WO2019122731A1

WO2019122731A1 - Method for separating colloidal zirconium hydroxide contained in an acid aqueous solution

Info

Publication number: WO2019122731A1
Application number: PCT/FR2018/053433
Authority: WO
Inventors: Laure Dehuyser; Alexandre MICHAUT; Christelle MARTEAU; Pascal Nardoux
Original assignee: Orano Mining
Priority date: 2017-12-22
Filing date: 2018-12-20
Publication date: 2019-06-27
Also published as: FR3075778B1; FR3075778A1

Abstract

The invention relates to a method for separating colloidal zirconium hydroxide contained in an acid aqueous suspension with a pH of more than 3, comprising a step a) in which the aqueous suspension is subjected to a solid/liquid separation on a support in the presence of calcium sulfate, thereby obtaining an acid liquor containing pure zirconium and a solid phase containing calcium sulfate and zirconium hydroxide.

Description

Process for separating colloidal zirconium hydroxide

contained in an acidic aqueous solution

The present invention generally relates to a process for separating the colloidal zirconium hydroxide contained in an acidic aqueous suspension. In particular, the field of the invention is part of the process for separating zirconium contained in a uraniferous acidic aqueous solution which may optionally contain sulfate ions. 0 State of the art

Depending on the technologies used and the nature of the ores from which the uranium is extracted, the uranium-containing concentrates produced may contain up to 3% by mass of zirconium relative to uranium. A high concentration of zirconium in uraniferous concentrates is undesirable because it hinders the proper operation of the treatment and purification processes of these concentrates (eg clogging of the units). Moreover, these impurities tend to precipitate simultaneously with the uranium during the treatment of these concentrates, leading then to the production of a uranium which is not sufficiently pure for nuclear applications. Thus, ASTM standard C967-13 fixes for zirconium a Zr / U mass ratio of less than 0.1% in uraniferous concentrates.

Processes have therefore been developed to allow the production of uraniferous concentrates complying with the standards imposed, these processes generally having the objective of selectively separating uranium from the impurities contained in the uranium solution.

For example, document FR 2 533 907 or its counterpart

US Pat. No. 5,524,001, which discloses a process for the purification of uraniferous aqueous solutions containing, as impurities, zirconium and / or hafnium and also at least one of the anions S0 ₄ ² , NO 3, Cl and F which act as agents complexing agents of uranium and impurities. The process of FR 2 533 907, which is based on the precipitation of impurities by means of an alkaline agent, consists of the following steps: a) the uranium solution is brought to a temperature at least equal to 40 ° C, b) the content of the complexing agent is adjusted in such a way that the molar ratio [complexing agent] / [U + Zr and / or soluble Hf ] at least 3,

c) the pH of the solution is adjusted to between 2.2 and 4.3 by introducing an alkaline agent.

The suspension resulting from stage c) is treated by a solid / liquid separation step leading to the production of a purified uranium solution.

Another method is described in document FR 2,553,750, or its US counterpart 4,675,166, for treating a carbonate-containing uranium solution containing zirconium and / or hafnium and / or molybdenum. The process according to FR 2 553 750 consists in selectively precipitating zirconium and / or hafnium and / or molybdenum without precipitating the uranium by treating the carbonated solution with a dilute alkaline aqueous solution containing not more than 17 g / L of at least one alkaline hydroxide (expressed in OH) so as to reach a pH greater than 11.

US 4,330,509 teaches a process for separating and recovering zirconium from an aqueous solution of zirconium and uranium having a pH of between 0.2 and 1.0, in which the solution is treated with an agent precipitation agent selected from tartaric acid or tartrate, the amount of precipitation agent employed being sufficient to selectively precipitate zirconium as tartrate with limited uranium precipitation.

All methods for purifying uranium-containing liquid concentrates that involve zirconium precipitation in the form of hydroxide face implementation problems at the industrial level. Indeed, the zirconium precipitates are in colloidal form, with a particle size generally between a few nanometers and a few microns, making the separation operation solid / liquid complicated, long and expensive to implement. For example, separation by settling is not applicable industrially because it requires, to be effective, a residence time too long for the suspension in the settler. A separation under pressure on a filter medium (for example by means of a filter press, rotary filter or by micro or nanofiltration) is possible, but in this case, one is confronted with the phenomenon of clogging of the filter medium by the colloidal particles of zirconium hydroxide. In order to reduce the filtration time and / or prevent a rapid clogging of the membranes, it is possible to use compounds that help in the separation, for example diatomaceous earths. However, the use of such additives has an impact on the operating expenses of the process (additive costs).

In general, an object of the invention is therefore to provide a process for separating zirconium contained in an aqueous suspension, which is easy to implement and requires low investment. More particularly, the invention aims to provide an industrially applicable process for the treatment of zirconium containing uraniferous acid solutions as an impurity.

