7
Si	1965	330	6.0
Zr	269	37	7.3
SO4	24824	<7000	>3.5

TABLE 3

Considering the whole of these results, it may be considered that the conversion of the Concentrate 1 into ÜO₄ hydrate is total and that the précipitation yield is close to 100%.

The concentrations of impurities in the final product show that the method according to the invention has allowed significant purification of the initial concentrate.

In other words, the method according to the invention gave the possibility of removing the essential part of the chemîcal éléments présent as impurities in the initial concentrate.

Example 2 : Tests on the Concentrate 2.

In this example, précipitation of uranium peroxide is carried out under the same conditions as in Example 1 but on the Concentrate 2.

The same behavior of the reaction medium is observed as in Example 1, i.e.: exothermic reaction, stabilization of the pH towards 1.6 at the end of the reaction but the kinetics is much slower. The duration of the method until the stopping of the stirring which

was 7	hours (3h30mins +3h30mins)	in Example 1, is	24
hours	in Example 2.
	The humidity content	of the CJO4 hydrate cake	is
higher	than in Example 1.	This	humidity content	is

actually 78% instead of 63%. This différence is perhaps related to the size of the UO₄ hydrate needles which are much larger than in the case of the Concentrate 1 (see Fig. 5).

The uranium content of the filtrate is 9 mg/L.

Analyses of the obtained solid were carried out by X-ray diffraction (XRD) (see Fig. 2) and by scanning électron microscopy (SEM) (see Fig. 5).

In Fig. 5, 1 to 2 pm needles of UO₄.2H2O are observed.

The measured impurity contents in the purified

U0₄ hydrate are given in the following Table 4 :

	U3Oa (ppm/U)	Purified UO4 hydrate (ppm/U)	Decontaminating factor [ ]init / [ ]final
Mo	943	103	9.2
Na	17	4	4.3

TABLE 4

The molybdenum content is still high as compared with the ASTM spécification and it does not seem possible to reduce it even in the case when O/ complexing agents are used which promote local dissolution kinetics of the uranium (see Examples 3 and

4) .

However, it is no doubt possible to improve the removal of contamination by Mo by intensive washings of the UO₄ hydrate cake.

Washing with a washing ratio « wash ratio » of 10 may give the possibility of attaining a Mo content close to 10 ppm/U, which might be acceptable in the case of a supplementary purification downstream from the method (absorption of the impurities in UFe) .

Considering the whole of these results, it may be considered that the conversion of the Concentrate 2 is as satisfactory as the one of the Concentrate 1 but it is slower and requires an intensive washing step for obtaining sufficient decontamination.

Example 3 : Tests on the Concentrate 3 with the addition of sulfates.

The conversion of the Concentrate 3 was tested according to the operating procedure of Example 1 but the experimental results show that in this case, conversion is not possible.

The operating procedure was therefore modified by adding sulfates to the concentrate in order to complex the uranium and allow its conversion hydrate.

The tested molar ratios are :

S/U = [0.125

1] .C*J

A · *

These tests were conducted on smaller amounts of U, i.e. a few grams in a stirred beaker with small volumes of solution (10 mL).

The targeted concentration in the beaker is 250 gU/L.

30% oxygenated water is gradually added to the powder suspended beforehand in demineralized water, with a molar ratio H2O2/U=2.

The sulfates are added in the form of sulfuric acid, so that the molar ratio S/U is equal to 0.125 which is the optimum value of this ratio, which corresponds to a concentration of sulfates in solution of 13 g/L.

After 8 hours, the initial concentrate is compietely converted into UO4 hydrate.

The pH varies during the reaction but is globally stable and equal to 2.

The formed hydrated UO4 is filtered on a filter paper by gravity but is not subsequently washed.

The uranium content in the filtrate is 28 mg/L.

The analysis of the purified uranium given in Table 5 below shows that certain impurities such as Mo or W are removed, decontaminated but on the other hand sulfur is not removed, purified.

	U3oe (ppm/U)	Purified UO4 (ppm/U)	Decontamination factor [ ]init /[ ]final
Mo	1017	82	12.4
W	56	26	2.2
S	761	722	1.05

TABLE 5 C-J

The results are therefore satisfactory, in particular as regards the kinetics, which is doser to that observed in the case of Example 1.

But like in the case of Example 2, intensive washing seems to be necessary in order to perfect removal of contamination of the uranium by impurities.

Example 4 : Tests on the Concentrate 4 with addition of oxalates.

As in the case of the preceding example, tests were conducted by using oxalic acid for accelerating conversion of the Concentrate 4.

The tested molar ratios are

C₂O₄/U = [0.05 - 1].

The targeted concentration in the beaker is 50 gü/L.

The 30% oxygenated water is gradually added to the powder suspended beforehand in demineralized water, with a molar ratio H₂O₂/U=2.

Oxalic acid is added to the medium so that the molar ratio C₂O₄/U is equal to 0.025, which corresponds to a concentration of oxalates in solution of 2.6 g/L.

After 11 hours, the initial U3Û_B is completely converted to hydrated U0₄.

The pH does not fall below 2.

The hydrated UO₄ formed is filtered on a paper filter by gravity but is not washed subsequently.

The uranium content in the filtrate is very high, i.e. 520 mg/L, and may be explained by the highly complexing nature of the oxalate ions.<3-J * ·

Additional tests were therefore conducted with an oxalate/sulfate mixture in order to attempt to reduce the uranium release, since the sulfates are less complexing than oxalates.

In the case of a 1/3 oxalic acid - 2/3 sulfuric acid mixture, the conversion rate is 95% after 8hours 30mins.

The releases are reduced to 330 mg/L and therefore remain approximately ten times higher as compared with tests conducted without addition of oxalic acid.

The analysis of the purified uranium (see XRD spectrum: Fig. 3) when a 1/3 oxalic acid- 2/3 sulfuric acid mixture is used, is given in Table 6 below:

1/3 oxalic acid - 2/3 sulfuric acid mixture	u308 (ppm/ü)	Purified UO< hydrate (ppm/U)	Decontamination factor [ ]init / [ ]final
Mo	1011	132	7.7
W	18	6	3
S	406	562	-
V	42	44	-
Zr	1019	953	1.1

TABLE 6

The decontamination factors observed for W and

Mo are comparable with those observed in the tests conducted with sulfuric acid alone.

Moreover, this treatment does not give the possibility of decontaminating, removing the following éléments: S, V, Zr.

The operating procedure applied in this example, therefore seems to be less adapted than theC*/ ' k one used in Example 3 in the case of highly impure oxides.

Example 5 : Tests on the Concentrate 2 with ultrasonic waves.

Tests were carried out by placing the beaker in an ultrasonic bath (35kHz) in order to accelerate conversion of the Concentrate 2 according to the operating procedure of Example 3.

The conversion into UO4 hydrate is carried out much more rapidly than in Example 2, i.e. within 2 hours instead of 24 hours, but the observed increase in température induces an overconsumption of H₂O₂, which excludes any optimization of this molar ratio (H₂O₂/U) with the penalty of reducing the conversion rate.

Moreover, it is seen that the final product is practically dry (no filtrate).

By using ultrasonic waves, it is therefore possible to significantly accelerate the conversion kinetics and to reduce notably the amounts of water in the purified UO₄ hydrate, which may hâve an advantage by simplifying the method, with potential suppression of a filtration step.

This operating procedure may be optimized (preliminary milling, injection of an inert gas, high/low frequencies ultrasonic waves combinations, control of the température by cooling the reaction medium, increase in the L/S ratio).

In the following Example 6, focus is laid on the conversion of UChZzJ

Λ J* 4

Example 6 : Tests on the Concentrate 5.

Uranium peroxide précipitation tests were conducted under the same conditions as in Example 1 but with the Concentrate 5 by adding additional calcination and washing steps according to the following scheme:

Calcination

H₂O₂ précipitation + washings

Scheme 1

The step for calcination of impure UO4 hydrate into UO3 at 200°C for 3 hours allows modification of the structure of the uranium in order to be able to then purify it more easily from its impurities during the subséquent steps.

The température of the calcination is a sensitive parameter.

Indeed, a test conducted at a température of less than 200°C (150°C) demonstrated dégradation of the results obtained by the method according to the invention as regards the purification level of the uranium towards certain impurities such as sodium (imperfect calcination of impure UO₄ hydrate into UO3, residual fraction of impure UO4 hydrate which is more difficult to decontaminate as regards sodium removal).

The second step consists of recrystallizing UO3 into UO4 hydrate according to the initial operating procedure of Example 1 and then of washing the obtained<3J *

cake with higher washing ratios (« Wash Ratios » or « WR ») than previously.

Thus, a maximum wash ratio WR of 25 was applied but an optimum wash ratio may be located between 1 and 5 10.

The washing is carried out with demineralized water for which the pH is adjusted to 2.5 with sulfuric acid.

The sélection of the acidity of the pH is guided by the efficiency of the purification in the washing step.

Indeed, a decrease in the purification level of the uranium with regard to sodium is observed if the washing is carried out with distilled water without any 15 sulfuric acid.

This is explained by the complexing nature of the sulfates towards impurities such as sodium.

The washings are carried out with the method of successive repulping operations, with a repulping 20 duration of 600 seconds, and the UO4 hydrate suspensions are filtered between two repulping operations on a Büchner (filter: 0=142mm;

porosity = 0.45pm).