Summary of the invention

The present invention thus relates to a process for separating zirconium in hydroxide form, contained in an acidic aqueous suspension at a pH greater than 3, comprising a step a) in which a solid separation is carried out on the aqueous suspension. liquid on support in the presence of calcium sulphate. At the end of this solid / liquid separation step a), a purified zirconium acid liquor and a solid phase containing calcium sulphate and zirconium hydroxide are obtained. In the context of the present invention, the term "zirconium purified acidic liquor" refers to an acidic liquid phase whose zirconium concentration is less than or equal to 100 mg / L, preferably less than or equal to 50 mg / L and preferential less than or equal to 10 mg / L.

The inventors have surprisingly found that it is possible to separate zirconium in the form of colloidal hydroxide from an acidic aqueous suspension by a solid / liquid separation method on a support using calcium sulphate as agent for helps with separation. Thus it has been observed that in the presence of calcium sulphate, the separation time, in particular by pressure or vacuum filtration, of suspensions containing zirconium hydroxide in colloidal form can be greatly reduced so that the process becomes applicable to the industrial scale. Preferably, the pH of the suspension treated in step a) is greater than 3.1, or even greater than 3.5 and preferably greater than 3.7.

Preferably, the weight ratio [CaSO 4] / [Zr] in the suspension to be separated is such that the filtration rate measured at 25 ° C., for a filtration system under pressure or under a given vacuum, is greater than or equal to 300 L suspension / hour / m ² support.

Preferably, step a) of solid / liquid separation on support is carried out on an aqueous suspension whose weight ratio [CaSO ₄ ] / [Zr] is at least 75 g / g and preferably from minus 100 g / g.

According to the invention, the acid suspension to be purified may contain sulphate ions at a content which is generally between 10 and 60 g / l.

According to the invention, the acidic aqueous suspension which is treated may also contain uranium (solubilized in the suspension). When the suspension contains uranium, the pH of said suspension is preferably between 3.1 and 4.2, or even between 3.5 and 4 and preferably between 3.7 and 3.8, so that the uranium remains in soluble form in the suspension of zirconium hydroxide.

Thus, the process makes it possible to purify uraniferous aqueous solutions which are then used for the production of solid uranium concentrates, for example in the form of uranium oxide U0 _4, sodium diuranate Na ₂ U ₂ O ₇ (SDU ), magnesium diuranate MgU ₂ 0 ₇ or ammonium diuranate (NH ₄₎ ₂ U ₂ 0 ₇ (ADU).

The process according to the invention finds particular application in the treatment of uraniferous acid solutions containing zirconium in soluble form and sulfate ions at a level of 10 to 60 g / l. Thus, according to a preferred embodiment, the method comprises:

A step a ') in which there is provided an aqueous solution containing zirconium and uranium ions and having a pH of less than 3; then

A step a ") in which the pH of said solution is adjusted between 3.1 and 4.2 so as to selectively precipitate the zirconium hydroxide while limiting the Uranium precipitation and produce a uranium-containing suspension containing zirconium hydroxide.

The uraniferous suspension containing the colloidal zirconium hydroxide is then treated in accordance with step a) by a solid / liquid supported separation in the presence of calcium sulfate to provide a purified zirconium acidic uraniferous liquor and a solid consisting essentially of calcium sulphate and zirconium hydroxide.

The term "selective precipitation" is understood to mean a precipitation step leading to the precipitation of at least 90% by weight of zirconium while limiting the precipitation of solubilized uranium, that is to say less than 5% in mass of uranium is precipitated.

Generally, the purified zirconium acidic uraniferous acid liquor from step a) has a mass ratio Zr / U which is less than 0.1%, so that it can be used for the production of solid uranium concentrate for the manufacture of nuclear fuels.

The process according to the invention is especially suitable for purifying a uranium-uraniferous eluate originating from a uranium extraction unit by solvent or by ion exchange (on an ion exchange resin). The method according to the invention can therefore be integrated in a uranium ore treatment process comprising, in particular, an acid attack on the sulfuric acid of the ore, followed by an extraction of the uranium by solvent or on an ion exchange resin. and finally stripping with an aqueous solution, for example with sodium chloride, resulting in a sulfated acid uraniferous eluate (sulfate ions from the acid etching of the ore).

When the uraniferous aqueous solution contaminated with zirconium ions additionally contains sulphate ions, it is possible during step a '') or before step a) to precipitate in situ the calcium sulphate by addition of calcium hydroxide. This embodiment is advantageous in terms of operating costs because the calcium hydroxide serves not only to fix the pH of the aqueous uraniferous acid solution to induce the precipitation of zirconium hydroxide, but also to generate in situ the calcium sulphate which is used as a pre-layer during step a) solid / liquid separation on support.

Thus, the invention relates to a process for treating a sulfated acid uranium solution containing soluble zirconium and uranium and having an initial pH of less than 3, the process comprising the following steps:

a ') is provided said sulfated acid uraniferous solution;

a ") the pH of said solution is set to a value between 3.1 and 4.2 by addition of calcium hydroxide so as to selectively precipitate zirconium hydroxide and calcium sulfate;

A) is carried out on the suspension obtained in step a ") a solid / liquid separation on support so as to provide a purified acidic zirconium liquor and a solid phase containing essentially calcium sulfate and zirconium hydroxide The [CaSO 4] / [Zr] mass ratio of said suspension is such that the filtration rate, for a given pressure or vacuum filtration system, is greater than 300 L suspension / hour / m ² of support.