With this method, the précipitation kinetics 25 are fast, i.e. less than 5 hours (an estimation of the minimum conversion time is located around 1 hour), and the analyses of the obtained solid show that the conversion is actually total.

The reaction is exothermic and the pH increases at the beginning of the reaction and then returns to its initial value, which confirms the assumptions of « >

précipitation of hydrated UO4 catalyzed by the acid according to the following reaction scheme:

Formation of the uranyl ion (Reaction intermediate):

UO₃ + 2H⁺-> UO2 ^z+ + H2O

Précipitation of the UO4 hydrate:

UO₂ ²⁺ + nH₂O + H₂O₂-* UO₄,nH₂O + 2H⁺

Equation balance:

UO₃ + nH₂O + H₂O₂ - υθ«,ηΗ₂Ο+ H₂O

The targeted concentration in the reactor was 100 gU/L in this example, but other tests hâve shown that the optimum concentration should be found in the interval 100 - 200 gU/L.

Actually, a régression of the purification level of sodium is observed if the targeted [U] is greater than 200 gü/L.

This régression is the effect of the decrease in the spécifie surface area of the UO₄ hydrate with the concentration.

The oxygenated water excess used in this example is the same as in Example 1 but the optimum molar ratio H₂O₂/U is located between 1 and 3.

The humidity level of the hydrated UO4 cake is 46%.

The morphology and the size of the hydrated UO4 grains are different from the others hydrated UO4 produced in the previous examples; the powder consista of small nanometric agglomérâtes, for example from 100 nm to 200 nm (see Fig. 6) .OJ >· ι

The uranium content in the filtrate is

5.5 mg/L.

As shown in the following Table 7, the purity of the obtained U0₄ hydrate is highly satisfactory with respect to the constraints on the method related to the

ASTM standards, and to the Comurhex® fluorination method (CX method).

	Impure hydrated UO4 (ppm/U)	Purified hydrated ÜO4 (ppm/U)	ASTM standard	CX method limit
Na	404	4		40
Ca	407	58
Mo	9	1	1.4
V	5	4	1.4
W	2	1	1.4
Cr	30	3.9	10

TABLE 7/>J

1 JUIN 2013

CABINET CAZENAVE sari

Propriété Industrielle

8.P. 500 YAOUNDE, Cameroun

Tôt. 22 21 32 89 - Fax: 22 20 64 14

E-mail: cabinetcazenave@iccnet.cm

Claims (4)

1. A method for converting ÜO₃ and/or U₃O₈ into hydrated UO₄ of formula UO₄,nH₂O wherein n is 2 or 4, comprising the following successive steps:

a) preparing an aqueous suspension of a UO₃ powder and/or a U₃O₈ powder;

b) adding hydrogen peroxide H₂O₂ to the aqueous suspension of a UO₃ and/or U₃O₈ powder, converting the UO₃ and/or U₃O_e into hydrated UO₄ and precipitating, crystallizing the hydrated UO₄ in the suspension;

c) recovering the precipitate, crystals of UO₄ hydrate;

d) optionally, washing the recovered UO₄ hydrate precipitate, crystal(s);

e) optionally, repeating step d) ;

f) optionally, drying the precipitate, the crystals;

wherein the addition of H₂O₂ to the aqueous suspension is carried out so that the suspension contains a stoichiometric excess of H₂O₂ relatively to the stoichiometry of the reaction from UO₃:

UO₃ + nH₂O+ H₂O₂ UO₄,nH₂O + H₂O (1) or of the reaction from U₃O₈

UO₂,e? + 1.33 H₂O₂ + nH₂O -> UO₄,nH₂O + H₂O (2), and the pH of the suspension is maintained in steps a) and b) at a value comprised between 2 and 3.

2. The method according to claim 1, wherein the pH of the suspension is adjusted during step a) to »’ » a value comprised between 2 and 3 by adding an acid to the suspension.

3. The method according to claim 1, wherein the acid is selected from oxalic acid, sulfuric acid and mixtures thereof. 4 . The method according to claim 1, wherein

the stoichiometric excess of H₂O₂ is from more than 1 to 10, preferably from 1.5 to 3, relatively to the stoichiometry of the reaction (1) and from more than 1.33 to 10 relatively to the stoichiometry of reaction (2) .

5. The method according to any one of the preceding claims, wherein the hydrogen peroxide is added as an aqueous solution at a concentration of 30% to 70% by weight.

6. The method according to any one of the preceding claims, wherein the aqueous suspension of UO3 and of U₃O₈ has a uranium concentration from 10 to

500 and for g/L (gü/L), from 10 to example 250 preferably from 100 to 200 g/L to for 200 U0₃, g/L, 500 g/L, g/L for preferably U₃O₈. from 100 7 . The method according to any one of the preceding claims, where in steps a) and b) are carried

out with stirring.