When calcium hydroxide is used as an agent for adjusting the pH, it is preferably implemented in the form of a lime milk, the content of Ca (OH) ₂ is generally between 50 and 500 g / L.

It is of course possible, before step a) or during step a), to add calcium sulphate to the suspension or to the aqueous acid solution, respectively. also combine an in situ precipitation of calcium sulphate with an addition of calcium sulphate, for example in the case where the solution / suspension has an insufficient sulphate ion content to produce a sufficient amount of calcium sulphate (by reaction with calcium ions (Ca ²⁺ )) to allow easy solid / liquid separation This procedure is also applicable when the amount of calcium hydroxide added is limited by the final pH so that the Ca ²⁺ ions supplied are then in insufficient quantity to produce the necessary amount of calcium sulfate to achieve a facilitated solid / liquid separation. Generally step a) is carried out at a temperature between 10 and 45 ° C, preferably between 20 and 40 ° C and more preferably between 25 and 35 ° C.

According to the invention, for solid / liquid separation step a), centrifugation or filtration under vacuum or under pressure is preferably used.

According to a preferred embodiment, the method according to the invention further comprises at least one step b) in which the solid phase obtained at the end of step a) is brought into contact with an acid solution having a pH that is included. between0.1-3.2, followed by at least one solid / liquid separation step c) to provide a solid phase containing essentially calcium sulfate and zirconium and an acidic liquid flux containing uranium. The acidic solution is preferably a solution of a mineral acid which may be selected from H ₂ SO ₄ , HCl and HNO ₃ , preferably the mineral acid is H ₂ SO ₄ .

The implementation of steps b) and c) thus contributes to improving the uranium yield of the process by recovering the uranium contained not only in the mother liquor impregnated in the solid phase but also possibly precipitated during the stage. In the context of the invention, step c) which aims to pass the uranium in the liquid phase may consist of washing or suspending the solid phase with the solution. acid.

At the end of step c), an acid-containing liquid flow containing uranium is collected which may optionally be mixed with the uranium-containing liquor from step a), the mixture being advantageously treated to precipitate the uranium and produce a concentrate of uranium.

According to one embodiment, the process according to the invention comprises a step d) in which the uranium contained in the purified acidic uranium liquor is precipitated in the form of uranium oxide U0 ₄ and sodium diuranate Na ₂ U ₂ 0 ₇ (SDU), magnesium MgU ₂ 0 ₇ or ammonium (NH ₄₎ ₂ U ₂ 0 _7.

The process according to the invention is also useful for treating uraniferous acid solutions derived from the dissolution of solid uranium concentrates. containing as zirconium impurity and for which the mass ratio Zr / U does not meet the standard imposed by refiners, which is generally less than 0.1%. For this purpose, a step of dissolving the uraniferous concentrate is carried out by bringing said concentrate into contact with an acidic solution (H 2 SO 4, HCl or HNO 3) whose pH is between 3.1 and 4.2 (preferably between 3.5 and 4 and preferably between 3.7 and 3.8) so as to pass the uranium in solution and precipitate the zirconium hydroxide in colloidal form. Calcium sulphate is then added to the suspension obtained or calcium sulphate is precipitated in situ if the suspension contains sulphate ions (for example provided by the sulfuric acid solution used for the dissolution of the uranium concentrate). . The mixture is subjected to the solid / liquid separation step on support described above.

According to another embodiment, the uraniferous concentrate is dissolved with an acid solution at a pH of less than 3 so as to dissolve the uranium and zirconium. The zirconium is then selectively precipitated in hydroxide form by adjusting the pH to a value between 3.1 and 4.2, preferably between 3.5 and 4 and preferably between 3.7 and 3.5. and 3.8. Calcium sulphate is then added to the suspension obtained or calcium sulphate is precipitated in situ if the suspension contains sulphate ions (for example provided by the sulfuric acid solution used for the dissolution of the uranium concentrate). . Finally, the step of solid / liquid separation on support on the suspension is carried out.

Detailed description of the invention

The other features and advantages of the invention will become apparent on reading the following description, given solely by way of illustration and without limitation, and with reference to the drawing of FIG.

• Figure 1 is a block diagram of a method according to a preferred embodiment for the treatment of a zirconium-contaminated uraniferous acid solution. In general, the process according to the invention is useful for treating acidic aqueous suspensions containing as impurities zirconium, in the form of zirconium hydroxide Zr (OH) ₄ of colloidal structure, which is difficult to separate from the liquid phase, for example by settling, because of their extremely fine structure.

In order to overcome the difficulty of separating such colloidal precipitates, the solid / liquid carrier separation step according to the invention is carried out in the presence of calcium sulphate. Without being bound by any theory, the calcium sulfate particles would promote the agglomeration of the colloidal particles of zirconium hydroxide, thus facilitating their separation from the liquid phase. Preferably, the amount of calcium sulphate involved is such that the filtration rate measured at 25 ° C., for a filtration system under vacuum or under a given pressure, is greater than or equal to 300 L of suspension / hour / m. ² of support. For example, the suspension to be separated has a mass ratio [CaSO ₄ ] / [Zr] which is preferably at least 75 g / g and more preferably at least 100 g / g.

According to the invention, the term "in the presence of calcium sulphate" refers to the fact that at the time of the solid / liquid separation operation on support, the calcium sulphate is present in the colloidal suspension of zirconium hydroxide . In this context, the calcium sulfate may be added to said colloidal solution / suspension and / or precipitated in situ therefrom.

The suspension containing the zirconium hydroxide treated in step a) of the process preferably has a pH greater than 3.1, and more preferably greater than 3.7, so that zirconium is quantitatively precipitated in the form of colloidal Zr (OH) ₄ hydroxide.

The solid / liquid carrier separation step which is preferably conducted dynamically under the action of a motive force other than gravity may be selected from centrifugation or filtration under vacuum or under pressure. example with a filter press, drum or tape. The separation step is preferably a filtration step in which the liquid of the uranium slurry is sucked through a filter element and the calcium sulfate will entrain the zirconium hydroxide precipitates so as to form a precursor. layer on the filter media. Thus, according to the invention calcium sulfate economically replaces the diatomaceous earths that are commonly used for this application.

The separation process according to the invention is particularly suitable for purifying zirconium acidic and sulphated uraniferous solutions having a pH of less than 3. Such solutions can be eluates from solvent extraction units of uranium or by ion exchange resin or solutions obtained by dissolution in an acid medium of uranium solid concentrate (also referred to as "yellow cake") contaminated with zirconium.

These solutions have a typical composition described in Table 1 0 below:

Table 1

The principle of the zirconium separation process is now described with reference to Figure 1 which relates to a particular embodiment suitable for treating an acidic and sulfated uranium solution (i.e. containing soluble uranium).

When the pH of the uranium solution to be treated is less than 3, the pH of the solution is then adjusted in order to selectively precipitate zirconium (step a "). Within the scope of the invention, the terms "selective precipitation" means a precipitation step leading to the precipitation of at least 90% by mass of zirconium while limiting the precipitation of solubilized uranium, that is to say less than 5% by mass of Uranium is precipitated. The acidic uranium solution to be treated is fed via line 1 into a precipitation reactor 2 of zirconium. This selective precipitation step is carried out by adjusting the pH of the uranium solution in a range between 3.1 and 4.2, preferably between 3.5 and 4 and preferably between 3.7 and 3.8. By addition of an alkaline agent containing hydroxide ions. The precipitation step is conducted at a temperature generally between 10 and 45 ° C. Preferably, the selective precipitation of zirconium hydroxide is carried out at room temperature (for example between 20 and 25 ° C) so that it is possible to make accurate pH measurements, possibly continuously, with measuring equipment. 0 industrial classics. Advantageously, the device for adding hydroxide ions is slaved to a pH meter so that the regulation can be performed for example to 0.1 pH unit.

The alkaline agent which is added via line 3 to the precipitation reactor 2 may be selected from sodium hydroxide, potassium hydroxide, magnesium hydroxide and calcium hydroxide. Preferably, calcium hydroxide is used. The calcium hydroxide is preferably used in the form of an aqueous solution (also known as whitewash) to facilitate its mixing in the acidic uraniferous solution to be treated. Preferably the alkaline agent is employed in a concentrated form to avoid dilution and thus limit the amount of slurry to be treated in the subsequent solid / liquid separation step a).

The precipitation step a ") is for example carried out in a stirred reactor well known to those skilled in the art.

After addition of the alkaline agent, the uraniferous solution is preferably left to ripen, preferably with stirring, for a sufficient time (for example between 30 minutes and 3 hours, preferably between 1 and 2 hours) to quantitatively precipitate the solution. zirconium hydroxide, i.e., at least about 90% by weight of the zirconium is precipitated.

With reference to FIG. 1, the uraniferous suspension containing colloidal zirconium hydroxide resulting from the ripening phase is sent to step a) of solid / liquid separation on support which is carried out in the presence of sulphate of calcium and with a mass ratio [CaSO 4] / [Zr] in the suspension sufficient to allow solid / liquid separation with a sufficiently high filtration rate from the industrial point of view. In the context of the invention, the mass ratio [CaSO 4] / [Zr] will be considered sufficient when the filtration rate measured at 25 ° C. is greater than or equal to 300 L of suspension / hour / m ² of support for a filtration system under vacuum or under pressure. Preferably, the weight ratio [CaSO 4] / [Zr] is at least 75 (g / g) and more preferably at least 100 (g / g).

According to one embodiment, the calcium sulphate, the presence of which is obligatory in step a), is not formed in situ in the uraniferous solution but is added before step a). As indicated in FIG. 1, the calcium sulphate, whatever its crystallographic form (dihydrate, hemihydrate or anhydrous, but preferably in dihydrate form), is either mixed with the uranium solution via line 4 upstream of the stage zirconium precipitation (step a ")), either brought directly into the precipitation reactor 2 via line 5 or finally added via line 6 to the uraniferous suspension which is withdrawn from the precipitation reactor via line 7, for example by means of an online mixture.

According to a preferred embodiment, the calcium sulphate is produced in situ, that is to say in the same reactor 2 as the one in which the precipitation of the zirconium hydroxide takes place, by reaction between the calcium ions ( Ca ²⁺ ) and sulphate ions present in the uraniferous solution. This embodiment is advantageous from the economic point of view, not only in terms of raw material costs because calcium hydroxide is an inexpensive alkaline agent, but also in terms of investments because it is not necessary to provide a dedicated calcium sulfate distribution system.

Finally, it is of course possible to combine the two embodiments mentioned above, namely an in situ precipitation of calcium sulphate with an additional external supply of solid calcium sulphate.

After optionally a ripening phase which can generally last between 30 minutes and 3 hours, preferably between 1 and 2 hours, the uraniferous suspension which contains calcium sulphate is sent to a mixing unit. solid / liquid separation 8 to provide a solid phase comprising calcium sulfate in admixture with zirconium hydroxide and a liquid phase containing uranium in solution. The effectiveness of the solid / liquid separation is improved due to the presence of calcium sulfate which advantageously plays the role of aiding separation.

By way of non-limiting example, the solid / liquid separation unit 8 may use a technology of the filter press, drum filter, belt filter or rotary table filter type, which implements a support or filter media (a natural or synthetic fabric) that the skilled person knows to choose according to the particle size of the solid to be separated. The solid / liquid supported separation is carried out under pressure or under vacuum, at a temperature generally of between 10 and 45 ° C., preferably between 20 and 40 ° C. and more preferably between 25 and 35 ° C.

Still with reference to FIG. 1, at the outlet of the solid / liquid separation unit 8, two streams are collected, namely a liquid stream 9 that can be designated by the term filtrate or uraniferous liquor and a stream of solid wet (or wet cake) consisting essentially of calcium sulfate and zirconium hydroxide impregnated with the uraniferous liquor (filtrate).

At the end of steps a ") and a), a uranium 9 liquor is obtained which advantageously has a Zr / U mass ratio of less than 0.1%, compatible with ASTM standard C967-08, and which is then recoverable. in the production of uranium concentrate.

According to a preferred embodiment represented in FIG. 1, the process comprises an additional step of recovering uranium which is not only present in the impregnating liquid of the wet cake but also the uranium which would have precipitated when the precipitation step of zirconium hydroxide. The recovery of the uranium from the wet cake comprises a step b) of bringing the cake into contact with an acid solution whose pH is between 3.1 and 4.2, preferably between 3.5 and 4, and preferably between 3.7 and 3.8. This step b) is followed by a solid / liquid separation step c).

According to a ^1st embodiment, the step of recovering the uranium0 comprises a step c) of suspending (repulping) the wet cake 10, at a temperature preferably between 10 and 45 ° C, in an acidic solution (for example sulfuric acid, hydrochloric or nitric acid) so as to generate a suspension whose pH is between 3.1 and 4.2, preferably between 3.5 and 4 and preferably between 3.7 and 3.8. We are thus in the optimal conditions for the dissolution of uranium without solubilizing calcium sulphate and zirconium hydroxide. This repulping operation is for example carried out at ambient temperature for a period of between 30 minutes and 5 hours, preferably between 1 and 4 hours. As shown in FIG. 1, the wet cake 10 is sent to a contacting unit 11 which is supplied with an acidic solution via line 12. The acid suspension which is withdrawn from unit 11 via line 13 is sent to a solid / liquid separation unit 14 (step c)), from which a wet cake containing essentially calcium sulphate and zirconium and an acidic liquid flux containing uranium in solution is recovered.

According to an alternative embodiment of the step of recovering the uranium contained in the wet cake, a washing is carried out (step c) which may advantageously consist of a spraying of the washing acid solution on the circulating wet cake. on a belt filter, so as to perform the steps of contacting c) and solid / liquid separation c) with a single industrial tool.

At the end of the solid / liquid separation step c), the washed wet cake 15 is removed from the process while the acidic uranium-containing liquid stream in solution is withdrawn through line 16. The liquid acid stream containing uranium may be, after possibly a stage of concentration of uranium, mixed with the uranium 9 liquor.

According to the invention, the process may comprise a step d) in which the uranium contained in the acidic liquor obtained after step a) is precipitated. This step d) may for example consist of a precipitation of uranium in the form of uranium oxide U0 ₄ , sodium diuranate Na ₂ U ₂ O ₇ (SDU), magnesia MgU ₂ O ₇ or ammonium (NH ₄ ) ₂ U ₂ 07.

With reference to FIG. 1, the uraniferous acid liquor 9 is sent to a preparation unit for the solid uranium concentrate 17, which generally comprises at least one precipitation reactor and a solid / liquid separation device, from which a solid (or concentrate) uraniferous 18 and an effluent 19 are recovered.

Example 1

Example 1 illustrates the influence of pH on the precipitation of zirconium from a synthetic initial pH solution of 0.6, containing 60 g / L [SO ₄ ² ] and having a Zr content of 0.980 g / L. In 200 ml of demineralized water, 0.78 g of Zr (SO 4) 2.4H ₂ O, 15 g of anhydrous Na ₂ SO ₄ was then added. Concentrated sulfuric acid H ₂ SO ₄ is added to bring the pH to 0.6 in order to allow the quantitative dissolution of the Zr (and 0 to approach the pH of an eluate resulting from an extraction with NaCl, in a solvent workshop).

200 g / L NaOH sodium hydroxide solution is added to said solution and the Zr content in solution is monitored by means of a portable X-ray fluorescence apparatus. For this, at different times, is taken with a syringe an aliquot of the reaction medium. The suspension taken is filtered by means of an adaptable filter on the syringe, so as to recover a liquid fraction (filtrate) whose content of soluble zirconium (i.e. not precipitated) is measured by X-ray fluorescence.

Table 2 below gives the Zr content in the filtrate (after solid / liquid separation of the suspension) as well as the pH of the suspension as a function of time.

0 Table 2

* Adding soda At T0, sodium hydroxide is gradually added to the solution until a pH of 3 is reached after T0 + 3h. At pH 3, there is a precipitation of zirconium which is not quantitative, even after 24 hours of aging (passage from 0.980 to 0.782 mg / L, ie a precipitation yield of Zr of 23% by weight).

When the pH of the solution (suspension) is increased to 3.5, the precipitation yield of Zr is 50.5% by weight. On the other hand, when the pH of the suspension is fixed at 3.7, a quantitative precipitation of zirconium hydroxide is observed, which results in a content of 68 ppm of Zr in the filtrate, ie a precipitation yield of greater than 93%. in mass.

Another test was carried out by fixing the pH of the solution at 3.8 during which zirconium was found to precipitate quantitatively in one hour, with a yield of 98% by weight (20 ppm of residual Zr measured in the filtrate). . Example 2

Example 2 below illustrates the positive effect of the presence of calcium sulfate on the solid / liquid separation of a suspension containing a colloidal precipitate of zirconium hydroxide.

Three tests (1, 2 and 3) were carried out in parallel on a solution (2000 ml) containing 1 g / l of Zr, 60 g / l of S0 ₄ ² and at pH = 0.6. Soda ash concentrated to 200 g / L is added to cause the precipitation of zirconium hydroxide form.

In Run 1, the pH of the solution is adjusted to 2.5 and it is found that zirconium does not precipitate at this pH.

For runs 2 and 3, the pH was set at 4.4 and a turbidity in the beakers corresponding to the precipitate of colloidal zirconium hydroxide was observed.

Stirring is maintained for one hour for the precipitation to be quantitative.

For test 3, after stirring for one hour, gypsum is added to the suspension, equivalent to 28 g of CaSO ₄ , ie a mass ratio [CaSO ₄ ] / [Zr] of 140 (g / g) and the stirring is left an extra hour. The suspensions of tests 2 and 3 were filtered on a filtration system equipped with a MF-Millipore ™ membrane with a diameter of 47 mm and a porosity of 0.45 μm, under a vacuum of 850 mbar and at a temperature of 25 ° C. . In Table 3, for tests 2 and 3, the filtration time and the zirconium content in the filtrate which has been measured by X-ray fluorescence are given.

Table 3

It is found that in the test 2 the filtration is very difficult and after 0 2:30, only 100 mL (over 200 mL) of the suspension are passed through the filtration system. This filterability problem is attributable to the precipitate of colloidal zirconium hydroxide which settles slowly in the filtering tulip and which gradually clogs the filter, preventing the rest of the suspension from passing. On the other hand, for the test 3, the filtration of the 200 ml of suspension in the presence of gypsum was carried out in 18 minutes, corresponding to a filtration rate of greater than 300 L of suspension / hour / m ² of filter (in this case the calculated speed is approximately 384 L suspension / hour / m ² ). Gypsum therefore has a crucial pre-layer role in the separation of the colloidal precipitate of zirconium hydroxide. In addition it has been noted that zirconium hydroxide tends to decant rapidly in the tulip by entrainment by calcium sulfate.

Example 3

Example 3 was carried out to evaluate the effect of the amount of gypsum on the filtration time.

200 ml of a solution containing 1 g / L of Zr, 60 g / l of SO ₄ ² and an initial pH of 0.6 were first prepared for each test. The pH of the solution is then adjusted to 3.8 by adding a sodium hydroxide solution at 200 g / L in order to precipitate the zirconium hydroxide. Gypsum is then added to the suspensions (5 to 28.8 g of CaSO ₄ per 200 ml of suspension) and the suspensions are then filtered under the conditions of Example 2 so as to provide a filtrate which is analyzed by ICP-AES. (Atomic Emission Spectrometry with Inductively Coupled Plasma).

Table 4 gives the filtration time, which was measured under the same conditions as test 3, depending on the amount of calcium sulfate (in the form of gypsum) introduced into the suspension.

0 Table 4

For each test, it is found that zirconium precipitated quantitatively and that above 15 grams of CaSO ₄ in the suspension, a mass ratio [CaSO ₄ ] / [Zr] greater than 75 (g / g), filtration of the suspension is easy. 5

Example 4

Example 4 illustrates an in situ precipitation of calcium sulfate and takes place concurrently with the precipitation of zirconium hydroxide from a zirconium acid solution. Three suspensions of calcium hydroxide (milk of lime) were prepared at 50 g / l, 100 g / l and 200 g / l of Ca (OH) _{2, respectively} .

The milks of lime serve as an alkaline solution for setting the pH of 200 ml of a zirconium acid solution (initial pH 0.6) containing 1 g / l of Zr and 60 g / l of SO ₄ ² 5 pH of 3.8. After adjusting the pH of the zirconium acid solution, the latter is allowed to mature with stirring for 4 hours and the suspension is then filtered under the same conditions as in Example 2.

Precipitation results and filtration times are noted in Table 5.

0

Table 5

It is found that 4 hours after the alkalization step at pH 3.8, the precipitation yield of zirconium is substantially identical for the tests 10.5 11 and 12 and is greater than 99%. Finally, it is noted that the filtrations proceeded very rapidly thanks to the presence of the calcium sulphate formed in situ, with filtration times of between 3 and 5 minutes, ie with filtration rates of 2300 and 1300 L respectively. hour / m ² of support. 0 Example 5

Example 5 illustrates that the process according to the invention is suitable for carrying out a selective separation of the zirconium contained in an acidic uranium solution.

Test 13 was carried out in order to verify that at pH 3.8 the uranium does not precipitate from a solution whose initial pH is 0.6 and containing 60 g / l of SO ₄ ² and 32.7 g / L U. The solution was prepared by adding in 200 mL of demineralized water:

• 21 mL of uranyl nitrate solution at 332 g / L U,

15 g of anhydrous Na ₂ SO ₄ .

The pH of the uranium solution is then adjusted to 3.8 by adding 0 sodium hydroxide to 200 g / l. It is found that at this pH, the solution remains clear and that there is no precipitation of uranium. To this clear solution was then added 28.8 g CaSO ₄ and the suspension was filtered according to Example 2.

For Run 14 a solution at pH 0.6 was prepared containing 60 g / L SO ₄ ^2- , 0.77 g / L Zr and 32.7 g / L U. In 200 ml of demineralised water, therefore, 0.78 g of Zr (SO ₄ ) ₂ .4H ₂ O was added,

• 21 mL of uranyl nitrate solution at 332 g / L U,

15 g of anhydrous Na ₂ SO ₄ .

This solution is representative of an acidic aqueous solution obtained by dissolving in sulfuric acid medium a uranium concentrate or a uranium eluate, resulting from a solvent workshop.

The pH of the uranium solution is adjusted to the value of 3.8 by addition of sodium hydroxide at 200 g / l. A slight cloudiness is observed in the solution indicating the precipitation of zirconium hydroxide.

After stirring for 3 hours, 28.8 g of CaSO _{4 are added} to the uranium suspension and the mixture, whose mass ratio [CaSO ₄ ] / [Zr] is equal to 144 (g / g), is filtered as in Example 2

Table 6 gives the filtration time of the uraniferous solution and suspension of tests 13 and 14 as well as the Zr and U contents of the filtrates which were measured by ICP-AES.

0

Table 6

From Table 6, it is concluded that it is possible to purify a solution of uranium by zirconium by selective precipitation, followed by separation by filtration on the support of the suspension containing the colloidal zirconium hydroxide and the sulphate. calcium with a filtration rate greater than 300 L of solution / hour / m ² (in the example the filtration rate is estimated at 430 L of solution / hour / m ² ).

Example 6

Example 6 illustrates an in situ precipitation of calcium sulfate in a uraniferous solution containing sulfate ions and zirconium.

Example 6 corresponds to test 14 of Example 5 except that the alkaline agent added to adjust the pH to 3.8 is a lime milk with 200 g / L Ca (OH) ₂ replacing 200 g / l of sodium hydroxide and that calcium sulphate is not added to the suspension after alkalinization.

After adding the milk of lime to the uranium suspension, it is left stirring for 3 hours and then filtered according to the method described in Example 2. At the end of the filtration step which lasted 7 minutes, we recover:

• a wet cake and

0 · a uranium filtrate containing 28.2 g / L U (ICP-AES measurement) and 28.9 mg / L Zr

(ICP-AES measurement), a Zr / U ratio of 0.1%.

After drying the wet cake at 105 ° C. for 48 hours, approximately 10 g of solid consisting essentially of calcium sulphate is obtained. The ICP analysis of the solid after mineralization indicates that the calcium sulfate contains 12.8 g Zr / kg solid and 53.9 g U / kg solid. The uranium detected in the cake corresponds to uranium mainly from the mother liquor which was impregnated in the wet cake.

The uranium filtrate was used in a precipitation reaction of uranium in the presence of hydrogen peroxide to form uranyl peroxide U0 ₄ .

Example 7

8.2 g of a calcium sulphate wet cake containing 135 g U / kg and 11.1 g Zr / kg are put in contact (repulpage) three times with a solution of sulfuric acid at pH 3.5 and with a ratio [liquid] (mL) / [solid] (g) of 36 (ml / g). After each repulping, the suspension is filtered according to the method of Example 2 so as to recover a filtrate.

Table 7 gives the contents (determined by ICP-AES) at U and Zr of the three wash filtrates collected at the end of the three acid repulping steps.

Table 7

The analytical results indicate that the recovery of uranium by three successive repulpings in acidic medium at pH 3.5 is quantitative and is also selective in that the presence of zirconium in the filtrates is not detected. (less than the detection limit of the analytical method).

Example 8

Example 8 groups together tests in which the concentration of sulphate ions of a zirconium solution at pH 0.6 and containing 0.9 g / L of Zr. The zirconium solution is prepared by adding 0.78 g of Zr (SO ₄ ) ₂ .4H ₂ O to 0.75 g of demineralized water and the concentration of sulphate ions is adjusted by addition of anhydrous Na ₂ SO ₄ .

The pH of the zirconium solutions of tests 15 to 20 were then brought to 3.8 by addition of a lime milk to 200 g / L of Ca (OH) ₂ .

After maturing for one hour with stirring, the suspension is filtered with a Buchner filtration system equipped with a Whatman ™ filter with a diameter of 90 mm and a porosity of lpm, under a vacuum of 850 mbar.

Table 8 groups together the filtration times of tests 15 to 20 and the Zr concentration (determined by ICP-AES) of the filtrates recovered after filtration.

Table 8

* in test 15, the sulphates are only provided by the zirconium sulphate and the sulfuric acid which made it possible to fix the initial pH at 0.6

It emerges from Example 8 that it is possible to precipitate the zirconium contained in a solution loaded with sulphate ions, the latter are advantageously used to precipitate in situ the calcium sulphate. The presence of the latter in the suspension has the effect of facilitating the solid / liquid separation by reducing the filtration times.

Claims

1. Process for separating the colloidal zirconium hydroxide contained in an acidic aqueous suspension of pH greater than 3, comprising a step a) da ns

Which is carried out on the aqueous suspension a solid / liquid separation on support in the presence of calcium sulfate, after which there is obtained a purified acidic liquor zirconium and a solid phase containing calcium sulfate and hydroxide of zirconium.

2. Method according to claim 1, in which the weight ratio [CaSO 4] / [Zr] of said suspension is such that the filtration rate measured at 25 ° C, for a given filtration system under pressure or under vacuum, is greater than or equal to 300 L of suspension / hour / m ² of support.

3. Method according to one of claims 1 or 2, wherein the aqueous acidic suspension further contains sulfate ions at a content of between 10 and 60 g / l. 5

4. Method according to one of claims 1 to 3, wherein the acidic aqueous suspension further contains uranium and wherein at the end of step a) is obtained a purified zirconium uraniferous acid liquor.

5. The method of claim 4, wherein the uranium content is between 5 and 50 g / l of uranium.

6. The process according to one of claims 4 or 5, comprising the steps preliminary to step a) following:

A step a ') in which is provided an aqueous solution containing zirconium and uranium soluble and pH less than 3; then

A step a '') in which the pH of said solution is adjusted between 3.1 and 4.2 so as to selectively precipitate zirconium hydroxide.

7. Method according to one of claims 3 to 5, wherein before step a) is added to the acidic aqueous suspension of calcium hydroxide so as to precipitate calcium sulfate and / or calcium sulfate.

The process according to claim 6, wherein in step a ") calcium hydroxide is added to the aqueous acid solution so as to precipitate calcium sulfate and / or calcium sulfate.

9. Process according to claims 7 or 8, in which a lime milk is added to

A content of Ca (OH) ₂ of between 50 and 500 g / l.

The method according to one of the preceding claims, wherein the [CaSO 4] / [Zr] mass ratio of said acidic aqueous suspension is at least 75 g / g and preferably at least 100 g / g.

11. A process according to any one of claims 4 to 10, wherein the purified zirconium uraniferous acidic liquor has a Zr / U mass ratio of less than 0.1%.

12. Method according to one of claims 4 to 11, comprising at least one step b) wherein is brought into contact the solid phase from step a) with an acid solution having a pH between 3.1 and 4 , 2 and at least one step c) in which a solid / liquid separation is carried out so as to provide a solid phase containing essentially calcium sulphate and zirconium and a liquid stream containing uranium.

13. The method of claim 12, wherein the contacting of step c) is selected from washing and suspending the solid phase.

14. A process according to any one of claims 4 to 13, wherein the solution or uraniferous aqueous suspension is selected from a uraniferous acid eluate from a solvent extraction unit or by ion exchange and an acid solution obtained. by dissolving a solid concentrate of uranium.